Radium isotopes as indicators of adsorption–desorption interactions and barite formation in groundwater

Borewater monitoring near the Ranger U mine tailings dam has revealed deterioration in water quality in several bores since 1981, with increases in sulphate concentrations of up to 1000 times being observed. In some cases, significant increases in 226Ra concentrations also occurred. In this study, isotopes of Ra, Th and Ac were measured in borewater in 1988–1993. For the most seepage-affected bores, the concentrations of 223Ra, 224Ra and 228Ra as well as the nuclidic ratios 228Ra/226Ra, 223Ra/226Ra and 224Ra/228Ra generally increased. Time-series concentration increases were observed for Sr but not for Ba. 227Ac concentrations increased with time, but not sufficiently to account for the increasing 223Ra concentrations. It is concluded that increases observed in Ra isotope concentrations arise from competition for cation adsorption sites in the vicinity of the bore rather than by direct transport of Ra from the tailings. Formation of a barite solid phase is occurring in the groundwater and causing the removal of some Ra from solution, with rapid replenishment of the shorter-lived isotopes from their parents.