Mobility of radionuclides in high chloride environments: a case study of waters within and near the Delaware Basin, southeastern New Mexico

Concentrations of naturally occurring isotopes of uranium, thorium, radium and radon were measured in freshwaters and in sodium-chloride brines near the site of the Waste Isolation Pilot Plant (WIPP) located in southeastern New Mexico. Supplemental water chemistry analyses (chloride, alkalinity, P/sub CO2/, ΣCO2, Fe, Mn, ΣH2S) were made to aid in interpreting the natural radionuclide results. Three features of radionuclide mobility are evident from our results: (1) There is a slight tendency for U and Ra concentrations to correlate with the chloride content of the water samples. Whether this results from complexation by Cl- ions or cation exchange competition for adsorption sites cannot be resolved with the available information. (2) Much more dramatic than the correlation with Cl- concentration is the effect of the redox state of the waters on U and Ra concentrations. Chemically reducing groundwaters contain much lower U concentrations and much higher Ra concentrations than were measured in oxic and suboxic samples. Calculated retardation factors of about 1 for Ra indicate that this element can migrate freely in anoxic brines. (3) Low chemical recoveries of Th, and to a lesser extent U, by methods that work well with seawater samples suggest that these elements may be present in an unreactive dissolved or colloidal complex with organic matter. A similar situation has been observed for Pu in freshwaters containing high DOC concentrations by other investigators. The unreactive nature of the U and Th in the brines suggests that even though the concentrations are low, the U and Th that are present in the water may be quite mobile because of their unrective nature. 16 figs., 13 tabs