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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Uraniferous Waters of Southeastern Colorado: A Function of Geology, Climate and Land Use

by

Robert A. Zielinski (U.S Geological Survey, Denver, CO) and Sigrid Asher-Bolinder (U.S Geological Survey, Denver, CO)

Abstract

The effect of local geology and land-use practices on dissolved uranium in water is indicated by reconnaissance sampling of surface waters and some springs along the Arkansas River valley of southeastern Colorado. The rate of increase of uranium concentration with distance downriver increases markedly as the river flows from predominantly undeveloped lands underlain by igneous and metamorphic rocks to agriculturally developed lands underlain by marine shale and limestone. An additional abrupt increase in dissolved uranium is observed along the section of river where discharge is greatly reduced because of extensive diversions for irrigation and where remaining flow is largely composed of irrigation return water. Water collected along this last section of river is anomalously enriched in dissolved uranium compared to the average concentration reported in water from outside of the irrigated areas. As is commonly observed, dissolved uranium shows particularly strong positive correlation with specific conductance. Uranium also shows strong positive correlations with sodium, magnesium, sulfate and chloride that contribute heavily to total dissolved solids and specific conductance in these waters. Lithium, boron, strontium and selenium are also positively correlated with uranium. The combination of natural weathering of uranium-bearing soils and sediments, extensive soil leaching by irrigation return waters, and evaporative concentration in a semiarid climate produce concentrations of dissolved uranium that may threaten local water supplies.

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