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I levels and by routes of administration that are considered relevant to worker exposure Benzene is identified by NIOSH as an occupational carcinogen with a STEL of 1 part per k , million (ppm) and an IDLH of 3000 ppm Exposure limits for toluene are STEL of 150 ppm and IDLH of 2000 ppm, exposure limits for ethylbenzene are STEL of 125 ppm and IDLH of 2000 ppm, exposure limits of xylenes are STEL of 150 ppm and IDLH of 1000 ppm No exposure limits have been established for gasoline, but gasoline 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-m-100,000 incremental ' cancer risk level 111000 of the No-Observable-Effect Level is used for reproductive � toxicants These Proposition 65 limits are established as a dose in units 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 are 3 5 parts per billion (ppb) for benzene and 3500 ppb for toluene Ethylbenzene, xylenes, and gasoline were not ' included on the Proposition 65 list f Gasoline and its constituents are generally not mordmantly 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 migration due to percolation of surface water and/or vapor-phased migration in relatively permeable soils ' 30 EVALUATION OF REMEDIAL ALTERNATIVES 3.1 Remedial Alternatives for Soil And Groundwater Soil remediation alternatives include passive remediation, source excavation and offsite � disposal, in-situ bioremediation, and vapor extraction with groundwater monitoring Groundwater remediation alternatives include no action/monitoring, air-sparging, groundwater extraction and treatment, and natural attenuation and monitoring after source ' removal 3.2 Selection Criteria In accordance with SJCPHS-EHDidelin gu es, Ground Zero has considered at least two remediation alternatives The criteria used to evaluate potential remedial options for the site include level of protection, relative effectiveness, treatment time and time constraints, cost, and future liability The following general response alternatives for the site have been 16 considered ' groundLL%rmklcap 9