DELINEATION OF SELENIFEROUS ROCKS IN THE GUNNISON RIVER BASIN, COLORADO BY GAMMA-RAY SPECTROMETRY

Selenium naturally exists in the Mancos Shale and derived soils, both of which are present in large areas of the Gunnison River Basin. High concentrations of dissolved selenium have been detected in the Uncompahgre and lower Gunnison Rivers. Selenium is known to be concentrated in organic black shale if a source of selenium was available during the deposition of the shale. Additionally, the natural radioelements uranium, thorium, and potassium are also concentrated in organic black shale. Volcanic ash was deposited in interior seaways during Cretaceous time and incorporated into the sediments that later formed the Mancos Shale. This volcanic ash commonly contains selenium, uranium, thorium, and potassium, and thus could be the source for concentrations in the organic-rich parts of the Mancos Shale. It is possible that the main selenium source areas in the Gunnison Basin are restricted to the more organic-rich parts of the Mancos Shale, and may also be associated with enrichment of uranium, thorium, and potassium. If this is so, the areas of exposure of these rocks, and possibly their derivative soils, can be detected and mapped using a gamma-ray spectrometry survey. This is a non-invasive process that does not require time-consuming sampling and chemical analysis. To test this hypothesis data were collected from 28 sites in the Gunnison Basin to identify selenium sources by use of a four-channel hand held gamma-ray spectrometer. In addition, bedrock samples were collected at each site and were analyzed for selenium, uranium, thorium and potassium. Comparisons of selenium and uranium analytical data show a strong linear correlation between the two elements. A similar correlation is observed between analytical data for selenium and field gamma-ray counts for uranium. These relationships indicate that the distribution of selenium in particular strata of the Mancos Shale and in soils derived from the Mancos Shale can be mapped rapidly and efficiently by using an airborne gamma radiation survey. Such detailed mapping would provide important guidelines for the implementation of best-management practices and selenium remediation in the Gunnison Basin watershed. The radioisotope/selenium relationship so demonstrated could also be applied to other high-selenium black-shale areas in the United States.