Pathways of Acid Mine Drainage to Clear Lake: Implications for Mercury Cycling

Abstract

Abstract. Pore fluids from Clear Lake sediments collected near the abandoned Sulphur Bank Mercury Mine have low pH (locally ,4) and elevated sulfate (!197 mmol/L), aluminum (!52 mmol/L), and iron (!28 mmol/L) contents derived from oxidation of sulfide minerals at the mine site. Acid mine drainage (AMD) is entering Clear Lake by advective subsurface flow nearest the mine and by diffusion at greater distances. Oxygen and hydrogen isotope ratios, combined with pore fluid compositions, constrain the sources and pathways of contaminated fluids. Sediment cores taken nearest the mine have the highest concentrations of dissolved sulfate, aluminum, and iron, which are contributed by direct subsurface flow of AMD from sulfide-bearing waste rock. Sediment cores as far as 100 m west of the Clear Lake shoreline show the presence of AMD that originated in the acidic lake that occupies the abandoned Herman Pit at the mine site. High sulfate content in the AMD has the potential to promote the activity of sulfate-reducing bacteria in the organic-rich lake sediments, which leads to methylation of Hg þ2 , making it both more toxic and bioavailable. Quantitative depletion of pore water sulfate at depth and sulfur isotope values of diagenetic pyrite near 0ø indicate that sulfate availability limits the extent of sulfate reduction in the lake sediments away from the mine. Profiles of pore water sulfate in the sediments near the mine show that excess sulfate is available to support the activity of sulfate-reducing bacteria near the mine site. Enriched isotope values of dissolved sulfate (as high as 17.1ø) and highly depleted isotope values for diagenetic pyrite (as low as À22.6ø) indicate active bacterial sulfate reduction in the AMDcontaminated sediments. Sulfate-and iron-rich acid mine drainage entering Clear Lake by shallow subsurface flow likely needs to be controlled in order to lower the environmental impacts of Hg in the Clear Lake ecosystem.

Anthropogenic Stressors and Changes in the Clear Lake Ecosystem as Recorded in Sediment Cores

“…Observations by divers and sonar confirm there is an ill-defined sediment-water interface in Clear Lake due to the flocculent nature of the organic-rich sediments, a zone identified as the ''nepheloid layer'' by Chamberlin et al (1990). This layer is particularly evident near the mine where the bottom sediments are highly heterogeneous due to disposal of mine wastes ranging from boulder-sized material to finely powdered tailings (see bathymetry contours near the mine in Shipp and Zierenberg [2008]). These effects make detailed correlation of the stratigraphy of the long cores above 1927 difficult, but the similarity of the major geochemical trends is apparent.…”

“…Approximately 25% of the surface of the regraded waste piles contains sulfide minerals in sufficient concentrations to prevent the growth of annual vegetation (Suchanek et al 2008b, d, in press; P. J. Richerson and T. H. Suchanek, personal observations). Mercury and mine-related contaminants have entered the lake by four main routes: atmospheric deposition, erosion of waste rock piles, deliberate dumping/bulldozing of tailings and waste rock into the lake (Suchanek et al 2008d) and subsurface acid rock drainage flow (Shipp and Zierenberg 2008). Even after mining ceased, sheetwash erosion and mass wastage from wave action undercut the shoreline waste rock piles until a U.S. EPA remediation project in 1992.…”

“…Even after mining ceased, sheetwash erosion and mass wastage from wave action undercut the shoreline waste rock piles until a U.S. EPA remediation project in 1992. The mine currently discharges acid rock drainage into the lake, which we believe is the most ecologically significant source of ongoing Hg loading (see further discussion in Shipp andSuchanek et al 2008b, d ).…”

Use of Tracers to Quantify Subsurface Flow Through a Mining Pit

“…The water chemistry of the pit is highly complex (Shipp and Zierenberg 2008). F. Goff and D. Bergfeld (unpublished manuscript) evaluated the chemistry at two locations at opposite ends of Herman Pit and of Clear Lake from two locations adjacent to the mine at the north and south ends of the rip-rapped shoreline.…”

“…When mining operations (and concurrent dewatering) ceased in the 1950s, the pit filled with water. The source of this water is overland flow from the surrounding watershed during storms, meteoric groundwater flow throughout the year, surface seeps, subterranean streams, and geothermal springs at the bottom of the pit (Shipp and Zierenberg 2008). The subsurface characteristics between Herman Pit and Clear Lake are extremely heterogeneous.…”