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Ms. Vicki McCartney Project No. 8800401A.0201 <br /> 1 October 26, 2004 <br /> Page 7 <br /> pathway is hydrocarbon volatilization from smear zone soils and groundwater to <br /> ambient air. The potential exposure pathway for utility maintenance workers is <br /> dermal contact with hydrocarbon-impacted soil and groundwater. <br /> r Based on these potential exposure pathways, and according to the San Francisco <br /> Bay RWQCB's Screening For Environmental Concerns At Sites <br /> With Contaminated Soil and Groundwater, dated July 2003 (updated <br /> September 4, 2003), no significant risk is present at the site. The groundwater <br /> screening levels for the evaluation of potential indoor air impacts, assuming <br /> 1 residential land use, which is more conservative than the current commercial use, <br /> are 24,000 µg/1 -for MtBE and 5,300 µg/l for benzene. The direct exposure <br /> screening levels for the construction/trench worker scenario are 2,800 parts per <br /> million (ppm) for MtBE and 17 ppm for benzene. <br /> Based on the remaining petroleum hydrocarbon concentrations currently reported <br /> j •__ <br /> at the site, along with declining concentration trends in groundwater, there does <br /> not appear to be a significant threat to human health or the environment, or to <br /> any other potential beneficial uses of shallow groundwater beneath the site. <br /> ■ Appropriate conclusions and recommendations for the next phase of work. <br /> The extent of impact to soil and groundwater appears delineated. Additional <br /> assessment work is not warranted at this time. In order to facilitate timely <br /> 1 closure of the site, active remedial action appears appropriate. <br /> ■ An updated Site Conceptual Model illustrating site conditions showing the extent <br /> of known soil and groundwater impact relative to the leaking UST system and the <br /> relationship between contaminants and potential receptors. <br /> r� <br /> Figure 7 presents a site conceptual model of the site illustrating existing site - V <br /> � t <br /> conditions. <br /> a_ <br /> 7j <br /> t <br /> f 5 <br /> !_J v <br /> Y N:IsacDP120041Pp188018800401A.4ad.doc Shaw Environmental,Inc. <br />