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f t Stantec April 24, 2009 Ms. Kristi Shelton decreased. Low concentrations of diesel have also been reported in MW-2. The site layout is illustrated on Figure 2. The nature of chemical impacts to soil and groundwater is consistent with a historical source of fuel hydrocarbons. Volatile constituents such as BTEX and MTBE have never been detected in soil or groundwater samples, and analysis of historical soil samples have typically identified a non-typical gasoline chromatogram pattern indicative of a weathered fuel product. Based on observed groundwater chemical conditions, Stantec recommended the Site be evaluated for regulatory case closure. Following submittal of the August 2008 groundwater monitoring report, the Board concurred with Stantec's recommendation for site closure. However, during preparation of the closure documents, Board staff determined that concentrations of fuel hydrocarbons remaining in soils required remediation or filing of a deed notice. Specifically, Board staff cited concentrations of petroleum hydrocarbons in excess of gross contamination screening levels established by the San Francisco Bay Regional Water Quality Control Board (SF Bay Board). During a subsequent teleconference, Board staff agreed that residual petroleum hydrocarbons present in groundwater did not require additional monitoring. As presented in this Work Plan, Stantec proposes using the gross contamination Environmental Screening Levels(ESLs) developed by the SF Bay Board as remediation goals for Site soils. Based on a comparison of historical soil chemical data to the gross contamination ESLs for residential soil, Stantec estimates that the cleanup levels can be achieved by excavating the upper 10 feet of soil across an area approximately 15 feet by 15 feet, centered on well MW-1. The area of proposed excavation is illustrated on Figure 3. Chemical- specific cleanup goals are presented below. PROPOSED REMEDIATION GOALS Stantec proposes using the residential soil gross contamination ESLs as the soil remediation goals. Constituents of concern in site soils are total petroleum hydrocarbons as gasoline (TPHg), diesel (TPHd)and motor oil (TPHmo). The ESLs are differentiated for shallow soils(3 meters deep or less) and deep soils (deeper than 3 meters). The ESLs for TPH constituents in shallow and deep soils are as follows(RWQCB, 2008): ❑ TPHg: 100 milligrams per kilogram (mg/kg)for shallow soils, and 5,000 mg/kg for deep soils. ❑ TPHd: 100 mg/kg for shallow soils, and 5,000 mg/kg for deep soils. ❑ TPHmo: 500 mg/kg for shallow soils, and 5,000 mg/kg for deep soils. Confirmation samples collected during the 2007 excavation reported concentrations of TPHg, TPHd, and TPHmo at depths of 3 to 8 feet which exceeded the proposed cleanup goals. Additional sampling to depths of 27 feet bgs completed during the subsequent investigation confirmed that soils in the upper 8 feet bgs exceed screening levels. Stantec proposes excavating the area illustrated on Figure 3 to approximately 11 feet bgs, and applying the gross contamination ESL of 5,000 mg/kg for each TPH constituent as the cleanup level.