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

The large-scale production and use of chlorinated aliphatic hydrocarbo ns (CHC) in the United States has resulted in numerous documented case s of soil and ground water contamination. Potential chronic exposure f rom 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 carcinogen s. Most CHCs are very mobile in the subsurface environment, either as volatilized constituents in the vadose zone due to their high vapor pr essure and low water solubility; as dissolved constituents in ground w ater since they are not strongly adsorbed by aquifer materials; and as dense nonaqueous phase liquids (DNAPLs) in ground water, primarily be cause they have low Water solubility and densities greater than water. As free-phase chemicals (i.e., DNAPLs), CHCs may pool on low-permeabi lity subsurface stratum and cause shrinkage and cracking of clay miner als, thereby significantly increasing the stratum's hydraulic conducti vity and allowing rapid CHC breakthrough and migration Although labora tory and field studies have demonstrated this effect, additional inves tigations are needed to determine the impact CHC DNAPLs have on low-pe rmeability in situ sediments. CHCs are persistent environmental contam inants because they resist chemical and biological degradation. The av erage half-life for CHC abiotic transformations ranges from two months to greater-than 10(10) years. Chlorinated ethenes are the most resist ant CHCs to abiotic degradation. Biodegradation half-lives for CHCs ma y vary considerably but range from about two weeks to eight months or more. Anaerobic reductive dehalogenation is recognized as a primary CH C biodegradation process in ground water, although aerobic CHC biodegr adation may be a significant process in the shallow subsurface. Techno logical applications of remedial processes for stimulating in situ CHC biodegradation in soils and ground water need to be developed, partic ularly with regard to CHCs entrained as residual saturation.