Effects of incomplete remediation of NAPL-contaminated aquifers: experimental and numerical modeling investigations

The benefits of partial source zone treatment of non-aqueous phase liquids (NAPL)-contaminated sites (not fully removing the entrapped free-phase NAPL sources) with respect to achieving cleanup goals and reducing concentrations of dissolved constituents in downstream plumes are being debated. Uncertainty associated with the removal of NAPLs from source zones could be attributed to a number of factors including lack of information on the extent or timing of spills, complex entrapment configurations created by unstable behavior (fingering), geologic heterogeneity, and unavailability of accurate techniques for characterizing these heterogeneities, and uncertainty in locating source zone and estimating NAPL mass. Data for the resolution of issues related to benefits of partial source zone treatment are not expected to come from field sites. Laboratory studies in intermediate-scale test tanks can provide accurate data sets to investigate this issue, as it is possible to conduct controlled experiments under known conditions of aquifer heterogeneity. At this scale, source depletion and downstream concentrations in dissolved plumes can be monitored during remediation. The data generated in controlled experiments are used to validate numerical models to conduct theoretical analysis. This paper discusses this approach and presents results from such a study where the benefits of partial source zone treatment using surfactants were evaluated using intermediate-scale testing and numerical modeling. Results from both experiment and numerical simulations agreed conceptually where they suggested that a very large fraction of NAPL has to be removed from the entrapment zone to significantly reduce downstream plume concentrations.