Effects of nitrate addition on water column methylmercury in Occoquan Reservoir, Virginia, USA

Photochemical demethylation of methylmercury (MeHg) in aquatic systems: A review of MeHg species, mechanisms, and influencing factors

Photodemethylation is the major pathway of methylmercury (MeHg) demethylation in surface water before uptake by the food chain, whose mechanisms and influence factors are still not completely understood. Here, we review the current knowledge on photodemethylation of MeHg and divide MeHg photolysis into three pathways: (1) direct photodemethylation, (2) free radical attack, and (3) intramolecular electron or energy transfer. In aquatic environments, dissolved organic matter is involved into all above pathways, and due to its complex compositions, properties and concentrations, DOM poses multiple functions during the PD of MeHg. DOM-MeHg complex (mainly by sulfur-containing molecules) might weaken the C-Hg bond and enhance PD through both direct and indirect pathways. In special, synergistic effects of both strong binding sites and chromophoric moieties in DOM might lead to intramolecular electron or energy transfer. Moreover, DOM might play a role of radical scavenger; while triplet state DOM, which is generated by chromophoric DOM under light, might become a source of free radicals. Apart from DOMs, transition metals, halides, NO3-, NO2-, and carbonates also act as radical initialaters or scavengers, and significantly pose effects on radical demethylation, which is generally mediated by hydroxyl radicals and singlet oxygen. Environmental factors such as pH, light wavelength, light intensity, dissolved oxygen, salinity, and suspended particles also affect the PD of MeHg. This study assessed previously published works on three major mechanisms, with the goal of providing general estimates for photodemethylation under various environment factors according to know effects, and highlighting the current uncertainties for future research directions.

Biogeochemical and hydrologic synergy control mercury fate in an arid land river-reservoir system

Reservoirs in arid landscapes provide critical water storage and hydroelectric power but influence the transport and biogeochemical cycling of mercury (Hg). Improved management of reservoirs to mitigate the supply and uptake of bioavailable methylmercury (MeHg) in aquatic food webs will benefit from a mechanistic understanding of inorganic divalent Hg (Hg(II)) and MeHg fate within and downstream of reservoirs. Here, we quantified Hg(II), MeHg, and other pertinent biogeochemical constituents in water (filtered and associated with particles) at high temporal resolution from 2016-2020. This was done (1) at inflow and outflow locations of three successive hydroelectric reservoirs (Snake River, Idaho, Oregon) and (2) vertically and longitudinally within the first reservoir (Brownlee Reservoir). Under spring high flow, upstream inputs of particulate Hg (Hg(II) and MeHg) and filter-passing Hg(II) to Brownlee Reservoir were governed by total suspended solids and dissolved organic matter, respectively. Under redox stratified conditions in summer, net MeHg formation in the meta- and hypolimnion of Brownlee reservoir yielded elevated filter-passing and particulate MeHg concentrations, the latter exceeding 500 ng g^-1 on particles. Simultaneously, the organic matter content of particulates increased longitudinally in the reservoir (from 9-29%) and temporally with stratified duration. In late summer and fall, destratification mobilized MeHg from the upgradient metalimnion and the downgradient hypolimnion of Brownlee Reservoir, respectively, resulting in downstream export of elevated filter-passing MeHg and organic-rich particles enriched in MeHg (up to 43% MeHg). We document coupled biogeochemical and hydrologic processes that yield in-reservoir MeHg accumulation and MeHg export in water and particles, which impacts MeHg uptake in aquatic food webs within and downstream of reservoirs.

Historic land use and sedimentation in two urban reservoirs, Occoquan Reservoir and Lake Manassas, Virginia, USA

Human population growth and subsequent land use intensification are closely linked to contemporary increases in sediment and associated contaminants fluxes to fluvial systems, lakes, reservoirs, and coastal zones worldwide. In most urban areas, reservoirs that are the main source of fresh water supply, if not effectively managed, suffer from water quality decline and loss of capacity associated with accelerated siltation. This study analyzes watershed soil losses and sediment accumulation rates in two reservoirs in the Occoquan river basin, a sub-watershed of the Chesapeake Bay in the suburbs of the greater Washington, DC area. Lake Manassas is located in the upper reaches of the basin, characterized by mixed land use and cover of mostly forest, residential areas, and agriculture, whereas Occoquan Reservoir is located in the more urbanized lower reach of the basin in the heavily populated suburban zone south of Washington, DC. Five sediment cores from each lake were used in ²¹⁰Pb-based sediment accumulation rates analysis, and GIS-based Revised Soil Loss Equation (RUSLE) model and a sediment delivery ratio (SDR) were used to evaluate basin soil losses and sediment fluxes to the fluvial systems. ²¹⁰Pb sediment accumulation rate estimates in Occoquan Reservoir range from 0.26 g cm^-2 year^-1 in the upper reaches to 0.37 g cm^-2 year^-1 in the lower reaches. Lake Manassas also had comparable accumulation values ranging from 0.22 to 0.40 g cm^-2 year^-1. RUSLE/SDR estimated watershed sediment fluxes were 0.26 Mg ha^-1 year^-1 (Mg-mega gram) in the upper watershed, which is significantly higher than 0.07 Mg ha^-1 year^-1 estimates for the lower reaches of the watershed. The variability in the reservoirs' sediment accumulation rates and basin soil losses reflects the variability of land use and cover, basin slopes, and erosion mitigation efforts within the watershed. The lower reaches, though more urbanized, have well-developed storm drain systems limiting run-off related soil losses. The well-managed riparian zones surrounding both reservoirs also limit sediment fluxes, hence the relatively low sediment accumulation rates. Although surficial sediment sources seem to be well managed, some of these efforts might be associated with the uptick in intrinsic sediment sources, leading to localized high sediment accumulation in the mouth of tributaries draining the high-intensity urban areas of the basin.

Nitrates in the environment: A critical review of their distribution, sensing techniques, ecological effects and remediation

Nitrate pollution is eminent in almost all the developing nations as a result of increased natural activities apart from anthropogenic pollution. The release of nitrates in more than critical quantities into the water bodies causes accretion impacts on living creatures, environmental receptors, and human vigour by accumulation through the food chain. Nitrates have recently acquired researchers' huge attention and extend their roots in environmental contamination of surface and groundwater systems. The presence of nitrate in high concentrations in surface and groundwater triggers several health problems, for instance, methemoglobinemia, diabetes, eruption of infectious disorders, harmfully influence aquatic organisms. Sensing nitrate is an alternate option for monitoring the distribution of nitrate in different water bodies. Here we review electrochemical, spectroscopic, and electrical modes of nitrate sensing. It is concluded that, among the various sensors discussed in this review, FET sensors are the most desirable choice. Their sensitivity, ease of use and scope for miniaturisation are exceptional. Advanced functional materials need to be designed to satiate the growing need for environmental monitoring. Different sources of nitrate contamination in ground and surface water can be estimated using different techniques such as nitrate isotopic composition, co contaminants, water tracers, and other specialized techniques. This review intends to explore the research work on remediation of nitrate from wastewater and soil using different processes such as reverse osmosis, chemical denitrification, biological denitrification, ion exchange, electrodialysis, and adsorption. Denitrification proves as a promising alternative over previously reported techniques in terms of their nitrate removal because of its high cost-effectiveness.

Mercury pollution in the coastal Urmia aquifer in northwestern Iran: potential sources, mobility, and toxicity

The concentration of total dissolved mercury (Hg_T) in surface and groundwater resources in the coastal parts of Urmia aquifer (NW of Iran) was investigated to identify the possible sources and sinks of mercury and the geochemical mechanisms controlling its mobilization. The distribution of water samples on the Piper diagram demonstrates that most samples have the Ca-Mg-HCO₃ facies. From 62 water samples collected in this area, one sample contained Hg_T concentrations exceeding the maximum contaminant level recommended by the WHO (6 μg/L). The principal component analysis (PCA) produced five principal components. The positive moderate correlation of Hg_T with EC, Cl, K, Mg, and Na indicated that the weathering of geological formations was one of the main sources of mercury in groundwater samples. Position of water samples in Eh-pH regions where microorganisms involved in mercury methylation and mineralization were potentially active demonstrated that the aquifer had undergone sulfate reduction and had reached the final stage of the terminal electron accepting process (TEAP) sequence in the methane production processes which are limited to only 37% of the water samples that have anaerobic conditions. Some Hg-bearing species are in nonequilibrium geochemical conditions. The supersaturation of water samples with magnetite and goethite indicated that these Fe-bearing minerals could act as the strong reducing agents for the reduction of Hg(II) to Hg(0).

Formation and transformation of manganese(III) intermediates in the photochemical generation of manganese(IV) oxide minerals

As important natural oxidants and adsorbents, manganese (Mn) oxide minerals affect the speciation, bioavailability and fate of pollutants and nutrient elements. It was found that birnessite-type Mn(IV) oxide minerals can be formed in the presence of NO₃^- and solar irradiation. However, the photochemical formation and transformation processes from Mn²⁺ to Mn(IV) oxide minerals remain unclear. In this work, the Mn(IV) oxide minerals were confirmed to be photochemically formed mainly due to the disproportionation of Mn(III) intermediates generated from the oxidation of Mn²⁺ in the presence of NO₃^- under UV light irradiation. The oxidation rate of Mn²⁺ to Mn(IV) oxide minerals decreased with increasing initial Mn²⁺ concentration due to the lower disproportionation rate. The increase in NO₃^- concentration, pH and temperature promoted Mn²⁺ photochemical oxidation. The photochemical formation rate of Mn(IV) oxide minerals increased with increasing ligand concentrations at low ligand concentrations. Ligands affected the formation of Mn(IV) oxide minerals by promoting the formation and reducing the reactivity of Mn(III) intermediates. Overall, this work reveals the important role of Mn(III) intermediates in the formation of natural Mn oxide minerals.

The remediation of urban freshwater sediment by humic-reducing activated sludge

Organic pollution of urban rivers caused by stormwater discharge is a global problem. Traditional bioremediation of organic matters (OM) by aerobes could be restrained in anaerobic environments, which usually occurr in polluted river sediments. In this study, an anaerobic remediation technology has been developed to enhance the in-situ removal of organic matters in river sediments, humic-reducing sludge (HRS) was adapted from traditional activated sludge; it exhibited a strong humic-reducing ability. Nitrate and biostimulants were used to stimulate HRS. The change of microbial community between AQDS-adapted and non-AQDS-adapted was analyzed, and the effect of HRS augmentation and Nitrate/biostimulant addition on TOM removal were discussed from the perspective of light and heavy fraction organic matters (LFOM and HFOM). The results have indicated that, after adaptation, HRS had increased the bacterial population of Anaerolineales and Desulfuromonadales, which was related to the carbon metabolism and electron-transfer ability. On the other hand, the adaptation decreased the population of bacteria related to the sulfur/sulfate circulation. This characteristic of the HRS was potentially benificial to reducing the occurrence of black-odor phenomenon. Also, the removal efficiency of TOM in sediment was significantly improved by using HRS because HRS could facilitate the removal of HFOM. Fourier Transform Infrared Spectroscopy (FTIR) analysis indicated that the advantage of decomposing HFOM using HRS resulted from the fact that the HFOM contained redox mediators to facilitate humic-reducing respiration. In addition, nitrate appeared to be crucial for the enhancement of HRS in sediments. These findings have allowed for the development of a technology for in-situ anaerobic remediation of urban river sediments. They could also help to understand humic-reducing mechanism in the sediment during anaerobic bioremediation.

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.

Manganese(iv) oxide amendments reduce methylmercury concentrations in sediment porewater

Manganese(iv) oxide (pyrolusite, birnessite) mineral amendments can reduce dissolved MeHg concentrations in sediment theoretically by inhibiting microbial sulfate reduction, which is a major methylation pathway in sediments. Anaerobic sediment slurry microcosms in which Hg methylation was stimulated by addition of labile organic carbon (acetate) and HgCl2 showed that manganese(iv) oxide reduced the percent MeHg in slurry porewater (filtered), by 1-2 orders of magnitude relative to controls. Sediment-water mesocosms with pyrolusite or birnessite either directly mixed into the top 5 cm or applied in a thin (5 cm) sand layer over sediment showed reductions in percent MeHg in porewater of 66-69% for pyrolusite and 81-89% for birnessite amendment. A thin sand layer alone resulted in 65% reduction. CO2 respirometry experiments showed that the amendments stimulated microbial activity. Microbial community census by PCR and DNA sequencing indicated that the addition of Mn(iv) oxides did not significantly alter the indigenous sediment microbial community structure, although a small increase in abundance of iron and manganese reducers was observed after a 2 week incubation period. The mechanism of decreasing MeHg relative to Hg concentrations in porewater likely involved an increase in the importance of Mn(iv) reduction (relative to sulfate reduction) in heterotrophic microbial metabolism in the sediments amended with Mn(iv) oxides. Manganese reduction was confirmed as the predominant biogeochemical redox process by microelectrode voltammetry profiling of the sediment microcosms, although adsorption to Mn oxide surfaces, enhanced MeHg demethylation, and abiotic reduction of Mn(iv) also may have been involved in reducing percent MeHg and suppressing net MeHg production. These results represent a novel approach for mitigating MeHg impacts from sediments with potential applicability to a range of aquatic settings including intertidal zones, tidal marshes, seasonal wetlands, reservoirs, and lakes.

Characterization of manganese oxide amendments for in situ remediation of mercury-contaminated sediments

Addition of Mn(iv)-oxide phases pyrolusite or birnessite was investigated as a remedial amendment for Hg-contaminated sediments. Because inorganic Hg methylation is a byproduct of bacterial sulfate reduction, reaction of Mn(iv) oxide with pore water should poise sediment oxidation potential at a level higher than favorable for Hg methylation. Changes in Mn(iv)-oxide mineralogy and oxidation state over time were investigated in sediment tank mesocosm experiments in which Mn(iv)-oxide amendment was either mixed into Hg-contaminated sediment or applied as a thin-layer sand cap on top of sediment. Mesocosms were sampled between 4 and 15 months of operation and solid phases were characterized by X-ray absorption spectroscopy (XAS). For pyrolusite-amended sediments, Mn(iv) oxide was altered to a mixture of Mn(iii)-oxyhydroxide and Mn, Fe(iii, ii)-oxide phases, with a progressive increase in the Mn(ii)-carbonate fraction over time as mesocosm sediments became more reduced. For birnessite-amended sediments, both Mn(iii) oxyhydroxide and Mn(ii) carbonate were identified at 4 months, indicating a faster rate of Mn reduction compared to pyrolusite. After 15 months of reaction, birnessite was converted completely to Mn(ii) carbonate, whereas residual Mn, Fe(iii, ii)-oxide phases were still present in addition to Mn(ii) carbonate in the pyrolusite mesocosm. In the thin-layer sand cap mesocosms, no changes in either pyrolusite or birnessite XAS spectra were observed after 10 months of reaction. Equilibrium phase relationships support the interpretation of mineral redox buffering by mixed-valent (Mn, Fe)(iii, ii)-oxide phases. Results suggest that amendment longevity for redox buffering can be controlled by adjusting the mass and type of Mn(iv) oxide applied, mineral crystallinity, surface area, and particle size. For a given site, amendment capping versus mixing with sediment should be evaluated to determine the optimum treatment approach, which may vary depending on application constraints, rate of Mn(iv) oxide transformation, and frequency of reapplication to maintain desired oxidation state and pH.

Effect of oxygen, nitrate and aluminum addition on methylmercury efflux from mine-impacted reservoir sediment

Extensive contamination of aquatic ecosystems with mercury (Hg) has led to a growing interest in developing in situ management strategies to repress Hg bioaccumulation in aquatic biota in reservoirs. This study used experimental chamber incubations to assess the impact of three potential treatments, oxygen addition, nitrate addition and aluminum addition, to reduce the flux of toxic methylmercury (MeHg) from profundal reservoir sediment. The study sites, Almaden Lake and Guadalupe Reservoir, are located downstream of the historic New Almaden mining district in Santa Clara Valley, California, USA. In the first experiment (experiment 1), replicate chambers from both sites were incubated sequentially under aerobic and anaerobic conditions. At both sites, mean anaerobic fluxes of MeHg were higher than aerobic fluxes (Almaden: 11.0 vs. -2.3 ng/m²·d; Guadalupe: 22.3 vs 5.5 ng/m²·d), and anaerobic MeHg fluxes correlated with rates of sediment sulfate uptake, highlighting the linkage between MeHg production and microbial sulfate reduction. Under aerobic conditions, sediment from Guadalupe Reservoir released Hg(II), iron and sulfate, suggesting the oxidative dissolution of Hg-bearing sulfide minerals. A follow-up study at Almaden Lake (experiment 2) found that mean MeHg fluxes under aerobic conditions (5 ng/m²·d) and anoxic (nitrate-rich) conditions (1.7 ng/m²·d) were lower than anaerobic conditions (174 ng/m²·d), but aluminum addition had little effect (105 ng/m²·d) on MeHg flux. In both anaerobic and aluminum treated chambers, MeHg flux turned negative during the second half of the incubation, suggesting that highly reduced, sulfidic conditions lowered net methylation, possibly by enhancing demethylation or repressing Hg(II) bioavailability for methylation.