[en] Previously reported concentrations of U in well and surface waters of the Arkansas River drainage in southeastern Colorado approach or exceed the proposed EPA drinking water standard of 20 ppb U with values as high as 230 ppb. The authors sampled water, solid and bedrock in the drainage to determine the influence of natural weathering and agricultural practices on the amount of dissolved U. Waters are of the Na-Ca-SO₄ type and are saturated with gypsum and calcite. Dissolved U correlates positively with conductivity and occurs as uranyl carbonate complexes. In nonirrigated areas U is dissolved during oxidative chemical weathering of sulfidic marine shales (U=2-12 ppm) and their derivative soils, and can be concentrated to as much as 100 ppb in ponded surface water and shallow ground water. Irrigation increase the rate of natural weathering and enhances soil-water contact as irrigation waters return to the river and the alluvial aquifer for reuse downstream. In irrigated areas limited application of phosphate-bearing fertilizer containing as much as 150,000 ppb U also may contribute some uranium to local waters