Abstract
BackgroundA realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic 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 biogenic solids at the Lava Cap Mine Superfund (LCMS) site, a historic "Mother Lode" gold mine. Principal component analysis (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 arsenic speciation in XAFS spectra of tailings and biogenic solids.ResultsThe 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 the major 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 the major 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 ore samples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying and wetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weathering were identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3•3H2O). Existence of the former species is not in question, but the presence of the latter species was not confirmed by additional measurements, so its identification is less certain. The linear combination, least-squares fits totals of several samples deviate by more than ± 20% from 100%, suggesting that additional phases may be present that were not identified or evaluated in this study.ConclusionsSub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe3+-oxyhydroxides and arseniosiderite, if sufficient organic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization of secondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocs can sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of As such as mine adit water, but the fate of As associated with natural accumulations of floc material needs to be assessed.
Highlights
A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California’s famed “Mother Lode” region
Identification and quantification of As species in lode gold mine wastes is a critical step in a realistic estimation of health risks associated with increased exposures because (a) there is a wide range in solubility among solid forms of As [3,4,5] and (b) the dissolved, inorganic forms of As pose a high cancer risk [6]
Principal component analysis (PCA) and linear combination (LCLS) Fit Results PCA was performed with the goal of determining the best reference spectra for solid-phase As species present in the Lava Cap Mine Superfund (LCMS) as well as to identify differences among the samples
Summary
A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California’s famed “Mother Lode” region. Knowledge of arsenic (As) species in mine wastes and in mining-impacted areas is especially important in the heavily mined western foothills of the Sierra Nevada, California, because the recreational and residential development that has occurred in this region over the past decades has the potential to increase human and ecosystem exposure to inorganic As, a known carcinogen [1]. One of the key factors in the amount of As assimilated into the body is the partitioning of As between solid phases (food, soil, mine waste) and aqueous phases (natural waters as well as lung, gastric, and intestinal fluids) Both in vitro bioaccessibility and in vivo bioavailability studies of As in mine wastes demonstrate that these parameters vary as a function of the oxidation state and local coordination chemistry (i.e., the species) of As present [10,11,12].
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