GEOCHEMISTRY AND HYDROGEOLOGY OF ARTESIAN FLOWS OF ACID MINE DRAINAGE (AMD): CLARION RIVER WATERSHED, NORTHWESTERN PENNSYLVANIA

Numerous abandoned gas wells drilled during the late 1800’s through the mid 1900’s in the Clarion River watershed are acting as conduits for the artesian flow of iron-polluted waters to the surface. These shallow wells are located near surface mines that were mined during the 1940’s through the 1970’s with little if any surface reclamation. The geochemistry of the artesian well discharges suggests that they are originating in the open pit mine spoils located at the headwaters of tributaries to the Clarion River. Surface water filters through exposed mine spoils and weathers pyrite (FeS2) producing water with low pH and elevated sulfate, acidity, and Al. The polluted water flows down through the fractured pit pavement into lower stratigraphic units and then flows laterally and down the synclinal structures through moderate to high productivity aquifers. The contaminated groundwater artesians to the surface through natural fissures or abandoned gas wells. Samples collected from twenty artesian discharges had pH values of 4.7 to 6.1, alkalinity concentrations of 26 to 110 mg/L as CaCO3, sulfate concentrations of 33 to 1254 mg/L, and iron concentrations of 10 to 215 mg/L. Aluminum concentrations were less than 1 mg/L. Most of the samples were strongly acidic due to the presence of high concentrations of Fe. The change in chemistry between the surface mines and the artesian discharges is attributed to reaction with iron carbonate minerals such as siderite. During the course of our investigation, one of our study areas became the focus of well plugging activities. The plugging of abandoned oil/gas wells has become a remediation tool for AMD-polluted watersheds in northwestern PA. However, hydrologic conditions are rarely considered before well plugging efforts. The effect of plugging efforts on eight artesian flowing gas wells was monitored. Four wells were successfully plugged. Flow shifted rapidly from plugged wells to unplugged wells. The rapid response suggests that the wells drain a shallow contaminated aquifer. Because of the flow transfer, the plugging program had minimal remedial effect on the targeted watershed. Fortunately, the chemistry of the artesian flows is ideally suited for reliable passive treatment.