Hexavalent Chromium (Cr(VI)) in WI Groundwater: Identifying Factors Controlling the Natural Concentration and Geochemical Cycling in a Diverse Set of Aquifers

This proposal addresses the specific problem that detectable concentrations of hexavalent chromium (Cr(VI)) have been measured in drinking water sourced from Wisconsin groundwater. Although chromium (Cr) is a naturally occurring in groundwater, Cr(VI) – Cr in the +6 oxidation state – is a known carcinogen. The biogeochemical cycling of Cr between the +6 oxidation sate and the more benign +3 oxidation state (Cr(III)), as well as interaction with aquifer material at ambient groundwater conditions, needs to be fully assessed to determine potential human health concerns within certain aquifers of Wisconsin. We hypothesize that mineralized edges of three major geological basins of Wisconsin provide conditions favorable to the formation of Cr(VI). Our objective is to characterize the major aquifers of Wisconsin as to their natural background concentrations and release rates of total Cr and Cr(VI) through the use of controlled laboratory experiments on freshly collected aquifer cuttings obtained from drilling wells. We will use reactor studies to evaluate oxidation state specific release and dissolution rates from aquifer material, as well as reactors amended with reference material, which will be measured during timed kinetic studies to provide a mechanistic understanding of Cr(VI) cycling. Detailed chemical and oxidation state speciation measurements of both solution-phase and solid phase materials will be done using state-of-the-art low level analyses and established methods for a series physical, inorganic and organic parameters. By integrating the data from the reactor studies and detailed chemical and oxidation measurements, we will develop a mechanistic model of Cr release from the contrasting aquifers. Our findings will help both private well owners, public utilities and agencies better assess human health concerns regarding exposure to Cr(VI) and allow for predictions as to which aquifers are disposed to greater Cr(VI) release and formation under both natural and anthropogenic perturbation.