Ability of subsoils to buffer extremely acidic simulated coal-pile leachates

Coal piles stored directly on soil surfaces may pose a risk of ground-water contamination, particularly when extremely acidic coal-pile leachates, which contain elevated concentrations of heavy metals, are generated. Soil-column experiments were run to evaluate the ability of a sand, a silt loam, and a silt loam amended with 4% CaCO3 to buffer dilute H2SO4 solutions (pH 3.5 and 1.5) and a synthetic coal-pile leachate (a pH 1.5 H2SO4 solution containing metal sulfate salts). Both silt-loam soils were able to buffer the pH 3.5 H2SO4 solution for the entire leaching period (more than 50 pore volumes), but the sand was not. The buffering capacities of all three soils were insufficient to neutralize pH 1.5 acid and synthetic coal-pile leachates. The sand exhibited the least buffering, and the silt loam at a low flow rate and the silt loam amended with CaCO3 showed the greatest buffering. The buffering capacities of both silt loam soils were exhausted more rapidly when leached with synthetic coal-pile leachate rather than pH 1.5 H2SO4 solution. This suggests that soil-buffering studies using pure acid solutions may underpredict contamination due to coal-pile leachates. The results indicate that there is a potential for ground-water contamination due to coal-pile storage, particularly where leachates are acidic ad coarse textured soils exist. Coal storage practices should be adopted that protect soils and ground water from acidification and metals contamination