Abstract: Irrigation Leaches Uranium from Soils and Shales and Concentrates It in Arkansas Valley Waters of Southeastern Colorado

Sigrid Asher-Bolinder, Robert A. Zielinski

Irrigation of the semiarid Arkansas River Valley with oxidizing, alkaline waters accelerates the release of uranium from pyritic, calcareous marine shales and their derivative soils. Irrigation waters oxidize pyrite grains in shales and soils, producing localized, transient acidity that enhances liberation of uranium from the shale fraction. Uranium is stabilized in the still alkaline and oxidizing waters by the formation of uranyl-carbonate complexes.

Leaching experiments show that 30-60 percent more uranium islabile in irrigated shaley soils than in their similar unirrigated counterparts or in fresh shale bedrock. Less shaley soils, however, transmit oxidizing, alkaline irrigation waters downward to leach uranium from weathered shale bedrock. Seeps and springs that emerge at the soil-shale interface can contain 20-150 micrograms/L uranium; the proposed USEPA drinking water standard is 20 micrograms/L.

Irrigation also accelerates uranium concentration and transport in surface and groundwaters by increasing the frequency and duration of rock-water and soil-water interactions. Artificially elevated water tables, reuse of irrigation runoff, and recycling of shallow groundwater through irrigation pumping all increase the evaporative concentration of dissolved uranium and the runoff and return of high uranium waters to deeper aquifers.

Phosphate-rich liquid fertilizers containing as much as 150 mg/L uranium are used in parts of the Arkansas River Valley, and they are potential sources of additional uranium to surface and ground waters. Preliminary results of field and laboratory studies suggest that this added uranium may be sequestered in shallow soil horizons as relatively insoluble protoapatite-like compounds that formed through the combination of added phosphate and dissolved calcium in pore waters.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994