Impact of Non-Aqueous Phase Liquids (NAPLs) on Groundwater Remediation

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Nonaqueous Phase Liquids (NAPLs) are immiscible (undissolved) hydrocarbons in the subsurface that exhibit different behavior and properties than dissolved contaminant plumes. NAPLs have a tremendous impact on the remediation of contaminated aquifers, as it is very difficult or impossible to remove all of the NAPL from a hazardous waste site once released to the subsurface. Although many NAPL removal technologies are currently being tested, to date there have been few field demonstrations where sufficient NAPL has been successfully removed from the subsurface to restore an aquifer to drinking water quality (EPA, 1992a). The residual NAPL that remains trapped in the soil/aquifer matrix acts as a continuing source of dissolved contaminants to ground water, and effectively prevents the restoration of NAPL-affected aquifers for tens or hundreds of years.

This is particularly true for groundwater pump-and-treat systems, the most common remediation technology for addressing contaminated aquifers. This technology pumps groundwater out of contaminated zones to remove dissolved contaminants and, if present, to slowly dissolve any trapped NAPLs. The pumped water is then treated on the surface to remove or destroy the dissolved contaminants.

To help designers of pump-and-treat systems evaluate the impact of NAPLs on groundwater remediation, a simple design model has been developed that provides the user with the number of recovery/injection wells and time required to reach cleanup standards. The method uses 1) dissolution data from a simplified dissolution model based on the work of Powers et al. (1994) or field data from dissolution pilot tests, 2) an assumed well configuration typical for pump-and-treat systems, and 3) relationships of concentration vs. pore volumes of clean water flushed. In addition, design charts for zones with only dissolved-phase groundwater contamination are also presented. These design tools are being incorporated into a large computerized Decision Support System now being developed by one of the authors (Rifai et al., 1994) for evaluating U.S. Air Force groundwater pump-and-treat systems.

The design models were used in an example application to evaluate remediation alternatives for a hypothetical NAPL-contaminated site. This example serves to demonstrate the simple use of the models for the rapid estimation of pump-and-treat system design requirements.

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