Decreasing aqueous mercury concentrations to meet the water quality criterion in fish: examining the water-fish relationship in two point-source contaminated streams

Integrated watershed process model for evaluating mercury sources, transport, and future remediation scenarios in an industrially contaminated site

We used the Soil Water Assessment Tool (SWAT) as a framework to develop an empirical Hg flux model for Upper East Fork Poplar Creek (UEFPC), a Hg-contaminated watershed in Oak Ridge, Tennessee. By integrating long-term Hg monitoring data with simulated flow and suspended solid loads in a site-specific empirical Hg transport model, we (1) quantified the spatial, temporal, and flow regime controls on daily Hg flux (adjusted R² = 0.82) and (2) made predictions about Hg flux under future climate, land use, and management scenarios. We found that 62.79% of the average daily Hg flux in the watershed is currently driven by base flow, whereas variability in Hg flux is driven by storm and extreme flow. We estimate an average annual Hg flux of 28.82 g day^-1 leaving the watershed under baseline precipitation, with an estimated 43.73% reduction in daily Hg flux under drought conditions and a 296% increase in daily Hg flux in extreme precipitation scenarios. We estimated that a new mercury treatment facility would result in a 24.7% reduction in Hg flux under baseline conditions and a 33.4% reduction under extreme precipitation scenarios. The study demonstrated the merit of this approach, which can be replicated for sites where information on flow, suspended solids, and Hg concentrations is available.

Mitigating the impact of mercury contaminants in fish and other seafood-A review

Mercury (Hg) is a well-known toxicant which enters the marine environment by both natural and anthropogenic sources. Consumption of fish and other seafood that contain methylmercury (MeHg) is a leading source of Hg exposure in humans. Considerable efforts have been made to mitigate the Hg presence and reduce its risks to humans. In this review the acknowledged methods of mitigation are summarized such as regulation and maximum allowable limits, and culinary treatments. In addition, selected industrial level trials are reviewed, and studies on Hg intoxication and the protective effects of the essential trace element, selenium (Se), are discussed. In view of the available literature, Hg reduction in fish and other seafood on a large industrial scale still is largely unsuccessful. Hence, more research and further attempts are necessary in order to better mitigate the Hg problem in fish and other seafood products.

Mercury bioaccumulation in stream fish from an agriculturally-dominated watershed

Bioaccumulation of mercury in freshwater fish is a complex process driven by environmental and biological factors. In this study, we assessed mercury in fish from four tributaries to the Red Deer River, Alberta, Canada, which are characterized by high surface water mercury concentrations. We used carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes to examine relationships between fish total mercury (THg) concentrations, food web dynamics and patterns in unfiltered THg and methylmercury (MeHg) concentrations. We found that THg concentrations exceeded the tissue residue quality guideline for the protection of wildlife consumers in 99.7% of fish sampled. However, while the surface water THg concentration was highest in Michichi Creek and the MeHg concentration was consistent across streams, patterns of fish THg concentrations varied depending on species. Furthermore, body size and trophic level were only correlated with THg concentrations in white sucker (Catostomus commersoni) and Prussian carp (Carrasius gibelio). The results of this study suggest that mercury poses a risk to the health of piscivorous wildlife in the Red Deer River watershed. Despite high THg concentrations in these streams, mercury bioaccumulation is not driven by environmental inorganic mercury concentrations. Additionally, commonly cited factors associated with mercury concentrations in fish, such as body size and trophic level, may not strongly influence bioaccumulation in these stream ecosystems.

The assessment and remediation of mercury contaminated sites: A review of current approaches

Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production-such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.

Methylmercury sorption onto engineered materials

Four commercially available sorbents (BioChar (BC), ThiolSAMMS^® (TS), SediMite (SM), and Organoclay™ PM-199 (OC-199)) were tested for their ability to sorb methylmercury (MeHg) and MeHg complexed with dissolved organic matter (DOM). Testing sorption behavior with DOM is more representative of the environmental conditions and mercury speciation expected during in-situ remediation efforts. Isotherms were fit using a robust, iterative re-weighting scheme. This fitting approach improves upon the traditionally used indirect sorption method by removing the dependence between aqueous and solid phase concentrations in isotherm fitting. Developed isotherms show that without DOM, BC, TS, and SM adsorbed similar amounts of MeHg while OC-199 sorbed substantially less MeHg. Below an equilibrium concentration of 5.6 ng L^-1 BC was the best performing sorbent, between 5.6 and 20.9 ng L^-1 SM sorbed the most MeHg, and above an equilibrium concentration of 20.9 ng L^-1 TS outperformed the other sorbents. BC and OC-199 showed indication of MeHg sorption saturation over the tested concentration range of 3.5-680 ng L^-1. With DOM, SM outperformed the other sorbents at equilibrium concentrations less than 0.98 ng L^-1 and TS was the superior MeHg:DOM sorbent at higher concentrations. MeHg:DOM sorption was controlled by DOM-sorbent interactions. DOM decreased MeHg sorption onto BC and SM whereas TS exhibited similar sorption with and without DOM. OC-199 had slightly higher MeHg uptake with DOM. East Fork Poplar Creek (EFPC), an industrially Hg contaminated site, was used as a case study example to build a relationship between aqueous and fish MeHg concentrations and subsequently compare the cost of sorbent materials required to meet regulatory objectives. For this case study, SM provided the most cost-effective sorbent option for in-situ remediation efforts to reduce aqueous MeHg concentrations.

Divalent Mercury in Dissolved Organic Matter Is Bioavailable to Fish and Accumulates as Dithiolate and Tetrathiolate Complexes

The freshwater cyprinid Tanichthys albonubes was used to assess the bioavailability of divalent mercury (Hg(II)) complexed in dissolved organic matter (DOM) to fish. The fish acquired 0.3 to 2.2 μg Hg/g dry weight after 8 weeks in aquaria containing DOM from a Carex peat with complexed mercury at initial concentrations of 14 nM to 724 nM. Changes in the relative proportions of dithiolate Hg(SR)₂ and nanoparticulate β-HgS in the DOM, as quantified by high energy-resolution XANES (HR-XANES) spectroscopy, indicate that Hg(SR)₂ complexes either produced by microbially induced dissolution of nanoparticulate β-HgS in the DOM or present in the original DOM were the forms of mercury that entered the fish. In the fish with 2.2 μg Hg/g, 84 ± 8% of Hg(II) was bonded to two axial thiolate ligands and one or two equatorial N/O electron donors (Hg[(SR)₂+(N/O)_1-2] coordination), and 16% had a Hg(SR)₄ coordination, as determined by HR-XANES. For comparison, fish exposed to Hg²⁺ from 40 nM HgCl₂ contained 10.4 μg Hg/g in the forms of dithiolate (20 ± 10%) and tetrathiolate (23 ± 10%) complexes, and also Hg _xS _y clusters (57 ± 15%) having a β-HgS-type local structure and a dimension that exceeded the size of metallothionein clusters. There was no evidence of methylmercury in the fish or DOM within the 10% uncertainty of the HR-XANES. Together, the results indicate that inorganic Hg(II) bound to DOM is a source of mercury to biota with dithiolate Hg(SR)₂ complexes as the immediate species bioavailable to fish, and that these complexes transform in response to cellular processes.

Aquatic monitoring programs conducted during environmental impact assessments in Canada: preliminary assessment before and after weakened environmental regulation

Aquatic monitoring programs are imperative for the functioning of the environmental impact assessment (EIA) process and a cornerstone for industrial compliance in Canada. However, in 2012, several leading pieces of federal environmental legislation (e.g., Canadian Environmental Assessment Act c.19, s. 52, 2012) were drastically altered, effectively weakening levels of environmental protection for aquatic ecosystems during project developments. This paper assesses the impact of CEAA 2012 on aquatic monitoring programs (and subsequent monitoring data reporting) across Canada for ten projects (five completed pre-CEAA 2012 and five completed post-CEAA 2012). Projects included four energy and six mining projects and were selected based on the following criteria: (i) representative of Canada's resource economy; (ii) project information was publicly available; and (iii) strong public interest. Projects pre- and post-CEAA 2012 exhibited few apparent differences before and after environmental regulatory changes. However, wide discrepancies exist in numbers and types of parameters reported, along with a lack of consistency in reporting. Projects pre-CEAA 2012 provided more follow-up monitoring commitments. Although qualitative differences remain inconclusive, this paper highlights requirements for further assessment of aquatic monitoring and follow-up programs in Canada. Recommendations for the government to consider during reviews of the federal environmental assessment processes include (i) improved transparency on the Canadian Environmental Assessment Agency website ( https://www.ceaa-acee.gc.ca/ ); (ii) creation of a legally binding standardized aquatic monitoring program framework to ensure that all Canadian aquatic ecosystems are monitored with equal rigour; and (iii) commitments and justification related to frequency of aquatic monitoring of water quality.

The effects of a stannous chloride-based water treatment system in a mercury contaminated stream

We assessed the impacts of an innovative Hg water treatment system on a small, industrially-contaminated stream in the southeastern United States. The treatment system, installed in 2007, removes Hg from wastewater using tin (Sn) (II) chloride followed by air stripping. Mercury concentrations in the receiving stream, Tims Branch, decreased from >100 to ∼10 ng/L in the four years following treatment, and Hg body burdens in redfin pickerel (Esox americanus) decreased by 70% at the most contaminated site. Tin concentrations in water and fish increased significantly in the tributary leading to Tims Branch, but concentrations remain below levels of concern for human health or ecological risks. While other studies have shown that Sn may be environmentally methylated and methyltin can transfer its methyl group to Hg, results from our field studies and sediment incubation experiments suggest that the added Sn to the Tims Branch watershed is not contributing to methylmercury (MeHg) production or bioaccumulation in this system. The stannous chloride treatment system installed at Tims Branch was effective at removing Hg inputs and reducing Hg bioaccumulation in the stream, but future studies are needed to assess longer term impacts of Sn on the environment.

Health risk and significance of mercury in the environment

Mercury (Hg) has long been recognised as a global pollutant, because it can remain in the atmosphere for more than 1 year. The mercury that enters the environment is generally acknowledged to have two sources: natural and anthropogenic. Hg takes three major forms in the environment, namely methyl-Hg (MeHg), Hg(0) and Hg(2+). All three forms of Hg adversely affect the natural environment and pose a risk to human health. In particular, they may damage the human central nervous system, leading to cardiovascular, respiratory and other diseases. MeHg is bioavailable and can be bioaccumulated within food webs. Therefore, several methods of eliminating Hg from the soil and the aquatic system have been proposed. The focus of this article is on phytoremediation, as this technique provides a low-cost and environmentally friendly alternative to traditional methods.

Identification of multiple mercury sources to stream sediments near Oak Ridge, TN, USA

Sediments were analyzed for total Hg concentration (THg) and isotopic composition from streams and rivers in the vicinity of the Y-12 National Security Complex (Y12) in Oak Ridge, TN (USA). In the stream directly draining Y12, where industrial releases of mercury (Hg) have been documented, high THg (3.26 to 60.1 μg/g) sediments had a distinct Hg isotopic composition (δ(202)Hg of 0.02 ± 0.15‰ and Δ(199)Hg of -0.07 ± 0.03‰; mean ± 1SD, n = 12) compared to sediments from relatively uncontaminated streams in the region (δ(202)Hg = -1.40 ± 0.06‰ and Δ(199)Hg of -0.26 ± 0.03‰; mean ± 1SD, n = 6). Additionally, several streams that are nearby but do not drain Y12 had sediments with intermediate THg (0.06 to 0.21 μg/g) and anomalous δ(202)Hg (as low as -5.07‰). We suggest that the low δ(202)Hg values in these sediments provide evidence for the contribution of an additional Hg source to sediments, possibly derived from atmospheric deposition. In sediments directly downstream of Y12 this third Hg source is not discernible, and the Hg isotopic composition can be largely explained by the mixing of low THg sediments with high THg sediments contaminated by Y12 discharges.

The presence of EU priority substances mercury, hexachlorobenzene, hexachlorobutadiene and PBDEs in wild fish from four English rivers

Since 2007 about 200 to 300 fish per year--generally roach (Rutilus rutilus) also a few bleak (Alburnus alburnus) and eels (Anguilla anguilla)--have been collected from a number of English river sites and stored at -80°C to build up a Fish Tissue Archive as a resource for the monitoring of pollutants. Some of the fish from the Fish Tissue Archive from the years 2007-2011 were analyzed for substances in current and proposed European legislation regarding environmental quality standards (EQS) in biota. It was found that mercury exceeded the EU EQS of 20 μg/kg in 79% of samples with an average and median of 31 and 29 μg/kg. The legacy fungicide hexachlorobenzene (HCB) was below the EQS of 10 μg/kg in all fish analyzed, with a maximum of 6 μg/kg in some eels. The legacy solvent hexachlorobutadiene (HCBD) was below the EQS of 55 μg/kg, being <0.2 μg/kg in all samples where it was measured. The sums of six polybrominated diphenyl ethers (PBDEs) were several orders of magnitude higher than the new proposed 0.0085 μg/kg biota EQS. This study showed that the regular collection and analysis of whole body homogenate samples of relatively small native pelagic fish is suitable for the monitoring of contaminants capable of bioaccumulation. With regard to current or proposed EQS for EU countries, mercury and potentially PBDE in fish are of some concern in these English rivers.

Sources of mercury in a contaminated stream--implications for the timescale of recovery

Mercury contamination in East Fork Poplar Creek in Tennessee arises from dissolved mercury exiting a headwater industrial complex and residual mercury in the streambed and soil throughout the watershed downstream. The headwater inputs generate chronic base flow concentrations of total mercury of about 1,000 ng/L, but most of the annual export of mercury from the system appears to originate farther downstream. Effective targeting of remedial efforts requires determining how long downstream sources might continue to contaminate the system following elimination of the headwater mercury inputs. The authors calculations suggest that (1) contaminated soils and sediments account for >80% of the annual mercury export from the entire watershed, with most export occurring during wet weather events; (2) bank erosion and resuspension of streambed particulates are the major mercury sources maintaining high annual mercury export rates; and (3) the inventory of particle-associated mercury in the streambed was not large enough to sustain the estimated export rates for more than a few years. The authors findings imply that to prevent waterborne mercury contamination in this system from continuing for decades, remedial actions will have to control the headwater mercury source that sustains day-to-day base flow mercury concentrations and the riparian stream-bank sources that generate most of the mercury export from the system.