FIGURE 3-1 Environmental contamination from solvents and exposure pathways. Source: EPA 1989.

FIGURE 3-1 Environmental contamination from solvents and exposure pathways. Source: EPA 1989.

Contamination of drinking-water supplies is of greater health concern. In past years, halocarbons were generally regarded as water-insoluble. It is now recognized that they are soluble in water to a small extent. Maximum solubilities, for example, range from 150 mg/L (or parts per million) for PCE to 4,800 mg/L for methylene chloride. Concentrations typically found in finished drinking water in the United States range from parts per trillion to parts per billion (Moran et al. 2007).

VOCs are found as contaminants of surface water and groundwater. Concentrations diminish rapidly after VOCs enter bodies of water, primarily because of dilution and evaporation. Halocarbons rise to the surface or sink to the bottom, depending on their density. Halocarbons on the surface largely evaporate. The movement of halocarbons on the bottom depends on their solubilization in water and their mixing by currents or wave action; mixing causes them to reach the surface. Hydrocarbon solvents spilled onto the ground largely evaporate, although some can permeate soil and migrate through it until reaching groundwater or an impermeable layer. Migration of solvents through sandy soil of low organic content is most rapid and extensive (Munnecke and Van Gundy 1979). Solvents in groundwater tend to remain trapped until the water reaches the surface, although some are subject to microbial modification. PCE and TCE, for example, undergo reductive dehalogenation by microorganisms to a small extent to cis- and trans-1,2-DCE, vinyl chloride, and other products (Smith and Dragun 1984; McCarty 1993). Thus, halocarbon-contaminated groundwater usually contains a relatively high proportion of parent compounds and small amounts of microbial degradation products.

EXTERNAL EXPOSURE

People are exposed to halocarbons and other VOCs in water by three major routes: inhalation, skin contact, and ingestion. A number of studies have looked at the relative importance of those routes. Weisel and Jo (1996) based estimates of internal doses of TCE and chloroform received from showering on results of experiments with human subjects. They concluded that inhalation and dermal exposure resulted in an internal dose of each chemical comparable with the dose ingested in 2 L of water. Gordon et



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