Hydrogeological and Geochemical Controls on Radium and Uranium in the St. Peter Sandstone Aquifer in the Middle Illinois Water Supply Planning Region

The St. Peter Sandstone is part of the Cambrian-Ordovician aquifer system in the Midwestern U.S. and is a major aquifer covering a large area, including central Illinois. R echarge to the St. Peter Sandstone during the Pleistocene and Holocene has come from multiple locations and displaced in situ brines, which have altered the geochemistry. Radium (Ra) levels are above the drinking water standard of 5 picocuries per liter (pCi/L) in many community water supply wells open to the St. Peter Sandstone. The purpose of this research was to examine sources and sinks of Ra and other radioisotopes in the Middle Illinois water supply planning region.Both 226Ra and 228Ra are found in the groundwater, indicating that both uranium and thorium (238U and 232Th) decay chains are important in the aquifer. Many of the wells sampled were open to formations overlying and underlying the St. Peter Sandstone, and shale facies in those units may be sources of U to the St. Peter. Radon (222Rn) data indicate that most of the 226Ra is associated with the solid phase. In a region where the St. Peter Sandstone is at or near the land surface and active recharge is occurring, adsorption to Fe- and/or Mn-oxyhydroxides appears to be the major control on Ra concentrations in solution. High sulfate (SO42-) concentrations in the confined regions of the aquifer indicate that barite solubility is a control on Ra concentrations in these regions. Sorption/desorption reactions also likely play a role in Ra solubility.Pleistocene meltwater recharge has exerted a strong influence on the Ra and U geochemistry in the St. Peter Sandstone. Multiple episodes of fresh Pleistocene water that penetrated far into the St. Peter appear to have brought in oxygen that caused U to be solubilized, transported, and then re-precipitated out of solution once more reducing conditions were re-established. Fractionation of U isotopes occurred as a result of the fluctuating redox conditions and is especially pervasive in the Central region of our study area. These impacts of Pleistocene recharge may be prevalent locally due to the proximity of the aquifer’s northward recharge zone where the St. Peter Sandstone outcrops and subcrops.