Methylmercury degradation and exposure pathways in streams and wetlands impacted by historical mining

The authors acknowledge financial support from the National Science Foundation: EAR-1226741 (to M.B.S.) and EAR-1225630 (to J.D.B.).Monomethyl mercury (MMHg) and total mercury (THg) concentrations and Hg stable isotope ratios (δ202Hg and Δ199Hg) were measured in sediment and aquatic organisms from Cache Creek (California Coast Range) and Yolo Bypass (Sacramento Valley). Cache Creek sediment had a large range in THg (87 to 3870 ng/g) and δ202Hg (− 1.69 to − 0.20‰) reflecting the heterogeneity of Hg mining sources in sediment. The δ202Hg of Yolo Bypass wetland sediment suggests a mixture of high and low THg sediment sources. Relationships between %MMHg (the percent ratio of MMHg to THg) and Hg isotope values (δ202Hg and Δ199Hg) in fish and macroinvertebrates were used to identify and estimate the isotopic composition of MMHg. Deviation from linear relationships was found between %MMHg and Hg isotope values, which is indicative of the bioaccumulation of isotopically distinct pools of MMHg. The isotopic composition of pre-photodegraded MMHg (i.e., subtracting fractionation from photochemical reactions) was estimated and contrasting relationships were observed between the estimated δ202Hg of pre-photodegraded MMHg and sediment IHg. Cache Creek had mass dependent fractionation (MDF; δ202Hg) of at least − 0.4‰ whereas Yolo Bypass had MDF of + 0.2 to + 0.5‰. This result supports the hypothesis that Hg isotope fractionation between IHg and MMHg observed in rivers (− MDF) is unique compared to + MDF observed in non-flowing water environments such as wetlands, lakes, and the coastal ocean.PostprintPeer reviewe

Harmful Elements in Estuarine and Coastal Systems

Estuaries and coastal zones are dynamic transitional systems which provide many economic and ecological benefits to humans, but also are an ideal habitat for other organisms as well. These areas are becoming contaminated by various anthropogenic activities due to a quick economic growth and urbanization. This chapter explores the sources, chemical speciation, sediment accumulation and removal mechanisms of the harmful elements in estuarine and coastal seawaters. It also describes the effects of toxic elements on aquatic flora and fauna. Finally, the toxic element pollution of the Venice Lagoon, a transitional water body located in the northeastern part of Italy, is discussed as a case study, by presenting the procedures adopted to measure the extent of the pollution, the impacts on organisms and the restoration activities

Inorganic mercury (Hg

The species composition and the size structure of natural planktonic food webs may provide essential information to understand the fate of mercury and, in particular, the bioaccumulation pattern of Hg2+ in the water column of lake ecosystems. Heterotrophic and autotrophic picoplankton and phytoplankton are the most important entry points for Hg in aquatic ecosystems since they concentrate Hg2+ and MeHg from ambient water, making them available to planktonic consumers at higher trophic levels of lake food webs. In this investigation we studied the uptake of 197Hg2+ in natural plankton assemblages from four Andean lakes (Nahuel Huapi National Park, Patagonia, Argentina), comprised in the size fractions 0.2-2.7 μm (picoplankton), 0.2-20 μm (pico and nanoplankton) and 20-50 μm (microplankton) through experiments using Hg2+ labeled with 197Hg2+. The experimental results showed that the uptake of Hg2+ was highest in the smallest plankton fractions (0.2-2.7 μm and 0.2-20 μm) compared to the larger fraction comprising microplankton (20-50 um). This pattern was consistent in all lakes, reinforcing the idea that among pelagic organisms, heterotrophic and autotrophic bacteria with the contribution of nanoflagellates and dinoflagellates constitute the main entry point of Hg2+ to the pelagic food web. Moreover, a significant direct relationship was found between the Hg2+ uptake and surface index of the planktonic fractions (SIf). Thus, the smaller planktonic fractions which bore the higher SI were the major contributors to the Hg2+ passing from the abiotic to the biotic pelagic compartments of these Andean lakes

Inorganic mercury (Hg2+) uptake by different plankton fractions of Andean Patagonian lakes (Argentina)

The species composition and the size structure of natural planktonic food webs may provide essential information to understand the fate of mercury and, in particular, the bioaccumulation pattern of Hg2+ in the water column of lake ecosystems. Heterotrophic and autotrophic picoplankton and phytoplankton are the most important entry points for Hg in aquatic ecosystems since they concentrate Hg2+ and MeHg from ambient water, making them available to planktonic consumers at higher trophic levels of lake food webs. In this investigation we studied the uptake of 197Hg2+ in natural plankton assemblages from four Andean lakes (Nahuel Huapi National Park, Patagonia, Argentina), comprised in the size fractions 0.2-2.7 μm (picoplankton), 0.2-20 μm (pico and nanoplankton) and 20-50 μm (microplankton) through experiments using Hg2+ labeled with 197Hg2+. The experimental results showed that the uptake of Hg2+ was highest in the smallest plankton fractions (0.2-2.7 μm and 0.2-20 μm) compared to the larger fraction comprising microplankton (20-50 um). This pattern was consistent in all lakes, reinforcing the idea that among pelagic organisms, heterotrophic and autotrophic bacteria with the contribution of nanoflagellates and dinoflagellates constitute the main entry point of Hg2+ to the pelagic food web. Moreover, a significant direct relationship was found between the Hg2+ uptake and surface index of the planktonic fractions (SIf). Thus, the smaller planktonic fractions which bore the higher SI were the major contributors to the Hg2+ passing from the abiotic to the biotic pelagic compartments of these Andean lakes

Environmental Factors Affecting Mercury in Camp Far West Reservoir, California, 2001–03

This report documents water quality in Camp Far West Reservoir from October 2001 through August 2003. The reservoir, located at approximately 300 feet above sea level in the foothills of the northwestern Sierra Nevada, California, is a monomictic lake characterized by extreme drawdown in the late summer and fall. Thermal stratification in summer and fall is coupled with anoxic conditions in the hypolimnion. Water-quality sampling was done at approximately 3-month intervals on eight occasions at several stations in the reservoir, including a group of three stations along a flow path in the reservoir: an upstream station in the Bear River arm (principal tributary), a mid-reservoir station in the thalweg (pre-reservoir river channel), and a station in the deepest part of the reservoir, in the thalweg near Camp Far West Dam. Stations in other tributary arms of the reservoir included those in the Rock Creek arm of the reservoir, a relatively low-flow tributary, and the Dairy Farm arm, a small tributary that receives acidic, metal-rich drainage seasonally from the inactive Dairy Farm Mine, which produced copper, zinc, and gold from underground workings and a surface pit. Several water-quality constituents varied significantly by season at all sampling stations, including major cations and anions, total mercury (filtered and unfiltered samples), nitrogen (ammonia plus organic), and total phosphorus. A strong seasonal signal also was observed for the sulfur-isotope composition of aqueous sulfate from filtered water. Although there were some spatial differences in water quality, the seasonal variations were more profound. Concentrations of total mercury (filtered and unfiltered water) were highest during fall and winter; these concentrations decreased at most stations during spring and summer. Anoxic conditions developed in deep parts of the reservoir during summer and fall in association with thermal stratification. The highest concentrations of methylmercury in unfiltered water were observed in samples collected during summer from deep-water stations in the anoxic hypolimnion. In the shallow (less than 14 meters depth) oxic epilimnion, concentrations of methylmercury in unfiltered water were highest during the spring and lowest during the fall. The ratio of methylmercury to total mercury (MeHg/HgT) increased systematically from winter to spring to summer, largely in response to the progressive seasonal decrease in total mercury concentrations, but also to some extent because of increases in MeHg concentrations during summer. Water-quality data for Camp Far West Reservoir are used in conjunction with data from linked studies of sediment and biota to develop and refine a conceptual model for mercury methylation and bioaccumulation in the reservoir and the lower Bear River watershed. It is hypothesized that MeHg is produced by sulfate-reducing bacteria in the anoxic parts of the water column and in shallow bed sediment. Conditions were optimal for this process during late summer and fall. Previous work has indicated that Camp Far West Reservoir is a phosphate-limited system—molar ratios of inorganic nitrogen to inorganic phosphorus in filtered water were consistently greater than 16 (the Redfield ratio), sometimes by orders of magnitude. Therefore, concentrations of orthophosphate were expectedly very low or below detection at all stations during all seasons. It is further hypothesized that iron-reducing bacteria facilitate release of phosphorus from iron-rich sediments during summer and early fall, stimulating phytoplankton growth in the fall and winter, and that the MeHg produced in the hypolimnion and metalimnion is released to the entire water column in the late fall during reservoir destratification (vertical mixing). Mercury bioaccumulation factors (BAF) were computed using data from linked studies of biota spanning a range of trophic position: zooplankton, midge larvae, mayfly nymphs, crayfish, threadfin shad, bluegill, and spotted bass. Significant increases in total mercury in tissue with increasing organism size were observed for all three fish species and for crayfish. The BAF values were computed using the average methylmercury concentration (wet) in biota divided by the arithmetic mean concentration of methylmercury in filtered water (0.04 nanograms per liter). As expected, the BAF values increased systematically with increasing trophic position. Values of BAF were 190,000 for zooplankton; 470,000 to 930,000 for three taxa of invertebrates; 2.7 million for threadfin shad (whole body); 4.2 million for bluegill (fillet); and 10 million for spotted bass (fillet). The BAF values are high compared with those for biota in other reservoirs in northern California and elsewhere, indicating relatively efficient biomagnification of mercury in Camp Far West Reservoir