FATE OF URANIUM FROM ARKANSAS RIVER WATER IN IRRIGATED AGRICULTURE, SOUTHWEST KANSAS

Saline water in the Arkansas River in southeast Colorado and southwest Kansas contains a high concentration of dissolved uranium. The source of both the salinity and uranium is primarily weathering of marine shales in eastern Colorado. Evapotranspiration associated with irrigation and shallow reservoirs in eastern Colorado increases the salinity and uranium concentration of river water entering Kansas. The river water is diverted for irrigation and infiltrates to and contaminates the underlying High Plains aquifer, also used for irrigation, in southwest Kansas. We conducted a study to determine the fate of uranium in agricultural irrigation, i.e., the distribution in soils and crops irrigated with the river water and groundwater. Uranium concentration in low river flows can be up to 90 µg/L, and in the moderate to high flows diverted for irrigation during the study was 12-25 µg/L. Uranium in irrigation well waters associated with the fields from which soils and crops were sampled was 25-102 µg/L. Uranium concentrations in water and dilute acid extracts of the soils sampled at two depths were 3-99 µg/kg and 609-2,200 µg/kg, respectively, on a dry soil basis. The grain, non-grain, and roots of the grain crops sampled (corn, soybean, sorghum) and digested in acid contained uranium concentrations of ~1-22 µg/kg, 18-256 µg/kg, and 145-1,450 µg/kg, respectively, on a dry plant matter basis; alfalfa above ground material and roots contained 88-192 µg/kg and 603-833 µg/kg, respectively. The results indicate that uranium is preferentially stored in the crop roots and is lowest in the grain of the three plant parts. The relative accumulation of uranium in the different media was compared based on the ratio of uranium to total cation concentration (U/Cat). This ratio was lower in the soil water extracts and generally as high in the soil acid extracts as in the irrigation water. It was lower in all plant parts (except for the roots of three samples) than in the irrigation water. The U/Cat ratio in the grain and non-grain above-ground material was lower than, and in the roots was in the same range as, in the water and acid extracts of soils. Thus, the crop plant parts used for forage do not substantially bioaccumulate uranium from the irrigation water or uranium biologically available from the soil compared to total cation concentrations.