Arsenic species in weathering mine tailings and biogenic solids at the Lava Cap Mine Superfund Site, Nevada City, CA

摘要

Background: A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived fromhistoric mining operations is a major challenge to redevelopment of California’s famed “Mother Lode” region.Arsenic, a known carcinogen, occurs in multiple solid forms that vary in bioaccessibility. X-ray absorption fine-structure spectroscopy (XAFS) was used to identify and quantify the forms of As in mine wastes and biogenicsolids at the Lava Cap Mine Superfund (LCMS) site, a historic “Mother Lode” gold mine. Principal componentanalysis (PCA) was used to assess variance within water chemistry, solids chemistry, and XAFS spectral datasets.Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenicspeciation in XAFS spectra of tailings and biogenic solids.Results: The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area. Iron, dissolved oxygen, alkalinity, specific conductivity, and As were themajor variables in the water chemistry PCA. Arsenic was, on average, 14 times more concentrated in biologically-produced iron (hydr)oxide than in mine tailings. Phosphorous, manganese, calcium, aluminum, and As were themajor variables in the solids chemistry PCA. Linear combination fits to XAFS spectra indicate that arsenopyrite(FeAsS), the dominant form of As in ore material, remains abundant (average: 65%) in minimally-weathered oresamples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying andwetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weatheringwere identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3·3H2O). Existence ofthe former species is not in question, but the presence of the latter species was not confirmed by additionalmeasurements, so its identification is less certain. The linear combination, least-squares fits totals of several samplesdeviate by more than ± 20% from 100%, suggesting that additional phases may be present that were notidentified or evaluated in this study.Conclusions: Sub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe~(3+) -oxyhydroxides and arseniosiderite, if sufficientorganic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization ofsecondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary andsecondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocscan sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of Assuch as mine adit water, but the fate of As associated with natural accumulations of floc material needs to beassessed.