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Chevron Products Company 12 Former Chevron Service Station #9-5775, 301 West Kettleman Lane, Lodi, California June 5, 2001 Concentrations of MtBE are also shown to decrease to the south and southwest (Figures N-5 and • N-6) 4.3 Geologic Cross Section Maps Based on observations during drilling activities, the site vicinity is underlain by sandy silts, clayey silts, silty sands, clayey sands, and sands of varying amounts. Numerous water -bearing zones consisting primarily of sands and silty to clayey sands appear from first encountered water (approximately 58 feet bgs) to a depth of approximately 220 feet bgs Low permeability zones were encountered at various depths and locations, but due to the presence of water in these zones at varying locations, they do not appear to act as confining layers A cross section location map and geologic cross section A -A' are presented as Figures 12A and 12B, respectively A cross section location map and geologic cross section B -B' are presented as Figures 13A and 13B, respectively A geologic cross section for the site from C -C' to a maximum depth of 217 feet bgs is presented as Figure 14 Depth discrete and groundwater monitoring well analytical data are also presented on the cross sections Historical boring logs and well construction details for MW -1 through MW -20B, EX -1 through EX -3, and VE -1 through VE -4 can be found in Attachment O 4.4 Exposure Pathway Model An exposure pathway model has been developed to illustrate likely sources, exposure pathways, 40 and receptors (Figure 16) Potential exposure pathways were evaluated using a Tier I and 2 Risk Assessment summarized below and presented in full within Attachment P 0 4 4 1 Summary and Conclusions A conservative, tiered risk-based assessment was conducted to evaluate on-site exposures from site VOCs via soil and groundwater volatilization into indoor air The majority of VOCs (including benzene, toluene, ethyl -benzene, xylenes, MtBE, DIPE, EtBE, TAME, 1,2 -DCA, TCE, P-1PMB, and PCE) detected in site soil and groundwater were eliminated from further evaluation in Tier 1 Based on the Tier 2 analysis, which included site specific fate and transport modeling and evaluation of the impact to indoor air, the four chemicals evaluated in Tier 2 (including methanol, ethanol, TBA, and EDB) were also found to not pose a significant risk to human health and welfare. 5.0 EVALUATION OF EXISTING REMEDIAL APPROACH 5.1 GWE System Operation The groundwater extraction (GWE) system extracts groundwater from site wells EX -1, MW- 14/EX-2, and EX -3 Extraction and treatment of groundwater from EX -1, EX-2/MW-14, and EX - I IChevron1957751ParFrpl9-5775PARFRP2 doc