Arsenic is a well-documented drinking water problem due to its ubiquity in the Earth’s crust. However, few studies have investigated its mobility and fate in high bicarbonate waters. A “hot spot” for As contamination in groundwater from a carbonate-bearing glacial drift aquifer near Hell Michigan was chosen for such a study. This area hosts carbonate-rich, Wisconsin age, glacial drift deposits (mainly moraines and kames) resting in part, atop, a fluvial to marginal marine Mississippian age unit, the Marshall Sandstone, noted for locally high concentrations of As in arsenopyrite. The moraines, comprised of silts, sands and gravels and the kames of sorted sands and gravels, permit a largely unconfined aquifer. Thick podzolic soils are established on the drift, aided by the glacial erosion of the Marshall Sandstone. The topography affects groundwater discharge patterns, which can be locally focused into carbonate precipitating fens, commonly drained by streams.

Piezometers, installed across the fen and lysimeters, installed atop a moraine on either side of the fen, were used for soil water collection. Ground and surface waters were sampled and complete geochemical analyses were conducted. Soil profiles and carbonate-rich fen deposits were analyzed focusing on depth dependent variation in total metal and As concentrations. The soil leaches show As in the soil column is mostly in the B-horizon, associated with dissolution of Fe-Al oxyhydroxides. The groundwaters, generally low in oxygen (<1% saturation), have widely varied As values, many as high as 30ug/l. Relatively low As concentrations are observed in soil waters and the shallow water systems analyzed (streams, fens). Although high As groundwater can source the streams and fens, As is effectively scrubbed by Fe-oxyhydroxides in these oxic surface systems.

High As groundwater typically has the lowest Fe concentrations, suggesting that simple mobilization from Fe-oxyhyroxides is not the main As source. Our current working hypothesis is that As was liberated to groundwater from oxidative dissolution of As-bearing pyrite in relatively fresh glacial drift. In fact, As-rich water seems to be localized from aquifers interspersed with thick gray clay layers. Work is ongoing to refine our understanding of the regional hydrogeochemistry controls on high As groundwater.