Air and Water Pollutants Continued	Water Pollution Drinking water contains complex mixtures of known and suspected carcinogens, including asbestos, metals, radioactive substances, and industrial chemicals. In addition, the process of treating water may create small quantities of chemicals that have been linked to cancer in laboratory animals. Chlorination byproducts, including chloroform, other trihalomethanes (or THMs), and other compounds, can be formed when chlorine used to purify drinking water reacts with organic compounds in water. Even at levels normally found in some chlorinated city water supplies with high levels of organic material, there is suspicion that the byproducts may increase the risk of gastrointestinal and urinary tract cancers (Crump and Guess, 1982; Cantor et al., 1987). To reduce the levels of chlorination byproducts, water is often filtered to reduce the amount of chlorine needed for purification. A few other industrial chemicals, known to cause cancer in humans, have been found occasionally in drinking water (National Research Council, 1977). Vinyl chloride, for example, may be introduced into drinking water from industrial plants or, in very small amounts, by seepage from polyvinyl chloride piping used in some water distribution systems. Benzene and bis(2-chloroethyl)ether are other carcinogens that are occasionally found in drinking water. Nitrates themselves do not cause cancer, but they can combine in the body with certain amines to form nitrosamines, many of which are powerful carcinogens in animal models. In most circumstances, ingested nitrate comes primarily from food, but water can be the primary source of consumed nitrate in places where nitrate in drinking water is close to, or above, the maximum level set by the EPA. Nitrates are seldom eliminated by the water treatment process. The evidence linking nitrate to human cancer is weak. Some studies point to an association; others do not. Therefore, if there is a connection, the link is not likely to be strong (Fraser et al., 1980). Asbestos fibers are widely distributed in water supplies in this country, with higher levels often found near cities and industrial centers. But studies have not shown consistently that asbestos in drinking water affects cancer risk (Working Group on Ingested Asbestos, 1987). The trace metals arsenic, chromium, and nickel are found in drinking water in varying amounts. They may come from industrial plants and mines; by seepage from soil or piping; by mineralization from rocks; or from water treatment processes. High levels of arsenic in drinking water in Taiwan and some other countries have been linked with several types of cancer (bladder, kidney, lung, nasal cavity, and liver in both sexes, and prostate) (Chen and Wang, 1990; Wu et al., 1989). However, the levels of arsenic in almost all U.S. drinking waters are considered low enough not to constitute a public health threat. Depending on local rock type and on the handling of radioactive compounds by nearby industries, hospitals, and nuclear power plants, radioactive substances may be found in the water supply. While the radioactive strontium and radium found in some waters can accumulate in bone tissue, the low cumulative dose from radium would result in so few fatal bone cancers that they would probably not be detected in epidemiologic studies. Naturally occurring radon gas is found dissolved in water in some parts of the United States. Ingestion of radon in water does not pose much of a direct hazard, because of its low concentration; but radon can be released into household air via showers, washing machines, and other water usage. In some cases, waterborne radon released into the air can contribute substantially to domestic airborne levels. Based on the results of studies of underground miners exposed to much higher levels of radon, elevated levels of radon in houses is suspected to contribute to lung cancer risk (Samet, 1989). Water that percolates below the earth's surface, known as groundwater, is the source of spring and well water. Seepage of pesticides, industrial solvents, and other industrial chemicals such as polychlorinated biphenyls (PCBs) into the underground aquifers (rock formations that hold water) causes further contamination of the groundwater supply. Burying hazardous wastes on land is the most common method of disposal in this country because it is inexpensive. Disposal sites can leak, however, and contamination of groundwater near the older sites has been documented in many locations. Therefore, the EPA has instituted more rigorous guidelines regulating hazardous waste disposal. Under EPA guidelines, the major U.S. water supplies are monitored regularly for a number of carcinogens. National Cancer Institute scientists have been studying the possible link between cancer and drinking water quality (Cantor, 1990). The available evidence suggests that the cancer risk posed by contaminants in our drinking water is relatively small.
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