THE IMPACT OF ACID MINE AND SEWAGE DISCHARGE ON THE AQUEOUS GEOCHEMISTRY OF MAHANOY CREEK, EAST-CENTRAL PENNSYLVANIA

Mahanoy Creek, a tributary of the Lower Susquehanna River, is located in east-central Pennsylvania. Long-term anthracite mining in the region has degraded the watershed through acid mine drainage, resulting from water interacting with the surface mine waste or flowing through the abandoned underground mines. The acidic groundwater reaches the surface by either tunnel outlets (e.g. Centralia Tunnel) or surface seeps (e.g. Potts Mine Seep).

In July 2010, samples were collected at locations along Mahanoy Creek, its tributaries, and several acid mine discharge sites. Two tributaries are considered reference streams for this study as preliminary data indicate that their waters and sediments have very low levels of major ions, Fe, and Mn. Additional tributaries are considered mixed-water tributaries, as the sampling locations are downstream of water/sewage treatment plants and possible inputs of acid mine drainage. The Cl/F ratios indicate that there are sources of sewage in Mahoney Creek.

With over 30 known sources of acidic mine inputs into Mahanoy Creek, the pH of the main stem is higher than expected (pH 5.1-8.1). Accelerated water-rock interactions combined with inputs from treated sewage and leaky sewage disposal systems may be buffering the acid mine drainage throughout the Mahoney Creek watershed. Sodium, potassium, and chloride (in meq/L) do not balance (as they would if sewage and road salt were the main contributors), suggesting other sources of Na and K such as weathering of the geology. The acidic waters may be accelerating weathering in the watershed, which, in turn, may be buffering the acid mine drainage.

Red-orange coatings, presumably iron oxy-hydroxides deposits (FeOOH), were observed at many of the acid mine drainage and Lower Mahanoy Creek sites. Most of the water samples contain elevated amounts of dissolved Fe, and the sediment extracts have high levels of absorbed iron and manganese. The acid mine drainage water and sediment samples have the highest concentrations of Fe and Mn. Overall, Mahanoy Creek has elevated levels of dissolved Fe and Mn, which increases after acidic mine waters discharge into the main stem. Using PHREEQ, we determined that the majority of the water samples are supersaturated with respect to Geothite (FeOOH).