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Work Plan for Additional Subsurface Assessment and Remedial Action—Former Unocal Station#0187 November 27,2001 The dissolved hydrocarbon plume in Zone C is delineated to the west (upgradient), north, and southeast, but not downgradient (east to northeast) of the Unocal site. Dissolved hydrocarbons were detected in wells U-10, NP-2-116, U-12, U-19, and U-21, with a trace concentration of benzene detected in well U-17 (0.97 parts per billion, or ppb). Total Petroleum Hydrocarbons as gasoline (TPHg) and benzene concentrations detected in Zone C wells range from 170 to 710 ppb, and 0.97 to 120 ppb, respectively, with concentrations diminishing toward the perimeter of the plume. Groundwater Monitoring and Sampling Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994. During the most recent sampling event on August 13 through 15, 2001, TPHg range hydrocarbons were reported at concentrations ranging from 84 to 10,000 ppb, and benzene concentrations ranged from 0.97 to 3,800 ppb. MtBE was not detected in any well by EPA Method 8260B. The highest concentrations of TPHg range hydrocarbons and benzene were detected in well NP-2-60. Depth to groundwater ranged from 23.77 to 28.48 feet below top of casing, and the groundwater flow direction was in a northeasterly direction at a gradient ranging from 0.002 to 0.01. Quarterly sampling and monitoring of the wells is performed by GR. Historical groundwater chemical analytical data are summarized in the most recent GR report (Third Quarter 2001 Groundwater Monitoring and Sampling Report, dated September 27, 2001). Remedial Actions GR personnel performed a five-day vapor extraction (VE)pilot test at the site from November 27 through December 1, 2000. The VE system operated for a total of 40 hours, and soil vapors were extracted from wells UV-1, UV-2, and UV-3. The purpose of the VE pilot 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 conclusion of the VE pilot test was that VE technology is not appropriate for this site. From December 2000 through July 2001, GR performed an interim remedial action test of the existing ozone microsparging system. The system was operated continuously over this period of time and groundwater samples were collected from surrounding groundwater monitoring wells to evaluate the effectiveness of the system. Groundwater samples were collected weekly for the first three months, then every two weeks through the end of the test. The results of the test show that the system was successful in significantly reducing dissolved hydrocarbon concentrations in the groundwater beneath the site, and there were some indications that the treatment system might be reducing concentrations in downgradient wells. Based on these results, the system continues to operate. SCOPE OF WORK In order to further delineate the downgradient extent of the dissolved hydrocarbon plume in Zone C, and to expand the ozone microsparging remedial system, GR proposes to install one additional off-site groundwater monitoring well, and install and operate two additional on-site and six off-site ozone microsparging wells at the locations shown on Figures 2 and 3. GR Field Methods and Procedures are included in Appendix A. 240042.11 3