FAT OF CHLORINATED ALIPHATIC-HYDROCARBONS IN THE VADOSE ZONE AND GROUND-WATER

The large-scale production and use of chlorinated aliphatic hydrocarbons (CHC) in the United States has resulted in numerous documented cases of soil and ground water contamination. Potential chronic exposure from contaminated soil and ground water, as well as from products used in the home, is sufficient reason to develop a thorough understanding of the environmental fate of these known and possible human carcinogens. Most CHCs are very mobile in the subsurface environment, either as volatilized constituents in the vadose zone due to their high vapor pressure and low water solubility; as dissolved constituents in ground water since they are not strongly adsorbed by aquifer materials; and as dense nonaqueous phase liquids (DNAPLs) in ground water, primarily because they have low Water solubility and densities greater than water. As free-phase chemicals (i.e., DNAPLs), CHCs may pool on low-permeability subsurface stratum and cause shrinkage and cracking of clay minerals, thereby significantly increasing the stratum's hydraulic conductivity and allowing rapid CHC breakthrough and migration Although laboratory and field studies have demonstrated this effect, additional investigations are needed to determine the impact CHC DNAPLs have on low-permeability in situ sediments. CHCs are persistent environmental contaminants because they resist chemical and biological degradation. The average half-life for CHC abiotic transformations ranges from two months to greater-than 10(10) years. Chlorinated ethenes are the most resistant CHCs to abiotic degradation. Biodegradation half-lives for CHCs may vary considerably but range from about two weeks to eight months or more. Anaerobic reductive dehalogenation is recognized as a primary CHC biodegradation process in ground water, although aerobic CHC biodegradation may be a significant process in the shallow subsurface. Technological applications of remedial processes for stimulating in situ CHC biodegradation in soils and ground water need to be developed, particularly with regard to CHCs entrained as residual saturation.