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17 June 1997 AGE Project No. 95-0118 ! . . Page 3 of 4 4.0. CONCLUSIONS AND RECOMMENDATIONS - 4.1. CONCLUSIONS Based upon results of ground water monitoring well sampling, AGE concludes the following: • The relative ground water elevation has increased an average 1.4 feet during the last quarter. • The plume of hydrocarbon-impacted ground water now extends to the location of monitoring well MW-5. The plume is delineated on the southwest, southeast and northwest, relative to the former UST pit. The plume is less well defined north of the former UST pit. The concentrations of dissolved hydrocarbons in MW-2 have increased compared to the } results of first quarter_of 1997; but decreased in MW-3. • The continued hydrocarbon-concentration flucuations on the site on the site indicates that the plume of benzene may not be stable and the occurrence of petroleum hydrocarbons in MW-5 may indicate that the dissolved phase plume is migrating toward the northeast. • MTBE was quantified in the sample from well MW-2, above the California Department of j Health Services (DHS) action level of 35 ppb. However, MTBE was not detected in any, other well. 4.2. RECOMMENDATIONS Based upon the results of the investigation, AGE recomiuendations the following: The next quarterly ground water sampling event should be conducted in July of 1997. • Feasibility testing has indicated that in-situ air sparging.(IAS), combined with soil vapor !.- extraction, would be a cost-effective remediation method for this site. If remediation is required, we recoinniend the installation of an SVEIIAS system,pending approval of costs from the California Water Resources Control Board: If active soil and ground water remediation will not be required at the site, then a long-term ground water monitoring plan with a site-specific dissolved hydrocarbon concentration goal for certified site closure should be established.