Application of Ground Water Fate and Transport Models to Evaluate Contaminant Mass Flux and Remedial Options for a MTBE Plume on Long Island, NY

摘要

A petroleum hydrocarbon release, first discovered in 1990 at a gasoline service station on Long Island, NY has resulted in a large dissolved-phase MTBE plume. Possible downgradient ground water receptors include two municipal well fields and a stream/lake system. Recent delineation efforts indicate that the dissolved-phase MTBE plume is likely over 8,000 feet long, with a diving vertical profile resulting principally from concentrated recharge at a nearby storm water recharge basin. The hydrogeologic setting of the area consists of a surficial Pleistocene glacial sand aquifer overlying a thin but areally-extensive clay located at a depth of approximately 100 feet. The aquifer has a hydraulic conductivity of approximately 500 feet/day, with an average ground water flow velocity of 3 to 6 feet/day. A large storm water recharge basin is located approximately 700 feet downgradient of the source area and appears to have a significant impact on the vertical movement of the MTBE plume. MODFLOW/MT3D computer models were used to evaluate the possible extent and magnitude of the plume, the risk to nearby ground water receptors, and to predict the effects of various remedial strategies. The models incorporate several key factors including: 1) MTBE mass loading rate at the source area, 2) concentrated recharge at the storm water recharge basin, 3) leakage through the underlying clay, 4) a biodegradation zone along the MTBE flow path, 5) MTBE mass and concentration discharged to receptors, and 6) MTBE mass flux. The resulting predictions indicated no impact on the public supply wells, and a projected peak discharge to the stream/lake system below applicable standards. Future uses of the model will include supporting the selection of remedial solutions, monitoring future progress of the chosen solution, and aiding in the evaluation of MTBE biodegradation at the site.