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r^ I Ms. Vicki McCartney ProjectNo.: 880.040.1A.0110 May 23, 2003 Page 5 Potential exposure receptors include current and future workers and customers of the subject station at 3250 West Hammer Lane, motorists, pedestrians, and utility maintenance workers. The potential exposure mediums are ambient air and soil.and groundwater in potential future excavation areas. The major exposure pathway is hydrocarbon volatilization from smear zone soils and groundwater to ambient air. The potential exposure pathway for utility maintenance workers is dermal contact with hydrocarbon-impacted soil and groundwater. Dissolved Mass Balance Calculation Appendix E contains calculations of total mass .of TPHg, benzene, and MtBE in groundwater in the area currently delineated beneath the site. These calculations are based on analytical data presented in Shaw's Second Quarter _2003 Groundwater Monitoring Report, dated April 24, 2003. Since the majority of TPHg, benzene, and MtBE molecules tend to concentrate near the top of the groundwater column, without Ks consideration of possible downward flow or gradient effects, the volumes used for mass calculations are based on a zone three feet thick. The acreage concentrations used in the calculations are based on the mean of each zone, or an approximate mean if only one or I �5 no data points were in the zone, which may slightly overestimate the actual concentrations present in that zone. The total mass of TPHg, benzene, and MtBE in groundwater is approximately 0.39, 0.074, and 2.90 pounds, respectively. q� Other Environmental Issues The dissolved hydrocarbon plume has not been delineated to the northeast of well MW-4 (downgradient). The vertical extent of the hydrocarbon.plume has also not been fully delineated. Cross sections illustrating current conditions at the site are presented in Appendix F. Several underground utility lines are present near the subject site, including and due to shallow groundwater conditions, utility trenches may act as a preferential pathway. As part of this phase of the investigation; Shaw will obtain additional information on the '4 location of buried utility lines in the immediate vicinity of the site. Figure 3 shows the locations of buried utility lines beneath the subject site. SHAW ENVIRONMENTAL,INC. N:\sacDP\2003\Prj\880\8800401 A.3 ac.doc a .