1 research outputs found
Isotopic studies in the natural sources of radium in groundwater in Illinois
Stable and radioactive isotopes in groundwater were studied in an investigation of the natural geologic sources of high concentrations of ²²⁶Ra and ²²⁸Ra in confined aquifers in the Cambrian and Ordovician bedrock of northern Illinois. The covariation of ¹⁸0 and D determined that the groundwater has a meteoric isotopic composition. Groundwater in unconfined aquifers has ¹⁸0 values (-6.6 to -7.9⁰/₀₀) that are similar to contemporary meteoric water. However, a source of recharge related to glaciation is required for groundwater in confined aquifers of the Cambrian and Ordovician that is significantly depleted in ¹⁸0 (¹⁸0 values range to -12.7⁰/₀₀ and are less than -9⁰/₀₀ over large regions) . The covariation of ³⁴S and ¹⁸0 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 present in large concentrations in confined aquifers but 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 ²³⁴U/²³⁸U activity ratio in groundwater from the Cambrian and Ordovician are unexpectedly high; values range from 2.1 to 40.7. The lowest ratios occur in primary recharge zones. 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 ²³⁴U concentrations supports the concept that glacial recharge has contributed to the high ratios. Radiological and geochemical mechanisms that partition ²³⁸U, ²³⁴U and ²³⁰Th on the sandstone matrix are important to the dissolved ²²⁶Ra concentration.U.S. Department of the InteriorU.S. Geological Surve