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i 1 ' Feasibility Study Report 1140 W Hammer Lane, Stockton, CA limits established by National Institute for Occupational Safety and Health (NIOSH) A short term ' exposure limit(STEL) is designated by a 15-minute time-weighted average (TWA) exposure in air that should not be exceeded at any time during a workday Immediately dangerous to life or health ' concentrations (IDLHs) represent the maximum concentration in air from which, in the event of respirator failure, one could escape within 30 minutes without a respirator and without experiencing any escape-impainng or irreversible health effects Benzene has been classified by the Amencan Conference of Governmental Industrial Hygienists (ACGIH) as an A2 carcinogen (suspected human carcinogen) A suspected human carcinogen is carcinogenic in experimental animals at dose levels and ' by routes of adrnirustration that are considered relevant to worker exposure Benzene is identified by NIOSH as an occupational carcinogen with an established exposure limit of 0 1 parts per million (ppm) ' in air, a STEL of 1 part per million (ppm) and an IDLH of 500 ppm Exposure limits for toluene are 100 ppm, STEL of 150 ppm, and IDLH of 500 ppm Exposure limits for ethylbenzene are 100 ppm, STEL of 125 ppm and IDLH of 800 ppm Exposure limits for xylenes are 100 ppm, STEL of 150 ppm and IDLH of 900 ppm No exposure limits have been established for gasoline, but gasoline 10 contains benzene, toluene, ethylbenzene, and xylenes (BTEX) Proposition 65 in the state of California established limits under the California Safe Drinking Water and Toxic Enforcement Act of 1986 for known human carcinogens and reproductive toxins For carcinogens, No-Significant-Risk Levels are set at the 1-in-100,000 incremental cancer nsk level 111000 of the No-Observable-Effect Level is used for reproductive toxicants Proposition 65 lirrvts are ' established as a dose in uruts of micrograms per day of exposure These limits are converted into regulatory levels in water by assuming two liters per day water consumption and 100 percent exposure ' to the chemical from drinking water (CRWQCB, 1993) The established Proposition 65 levels in water are 3 5 parts per billion (ppb) for benzene, and 3,500 ppb for toluene Ethylbenzene, xylenes and gasoline were not included on the Proposition 65 Inst or significant risk levels have not been established Gasoline and its constituents are generally not inordinately persistent in the subsurface environment due to their volatile nature and amenability to biodegradation by naturally occurring bacteria The potential modes of migration of contaminants in the vadose zone at the subject site are vertical C \Duua1C.R0UNDLL\KN0WLC5 HAMTEPORrS\Fumbi]ayRLportdo�