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<br />- 11 - <br /> <br />. <br /> <br />Until recently, people viewed drinking water drawn from <br />tne ground as a pristine resource, unspoiled by human activi- <br />ties. Most believed that soils are capable of binding and <br />holding chemicals applied to their surfaces. While this is true <br />for some chemicals, we have learned over the past few years that <br />it is definitely not true for several important and widely used <br />classes of chemicals, such as low molecular-weight organic sol- <br />vents. Among those of primary concern are such common chemical <br />solvents as tri- and tetrachloroethylene, benzene, and chlorinated <br />benzenes. <br /> <br />This new understanding of the vulnerability of ground water <br />to contamination by man-made chemicals is significant because <br />concentrations of these substances in ground water are orders <br />of magnitude higher than those generally found in surface water. <br />This is particularly disturbing in light of the fact that ground <br />water is often used with little or no treatment. Many of the <br />most troublesome chemicals are toxic, and some have been linked <br />to cancer in animal tests. For example, the suspected carcinogen <br />trichloroethylene (TCE), has been found--if only rarely--at levels <br />as high as 500,000 ppb (parts-per-billion) in heavily contaminated <br />ground water. More typical concentrations in groand water are <br />significantly less than 100 ppb, but surface water concentrations <br />seldom exceed 1 ppb. EPA.'s draft Health Advisory recommends safe <br />levels of TCE in drinking water at 2,000 ppb for an e~posure <br />duration of 1 day, and at 80 ppb for a duration of one to two <br />years, based on toxic effects. . The companion guidance on cancer <br />risks projects excess lifetime risk due to drinking water contami- <br />nated by TCE to be one in a million at an exposure level of 2.8 <br />ppb.3/ <br /> <br />2. THE SOURCES OF GROUND-WATER CONTAMINATION <br /> <br />The diversitv and number of existinq and potential sources <br />of ground-water contamination are surprisinalv laroe. There <br />are three categories of sources of ground-w~ter contamination: <br />waste disposal, non-disposal use of chemicals on the surface <br />of the land and overpumping or depletion. As a result of <br />differences in hydrogeological conditions and ground-water <br />use, the threat posed by these sources varies greatly with <br />each specific site. <br /> <br />States assess ground-water contamination problems as <br />follows:i/ <br /> <br />Major Problems: Industrial landfills/lagoons; Municipal land- <br />fills/lagoons; Underground storage tanks; and Chemical oil anc <br />brine spills. <br /> <br />C~06 <br />