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Mangal V, Lam WY, Emilson EJS, Mackereth RW, Mitchell CPJ. The molecular diversity of dissolved organic matter in forest streams across central Canadian boreal watersheds. Environ Sci Process Impacts 2024. [PMID: 38690707 DOI: 10.1039/d3em00305a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/02/2024]
Abstract
Small headwater streams can mobilize large amounts of terrestrially derived dissolved organic matter (DOM). While the molecular composition of DOM has important controls on biogeochemical cycles and carbon cycling, how stationary landscape metrics affect DOM composition is poorly understood, particularly in relation to non-stationary effects from hydrological changes across seasons. Here, we apply a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and absorbance spectroscopy to characterize stream DOM from 13 diverse watersheds across the central Canadian boreal forests and statistically relate DOM compositional characteristics to landscape topography and hydrological metrics. We found that watershed runoff across different surface physiographies produced DOM with distinctly different chemical compositions related to runoff pH. Specifically, streams in sandy soil watersheds contained more abundant aromatic, nitrogenated and sulfurized fractions of DOM, likely due to a combination of lower soil capacity to absorb DOM than other soil types and high conifer forest coverage that generated acidic litterfall in more sandy watersheds. In contrast, streams with more neutral pH in watersheds with shallow soils had DOM resembling low oxidized phenolic molecules mainly due to increased brush/alder and deciduous vegetation coverage in relatively steeper watersheds. However, as precipitation and flows increased in the fall, the overall water chemistry of streams became more similar as runoff pH increased, the overall chemical diversity of DOM in streams decreased, and stream DOM resembled fresher, lower molecular weight lignin material likely originating from freshly produced leaf litter. Together, our findings show that during hydrologically disconnected periods, pH and landscape characteristics have important controls on the mobilization of aromatic DOM but that many landscape-specific characteristics in the Canadian boreal forest are less influential on DOM processing during wetter conditions where chemically similar, plant-derived DOM signatures are preferentially mobilized. These findings collectively help predict the composition of DOM across diverse watersheds in the Canadian boreal to inform microbial and contaminant biogeochemical processes in downstream ecosystems.
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Affiliation(s)
- V Mangal
- Chemistry Department, Brock University. 1812 Sir Isaac Brock Way, St. Catherine's, ON, L2S 3A1, Canada.
| | - W Y Lam
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada.
| | - E J S Emilson
- Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, 1219 Queen St. E., Sault Ste. Marie, ON, P6A 2E5, Canada
| | - R W Mackereth
- Centre for Northern Forest Ecosystem Resources, Ontario Ministry of Natural Resources and Forestry, 421 James St. S., Thunder Bay, ON, P7E 2V6, Canada
| | - C P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada.
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2
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Zhong H, Zhou H, Li Y, Li C, Tsui MTK, Mitchell CPJ, Zhou Y, Yang Y, Chen L, Ren H, Tang W. Methylmercury photodegradation in paddy water: An overlooked process mitigating methylmercury risks. Water Res 2024; 253:121332. [PMID: 38377924 DOI: 10.1016/j.watres.2024.121332] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/15/2024] [Revised: 02/15/2024] [Accepted: 02/16/2024] [Indexed: 02/22/2024]
Abstract
Photodegradation is critical to reduce the potent neurotoxic methylmercury (MeHg) in water and its subsequent accumulation along food chains. However, this process has been largely ignored in rice paddies, which are hotspots of MeHg production and receive about a quarter of the world's developed freshwater resources. Here, we reported that significant MeHg photodegradation, primarily mediated by hydroxyl radicals, occurs in the overlying water during rice growth. By incorporating field-measured light interception into a rice paddy biogeochemistry model, as well as photodegradation rates obtained from 42 paddy soils stretching ∼3500 km across China, we estimated that photodegradation reduced MeHg concentrations in paddy water and rice by 82 % and 11 %, respectively. Without photodegradation, paddy water could be a significant MeHg source for downstream ecosystems, with an annual export of 178 - 856 kg MeHg to downstream waters in China, the largest rice producer. These findings suggest that photodegradation in paddy water is critical for preventing greater quantities of MeHg entering human food webs.
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Affiliation(s)
- Huan Zhong
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China
| | - Hangyu Zhou
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China
| | - Yanbin Li
- Key Laboratory of Marine Chemistry Theory and Technology (Ministry of Education) and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong Province, PR China.
| | - Chengjun Li
- Institute of Environmental Research at Greater Bay Area, Guangzhou University, Guangzhou, Guangdong Province, PR China
| | - Martin Tsz-Ki Tsui
- School of Life Sciences, Earth and Environmental Sciences Programme, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, PR China; State Key Laboratory of Marine Pollution, City University of Hong Kong, Kowloon Tong, Kowloon, Hong Kong SAR, PR China
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada
| | - Yang Zhou
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China
| | - Yanan Yang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China
| | - Long Chen
- Key Laboratory of Geographic Information Science (Ministry of Education), School of Geographic Sciences, East China Normal University, Shanghai, PR China
| | - Hongqiang Ren
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China
| | - Wenli Tang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, Jiangsu Province, PR China.
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3
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Lam WY, Mackereth RW, Mitchell CPJ. Mercury concentrations and export from small central Canadian boreal forest catchments before, during, and after forest harvest. Sci Total Environ 2024; 912:168691. [PMID: 37996028 DOI: 10.1016/j.scitotenv.2023.168691] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2023] [Revised: 11/15/2023] [Accepted: 11/16/2023] [Indexed: 11/25/2023]
Abstract
Northern boreal forests are a strong sink for mercury (Hg), a global contaminant of significant concern to wildlife and human health. Mercury stored in forest soils can be mobilized via runoff and erosion, and under suitable conditions can be methylated to its much more bioaccumulative form, methylmercury. Forest harvesting can affect the mobilization and methylation of Hg, though the direction and magnitude of the impact is unclear or conflicting across previous studies. This study examined 5 harvested and 2 reference watersheds in northwestern Ontario, Canada, before, during, and after harvest to quantify changes in stream total and methylmercury concentration and loads and identified potential landscape and management factors that contribute to differences in stream response. In watersheds where streams were buffered by natural vegetation (≥30 m), no significant changes in total Hg or methylmercury concentrations or loads were observed. Significant increases in methylmercury concentrations and loads were observed downstream of a stream crossing in a watershed where the relatively small stream was unmapped and therefore only buffered by a 3 m machine exclusion zone. These results show that when current best management practices that minimize soil and water disturbance are followed, harvest can have a minimal impact on total and methylmercury loads, even in extensively harvested watersheds. However, there is a need for improved mapping of small streams to ensure best management practices are applied adequately across the landscape.
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Affiliation(s)
- W Y Lam
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, Canada
| | - R W Mackereth
- Centre for Northern Forest Ecosystem Research, Ontario Ministry of Natural Resources and Forestry, Thunder Bay, ON, Canada
| | - C P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, Canada.
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4
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Huang H, Mackereth RW, Mitchell CPJ. Impacts of forest harvesting on mercury concentrations and methylmercury production in boreal forest soils and stream sediment. Environ Pollut 2024; 341:122966. [PMID: 37981183 DOI: 10.1016/j.envpol.2023.122966] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/25/2023] [Revised: 11/13/2023] [Accepted: 11/14/2023] [Indexed: 11/21/2023]
Abstract
Methylmercury (MeHg) is the most neurotoxic and bioaccumulative form of mercury (Hg) present in the terrestrial and aquatic food sources of boreal ecosystems, posing potential risks to wildlife and human health. Harvesting impacts on Hg methylation and MeHg concentrations in forest soils and stream sediment are not fully understood. In this study, a field investigation was carried out in 4 harvested and 2 unharvested boreal forest watersheds, before and after harvest, to better understand impacts on Hg methylation and MeHg concentration in soils and stream sediment, including their responses to different forest management practices. Changes in total Hg (THg) and MeHg concentrations, first-order potential rate constants for Hg methylation and MeHg demethylation (Kmeth and Kdemeth) as well as total carbon content and carbon-to-nitrogen ratio post-harvest in upland, wetland and riparian soils and stream sediment were assessed and compared. Increases in MeHg production were minimal in upland, wetland or riparian soils after harvest. Sediment in streams with minor buffer protection (∼3 m), greater fractions (>75%) of harvested watershed area and more road construction had significantly increased THg and MeHg concentrations, %-MeHg, Kmeth and total carbon content post-harvest. From these patterns, we infer that inputs of carbon and inorganic Hg into harvest-impacted stream sediment are likely sourced from the harvested upland areas and stimulate in situ MeHg production in stream sediment. These findings indicate the importance of stream sediment as potential MeHg pools in harvested forest watersheds. The findings also demonstrate that forest management practices aiming to mitigate organic matter and Hg inputs to streams can effectively alleviate harvesting impacts on Hg methylation and MeHg concentrations in stream sediment.
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Affiliation(s)
- Haiyong Huang
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, Canada
| | - Robert W Mackereth
- Centre for Northern Forest Ecosystem Research, Ontario Ministry of Natural Resources and Forestry, Thunder Bay, ON, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, Canada.
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5
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Tang W, Bai X, Zhou Y, Sonne C, Wu M, Lam SS, Hintelmann H, Mitchell CPJ, Johs A, Gu B, Nunes L, Liu C, Feng N, Yang S, Rinklebe J, Lin Y, Chen L, Zhang Y, Yang Y, Wang J, Li S, Wu Q, Ok YS, Xu D, Li H, Zhang XX, Ren H, Jiang G, Chai Z, Gao Y, Zhao J, Zhong H. A hidden demethylation pathway removes mercury from rice plants and mitigates mercury flux to food chains. Nat Food 2024; 5:72-82. [PMID: 38177223 DOI: 10.1038/s43016-023-00910-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2023] [Accepted: 12/01/2023] [Indexed: 01/06/2024]
Abstract
Dietary exposure to methylmercury (MeHg) causes irreversible damage to human cognition and is mitigated by photolysis and microbial demethylation of MeHg. Rice (Oryza sativa L.) has been identified as a major dietary source of MeHg. However, it remains unknown what drives the process within plants for MeHg to make its way from soils to rice and the subsequent human dietary exposure to Hg. Here we report a hidden pathway of MeHg demethylation independent of light and microorganisms in rice plants. This natural pathway is driven by reactive oxygen species generated in vivo, rapidly transforming MeHg to inorganic Hg and then eliminating Hg from plants as gaseous Hg°. MeHg concentrations in rice grains would increase by 2.4- to 4.7-fold without this pathway, which equates to intelligence quotient losses of 0.01-0.51 points per newborn in major rice-consuming countries, corresponding to annual economic losses of US$30.7-84.2 billion globally. This discovered pathway effectively removes Hg from human food webs, playing an important role in exposure mitigation and global Hg cycling.
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Affiliation(s)
- Wenli Tang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Xu Bai
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China
- University of Chinese Academy of Sciences, Beijing, China
| | - Yang Zhou
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Christian Sonne
- Department of Ecoscience, Arctic Research Centre, Aarhus University, Roskilde, Denmark.
- Sustainability Cluster, School of Engineering, University of Petroleum and Energy Studies, Dehradun, India.
| | - Mengjie Wu
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Su Shiung Lam
- Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, Kuala Nerus, Malaysia
- Center for Global Health Research (CGHR), Saveetha Institute of Medical and Technical Sciences (SIMATS), Saveetha University, Chennai, India
| | - Holger Hintelmann
- Department of Chemistry and School of the Environment, Trent University, Peterborough, Ontario, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada
| | - Alexander Johs
- Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
| | - Baohua Gu
- Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
| | - Luís Nunes
- Faculty of Sciences and Technology, Civil Engineering Research and Innovation for Sustainability Center, University of Algarve, Faro, Portugal
| | - Cun Liu
- Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China
| | - Naixian Feng
- College of Life Science and Technology, Jinan University, Guangzhou, China
| | - Sihai Yang
- State Key Laboratory of Pharmaceutical Biotechnology, School of Life Sciences, Nanjing University, Nanjing, China
| | - Jörg Rinklebe
- School of Architecture and Civil Engineering, Institute of Foundation Engineering, Water and Waste Management, Laboratory of Soil and Groundwater Management, University of Wuppertal, Wuppertal, Germany
| | - Yan Lin
- Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, College of Environmental Science and Engineering, Nankai University, Tianjin, China
| | - Long Chen
- Key Laboratory of Geographic Information Science (Ministry of Education), School of Geographic Sciences, East China Normal University, Shanghai, China
| | - Yanxu Zhang
- School of Atmospheric Sciences, Nanjing University, Nanjing, China
| | - Yanan Yang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Jiaqi Wang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Shouying Li
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Qingru Wu
- State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China
- State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing, China
| | - Yong Sik Ok
- Korea Biochar Research Center, APRU Sustainable Waste Management Program and Division of Environmental Science and Ecological Engineering, Korea University, Seoul, Republic of Korea
| | - Diandou Xu
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China
| | - Hong Li
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China
| | - Xu-Xiang Zhang
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Hongqiang Ren
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China
| | - Guibin Jiang
- State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
| | - Zhifang Chai
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China
- State Key Laboratory of Radiation Medicine and Protection, Soochow University, Suzhou, China
| | - Yuxi Gao
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China.
| | - Jiating Zhao
- Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Beijing, China.
- Department of Environmental Science, Zhejiang University, Hangzhou, China.
| | - Huan Zhong
- School of the Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China.
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6
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Harrow-Lyle TJ, Lam WY, Emilson EJS, Mackereth RW, Mitchell CPJ, Melles SJ. Watershed characteristics and chemical properties govern methyl mercury concentrations within headwater streams of boreal forests in Ontario, Canada. J Environ Manage 2023; 345:118526. [PMID: 37418824 DOI: 10.1016/j.jenvman.2023.118526] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/08/2023] [Revised: 06/21/2023] [Accepted: 06/25/2023] [Indexed: 07/09/2023]
Abstract
Methyl mercury (MeHg) concentrations in boreal headwater streams are influenced by complex natural processes and disturbances such as forestry management. Understanding drivers of MeHg within boreal streams in Ontario, Canada, is of particular interest as there are legacy MeHg concerns. However, models accounting for the complexity of underlying processes have not yet been developed. We assessed how catchment characteristics and stream water chemistry influence MeHg concentrations within 19 watersheds of the Dryden - Wabigoon Forest in Ontario, Canada, using a structural equation modelling (SEM) approach. Despite the study area encompassing a large variation of boreal forest watersheds in the Canadian Shield, our SEM had substantial explanatory power across the region (χ251 = 45.37, p-value = 0.70, R2 = 0.75). Nitrate concentrations (p-value <0.001), water temperature (p-value = 0.002), and the latent watershed characteristic (p-value <0.001) had a positive influence on MeHg concentrations once variable interactions were accounted. Due to the inherent strengths of applying an SEM approach, we describe two plausible pathways driving MeHg concentrations: 1) indirect effect of forest-derived nutrients increases in-situ MeHg production in Dryden - Wabigoon Forest streams, and 2) direct supply of MeHg from inundated soils following consistent precipitation and inundation events (i.e., fill, sit, and spill).
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Affiliation(s)
- Tyler J Harrow-Lyle
- Department of Chemistry and Biology, Toronto Metropolitan University, 43 Gerrard St, Toronto, Ontario, M5B 2K, Canada.
| | - Wai Ying Lam
- University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada.
| | - Erik J S Emilson
- Natural Resources Canada, Canadian Forest Service, 1219 Queen Street E., Sault Ste. Marie, Ontario, P6A 2E5, Canada.
| | - Robert W Mackereth
- Ministry Natural Resources and Forestry, 421 James St., Thunder Bay, Ontario, P7E 2V6, Canada.
| | - Carl P J Mitchell
- University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada.
| | - Stephanie J Melles
- Department of Chemistry and Biology, Toronto Metropolitan University, 43 Gerrard St, Toronto, Ontario, M5B 2K, Canada.
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7
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Huang H, Mitchell CPJ. Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment. Sci Total Environ 2023:164447. [PMID: 37245803 DOI: 10.1016/j.scitotenv.2023.164447] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/07/2023] [Revised: 05/21/2023] [Accepted: 05/22/2023] [Indexed: 05/30/2023]
Abstract
Terrestrial ecosystems store large amounts of Hg, which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.
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Affiliation(s)
- Haiyong Huang
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
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8
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McCarter CPR, Sebestyen SD, Coleman Wasik JK, Engstrom DR, Kolka RK, Jeremiason JD, Swain EB, Monson BA, Branfireun BA, Balogh SJ, Nater EA, Eggert SL, Ning P, Mitchell CPJ. Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater. Environ Sci Technol 2022; 56:17615-17625. [PMID: 36445185 DOI: 10.1021/acs.est.2c02621] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/16/2023]
Abstract
Changes in sulfate (SO42-) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42- export from a peatland-dominated catchment as a function of changing SO42- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO42- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42- deposition declines.
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Affiliation(s)
- Colin P R McCarter
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, OntarioM1C 1A4, Canada
| | - Stephen D Sebestyen
- USDA Forest Service Northern Research Station, Grand Rapids, Minnesota55744, United States
| | - Jill K Coleman Wasik
- Department of Plant and Earth Science, University of Wisconsin - River Falls, 410 S. 3rd Street, River Falls, Wisconsin54022, United States
| | - Daniel R Engstrom
- St. Croix Watershed Research Station, Science Museum of Minnesota, 16910 152nd Street N., Marine on St. Croix, Minnesota55047, United States
| | - Randall K Kolka
- USDA Forest Service Northern Research Station, Grand Rapids, Minnesota55744, United States
| | - Jeff D Jeremiason
- Department of Chemistry, Gustavus Adolphus College, 800 W College AvenueSt. Peter, Minnesota56082, United States
| | - Edward B Swain
- Department of Fisheries, Wildlife, and Conservation Biology, University of Minnesota, St. Paul, Minnesota55108, United States
| | - Bruce A Monson
- Minnesota Pollution Control Agency, 520 Lafayette Road North, Saint Paul, Minnesota55155, United States
| | - Brian A Branfireun
- Department of Biology, The University of Western Ontario, London, OntarioN5B 2A7, Canada
| | - Steven J Balogh
- Metropolitan Council Environmental Services, 2400 Childs Road, Saint Paul, Minnesota55106, United States
| | - Edward A Nater
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota55108, United States
| | - Susan L Eggert
- USDA Forest Service Northern Research Station, Grand Rapids, Minnesota55744, United States
| | - Paris Ning
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, OntarioM1C 1A4, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, OntarioM1C 1A4, Canada
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9
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Negrazis L, Kidd KA, Erdozain M, Emilson EJS, Mitchell CPJ, Gray MA. Effects of forest management on mercury bioaccumulation and biomagnification along the river continuum. Environ Pollut 2022; 310:119810. [PMID: 35940481 DOI: 10.1016/j.envpol.2022.119810] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/07/2022] [Revised: 07/15/2022] [Accepted: 07/16/2022] [Indexed: 06/15/2023]
Abstract
Forest management can alter the mobilization of mercury (Hg) into headwater streams and its conversion to methylmercury (MeHg), the form that bioaccumulates in aquatic biota and biomagnifies through food webs. As headwater streams are important sources of organic materials and nutrients to larger systems, this connectivity may also increase MeHg in downstream biota through direct or indirect effects of forestry on water quality or food web structure. In this study, we collected water, seston, food sources (biofilm, leaves, organic matter), five macroinvertebrate taxa and fish (slimy sculpin; Cottus cognata) at 6 sites representing different stream orders (1-5) within three river basins with different total disturbances from forestry (both harvesting and silviculture). Methylmercury levels were highest in water and some food sources from the basin with moderate disturbance (greater clearcutting but less silviculture). Water, leaves, stoneflies and fish increased in MeHg or total Hg along the river continuum in the least disturbed basin, and there were some dissipative effects of forest management on these spatial patterns. Trophic level (δ15N) was a significant predictor of MeHg (and total Hg in fish) within food webs across all 18 sites, and biomagnification slopes were significantly lower in the basin with moderate total disturbance but not different in the other two basins. The elevated MeHg in lower trophic levels but its reduced trophic transfer in the basin with moderate disturbance was likely due to greater inputs of sediments and of dissolved organic carbon that is more humic, as these factors are known to both increase transport of Hg to streams and its uptake in primary producers but to also decrease MeHg bioaccumulation in consumers. Overall, these results suggest that the type of disturbance from forestry affects MeHg bioaccumulation and trophic transfer in stream food webs and some longitudinal patterns along a river continuum.
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Affiliation(s)
- Lauren Negrazis
- Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada
| | - Karen A Kidd
- Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada; School of Earth, Environment and Society, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada.
| | - Maitane Erdozain
- Canadian Rivers Institute and Biology Department, University of New Brunswick, 100 Tucker Park Road, Saint John, New Brunswick E2L 4L5, Canada
| | - Erik J S Emilson
- Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, 1219 Queen St. East, Sault Ste. Marie, Ontario P6A 2E5, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada
| | - Michelle A Gray
- Canadian Rivers Institute, Faculty of Forestry and Environmental Management, University of New Brunswick, 28 Dineen Drive, Fredericton, New Brunswick E3B 5A3, Canada
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10
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Huang H, Mangal V, Rennie MD, Tong H, Simpson MJ, Mitchell CPJ. Mercury methylation and methylmercury demethylation in boreal lake sediment with legacy sulphate pollution. Environ Sci Process Impacts 2022; 24:932-944. [PMID: 35532885 DOI: 10.1039/d2em00064d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Sulphate and dissolved organic matter (DOM) in freshwater systems may regulate the formation of methylmercury (MeHg), a potent neurotoxin that biomagnifies in aquatic ecosystems. While many boreal lakes continue to recover from decades of elevated atmospheric sulphate deposition, little research has examined whether historically high sulphate concentrations can result in persistently elevated MeHg production and accumulation in aquatic systems. This study used sediment from a historically sulphate-impacted lake and an adjacent reference lake in northwestern Ontario, Canada to investigate the legacy effects of sulphate pollution, as well as the effects of newly added sulphate, natural organic matter (NOM) of varying sulphur content and a sulphate reducing bacteria (SRB) inhibitor on enhancing or inhibiting the Hg methylation and demethylation activity (Kmeth and Kdemeth) in the sediment. We found that Kmeth and MeHg concentrations in sulphate-impacted lake sediment were significantly greater than in reference lake sediment. Further adding sulphate or NOM with different sulphur content to sediment of both lakes did not significantly change Kmeth. The addition of a SRB inhibitor resulted in lower Kmeth only in sulphate-impacted sediment, but methylation was not entirely depressed. Methylmercury demethylation potentials in sediment were consistent across lakes and experimental treatments, except for some impacts related to SRB inhibitor additions in the reference lake sediment. Overall, a broader community of microbes beyond SRB may be methylating Hg and demethylating MeHg in this system. This study reveals that legacies of sulphate pollution in boreal lakes may persist for decades in stimulating elevated Hg methylation in sediment.
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Affiliation(s)
- Haiyong Huang
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
| | - Vaughn Mangal
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
| | - Michael D Rennie
- Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada
| | - Huan Tong
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
- Environmental NMR Centre, University of Toronto Scarborough, Ontario, Canada
| | - Myrna J Simpson
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
- Environmental NMR Centre, University of Toronto Scarborough, Ontario, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Ontario, Canada.
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11
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Pierce CE, Furman OS, Nicholas SL, Wasik JC, Gionfriddo CM, Wymore AM, Sebestyen SD, Kolka RK, Mitchell CPJ, Griffiths NA, Elias DA, Nater EA, Toner BM. Role of Ester Sulfate and Organic Disulfide in Mercury Methylation in Peatland Soils. Environ Sci Technol 2022; 56:1433-1444. [PMID: 34979084 DOI: 10.1021/acs.est.1c04662] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
We examined the composition and spatial correlation of sulfur and mercury pools in peatland soil profiles by measuring sulfur speciation by 1s X-ray absorption near-edge structure spectrocopy and mercury concentrations by cold vapor atomic fluorescence spectroscopy. Also investigated were the methylation/demethylation rate constants and the presence of hgcAB genes with depth. Methylmercury (MeHg) concentration and organic disulfide were spatially correlated and had a significant positive correlation (p < 0.05). This finding is consistent with these species being products of dissimilatory sulfate reduction. Conversely, a significant negative correlation between organic monosulfides and MeHg was observed, which is consistent with a reduction in Hg(II) bioavailability via complexation reactions. Finally, a significant positive correlation between ester sulfate and instantaneous methylation rate constants was observed, which is consistent with ester sulfate being a substrate for mercury methylation via dissimilatory sulfate reduction. Our findings point to the importance of organic sulfur species in mercury methylation processes, as substrates and products, as well as potential inhibitors of Hg(II) bioavailability. For a peatland system with sub-μmol L-1 porewater concentrations of sulfate and hydrogen sulfide, our findings indicate that the solid-phase sulfur pools, which have a much larger sulfur concentration range, may be accessible to microbial activity or exchanging with the porewater.
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Affiliation(s)
- Caroline E Pierce
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108, United States
| | - Olha S Furman
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108, United States
| | - Sarah L Nicholas
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108, United States
| | - Jill Coleman Wasik
- Plant and Earth Science Department, University of Wisconsin River Falls, River Falls, Wisconsin 54022, United States
| | - Caitlin M Gionfriddo
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
| | - Ann M Wymore
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
| | - Stephen D Sebestyen
- USDA Forest Service, Northern Research Station, Grand Rapids, Minnesota 55744, United States
| | - Randall K Kolka
- USDA Forest Service, Northern Research Station, Grand Rapids, Minnesota 55744, United States
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Scarborough, Ontario M1C 1A4, Canada
| | - Natalie A Griffiths
- Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
| | - Dwayne A Elias
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
| | - Edward A Nater
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108, United States
| | - Brandy M Toner
- Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota 55108, United States
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12
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Strickman RJ, Larson S, Huang H, Kakouros E, Marvin-DiPasquale M, Mitchell CPJ, Neumann RB. The relative importance of mercury methylation and demethylation in rice paddy soil varies depending on the presence of rice plants. Ecotoxicol Environ Saf 2022; 230:113143. [PMID: 34998262 DOI: 10.1016/j.ecoenv.2021.113143] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/22/2021] [Revised: 12/26/2021] [Accepted: 12/28/2021] [Indexed: 06/14/2023]
Abstract
Neurotoxic methylmercury (MeHg) accumulates in rice grain from paddy soil, where its concentration is controlled by microbial mercury methylation and demethylation. Both up- and down-regulation of methylation is known to occur in the presence of rice plants in comparison to non-vegetated paddy soils; the influence of rice plant presence/absence on demethylation is unknown. To assess the concurrent influence of rice plant presence/absence on methylation and demethylation, and to determine which process was more dominant in controlling soil MeHg concentrations, we maintained six rhizoboxes of paddy soil with and without rice plants. At the peak of plant growth, we simultaneously measured ambient MeHg, ambient inorganic mercury (IHg), and potential rate constants of methylation and demethylation (Kmeth and Kdemeth) in soil using stable isotope tracers and ID-GC-ICPMS. We also measured organic matter content, elemental S, and water-extractable sulfate. We found MeHg concentrations were differentially controlled by MeHg production and degradation processes, depending on whether plants were present. In non-vegetated boxes, MeHg concentration was controlled by Kmeth, as evidenced by a strong and positive correlation, while Kdemeth had no relation to MeHg concentration. These results indicate methylation was the dominant driver of MeHg concentration in non-vegetated soil. In vegetated boxes, Kdemeth strongly and negatively predicted MeHg concentration, indicating that demethylation was the dominant control in soil with plants. MeHg concentration, Kmeth, and % MeHg all had significantly less variance in vegetated than in non-vegetated soils due to a consistent elimination of greater values. This pattern suggests that reduced MeHg production capacity was a secondary control on MeHg concentrations in vegetated soils. We observed no difference in the magnitude or variance of Kdemeth between treatments, suggesting that demethylation was robust to soil chemical conditions influenced by the plant, perhaps because of a wider taxonomic diversity of demethylators. Our results suggest that methylation and demethylation processes could both be leveraged to alter MeHg concentrations in rice paddy soil.
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Affiliation(s)
- R J Strickman
- Department of Civil and Environmental Engineering, University of Washington, Seattle, USA.
| | - S Larson
- Department of Civil and Environmental Engineering, University of Washington, Seattle, USA
| | - H Huang
- University of Toronto Scarborough, Ontario, Canada
| | - E Kakouros
- US Geological Survey, Menlo Park, Palo Alto, CA, USA
| | | | | | - R B Neumann
- Department of Civil and Environmental Engineering, University of Washington, Seattle, USA
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13
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Ni FJ, Bhavsar SP, Poirier D, Branfireun B, Petro S, Arts MT, Chong-Kit R, Mitchell CPJ, Arhonditsis GB. Impacts of water level fluctuations on mercury concentrations in hydropower reservoirs: A microcosm experiment. Ecotoxicol Environ Saf 2021; 220:112354. [PMID: 34116335 DOI: 10.1016/j.ecoenv.2021.112354] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/12/2020] [Revised: 04/02/2021] [Accepted: 05/15/2021] [Indexed: 06/12/2023]
Abstract
Hydropower generation, a renewable source of electricity, has been linked to elevated methylmercury (MeHg) concentrations in impoundments and aquatic biota. This study investigates the impact of water level fluctuations (WLF) on MeHg concentrations in water, sediment, and fish. Using a set of controlled microcosm experiments emulating the drawdown/refill dynamics and subsequent sediment exposure to air experienced in reservoirs, we demonstrate that less frequent WLFs, and/or increased exposure of sediment to air, can lead to elevated MeHg concentrations in sediment, and total mercury (THg) and MeHg concentrations in water. In examining the effects of WLF frequency (two-day, weekly, and monthly), the monthly treatment displayed the highest THg and MeHg water levels, while the weekly treatment was characterized by the highest MeHg levels in the sediment. Our work supports emerging evidence that longer duration between WLF creates a larger surface area of sediment exposed to air leading to conditions conducive to higher MeHg concentrations in sediments and water. In contrast, THg, MeHg, and fatty acid trends in fish were largely inconclusive characterized by similar among-treatment effects and minimal temporal variability over the course of our experiment. This result could partly be attributed to overall low mercury levels and simple "worm-forage fish" food web in our experiment. To elucidate the broader impacts of water fluctuations on aquatic chemistry and biota, other factors (e.g., longer WLF cycles, dissolved organic matter, temperature, more complex food webs) which modulate both methylation rates and food web dynamics must be considered.
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Affiliation(s)
- Felicity J Ni
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada
| | - Satyendra P Bhavsar
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada; Ontario Ministry of Environment, Conservation, and Parks, Toronto, Ontario M9P 3V6, Canada.
| | - David Poirier
- Ontario Ministry of Environment, Conservation, and Parks, Toronto, Ontario M9P 3V6, Canada
| | - Brian Branfireun
- Department of Biology and Centre for Environment & Sustainability, Western University, London, Ontario N6A 3K7, Canada
| | - Steve Petro
- Ontario Ministry of Environment, Conservation, and Parks, Toronto, Ontario M9P 3V6, Canada
| | - Michael T Arts
- Department of Chemistry & Biology, Ryerson University, Toronto, Ontario M5B 2K3, Canada
| | - Richard Chong-Kit
- Ontario Ministry of Environment, Conservation, and Parks, Toronto, Ontario M9P 3V6, Canada
| | - Carl P J Mitchell
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada
| | - George B Arhonditsis
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario M1C 1A4, Canada.
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14
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Snow MA, Feigis M, Lei YD, Mitchell CPJ, Wania F. Development, characterization, and testing of a personal passive sampler for measuring inhalation exposure to gaseous elemental mercury. Environ Int 2021; 146:106264. [PMID: 33227582 DOI: 10.1016/j.envint.2020.106264] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2020] [Revised: 11/02/2020] [Accepted: 11/03/2020] [Indexed: 06/11/2023]
Abstract
Inhalation of gaseous elemental mercury (GEM) is an occupational exposure concern for workers handling elemental mercury or mercury-containing waste. GEM is also often present near historically mercury-contaminated sites, potentially resulting in low-level, chronic exposure of the wider population. Here we introduce a passive sampler for personal GEM monitoring which combines a radial porous diffusive barrier with an activated carbon sorbent. A total mercury analyzer is used to quantify GEM sorbed to the carbon by thermal decomposition, amalgamation, and atomic absorption spectroscopy. A sampling rate of 0.070 m3/day was determined by calibrating the sampler at low and high concentrations. Deployments lasting 8 h result in limits of quantification well below 200 ng/m3. The sampler has a measurement range of at least four orders of magnitude. Derived air concentrations were not statistically significantly different from those obtained by active air sampling but were more precise than those obtained using a personal pump. If properly stored, the sampler maintains low blank levels in high GEM environments. Affordability, sturdiness, simplicity, and the wide availability of total mercury analyzers make this sampler highly suited for monitoring GEM inhalation exposure, including in developing countries.
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Affiliation(s)
- Melanie A Snow
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Michelle Feigis
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Ying Duan Lei
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Frank Wania
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada.
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15
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Jones DS, Johnson NW, Mitchell CPJ, Walker GM, Bailey JV, Pastor J, Swain EB. Diverse Communities of hgcAB+ Microorganisms Methylate Mercury in Freshwater Sediments Subjected to Experimental Sulfate Loading. Environ Sci Technol 2020; 54:14265-14274. [PMID: 33138371 DOI: 10.1021/acs.est.0c02513] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Methylmercury (MeHg) is a bioaccumulative neurotoxin produced by certain sulfate-reducing bacteria and other anaerobic microorganisms. Because microorganisms differ in their capacity to methylate mercury, the abundance and distribution of methylating populations may determine MeHg production in the environment. We compared rates of MeHg production and the distribution of hgcAB genes in epilimnetic sediments from a freshwater lake that were experimentally amended with sulfate levels from 7 to 300 mg L-1. The most abundant hgcAB sequences were associated with clades of Methanomicrobia, sulfate-reducing Deltaproteobacteria, Spirochaetes, and unknown environmental sequences. The hgcAB+ communities from higher sulfate amendments were less diverse and had relatively more Deltaproteobacteria, whereas the communities from lower amendments were more diverse with a larger proportion of hgcAB sequences affiliated with other clades. Potential methylation rate constants varied 52-fold across the experiment. Both potential methylation rate constants and % MeHg were the highest in sediments from the lowest sulfate amendments, which had the most diverse hgcAB+ communities and relatively fewer hgcAB genes from clades associated with sulfate reduction. Although pore water sulfide concentration covaried with hgcAB diversity across our experimental sulfate gradient, major changes in the community of hgcAB+ organisms occurred prior to a significant buildup of sulfide in pore waters. Our results indicate that methylating communities dominated by diverse anaerobic microorganisms that do not reduce sulfate can produce MeHg as effectively as communities dominated by sulfate-reducing populations.
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Affiliation(s)
- Daniel S Jones
- BioTechnology Institute, University of Minnesota, Saint Paul 55108, Minnesota, United States
- Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis 55455, Minnesota, United States
- Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro 87801, New Mexico, United States
- National Cave and Karst Research Institute, Carlsbad, New Mexico 88220, United States
| | - Nathan W Johnson
- Department of Civil Engineering, University of Minnesota Duluth, Duluth, Minnesota 55812, United States
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto-Scarborough, Toronto, Ontario M1C 1A4, Canada
| | - Gabriel M Walker
- Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis 55455, Minnesota, United States
| | - Jake V Bailey
- Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis 55455, Minnesota, United States
| | - John Pastor
- Department of Biology, University of Minnesota Duluth, Duluth, Minnesota 55812, United States
| | - Edward B Swain
- Minnesota Pollution Control Agency, Saint Paul, Minnesota 55155, United States
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16
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Szponar N, McLagan DS, Kaplan RJ, Mitchell CPJ, Wania F, Steffen A, Stupple GW, Monaci F, Bergquist BA. Isotopic Characterization of Atmospheric Gaseous Elemental Mercury by Passive Air Sampling. Environ Sci Technol 2020; 54:10533-10543. [PMID: 32786342 DOI: 10.1021/acs.est.0c02251] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Tracing emission sources and transformations of atmospheric mercury with Hg stable isotopes depends on the ability to collect amounts sufficient for reliable quantification. Commonly employed active sampling methods require power and long pumping times, which limits the ability to deploy in remote locations and at high spatial resolution and can lead to compromised traps. In order to overcome these limitations, we conducted field and laboratory experiments to assess the preservation of isotopic composition during sampling of gaseous elemental mercury (GEM) with a passive air sampler (PAS) that uses a sulfur-impregnated carbon sorbent and a diffusive barrier. Whereas no mass independent fractionation (MIF) was observed during sampling, the mass dependent fractionation (MDF, δ202Hg) of GEM taken up by the PAS was lower than that of actively pumped samples by 1.14 ± 0.24‰ (2SD). Because the MDF offset was consistent across field studies and laboratory experiments conducted at 5, 20, and 30 °C, the PAS can be used for reliable isotopic characterization of GEM (±0.3‰ for MDF, ±0.05‰ for MIF, 2SD). The MDF offset occurred more during the sorption of GEM rather than during diffusion. PAS field deployments confirm the ability to record differences in the isotopic composition of GEM (i) with distance from point sources and (ii) sampled at different background locations globally.
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Affiliation(s)
- Natalie Szponar
- Department of Earth Sciences, University of Toronto, M5S 2B1 Toronto, Canada
| | - David S McLagan
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, M1C 1A4 Toronto, Canada
| | - Robert J Kaplan
- Department of Earth Sciences, University of Toronto, M5S 2B1 Toronto, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, M1C 1A4 Toronto, Canada
| | - Frank Wania
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, M1C 1A4 Toronto, Canada
| | - Alexandra Steffen
- Air Quality Processes Research Section, Environment and Climate Change Canada, M3H 5T4 Toronto, Canada
| | - Geoff W Stupple
- Air Quality Processes Research Section, Environment and Climate Change Canada, M3H 5T4 Toronto, Canada
| | - Fabrizio Monaci
- Department of Life Sciences, University of Siena, via Mattioli 4, 53100 Siena, Italy
| | - Bridget A Bergquist
- Department of Earth Sciences, University of Toronto, M5S 2B1 Toronto, Canada
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17
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Haynes KM, Kane ES, Potvin L, Lilleskov EA, Kolka RK, Mitchell CPJ. Impacts of experimental alteration of water table regime and vascular plant community composition on peat mercury profiles and methylmercury production. Sci Total Environ 2019; 682:611-622. [PMID: 31129544 DOI: 10.1016/j.scitotenv.2019.05.072] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/31/2019] [Revised: 04/22/2019] [Accepted: 05/06/2019] [Indexed: 06/09/2023]
Abstract
Climate change is expected to alter the hydrology and vascular plant communities in peatland ecosystems. These changes may have as yet unexplored impacts on peat mercury (Hg) concentrations and net methylmercury (MeHg) production. In this study, peat was collected from PEATcosm, an outdoor, controlled mesocosm experiment where peatland water table regimes and vascular plant functional groups were manipulated over several years to simulate potential climate change effects. Potential Hg(II) methylation and MeHg demethylation rate constants were assessed using enriched stable isotope incubations at the end of the study in 2015, and ambient peat total Hg (THg) and MeHg concentration depth profiles were tracked annually from 2011 to 2014. Peat THg and MeHg concentrations and the proportion of THg methylated (%MeHg) increased significantly within the zone of water table fluctuation when water tables were lowered, but potential Hg(II) methylation rate constants were similar regardless of water table treatment. When sedges dominate over ericaceous shrubs, MeHg concentrations and %MeHg became significantly elevated within the sedge rooting zone. Increased desorption of Hg(II) and MeHg from the solid phase peat into pore water occurred with a lowered water table and predominant sedge cover, likely due to greater aerobic peat decomposition. Deeper, more variable water tables and a transition to sedge-dominated communities coincided with increased MeHg accumulation within the zone of water table fluctuation. Sustained high water tables promoted the net downward migration of Hg(II) and MeHg. The simultaneous decrease in Hg(II) and MeHg concentrations in the near-surface peat and accumulation deeper in the peat profile, combined with the trends in Hg(II) and MeHg partitioning to mobile pore waters, suggest that changes to peatland hydrology and vascular plant functional groups redistribute peat Hg(II) and MeHg via vertical hydrochemical transport mechanisms.
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Affiliation(s)
- Kristine M Haynes
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada; University of Toronto, Department of Geography, 100 St. George Street, Toronto, Ontario M5S 3G3, Canada.
| | - Evan S Kane
- Michigan Technological University, School of Forest Resources and Environmental Science, Houghton, MI 49931, USA; USDA Forest Service Northern Research Station, Houghton, MI 49931, USA
| | - Lynette Potvin
- USDA Forest Service Northern Research Station, Houghton, MI 49931, USA
| | - Erik A Lilleskov
- USDA Forest Service Northern Research Station, Houghton, MI 49931, USA
| | - Randall K Kolka
- USDA Forest Service Northern Research Station, Grand Rapids, MN 55744, USA
| | - Carl P J Mitchell
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada; University of Toronto, Department of Geography, 100 St. George Street, Toronto, Ontario M5S 3G3, Canada
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18
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Jones DS, Walker GM, Johnson NW, Mitchell CPJ, Coleman Wasik JK, Bailey JV. Molecular evidence for novel mercury methylating microorganisms in sulfate-impacted lakes. ISME J 2019; 13:1659-1675. [PMID: 30809010 DOI: 10.1038/s41396-019-0376-1] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/31/2018] [Revised: 02/02/2019] [Accepted: 02/08/2019] [Indexed: 11/09/2022]
Abstract
Methylmercury (MeHg) is a bioaccumulative neurotoxin that is produced by certain anaerobic microorganisms, but the abundance and importance of different methylating populations in the environment is not well understood. We combined mercury geochemistry, hgcA gene cloning, rRNA methods, and metagenomics to compare microbial communities associated with MeHg production in two sulfate-impacted lakes on Minnesota's Mesabi Iron Range. The two lakes represent regional endmembers among sulfate-impacted sites in terms of their dissolved sulfide concentrations and MeHg production potential. rRNA amplicon sequencing indicates that sediments and anoxic bottom waters from both lakes contained diverse communities with multiple clades of sulfate reducing Deltaproteobacteria and Clostridia. In hgcA gene clone libraries, however, hgcA sequences were from taxa associated with methanogenesis and iron reduction in addition to sulfate reduction, and the most abundant clones were from unknown groups. We therefore applied metagenomics to identify the unknown populations in the lakes with the capability to methylate mercury, and reconstructed 27 genomic bins with hgcA. Some of the most abundant potential methylating populations were from phyla that are not typically associated with MeHg production, including a relative of the Aminicenantes (formerly candidate phylum OP8) and members of the Kiritimatiellaeota (PVC superphylum) and Spirochaetes that, together, were more than 50% of the potential methylators in some samples. These populations do not have genes for sulfate reduction, and likely degrade organic compounds by fermentation or other anaerobic processes. Our results indicate that previously unrecognized populations with hgcAB are abundant and may be important for MeHg production in some freshwater ecosystems.
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Affiliation(s)
- Daniel S Jones
- BioTechnology Institute, University of Minnesota, St. Paul, MN, USA. .,Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA. .,Dept. of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM, USA.
| | - Gabriel M Walker
- Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA
| | - Nathan W Johnson
- Department of Civil Engineering, University of Minnesota Duluth, Duluth, MN, USA
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto - Scarborough, Toronto, ON, Canada
| | - Jill K Coleman Wasik
- Department of Plant and Earth Science, University of Wisconsin River Falls, River Falls, WI, USA
| | - Jake V Bailey
- Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA
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19
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Turner RR, Mitchell CPJ, Kopec AD, Bodaly RA. Tidal fluxes of mercury and methylmercury for Mendall Marsh, Penobscot River estuary, Maine. Sci Total Environ 2018; 637-638:145-154. [PMID: 29751297 DOI: 10.1016/j.scitotenv.2018.04.395] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2017] [Revised: 04/27/2018] [Accepted: 04/29/2018] [Indexed: 05/28/2023]
Abstract
Tidal marshes are both important sites of in situ methylmercury production and can be landscape sources of methylmercury to adjacent estuarine systems. As part of a regional investigation of the Hg-contaminated Penobscot River and Bay system, the tidal fluxes of total suspended solids, total mercury and methylmercury into and out of a regionally important mesohaline fluvial marsh complex, Mendall Marsh, were intensively measured over several tidal cycles and at two spatial scales to assess the source-sink function of the marsh with respect to the Penobscot River. Over four tidal cycles on the South Marsh River, the main channel through which water enters and exits Mendall Marsh, the marsh was a consistent sink over typical 12-h tidal cycles for total suspended solids (8.2 to 41 g m-2), total Hg (9.2 to 47 μg m-2), total filter-passing Hg (0.4 to 1.1 μg m-2), and total methylmercury (0.2 to 1.4 μg m-2). The marsh's source-sink function was variable for filter-passing methylmercury, acting as a net source during a large spring tide that inundated much of the marsh area and that is likely to occur during approximately 17% of tidal cycles. Additional measurements on a small tidal channel draining approximately 1% of the larger marsh area supported findings at the larger scale, but differences in the flux magnitude of filter-passing fractions suggest a highly non-conservative transport of these fractions through the tidal channels. Overall the results of this investigation demonstrate that Mendall Marsh is not a significant source of mercury or methylmercury to the receiving aquatic systems (Penobscot River and Bay). While there is evidence of a small net export of filter-passing (<0.4 μm pore size) methylmercury under some tidal conditions, the mass involved represents <3% of the mass of filter-passing methylmercury carried by the Penobscot River.
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Affiliation(s)
- R R Turner
- RT Geosciences Inc, 3398 Kingburne Dr., Cobble Hill, B.C. V0R 1L5, Canada.
| | - C P J Mitchell
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada.
| | - A D Kopec
- Penobscot River Mercury Study, 479 Beechwood Ave., Old Town, ME 04468, USA.
| | - R A Bodaly
- Penobscot River Mercury Study, 115 Oystercatcher Place, Salt Spring Island, B.C. V8K 2W5, Canada
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Parajulee A, Lei YD, Cao X, McLagan DS, Yeung LWY, Mitchell CPJ, Wania F. Comparing winter-time herbicide behavior and exports in urban, rural, and mixed-use watersheds. Environ Sci Process Impacts 2018; 20:767-779. [PMID: 29578561 DOI: 10.1039/c7em00596b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The presence of pesticides in streams in winter, five to six years following bans on their municipal use suggests that complicated transport behaviour, such as subsurface retention and/or accumulation of pesticides and its release during storms, could be important for understanding recovery time frames following bans or legislation that aim to reduce chemical inputs. We investigated late fall and winter dynamics of four herbicides in paired urban and rural watersheds in Toronto, Canada during rainfall and snowmelt. The range of average concentrations and loads of the sum of atrazine, metolachlor, 2,4-D and mecoprop overlapped in the two types of watersheds, with slightly higher average concentrations in the rural watershed. Relatively consistent herbicide concentration-discharge patterns (i.e. dilution) were observed in the urban sub-watersheds during rainfall, while concentration-discharge patterns were much more variable in the rural watershed. This suggests relatively uniform transport pathways across the urban sub-watersheds, compared to the rural watershed. Concentration-discharge patterns of the neutral herbicides atrazine and metolachlor were similar in both watersheds during snowmelt, though varying discharge patterns resulted in divergent timings of peak concentrations. In contrast, the acidic pesticides 2,4-D and mecoprop, which are primarily associated with urban uses, showed much more variable behavior across both watersheds and merit further investigation. Overall, this work highlights the need to consider pesticide dynamics throughout the year in order to more thoroughly assess the long-term efficacy of legislation governing their use.
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Affiliation(s)
- Abha Parajulee
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada.
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21
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Strickman RJ, Mitchell CPJ. Mercury methylation in stormwater retention ponds at different stages in the management lifecycle. Environ Sci Process Impacts 2018; 20:595-606. [PMID: 29376168 DOI: 10.1039/c7em00486a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Stormwater retention ponds effectively manage erosion, flooding, and pollutant loadings, but are also sources of methylmercury (MeHg), a bioaccumulative neurotoxin which is produced by anaerobic aquatic microorganisms. Stormwater retention ponds have a 10-15 year working life, after which they are dredged and reflooded. In this study, we related MeHg biogeochemistry to the different stages of the management lifecycle. In a new, a dredged, and a mature stormwater retention pond, we measured MeHg and inorganic mercury (IHg) concentrations, and the potential for MeHg formation (Kmeth), during the early summer, peak summer, and fall of 2013. In our study sites, MeHg concentrations appear to be driven by mercury (Hg) methylation, indicated by significant correlations between Kmeth values and MeHg concentrations and the percent of Hg present as MeHg. Relationships between Hg variables and ancillary biogeochemistry suggest that Hg methylation is carried out by sulfate reducing bacteria, but that the process is modulated by the supply of IHg substrate, sediment total and labile organic carbon, and possibly competition with nitrate reducers. Wetlands at different points in the management lifecycle differ in terms of their MeHg biogeochemistry. The organic matter-poor new wetland had low MeHg production (mean Kmeth 0.014 per day) and sediment concentrations (mean 0.015 ng g-1), while the mature wetland both produced and accumulated MeHg about five times more actively. Methylmercury production capacity was only temporarily reduced in the reflooded sediments of the dredged wetland, which experienced rapid increases in Kmeth values from low (mean 0.015 per day) immediately after dredging, to values similar to those in the mature wetland after five months. This pattern may have been related to recolonization of the sediments with mercury methylators or increased microbial activities in response to the addition of fresh organic matter. Additional studies should focus on the applicability of these patterns to stormwater retention ponds in other areas, and particularly investigate the effects of stormwater pond dredging on their microbial ecology and MeHg biogeochemistry.
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Affiliation(s)
- R J Strickman
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1285 Military Trail, Toronto, Ontario M4C 1A4, Canada.
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22
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Parajulee A, Lei YD, Kananathalingam A, Mitchell CPJ, Wania F. Investigating the Sources and Transport of Benzotriazole UV Stabilizers during Rainfall and Snowmelt across an Urbanization Gradient. Environ Sci Technol 2018; 52:2595-2602. [PMID: 29429338 DOI: 10.1021/acs.est.8b00552] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/14/2023]
Abstract
Benzotriazole UV stabilizers (BT-UVs) have attracted increasing attention due to their bioaccumulative nature and ubiquitous presence in surface waters. We apply high-frequency sampling in paired watersheds to describe, for the first time, the behavior of BT-UVs in stream channels during snowmelt and rainfall. Relative to a largely agricultural watershed, concentrations of BT-UVs in an urban watershed were 4-90 times greater during rainfall and 3-21 times greater during snowmelt. During rainfall, a decrease in BT-UV concentrations on particles with increasing suspended sediments and streamflow occurred at all urban sites due to input of relatively clean sediments, while both decreases and increases were observed at rural sites. Where increases occurred in the rural watershed, road sediments were consistently suggested as the source. Contrasts between the urban and rural sites were also observed during snowmelt. While BT-UV concentrations on particles peaked with peak suspended sediment levels at urban stream sites, the opposite was true at rural stream sites. This appeared to be driven partially by different snowpack melt rates in the two watersheds, with earlier melt and presumably higher streamflow facilitating suspension or erosion of more contaminated sediment in the urban stream. In general, it appears that relatively high, consistent emissions in the form of informal (plastic) debris disposal by consumers or industrial releases have likely led to more homogeneous BT-UV profiles and temporal behavior in the urban watershed. In the rural watershed, low emissions instead entail that emissions variability is more likely to translate to variability in chemical profiles and temporal behavior.
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Affiliation(s)
- Abha Parajulee
- Department of Physical and Environmental Sciences , University of Toronto Scarborough , 1265 Military Trail , Toronto , Ontario M1C 1A4 , Canada
| | - Ying Duan Lei
- Department of Physical and Environmental Sciences , University of Toronto Scarborough , 1265 Military Trail , Toronto , Ontario M1C 1A4 , Canada
| | - Ajitha Kananathalingam
- Department of Physical and Environmental Sciences , University of Toronto Scarborough , 1265 Military Trail , Toronto , Ontario M1C 1A4 , Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences , University of Toronto Scarborough , 1265 Military Trail , Toronto , Ontario M1C 1A4 , Canada
| | - Frank Wania
- Department of Physical and Environmental Sciences , University of Toronto Scarborough , 1265 Military Trail , Toronto , Ontario M1C 1A4 , Canada
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Hsu-Kim H, Eckley CS, Achá D, Feng X, Gilmour CC, Jonsson S, Mitchell CPJ. Challenges and opportunities for managing aquatic mercury pollution in altered landscapes. Ambio 2018; 47:141-169. [PMID: 29388127 PMCID: PMC5794684 DOI: 10.1007/s13280-017-1006-7] [Citation(s) in RCA: 128] [Impact Index Per Article: 21.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
The environmental cycling of mercury (Hg) can be affected by natural and anthropogenic perturbations. Of particular concern is how these disruptions increase mobilization of Hg from sites and alter the formation of monomethylmercury (MeHg), a bioaccumulative form of Hg for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of MeHg in the environment. The objective of this paper is to synthesize the scientific understanding of how Hg cycling in the aquatic environment is influenced by landscape perturbations at the local scale, perturbations that include watershed loadings, deforestation, reservoir and wetland creation, rice production, urbanization, mining and industrial point source pollution, and remediation. We focus on the major challenges associated with each type of alteration, as well as management opportunities that could lessen both MeHg levels in biota and exposure to humans. For example, our understanding of approximate response times to changes in Hg inputs from various sources or landscape alterations could lead to policies that prioritize the avoidance of certain activities in the most vulnerable systems and sequestration of Hg in deep soil and sediment pools. The remediation of Hg pollution from historical mining and other industries is shifting towards in situ technologies that could be less disruptive and less costly than conventional approaches. Contemporary artisanal gold mining has well-documented impacts with respect to Hg; however, significant social and political challenges remain in implementing effective policies to minimize Hg use. Much remains to be learned as we strive towards the meaningful application of our understanding for stakeholders, including communities living near Hg-polluted sites, environmental policy makers, and scientists and engineers tasked with developing watershed management solutions. Site-specific assessments of MeHg exposure risk will require new methods to predict the impacts of anthropogenic perturbations and an understanding of the complexity of Hg cycling at the local scale.
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Affiliation(s)
- Heileen Hsu-Kim
- Department of Civil & Environmental Engineering, Duke University, 121 Hudson Hall, Box 90287, Durham, NC 27708 USA
| | - Chris S. Eckley
- U.S. Environmental Protection Agency, Region-10, 1200 6th Ave, Seattle, WA 98101 USA
| | - Dario Achá
- Unidad de Calidad Ambiental, Instituto de Ecología, Carrera de Biología, Universidad Mayor de San Andrés, P.O. Box 10077, La Paz, Bolivia
| | - Xinbin Feng
- State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002 China
| | - Cynthia C. Gilmour
- Smithsonian Environmental Research Center, 647 Contees Wharf Rd, Edgewater, MD 21037-0028 USA
| | - Sofi Jonsson
- Department of Environmental Science and Analytical Chemistry, Stockholm University, Svante Arrhenius väg 8, 11418 Stockholm, Sweden
| | - Carl P. J. Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4 Canada
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Tavshunsky I, Eggert SL, Mitchell CPJ. Accumulation of Methylmercury in Invertebrates and Masked Shrews (Sorex cinereus) at an Upland Forest-Peatland Interface in Northern Minnesota, USA. Bull Environ Contam Toxicol 2017; 99:673-678. [PMID: 29063129 DOI: 10.1007/s00128-017-2198-z] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/04/2017] [Accepted: 10/16/2017] [Indexed: 06/07/2023]
Abstract
Mercury (Hg) methylation is often elevated at the terrestrial-peatland interface, but methylmercury (MeHg) production at this "hot spot" has not been linked with in situ biotic accumulation. We examined total Hg and MeHg levels in peat, invertebrates and tissues of the insectivore Sorex cinereus (masked shrew), inhabiting a terrestrial-peatland ecotone in northern Minnesota, USA. Mean MeHg concentrations in S. cinereus (71 ng g-1) fell between concentrations measured in spiders (mean 70-140 ng g-1), and ground beetles and millipedes (mean 29-42 ng g-1). Methylmercury concentrations in S. cinereus increased with age and differed among tissues, with highest concentrations in kidneys and muscle, followed by liver and brain. Nearly all Hg in S. cinereus was in the methylated form. Overall, the high proportional accumulation of MeHg in peat at the site (3.5% total Hg as MeHg) did not lead to particularly elevated concentrations in invertebrates or shrews, which are below values considered a toxicological risk.
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Affiliation(s)
- Ilana Tavshunsky
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, Canada
| | - Susan L Eggert
- United States Forest Service, Northern Research Station, 1831 Hwy 169 E, Grand Rapids, MN, 55744, USA
| | - Carl P J Mitchell
- Department of Physical & Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, Canada.
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25
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Parajulee A, Lei YD, Kananathalingam A, McLagan DS, Mitchell CPJ, Wania F. The transport of polycyclic aromatic hydrocarbons during rainfall and snowmelt in contrasting landscapes. Water Res 2017; 124:407-414. [PMID: 28783496 DOI: 10.1016/j.watres.2017.07.074] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/17/2017] [Revised: 07/11/2017] [Accepted: 07/29/2017] [Indexed: 06/07/2023]
Abstract
Though it has been established that stream concentrations of polycyclic aromatic hydrocarbons (PAHs) in urban watersheds can be much greater than those in less developed watersheds, knowledge of transport mechanisms is lacking, particularly in temperate, Northern climates with seasonal snow packs. We combine high-resolution stream water sampling with air, suspended solid and stream flow monitoring to investigate the source to stream transport of PAHs during rainfall and snowmelt in paired watersheds with contrasting land use. Despite similar particle loads, contamination of particles that is 8-48 times higher in the urban watersheds leads to area-normalized loads of PAHs that are 6-82 times greater than in the agricultural watersheds. In the urban watershed, average volumetric storm flow concentrations increase with longer antecedent dry period that allows build-up of PAHs on watershed surfaces. Cluster analysis suggests road dust is a minor source of suspended solid-bound PAHs in more agricultural watersheds during rainfall. During snowmelt, earlier peaks in concentration in the urban watershed are likely due to melt from snow packs and snow banks travelling quickly to the stream network via impervious surfaces and sewer drains. While road-derived inputs also appear to be important during snowmelt in the agricultural watershed, relatively delayed peak concentrations result from delayed inputs from snow packs in more pervious areas of the watershed.
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Affiliation(s)
- Abha Parajulee
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada
| | - Ying Duan Lei
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada; Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada
| | - Ajitha Kananathalingam
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada
| | - David S McLagan
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada
| | - Frank Wania
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada.
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Parajulee A, Lei YD, De Silva AO, Cao X, Mitchell CPJ, Wania F. Assessing the Source-to-Stream Transport of Benzotriazoles during Rainfall and Snowmelt in Urban and Agricultural Watersheds. Environ Sci Technol 2017; 51:4191-4198. [PMID: 28351142 DOI: 10.1021/acs.est.6b05638] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
While benzotriazoles (BTs) are ubiquitous in urban waters, their sources and transport remain poorly characterized. We aimed to elucidate the origin and hydrological pathways of BTs in Toronto, Canada, by quantifying three BTs, electrical conductivity, and δ18O in high-frequency streamwater samples taken during two rainfall and one snowmelt event in two watersheds with contrasting levels of urbanization. Average concentrations of total BTs (∑BT) were 1.3 to 110 times higher in the more urbanized Mimico Creek watershed relative to the primarily agricultural and suburban Little Rouge Creek. Strong correlations between upstream density of major roads and total BT concentrations or BT composition within all events implicate vehicle fluids as the key source of BTs in both watersheds. Sustained historical releases of BTs within the Mimico Creek watershed have likely led to elevated ∑BT in groundwater, with elevated concentrations observed during baseflow that are diluted by rainfall and surface runoff. In contrast, relatively constant concentrations, caused by mixing of equally contaminated baseflow and rainfall/surface runoff, are observed in the Little Rouge Creek throughout storm hydrographs, with an occasional first flush occurring at a subsite draining suburban land. During snowmelt, buildup of BTs in roadside snowpiles and preferential partitioning of BTs to the liquid phase of a melting snowpack leads to early peaks in ∑BT in both streams, except the sites in the Little Rouge Creek with low levels of vehicle traffic. Overall, a history of BT release and land use associated with urbanization have led to higher levels of BTs in urban areas and provide a glimpse into future BT dynamics in mixed use, (sub)urbanizing areas.
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Affiliation(s)
- Abha Parajulee
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Ying Duan Lei
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Amila O De Silva
- Water Science and Technology Directorate, Environment and Climate Change Canada , 867 Lakeshore Road, Burlington, Ontario L7S 1A1, Canada
| | - Xiaoshu Cao
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Frank Wania
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
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Bailey LT, Mitchell CPJ, Engstrom DR, Berndt ME, Coleman Wasik JK, Johnson NW. Influence of porewater sulfide on methylmercury production and partitioning in sulfate-impacted lake sediments. Sci Total Environ 2017; 580:1197-1204. [PMID: 28024742 DOI: 10.1016/j.scitotenv.2016.12.078] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/24/2016] [Revised: 12/12/2016] [Accepted: 12/12/2016] [Indexed: 06/06/2023]
Abstract
In low-sulfate and sulfate-limited freshwater sediments, sulfate loading increases the production of methylmercury (MeHg), a potent and bioaccumulative neurotoxin. Sulfate loading to anoxic sediments leads to sulfide production that can inhibit mercury methylation, but this has not been commonly observed in freshwater lakes and wetlands. In this study, sediments were collected from sulfate-impacted, neutral pH, surface water bodies located downstream from ongoing and historic mining activities to examine how chronic sulfate loading produces porewater sulfide, and influences MeHg production and transport. Sediments were collected over two years, during several seasons from lakes with a wide range of overlying water sulfate concentration. Samples were characterized for in-situ solid phase and porewater MeHg, Hg methylation potentials via incubations with enriched stable Hg isotopes, and sulfur, carbon, and iron content and speciation. Porewater sulfide reflected historic sulfur loading and was strongly related to the extractable iron content of sediment. Overall, methylation potentials were consistent with the accumulation of MeHg on the solid phase, but both methylation potentials and MeHg were significantly lower at chronically sulfate-impacted sites with a low solid-phase Fe:S ratio. At these heavily sulfate-impacted sites that also contained elevated porewater sulfide, both MeHg production and partitioning are influenced: Hg methylation potentials and sediment MeHg concentrations are lower, but occasionally porewater MeHg concentrations in sediment are elevated, particularly in the spring. The dual role of sulfide as a ligand for inorganic mercury (decreasing bioavailability) and methylmercury (increasing partitioning into porewater) means that elucidating the role of iron and sulfur loads as they define porewater sulfide is key to understanding sulfate's influence on MeHg production and partitioning in sulfate-impacted freshwater sediment.
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Affiliation(s)
- Logan T Bailey
- Water Resources Science Program, University of Minnesota, United States
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Canada
| | - Daniel R Engstrom
- St. Croix Watershed Research Station, Science Museum of Minnesota, United States
| | - Michael E Berndt
- Minnesota Department of Natural Resources, Division of Lands and Minerals, United States
| | - Jill K Coleman Wasik
- St. Croix Watershed Research Station, Science Museum of Minnesota, United States
| | - Nathan W Johnson
- Water Resources Science Program, University of Minnesota, United States; Department of Civil Engineering, University of Minnesota Duluth, United States.
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Strickman RJ, Mitchell CPJ. Methylmercury production and accumulation in urban stormwater ponds and habitat wetlands. Environ Pollut 2017; 221:326-334. [PMID: 27939209 DOI: 10.1016/j.envpol.2016.11.082] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2016] [Revised: 11/20/2016] [Accepted: 11/29/2016] [Indexed: 06/06/2023]
Abstract
Stormwater management ponds and created habitat wetlands effectively manage erosion, flooding, and pollutant loadings while providing biodiversity and aesthetic benefits, but these structures are also potential sources of methylmercury (MeHg), a bioaccumulative neurotoxin. While MeHg accumulation has been confirmed in habitat wetlands, the extent of MeHg production and accumulation in stormwater ponds is unknown. Additionally, the fine-scale spatial variation in MeHg in these wetlands has never been explored despite the possibility that cycles of wetting and drying, and the presence of aquatic plants may stimulate methylation at their margins. To address these knowledge gaps, we compared MeHg and inorganic mercury concentrations, the percent of total mercury present as MeHg (%MeHg), and potential mercury methylation rate constants (Kmeth) in the sediments of terrestrial-aquatic transects through several stormwater and habitat wetlands. We present novel evidence confirming the in situ production of MeHg in both stormwater ponds and habitat wetlands, but observe no systematic differences across the terrestrial-aquatic gradient, suggesting that routine variations in water level do not alter MeHg production and accumulation. Stormwater ponds effectively trap mercury while converting relatively little to MeHg, as evidenced by lower MeHg concentrations, %-MeHg, and Kmeth values than habitat wetlands, but often greater inorganic Hg concentrations. The relationship of aquatic vegetation to MeHg accumulation is weak and ambiguous, suggesting plants are not strong drivers of MeHg biogeochemistry in these systems. Although the MeHg hazard associated with individual artificial wetlands is low, they may be important sources of MeHg at the landscape level.
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Affiliation(s)
- R J Strickman
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1285 Military Trail, Toronto, Ontario M4C 1A4, Canada
| | - C P J Mitchell
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1285 Military Trail, Toronto, Ontario M4C 1A4, Canada.
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29
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Strickman RJ, Mitchell CPJ. Accumulation and translocation of methylmercury and inorganic mercury in Oryza sativa: An enriched isotope tracer study. Sci Total Environ 2017; 574:1415-1423. [PMID: 27542632 DOI: 10.1016/j.scitotenv.2016.08.068] [Citation(s) in RCA: 54] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2016] [Revised: 08/08/2016] [Accepted: 08/09/2016] [Indexed: 06/06/2023]
Abstract
Methylmercury (MeHg) accumulation in rice is an emerging human health issue, but uptake pathways and translocation into the grain remain poorly understood. We grew Oryza sativa plants in pots of wetland soil amended with an enriched mercury isotope (94.3% 200Hg) tracer, alongside unvegetated control pots, and assessed both ambient and tracer MeHg and inorganic Hg (IHg) concentrations in soil and plant tissues at three growth stages. Based on similar ratios of ambient:tracer MeHg concentrations in soil and plant tissues, we provide the first direct evidence that MeHg is first synthesized in saturated soil and subsequently translocated to rice grains. There is no evidence of in planta methylation of IHg, but significant losses of MeHg from plant tissues between flowering and maturity indicates likely in planta demethylation. In this greenhouse experiment, lower percent of tracer MeHg in vegetated soils at late growth stages suggests that rice plants reduce the net MeHg accumulation capacity of soils, although the mechanism remains unclear. For IHg, roots accumulated Hg from the soil, straw from the soil and the atmosphere, and grain almost entirely from the atmosphere. Management strategies that aim to reduce MeHg accumulation in rice should focus on mercury methylation in paddy soils, but IHg reductions will depend on regional controls of atmospheric Hg.
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Affiliation(s)
- R J Strickman
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada
| | - C P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada.
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Strickman RJS, Fulthorpe RR, Coleman Wasik JK, Engstrom DR, Mitchell CPJ. Experimental sulfate amendment alters peatland bacterial community structure. Sci Total Environ 2016; 566-567:1289-1296. [PMID: 27267720 DOI: 10.1016/j.scitotenv.2016.05.189] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/03/2016] [Revised: 05/26/2016] [Accepted: 05/26/2016] [Indexed: 06/06/2023]
Abstract
As part of a long-term, peatland-scale sulfate addition experiment, the impact of varying sulfate deposition on bacterial community responses was assessed using 16S tag encoded pyrosequencing. In three separate areas of the peatland, sulfate manipulations included an eight year quadrupling of atmospheric sulfate deposition (experimental), a 3-year recovery to background deposition following 5years of elevated deposition (recovery), and a control area. Peat concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, were measured, the production of which is attributable to a growing list of microorganisms, including many sulfate-reducing Deltaproteobacteria. The total bacterial and Deltaproteobacterial community structures in the experimental treatment differed significantly from those in the control and recovery treatments that were either indistinguishable or very similar to one another. Notably, the relatively rapid return (within three years) of bacterial community structure in the recovery treatment to a state similar to the control, demonstrates significant resilience of the peatland bacterial community to changes in atmospheric sulfate deposition. Changes in MeHg accumulation between sulfate treatments correlated with changes in the Deltaproteobacterial community, suggesting that sulfate may affect MeHg production through changes in the community structure of this group.
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Affiliation(s)
- R J S Strickman
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada
| | - R R Fulthorpe
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada
| | - J K Coleman Wasik
- St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, MN, United States
| | - D R Engstrom
- St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, MN, United States
| | - C P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada.
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Johnson NW, Mitchell CPJ, Engstrom DR, Bailey LT, Coleman Wasik JK, Berndt ME. Methylmercury production in a chronically sulfate-impacted sub-boreal wetland. Environ Sci Process Impacts 2016; 18:725-734. [PMID: 27224550 DOI: 10.1039/c6em00138f] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Increased deposition of atmospheric sulfate exacerbates methylmercury (MeHg) production in freshwater wetlands by stimulating methylating bacteria, but it is unclear how methylation in sub-boreal wetlands is impacted by chronically elevated sulfate inputs, such as through mine discharges. The purpose of our study is to determine how sulfate discharges to wetlands from iron mining activities impact MeHg production. In this study, we compare spatial and temporal patterns in MeHg and associated geochemistry in two wetlands receiving contrasting loads of sulfate. Two orders of magnitude less sulfate in the un-impacted wetland create significant differences in acid-volatile sulfide and porewater sulfide; however, dissolved and solid-phase MeHg concentrations and methylation rate potentials (Kmeth) are statistically similar in both wetlands. Permitted mine pumping events flood the sulfate-impacted wetland with very high sulfate waters during the fall. In contrast to observations in sulfate-limited systems, this large input of sulfate to a chronically sulfate-impacted system led to significantly lower potential relative methylation rates, suggesting a predominance of demethylation processes over methylation processes during the sulfate loading. Overall, short-term measurements of methylation and demethylation potential are unrelated to gross measures of long-term MeHg accumulation, indicating a decoupling of short- and long-term process measurements and an overall disequilibrium in the systems. High sulfide accumulation, above ∼600-800 μg l(-1) sulfide, in the sulfate-impacted system lowers long-term MeHg accumulation, perhaps as a result of less bioavailable Hg-S complexes. Although continued research is required to determine how sulfate-limited freshwater wetlands might respond to new, large inputs of high-sulfate runoff from mining operations, chronically impacted wetlands do not appear to continually accumulate or produce MeHg at rates different from wetlands unimpacted by mining.
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Affiliation(s)
- Nathan W Johnson
- Department of Civil Engineering, University of Minnesota Duluth, USA.
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Tetzlaff D, Buttle J, Carey SK, van Huijgevoort MHJ, Laudon H, McNamara JP, Mitchell CPJ, Spence C, Gabor RS, Soulsby C. A preliminary assessment of water partitioning and ecohydrological coupling in northern headwaters using stable isotopes and conceptual runoff models. Hydrol Process 2015; 29:5153-5173. [PMID: 27656040 PMCID: PMC5012127 DOI: 10.1002/hyp.10515] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2015] [Accepted: 04/10/2015] [Indexed: 05/03/2023]
Abstract
We combined a conceptual rainfall-runoff model and input-output relationships of stable isotopes to understand ecohydrological influences on hydrological partitioning in snow-influenced northern catchments. Six sites in Sweden (Krycklan), Canada (Wolf Creek; Baker Creek; Dorset), Scotland (Girnock) and the USA (Dry Creek) span moisture and energy gradients found at high latitudes. A meta-analysis was carried out using the Hydrologiska Byråns Vattenbalansavdelning (HBV) model to estimate the main storage changes characterizing annual water balances. Annual snowpack storage importance was ranked as Wolf Creek > Krycklan > Dorset > Baker Creek > Dry Creek > Girnock. The subsequent rate and longevity of melt were reflected in calibrated parameters that determine partitioning of waters between more rapid and slower flowpaths and associated variations in soil and groundwater storage. Variability of stream water isotopic composition depends on the following: (i) rate and duration of spring snowmelt; (ii) significance of summer/autumn rainfall; and (iii) relative importance of near-surface and deeper flowpaths in routing water to the stream. Flowpath partitioning also regulates influences of summer evaporation on drainage waters. Deviations of isotope data from the Global Meteoric Water Line showed subtle effects of internal catchment processes on isotopic fractionation most likely through evaporation. Such effects are highly variable among sites and with seasonal differences at some sites. After accounting for climate, evaporative fractionation is strongest at sites where lakes and near-surface runoff processes in wet riparian soils can mobilize isotopically enriched water during summer and autumn. Given close soil-vegetation coupling, this may result in spatial variability in soil water isotope pools available for plant uptake. We argue that stable isotope studies are crucial in addressing the many open questions on hydrological functioning of northern environments. © 2015 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.
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Affiliation(s)
- Doerthe Tetzlaff
- Northern Rivers Institute, School of Geosciences University of Aberdeen Aberdeen AB24 3UF UK
| | - James Buttle
- Department of Geography Trent University 1600 West Bank Drive Peterborough Ontario Canada
| | - Sean K Carey
- School of Geography and Earth Sciences McMaster University 1280 Main St. W Hamilton Ontario L8S 4 K1 Canada
| | | | - Hjalmar Laudon
- Swedish University of Agricultural Sciences Department of Forest Ecology and Management Umeå SE-90183 Sweden
| | - James P McNamara
- Department of Geosciences Boise State University Boise ID 83725 USA
| | - Carl P J Mitchell
- Department of Physical and Environmental Sciences University of Toronto Scarborough 1265 Military Trail Toronto Ontario M1C 1A4 Canada
| | - Chris Spence
- National Hydrology Research Centre Environment Canada 11 Innovation Blvd. Saskatoon Saskatchewan Canada S7N 3H5
| | - Rachel S Gabor
- Northern Rivers Institute, School of Geosciences University of Aberdeen Aberdeen AB24 3UF UK
| | - Chris Soulsby
- Northern Rivers Institute, School of Geosciences University of Aberdeen Aberdeen AB24 3UF UK
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Hoggarth CGJ, Hall BD, Mitchell CPJ. Mercury methylation in high and low-sulphate impacted wetland ponds within the prairie pothole region of North America. Environ Pollut 2015; 205:269-277. [PMID: 26099458 DOI: 10.1016/j.envpol.2015.05.046] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2015] [Revised: 04/21/2015] [Accepted: 05/21/2015] [Indexed: 06/04/2023]
Abstract
Using enriched stable (201)Hg injections into intact sediment cores, we provide the first reported Hg methylation potential rate constants (km) in prairie wetland ponds (0.016-0.17 d(-1)). Our km values were similar to other freshwater wetlands and did not differ in ponds categorized with high compared to low surface water concentrations of sulphate. Sites with high sulphate had higher proportions of methylmercury (MeHg) in sediment (2.9 ± 1.6% vs. 1.0 ± 0.3%) and higher surface water MeHg concentrations (1.96 ± 1.90 ng L(-1)vs. 0.56 ± 0.55 ng L(-1)). Sediment-porewater partitioning coefficients were small, and likely due to high ionic activity. Our work suggests while km measurements are useful for understanding mercury cycling processes, they are less important than surface water MeHg concentrations for assessing MeHg risks to biota. Significant differences in MeHg concentrations between sites with high and low sulphate concentrations may also inform management decisions concerning wetland remediation and creation.
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Affiliation(s)
- Cameron G J Hoggarth
- Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada
| | - Britt D Hall
- Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada.
| | - Carl P J Mitchell
- Department of Physical and Environmental Science, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canada
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Mazur MEE, Eckley CS, Mitchell CPJ. Susceptibility of Soil Bound Mercury to Gaseous Emission As a Function of Source Depth: An Enriched Isotope Tracer Investigation. Environ Sci Technol 2015; 49:9143-9149. [PMID: 26151306 DOI: 10.1021/acs.est.5b01747] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Soil mercury (Hg) emissions are an important component of the global Hg cycle. Sunlight induced photoreduction of oxidized Hg to gaseous elemental Hg is an important mechanism controlling emissions from the soil surface, however we currently understand little about how subsurface Hg stores participate in gaseous Hg cycling. Our study objective was to investigate the ability of Hg at deeper soil depths to participate in emissions. Soil fluxes were measured under controlled laboratory conditions utilizing an enriched stable Hg isotope tracer buried at 0, 1, 2, and 5 cm below the surface. Under dry and low-light conditions, the Hg isotope tracer buried at the different depths participated similarly in surface emissions (median flux: 7.5 ng m(-2) h(-1)). When the soils were wetted, Hg isotope tracer emissions increased significantly (up to 285 ng m(-2) h(-1)), with the highest fluxes (76% of emissions) originating from the surface 1 cm amended soils and decreasing with depth. Mercury associated with sandy soil up to 6 cm below the surface can be emitted, clearly demonstrating that volatilization can occur via processes unrelated to sunlight. These results have important implications for considering how long older, legacy soil Hg contamination continues to cycle between soil and atmosphere.
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Affiliation(s)
- Maxwell E E Mazur
- †University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
| | - Chris S Eckley
- ‡U.S. Environmental Protection Agency, Region 10, 1200 Sixth Avenue, Seattle, Washington 98101, United States
| | - Carl P J Mitchell
- †University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
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Mazur M, Mitchell CPJ, Eckley CS, Eggert SL, Kolka RK, Sebestyen SD, Swain EB. Gaseous mercury fluxes from forest soils in response to forest harvesting intensity: a field manipulation experiment. Sci Total Environ 2014; 496:678-687. [PMID: 24993512 DOI: 10.1016/j.scitotenv.2014.06.058] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2014] [Revised: 06/13/2014] [Accepted: 06/13/2014] [Indexed: 06/03/2023]
Abstract
Forest harvesting leads to changes in soil moisture, temperature and incident solar radiation, all strong environmental drivers of soil-air mercury (Hg) fluxes. Whether different forest harvesting practices significantly alter Hg fluxes from forest soils is unknown. We conducted a field-scale experiment in a northern Minnesota deciduous forest wherein gaseous Hg emissions from the forest floor were monitored after two forest harvesting prescriptions, a traditional clear-cut and a clearcut followed by biomass harvest, and compared to an un-harvested reference plot. Gaseous Hg emissions were measured in quadruplicate at four different times between March and November 2012 using Teflon dynamic flux chambers. We also applied enriched Hg isotope tracers and separately monitored their emission in triplicate at the same times as ambient measurements. Clearcut followed by biomass harvesting increased ambient Hg emissions the most. While significant intra-site spatial variability was observed, Hg emissions from the biomass harvested plot (180 ± 170 ng m(-2)d(-1)) were significantly greater than both the traditional clearcut plot (-40 ± 60 ng m(-2)d(-1)) and the un-harvested reference plot (-180 ± 115 ng m(-2)d(-1)) during July. This difference was likely a result of enhanced Hg(2+) photoreduction due to canopy removal and less shading from downed woody debris in the biomass harvested plot. Gaseous Hg emissions from more recently deposited Hg, as presumably representative of isotope tracer measurements, were not significantly influenced by harvesting. Most of the Hg tracer applied to the forest floor became sequestered within the ground vegetation and debris, leaf litter, and soil. We observed a dramatic lessening of tracer Hg emissions to near detection levels within 6 months. As post-clearcutting residues are increasingly used as a fuel or fiber resource, our observations suggest that gaseous Hg emissions from forest soils will increase, although it is not yet clear for how long such an effect will persist.
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Affiliation(s)
- M Mazur
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, ON M1C 1A4, Canada
| | - C P J Mitchell
- University of Toronto Scarborough, Department of Physical and Environmental Sciences, 1265 Military Trail, Toronto, ON M1C 1A4, Canada.
| | - C S Eckley
- Meteorological Service of Canada, Environment Canada, 4905 Dufferein Street, Toronto, ON M3H 5T4, Canada
| | - S L Eggert
- Northern Research Station, USDA Forest Service, 1831 Hwy 169 E, Grand Rapids, MN 55744, United States
| | - R K Kolka
- Northern Research Station, USDA Forest Service, 1831 Hwy 169 E, Grand Rapids, MN 55744, United States
| | - S D Sebestyen
- Northern Research Station, USDA Forest Service, 1831 Hwy 169 E, Grand Rapids, MN 55744, United States
| | - E B Swain
- Minnesota Pollution Control Agency, St. Paul, MN 55155, United States
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Vidon PG, Mitchell CPJ, Jacinthe PA, Baker ME, Liu X, Fisher KR. Mercury dynamics in groundwater across three distinct riparian zone types of the US Midwest. Environ Sci Process Impacts 2013; 15:2131-2141. [PMID: 24113840 DOI: 10.1039/c3em00254c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Although the intense biogeochemical gradients present in riparian zones have the potential to affect mercury (Hg) cycling, Hg dynamics in riparian zones has received relatively little attention in the literature. Our study investigated groundwater filtered total mercury (THg) and methylmercury (MeHg) dynamics in three riparian zones with contrasting hydrogeomorphic (HGM) characteristics (till, alluvium, outwash) in the US Midwest. Despite high Hg deposition rates (>16 μg m(-2)) in the region, median THg (<1.05 ng L(-1)) and MeHg (<0.05 ng L(-1)) concentrations were low at the study sites. Methylmercury concentrations were significantly (p < 0.05) correlated to THg (R = 0.82), temperature (R = 0.55), and dissolved organic carbon (DOC) (R = 0.62). THg also correlated with groundwater DOC (R = 0.59). The proportion of MeHg in THg (%MeHg) was significantly correlated to temperature (R = 0.58) and MeHg (R = 0.50). Results suggest that HGM characteristics, the presence of tile drains, and the propensity for overbank flooding at a riparian site determined the extent to which stream water Hg concentrations influenced riparian groundwater Hg levels or vice versa. Differences in hydrogeomorphic characteristics between sites did not translate however in significant differences in groundwater MeHg or %MeHg. Overall, widespread Hg contamination in the most common riparian hydrogeomorphic types of the US Midwest is unlikely to be a major concern. However, for frequently flooded riparian zones located downstream from a potentially large source of Hg (e.g., concentrated urban development), Hg concentrations are likely to be higher than at other sites.
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Affiliation(s)
- Philippe G Vidon
- Department of Forest and Natural Resources Management, The State University of New York College of Environmental Science and Forestry (SUNY-ESF), 1 Forestry Drive, Syracuse, NY 13210, USA.
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Dhal S, Mitchell CPJ. Saltwater flotation for more efficient matrix separation of wetland macroinvertebrates does not affect total mercury or methylmercury concentrations. Environ Toxicol Chem 2013; 32:1233-1236. [PMID: 23440748 DOI: 10.1002/etc.2176] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/02/2012] [Revised: 09/24/2012] [Accepted: 01/02/2013] [Indexed: 06/01/2023]
Abstract
The authors compared benthic wetland invertebrate matrix separation techniques (handpicking vs saltwater flotation) to test for effects on invertebrate mercury concentrations. Neither total mercury nor methylmercury concentrations differed significantly between techniques across 8 taxa. Matrix separation by the flotation technique took significantly less time and resulted in significantly greater abundance recovery in some taxa. The authors conclude that the saltwater-based flotation technique does not lead to mercury contamination or analytical interference issues.
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Affiliation(s)
- Suman Dhal
- Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada
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Sinclair KA, Xie Q, Mitchell CPJ. Methylmercury in water, sediment, and invertebrates in created wetlands of Rouge Park, Toronto, Canada. Environ Pollut 2012; 171:207-215. [PMID: 22940274 DOI: 10.1016/j.envpol.2012.07.043] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/25/2012] [Revised: 06/14/2012] [Accepted: 07/25/2012] [Indexed: 06/01/2023]
Abstract
Thousands of hectares of wetlands are created annually because wetlands provide beneficial ecosystem services. Wetlands are also key sites for production of the bioaccumulative neurotoxin methylmercury (MeHg), but little is known about MeHg production in created systems. Here, we studied methylmercury in sediment, water, and invertebrates in created wetlands of various ages. Sediment MeHg reached 8 ng g(-1) in the newest wetland, which was significantly greater than in natural, control wetlands. This trend was mirrored in several invertebrate taxa, whose concentrations reached as high as 1.6 μg g(-1) in the newest wetland, above levels thought to affect reproduction in birds. The MeHg concentrations in created wetland invertebrate taxa generally decreased with increasing wetland age, possibly due to a combination of deeper anoxia and less organic matter accumulation in younger wetlands. A short-term management intervention and/or improved engineering design may be necessary to reduce the mercury-associated risk in newly created wetlands.
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Affiliation(s)
- Kathleen A Sinclair
- Department of Physical and Environmental Science, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON, Canada M1C 1A4.
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Mitchell CPJ, Kolka RK, Fraver S. Singular and combined effects of blowdown, salvage logging, and wildfire on forest floor and soil mercury pools. Environ Sci Technol 2012; 46:7963-70. [PMID: 22747193 DOI: 10.1021/es300133h] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
A number of factors influence the amount of mercury (Hg) in forest floors and soils, including deposition, volatile emission, leaching, and disturbances such as fire. Currently the impact on soil Hg pools from other widespread forest disturbances such as blowdown and management practices like salvage logging are unknown. Moreover, ecological and biogeochemical responses to disturbances are generally investigated within a single-disturbance context, with little currently known about the impact of multiple disturbances occurring in rapid succession. In this study we capitalize on a combination of blowdown, salvage logging and fire events in the sub-boreal region of northern Minnesota to assess both the singular and combined effects of these disturbances on forest floor and soil total Hg concentrations and pools. Although none of the disturbance combinations affected Hg in mineral soil, we did observe significant effects on both Hg concentrations and pools in the forest floor. Blowdown increased the mean Hg pool in the forest floor by 0.76 mg Hg m(-2) (223%). Salvage logging following blowdown created conditions leading to a significantly more severe forest floor burn during wildfire, which significantly enhanced Hg emission. This sequence of combined events resulted in a mean loss of approximately 0.42 mg Hg m(-2) (68% of pool) from the forest floor, after conservatively accounting for potential losses via enhanced soil leaching and volatile emissions between the disturbance and sampling dates. Fire alone or blowdown followed by fire did not significantly affect the total Hg concentrations or pools in the forest floor. Overall, unexpected consequences for soil Hg accumulation and by extension, atmospheric Hg emission and risk to aquatic biota, may result when combined impacts are considered in addition to singular forest floor and soil disturbances.
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Affiliation(s)
- Carl P J Mitchell
- Department of Physical and Environmental Sciences, University of Toronto Scarborough , 1265 Military Trail, Toronto, Ontario, Canada.
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Ahmed F, Bibi MH, Asaeda T, Mitchell CPJ, Ishiga H, Fukushima T. Elemental composition of sediments in Lake Jinzai, Japan: assessment of sources and pollution. Environ Monit Assess 2012; 184:4383-96. [PMID: 21800061 DOI: 10.1007/s10661-011-2271-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2011] [Accepted: 07/15/2011] [Indexed: 05/16/2023]
Abstract
Bottom sediments from Lake Jinzai in southwest Japan were analyzed to determine their chemical compositions and to assess the potential for ecological harm by comparison with sediment quality guidelines. The pollution status of lake sediments was evaluated by employing contamination factor (CF), pollution load index (PLI), and geoaccumulation index (I(geo)), focusing on a suite of elements in lakebed and core sediments. Elevated concentrations of As, Pb, Zn, Cu, TOC, N, and P were present in several layers of the upper core and other surface sediments. The elevated metal concentrations are likely related to the fine-grained nature of the sediments, reducing bottom conditions produced by abundant organic matter, and possibly minor non-point anthropogenic sources. Moreover, correlations between the concentrations of trace metals and organic carbon, nitrogen, phosphorus, and iron, suggest that these elements play a role in controlling abundances. Calculated CF, PLI, and I(geo) indicate that the sediments are strongly polluted with respect to As, moderately to strongly polluted with Zn, and moderately polluted with Pb and Cu. Metal concentrations exceed the New York State Department of Environmental Conservation (NYSDEC) lowest effect level and the Canadian Council of Ministers of the Environment (CCME) interim sediment quality guidelines that indicate moderate impact on aquatic organisms in the study area.
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Affiliation(s)
- Faruque Ahmed
- Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, 305-8572, Japan.
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Coleman Wasik JK, Mitchell CPJ, Engstrom DR, Swain EB, Monson BA, Balogh SJ, Jeremiason JD, Branfireun BA, Eggert SL, Kolka RK, Almendinger JE. Methylmercury declines in a boreal peatland when experimental sulfate deposition decreases. Environ Sci Technol 2012; 46:6663-6671. [PMID: 22578022 DOI: 10.1021/es300865f] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Between 2001 and 2008 we experimentally manipulated atmospheric sulfate-loading to a small boreal peatland and monitored the resulting short and long-term changes in methylmercury (MeHg) production. MeHg concentrations and %MeHg (fraction of total-Hg (Hg(T)) present as MeHg) in the porewaters of the experimental treatment reached peak values within a week of sulfate addition and then declined as the added sulfate disappeared. MeHg increased cumulatively over time in the solid-phase peat, which acted as a sink for newly produced MeHg. In 2006 a "recovery" treatment was created by discontinuing sulfate addition to a portion of the experimentally treated section to assess how MeHg production might respond to decreased sulfate loads. Four years after sulfate additions ceased, MeHg concentrations and %MeHg had declined significantly from 2006 values in porewaters and peat, but remained elevated relative to control levels. Mosquito larvae collected from each treatment at the end of the experiment exhibited Hg(T) concentrations reflective of MeHg levels in the peat and porewaters where they were collected. The proportional responses of invertebrate Hg(T) to sulfate deposition rates demonstrate that further controls on sulfur emissions may represent an additional means of mitigating Hg contamination in fish and wildlife across low-sulfur landscapes.
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Affiliation(s)
- Jill K Coleman Wasik
- Water Resources Science Graduate Program, University of Minnesota, 173 McNeal Hall, 1985 Buford Avenue, St Paul, Minnesota 55108, United States.
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Haynes KM, Mitchell CPJ. Inter-annual and spatial variability in hillslope runoff and mercury flux during spring snowmelt. ACTA ACUST UNITED AC 2012; 14:2083-91. [DOI: 10.1039/c2em30267e] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Mann E, Meyer T, Mitchell CPJ, Wania F. Mercury fate in ageing and melting snow: development and testing of a controlled laboratory system. ACTA ACUST UNITED AC 2011; 13:2695-702. [PMID: 21858316 DOI: 10.1039/c1em10297d] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A snow cover can modify when, to what extent, and in what form atmospherically deposited mercury is released to the underlying surface media and/or back to the atmosphere. Investigations of mercury transport and transformation processes in snow packs are hampered by the difficulty in controlling experimental and melt conditions and due to the huge variability in the composition and physical structure of environmental snow packs. A method was developed that allows the detailed mechanistic investigation of mercury fate in snow that is made, aged and melted under controlled laboratory conditions. A number of control samples established that mercury in indoor air, scavenged during the snow making process, constitutes the dominant source of mercury in the artificial snow. No addition of mercury is required. The amount of mercury in fresh snow was quantitatively (102 and 106% in two experiments) recovered in the dissolved and particulate fractions of the melt water and the vessel head space, confirming a mass balance for mercury and the absence of unquantifiable mercury sources and sinks in the experimental system. In snow made from unmodified tap water, more than half of the mercury present in the snowpack was recovered from the bottom of the snow vessel after all of the snow had melted. Such late elution is indicative of mercury being mostly associated with particles that are filtered by, and retained in, the shrinking snowpack. Addition of salt to the snow-making water at an environmentally realistic pH notably shifted the distribution of mercury in the snowpack from the particulate to the dissolved phase, resulting in more than 60% of the mercury eluting in the dissolved phase of early melt water fractions.
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Affiliation(s)
- Erin Mann
- Department of Chemistry, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada
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Richardson MC, Mitchell CPJ, Branfireun BA, Kolka RK. Analysis of airborne LiDAR surveys to quantify the characteristic morphologies of northern forested wetlands. ACTA ACUST UNITED AC 2010. [DOI: 10.1029/2009jg000972] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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Affiliation(s)
- Carl P. J. Mitchell
- Department of Physical and Environmental Sciences; University of Toronto Scarborough; Toronto, Ontario Canada
- Smithsonian Environmental Research Center; Edgewater Maryland USA
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Mitchell CPJ, Branfireun BA, Kolka RK. Spatial characteristics of net methylmercury production hot spots in peatlands. Environ Sci Technol 2008; 42:1010-6. [PMID: 18351065 DOI: 10.1021/es0704986] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
Many wetlands are sources of methylmercury (MeHg) to surface waters, yet little information exists about the distribution of MeHg within wetlands. Total mercury (THg) and MeHg in peat pore waters were studied in four peatlands in spring, summer, and fall 2005. Marked spatial variability in the distribution of MeHg, and %MeHg as a proxy for net MeHg production, was observed, with highest values occurring in discrete zones. We denote these zones "MeHg hot spots", defined as an area where the pore water %MeHg exceeded the 90th percentile of the data set (n=463) or >22% of THg as MeHg. MeHg hot spots occurred near the interface between peatland and the upland watershed with few exceptions. The %MeHg in pore water was significantly less in peatland interiors compared to upland-peatland interface zones, with the significance of these differences related to the delineation of the boundary between the two areas. Although further research is necessary, our data suggest that the occurrence of MeHg hot spots is related to the transport of solutes in upland runoff to the peatland perimeter and not to the accumulation of MeHg in this zone as a result of transport from either the peatland interior or the surrounding upland watershed. These findings augment the understanding of peatland MeHg production in upland-peatland watersheds, provide guidance for more accurate quantification of MeHg pool sizes in the landscape, and a spatial framework forthe further study of mercury methylation processes in peatlands.
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Affiliation(s)
- Carl P J Mitchell
- Department of Geography, University of Toronto, 3359 Mississauga Road, Mississauga, Ontario, Canada.
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