GEOCHEMICAL CONTROLS ON SOLUTE CONCENTRATIONS AT AN ACID PRODUCING SURFACE MINE FOLLOWING THE IN-SITU INJECTION OF ALKALINITY

The former DLM coal mine is located in southern Upshur County, W.V. and has been the focus of continued acid mine drainage (AMD) remediation by the WVDEP for the past twenty six years. The mine-spoil aquifer used in this study currently discharges AMD that is characteristic of pyrite oxidation and aluminosilicate mineral dissolution. Throughout the summer of 1999, Ca(OH)₂ was injected into the aquifer through an excavated trench. Baseline aqueous chemistry was sampled throughout the aquifer prior to injection but subsequent sampling did not occur until June of 2010. This provided the opportunity to analyze long term effects of in-situ alkaline injections, and geochemical processes that develop over time.

Analysis was conducted with the geochemical modeling program PHREEQC to determine the reactions along the geochemical pathway which intercepted the injection trench. This included speciation-solubility modeling during which saturation indices were calculated to assess mineral solubility. Inverse modeling was used to supply a quantitative solution of mole transfer between aqueous and mineral phases along the reaction path. The mineralogy of the aquifer was determined in a previous study, and was used to supply the primary mineral phases for the inverse model.

Inverse modeling, coupled with analytical data, showed that aluminosilicate dissolution is initially an important process in the aquifer. After the reaction path intersects the injection trench, pH is buffered by carbonate minerals and then amorphous Al(OH)₃ further down hydraulic gradient. It was concluded that injection of alkalinity in 1999 had a persistent effect throughout 2010 and modeling provided insight into the geochemical processes controlling solute concentrations.