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Work Plan for Additional Site Assessment and Interim Remediation/Feasibility Testing ARCO Facility No.5469 Stockton,California Page 4 To evaluate the downgradient extent of the dissolved phase petroleum hydrocarbons and oxygenates, Delta proposes to install a well cluster (MW-11 and MW-12) to the east of the site (see Figure 3). In addition, Delta proposes to install a well adjacent to GZA well MW-11 (screened between 53 and 73 feet bgs) to assess the lateral extent of impact in the upper sandy unit. The shallow wells will be screened between 35 and 55 feet bgs to monitor the upper sandy unit. The deeper well will be screened between 60 and 75 feet bgs to monitor the lower sand unit. To assess the vertical extent of hydrocarbons beneath the site, Delta proposes to advance one Cone Penetrometer Test (CPT) boring onsite (see Figure 3) to a depth of 120 feet bgs, and collect depth discrete soil and groundwater samples. This CPT boring is located downgr adient of the UST complex and will extend approximately 50 feet deeper than the base of any well that shows hydrocarbon impact(MW-10). Interim Remediation/Feasibility Testin Considering the known vertical and lateral extent of impact to soil and groundwater in the vicinity of the site, the primary objective of the interim remediation/feasibility testing is to evaluate an alternative to mitigate the impact originating from the ARCO site, while not adversely affecting the adjacent plumes. Delta proposes to initiate interim remediation/feasibility testing to evaluate the results of ozone injection. Delta proposes to inject ozone (C-SpargerTM) at five locations (see figure 4) through sparge wells installed to a depth of 55 feet bgs. Review of published literature indicates that this technology combines the unit operations of air stripping and oxidative decomposition into a single process. Complete ozonation of petroleum hydrocarbons should result in formation of carbon dioxide and water. Ozone injection conducted at low flow rates and pressures, is pulsed on a predetermined cycle. Therefore, vapor extraction or recovery is not required and there is low risk of groundwater mounding at the sparge wells. Given the site geology of silts to clayey silts, relatively flat gradient of 0.0022 ft./ft. (fourth quarter 2001, sampling event), and the presence of other petroleum hydrocarbon plumes in the vicinity of the site, Delta does not propose to test conventional remedial alternatives such as groundwater pumping, soil vapor extraction, and air sparging at this time. Delta is concerned that conducting a groundwater pump test could potentially draw contamination from the nearby plumes to the ARCO facility, and therefore is not proposing a groundwater pump test. Review of the impact to soil samples collected in the upper 40 feet did not indicate significant impact to warrant remediation of the vadoze zone. Moreover, the upper 40 feet predominantly consists of clayey silts, which are not typically conducive to soil vapor extraction. The presence of interbedded sand units, P:\ARC0154691Work Plan15469 wrkpin 03-2002.doc