Geologic and Geochemical Controls on Arsenic in Groundwater in Northeastern Wisconsin

Abstract: This project involved geologic and hydrogeologic approaches to investigate the release of naturally occurring arsenic concentrations to well water in a confined sandstone aquifer in northeastern Wisconsin. Previous work identified sulfide mineralization, concentrated at the contact of the Sinnipee Group and the underlying St. Peter sandstone (Ancell Group), as the primary source of arsenic to the aquifer. Geologic mapping presented in this report indicates a high degree of variability in the distribution of iron-sulfide mineralization at the Sinnipee-St. Peter contact and a high degree of variability of arsenic within the mineralization. Overall, regional trends of increasing arsenic concentration and morphotypes of mineralization occur in outcrops in the St. Peter sandstone from north to south along the Fox River Valley. At a smaller scale, we observed trends of increasing arsenic concentration in the St. Peter sandstone where the sandstone pinches out. Arsenic concentrations in non-mineralized areas of the sandstone average less than 5 parts per million (ppm), while concentrations in mineralized zones vary from 10s to 100s ppm. Alteration of iron-sulfide mineralization is apparently followed by the precipitation of goethite, an iron oxide. In many cases, the iron oxide weathering products of the sulfide mineralization have higher associated arsenic concentrations than the iron-sulfide mineralization.