Abstract
The Tangascootack #1 passive treatment system was constructed in 1998 to treat an acid discharge from an abandoned surface mine in Clinton Co., PA. The discharge averaged about 155 L/min with pH 4.0, acidity 235 mg/L CaCO3, Fe 3.7 mg/L, Al 24 mg/L and Mn 68 mg/L. The vertical flow pond had dimensions of 48 x 20 m and was preceded by a wetland and followed by an oxidation/settling pond. The system produced net alkaline effluent for a few months, but by 1 year after construction, the effluent acidity was 120 mg/L, and flows decreased to 20-60 L/min. Plugging by Al precipitate was suspected, despite occasional flushing. In 2003, investigation showed partial plugging by cattail roots in the effluent pipe, but after removal of the roots, head loss across the VFP was less than 2 cm at flows of about 20 L/min, indicating that plugging was not severe. The VFP was drained and the compost and limestone (2.5 cm dia..) were excavated along several trenches. Compost, originally designed as 30 cm thick, varied markedly from 8 to 25 cm thick. In thick zones, basal compost was coated by black Fe sulfides. The limestone was coated nearly everywhere for at least 25 cm beneath the compost with gelatinous white Al-bearing precipitate mixed with quartz-rich silt. A layer of gypsum beneath the Al precipitate covered the limestone in places. An open joint in the underdrain may have somewhat concentrated flow. The limestone layer retained considerable pore space and permeability. In spite of the flushing and open joint, Al coating was approximately the same over underdrain pipes as elsewhere. Also, the Al coating was present nearly everywhere, under both thick and thin compost, indicating that channeling was minimal. Measurements in 1999 and 2003 show that the system was removing acidity at reasonable rates of 45 to 60 g/m 2 /day.
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