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SITE CONCEPTUAL MODEL Shell-branded Service Station 6131 Pacific Avenue, Stockton, California . bye 10 2.5 BTEX Plume Stability and TPHg concentrations are primarily below detection limits in wells Concentration Trends MW-1, MW-2,,and MW-5 and are decreasing in wells MW-3, MW-4, and MW-10.°TPHg concentration trends are stable in wells VEW-2, MW-7, MW-8, MW-9, and MW-11 and inconclusive in well MW-6. Benzene concentrations are primarily below detection limits in wells MW-1, MW-2, and MW-11 and are decreasing in wells MW-3, MW- 4, and MW-6. Benzene concentrations are stable in wells VEW-2, MW-5, MW-7, MW-8, MW-9, and MW-10.These concentration trends suggest a stable to shrinking plume. 2.6 Groundwater Definition Status The lateral extent of MTBE is defined in all directions around the site MTBE by wells MW-1, MW-8, MW-7, MW-6, MW-3, MW-5,and MW-2. 2.7 MTBE Plume Stability and MTBE is historically ND in wells VEW-2, MW-1, MWl2 MW-3 and- - n Concentration Trends MW-5 through MW-11. Concentrations are decreasing in MW 4. This trend suggests a shrinking plume. 2.8 Groundwater Flow Direction, From 1990 to present,depth to water has ranged from 30.72 to Depth Trends and Gradient 59.95 fbg in the monitoring wells. Overall depth to water is rising. Trends The groundwater flow direction varies, although it is predominantly easterly. A rose diagram is included on Figure 2. Historically the gradient has varied from 0.0005 to 0.01. From 2001 to present the gradient has varied from 0.002 to 0.004. 2.9 Stratigraphy and Hydrogeology The site is underlain by layers of clay(CL),silt(ML),clayey sand (SC),silty sand (SM), and sand (SP). Soil types vary from boring to boring without a consistent stratigraphic order of soil types. Boring logs are included in Attachment 2. 2.10 Preferential Pathways Analysis Aegis identified onsite utility locations. A map showing these locations is included in Attachment 3. All of the underground utilities are located along the southwestern property boundary. 2.11 Other Pertinent Issues None 3 Remediation Status 3.1 Remedial Actions Taken Aegis conducted an extended pilot vapor extraction test during December 1991. 3.2 Area Remediated The former UST com lex area. 3.3 Remediation Effectiveness Unknown 4 Well and Sensitive Receptor -v Surve •�t= 4.1 Designated Beneficial Water Existing and potential beneficial uses include municipal and O Use domestic water supply, agricultural supply, industrial service supply, and industrial process supply(CVRWQCB Basin Plan). 4.2 Shallow Groundwater Use 3 domestic wells (<200 fbg)were identified within 2,000 feet of the site. Cambria confirmed that the 2 nearest the site are not used. The remainingwell is approximately 2,000 feet southwest of the site. 4.3 Deep Groundwater Use 1 municipal well (screened from 210-262 fbg)was identified 2,000 G,. feet northwest of the site. 4.4 Well Survey Results As noted above, 1 domestic well and 1 municipal well were identifie 2,000 feet from the site. 4.5 Likelihood of Impact to Wells Because the wells are located upgradient to crossgradient of the site at distances of 2,000 feet, it is highly unlikely that site activities would impact these wells. 0854 Page 2 of 3 Date Modified: 10/3/2003