Uranium and radium mobility in groundwaters and brines within the delaware basin, Southeastern New Mexico, U.S.A.

Concentrations of naturally occurring isotopes of U and Ra were measured in fresh waters, groundwaters and in sodium chloride brines within and near the Delaware Basin of southeastern New Mexico. We sought to determine the effect of high chloride concentrations in a wide range of redox conditions on the mobility of U and Ra in natural waters. Two important features of radionuclide mobility are evident from our results: 1. (1) There is a slight tendency for U and Ra concentrations to correlate with the chloride content of the water samples. Thermodynamic speciation calculations suggest that this may result from complexation of these elements by Cl ^-1 ions. 2. (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 contained much lower U concentrations and much higher Ra concentrations than were measured in oxic and suboxic samples. The low U values are consistent with the expected reduction of U (VI) to insoluble U (IV). We suggest that the increased Ra concentrations result from mobilization of Mn which eliminates the strongest adsorption sites for Ra thereby greatly enhancing its concentration in the anoxic waters.