NATURAL ARSENIC CONTAMINATION OF ALLUVIAL AQUIFERS BY LINKED GEOLOGICAL AND MICROBIAL PROCESSES

We integrate groundwater geochemistry, mineralogy, microbiology, and numerical modeling techniques to study the biogeochemical linkages between Fe, S, and As in shallow alluvial aquifers in Bangladesh and Taiwan. High concentations of As in groundwater generally correlate with elevated Fe, Mn, alkalinity, and dissolved H₂ gas contents. Dissolved inorganic carbon (DIC) in As- and Fe-rich groundwaters is enriched in ¹³C, indicating that bacteria preferentially use the lighter carbon for metabolism. The geochemical correlations, carbon isotopic signatures, elevated dissolved H2 of groundwater (2.1-2.4 nM), and high organic carbon (TOC) contents (>2%) of sediments support the hypothesis that elevated As, Fe, Mn concentrations may be derived from bacterial iron and manganese reduction. Moreover, As-rich groundwaters have high fluorescent intensity (FI) and our analysis showed that the fluorescent humic substances have one primary emission spectrum of 415 nm associated with special function groups. Further work is needed to characterize the potential role of these functional groups in Fe reduction and As release. Groundwaters in Bangladesh and Taiwan commonly occur a substantial distance from their ultimate source of arsenic, apparently in associated mountain ranges, where chemical weathering of As-bearing minerals may be enhanced by rapid tectonic uplift. We propose a new model of As cycling that invokes active erosion of mountain belts, transport of arsenic by surface waters, adsorption of As by stream sediments, and deposition of stream sediments and organic matter in alluvial deposits. Subsequently, Fe-reducing bacteria present in alluvial aquifers cause the release of sorbed As to groundwater under moderately reducing conditions. Arsenic may be removed by Fe-S solid phases under local biogenic sulfate reduction not limited by initial low sulfate concentrations. The proposed model was tested through theoretical geochemical modeling for the speciation, solubility, and reactivity of arsenic in Fe-S-As-H₂O systems. Groundwater microbiology and ion microprobe isotope analyses of bio-minerals are currently underway to further characterize the biological fingerprints of microbial processes involved in As biotransformation.