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GEOMATRIX RWQCB water quality objectives (WQOs). The grab groundwater sample collected from the off-site and downgradient location GMX-6 contained low concentrations of TPHg, TPHd, and benzene. Based on previous site reconnaissance activities and discussions with facility person- nel, GMX-6 is located adjacent to a former gasoline station.3 The constituents detected in the GMX-6 sample may be related to this off-site source and not from residual crude oil originating from the OVP (Geomatrix, 2000). 3.0 INVESTIGATIVE OBJECTIVES AND APPROACH Based on the analytical results of the grab groundwater sample collected from GMX-6, the RWQCB requested additional off-site soil and groundwater assessment. ChevronTexaco sub- mitted the Work Plan(Geomatrix, 2003a) and Amendment (Geomatrix, 2003b) in response to the RWQCB's request. The RWQCB subsequently approved the Work Plan in a February 23, 2003 letter and the Amendment in an email. In August 2003, Geomatrix implemented the off-site investigation which included soil and groundwater sampling. Sampling locations are shown on Figure 3. Specific objectives of the off-site investigation are summarized below. • Assess the lateral extent of petroleum hydrocarbon-affected soil. Borings GMX-10, GMX-11, and GMX-12 were advanced to assess the off-site lateral extent of petroleum-affected soil. • Assess the lateral extent of petroleum hydrocarbon-affected groundwater. Groundwater samples were collected from on-site monitoring well MW-3 and from off-site borings GMX-10, GMX-11, and GMX-12 to assess groundwater quality at the site and downgradient(north) of the site. 4.0 FIELD AND ANALYTICAL METHODS Prior to conducting field activities, Geomatrix obtained a boring permit from the San Joaquin County Public Health Services (SJC-PHS) (Appendix A), notified Underground Service Alert (USA), and contracted with Cruz Brothers Sub-Surface Locators, Inc., of Milpitas, California, to perform an underground utility clearance at each boring location. Soil boring, groundwater sampling, and analytical methods are described in Sections 4.1, 4.2, and 4.3, respectively. 2 Water Quality Objectives(WQOs)for petroleum hydrocarbons are presented in A Compilation of Water Quality Goals,California Environmental Protection Agency,Regional Water Quality Control Board,Central Valley Region,by Jon B.Marshack,August 2003.;WQOs for chlorinated VOCs are based on United States Environmental Protection Agency Maximum Contaminant Levels(MCLS). 3 Based on a communication from Mr.Don Guzenhauser,facility representative for the 35275 Welty Road Site,a Union Gasoline Station was present from at least 1972 to the mid-1980s. I:\Doc_Safe\6000s\6447.001\Offsite Rpt\US Can SoilGW Rpt 2003.doc 5