Geochemical and hydrogeological controls on radium and barium in deep sandstone aquifers of Illinois

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 (super 226) Ra and (super 228) Ra were detected in all samples, though (super 226) Ra tended to be the dominant isotope. (super 222) Rn, a daughter of (super 226) Ra, generally had a positive relationship with (super 226) Ra although it typically had concentrations one to two orders of magnitude higher than (super 226) Ra. This suggests the presence of significant amounts of (super 226) Ra on the aquifer matrix. Dissolved (super 238) U, the parent of (super 226) Ra, did not appear to be correlated with (super 226) Ra 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 delta D and delta (super 18) O. In general, the greater the amount of Pleistocene recharge detected, the lower the radium concentration.