SITE INFORMATION AND CORRESPONDENCE_FILE 2

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installed the necessary wells for this remedial approach. From January to April 1991, wells IW 1, PI, P2, V 1,V2, and V4 were installed onsite, and wells RW4,V3, MW 11, and MW 12 were installed offsite. Aquifer tests as well as injection/percolation tests were performed at the site to gain a better understanding of the hydraulic properties of soil underlying the site. The injection/percolation tests were performed to assess if a hydraulic barrier could be created which would prevent the migration of upgradient hydrocarbon-impacted groundwater from the Shell service station to the former Exxon site during groundwater extraction onsite (EA 1991a). Due to problems in securing the permit for re-injection of treated groundwater,this remedial option was not implemented. Liquid-phase hydrocarbons (LPH) had been found in wells MW3, MW6, RW3, VI, V2, and V4, as documented in quarterly groundwater monitoring reports. In December 1992, passive skimmers were installed in wells RW3 and V 1 to recover LPH. A soil vapor extraction and air sparging (SVE/AS) system with two air sparging wells (SWI and SW2) was installed in May 1994. The SVE/AS system operated from May 1994 to November 1996 and recovered petroleum hydrocarbon vapors from wells MW2, MW3,MWS, and MW6 (screened from 10 to 30 bgs), wells VI and V2 (screened from 20 to 40 feet bgs), and well V4 (screened from 25 to 40 feet bgs). A total of approximately 14,500 pounds of petroleum hydrocarbons was removed from the vadose zone by the SVE/AS system(EA 1997). In November 1996, a vacuum vaporizer well system, known as UVB (Underdruck Verdampfer Brunnen), was installed for groundwater remediation at the site. The installation of the UVB system consisted of overdrilling existing groundwater recovery well RW3 to install the dual-screen UVB well for groundwater extraction-injection, and the installation of adjacent monitoring wells UVB-MW 1 and UVB- MW2 to monitor system performance (EA 1997). This remedial option was selected as it prevented migration from an upgradient offsite source, while remediating the onsite groundwater hydrocarbon concentrations. Between 6 February 1997 and 22 September 2000, a total of 1,890.2 pounds of petroleum hydrocarbons as gasoline (TPH-g) was removed from groundwater by the UVB system based on laboratory analytical results, and 821.5 pounds based on field measurements. The operational measurements of the UVB system showed that it had reached the asymptotic stage of operation. The UVB system was therefore shut down as referenced in ETIC's letter dated 29 October 2000 (ETIC 2000c). The shutdown of the UVB system was approved by the SJCPHS in a letter received in December 2000. RBCA A review of the historical data at the site in comparison to generally accepted data needs to perform a risk-based corrective action (RBCA) analysis identified two data gaps: (1) soil gas data, and (2) a well survey. These tasks were performed and the RBCA analysis indicated the following: (1) The risk associated with commercial/industrial exposure to detected levels of hydrocarbons in surface soils is protective of target levels adopted by the USEPA; (2) the risk associated with commercial/industrial and residential exposure to detected levels of hydrocarbons and MTBE in soil vapor beneath the site is protective of target risk levels adopted by the USEPA; (3) the construction worker exposure risk associated with a single historical benzene concentration in a soil sample exceeds the USEPA-defined target risk range, and therefore an appropriate health and safety plan should be developed,ensuring protection of future construction workers at the site. G:VmjmU\73%2TUBL1G 9m n&V C.dm 2