HR: 16:40h
AN: H14D-03 INVITED    [Abstracts]
TI: Effects of Pumping and Well Disinfection on Arsenic Release to Well Water
AU: * Gotkowitz, M
EM: mbgotkow@wisc.edu
AF: Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705, United States
AU: Shelobolina, E
EM: shelobolina@wisc.edu
AF: Department of Geology & Geophysics, University of Wisconsin - Madison, 1215 W Dayton St., Madison, WI 53706, United States
AU: Roden, E E
EM: eroden@geology.wisc.edu
AF: Department of Geology & Geophysics, University of Wisconsin - Madison, 1215 W Dayton St., Madison, WI 53706, United States
AB: In areas of northeastern Wisconsin, arsenic-bearing sulfides and iron oxides are distributed throughout a sandstone aquifer used for domestic water supplies. Aqueous arsenic concentrations exceed 10 μg/L in approximately 20% of wells in this region. These wells are often subjected to in situ chlorine disinfection to control nuisance or pathogenic bacteria. Field-based experiments investigating the effects of pumping and well disinfection showed that under non-pumping conditions, the geochemical environment in a domestic well is strongly reducing. Aqueous arsenic ranged from 10 to 18 μg/L, and the number of all tested groups of microorganisms (As(III)-reducing, Fe(III)-reducing, sulfate-reducing, As(V)-oxidizing, and aerobic microorganisms) increased 0.3 to 2.4 orders of magnitude in the well water under non-pumping conditions. The diverse populations of anaerobic and aerobic microorganisms reflect the complexity of the borehole environment. The number of Fe(III)-reducing bacteria correlates with As(III) concentrations, suggesting that microbially- facilitated reduction of iron (hydr)oxides contributes to the relatively rapid rise in aqueous arsenic observed under non-pumping conditions. Pumping the well introduces up to 1 mg/L of oxygen into the well water. The change in redox imposed by pumping decreased the number of anaerobic As(III)-reducing, Fe(III)-reducing, and sulfate-reducing microorganisms by 1 to 1.7 orders of magnitude. Aqueous arsenic also decreased during pumping (<7 μg/L), indicating that low-arsenic groundwater recharges the well. Chlorine disinfection produced strongly oxidizing conditions in the well for one hour. Treatment reduced the numbers of all microorganisms tested, but the populations recovered within three weeks. This suggests that either fresh formation water re-inoculated the well or that biofilm and scale in the well harbored some microbes from the disinfectant. Post-disinfection arsenic concentrations were similar to those measured prior to treatment. Although arsenic release under non-pumping conditions appears to be microbially mediated, well disinfection did not provide a sustained improvement in well- water quality.
DE: 1831 Groundwater quality
DE: 1834 Human impacts
SC: Hydrology [H]
MN: 2007 Fall Meeting