AQUIFER CHARACTERIZATION TECHNIQUES FOR AVOIDING RADIUM-ENRICHED GROUNDWATER

The regionally extensive, complexly layered sedimentary formations in the Midwestern Cambrian-Ordovician aquifer system (C-OAS) supply water to a large number of communities in the north-central United States. However, many wells completed in this system produce water with combined radium (Ra²²⁶ + Ra²²⁸) at activities close to or exceeding the Maximum Contaminant Level of 5 pCi/L. In Wisconsin, more than 95 public water systems have implemented remedial measures to comply with this standard. Remediation strategies include water treatment by ion exchange, blending high- and low-Ra waters, switching from groundwater to surface water, and various well reconstruction techniques.

This study, conducted in south-central Wisconsin, examines the elemental composition of aquifer solids and the aqueous geochemistry within discrete hydrostratigraphic intervals in the C-OAS. We found that aquifer and aquitard sequences are stratified with respect to Ra(II) activity and geochemical conditions. Elevated total dissolved solids are positively associated with Ra(II) in the unconfined, oxic aquifer. Anoxic conditions correlated with elevated Ra(II) in the underlying confined system. This suggests that multiple sources of radium and more than one geochemical mechanism increase Ra(II) mobility in this study area.

These findings provide insight into examples of successful and un-successful municipal well reconstruction projects. In some instances, extending casing across shale layers has reduced Ra by limiting sources of U and Th from fine-grained sediments. However, the same approach implemented in a different public system is apparently causing Ra activities to increase over time. This work provides geologic and geochemical insight useful to design wells that avoid Ra-enriched groundwater or to evaluate mitigation strategies in existing wells affected by elevated Ra.