Report on the Preliminary Investigation of Arsenic in Groundwater Near Lake Geneva, Wisconsin

Project Summary: The purpose of this preliminary investigation is to characterize the hydrogeologic and groundwater quality conditions prevalent in the vicinity of the Wood School, between the towns of Lake Geneva and Williams Bay, Wisconsin (Figure |). Results of groundwater sampling conducted by Wisconsin Department of Natural Resources (WDNR) personnel in 1998 showed total arsenic concentrations ranging from 15 to 120 ug/L in wells completed in the Silurian aquifer. Some of these data suggested a correlation between well chlorination and an increase in arsenic concentration. The main objective of this project was to evaluate the nature and extent of arsenic in well water in this limited area. Additional objectives were to determine 1) if well chlorination was the cause of the apparent increase in arsenic levels, and if so, the duration of the effect and 2) if oxidation or reduction of arsenic-bearing sulfides is a primary cause of arsenic contamination of groundwater.

Based on the results of this study, groundwaters in all three of the aquifers (unlithified sands and gravel, shallow bedrock (Silurian), and Cambrian-Ordovician sandstones) in the study area are, to some extent, impacted by arsenic. Levels exceeding the current federal maximum contaminant limit (M.C.L.) of 50 ug/L were found only in wells that are open to the shallow bedrock aquifer. Each of the eight wells completed in shallow bedrock that were tested during this study contain arsenic. Concentrations in excess of the proposed M.C.L. of 5 ug/L were found in the unconfined sand and gravel aquifer that overlies the Silurian bedrock, and in the deep Cambrian sandstone aquifer. However, concentrations exceeding 5 ug/L do not appear to be widespread within either the sand and gravel or the deep sandstone aquifers.