Uranium-isotope systematics in groundwaters of the Rustler Formation, northern Delaware Basin, southeastern New Mexico

Abstract

Values for uranium concentration ((U)) and {sup 234}U/{sup 238}U activity ratio (AR) have been determined for groundwaters and host rocks from the Rustler Formation near the Waste Isolation Pilot Plant (WIPP) site. (U) varies from about 0.02 to 40 {times} 10{sup {minus}9} g/g, increasing westward across the WIPP site to Nash Draw, a dissolution valley underlain by outcrops of Rustler Formation evaporites. Large deviations from secular equilibrium (AR {approx} 1) in the groundwaters increase eastward from about 2 to 3 in Nash Draw to almost 12 in the eastern part of the WIPP site. (U) and AR variations cannot be completely explained by simple mixing due to congruent dissolution of uranium from rock (without isotopic fractionation). AR values typically increase along the flow path in a reducing environment, and the observed eastward increase in AR suggests a relict flow system whose dominant flow direction (eastward) was at high angles to that now observed. A westward decrease in AR coupled with a steady increase in (U) indicates not only that there was a change in flow direction since recharge, but that Rustler groundwater is now draining from areas of high potentiometric level low permeability near the WIPP site, without appreciable recharge.more » The maximum time required for this westward drainage is about 200,000 a. The minimum time required to achieve the highest observed ARs during the earlier episode of eastward flow from recharge in the west is 10,000 to 30,000 a. Radiocarbon and stable-isotope studies of the Rustler Formation near the WIPP indicate that the modern Rustler flow system is not at steady state, recharge being dominated by wetter climatic conditions in the Pleistocene. Uranium-isotope studies are consistent with these results, and further suggest that present flow directions are qualitatively different from those existing at the time of recharge. 36 refs., 17 figs., 3 tabs.« less