Dense Nonaqueous Phase Liquids (DNAPLs)
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Over 70 individual compounds have been identified as common constituents of DNAPLs. These compounds are all denser than water and can be grouped into very broad chemical classes, such as halogenated alkanes, halogenated alkenes, polychlorinated biphenyls (PCBs), and phthalates. However, even within one class, compounds that are closely related structurally may cause different toxic effects. Even two isomeric forms of the same compound may show differences in toxicological activity. Therefore, it is difficult to generalize the degree and extent of toxic effects that will be exerted by a class of compounds.
More than one class of compounds may be present in a DNAPL. A common mixture of classes, found in ground water at many NPL sites, includes the chlorinated alkenes and chlorinated alkanes. If ground water contaminated with these classes of compounds is used for drinking water purposes, the user will be potentially at risk to the effects of them all. There are no studies that directly characterize the health hazards of such a mixture, and models have not been developed to predict the distribution of the mixture in the body or its toxicological consequences. Conservatively, the effects of a mixture of 1,1,1-trichloroethane, 1,1-dichloroethane, trichloroethene (TCE), and tetrachloroethene, in the absence of directly applicable studies or models, are, of necessity, assumed to be the cumulative joint toxic action of the individual chemicals in the mixture.
Conceptually, health hazards associated with compounds present in a DNAPL could be additive, less than additive, or greater than additive. As an example of the latter, PCBs and chlorinated alkenes are structurally very different classes of chemicals. TCE, a chlorinated alkene, is extremely volatile and has little tendency to bioaccumulate, whereas PCBs are not volatile and tend to bioaccumulate. However, both PCBs and TCE exert toxic effects on the nervous system and liver, and there is evidence to suggest that there is a greater than additive effect of PCBs on TCE toxicity.
Complex mixtures, such as the coal tar creosotes, coal tars, and heavy fuel oils may be present in DNAPLs. Each of these mixtures contains many classes of compounds. Unlike the mixtures of alkanes and alkenes, there are animal studies and epidemiological studies that provide information on the toxicology of these mixtures. The toxic effects exerted by the coal tars, coal tar creosotes, and heavy fuel oils reflect those of the major chemical groups contained within each mixture. All of these mixtures contain considerable proportions of polycyclic aromatic hydrocarbons (PAHs), and some PAH compounds are known to be carcinogenic in laboratory animals. Benz(a)pyrene, one of the most toxic and well-studied PAHs, which is present in coal tar creosote, has been used as a positive control (used to produce tumors) in laboratory animals. However, one animal study indicated that coal tar creosote is a more potent carcinogen than benzo[a]pyrene alone. This suggests that the action of an individual compound in a mixture may not be predictive.