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I 2.0 BACKGROUND rzl� In May 1991, three underground storage tanks (USTs) were removed from the common tank pit shown on the Detailed Site flan, Figure 3. Strong petroleum hydrocarbon odors and discolored soil were noted in the tank pit. Confirmation soil samples were collected and laboratory analysis confirmed the presence of total petroleum hydrocarbons as gasoline (TPHg). Several subsurface investigations partially defined the extent of impacts in soil and groundwater; resulting in the installation. of nine groundwater monitoring wells (MW-1 through MW-9). The contaminants of concern include TPHg; benzene, toluene, ethylbenzene and xylenes (BTEX); methyl'tert-butyl ether (MTBE), tert-butanol (TBA), di-isopropyl ether (DIPE), ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) otherwise known as fuel oxygenate compounds (FOCs); 1,2-dichloroethane (1,2-DCA); and 1.2-dibromoethane (EDB). In September 2000, Secor International Incorporated (Secor) began remedial activities and oversaw t� the installation of six soil vapor extraction (SVE) wells (V1 through V6) and one air sparge (A. S ) well (AS-1) in or adjacent to the former tank pit at the Site (Figure 3). Soil samples collected from V-1,V-2,V-3, and AS-1 contained elevated concentrations'of BTEX, TPHg, and MTBE. a 2.1 Soil Vapor Extraction & Air Sparge Feasibility Testing In October 2000, Secor conducted a two-day SVE and AS feasibility test using wells V I through V6 and AS-]."Based on the test results, the amounts of TPHg and benzene removed during the SVE/AS ' feasibility ,test were 102 pounds and 1.9 pounds, respectively. Based on the induced vacuum response (0.2 inches of water at 40 feet) and the slight rise in groundwater levels in offsetting monitoring wells,the SVE radius of influence was estimated to be 40 feet. While the air sparging did " not result in an immediate increase in TPHg vapor concentrations the increase in dissolved oxygen Ygen concentration and slight rise in the water table in offsetting wells indicated a radius of influence of approximately 30 feet. Details related to. the feasibility test are presented in Secor's Problem Assessment Report dated November 8, 2000._ 2.2 Final Remediation Plan SECOR submitted a Final Remediation Plan (FRP) on March 2, 2001, that was approved by the California Regional Water Quality Control Board, Central Valley Region, (RW.QCB) in a letter dated March 26,2001. l Due to an expansion of the remedial objectives, Secor submitted" a" Work Plan for Remedial Approach and Well Installation dated May 2, 2002. The RWQCB approved the work plan in a letter dated May 13, 2002. The final remediation plan included 10 soil vapor extraction (SVE) wells (V1 through V 10 each approximately 30 feet deep) and 20 ozone-sparge wells (SI through S20, each -� approximately 45 feet deep). Project No.58643-06-01 -2- December 30,2004