U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Chemistry and Behavior

Halogenated Alkenes

As would be expected, the chlorinated ethenes have similar physical and chemical properties. All have relatively low log Koc and Kow values and are less viscous than water, indicating they are mobile in the subsurface. Their water solubilities fall in the range of 150 mg/L (tetrachloroethene [PCE]) to 1,100 mg/L (trichloroethene [TCE]), which are orders of magnitude above their risk-based health concentrations.

All the chloroethene DNAPL chemicals are anaerobically biodegradable in the presence of an appropriate microbial consortium, which may or may not be present at a specific site. The dichloroethenes can be biodegraded aerobically and under some circumstances, TCE can be abiotically co-metabolized.

All of the chloroethenes have relatively high Henry's constants and can be expected to form vapor plumes that emanate from the source area and the dissolved-phase plume, making them good candidates to cause vapor intrusion into buildings. In shallow aquifers, vapor plumes can be traced by soil gas survey.

None of the chloroethenes are expected to bioaccumulate and all have relatively short half lives in surface water (hours to days). All chloroethenes, except PCE, are degraded in the atmosphere by photooxidation and hydroxyl radicals in a relatively short time with half lives on the order of hours to a few days. PCE also degrades in the atmosphere with a half life of up to two months.

The 1,3-dichloropropenes have very similar fate properties as the chloroethenes. They have slightly higher solubilities (2,700 to 2,800 mg/L) and slightly lower log Koc and Kow values, so they are somewhat more mobile in the subsurface than the chloroethenes.

For Further Information

Adobe PDF LogoAFCEE Source Zone Initiative
Air Force Center for Environmental Excellence (AFCEE), 234 pp +appendix (273 pp), 2007

Key elements include a description of governing processes, illustrative laboratory studies, predictive models, and demonstrative field data, including a review of source control measures taken for TCE at Air Force Plant 4, near the former Carswell Air Force Base, TX. Results from this work indicate that transverse diffusion can drive contaminants into low permeability zones. Initially, this has the effect of attenuating contaminants in transmissive layers. After the DNAPL has been depleted, back diffusion from low permeability zones can sustain contaminant concentrations in transmissive layers in source zones and plumes.

Adobe PDF LogoPreliminary Conceptual Models of Chlorinated-Solvent Accumulation in Karst Aquifers
Wolfe, W. and C. Haugh
Water-Resources Investigations Report 01-4011, p. 157-162

The paper presents five preliminary DNAPL conceptual models that were developed for the karst regions of Tennessee but are intended to be transferable to similar karst settings elsewhere.The five models of DNAPL accumulation in karst settings are: (1) trapping in regolith; (2) pooling at the top of bedrock; (3) pooling in karst conduits; (4) pooling in bedrock diffuse-flow zones; and (5) pooling in isolation from active groundwater.

Review and Analysis of Chlorinated Solvent Dense Nonaqueous Phase Liquid Distributions in Five Sandy Aquifers
Parker, B., J. Cherry, S. Chapman, and M. Guilbeault
Vadose Zone Journal, 2003

This 22-page article examines the architectures of DNAPLs in five different sandy aquifers. The vertical profiling enables a close look at distribution and aging effects.

Adobe PDF LogoSustainability of Long-Term Abiotic Attenuation of Chlorinated Ethenes
M.M. Scherer, E. O'Loughlin, G.F. Parkin, R. Valentine, H. Al-Hosney, R. Handler, C. Just, P. Larese-Casanova, T. Pasakarnis, and S.L. Smith.
Strategic Environmental Research and Development Program, Project ER-1369, 48 pp, 2007

To identify abiotic degradation mechanisms that might contribute to the attenuation of chlorinated ethene plumes, the reduction of chlorinated ethenes by a series of chemically and microbially generated reductants was measured under a range of natural conditions. The effects of the various reductants were evaluated based on the extent and rate of TCE, PCE, and 1,2-DCE reduction in batch reactors.