Evaluation of Limestone Treatment of Acidic Mine Drainage in Swatara Creek Basin, Schuylkill County, Pennsylvania
ProblemAcidic mine drainage (AMD)from abandoned anthracite mines has degraded water resources in the 48 mi2 (square mile) northern Swatara Creek Basin. To neutralize the AMD, with a goal of remediating approximately 25 mi (67 percent) of degraded streams in the basin, a variety of low-maintenance or low-cost limestone treatment systems have been constructed (figure 1). Most of the limestone treatment systems were installed during fall 1996 and spring 1997. The treatments, which include limestone-sand dosing, open limestone channels, anoxic and oxic limestone drains, and limestone diversion wells, were constructed by the Schuylkill County Conservation District and the Swatara Creek Watershed Association, with technical assistance from the USGS and the PaDEP. Each treatment has different advantages and disadvantages; however, all suffer from uncertainties about efficiency and longevity of the treatment.
To determine the effectiveness of the various limestone treatments, the USGS has established flow and water-quality monitoring stations upstream and downstream of each treatment. To evaluate cumulative effects of AMD remediation throughout the northern Swatara Creek Basin, the USGS also is monitoring flow, water quality, and aquatic biology at key downstream locations on the mainstem of Swatara Creek.
CollaboratorsUSGS is working with the PaDEP Bureau of Mining and Reclamation and the Schuylkill County Conservation District, with matching funds from the USGS Federal-State Cooperative Program.
ActivitiesSince 1996, data on discharge or streamflow rate and water quality at 40 stations in the Swatara Creek basin have been collected periodically during base-flow and high-flow conditions to characterize untreated mine drainage, treatment-system performance, and cumulative downstream effects. Water samples routinely are analyzed for temperature, specific conductance (SC), dissolved oxygen, redox potential (Eh), pH, alkalinity, acidity, dissolved and total metals, sulfate and other major ions, nutrients, and suspended sediment. In addition, streamflow, temperature, SC, and pH are being monitored continuously at three streamgages on Swatara Creek. Two of these gages, at Newtown and Ravine, were installed in the spring-summer 1996. Aquatic biological surveys are being conducted annually during base-flow conditions at selected reaches on Swatara Creek, including the stations at Newtown and Ravine. The water-quality and biological data for the station at Ravine indicate cumulative effects of AMD remediation throughout the northern Swatara Creek Basin.
Limestone sand dosing and open limestone channels (figure 2) are the simplest treatment systems where limestone fragments are added directly to the stream channel semiannually or less frequently. Slow dissolution rates, armoring, burial, and transport of limestone from the channel during high flow are concerns. Limestone sand, which can dissolve rapidly because of its small size (<1/8 inch), was dumped into Coal Run (14 tons) between stations C4 and C6 on September 4, 1996, and into Lorberry Creek (150 tons) below station E2 on February 13-14, 1997. An open limestone channel was constructed within a 110-ft long segment of Swatara Creek at station B2 on March 21, 1997. A total of 44 tons of sand-size fragments and 70 tons of larger fragments (1-4 inches) were installed as a series of alternating berms extending part way across the 15-ft-wide channel from opposite sides of the stream.
A limestone drain (figure 3) is another relatively simple treatment method, which involves the burial of limestone in air-tight trenches that intercept acidic discharge water. Keeping carbon dioxide within the drain can enhance limestone dissolution and alkalinity production. Furthermore, keeping oxygen out of contact with the discharge water minimizes the potential for oxidation of dissolved iron and the consequent precipitation of solid iron hydroxide [Fe(OH)3], which could armor the limestone and clog the drains. Limestone drains were constructed on March 15, 1995, at station E3 to treat a small acidic discharge (10-30 gpm, oxic inflow; 44 tons limestone) along Lower Rausch Creek and on May 21, 1997, at station A1 to treat a larger discharge (50-200 gpm, anoxic inflow; 400 tons limestone) at the headwaters of Swatara Creek. The larger system was designed on the basis of results for the smaller system where pH increased from 3.5 to 6.5 through the drain during <3-hour residence time.
In a limestone diversion well (figure4), acidic water is diverted from upstream points and the hydraulic force of the piped flow is deflected upward through limestone fragments inside 4-ft diameter "wells." Hydraulic churning abrades limestone forming fine particles and preventing the buildup of hydroxide armoring. On November 14, 1995, a pair of diversion wells were installed to treat water diverted from Swatara Creek at station C2, and on July 13, 1997, a single diversion well was installed to treat water from Martin Run at station C8. Approximately 1 ton of limestone is consumed weekly by each operating diversion well. Diversion wells can be installed in series to treat large flows, but must be maintained by frequent refilling with fresh limestone.
Changes in water quality and effects of precipitated metal-hydroxide coatings on limestone dissolution rates will be evaluated and compared among the different limestone treatments in the basin to draw conclusions about critical factors affecting system performance. Hydrological, chemical, and biological data collected at the stations on Swatara Creek will be evaluated to determine the cumulative effects of AMD cleanup on downstream water quality and ecosystem recovery. An interpretive report on the performance of the limestone systems for AMD treatment and an interim report on the cumulative effects of AMD cleanup on downstream water quality will be written for the project during 1998-99. In spring-summer 1996, two streamflow stations were installed for continuous streamflow and water-quality monitoring and background data on streamflow and water quality at another 17 stations were collected during base-flow and high-flow conditions. In fall 1996-spring 1997, most of the limestone treatment systems were installed. System sizing was based on flow rates and chemistry determined by background monitoring and preliminary field trials.
Project ChiefChuck Cravotta