Date of this Version
Public water supply (PWS) wells currently contain dissolved uranium concentrations above the federally mandated maximum contaminant level (MCL) of 30 ppb (parts per billion) and dissolved arsenic concentrations above the 10 ppb MCL. Both uranium and arsenic are known to cause various forms of cancer in humans. Variations in total uranium concentrations in PWS wells in Nebraska indicate a relationship to the duration and rate of pumping in specific wells. Although total arsenic concentrations show some variability over time in specific wells, the relationship to pumping is not as clear. Previous studies show that iron and sulfur bacteria present in aquifer systems affect the redox state of both uranium and arsenic species. The chemical reactions contributing to the uranium and arsenic concentration variations observed in these PWS wells are hypothesized to be mediated by the microbial populations present within the groundwater. Chemical extractions indicate uranium and arsenic concentrations in well precipitates are more strongly bound to organic material than exchangeable metals. Scanning electron microscopy and microprobe analyses verified the presence of poorly-ordered iron oxyhydroxides bound to organic materials. These organometal complexes adsorb uranium and arsenic species. Microbial populations were characterized of groundwater samples and well screen precipitates collected from pump intakes. Phylogenetic techniques indicate diverse communities of iron-oxidizing, iron-reducing, and biofilm-forming bacteria within and around sampled PWS wells. Management of high uranium and arsenic concentrations in PWS wells may be enhanced by a thorough understanding of PWS well biogeochemistry and its ability to influence the behavior of uranium and arsenic.