URANIUM ISOTOPES IN PRISTINE BEDROCK SPRINGS AND MINE DRAINAGE IN COLORADO: A RECONNAISSANCE GEOCHEMICAL ASSESSMENT

Analysis of uranium isotopes (²³⁴U, ²³⁸U) and concentrations was combined with strontium isotopes (⁸⁷Sr/⁸⁶Sr), rare earth elements (REE), noble gases, stable isotopes (δ²H, δ¹⁸O_H2O, δ³⁴S, δ¹⁸O_SO4, δ¹³C), and groundwater age tracers in water from 11 pristine bedrock springs of the Upper Colorado River Basin and in mine drainage from the Colorado Mineral Belt to evaluate the utility of uranium as a complementary tracer of solute transport, groundwater flow, and mixing. Results indicate that activity ratios of ²³⁴U/²³⁸U and REE concentrations vary systematically in different aquifers of the Upper Colorado River Basin and may be used for end-member definition and mixing calculations, depending on redox conditions. Radiogenic ²³⁴U concentrations also increase with increasing groundwater age which supports the use of uranium isotopes as an indicator of residence times. Other processes potentially affecting ²³⁴U/²³⁸U activity ratios must be considered, however, and inclusion of other tracers such as ³H, SF₆, CFCs, or ¹⁴C are useful for confirming quantitative residence-time estimates. Radiogenic ²³⁴U is therefore suggested for use as a complementary, not standalone, residence-time indicator. Within the mine drainage and adjacent groundwater, ²³⁴U/²³⁸U activity ratios vs. uranium concentrations are distinctive and, when used in conjunction with groundwater ages, may be used to estimate pre-mining groundwater chemistry. Uranium activity ratios show minimal covariation with noble gas isotopes or ⁸⁷Sr/⁸⁶Sr despite radiogenic controls on each decay series, indicating that differing processes may control these isotopic systems. Overall, these preliminary results indicate that uranium isotopes are useful for conceptual hydrologic and geochemical model development and could be considered for investigations other than contaminated site assessments where uranium isotope analyses are more typically completed.