FIELD AND LABORATORY EVALUATION OF LIMESTONE DISSOLUTION FOR PASSIVE NEUTRALIZATION OF ACIDIC MINE DRAINAGE

Limestone drains were constructed in 1995, 1997, and 2000 to treat acidic mine drainage (AMD) from the Orchard, Buck Mountain, and Hegins discharges, respectively, in the Southern Anthracite Coalfield, east-central Pennsylvania. The flow rate and water quality of influent and effluent were monitored for 7 yr (through 2002), 8 yr (through present), and 5 yr (through present) at the Orchard, Buck Mountain, and Hegins limestone drains, respectively. The influent at these sites had average pH of 3.5, 4.6, and 3.5 and average net acidity of 30.3, 28.1, and 47.4 mg/L as CaCO₃, respectively. Effluent from each drain had higher average pH, alkalinity, and Ca, and lower average acidity, Fe, and Al concentrations than the influent. The long-term average flow multiplied by the difference between average concentrations of Ca for influent and effluent indicates average CaCO₃ fluxes of 1.5, 22.9, and 5.0 tonne/yr at the Orchard, Buck Mountain, and Hegins limestone drains, respectively. Nevertheless, the annual Ca and alkalinity loading rates have progressively declined with age of all three systems as the limestone has been consumed. Collapsible-container (cubitainer) testing indicated limestone dissolution rates generally were larger for conditions closed to the atmosphere than open conditions, but the rates for fresh, uncoated versus environmentally exposed, metal-hydroxide-coated limestone were comparable for a given condition. The dissolution rates as measured by cubitainer tests, after corrections for surface area and fluid volume, were in agreement with field flux data for alkalinity and dissolved Ca. Models developed on the basis of the cubitainer tests indicate (1) exponential declines in limestone mass and corresponding alkalinity loading rates with increased age of limestone drains and (2) potential for improved performance with enlargement, complete burial, and/or regular flushing of the systems.