Superfund Research Program
Enhancing Remediation of DNAPL-Contaminated Subsurface Systems
Release Date: 01/02/2002
By definition, dense non-aqueous phase liquids (DNAPLs) are liquids that are heavier than water and have a low aqueous solubility. DNAPLs, such as creosotes, coal tar, and the chlorinated solvents perchloroethylene (PCE), trichloroethylene (TCE) and dichloroethylene (DCE), are the most frequent sources of contamination at Superfund sites. The main reasons for the high incidence rate and the magnitude of groundwater contamination by DNAPLs are:
- For decades these compounds were widely used across industrial, commercial, and service sectors
- Both intentional and accidental releases of these compounds into the subsurface environment occurred routinely
- DNAPLs are long-lived in the subsurface environment
DNAPLs often contain compounds that cause a significant risk to human and ecological health. Once in the subsurface DNAPLs tend to migrate rapidly downward toward the bottom of the aquifer. A pool is formed when a migrating DNAPL is prevented from making further progress through the porous medium by an increase in capillary resistance. Because subsurface systems are heterogeneous, residual and pooled DNAPL typically has a complex morphological distribution, which makes remediation of such contamination a difficult challenge. In fact, the remediation of DNAPL-contaminated sites has proven to be one of the most difficult problems facing the Superfund program. Common remediation practices such as pump-and-treat, vapor extraction, gas stripping, and in situ biodegradation have been shown to be time-consuming, expensive and inefficient when applied to these sites.
SBRP-funded researchers at the University of North Carolina at Chapel Hill (UNC) continue to work towards the development and implementation of a novel class of methods for efficiently remediating certain DNAPL-contaminated subsurface systems. They have recently patented (1,2) the remediation methods described in Research Brief 61 and are currently developing improved methods to quantify and remediate DNAPL-contaminated Superfund sites.
In studies designed to increase our understanding of the relationship between site conditions and DNAPL pools, the UNC scientists performed a series of two- and three-dimensional experiments in heterogeneous porous medium systems. The remediation strategies studied involve the joint use of a dense brine barrier and controlled mobilization of trapped DNAPL using small-volume surfactant flushes.
These tests accomplished several objectives:
- The data generated in both the two- and three-dimensional heterogeneous porous media systems demonstrate that controlled mobilization strategies can work. 63-87% of the trapped DNAPL was removed with less than one pore volume of a surfactant solution in heterogeneous media.
- The researchers established methods to construct heterogeneous packings in both two- and three-dimensional cells. The flow cells and packing procedures developed at UNC are a significant improvement over the essentially one-dimensional flow domains used in most DNAPL remediation investigations. They allow the study of more complex flow phenomena resulting from effects such as lateral capillary trapping and flow bypassing.
- The researchers developed a new analytical technique to accurately quantify the fractional removal of DNAPL. Their use of C-14 labeling of the TCE injected into the test media combined with hexane extraction of the sample solutions overcame the analysis problems associated with high salt and surfactant concentrations.
- The two-dimensional experiments allowed for the collection of a detailed visual record of DNAPL pool formation and recovery.
- The three-dimensional experiments provided a rare opportunity to evaluate the efficiency of DNAPL recovery in a well-characterized, heterogeneous three-dimensional system, similar to field conditions.
The goal of the UNC research is to continue to develop and implement new approaches to remove DNAPLs from the subsurface. If these novel approaches are successful and put into practice, more economical means of contaminant source-zone remediation will be available, resulting in reduced risk to human and ecological populations. Such an advance would have a profound effect on the Superfund program and the restoration of DNAPL-contaminated sites.
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:
- Hill EH, Moutier M, Alfaro J, Miller CT. 2001. Remediation of DNAPL pools using dense brine barrier strategies. Environ Sci Technol 35(14):3031-3039. PMID:11478259
- Hilpert M, Miller CT. 2001. Pore-morphology-based simulation of drainage in totally wetting porous media. Adv Water Resour 24(3-4):243-255.
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