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Limestone Drains to Increase pH and Remove Dissolved Metals from an Acidic Coal-Mine Discharge in the Swatara Creek Watershed

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Water and rock samples were collected during March 1995-March 1996 and analyzed to explain changes in water chemistry as a result of dissolution, precipitation, and sorption reactions within the drains, and differences in chemical reactions among the three drains as a function of inflow rate and oxygenation. As water flowed through the drains, concentrations of O₂, sulfate, and magnesium did not change; pH and concentrations of alkalinity and calcium increased; and concentrations of acidity and of dissolved and suspended iron and aluminum decreased. During the 1-year monitoring period, the treated effluent, which had pH 6.2-7.0 and alkalinity 55-136 mg/L as CaCO₃, appeared clear and contained less than 5% of the influent concentrations of dissolved Fe and Al. Rust-colored Fe and Al hydroxides precipitated in the drains as pH increased to about 5.5 within about the first 10 to 20 ft (feet). Initially, rusty floc was visible only in water samples 5 ft from the inflow (pH <5), and 90-100% of the dissolved manganese, cobalt, nickel, and zinc (Mn, Co, Ni, and Zn) was transported through the 80-ft long drains. After 6 months, however, the rusty floc was visible in all samples <20 ft from the inflow (pH <5.5), and concentrations of Mn, Co, Ni, and Zn in effluent declined to about 50% of influent concentrations because of sorption of the metals on hydrous oxides of Fe, Al, and Mn that had accumulated within the drains. Concentrations of trace metals (Mn, Cu, Ni, Zn) relative to iron as coatings on suspended limestone or particulate indicated that sorption of the metals was more effective at higher pH (>5) in downflow parts of the drain than at lower pH (<4) in the upflow part of the drain. The trends were the same for all three drains, despite periodic O₂ pretreatment of drain 1, only.

Limestone slabs, which were suspended at the inflow and at downflow points within the drains, showed effects of dissolution and loosely bound accumulations of iron and aluminum hydroxide, particularly near the inflow to the drains where water quality changed rapidly. Despite thick (<0.04 inch) accumulations of iron hydroxide (armoring) on limestone at the inflow to the drain, limestone dissolution rates were greater at the inflow than at downflow points within the drain where such accumulations were negligible. The limestone dissolution rates decreased with increasing pH and decreasing partial pressure of CO₂, which is consistent with rate laws established for calcite dissolution in laboratory experiments. However, the rates estimated for the field experiments were about an order of magnitude less than rates determined by laboratory methods.

Results of the project were published as an invited paper, "Oxic limestone drains for treatment of dilute, acidic mine drainage" (Cravotta, 1998, in Proceedings of the 19th Annual Meeting of the Surface Mine Drainage Task Force, Morgantown, WV). Additional findings were included in the paper, "Hydrobiogeochemical interactions on calcite and gypsum in 'anoxic' limestone drains in West Virginia and Pennsylvania" (Robbins and others, 1997, in International Ash Utilization Symposium, University of Kentucky, Lexington, Ky.) The final comprehensive report, "Limestone drains to increase pH and remove dissolved metals from acidic mine drainage," has been accepted for publication in the journal Applied Geochemistry.

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