Geochemistry, hydrology, and environmental applications of uranium-series nuclides in the Platte River drainage basin

Dissolved uranium concentrations in the Platte River Valley result from recent weathering of uraniferous igneous rocks at the headwaters of the North and South Platte Rivers. Uranium in the Platte River and its tributaries exhibit concentrations and $\rm\sp{234}U/\sp{238}$U activity ratios reflecting variations in the contributions of sources due to variable flow. Mixing calculations using conservative uranium isotopes in the Platte indicate considerable variations in longitudinal and transverse mixing of tributary water, as well as temporal variations of ground water contributions. Elevated uranium concentrations in the shallow ground water in the Central Platte Valley have been associated with high dissolved solids linked to the formation of alkali soils. Higher and more variable uranium concentrations occur near the base of the alluvial aquifer upgradient of a ground water redox zone where contemporary reduction is indicated. The associations of dissolved solids, redox indicators, and stable isotopes are consistent with two upgradient sources of uranium. Acid-soluble uranium is up to ten times more abundant in the confining layer near the aquifer base and may indicate a source of U in the deeper aquifer. Selective extraction indicates that 70-90% of leachable uranium is associated with acetate and oxalate-extractable solids, probably in the form of hydrous iron oxides. Mobilization of bound uranium may be related to aerobic microbial respiration. Reduction of dissolved uranium is indicated by the presence of low concentrations of reduced uranium, probably in the form of UO$\sb2\sp-$ and is associated with Fe$\sp{+2}$ and H$\sb2$S. Microbial fermentation and enzymatic reduction of uranium is consistent with the presence of iron and sulfate-reducing bacteria. Radioactive disequilibrium of the U-series daughter, $\sp{222}$Rn, permits dating of young ground water residing in the subsurface for up to 15 days. Changes in radon activity in response to rapid surface water infiltration were used to estimate subsurface residence times. Relatively constant radon activity in deep monitoring wells suggested that radon remained in secular equilibrium, and residence times were in excess of the 15-day method limit. Highly variable radon activity in shallower ground water allowed estimation of ground water ages ranging from 4-14 days using disequilibrium theory