HR: 10:35h
AN: H32C-02    [Abstracts]
TI: Controls on Arsenic Concentrations in Ground Water Near Lake Geneva, Wisconsin
AU: * Root, T L
EM: tara@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706
AU: Bahr, J M
EM: jmbahr@geology.wisc.edu
AF: Department of Geology and Geophysics University of Wisconsin - Madison, 1215 W. Dayton St., Madison, WI 53706
AU: Gotkowitz, M B
EM: mbgotkow@facstaff.wisc.edu
AF: Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705
AB: Approximately 15% of wells open to Quaternary glacial and shallow bedrock aquifers near the town of Lake Geneva in southeastern Wisconsin have As concentrations above the U.S. Environmental Protection Agency standard of 10 ug/l. Comparison of groundwater As concentrations with well construction records indicates that As concentrations are highest in wells open to the deep Quaternary and shallow Silurian sediments; wells open to a shallow Quaternary aquifer are not significantly impacted by As. Geochemical analysis of core samples from a single borehole near Lake Geneva shows solid-phase As concentrations in the 2-20 mg/kg range throughout the entire thickness (310 ft) of Quaternary sediments. Direct identification of mineralogic As sources in such sediments is difficult; isolation of As-rich phases is problematic due to the heterogeneous mineralogy and relatively low As concentrations. X-ray diffraction is of limited use for identifying As sources in these core samples because As-bearing minerals such as (hydr)oxides are typically poorly crystalline and present at weight percents below the detection limit. More sophisticated spectroscopic techniques such as, infrared, Raman, and X-ray adsorption spectroscopy, have the potential to provide more detailed information about the solid-phase associations of As in aquifer sediments. However, these techniques require access to expensive instrumentation and analyses of heterogeneous natural samples can be difficult to interpret. As an alternative to direct identification of As sources, we used a variety of indirect methods to develop and test hypotheses regarding As sources and processes controlling As mobility in these heterogeneous aquifer sediments. Groundwater sampling, chemical extractions, batch experiments, aquifer testing, and flow modelling provided evidence about the controls on As concentrations in groundwater in the study area. The results of these techniques suggest that geologic, hydrogeologic, and geochemical factors combine to create reducing conditions in the deep Quaternary and shallow Silurian sediments where As is mobilized via reductive dissolution of (hydr)oxide minerals.
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting