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. SITE CONCEPTUAL MODEL Former Texaco Service Station/ARCO 440 West Charter Way, Stockton, California 2 7 MTBE Plume Stability and MTBE concentration trends are stable or declining in the wells associated Concentration Trends with this site The only exception to this is well MW-14 located offsite to the southwest It appears that MTBE levels in this well are likely due to an offsite source 2 8 Groundwater Flow Direction, Since 1992, depth to water beneath the subject site has ranged from 17 to 43 Depth Trends and Gradient fbg in the monitoring wells The groundwater flow direction is typically toward Trends the east/southeast with the groundwater gradient varying around 0 001 to 0 007 Since 1992 the groundwater elevation beneath the site has risen approximately 20 feet This groundwater rise peaked in 1999 and is now slowly declining The groundwater elevation has decreased approximately 5 feet since 1999 A graph of historical (1992 to 2003) water levels in well MW- 1 is included in Attachment 2 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 & SW) The predominant soil type beneath the site is fine grained sods (silt and clay) There are several coarse grained layers (sand) beneath the site that appear to be laterally continuous These coarse grained layers occur at approximate depths of 80 and 120 fbg and vary in thickness across the site Boring logs are included in Attachment 3 and site cross sections are included in Attachment 4 I 2 10 Preferential Pathways Analysis Since the depth to groundwater is deeper (>= 17 fbg) underground utilities are buried, it is highly unlikely there are preferential pathways in the subsurface as a result of underground utilities No natural preferential pathways (buried stream channels, etc) have been identified in the subsurface It appears contaminant transport is occurring through the naturally deposited sediments beneath the site 2 11 Other Pertinent Issues None 3 Remediation Status 31 Remedial Actions Taken Interim remediation, in the form of sod vapor extraction (SVE) using a thermo oxider, was performed in wells VEW-1 through VEW-4 from January 1996 to March 1998 Approximately 6,256 pounds of TPHg were removed from VEW-1 through VEW-4 3 2 Area Remediated The vicinity of VEW-1 through VEW-4 3 3 Remediation Effectiveness Influent concentrations were reduced from a high of 9,700 mg/m3 (February 1996) to <10 mg/m3 March 1998 4 Well and Sensitive Receptor Surve 41 Designated Beneficial Water Existing and potential beneficial uses include municipal and domestic water Use supply, agricultural supply, industrial service supply, and industrial process supply(CVRWQCB Basin Plan) 4 21 Shallow Groundwater Use INone identified in the vicinity of the site 4 31 Deep Groundwater Use I None identified in the vicinity of the site 1399 Page 2 of 3 Date Modified 11/2/2004