URANIUM VARIATIONS AND LOADS OF THE ARKANSAS RIVER IN THE HIGH PLAINS, USA

Arkansas River water in southeast Colorado and southwest Kansas is saline and mixed cation-sulfate type. Uranium concentration is also high and has been documented (e.g., Zielinski, 1995) to substantially increase in southeastern Colorado from the Rocky Mountain front across the High Plains as a result of leaching of uranium-bearing shales and derivative soils, and evapotranspiration associated with extensive irrigation based on diverted river water. Recent (2009) uranium data for Kansas indicate that the concentration range is about 27-80 µg/L for moderately high to low flows at the Colorado-Kansas state line; the drinking-water criterion is 30 µg/L. The U concentration remains about the same as in the flow from Colorado to as far east as flow exists in the river channel in southwest Kansas before completely seeping into the subsurface. Downstream where the river resumes flow in south-central Kansas, the uranium concentration is substantially lower. The uranium/sulfate ratio decreases appreciably in the river from the front range to east of Pueblo in Colorado, does not change substantially across the High Plains from Colorado to southwest Kansas, and then increases to nearly the same value in south-central Kansas as at the front range. Uranium and sulfate concentrations in the river water in Kansas are both highly correlated with specific conductance. This allows good estimates of concentrations and loads in the river at gaging stations with continuous conductance recording. The estimated mean and range in daily uranium concentration in Arkansas River flow at the Colorado-Kansas line in 2009 were 63 µg/L and 25-79 µg/L, respectively. The estimated mean and range in daily sulfate concentration at the state line for 2009 were 1,960 mg/L and 690-2,500 mg/L, respectively. The total uranium and sulfate loads for 2009 were 7 and 4.1 x 10⁶ metric ton, respectively. Essentially all of this saline, high-uranium water seeped into the alluvial and High Plains aquifers in southwest Kansas. This is enough to contaminate about 2.8 x 10⁸ m³ (230,000 acre-ft) of ground water to the drinking-water limit if a background uranium concentration of 5 µg/L were assumed. An investigation is in progress to assess the impact of the high-uranium river water on aquifer waters in southwest Kansas.