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SITE CONCEPTUAL MODEL Prepared by Cambria Environmental Technology,inc 2.7 MTBE Plume Stability and MTBE concentrations have been variable at the site. Since MTBE is no Concentration Trends longer a component of the fuel retailed at this service station(as of January 2003), MTBE concentrations are expected to decrease. Concentrations of MTBE in wells MW-1 and MW-6 have declined after initiation of periodic groundwater extraction GWE_ events in these wells. 2.8 Groundwater Flow Direction, Since 2003, depth to water beneath the subject site has ranged from 9.7 tcl Depth Trends and Gradient 13.5 fbg In the monitoring wells, and'groundwater elevations have range Trends from 11.44D 16.2 feet above mean sea level. The groundwater flow directio is typically toward the north or northwest with the groundwater gradien varying from 0.02 to 0.007. 2.9 Stratigraphy and Wydrogeology The predominant soil type beneath the site is fine-grained soils (silts an i clays) to the total depth explored of approximately 86 fbg. Silty sand an { sand-layers have been encountered that do not appear to be laterall ; continuous across the site. Boring logs are included in Attachment 3, and site cross section is included in Attachment 4. 2.10 Preferential Pathways Analysis No analysis has been performed. Since the groundwater flow directio f beneath this site Is consistently to the north (away from West Grant Lin Road and toward Interstate 205), it is unlikely that any underground utili trenches beneath West Grant Line Road would be acting as a preferentia pathway for the Shell plume. Additionally, given the fact that groundwater i typically deeper than 9 fbg at this site, and most underground utilities are buried less than 5 fbg,there are-few, if any, underground utility trenches tha could act as preferential pathways. 2.11 Other Pertinent Issues Several additional leaking underground fuel tank(LUFT)sites are located in the immediate vicinity of the site,.and the MTBE plume beneath the site is likely commingled with the plume from the upgradient 7-Eleven sites. Investigation reports for the 7-Eleven site, located south of the Shell site, have reported that MTBE from that site has migrated beneath the Shell site. i z - 3.1 Remedial Actions Taken Interim remediation of periodic GWE began in August 2005 from wells MW-1 and MW-6. A vacuum truck is used to extract groundwater from the we11 from dedicated stingers. Extracted groundwater is off-hauled and transporte to the Martinez Refinery for recycling. 3.2 Area Remediated Groundwater in the vicinity of wells MW-1 and MW-6, located downgradient of the dispensers and USTs at the site have been targeted for periodic GWE. 3.3 Remediation Effectiveness Groundwater analytical data from the first quarter of 2006 show a decline i MTBE concentration In wells MWA and MW-6. These data suggest periodic i GWE appears to be effective in reducing MTBE concentrations. 4.1 Designated Beneficial Water Existing and potential beneficial uses for groundwater include.municipal anc Use domestic water supply, agricultural supply, industrial service supply, anc industrial process supply(CVRWOCB Basin Plan). 4.2 Shallow Groundwater Use None identified. 4.31 Deep Groundwater Use lWater supply. 1661 Page 2 of 3 Date Modified:3/21/2006