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Work Plan for Additional Subsurface Assessment and Remedial Action—Former Unocal Station#2859 December 11,2001 MW-12(18 ppb), and MW-15 (17 ppb). Methyl tert-butyl ether(MtBE) was reported in well MW-14 at a concentration of 2.1 ppb by EPA Method 8260. Vapor Extraction Test OR performed a five-day vapor extraction (VE) test at the site from .Tune 18 through 22, 2001. The purpose of the VE test was to evaluate the feasibility of the VE technology and collect needed site specific data for the design of a VE system if deemed necessary. The VE test was performed for a total of 39 hours. An IC engine was used to extract vapors from wells VW-1,VW-2, and VW-3. Air samples were collected from the influent and effluent vapor streams. Petroleum hydrocarbon concentrations detected in the influent vapor stream were up to 311 ppmv of TPHg, and up to 2.2 ppmv of benzene. No detectable concentrations of MtBE were reported. The only well where significant concentrations of petroleum hydrocarbons were removed was well VW-3. Based on the VE monitoring data,no significant amount of TPHg or benzene were removed during the pilot test. It is estimated that less than one pound per day of petroleum hydrocarbons could be extracted. Vapor extraction would have little or no effect at this site. SCOPE OF WORK In order to further delineate the extent of the dissolved hydrocarbon plume in the vicinity of the subject site and to remediate groundwater beneath the site, OR proposes to install two additional off-site groundwater monitoring wells, and install and operate eight on-site ozone microsparging wells and supporting equipment at the locations shown on Figure 2. OR Field Methods and Procedures are included in Appendix A. To implement this scope of work, OR proposes the following seven tasks: Task 1. Pre-field Activities OR will update the site-specific safety plan and obtain the necessary well installation permits from the SJCPHS and encroachment permit from the City of Stockton. Underground Service Alert (USA) will be notified at least 48 hours prior to initiating field work. Task 2. Well Installation GR will install two groundwater monitoring wells at the locations shown on Figure 2. Drilling and well construction will be performed by a California licensed driller. The groundwater monitoring wells will be installed in well borings drilled to depths of approximately 60 and 90 feet bgs. Soil samples for description and possible chemical analysis will be obtained from the well borings at five-foot intervals, as a minimum. A OR geologist will monitor the drilling activities and prepare a log of each boring. Soil samples collected at 30 feet bgs and every 20 feet in depth thereafter will be submitted for chemical analysis as described in Task 6. 240072.06 3