Reconnaissance Study of Uranium in the South Platte River, Colorado

The South Platte River in Colorado drains areas of crystalline and sedimentary rocks. The water is a sodium-calcium-sulfate-chloride type throughout its length of flow in Colorado. The concentration of uranium in the water of the South Platte during the winter of 1969-1970 ranged from 5 ppb to 67 ppb, making it anomalously rich in uranium in comparison with most other rivers of the world. The concentration of uranium increases downstream, in contrast to the decrease in uranium concentration observed in other rivers that drain areas with known deposits of urinaum. The South Platte contains a higher concentration of uranium than either the Colorado or North Platte rivers, despite the fact that the latter two rivers drain ore-producing areas of the United States. It is likely that most of the uranium in the South Platte is contributed by uraniferous coal seams in the Cretaceous Laramie Formation and by uranium-rich black shales in the Cretaceous Pierre Formation. The possibility that undiscovered deposits of uranium ore are present in the drainage basin of the South Platte cannot be excluded, but no major deposits are known. Use of the parameter "incremental areal uranium-load " permits certain portions of the drainage basin to be recognized as contributors of anomalously large amounts of uranium to the river. In the headwaters of the South Platte the incremental areal uranium-load is a low 0.00018 kg U/day/kin 2, whereas for the increment of drainage between Weldona and Balzac, Colorado, the incremental areal uranium-load is 0.016 kg U/day/kin 2. This parameter may be useful for hydrogeochemical prospecting for uranium ore in other areas. The concentration of uranium in the interstitial water of the alluvium in cutoff meanders varies seasonally, but there is no clear-cut evidence for precipitation of uranium minerals. Measurements of Eh, pH, and total vanadium in interstitial waters indicate that neither uraninite, coffinite, nor carnotite is stable