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Ms Vicky McCartney Project No 880-040 IA <br /> December 8, 2003 <br />' Page 7 <br /> Potential exposure receptors include current and future workers and customers of the <br /> subject station at 3250 West Hammer Lane, motorists, pedestrians, and utility <br /> maintenance workers The potential exposure mediums are ambient air and soil and <br /> groundwater in potential future excavation areas The major exposure pathway is <br /> hydrocarbon volatilization from smear zone soils and groundwater to ambient air The <br /> potential exposure pathway for utility maintenance workers is dermal contact with <br />' hydrocarbon-impacted soil and groundwater <br /> Based on these potential exposure pathways, and according to the San Francisco Bay <br /> Regional Water Quality Control Board's Screening For Environmental Concerns At <br /> Sites With Contaminated Soil and Groundwater, dated July 2003 (updated September 4, <br />' 2003), no significant risk is present at the site The groundwater screening levels for the <br /> evaluation of potential indoor air impacts, assuming residential land use, which is more <br /> conservative than the current commercial use, are 24,000 ug/l for MtBE and 5,300 ug/l <br /> for benzene The direct exposure screening levels for the construction/trench worker <br /> scenario are 2,800 parts per million (ppm) for MtBE and 17 ppm for benzene <br />' Dissolved Mass Balance Calculation <br /> Table 2 contains calculations of total mass of dissolved MtBE in groundwater in the <br /> vicinity of the site These calculations are based on groundwater analytical data <br /> collected in June and July 2003 Since the majority of MtBE molecules appears present <br /> in the sand horizon between approximately 28 to 40 feet bgs, the volumes used for mass <br />' calculations are based on a zone 15 feet thick The average concentrations used in the <br /> calculations are based on the mean of each contoured zone in Figure 4, or an <br /> approximate mean if only one or no data points were in the zone, which may slightly <br /> overestimate the actual concentrations present in that zone The total mass of dissolved <br /> MtBE in groundwater is approximately 17 30 pounds f <br /> v <br /> N lsacdP12o031Pr1881880040 1 a 3aa doc <br />