GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF RADIUM OCCURRENCE IN PRIVATE AND PUBLIC DRINKING WATER SUPPLIES FROM THE CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM, EASTERN WISCONSIN

Naturally-occurring contaminants limit the quantity of groundwater from the Midwestern Cambrian-Ordovician aquifer system (MCOAS) available for public and private drinking water supplies. In eastern Wisconsin (WI), the MCOAS has elevated activities of naturally-occurring radium (Ra) where the aquifer transitions from regionally unconfined to regionally confined. In this region, geochemical conditions favor Ra mobility. However, studies examining Ra occurrence in this region have generally been limited to public drinking water systems.

In this study, we use a combined geochemical and isotopic approach to investigate the occurrence of Ra in shallow (<150 m) private wells and deep (>225 m) public drinking water wells along a local flow path in eastern WI, where the MCOAS transitions from regionally unconfined to confined. Multivariate cluster analysis was used to classify 32 groundwater samples in groups based on measured geochemical and isotopic parameters, where groups generally correspond with well depth and the natural evolution of groundwater. Deep wells are further along the groundwater flow path and their geochemistry reflects additional residence time in the subsurface that enhances rock-water interactions. Groundwater from shallow wells is typically younger, recently recharged, and less chemically evolved. Combined Ra activities in shallow wells range from 0.7 to 3.5 pCi/L. Combined Ra activities above the EPA Maximum Contaminant Level of 5 pCi/L are restricted to deep wells. Samples from deep wells are characterized by high ²³⁴U/²³⁸U ratios, high δ³⁴S_SO4 values, and low δ¹⁸O and δD values indicative of Pleistocene recharge and the transport of saline, sulfate-rich water from the Michigan Basin. Results expand the predicted extent and effects of Pleistocene glacial recharge on the mobility of natural contaminants in the WI MCOAS. Pleistocene recharge preserved by a local confining unit in the study area influences Ra occurrence in drinking water due to high specific conductance and reducing conditions, resulting in competitive sorption and the dissolution of Mn-Fe oxides. Thus, this study demonstrates that the occurrence of Ra and other natural contaminants in private and public drinking water supplies extracted from the MCOAS can be strongly influenced by the recharge history of the aquifer.