[en] Dissolved concentration of the natural radioisotopes ²²²Rn, ²²⁶Ra, ²²⁸Ra, ²³⁰Th, ²³²Th, ²³⁴U, and ²³⁸U and the element Ba (²⁺) were investigated in a study of high concentrations of ²²⁶Ra, ²²⁸Ra, and Ba (²⁺) in groundwater from the Cambrian and Ordovician bedrock of northern Illinois. The high radium and barium concentrations are present in the major aquifers - the sandstone bedrock. Therefore, remedial well construction measures will not lower the concentrations. The combined concentrations of ²²⁶Ra and ²²⁸Ra range from 2.3 to 50.2 pCi/L; the majority of analyses exceed the drinking water reulations of 50.0 pCi/L. The high Ba (²⁺) concentrations occur in groundwater that is depleted in dissolved SO₄^2- by anaerobic microbial reactions. A map presents the distribution of Ba ²⁺ in groundwater from the Cambrian and Ordovician bedrock in northeastern Illinois