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Benzene has been classified by the American Conference of Governmental Industrial Hygienists <br /> (ACGIH) as an A2 carcinogen (suspected human carcinogen) A suspected human carcinogen <br /> ' is carcinogenic in experimental animals at dose levels and by routes of administration that are <br /> considered relevant to worker exposure Benzene is identified by NIOSH as an occupational <br /> carcinogen with a STEL of 1 part per million (ppm) and an IDLH of 3000 ppm Exposure <br /> limits for toluene are STEL of 150 ppm and IDLH of 2000 ppm, exposure limits for <br /> ethylbenzene are STEL of 125 ppm and IDLH of 2000 ppm, exposure limits of xylenes are <br /> STEL of 150 ppm and IDLH of 1000 ppm No exposure limits have been established for <br /> gasoline, but gasoline contains benzene, toluene, ethylbenzene, and xylenes (BTEX) <br /> ' Proposition 65 in the state of California established limits under the California Safe Drinking <br /> ' Water and Toxic Enforcement Act of 1986 for known human carcinogens and reproductive <br /> toxins For carcinogens, No-Significant-Risk Levels are set at the 1-in-100,000 incremental <br /> cancer nsk level 1/1000 of the No-Observable-Effect Level is used for reproductive toxicants <br /> These Proposition 65 limits are established as a dose in units of micrograms per day of <br /> ' exposure These limits are converted into regulatory levels in water by assuming two liters per <br /> day water consumption and 100 percent exposure to the chemical from drinking water <br /> (CRWQCB, 1993) The established Proposition 65 levels are 3 5 parts per billion (ppb) for <br /> benzene, 3500 ppb for toluene, and 3 5 ppb for ethylbenzene Xylenes and gasoline were not <br /> ' included on the Proposition 65 list <br /> ' Gasoline and its constituents are generally not inordinantly persistent in the subsurface <br /> environment due to their volatile nature and amenability to biodegradation by naturally occurring <br /> ' bacteria The potential modes of migration of contaminants in the vadose zone at the subject <br /> site are vertical migration due to percolation of surface water and/or vapor-phased migration in <br /> ' relatively permeable soils <br /> 1500471REPORTSICAP 0644 FNL 6 <br /> 1 <br />