Modeling Release of Chemical Constituents in Surface Mine Runoff

Abstract

ABSTRACT THE dominant soluble inorganic compounds in geologic strata spoiled during surface mining of coal are gypsum CaS04-2H20) and magnesium sulfate (MgS04-nH20). The concentration of the soluble constituents in the runoff water or water stored in ponds are a function of the ratio of volume of water to weight of sediments, the thermodynamic interactions between solution ions and clay colloids, and the thermodynamic interactions between the various ions in the solution (water). The above consider the fact that water infiltration in many spoils is generally negligible. A computer model was developed based on thermodynamics of certain chemical reactions. The reactions include: (a) gypsum and magnesium sulfate solubility in water; (b) stability constants of the ion pairs calcium sulfate (CaSOJ), magnesium sulfate (MgSOS), potassium sulfate (KSO^, sodium sulfate (NaSO^, bisulfate (HSO^, ammonium sulfate (NH4SO~4), calcium chloride (CaCl+), and magnesium chloride (MgCl^); and (c) the exchange selectivity coefficients between solution ions and clay colloids for the Ca-Mg and Z-(CaH-Mg) systems, where Z deonotes any monovalent cation. The model was tested with data from the literature and dissolved constituent released into runoff water from coal spoils during simulated rainfall. The values predicted by the computer model correlated well with the experimental values. This project demonstrated that modeling water quality based on solution ion interactions, mineral precipitation-dissolution, and ion exchange has great potential for predictive purposes. This is especially true when multi-ion interactions are involved, which is the rule rather than the exception for natural water systems. Strength, weaknesses, and limitations of this particular model were also discussed.