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Biogeochemical behavior of dissolved arsenic and uranium concentrations in public water supply wells

Kevin J Mcvey, University of Nebraska - Lincoln

Abstract

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. Removal of the trace metals from groundwater systems may cost hundreds of thousands of dollars per town to achieve compliance with the MCLs. 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. Variable pumping conditions create an environment that is conducive to the growth of these microorganisms in and adjacent to the PWS wells. 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 using culture techniques and DNA methodology on groundwater samples and well screen precipitates collected from pump intakes. Samples cultured with selective media yield microorganisms representative of iron-oxidizing, iron-reducing, and sulfur-reducing bacterial groups. 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.^

Subject Area

Geology|Hydrology|Biogeochemistry

Recommended Citation

Mcvey, Kevin J, "Biogeochemical behavior of dissolved arsenic and uranium concentrations in public water supply wells" (2009). ETD collection for University of Nebraska - Lincoln. AAI3352340.
http://digitalcommons.unl.edu/dissertations/AAI3352340

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