HIGH URANIUM AND ARSENIC CONCENTRATION AFFECTS MICROBIAL DIVERSITY IN NEBRASKA PUBLIC WATER SUPPLY WELLS

Nebraska 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 uranium concentrations in PWS wells in Nebraska indicate a relationship to the duration and rate of pumping. Arsenic concentrations show some variability, but the relationship to pumping is not as clear. Chemical data indicate higher uranium and arsenic concentrations are associated with the presence of organic material, the accumulation of which is facilitated by the dynamic environment generated during pumping. We hypothesize that the chemical reactions contributing to the uranium and arsenic concentration variations observed in these PWS wells are mediated by the microbial populations present within the groundwater. Variable pumping conditions create an environment that is conducive to the growth of microorganisms in and adjacent to the PWS wells. Previous studies have shown that iron and sulfur bacteria present in aquifer systems affect the redox state of both uranium and arsenic species. Groundwater samples and inorganic precipitates from well screens were collected from pump intakes in order to characterize the microbial populations using culture techniques and DNA methodology. DNA was extracted and probed by molecular analysis for domain affiliation of resident microbes and the presence of genes permissive to redox reactions involving uranium and arsenic. All well samples cultured with selective media have yielded microorganisms representative of Fe oxidizing, Fe reducing, and S reducing bacterial groups. Management of high uranium and arsenic concentrations in PWS wells may be enhanced by a thorough understanding of groundwater microbial communities and their ability to influence the behavior of uranium and arsenic. This may provide alternative well management techniques.