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WWRC 95-12
Geochemical Processes and the Effects of Natural Organic Solutes on the Solubility of Selenium in Coal-Mine Backfill Samples from the Powder River Basin, Wyoming


This report describes geochemical processesc affecting the solubility of selenium and tbe effects natural organic solutes have on the solubility of selenium in surface coal-mine backfill aquifers. Three field sites selected for this study were located in reclaimed areas at two large surface coal mines in the Powder River Basin, Wyoming. These sites were selected on the basis of the known presence of selenium in ground water. Two backfill cores were collected from within an approximately 5-meter radius of each existing well site. X-ray diffraction analyses indicated that all samples contained quartz, kaolinite, potassium feldspar, illite, and muscovite. Backfill-core samples differed in elemental concentration, but were generally dominated by aluminum (14,400 to 49,000 milligrams per kilogram (mg/kg)), iron (3,330 to 23,200 mg/kg), and potassium (7,950 to 18,000 mg/kg). Backfill-core sample saturated paste extract selenium concentrations ranged from 1 to 156 micrograms per kilogram. Analytical results from the saturated-paste extracts indicated that the backfill-core samples were heterogeneous. No correlation was apparent between measured factors and depth, or selenium. Sequential partial dissolution techniques were used to partition selenium into six operationally defined phases. The organic phase accounted for less than 5 to 58 percent of the total selenium in backfill-core samples.

Water samples collected from each well were analyzed for concentrations of cations and anions. Selenium concentrations ranged from 3 to 125 micrograms per liter. Hydrogeologic monitoring is conducted on a regular basis at both mines. A general decline in selenium concentrations and concurrent increase in water levels has been observed at all sites. Dissolved organic carbon in all ground-water samples was dominated by hydrophobic and hydrophilic acids (38 to 84 percent). Hydrophobic and hydrophilic acids in the ground-water samples were isolated and concentrated for use in sorption/desorption studies to determine the effect of the acids on selenium sorption.

Three selenite sorption/desorption experiments were conducted using background solutions of distilled-deionized water, 0.1 molar calcium chloride, and isolated hydrophobic and hydrophilic acids. Selenite sorption was larger when 0.1 molar calcium chloride was used as a background solution as compared to distilled-deionized water. Selenite sorption generally was smaller in the presence of hydrophobic and hydrophilic acids, and smaller in the presence of hydrophilic acids than in the presence of hydrophobic acids.

Precipitation/dissolution studies were conducted to predict the solid phases controlling dissolved selenium concentrations and to evaluate the effect of dissolved organic carbon on selenium solubility in backfill-core samples. A geochemical speciation model was used to calculate ion activi- ties. Dissolved organic carbon had little effect on selenium speciation. Chemical speciation results suggested that backfill-core sample precipitation/ dissolution extracts were dominated by magnesium selenate ion pairs (55 to 90 percent of total selenium concentration). The presence of ion pairs may represent a potential for selenium mobility in backfill aquifers.

An oxidation-reduction (redox) controlling apparatus was constructed to control Eh and pH in water and backfill-core sample suspensions. The redox controlling apparatus successfully controlled redox of the samples. The response of selenite and selenate in water samples to the imposed redox conditions of the redox controlling apparatus differed from thermodynamic predictions. Selenite reduction in water samples occurred at a larger redox than thermodynamic predictions. Reduction of selenate in water samples did not occur at any of the redox levels tested.

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