For a private client involved in litigation, a model capable of predicting changes in water chemistry with time in an abandoned mine was developed by SSP&A. The purpose for the modeling task was to evaluate options for treatment of contaminated surface water and for partitioning of financial responsibility among the PRPs. The geochemical model involved two steps. First, the inflow of chemically-distinct water from various sources was modeled as a time-dependent mixing process using both PHREEQE and inflow data derived from a groundwater flow and transport model. The model allowed acid sulfate water to accumulate and to react with aluminosilicate minerals in the walls and floor of the mine and allowed for secondary minerals to form. Second, to simulate slow weathering reactions which remove protons from the water by silicate hydrolysis, the code MPATH was used. MPATH allows a kinetic treatment of dissolution and precipitation reactions in water-rock systems and thermodynamic treatment of complexation, ion association, and aqueous redox reactions. Because of the kinetic formulation of MPATH, slow processes can be modeled explicitly as a function of time. For example, simulations to 1000 years initially were made to determine the time scale of the weathering process. It was found that the time scale for neutralization of the acidity of the surface water and concomitant removal of heavy metals from the water could range from several decades to a few hundred years depending on the rate of deposition of windblown dust particles to the water surface.
This project is currently ongoing.