Transport and Fate of Chlorinated Hydrocarbons in the Vadose Zone — A Literature Review with Discussions on Regulatory Implications

Chlorinated hydrocarbons (CHCs) are one of the pollutant groups most commonly found in hazardous waste sites. Understanding the transport and fate of these compounds in the vadose zone is crucial to identifying pollution sources, assessing the threat to groundwater, and evaluating the need and adequacy of cleanup. CHCs exist in various physical phases in the subsurface, linked by interphasal processes. These, combined with unsaturated water flow in the vadose zone, which is complicated by the multifluid (air and water) situation, geologic heterogeneity, and a highly site-specific preferential flow pattern, create a possibly four-fluid flow condition, with interactions among different phases of CHCs and between chemicals and geologic materials. Superimposed on these are various biotic and abiotic transformations. Integration of these processes is being attempted by mathematical modeling; however, natural heterogeneity and site-specific factors render these efforts partially successful at best. Regulators are faced with considerable uncertainties and cannot expect simple solutions with the current level of understanding of subsurface processes.