Effect of Well‐Screen Placement on Recovery of Vertically Stratified Contaminants

Abstract

AbstractField experiments and numerical modeling showed that vertical placement of a pumping‐well screen in an appropriate location could enhance the recovery of dissolved contaminants from a distinct depth interval of an unconfined aquifer. Two single‐well contaminant recovery tests were performed for 72 hours each at Borden, Ontario in a thin, fine‐grained sand aquifer. A zone 1.5 m thick at the bottom of the aquifer was contaminated by landfill leachate. One screen was located entirely within the contaminated zone and the other had only half its length placed in the zone of contamination. In both tests, concentrations in the effluent were initially high, but asymptotically declined and approached the vertically averaged concentration of the aquifer. The decline in concentration occurred because converging flow in the vertical plane caused a reduction in the thickness of the plume near the well; that in turn decreased the ratio of contaminated to clean waters entering the well screen. The field results were simulated using SUNFLOWPT, a saturated‐unsaturated flow model with particle tracking. Predictions of the model successfully matched the observed drawdown and effluent concentration data. Propagation of simulated capture zones into the overlying uncontaminated aquifer waters are consistent with the observed declines in effluent concentrations. Two‐dimensional plan‐view modeling is unable to accurately simulate observed data at early times of pumping. In aquifers having stratified contamination, only models that incorporate vertical flow should be used to interpret or predict effluent concentrations from short‐duration tests conducted for the purpose of designing pump‐and‐treat remediation schemes. The results suggest that in some cases of long‐term cleanup, two screens that separately capture the clean and contaminated waters of the aquifer could significantly reduce the volume of water requiring treatment.