RELATIONSHIPS BETWEEN GROUND WATER AND AN ACIDIC PIT LAKE IN THE MIDDLE ANTHRACITE FIELD OF EASTERN PENNSYLVANIA

Ground water discharge from a water-filled abandoned surface anthracite mine near Freeland, Pennsylvania, may significantly contribute to the volume of acid mine drainage contamination received by a tributary of the Lehigh River. Hydrogeologic investigations of the acidic Pond Creek pit lake show a complex relationship between lake water, ground water inputs, and ground water outputs. The lake exhibits a low pH (4.1) with elevated acidity and sulfate concentrations (Fe = 0.69 mg/L, Al = 1.5 mg/L, Mn = 0.59 mg/L, and SO4 = 130 mg/L). A spring located 9.5 meters above the lake surface discharges cold (9 °C) ground water with notably higher concentrations of iron and sulfate (pH = 4.1, Fe = 2.4 mg/L, Al = 2.0 mg/L, Mn = 0.59 mg/L, and SO4 = 150 mg/L). Surface water from this spring becomes the largest surface water input to the lake. Current meter measurements on June 23rd and August 4th 2006 recorded spring discharge rates of 0.15 m3/sec and 0.08 m3/sec, respectively. Seismic refraction and electrical conductivity profiles above the spring identified discontinuous voids overlain by shallow silt which is consistent with historic maps that illustrate the presence of one or more mine tunnels in this area. The elevation of the spring implies that these voids do not directly connect to the lake, otherwise the spring would not exist. Despite several high rainfall events, the lake surface dropped over 2 meters between August 5th and October 2nd 2006, corresponding to an estimated volume loss of 190,000 m3. Because the lake surface continued to drop below the lake overflow elevation, ground water outflow from the lake was largely responsible for this decline. Between October 2nd and November 20th, the lake surface rebounded above its August 5th level. These large fluctuations highlight the variability of ground water input and output fluxes to the lake. Since the chemistry of the ground water outflow will be similar to the lake chemistry, efforts to improve water quality in the Lehigh River must address ground water quality throughout the recharge area. Pit lakes may provide a unique mechanism to capture and treat contaminated ground water prior to baseflow discharge.