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Passive Treatment of Acid Mine Drainage with Limestone
The water treatment performances of two anoxic limestone drains (ALDs) were evaluated. Anoxic limestone drains are buried beds of limestone that are intended to add bicarbonate alkalinity to flow-through acid mine drainage. Both ALDs received mine water contaminated with Fe2+ (216–279 mg L−1) and Mn (41–51 mg L−1). Flow through the Howe Bridge ALD increased alkalinity by an average 128 mg L−1 (CaCO3 equivalent) and Ca by 52 mg L−1, while concentrations of Fe, K, Mg, Mn, Na, and SO2−4 were unchanged. The Morrison ALD increased alkalinity by an average 248 mg L−1 and Ca by 111 mg L−1. Concentrations of K, Mg, Mn, and SO2−4 all decreased by an average 17%, an effect attributed to dilution with uncontaminated water. Iron, which decreased by 30%, was partially retained within the Morrison ALD. Calcite dissolution was enhanced at both sites by high PCO2. Untreated mine waters at the Howe Bridge and Morrison sites had average calculated PCO2 values of 6.39 kPa (10−1.20 atm) and 9.24 kPa (10−1.04 atm), respectively. At both sites, concentrations of bicarbonate alkalinity stabilized at undersaturated values (SICalcite = 10−1.2 at Howe Bridge and 10−0.8 at Morrison) after flowing through approximately half of the limestone beds. Flow through the second half of each ALD had little additional effect on mine water chemistry. At the current rates of calcite solubilization, 17.9 kg d−1 CaCO3 at Howe Bridge and 2.7 kg d−1 CaCO3 at Morrison, the ALDs have theoretical effective lifetimes in excess of 20 yr. By significantly increasing alkalinity concentrations in the mine waters, both ALDs increased metal removal in downstream constructed wetlands.