ABSTRACT
The regional occurrences of selected radioactive isotopes in groundwater were studied in an investigation of the natural geologic sources of high concentrations of 226Ra and 228Ra in groundwater from wells finished in the Cambrian and Ordovician bedrock in northern Illinois. The combined dissolved concentration of the two isotopes ranges from 2.0 to greater than 50.0 pCi/L. Over 100 public water supplies in northern Illinois exceed the U.S. EPA Interim Drinking Water Standard of 5.0 pCi/L. Most supply wells are over 1000 feet deep and open to receive groundwater from sandstone, dolomite and shale lithologics. The most productive aquifer units, the sandstones, provide the high dissolved radium concentrations.
Dissolved concentrations of 222Rn range from 40.0 to 1000 pCi/L. The combined dissolved concentrations of 238U and 234U range from less than 0.1 pCi/L to 8.0 pCi/L; concentrations greater then 1.0 pCi/L reflect marked enrichment in 234U. Over large regions of northern Illinois groundwater in the Cambrian-Ordovician bedrock is uniquely enriched in 234U
Dissolved concentrations of 226Ra are poorly correlated with dissolved concentrations of 238U, 234U, 222Rn and 228Ra. However, a significant source of dissolved 226Ra is the chemical Precipitation adsorption of the parent 238U, 234U and 230Th 404nuclides on silica surfaces of the sandstones. A significant source of dissolved 228Ra is the occurrence of 232Th enriched accessory minerals in the sandstone bedrock. The ionic strength of groundwater is an important control on the dissolution of 226Ra. Mechanisms important to the dissolution of both 226Ra and 228Ra are alpha recoil and in specific localities the solubility of barite.
