CRETACEOUS SEDIMENTARY BEDROCK AND ITS IMPACT ON URANIUM CONCENTRATIONS IN IRRIGATION RETURN-FLOWS TO THE ARKANSAS RIVER, SOUTHEASTERN COLORADO

Cretaceous-aged sedimentary rocks in the western United States are potential sources of uranium (U) in the groundwater systems of irrigated areas. In Southeastern Colorado, Cretaceous-aged shales and limestones may be leaching U into groundwater where those units underlie the irrigated valley along the Arkansas River. When exposed to oxidizing groundwater, reduced U (IV) undergoes oxidative dissolution, producing mobile aqueous U (VI). Elevated concentrations of U in the shallow return-flow groundwater is of concern as a non-point source of pollution because of loading issues in surface water flowing through Colorado and into Kansas. The overall goal of this study is to evaluate land and water best management practices to mitigate what appears to be a combination of natural processes impacted by human activity in this important agricultural area. We are examining the impact that the local geology is having on U concentrations in irrigation return-flows to the Arkansas River, spanning from John Martin Reservoir to the state line with Kansas.

In 2017, we installed eight monitoring wells in the irrigated area. In 2018, we installed two monitoring wells in the irrigated area, and two outside of the irrigated area. We recovered continuous core from the four boreholes drilled in 2018, which revealed that the underlying bedrock is dominantly composed of the Niobrara Formation, the Carlile Shale, and the Greenhorn Limestone. The Niobrara Formation, a clay-rich carbonate in southeastern Colorado, contains 1.0 to 15.3 mg/kg of U in split spoon and drill core samples, while the groundwater from wells in the Niobrara Formation contains 11 to 450 µg/L of U. The Carlile Shale, a formal unit of the Colorado Group, contains 0.8 to 9.8 mg/kg of U in drill core samples, while groundwater from wells in the Carlile Shale contains 15 to 280 µg/L of U. The Greenhorn Limestone, another formal unit of the Colorado Group, contains 2.7 to 8.3 mg/kg of U in split spoon and drill core samples, while the groundwater from wells in the Greenhorn Limestone contains 15 to 240 µg/L of U. Groundwater exchange with the river contributes to measured 85^th percentile river concentrations as high as 80 µg/L, substantially exceeding the USEPA chronic standard of 30 µg/L for drinking water.