Assessment of characteristics and remedial alternatives for abandoned mine drainage : case study at Staple Bend Tunnel unit of Allegheny Portage Railroad National Historic Site, Cambria County, Pennsylvania, 2004

Abstract

This report describes field, laboratory, and computational methods that could be used to assess remedial strategies for abandoned mine drainage (AMD). During April-June, 2004, the assessment process was applied to AMD from bituminous coal deposits at a test site in the Staple Bend Tunnel Unit of Allegheny Portage Railroad National Historic Site (ALPO-SBTU) in Cambria County, Pennsylvania. The purpose of this study was (1) to characterize the AMD quantity and quality within the ALPO-SBTU test site; (2) to evaluate the efficacy of limestone or steel slag for neutralization of the AMD on the basis of reaction-rate measurements; and (3) to identify possible alternatives for passive or active treatment of the AMD. The data from this case study ultimately will be used by the National Park Service (NPS) to develop a site remediation plan. The approach used in this study could be applicable at other sites subject to drainage from abandoned coal or metal mines. During April 2004, AMD from 9 sources (sites1, 1Fe, Fe, 2, 3, 3B, 5, 6, and 7) at the ALPO-SBTU test site had a combined flow rate of 1,420 gallons per minute (gal/min) and flow-weighted averages for pH of 3.3, net acidity of 55 milligrams per liter (mg/L) as CaCO3, and concentrations of dissolved sulfate, aluminum, iron, and manganese of 694 mg/L, 4.4 mg/L, 0.74 mg/L, and 1.2 mg/L, respectively. These pH, net acidity, sulfate, and aluminum values exceed effluent criteria for active mines in Pennsylvania. During April-June 2004, limestone and steel slag that were locally available were tested in the laboratory for their composition, approximate surface area, and potential to neutralize samples of the AMD. Although the substrates had a similar particle-size distribution and identical calcium content (43 percent as calcium oxide), the limestone was composed of crystalline carbonates and the slag was composed of silicate glass and minerals. After a minimum of 8 hours contact between the AMD and limestone or steel slag in closed containers (cubitainers), near-neutral effluent was produced. With prolonged contact between the AMD and limestone or steel slag, the concentrations of iron, aluminum, and most dissolved trace elements in effluent from the cubitainers declined while pH was maintained greater than 6.0 and less than 9.0. The cubitainer testing demonstrated (1) lower alkalinity production but higher pH of AMD treated with steel slag compared to limestone, and (2) predictable relations between the effluent quality, detention time, and corresponding flow rate and bulk volume for a bed of crushed limestone or steel slag in an AMD passive-treatment system. The process for evaluating AMD remedial strategies at the ALPO-SBTU test site involved the computation and ranking of the metal loadings during April 2004 for each of the AMD sources and a comparison of the data on AMD flow and chemistry (alkalinity, acidity, dissolved oxygen, ferric iron, aluminum) with published criteria for selection of passive-treatment technology. Although neutralization of the AMD by reaction with limestone was demonstrated with cubitainer tests, an anoxic limestone drain (ALD) was indicated as inappropriate for any AMD source at the test site because all had excessive concentrations of dissolved oxygen and (or) aluminum. One passive-treatment scenario that was identified for the individual or combined AMD sources involved an open limestone channel (OLC) to collect the AMD source(s), a vertical flow compost wetland (VFCW) to add alkalinity, and an aerobic wetland to facilitate iron and manganese oxidation and retention of precipitated solids. Innovative passive-system designs that direct flow upward through submerged layers of limestone and/or steel slag and that incorporate siphons for automatic flushing of solids to a pond also may warrant consideration. Alternatively, an active-treatment system with a hydraulic-powered lime doser could be employed instead of the VFCW or upflow system. Now, given these data on AMD flow and chemistry and identified remedial technologies, a resource manager can use a publicly available computer program such as "AMDTreat" to evaluate the potential sizes and costs of various remedial alternatives.