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I Work Plan for Additional Assessment S E C O R ARCO Service Station 2130 Stockton, California Page 4 o 15 SOIL EVALUATION Prior to conducting a soil vapor extraction test, the residual soil contamination was evaluated for all routes of exposure in order to protect human health , safety, and the environment. Table 5 presents a comparison of the soil concentrations to San Francisco Bay Regional Water Quality Control Board Environmental Screening Levels for residential, commercial, and construction workers. None of the soil concentrations appear to pose a risk via the indoor air pathway to any of the potential populations. The only potential residential risk is dermal contact to TPH. However, this comparison does not consider degradation . These soil sample results are from 1993 and have likely declined from the original concentrations. Soils deeper than 10 feet below ground surface (bgs) impacted with xylenes and TPH could potentially pose a risk to construction workers via dermal contact. However, the cited concentrations are from 1993 and 1995 and have likely declined . PROPOSED SCOPE OF WORK The scope of work outlined in this work plan includes the installation of additional groundwater monitoring wells and the procedures for conducting a bench scale study of the potential leachability of metals as a result of oxygen injection . The objective of the additional site assessment is to provide lateral and vertical delineation of the petroleum hydrocarbon plume. The following sections describe the installation of seven groundwater monitoring wells (MW- 14 through MW-20) . Recommendations in the Site Conceptual Model included the use of periodic oxygen injection to treat the petroleum hydrocarbons in the groundwater. The January 12, 2007 SJCHED letter requested a bench scale pilot test prior to approval of this remediation technology. As requested by the SJCEHD, SECOR contacted Jim Barton with the Regional Water Quality Control Board (RWQCB) regarding procedures for conducting this test. The objective of this bench scale study is to address concerns about the possible mobilization of general minerals in soils and the creation of degradation byproducts from the chemical oxidation petroleum hydrocarbon and fuel oxygenate. The specific objective of the study is to evaluate the efficiency of oxygen injection on methyl tert butyl ether (MtBE) , benzene, toluene, ethylbenzene, and xylenes (BTEX) and total petroleum hydrocarbons as gasoline (TPHg) . Task 1 - Preliminary Field Activities Prior to conducting the subsurface investigation , a site-specific Health and Safety Plan (HASP) for use by personnel during implementation of this work plan will be prepared. The HASP will address the proposed subsurface investigation activities, and a copy of the HASP will be available on-site during field work. Subcontractors performing field activities will also be provided a copy of the HASP prior to initiating work. The required monitoring well installation permits will be obtained from San Joaquin County Environmental Health Department (SJEHD) . All locations will be marked and Underground Service Alert (USA) will be notified at least 48 hours in advance of drilling . In addition , proposed boring locations will be cleared by a private utility locator and boring locations will be cleared by hand to approximately 5-feet bgs before machine drilling is initiated. Task 2 — Soil Boring Advancement , Soil Sampling and Groundwater Monitoring Well Installation In order to evaluate the current lateral extent of fuel- related hydrocarbons, MtBE , and 1 ,2- DCA four shallow zone (screen interval 30 to 50 feet bgs) and two deeper zoned (screen interval 60 to 75 feet bgs) groundwater monitoring wells (MW-14 through MW- 19) are proposed at the approximate locations shown on Figure 3. In addition , in order to delineate