Radium Mobility and the Age of Groundwater in Drinking-Water Supplies from the Cambrian-Ordovician Aquifer System

Groundwater ages which ranged from modern (< 50 yrs) to ancient (> 1 Myrs) were combined with raw water-quality data from 80 public-supply wells to evaluate the evolution of geochemical conditions that mobilize high concentrations of radium (Ra) in potable portions of the Cambrian-Ordovician (C-O) aquifer system. Findings indicate that it is the combination of anoxic, Fe-reducing conditions and increasing mineralization that favor the mobilization of Ra coupled with the length of time water is in the aquifer system that results in the frequent occurrence of combined Ra (²²⁶Ra + ²²⁸Ra) at concentrations exceeding the USEPA MCL of 5 pCi/L. Strongly correlated concentrations of ²²⁴Ra and ²²⁸Ra comprised a larger proportion of the total Ra (Ra_t = ²²⁴Ra + ²²⁶Ra + ²²⁸Ra) concentration in samples from upgradient recharge areas where arkosic sandstones are common, whereas ²²⁶Ra comprised the larger proportion in samples from downgradient confined regions. ²²⁶Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength (mineralization) indicating Ra is mobilized to solution from solid phases of the aquifer as sorption capacity is diminished. The rate of release of ²²⁶Ra from solid phases by alpha-recoil mechanisms exceeds the rate of Ra sequestration by adsorption processes or co-precipitation with barite. Although ²²⁶Ra occurred at concentrations greater than ²²⁴Ra or ²²⁸Ra, the ingestion exposure risk was greater for ²²⁸Ra owing to its greater toxicity. In addition, ²²⁴Ra added substantial gross alpha-particle radioactivity (GAA) to potable samples from the C-O aquifer system. Thus, monitoring for Ra radionuclides and GAA is equally critical in the recharge area as downgradient, with GAA measurements being completed within 72 h of sample collection to capture alpha-particle radiation from the short-lived ²²⁴Ra.