GEOLOGIC CONTROLS ON THE SPATIAL VARIABILITY OF ARSENIC CONCENTRATIONS IN GROUNDWATER IN WISCONSIN

Understanding the spatial variability of geologic factors that influence arsenic concentrations in groundwater is necessary if we are to make effective recommendations for preventing or reducing arsenic contamination in drinking water wells. Two studies are being conducted in eastern Wisconsin to characterize natural arsenic sources and factors influencing aqueous arsenic concentrations: our study addresses arsenic in groundwater in Quaternary sand and gravel and Silurian dolomite in southeastern Wisconsin and another involves groundwater derived from Cambrian-Ordovician sandstone and dolomite in northeastern Wisconsin (Schreiber, M.E., J.A. Simo, and P.G. Freiberg, Hydrogeol. J., 8, 161-176, 2000). The results of these studies demonstrate that a single model is not adequate to account for observed arsenic occurrences regionally, or even locally.

Regionally, arsenic sources differ depending on the lithology and mineralogy of the aquifers of interest. Arsenic-rich pyrite has been identified as the primary source of arsenic in the Cambrian-Ordovician aquifer in northeastern Wisconsin, while our work in the Quaternary and upper Silurian aquifers in southeastern Wisconsin indicates that arsenic is associated with oxide and/or clay minerals.

Locally, stratigraphic variability exerts significant control on the spatial distribution of aqueous arsenic concentrations. There are no wells yielding water with elevated arsenic concentrations completed at shallow depths in the Quaternary aquifer in our study area. Chemical signatures from both shallow and deep wells and the results of a pump test suggest that there is little groundwater flow between the deeper, arsenic-impacted groundwater system and the shallower, non-impacted system. Examination of rotosonic core revealed a clay-rich dark grayish brown till unit that likely constitutes an aquitard, restricting flow between the shallow and deep groundwater systems. The recognition that zones of non-arsenic impacted water exist within the Quaternary aquifer and that the local stratigraphy is an important control on where such zones occur is important in designing wells in a manner that prevents or reduces the risk of arsenic contamination.