SEDIMENT AND PRECIPITATE ACCUMULATION RATES IN A PASSIVE WETLAND AMD TREATMENT FACILITY

Moraine State Park, near Pittsburgh, PA, was at one time extensively mined with both underground and strip coal mines. A total of sixty nine mine entrances were sealed in the early 1970’s, in the 80’s it was determined that a number had failed. A passive wetland acid mine drainage (AMD) treatment system was installed in 1996 to combat a few of these failing seals, the exact number is unknown. Of the three ponds that make up the treatment system, pond two, the vegetation pond, is demonstrating the most signs of stress and possible failure. The ponds were designed to last twelve years with no required maintenance and are currently on their fourteenth year. Shallow water levels in pond two are believed to be attributed to the sediment build up over the years. Initially, hay bales were installed in pond two to direct flow and give the iron-laden water the proper residence time for natural attenuation. Water samples were taken at the inflow, outflow, each of the corners, and at the midpoint of each side of pond two. Chemistry suggests that sediments have overcome these guides and water is not attaining the proper residence time to settle out iron hydroxide precipitates, among other AMD contaminants. . The sides of the pond were registering little to no iron content, while the both the inflow and outflow measured about 1.5 mg/L. A preliminary sediment sample was taken at the inflow of pond 2 which clearly demonstrated significant precipitation of iron and sediment. After months of precipitate sampling at the water chemistry locations, the max amount of precipitate recorded was 0.13:1 precipitate to water ratio after a period of 25 days at the inflow. The minimum amount recorded was at the outflow, at 0.05:1 over a 35 day period.