Characterization and removal of DNAPL from sand and clay layered media

Important characterization and remediation techniques (e.g. partitioning tracer tests (PTT), cosolvent and surfactant flushing) have been developed over the years to deal with dense nonaqueous phase liquid (DNAPL) sources in the saturated zone. Unfortunately, subsurface media layering and heterogeneity pose a major challenge to the efficiency of these and other techniques that rely on flushing fluids through porous media. Using laboratory column experiments with both single media and layered-media columns and computer modeling of tracer breakthrough results, we examined the difficulty that layering poses to subsurface remediation and characterization techniques. Quantifying tetrachloroethylene (PCE) saturation in layered media was determined using PTT and effluent mass determinations. Conservative tracer breakthrough curves were used to determine permeability and the flow through layers before and after PCE contamination. The removal efficiency of alcohol flushing in the layered systems was also determined. Results showed that even a relatively simple layering with less than an order magnitude difference in permeability leads to difficulty in characterization and remediation using flushing technologies. These results suggest that much greater volumes of flushing solutions will be needed in heterogeneous environments to ensure adequate flushing of lower permeable lenses and layers.