Spatial-temporal characteristics of mercury and methylmercury in marine sediment under the combined influences of river input and coastal currents

Comparison of the Performance of ICP-MS, CV-ICP-OES, and TDA AAS in Determining Mercury in Marine Sediment Samples

Mercury (Hg) determination in marine sediment is an analytical challenge due to the toxicity of this element even at low concentrations (up to 130 μg kg-1 in marine sediments) and complex matrices. Therefore, it is necessary to use analytical techniques that have high sensitivity, selectivity, and low limits of quantification (LoQ). In this study, two methods that require sample treatment and one method with direct sampling were studied. The techniques studied were inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry with cold vapor generation (CV-ICP-OES), and atomic absorption spectrometry with thermodecomposition and amalgamation (TDA AAS) for Hg determination in marine sediment samples. Since ICP-MS has more studies in the literature, optimization with design of experiments was developed for CV-ICP-OES and TDA AAS. Although it was found to have low levels of instrumental LoQ for all three techniques, differences were found once the method LoQ was calculated. The calculation for method LoQ considers all analytical procedures executed, including sample treatment, which provides a 100-fold dilution for ICP-MS and CV-ICP-OES. The method LoQ obtained were 1.9, 165, and 0.35 μg kg-1 for ICP-MS, CV-ICP-OES, and TDA AAS, respectively. Comparing marine sediment sample analyses, Hg concentrations had no statistical difference when determined by ICP-MS and TDA AAS. It was not possible to determine Hg in marine sediment samples by CV-ICP-OES due to the high method LoQ obtained (165 μg kg-1). Although ICP-MS has the advantage of being a multielemental technique, it is high-value equipment and needs a large volume of argon, which has a high cost in the market, and it requires sample treatment. On the other hand, TDA AAS-based spectrometer DMA-80 performs direct sampling, avoiding the pretreatment stage, and has a relatively lower cost, both in terms of initial investment and maintenance, while maintaining the high sensitivity, accuracy, and precision required for Hg determination on marine sediment samples.

Quantifying the impact of anthropogenic emissions and aquatic environmental impacts on sedimentary mercury variations in a typical urban river

In urban and industrial regions, sedimentary mercury (Hg) serves as the crucial indicator for Hg pollution, posing potential risks to ecology and human health. The physicochemical processes of Hg in aquatic environments are influenced by various factors such as anthropogenic emissions and aquatic environmental impacts, making it challenging to quantify the drivers of total mercury (THg) variations. Here, we analyzed the spatiotemporal variations, quantified driving factors, and assessed accumulation risks of sedimentary THg from the mainstream of a typical urban river (Haihe River). THg in the urban region (37-3237 ng g-1) was significantly higher (t-test, p < 0.01) than in suburban (71-2317 ng g-1) and developing regions (156-916 ng g-1). The sedimentary THg in suburban and developing regions increased from 2003 to 2018, indicating the elevated atmospheric deposition of Hg. Together with the temperature, grain size of sediments, total organic carbon (TOC), the pH and salinity of water, 40 components of parent and substituted polycyclic aromatic hydrocarbons (PAHs) were first introduced to quantify the driver of sedimentary THg based on generalized additive model. Results showed that anthropogenic emissions, including three PAHs components (31%) and TOC (63%), accounted for 94% of sedimentary THg variations. The aquatic environmental impacts accounted for 5% of sedimentary THg variations. The geo-accumulation index of THg indicated moderate to heavy accumulation in the urban region. This study demonstrates that homologous pollutants such as PAHs can be used to trace sources and variations of Hg pollution, supporting their co-regulation as international conventions regulate pollutants.

Effects of hydrologic regimes on the loading and spatiotemporal variation of mercury in the microtidal river estuary

The potential effect of hydrological conditions on distribution and loadings of Hg species was investigated in the microtidal Hyeongsan River Estuary (HRE). Dissolved Hg (DHg) and dissolved methylmercury (DMeHg) from the creek receiving industrial wastes were effectively settled to sediment during the post-typhoon period, while persistent input from the Hg-contaminated creek without settling was observed during the dry periods. The event-based mean approach was applied to explore the hydrological effects on the annual flux of Hg. The largest inputs of DHg and particulate Hg (PHg) were found in the Hg-contaminated creek, and DHg input was higher in the dry than wet periods whereas PHg input was higher in the wet than dry periods. In sediment, Hg and MeHg concentrations decreased after the typhoon, attributed to erosion of surface sediments. Overall, the HRE serves as an effective sink of Hg that reduces the degree of Hg contamination in coastal water.

Assessing glyphosate and AMPA pesticides in the Ofanto River waters and sediments

In the present study, we determined glyphosate (GPS) and aminomethylphosphonic acid (AMPA) in the water and sediments of the Ofanto River (Italy), evaluating their transport from the mouth to the sea. Sediments were collected twice in 2021 during low and high tide; waters were sampled on a seasonal basis. The results showed the prevalence of GPS and AMPA in the water with concentrations equal to 190 and 3053 ng/l, respectively. We also found GPS and AMPA in the sediments with values of 0.95 and 11.34 ng/g. In water, pesticides were detected in all seasons with peaks in concentrations during summer and spring. A significant positive correlation between the pesticides in the sediments and the water pH and a negative correlation with salinity was observed. An estimation of the average loads revealed a discharge of 64.11 kg/yr. of GPS and 958.37 kg/yr. of AMPA from the river to the marine environment.

Distributions of total mercury and methylmercury and regulating factors in lake water and surface sediment in the cold-arid Wuliangsuhai Lake region

This study aimed to understand the occurrence of mercury in the water environment of typical cold and arid lakes and the regulating environmental factors. Water and surface sediment samples were collected from July to August, 2022 in the Wuliangsuhai Lake region for the analysis of total mercury (THg) and total methylmercury (TMeHg). Lake water THg and TMeHg ranged between 19.20 ~ 668.10 and 0.10 ~ 11.40 ng/L, respectively, exceeding China's environmental quality standards and contents of other lakes and reservoirs in China and other areas. Surface sediments showed lower mean THg and TMeHg of 261.85 and 0.18 μg/kg, respectively, with the former significantly exceeding the background value of Inner Mongolia and unpolluted natural lakes but lower than those of lakes affected by human factors, such as aquaculture. Sediments showed relatively low methylation and TMeHg (0.01-0.21%) concentrations. Correlation analysis identified salinity, total dissolved solids, conductivity, and redox potential as important factors affecting mercury speciation in water, whereas those in surface sediments were organic matter, pH, and total iron content. This study conducted preliminary research on the different species of Hg in Wuliangsuhai Lake water environment, which can provide scientific evidence for the specific treatment of Hg pollution in agriculture, or industry and other related fields. Our results suggest that upstream and downstream regulatory agencies should strengthen the regulation of agricultural and industrial production, moderately reduce human activities, and reduce the use of mercury-rich substances such as pesticides.

Significant impacts of river inputs on the distributions and transports of mercury and methylmercury in nearshore and open seas - Simulation based on field surveys and mass balance modeling

Rivers are important sources of Hg for adjacent seas, and seafood from nearshore waters is a major source of Hg exposure for humans. There is thus a key scientific concern regarding how much riverine Hg inputs influence Hg loads in nearshore waters as well as how far the impact range can extend from the river to the open sea. In addition, it is important to understand the influence of anthropogenic hydro-facilities and activities on Hg levels in downstream seas. Because of the concise mass exchange pattern between the seas and the previously demonstrated intensive Hg inputs under anthropogenic regulation from the Yellow River, the Bohai and Yellow Seas, which are key fishery and marine breeding areas for China, are an ideal research area for exploring the impacts of riverine Hg on nearshore and adjacent open seas. Field surveys were conducted in eight major rivers and two seas, and 433 water samples were collected. The main Hg input and output terms (rivers, ocean currents, underground discharge, sewage, coastal erosion, atmospheric deposition, surface evasion, sedimentation, and fisheries) were quantified in the Bohai and Yellow Seas. Owing to the high inputs from the Yellow and Yalu Rivers, elevated THg concentrations were found. Apart from direct MeHg discharge, riverine nutrients may also seemingly affect nearshore MeHg. Using mass balance models, we found that the Yellow River (9.8 t) was the dominant Hg source in the Bohai Sea, which accounted for more than half of all contributions, and the Bohai Sea played the role of a secondary source of Hg to the Yellow Sea, with a flux of 3.3 t. Anthropogenic hydro-activities in large rivers could significantly influence Hg outputs and loads in the nearshore and even open seas. This study provides useful information for water resource management applications to reduce potential MeHg risks.

Responses of the structure and function of microbes in Yellow River Estuary sediments to different levels of mercury

The health and stability of the estuary of the Yellow River ecosystem have come under increasing pressure from land-based inputs of heavy metals. While it is known that heavy metals affect the function and health of the microbial community, there remains little knowledge on the responses of the microbial community to heavy metals, particularly highly toxic mercury. The research aimed to characterize the responses of the sediment microbial community of the estuary of the Yellow River to different levels of mercury stress. Estuary sediment samples were collected for microbial community analysis, measurement of mercury [including total mercury (THg) and methylmercury (MeHg)], and measurement of other physicochemical factors, including pH, total organic carbon (TOC), sulfide, iron ratio (Fe3+/Fe2+), ammonium salt (NH4+), and biochemical oxygen demand (BOD). The application of 16S rRNA sequencing identified 60 phyla of bacteria, dominated by Proteobacteria, Firmicutes, and Bacteroidetes. Stations with higher THg or MeHg and lower microbial abundance and diversity were generally distributed further outside of the estuary. Besides mercury, the measured physicochemical factors had impacts on microbial diversities and distribution. Metagenomics assessment of three stations, representative of low, moderate, and high mercury concentrations and measured physicochemical factors, revealed the abundances and functions of predicted genes. The most abundant genes regulating the metabolic pathways were categorized as metabolic, environmental information processing, and genetic information processing, genes. At stations with high levels of mercury, the dominant genes were related to energy metabolism, signal transport, and membrane transport. Functional genes with a mercury-resistance function were generally in the mer system (merA, merC, merT, merR), alkylmercury lyase, and metal-transporting ATPase. These results offer insight into the microbial community structure of the sediments in the Yellow River Estuary and the microbial function of mercury resistance under mercury stress.

A risk-based approach to cumulative effects assessment for large marine ecosystems to support transboundary marine spatial planning: A case study of the yellow sea

Cumulative effects assessment (CEA) should be conducted at ecologically meaningful scales such as large marine ecosystems to halt further ocean degradation caused by anthropogenic pressures and facilitate ecosystem-based management such as transboundary marine spatial planning (MSP). However, few studies exist at large marine ecosystems scale, especially in the West Pacific seas, where countries have different MSP processes yet transboundary cooperation is paramount. Thus, a step-wise CEA would be informative to help bordering countries set a common goal. Building on the risk-based CEA framework, we decomposed CEA into risk identification and spatially-explicit risk analysis and applied it to the Yellow Sea Large Marine Ecosystem (YSLME), aiming to understand the most influential cause-effect pathways and risk distribution pattern. The results showed that (1) seven human activities including port, mariculture, fishing, industry and urban development, shipping, energy, and coastal defence, and three pressures including physical loss of seabed, input of hazardous substances, nitrogen, and phosphorus enrichment were the leading causes of environmental problems in the YSLME; (2) benthic organisms, fishes, algae, tidal flats, seabirds, and marine mammals were the most vulnerable ecosystem components on which cumulative effects acted; (3) areas with relatively high risk mainly concentrated on nearshore zones, especially Shandong, Liaoning, and northern Jiangsu, while coastal bays of South Korea also witnessed high risk; (4) certain risks could be observed in the transboundary area, of which the causes were the pervasive fishing, shipping, and sinking of pollutants in this area due to the cyclonic circulation and fine-grained sediments. In future transboundary cooperation on MSP, risk criteria and evaluation of existing management measures should be incorporated to determine whether the identified risk has exceeded the acceptable level and identify the next step of cooperation. Our study presents an example of CEA at large marine ecosystems scale and provides a reference to other large marine ecosystems in the West Pacific and elsewhere.

Riverine input of suspended particulate matter controls distribution, partitioning and transport of mercury and methylmercury in the Yellow River Estuary

Riverine mercury (Hg) is the largest global source of Hg in coastal oceans. The Yellow River delivers the majority of Hg to the semi-enclosed Bohai Sea, where Hg contamination adversely affects the surrounding heavily populated provinces in northern China. Mercury distribution patterns in the river-estuary interacting area provides essential information to understand the riverine Hg transport and biogeochemical cycling of Hg in the estuary. Analyzing the spatial distributions of total- (THg) and methyl-Hg (MeHg) in the lower end of Yellow River (∼105 km) and adjacent estuary, we found the dominant role of suspended particulate matter (SPM) in Hg transport, with 99.1% and 86.3% of THg and MeHg being in particulate phase. The SPM dynamics, such as transport, retention, sorting and sedimentation, governs Hg transport with water flow and particle-water partition of Hg. While THg decreased along the water flow to the river mouth with the settlement of particulate THg (about 27.5% onto the riverbed and the rest entering the sea), MeHg and particulate MeHg increased by 110% and 117%, respectively. This study highlights the distinct patterns in THg and MeHg distribution and transport and suggests potential Hg methylation and external MeHg input in the river-estuary mixed zone.

Mercury contamination in the water and sediments of a typical inland river - Lake basin in China: Occurrence, sources, migration and risk assessment

In the area affected by non-ferrous metal mining activities, mercury (Hg) contamination in the water and sediments posed potential risks to ecology and human health. In this study, river water and sediment samples were collected in the Zijiang river - South Dongting Lake basin to analyze Hg residues, identify potential Hg sources and evaluate the ecological and health risks posed by Hg contamination. In this study, the average concentrations of THg, PHg, DHg and DMeHg in river water were 38.05 ± 27.13 ng/L, 25.18 ± 26.83 ng/L, 12.88 ± 9.64 ng/L and 0.29 ± 0.07 ng/L, respectively. The THg and MeHg contents in sediments were 234.24 ± 152.93 µg/kg and 0.48 ± 0.16 µg/kg, respectively. The more enrichment of Hg in sediments was observed in the Zijiang River than in the South Dongting Lake, especially in the upstream and midstream regions. Two potential Hg sources in the basin were identified by correlation matrix, principal component analysis (PCA) and positive matrix factorization (PMF) model. The comparable Hg flux with other rivers worldwide was found in the Zijiang River (0.53 Mg/y). Furthermore, it was found by the delayed geochemical hazard (DGH) model that the ecological risk of Hg was more significant in the Zijiang River with more frequent transformation pathways. For different populations, the health risk values caused by Hg were all lower than the USEPA's guideline value. This study provided sound evidence for further control of Hg contamination.

Adsorption and environmental behavior of mercury on the sediment from the Yellow Sea of China

The Yellow Sea (YS) of China is facing severe mercury (Hg) pollution problems while the concentration of Hg in the sediment is relatively low compared to its high discharge intensity, whose mechanisms are still unclear. Here, we performed batch experiments to investigate the Hg adsorption capacity of the YS sediments. Freundlich isothermal adsorption simulation results showed that the parameters K_F (adsorption capacity constant) of the sediments were varied from 3.33 to 2.88 × 10⁴. Correlation analysis of K_F against the physicochemical properties and sequential extraction revealed that organic matter (OM) and particle size influenced the K_F for Hg. In addition, the calculated smaller Kd (distribution coefficient) values in the YS compared to other coastal seas indicate that at the sediment-water interface, Hg tends to be more present in porewater. There is also a significant positive correlation between K_F and Kd. We conclude that the low OM content of YS sediments is one of the main reasons for their weak adsorption capacity. These findings provide a scientific basis for the phenomenon that Hg entering the YS undergoes environmental behaviors to maintain low concentrations in the sediment, deepen the understanding of Hg cycling in the YS and improve long-term risk prediction capacity for Hg in marine environments.

Impacts of the invasive Spartina anglica on C-S-Hg cycles and Hg(II) methylating microbial communities revealed by hgcA gene analysis in intertidal sediment of the Han River estuary, Yellow Sea

We investigated the impact of invasive vegetation on mercury cycles, and identified microorganisms directly related to Hg(II) methylation using hgcA gene in vegetated mud flats (VMF) inhabited by native Suaeda japonica (SJ) and invasive Spartina anglica (SA), and unvegetated mud flats (UMF) in Ganghwa intertidal sediments. Sulfate reduction rate (SRR) and rate constants of Hg(II) methylation (K_m) and methyl-Hg demethylation (K_d) were consistently greater in VMF than in UMF, specifically 1.5, 2 and 11.7 times higher, respectively, for SA. Both K_m and K_d were significantly correlated with SRR and the abundance of sulfate-reducing bacteria. These results indicate that the rhizosphere of invasive SA provides a hotspot for Hg dynamics coupled with sulfate reduction. HgcA gene analysis revealed that Hg(II)-methylators were dominated by Deltaproteobacteria, Chloroflexi and Euryarchaeota, comprising 37.9%, 35.8%, and 6.5% of total hgcA gene sequences, respectively, which implies that coastal sediments harbor diverse Hg(II)-methylating microorganisms that previously underrepresented.

Historical mercury contamination in a major Latin American industrial and port complex: The case of the Santos estuary, Southeastern Brazil

The present study determined total mercury (Hg) in four ²¹⁰Pb dated sediment cores to assess the historical anthropogenic Hg accumulation in the Santos estuary, Southeastern Brazil. Background levels were identified using the deepest sections of the cores, corresponding to pre-industrial ages. Mercury distribution in the sediment cores (0.02-2.64 mg kg^-1) presented a large spatial and temporal variation. Contamination is highest in the upper estuary and indicates that the industrial hub, especially a chlor-alkali plant is the primary source of Hg. A contaminant trap effect is observed in this area associated with high fine sediment accumulation and Hg fluxes. The contamination pattern indicates that the regions not affected by direct inputs are influenced by reworking, resuspension, and transport of contaminated sediments by tidal flows. The Hg enrichment in the upper layers of the sediment cores demonstrates that the environmental actions fulfilled in the 1980s were insufficient to control Hg pollution in the Santos estuary.

Spatiotemporal partition dynamics of typical herbicides at a turbid river estuary

Organic pollutants are ubiquitous in estuarine areas, nonetheless, the transport mechanisms of herbicides in such areas are limited. Atrazine and acetochlor were analyzed in suspended particle matter (SPM), surface sediment, and surface water from the Yellow River estuary and the surrounding rivers and sea. Among these rivers, the Yellow River contributes the most herbicide flux to the sea annually. The herbicide concentrations in water and sediment decreased from the estuarine areas to the deep sea. The fugacity fraction values of atrazine exceeded 0.5 in the Yellow River estuary, which supported that the herbicides in sediment desorbed at the estuarine areas. The herbicide in the SPM showed high concentration in the outer sea and increased as a power function with decreasing SPM content. The increasing partition capacity indicated that the herbicides tended to sink into sediment, increasing the ecological risk posed by herbicides. The ecological risk of acetochlor deserves continuous attention.

Transport of mercury in a regulated high-sediment river and its input to marginal seas

Seafood is a major source of Hg exposure for humans, and rivers are an important source of Hg for adjacent seas. The Yellow River is an extremely high-sediment river that is heavily influenced by anthropogenic water facilities, especially sediment regulations, which might result in significant intra- and inter- year flux variations on pollutants. Overestimations of Hg flux were found in previous studies on high-sediment rivers, and a comprehensive study of Hg transport along such a river has not yet been attempted. In this study, we quantified the fates of Hg and methylmercury (MeHg) in the Yellow River and its main branches, as well as the impacts of anthropogenic activities, including deposition of contaminants in the reservoirs, diversion in the downstream, and sediment regulation, by sampling in 2017 and 2018,and comparing the two typical years with and without sediment regulation. The total Hg (THg) and MeHg concentrations in the Yellow River and its tributaries were high, and extremely high MeHg/THg ratios were found in the three sampled reservoirs. The river discharges substantial amounts of THg and MeHg into the Bohai Sea, and particulate Hg in the humid period accounts for a large proportion. A significant increase in Hg flux into the ocean was found in 2018 compared with that in 2017, which was primarily attributed to the altered hydrology caused by sediment regulation. Substantial amounts of Hg were impounded in the reservoirs, and only a small fraction was discharged downstream of the reservoir in the non-regulation years. Moreover, Hg fluxes to the coastal ocean were impacted by downstream water diversion in non-regulation years, whereas the contributions of diversions in regulation years were small. In conclusion, models showed that in recent 15 years, sediment regulation can enhance the Hg burden and risk to coastal ecosystem of Bohai Sea.