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Working To Restore Nature <br /> as an A2 carcinogen (suspected human carcinogen) A suspected human carcinogen is <br /> 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) in air and an IDLH of 3000 ppm. <br /> Exposure limits for toluene are STEL of 150 ppm and IDLH of 2000 ppm; exposure limits for <br /> t <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 risk level. 111000 of the No-Observable-Effect Level is used for reproductive toxicants <br /> Proposition 65 limits are established as a dose in units of micrograms per day of exposure <br /> These limits are converted into regulatory levels in water by assuming two liters per day water <br /> consumption and 100 percent exposure to the chemical from drinking water (CRWQCB, 1993). <br /> ' The established Proposition 65 levels in water are 3 5 parts per billion (ppb) for benzene, and <br /> 3500 ppb for toluene. Ethylbenzene, xylenes and gasoline were not included on the Proposition <br /> ' 65 list or significant risk levels have not been established. <br /> ' Gasoline and its constituents are generally not inordinately persistent in the subsurface <br /> ' environment due to their volatile nature and amenability to biodegradation by naturally occurring <br /> bacteria. <br /> ' The potential modes of migration of contaminants in the vadose zone at the subject site are <br /> ' vertical migration due to percolation of surface water and/or vapor-phased migration in relatively <br /> permeable sods. Dissolved contaminants are primarily transported by groundwater now <br /> ' Contaminant concentrations are generally reduced during transport within the saturated zone by <br /> the effects of dilution, dispersion, and attenuation <br /> 1500381REPOR'151C"-0794 DRF 15 <br /> 1 <br />