GEOCHEMISTRY, MINERALOGY, AND BIOACCESSIBILITY OF ARSENIC AND MERCURY IN MINE WASTE FROM SULPHUR BANK MERCURY MINE, LAKE COUNTY, CALIFORNIA

The Sulphur Bank Mercury Mine (SBMM), in Lake County, California, was mined intermittently from 1865 to 1957. Underground and surface-mining activity produced ~2.5 million cubic yards of waste material. In support of remediation efforts by the U.S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS) characterized the chemistry and mineralogy of mine waste piles at SBMM, and determined the bioaccessibility of arsenic (As) and mercury (Hg) using two in vitro leach tests. Four waste groups were considered: Ore piles; Tailings and south waste pile; Other waste rock piles; and the Waste Rock Dam (WRD) area, between Herman Impoundment and Clear Lake. An industrial plant that manufactured cinderblocks using tailings material and scoria from a nearby quarry was located on the south waste pile. Significant differences between the four waste groups were found using geochemistry and bulk mineralogy by x-ray diffraction. The minerals alunite, amorphous silica, buddingtonite (ammonium feldspar), goethite, hematite, jarosite, kaolinite, mica, plagioclase, pyroxene, and quartz were useful in distinguishing waste groups, as were major elements (Ca, Fe, K, Mg, P, and S), trace elements (As, Ba, Co, Cs, Ge, Li, Mn, Ni, Rb, Sn, Tl, W, and Zn), and all rare earth elements except La. Ore and Other waste rock pile material had mineralogy indicative of shallow mineralization (steam-heated zone), whereas tailings and WRD area material were indicative of deeper mineralization with less intense hydrothermal alteration. As and Hg bioaccessibility were evaluated in 15 mine waste composite samples using EPA method 1340 and the California Bioaccessibility method (CAB). Bioaccessibility results are reported as RBA (relative bioavailability) by dividing in vitro leachate concentration (solid basis) by total concentration. Total As ranged from 6 to 1,250 mg/kg in the 15 samples. Using EPA method 1340, only one of the 15 samples had detectable As, with 7% RBA As. Nine of the 15 samples had detectable As using the CAB test, with RBA As ranging from 5% to 33% (median 11.5%). Total Hg concentration in the 15 samples ranged from 17 to 15,100 mg/kg. Using EPA method 1340, RBA Hg ranged from 0.02% to 6.0% (median 0.24%); using the CAB method, RBA Hg ranged from 0.03% to 2.1% (median of 0.13%). These mine waste characterization data will be used by EPA to inform its remediation efforts at SBMM.