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Ms. Penny Fottrell GeoMa raix Chevron Environmental Management Company September 12, 2002 Page 3 depth of 48 feet bgs;affected soil was observed between depths of 9.5 and 46.5 feet b¢s. GP 120 was advanced to a depth of 16 feet bgs;affected soil was observed between 6.5 and 11 1 ' feet bgs. However,to confirm the vertical extent of petroleum hydrocarbons,we propose drilling in the vicinity of GP 96 and GP 120 to collect a sample beneath affected soil. • Delineation of the Lateral Extent ofAffected Soil—Petroleum hydrocarbon constituents were not observed or detected in samples from seven borings located approximately 50 to 100 feet to the northwest and southeast of borings that contained affected soil(GP 95,GP 141,GP 140, GP 118,GP 119,and GP 121;Figure 2). Therefore,the lateral extent of both areas has been defined to the northwest and southeast. However,additional data are needed to delineate the lateral extent to the northeast and southwest. r'! • Shallow Soil Data for Human Health Risk Assessment—Consistent with Califomia Environ- mental Protection Agency guidance,data from the top 10 feet of the site is typically used to evaluate health risks because it is considered likely that future use would only result in exposure to soil shallower than 10 feet. As part of the previous field effort,soil samples were only col- -- lected from the most visibly affected soil,which generally occurred at depths greater than 10 feet bgs. Geomatrix proposes to collect additional shallow soil samples(above 10 feet bgs)in the affected areas as well as northeast and southwest of the previous borings to assess potential health risk. • Delineation of the Lateral Extent ofAffected Groundwater—Grab groundwater samples were collected in borings GP 96,GP 97,GP 117,and GP 138. Petroleum hydrocarbon constituents were detected in all samples except the one collected from GP 138;however,separate phase petroleum hydrocarbons were observed in these samples and the reported concentrations are not representative of the dissolved fraction in groundwater. Therefore,Geomatrix will attempt to collect a grab groundwater sample that does not contain separate phase petroleum within the area of affected soil to assess dissolved concentrations of petroleum constituents. In addition, we propose collecting grab groundwater samples in the presume&upgradient(southwest)and downgradient(northeast)directions to assess potential impacts to groundwater. SCOPE OF WORK Geomatrix will advance a minimum of seven soil borings (MH3-1 to MH3-7, Figure 5) to collect data to address the data needs described above. Additional step-out borings may be drilled, depending on field observations as discussed below. CGS Prior to drilling, Underground Service Alert(USA)will be notified, the soil boring locations will 4I t,) be cleared by a private underground utility locator, and appropriate permits and access will be obtained from San Joaquin County Public Health Services,Environmental Health Division and Y " appropriate public agencies. Access for work within private property(e.g.,railroad right-of-way) will be obtained by ChevronTexaco. { 2 Groundwater gradient is based on water elevation data collected from dewatering wells and a reclamation water monitoring network in the site vicinity during 2001 and 2002.