Groundwater Flow and Solute Transport Modeling at a State Superfund Site, Northeastern St. Louis County, Minnesota

Abstract

This study identifies hydrogeologic and contaminant transport parameters in a glacial till environment. The study site was a mixed solid waste land disposal site from 1972 to 1990. A shallow, unconfined aquifer overlies Lower Precambrian bedrock. Saturated thickness varies from 5 to 20 feet, and the average linear groundwater velocity is 12 to 15 feet per year. The primary downgradient receptor is the Bear Island River. A finite-difference numerical flow model, calibrated to observed water table elevations, provided hydraulic head and aquifer parameter data for a solute transport model. Dispersion coefficients were determined by calibration of the transport model to observed chloride concentrations. The model was used to predict future plume configurations for benzene, methylene chloride, and tetrahydrofuran, using retardation factors computed from measured aquifer material organic carbon content. Retardation factor for chromium was determined by model calibration to observed concentration. This study found that the chloride and tetrahydrofuran plumes will reach the Bear Island River by year 2010. Benzene, methylene chloride, and chromium plumes are considerably more restricted in areal extent. None of these contaminants is expected to create a significant detrimental effect on river water quality.