GEOCHEMICAL AND HYDROGEOLOGICAL CONTROLS ON RADIUM AND BARIUM IN DEEP SANDSTONE AQUIFERS OF ILLINOIS (Invited Presentation)

The deep Cambrian-Ordovician sandstone aquifers in Illinois and the upper Midwest are heavily used for water supply, but they can have elevated concentrations of naturally occurring radium and barium. In the Chicago region in northeastern Illinois, many public water supplies have been dealing with these water quality issues for many years. We recently sampled 33 wells in the central part of Illinois southwest of the Chicago region to better understand the regional water quality of the deep sandstone aquifers. Both ²²⁶Ra and ²²⁸Ra were detected in all samples, though ²²⁶Ra tended to be the dominant isotope. ²²²Rn, a daughter of ²²⁶Ra, generally had a positive relationship with ²²⁶Ra although it typically had concentrations one to two orders of magnitude higher than ²²⁶Ra. This suggests the presence of significant amounts of ²²⁶Ra on the aquifer matrix. Dissolved ²³⁸U, the parent of ²²⁶Ra, did not appear to be correlated with ²²⁶Ra concentrations. Barium concentrations were generally not as high as observed in some parts of northeastern Illinois, and sulfate (barite) clearly controls its solubility. On the other hand, sulfate concentrations do not appear to be controlling radium concentrations. In some areas of central Illinois, there is a significant amount of Pleistocene recharge, as evidenced by relatively negative values of δD and δ¹⁸O. In general, the greater the amount of Pleistocene recharge detected, the lower the radium concentration.