Influence of porewater sulfide on methylmercury production and partitioning in sulfate-impacted lake sediments.

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.

A comparison of results from a hydrologic transport model (HSPF) with distributions of sulfate and mercury in a mine-impacted watershed in northeastern Minnesota.

The St. Louis River watershed in northeast Minnesota hosts a major iron mining district that has operated continuously since the 1890s. Concern exists that chemical reduction of sulfate that is released from mines enhances the methylation of mercury in the watershed, leading to increased mercury concentrations in St. Louis River fish. This study tests this idea by simulating the behavior of chemical tracers using a hydrologic flow model (Hydrologic Simulation Program FORTRAN; HSPF) and comparing the results with measured chemistry from several key sites located both upstream and downstream from the mining region. It was found that peaks in measured methylmercury (MeHg), total mercury (THg), dissolved organic carbon (DOC), and dissolved iron (Fe) concentrations correspond to periods in time when modeled recharge was dominated by active groundwater throughout the watershed. This helps explain why the timing and size of the MeHg peaks was nearly the same at sites located just upstream and downstream from the mining region. Both the modeled percentages of mine water and the measured sulfate concentrations were low and computed transit times were short for sites downstream from the mining region at times when measured MeHg reached its peak. Taken together, the data and flow model imply that MeHg is released into groundwater that recharges the river through riparian sediments following periods of elevated summer rainfall. The measured sulfate concentrations at the upstream site reached minimum concentrations of approximately 1 mg/L just as MeHg reached its peak, suggesting that reduction of sulfate from non-point sources exerts an important influence on MeHg concentrations at this site. While mines are the dominant source of sulfate to sites downstream from them, it appears that the background sulfate which is present at only 1-6 mg/L, has the largest influence on MeHg concentrations. This is because point sourced sulfate is transported generally under oxidized conditions and is not flushed through riparian sediments in a gaining stream watershed system.

Methylmercury production in a chronically sulfate-impacted sub-boreal wetland.

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.

Methylmercury and dissolved organic carbon relationships in a wetland-rich watershed impacted by elevated sulfate from mining.

Methylmercury (MeHg), dissolved organic carbon (DOC), and sulfate (SO(4)(=)) relationships were investigated in the mining-influenced St. Louis River watershed in northeast Minnesota. Fewer wetlands and higher SO(4)(=) in the mining region lead to generally lower availability and solubility of DOC in mining streams compared to non-mining streams. MeHg concentrations, however, are similarly low in mining and non-mining streams during low flow periods, implying that the extra DOC found in non-mining streams carries little MeHg with it during these periods. High water levels elevated MeHg concentrations in both stream types owing to release from wetlands of DOC species that contain MeHg and remain relatively soluble in streams with elevated ionic strength. In-river methylation appeared to be a negligible component of the MeHg budget for the St. Louis River during this study as MeHg and DOC concentrations were intermediate to those observed in its mining-influenced and wetland-dominated tributaries.

The origins of public concern with taconite and human health: Reserve Mining and the asbestos case.

Asbestos first became an issue to Minnesota's iron industry when it was revealed that mineral fibers similar to those in Reserve Mining's tailings were being found in drinking water for several communities that used Lake Superior as their primary water source. This discovery turned what had largely been an environmental court battle into a case concerning public health. The courts listened to much conflicting and uncertain scientific testimony on the size and distribution of the mineral fibers and on the potential health effects imposed by them. In April 1974, the plant was ordered to shut down by a federal judge but the company quickly appealed the decision. The appeals court granted a stay and ultimately ruled that the plant's closure could not be justified based on the unknown health effects of the mineral fibers since the consequences of such an action would have immediate and severe social and economic impacts. The plant was allowed to continue operation, but ordered to abate emissions to air around the plant and to switch to a land-based tailings disposal system. Much of the scientific uncertainty and public concern over mineral fibers in Minnesota's taconite industry remain today.