RADIUM MOBILITY AND THE AGE OF GROUNDWATER IN PUBLIC-DRINKING-WATER SUPPLIES FROM THE CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

Geochemical conditions that result in elevated concentrations of ²²⁴Radium (Ra), ²²⁶Ra, and ²²⁸Ra in potable parts of the Cambrian-Ordovician (C-O) aquifer system were investigated using raw-water-quality data and environmental tracers of groundwater age from 80 public-supply wells. Mean groundwater ages were estimated by calibration of tracers to lumped parameter models and ranged from less than 20 years in upgradient, unconfined areas to more than 1 million years in downgradient, confined areas. Anoxic, iron-reducing conditions and increased mineralization develop with time and mobilize Ra into solution resulting in the frequent occurrence of ²²⁶Ra + ²²⁸Ra at concentrations exceeding the U.S. Environmental Protection Agency Maximum Contaminant Level of 5 picocuries per liter. The distribution of the three Ra isotopes differed across the aquifer system. Concentrations of ²²⁴Ra and ²²⁸Ra were strongly correlated and comprised a larger proportion of the total Ra concentration in samples from upgradient, unconfined areas, where arkosic sandstones provide an enhanced source for progeny in the ²³²Thorium decay series. ²²⁶Ra comprised a larger proportion of the total Ra concentration in samples from downgradient confined regions, where carbonates provide an enhanced source for progeny in the ²³⁸Uranium decay series. ²²⁶Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength of groundwater (mineralization), indicating that the amount of ²²⁶Ra released from solid phases and retained in solution increases relative to the amount of Ra sequestered by sorption processes or co-precipitation with barite as sorption capacity and the concentration of barium decreases. 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 alpha-particle radioactivity to potable samples from the C-O aquifer system. Thus, monitoring for Ra isotopes and gross-alpha-activity (GAA) is as important in upgradient, unconfined areas as downgradient. GAA measurements made within 72 h of sample collection best capture alpha-particle radiation from the short-lived ²²⁴Ra.