AN INVESTIGATION OF THE SELENIUM CONCENTRATION IN THENARDITE EFFLORESCENCES ON MANCOS SHALE, WESTERN COLORADO

Selenium is a pollutant of ground and surface waters in areas underlain by the Mancos Shale in western Colorado. The Mancos Shale is a thick, Cretaceous-age marine shale that is exposed over 1,100 square miles in Montrose, Delta, and Mesa Counties. A white efflorescence, commonly called "alkali" by local residents, occurs sporadically and frequently appears in areas underlain by the lower third of the shale. This portion of the shale can contain high concentrations of selenium and is a source of selenium pollutants. Disturbed soils and areas receiving seepage from irrigation canals are common efflorescence sites. A limited amount of previous work indicated that the efflorescence is largely sodium sulfate. Because of the similarity between the selenate ion (SeO₄^-2) and the sulfate ion (SO₄^-2), we hypothesized that the white efflorescence might contain selenium.

We investigated the 106 samples of the white efflorescence collected in Montrose, Delta, and Mesa Counties, Colorado, using x-ray diffraction to determine the mineralogy and x-ray fluorescence to determine the selenium content. We purified the efflorescence prior to analysis by dissolving it in water, filtering out the particulate impurities, and evaporating it to dryness prior to analysis. We also investigated the selenium concentration of two wells that penetrated the Mancos Shale in Mesa County.

Mineralogically, the efflorescence is predominantly thenardite (Na₂SO₄), although small amounts of gypsum and bloedite (Na₂Mg(SO₄)₂.4H₂O) were also found. The samples varied considerably in selenium concentration: 22 percent contained less than 0.5 ppm Se, 39 percent contained between 0.5 and 5 ppm, 25 percent contained between 5 and 50 ppm Se, and 14 percent contained over 50 ppm Se with the highest value being 416 ppm Se. Our data suggests that the purest thenardite, which consists of a thick powdery accumulation above the soil, contains little Se compared to material that is intermixed with the surface soil itself. In comparison, the concentration of Se in the well cuttings ranged from 0 to 17 ppm. This suggests that selenium can concentrate in many of the areas where the efflorescence occurs and that these sites could be targets for remediation efforts.