Identifying the source of groundwater contaminants in West-Central Wisconsin, USA: Geochemical and mineralogical characterization of the Cambrian sandstone aquifer

Naturally-sourced groundwater quality issues are common, but there is rarely a geochemical and mineralogical dataset of aquifer properties for comparison. This study utilizes geochemical and mineralogical data to char-acterize naturally-occurring minerals that are potential groundwater contaminants in the Cambrian Eau Claire - Mount Simon aquitard-aquifer system of west-central Wisconsin, U.S.A. A high-resolution portable x-ray fluo-rescence elemental analysis was used for initial characterization of geochemistry and chemostratigraphy of well cutting and drill core samples. Then, a subset of sample materials was analyzed mineralogically and geochem-ically using XRD and ICP-MS, respectively. Elevated concentrations of arsenic, phosphorous, strontium, and various metals within the aquitard-aquifer sandstone were identified, mostly associated with sulfide minerals and iron (hydr)oxides and suggestive of Mississippi Valley-type mineralization. Similar elemental contaminants in surface and groundwater in the study area indicate that the observed trace element-bearing minerals are a natural source of groundwater contamination, most likely through release into groundwater during fluctuating redox and pH conditions near the water table. Co-occurrence of iron (hydr)oxide-coated sulfide minerals near the water table, and observations of sulfide oxidation post-drilling, suggests sulfides in these units are actively oxidizing. Well construction recommendations based on these results should mitigate current or future pumping of trace element-contaminated groundwater and in the vast majority of cases eliminate naturally-occurring contaminants as a potential source. This study provides an extensive baseline dataset of aquifer mineralogy and trace element composition (and an efficient approach for data collection) that is necessary for interpreting and attributing possible future groundwater quality issues.