Water Quality in the South Platte River Basin, Colorado, Nebraska, and Wyoming, 1992-95

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MAJOR ISSUES AND FINDINGS: What Factors Control the Occurrence of Uranium and Radon, and Are the Reported Concentrations a Concern?

Uranium is a naturally occurring radioactive element that is present in rocks throughout the South Platte River Basin. Radon gas is a product of the radioactive decay of uranium. Both of these constituents could be a health concern in the South Platte River Basin because of the radiotoxicity effects in humans. Uranium also is a heavy metal, which can cause liver damage. Radon may be a concern in drinking water, and radon also can enter indoor air from household water use, leading to exposure through inhalation. Uranium was detected in a majority of ground-water and bed-sediment samples and in some surface-water samples during the NAWQA study (Bruce and McMahon, 1998;Heiny and Tate, 1997). Radon also was detected in all ground-water samples (Bruce and McMahon, 1998).

High concentrations of uranium and radon in the South Platte River Basin are directly related to the local geology. The local bedrock, particularly the crystalline rocks (primarily granitic) in the mountains and marine shales and coal deposits in the plains, are naturally high in uranium. Sediments derived from crystalline rocks in the mountains are transported by the streams eastward onto the plains. Uranium concentrations in bed sediments were higher in the mountain streams than in the plains streams. There was no relation between mining impacts to the streams and uranium concentration in bed sediment. The highest uranium concentration in bed sediment (25 mg/g) was detected in the North St. Vrain Creek (naturally occurring), a mountain drainage that is relatively unaffected by humans.


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Uranium concentrations in streambed sediments are highest in mountain streams.

Median uranium concentrations in shallow ground water were higher in the plains than in the mountains, indicating a secondary source of dissolved uranium in the eastern part of the basin other than the mountain-derived alluvial sediments. Preliminary evidence points to the presence of natural uranium in coal deposits and shale bedrock of marine origin that are in contact with alluvial ground water. Samples from at least 10 wells exceeded the proposed USEPA maximum contaminant level (MCL) of 20 mg/L (equal to parts per billion) in each of the three study areas.


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Uranium occurs naturally in the South Platte River Basin as a result of weathering of crystalline and some sedimentary rocks.


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The source of the elevated radon and uranium concentrations is linked to the local geology.

Median radon concentrations in shallow ground water in the forested mountains were higher than in the plains. Currently (1998), there is no standard for radon in drinking water. USEPA withdrew the previous standard of 300 pCi/L pending further review. Median radon concentrations in ground water of the basin exceeded the previous USEPA standard. Once new standards for uranium and radon are finalized (expected by the year 2001), ground water in the South Platte River Basin may require special treatment before it can be used for human consumption. Analysis of fish liver tissue from numerous points in the basin indicated that uranium does not bioaccumulate (Heiny and Tate, 1997).

Uranium and radon levels in ground-water resources of the South Platte River Basin were high compared to proposed and historical USEPA drinking-water standards. Revised standards for both constituents are pending.

U.S. Geological Survey Circular 1167

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Suggested citation:
Dennehy, K.F., Litke, D.W., Tate, C.M., Qi, S.L., McMahon, P.B., Bruce, B.W., Kimbrough, R.A., and Heiny, J.S., 1998, Water Quality in the South Platte River Basin, Colorado, Nebraska, and Wyoming, 1992-95: U.S. Geological Survey Circular 1167, on line at <URL:>, updated October 15, 1998 .

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Last modified: Tue Sep 8 10:37:17 1998