Design of a ground-water extraction/reinjection system at a superfund site using MODFLOW

A sand and gravel quarry was simultaneously operated as a landfill. The underlying contaminated aquifer is unconfined at the landfill and becomes confined as it dips to the north. A ground-water model based on the MODFLOW program was developed to evaluate remediation alternatives and provide a basis for modifying the Record of Decision for the site. Hydraulic conductivities were contoured and continuously varied across the modeled area instead of defining zones of constant hydraulic conductivities. The model was calibrated by matching monthly water levels and travel times for nonadsorbing contaminants to migrate from the landfill to the extent of the contaminant plume. Drawdown data from a 72-hour pump test were used to confirm the model's ability to predict an accurate response to pumping stresses and to reduce the uncertainty of the model. Modeling indicated that ground-water recharge was required to prevent soil desaturation, minimize recontamination of ground water due to rebounding water levels, and to provide continuous flushing of the soils. The ground-water extraction system designed for the site, consisting of eight extraction wells and one recharge trench, was predicted to reverse the natural ground-water gradients observed at the edge of the contaminant plume, minimize travel distance for contaminants, and reduce contaminated ground-water flow through clean aquifer soils. Recharged water was predicted to flow through the most heavily contaminated soils and be captured primarily by two extraction wells closest to the landfill. Model results were used to successfully modify the existing Record of Decision design.