Passive Treatment of Acid Mine Drainage with Limestone Robert S. Hedin, George R. Watzlaf,* and Robert W. Nairn ABSTRACT The water treatment performancesof two anoxic limestone drains (ALDs)were evaluated. Anoxic limestone drains are buried beds limestone that are intended to add bicarbonate alkalinity to flow- through acid mine drainage. Both ALDs received mine water contami- nated with Fe 2+ (216-279 mg -I) and Mn ( 41-51 m g L -~). F low through the Howe Bridge ALD increased alkalinity by an average 128 mgL- ~ (CaCO3 equivalent) and Ca by 52 mgL - i, while concentrations of Fe, K, Mg, Mn, Na, and SO~- were unchanged. The Morrison ALD increased alkalinity by an average 248 mg L -I and Ca by III mg L-~. Concentrations of K, Mg, Mn, and SO~- 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 Pco~ values of 6.39 kPa (I0-1.z0 atm) and 9.24 kPa (I0 -I’°~ atm), respectively. At both sites, concentrations of bicarbonate alkalinity stabilized at undersaturated values (SIc~k~t~ I0 -I"~ at Howe Bridge and I0 -°’s 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-i CaCO3 at Howe Bridge and 2.7 kg d-i CaCO3at Morrison, the ALDs have theoretical effective lifetimes in excessof 20 yr. By significantly increas- ing alkalinity concentrations in the mine waters, both ALDs increased metal removal in downstream constructed wetlands. C OAL MINE DRAINAGES in the eastern USA are com- monly contaminated with dissolved Fe and Mn. The treatment of these polluted waters requires that the metal contaminants be precipitated and that the acidity be neutralized. In conventional mine drainage treatment systems, the water is treated with additions of highly alkaline chemicals such as NaOH, Ca(OH)2, CaO, Na2CO3, or NH3 (Skousen et al., 1990). These reagents promote metal removal reactions and neutralize the acid- ity, but are expensive, potentially dangerous, and when misused can result in the discharge of excessively alkaline water. Alternative passive processes that do not require regular chemical additions have attracted interest from mining, reclamation, and research groups (Brooks et al., 1985; U.S. Bureau of Mines, 1988; Hammer, 1989; Moshiri, 1993). Wetlandshave received particular atten- tion because of their tendency to decrease concentrations of Fe and Mnwhenthe mine water contains bicarbonate alkalinity (Brodie, 1990; Hedin and Nairn, 1990, 1993). The circumneutral environment that is associated with bicarbonate ion (pH 4.5-8.3) promotes Fe and Mn oxida- tion processes that are rapid enough to makethe wetland R.S. Hedin and G.R. Watzlaf, U.S. Bureau of Mines, Environ. Technol. Section, P.O. Box 18070, Pittsburgh, PA15236; and R.W. Nairn, U.S. Bureauof Mines, Environ. Technol. Section, P.O. Box 18070, Pittsburgh, PA 15236(current address: OhioState Univ., School of Natural Resources, Columbus, OH 43201). Received 12 Oct. 1994. *Corresponding author (watzlagr@ptbma.usbm.gov). Published in J. Environ. Qual. 23:1338-1345(1994). approach cost-effective for mine drainage treatment. The bicarbonate ion also acts as a buffer to neutralize the proton acidity released when these metals hydrolyze (Hedin et al., 1994). When mine water naturally contains sufficient alkalin- ity to offset the mineral acidity associated with dissolved Fe and Mn, passive treatment with a properly sized constructed wetland is sufficient. When mine water is acidic, successful passive treatment requires that alkalin- ity be added to the water. The most inexpensive alkaline source in coal mining regions is limestone (Table 1). the past, however, limestone has been rarely utilized in either passive or chemical mine water treatment systems, because it has a low solubility under atmospheric condi- tions, and it tends to become armoredwith ferric hydrox- ide (Wentzler and Aplan, 1972; USEPA, 1983). Recently, Turner and McCoy (1990) proposed that limestone could be used in passive mine water treatment systems if the Fe was in the ferrous (Fe 2÷) form and the mine water contacted the limestone in an anoxic environment. Under this condition, no armoring of the limestone should occur. Turner and McCoy (1990) de- scribed two systems in which acidic mine waters contami- nated with Fe and Mnwere rerouted through buried, limestone-filled trenches. Both trenches had alkaline dis- charges that were then directed into wetlands where metal contaminants precipitated. During the past 3 yr, dozens of limestone treatment systems, similar in concept to those originally described by Turner and McCoy (1990), have been constructed the Appalachian coal field. Because of the emphasis on anoxic conditions and the similarity of the manysystems to field drains, the systems have become known as anoxic limestone drains or ALDs. Surveys of the effluent chem- istry of ALDs have documented that many systems suc- cessfully generate alkalinity; however, large variation exists in the amount of alkalinity generated and in the effects that ALDs have on metal concentrations (Brodie et al., 1991; Faulkner and Skousen, 1993; Hedin and Watzlaf, 1994). The causes of variability in alkalinity generation and metal retention are unresolved. This paper reports detailed analyses of the perfor- mances of two ALDs constructed to treat mine water contaminated with ferrous Fe and Mn. Water quality at both sites was assessed at the ALD effluents and at walls placed within the two limestone beds. The water quality data collected during the 18- to 30-mo monitoring pro- grams provide the most detailed analyses of ALD perfor- mance yet presented. The results are used to identify factors of the ALD construction and mine water chemis- try that effect the generation of alkalinity, and the reten- tion of metals at the two sites. Abbreviations: ALD, anoxic limestone drains; PVC, polyvinyl chloride; ICP, inductively coupled argon plasma spectroscopy; CV,coefficient of variation; SI, saturation index; ISE, ion-sensitive electrode; DO, dissolved oxygen; IAP, ion activity product. 1338