URANIUM ISOTOPES IN THE PLATTE RIVER DRAINAGE-BASIN OF THE NORTH-AMERICAN HIGH-PLAINS REGION

The North and South Platte Rivers contribute significant concentration s of dissolved uranium to the Platte River system from the weathering of uraniferous rocks at their headwaters in Colorado and Wyoming. Uran ium isotopes measured in three groups of samples collected from the Pl atte River and its major tributaries exhibited a wide range of concent rations (0.27-31.7 mug/l) and activity ratios (1.03-1.72) reflecting v ariations in the contributions of different sources under differing fl ow conditions. Locally elevated uranium concentrations and higher acti vity ratios were associated with groundwater sources during low flow p eriods. Uranium concentrations in surface runoff were lower than in ba seflow and correlated significantly with dissolved solids. Uranium act ivity ratios in runoff were closer to equilibrium values than those in baseflow suggesting that dissolved uranium in runoff is less affected by disequilibrium processes. Mixing calculations using uranium concen trations and activity ratios indicate considerable variations in longi tudinal and transverse mixing of tributary water with Plate River wate r, and temporal variations of groundwater contributions (baseflow and irrigation return flow) in the Plate River system.