Superfund Research Program
Developing Methods for the Remediation of Dense Nonaqueous Phase Liquids
DNAPLs or dense nonaqueous phase liquids are a significant problem on Superfund sites. Approximately 60% of the sites on the National Priorities List are contaminated with these compounds, which include toxic chlorinated solvents such as trichlorethylene and carbon tetrachloride, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls. DNAPLs are common contaminants of soils and aquifers, where they are especially difficult to locate. Because of their high density in comparison to water, these industrial chemicals sink relatively quickly through permeable regions of the subsurface. As they migrate, they leave behind small amounts of chemical, which become trapped in small pores. DNAPLs also collect in pools when they reach impermeable materials.
Not surprisingly, DNAPLs are difficult to recover and treat in the subsurface, and efforts to clean up sites contaminated with these chemicals have been largely unsuccessful. Some of the methods used for DNAPL remediation include pump-and-treat, vapor extraction, gas stripping, cosolvent flushing, surfactant flushing, in situ biodegradation and thermal methods. All of these methods have two things in common that hinder their effectiveness: they require moving the chemical mass from the DNAPL phase to a different phase that is mobile, and they are all significantly affected by subsurface heterogeneity.
Researchers at the University of North Carolina have developed two alternative approaches to clean up DNAPLs. The concept behind both is simple -- rather than fight the mass transfer limitations that exist in subsurface systems to remove DNAPLs, these techniques exploit the potential for mobilization. For the first approach, the density of water is altered by injecting brine solutions such that the DNAPLs become lighter than water and float to the surface. The second approach is similar, using a less dense brine barrier to funnel mobilized DNAPLs to a partially screened well or wells. Both processes can use surfactants, which improve DNAPL recovery in highly heterogeneous areas of the subsurface.
Laboratory experiments performed to date in heterogeneous systems show encouraging results. High fractions of DNAPLs (exceeding 90% of the initial mass) are removed relatively quickly. Moreover, the remaining DNAPL has a high surface area to volume ratio, which makes the residual relatively easy to remove using existing methods. Follow up research is continuing to improve the effectiveness of these methods.
For More Information Contact:
Cass T Miller
University of North Carolina-Chapel Hill
Department of Environmental Sciences and Engineering
CB #7431, 3201 McGavran Greenberg Hall
Chapel Hill, North Carolina 27599-7431
To learn more about this research, please refer to the following sources:
- Miller CT, Hill EH, Moutier M. 2000. Remediation of subsurface systems contaminated with dense organic liquids. Environ Sci Technol 34(4):719-724.
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