Isotopic Studies of the Natural Sources of Radium in Groundwater in Illinois

摘要

Stable and radioactive isotopes in ground water were studied in an investigation of the natural geologic sources of high concentrations of Ra226 and Ra228 in confined aquifers in the Cambrian and Ordovician bedrock of northern Illinois. The covariation of delta 018 and delta D determined that a source of recharge related to glaciation is required for groundwater in confined aquifers of the Cambrian and Ordovician. The covariation of delta S34 and delta 018 in dissolved sulfates determined a mixing line between two sources; oxidation of sulfide minerals and dissolution of marine evaporites. Dissolved sulfates from evaporite sources are of a different isotopic composition than evaporites of Cambrian or Ordovician age. Glaciation may be important with regard to recharge of the sulfates. The U234/U238 activity ratio in groundwater from the Cambrian and Ordovician are unexpectedly high; values range from 2.0 to 40.7. In confined aquifers values are greater than 20 over large regions. Alpha recoil damage is a mechanism that contributes to the disequilibrium. However, the regional variation in activity ratios and in U234 concentrations supports the concept that glacial recharge has contributed to the high ratios.