Long-term performance of anoxic limestone drains

Ten anoxic limestone drains (ALDs), buried beds of limestone gravel that add alkalinity through dissolution of calcite, have been monitored for a decade. Influent and effluent water quality data have been analyzed to determine the long-term performance of each ALD for treating coal mine drainage. Alkalinity concentrations in the effluent of the ten ALDs ranged from 80–320 mg/L as CaCO3 with near maximum levels of alkalinity being reached after approximately 15 hours of detention. ALDs that received mine water containing less than 1 mg/L of both ferric iron and aluminum have continued to produce consistent concentrations of alkalinity since their construction. However, an ALD that received 21 mg/L of aluminum experienced a rapid reduction in permeability and failed within eight months.