Abstract Ground-water-quality changes from premining to postmining conditions at the Cordero coal mine were studied by geochemical-reaction-path modeling to explain the increased dissolved-solids concentrations in postmining ground water. Recent saturation of backfilled spoils has provided a source for data on spoil-water quality within the State. U.S. Geological Survey data collected from the Cordero coal mine were used to augment existing spoil and premining water-quality data previously collected by Cordero coal mine personnel. Overburden samples from the Cordero coal mine also were collected for quantitative mineralogical, batch-test, and chemical analyses.
Increased dissolved-solids concentration in ground water from premining to postmining conditions at the Cordero coal mine include the following geochemical observations: (1) Concentrations of calcium, sodium, magnesium, and sulfate were larger in postmining ground-water samples from coal-mine spoil than in premining ground water samples from the coal aquifer; and (2) calcite, dolomite, gypsum, and chalcedony are very close to equilibrium with the postmining ground water at the Cordero coal mine. Actual saturation-index values (log-transformed, unitless) calculated for the postmining ground water at the Cordero coal mine are 0.097 for calcite, 0.098 for dolomite, 0.117 for gypsum, and 0.065 for chalcedony. A saturation index is defined as the activity product of the dissolved species for a particular mineral divided by the equilibrium activity product for the same mineral. Therefore, a saturation index greater than zero would imply precipitation of a solid phase; whereas, a saturation index less than zero would imply dissolution of the solid phase.
Using geochemical-reaction-path modeling, measured changes in water quality from premining to postmining conditions at the Cordero coal mine were simulated. Equilibrium with respect to dolomite and calcite was maintained while the dissolution of gypsum was simulated until the postmining sulfate concentration measured in the sample collected from the Cordero coal mine spoil water was simulated. Measured versus calculated concentrations for calcium, sulfate, and magnesium in the Cordero coal mine spoil water then agreed within +/- 2 millimoles per liter or less. Evaluation of the batch-test leachate analyses and overburden mineralogy from the Cordero coal mine without regard to dissolution kinetics or limited dissolved oxygen, indicate that further degradation of the postmining ground water quality will not occur unless significant quantities of acid-forming material are contacted by the postmining ground waters.
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