ACIDIC-MINE-DRAINAGE PROJECTS IN PENNSYLVANIA
Submitted by Chuck Cravotta

PA237: Limestone Drains to Increase pH and Remove Dissolved
Metals from an Acidic Coal-Mine Discharge in the Swatara Creek Watershed

Problem: AMD from abandoned anthracite mines has degraded surface-water and ground-water resources in the Swatara Creek Basin (Susquehanna River Basin) in Schuylkill and Lebanon Counties, Pa. Enclosed drains filled with crushed limestone are low-cost systems that can be used to neutralize AMD. However, the chemistry of mine drainage in the basin is variable, and geochemical processes within limestone drains are poorly understood. Effects of iron and aluminum hydrolysis on limestone dissolution are critical. Precipitation of metal hydroxides can "armor" the limestone, effectively reducing the limestone surface area and hence the rate of alkalinity production. In contrast, H+ and CO2, which are hydrolysis products, can dissolve limestone, possibly countering effects of armoring, and trace metals may be removed from solution by sorption onto the metal hydroxides. An evaluation of the factors affecting chemical reactions within limestone drains is needed to resolve uncertainties about the optimum design of these treatment systems.

Collaborators: USGS-WRD is working with the Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation, with matching funds from the USGS Federal-State Cooperative Program.

Activities: In summer and fall 1994, water quality and flow rates were measured at several discharges from abandoned anthracite mines in the Swatara Creek Basin to assess the spatial variability of drainage quality and to select a site for construction of a pilot-scale treatment system. In winter 1995, at the collapsed opening to the Orchard Mine, three identical limestone drains, each containing 14 tons of crushed limestone, were constructed in parallel; access points in the drains enabled water, gas, and rock sampling. At the mine opening, acidic drainage (pH = 3.5; acidity = 80 mg/L as CaCO3) with low concentrations of dissolved oxygen (< 3.5 mg/L) was intercepted and diverted into the three drains. A static mixer and plumbing valves at the inflow enable aeration, deaeration (N2 sparging), or no pretreatment of the inflow to all three or to only one of the drains. Water, rock, and gas samples are being analyzed to explain changes in water chemistry as a result of dissolution, precipitation, and sorption reactions under closed-system conditions, and differences in chemical reactions among the three drains as a function of inflow rate and redox state. Microbiological factors affecting the rates of iron oxidation and hydrolysis and limestone dissolution also are being evaluated.

During March-July 1995, inflow to all three drains received the same pretreatment with N2 stripping to minimize concentrations of dissolved O2. As water flowed through the drains, pH and concentrations of alkalinity and calcium increased, concentrations of acidity and of dissolved and suspended iron and aluminum decreased, and concentrations of sulfate, magnesium, manganese, and trace metals did not change. Water at the inflow and outflow of the drains had pH 3.5 and 6.5-7.0, respectively. Dissolved iron and aluminum precipitated and settled in the drains as pH increased to about 5.5 within about the first 10 to 20 ft (feet) in the drains; however, dissolved trace metals passed through the 80-ft-long drain system. Limestone slabs, which were suspended at downflow points within the drains, show effects of dissolution and loosely bound accumulations of iron and aluminum hydroxide, particularly near the inflow to the drains where water quality changes rapidly.

During August 1995-March 1996, flow rates and redox state will be altered to evaluate their effects on chemical reactions in the drains. By increasing flow rates, metal hydroxide particulates may be suspended into the downflow part of the drain where pH is higher and where sorption of trace metals may be more effective than in the upflow part of the drain. By altering redox state, solubility of iron and effects of iron hydrolysis can be varied. Coupled geochemical reaction and transport models will be used to evaluate the data.

With involvement from Norrie Robbins and Gordon Nord (USGS-Geologic Division (GD), Red Slime Team), the iron and aluminum precipitate on limestone will be examined for chemical, mineralogical, and biological characteristics. The pH of fluid near the surface of the limestone is probably much greater than that of the bulk water. Tests will be conducted to determine if particular species of bacteria are promoting the removal of iron and aluminum from solution and if the microbial populations differ with respect to the bulk water pH. Glass slides, carbonate thin sections, and additional limestone slabs will be suspended in the drains to provide different substrates for precipitation of iron and aluminum hydroxides. The pH within the iron- and aluminum-hydroxide layer will be measured with microelectrodes, and bacteria associated with the precipitate will be identified to evaluate if the limestone surface provides a near-neutral microenvironment for bacterial growth and to determine any differences in the precipitation of iron and colonization by bacteria associated with the pH of the bulk solution or microenvironment.

Project Chief: Chuck Cravotta (tel.: 717-730-6963).