REMEDIATION OF ACID ROCK DRAINAGE FROM HIGHWAY CONSTRUCTION IN THE MARCELLUS SHALE, MIFFLIN COUNTY, PENNSYLVANIA

The PA Department of Transportation (PennDOT) excavated into the Marcellus Shale during the construction of the U.S. Route 22 bypass around Lewistown, PA. In the summer of 2004, acidic drainage was noted with water quality typical of severe acid coal mine drainage - acidity in the 1,600 to 2,900 mg/l range, 2,300 to 3,300 mg/l sulfate, and 676 to 976 mg/l iron. PennDOT subsequently began chemical treatment of the acidic drainage to prevent any further environmental drainage. The Marcellus Shale had an average sulfur content of 3.06% with up to 17% sulfur in the associated Tioga Bentonite beds.

This event is unusual because of the relative rarity of acid drainage from Devonian-age rocks as well as the mechanism for formation of the drainage. Most acid rock drainage (ARD) is formed when water flows through rock fills composed of pyritic rock. In this project, the fill was largely composed of limestone and did not create low pH drainage. Rather, the ARD was formed where the road cut exposed the Marcellus Shale. The easily weathered shale facilitates penetration of air and water into the unexcavated rock. Pyrite contained in the shale oxidizes and accumulates acidic sulfate salts that quickly dissolve during precipitation events, producing rapid flushes of acidic, highly mineralized water.

PennDOT and the PA Department of Environmental Protection (DEP) collaborated on a strategy to abate the acid rock drainage. The approach was to excavate as much of the Marcellus Shale as possible and to mix it with 400 lbs/yd3 of high-calcium lime to slow pyrite oxidation and neutralize acidity, similar to a technique used to prevent acidic drainage at surface coal mines. A slope failure required the removal of approximately 750,000 yards of rock, of which approximately 40,000 yards was Marcellus Shale. The Marcelllus was mixed with waste lime, and placed in a fill area enveloped with limestone waste rock. The remaining Marcellus exposed in the roadcut was coated with PennzSuppress, a paraffin derivative, to impede air and water entry, and will be covered with topsoil and planted. Horizontal borings into the underlying Onondaga and Old Port Formations will intercept and drain groundwater, preventing it from contacting the Marcellus.