GEOCHEMISTRY OF LABORATORY ANOXIC LIMESTONE DRAINS

Abstract

An anoxic limestone drain (ALD) is a passive treatment system for treating acid mine drainage (AMD). Historically it has been thought that AMD containing Fe{sup 3+} and Al{sup 3+} severely inhibits or stops limestone dissolution due to coating of limestone surfaces by precipitates generated during the neutralization process. Limestone dissolution in field ALDs is difficult to quantify because sampling of the water in ALDs at various locations is not possible, and fluctuations in flow and water chemistry affect limestone dissolution rates. Laboratory experiments were developed to determine the effects of Fe{sup 3+} and Al{sup 3+} precipitation on limestone dissolution and the controlling precipitation reactions. Synthetic AMD containing Fe{sup 3+} or Al{sup 3+} with and without sulfate was pumped through limestone-packed columns constructed with three sampling ports at equidistant intervals along the column. Water and sediments were periodically extracted for analysis at all sampling ports over a 12-hr period. Results show the majority of limestone dissolution occurred within the first 1.2 hrs of water-limestone contact. Limestone dissolution rate decreased with time and distance along the flow path. Higher concentrations of Fe{sup 3+} and Al{sup 3+} (increased in mineral acidity and ionic strength) enhanced limestone dissolution. Geochemical modeling predicted that solutions weremore » nearest equilibrium with respect to the amorphic metal hydroxide phases. Although solutions were periodically oversaturated with respect to sulfate containing minerals, but no x-ray identifiable sulfate minerals were found in the solid phase. The data suggest that smaller anoxic limestone drains may be used when the goal is to neutralize mineral acidity, thus reducing spatial requirements. However, if the goal is to treat AMD to NPDES limits, ALDs may not be a viable long term treatment alternative.« less