ARSENIC CONTAMINATION IN SOUTHEAST WISCONSIN: SOURCES OF ARSENIC AND MECHANISMS OF ARSENIC MOBILIZATION

Moderate to high levels of arsenic contamination occur in groundwater throughout eastern Wisconsin. Previous studies have shown that oxidative dissolution of arsenic-bearing sulfide minerals is the mechanism controlling high levels of arsenic contamination in the Fox River valley in east-central Wisconsin (Schreiber, M.E., J.A. Simo, and P.G. Freiberg, Stratigraphic and geochemical controls on naturally occurring arsenic in groundwater, eastern Wisconsin, USA, Hydrogeol. J., 8, 161-176, 2000). Preliminary work in southeast Wisconsin indicates that geologic and hydrogeologic conditions leading to a cluster of arsenic-impacted wells in this part of the state differ from those in the Fox River valley. However, neither the source of arsenic nor the mechanism of arsenic release to the groundwater in this region has been determined. This paper summarizes the status of an ongoing study, the objectives of which are 1) to identify the source(s) of arsenic contamination and 2) to determine the mechanism(s) controlling arsenic mobilization in southeast Wisconsin.

Ongoing work includes efforts to identify geologic and hydrogeologic controls on arsenic concentrations by comparing groundwater chemistry data to information about well construction, lithology, bedrock topography, stratigraphy, and groundwater flow regimes. Data sources for this analysis include existing groundwater chemistry data, archived core and cuttings samples, and well construction reports. Extensive groundwater sampling in arsenic-impacted areas will provide additional chemistry data. Detailed lithologic and mineralogic data will be obtained from core samples collected during rotasonic drilling.

Future work will include sequential extraction experiments on arsenic rich core samples to characterize the solid phases with which arsenic is associated. Bench-scale leaching experiments will be conducted to determine the chemical and environmental conditions that promote the mobilization of arsenic in the study area, and geochemical reaction models will be developed to further test hypotheses regarding processes controlling mobilization of arsenic. The results of this work will aid in developing guidelines for siting, constructing, and operating wells in a manner that prevents or reduces the likelihood of arsenic contamination.