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Commingled Plume#35 S E C 0 R Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1, 2006 Page 14 • 8.0 CONCLUSIONS AND RECOMMENDATIONS The objective of this Site Conceptual Model Is to summarize site activities and provide a guide for future assessment and remediation The following conclusions result from the previous remedial activities summarized in the preceding sections • Historical operations utilizing gasoline UST's and associated piping and dispensers at both the Chevron 9-4275 facility and the ARCO 2133 facility have resulted in the detections of petroleum hydrocarbons in soil and groundwater summarized above The majority of hydrocarbons in soil have been found in the area of the former underground storage tanks and dispensers A summary of the soil and groundwater analytical data, soil boring details, and groundwater gradient information are presented in Tables 1 through 6 • The nearest water supply well, City of Stockton Municipal Well #10, is approximately 100 feet to the east (crossgradient) However, the City of Stockton has not operated this well in at least 10 years and has no plans to use it in the future • Monitoring and sampling performed at the site since 1989 indicated the presence of TPHg, benzene, and MtBE dissolved in groundwater beneath the site The lateral extent of TPHg, benzene, and MtBE in groundwater has been delineated to low or non- detectable levels, except to the north • Based on groundwater samples collected from deep borings completed in March 2006, vertical delineation of the dissolved TPHg and benzene have been completed to low or non-detectable levels Although MtBE is not delineated vertically, the low concentrations reported in grab groundwater samples at 60 feet bgs in AS-9, AS-10, MW-1R, and MW-3R do not warrant further vertical investigation of groundwater • Following an examination of the site receptors and potential exposure mediums the only potentially complete exposure pathway is for a future construction worker to contact with impacted groundwater and subsurface sod Based on the above information SECOR recommends the following • The site should continue to be monitored and sampled on a quarterly basis to confirm groundwater concentrations and to evaluate plume stability The replacement wells (MW-1R through MW-3R, MW-5R, MW-7R, and MW-9R) have been added to the quarterly monitoring and sampling schedule • SECOR will continue to pursue offsite access north of the Chevron site to install monitoring well MW-10 to evaluate the northern extent of hydrocarbons in groundwater Once offsite access is granted and MW-10 is installed, SECOR will submit a report summarizing the well Installation and groundwater sampling activities • Following a review of the current and historical groundwater concentrations it appears that concentrations have been decreasing with time (Appendix 1) and the plume is stable Therefore, SECOR proposes that the SVEIAS system at the Chevron 9-4275 facility not be reinstalled In addition, the SVE system at the ARCO 2133 facility should not be restarted The AS system will continue to operate at the ARCO 2133 facility and SECOR will continue to maintain this system • 94275-ARCO 2133 SCM 9-2006 14