Uranium isotope disequilibrium in groundwaters of southeastern New Mexico and implications regarding age-dating of waters

Regional exploration of bedded salt for a radioactive waste repository in the Delaware Basin (Permian, New Mexico, USA) included boreholes into the evaporites and associated rocks. One such hole, ERDA No.6, encountered an accumulation of saturated NaCl-Na2SO4 brine accompanied by H2S-rich gas. This fluid, and fluids from other boreholes elsewhere in the area, have been characterized geochemically according to solute content, 18O/16O and D/H ratios and natural actinide content. Deviations from the equilibrium 234U/238U activity ratio (α) of 1.0 were found in all water samples. These deviations are used to affirm the isolation of ERDA No.6 and to establish bounds on the age of the ERDA No.6 fluid. A mathematical model for the age of ERDA No.6 water intrustion was formulated in terms of the following variables: initial α-value (α0) of the brine precursor waters, zero-order kinetic rate constants of leaching of 234Th and 238U from the rocks, and degree of leaching of 234Th from the rocks. The model allows calculation of α-values of brines as a function of time, leach rate and α0. Various combinations of α0's, leach fractions and leach rates indicate that the leach rate must be low. If no leaching is assumed and the α0 is the highest known α from the nearby Capitan Reef (α0=5.2 at present), the fluid is older than 880,000 years. ERDA No.6 brine has undergone more profound rock/fluid interactions, reflected in its solutes and stable isotopes, compared with younger meteoric Capitan waters. (author)