SITE INFORMATION AND CORRESPONDENCE

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Mr. Scott Hilyard - 2 - 6 July 2006 detected in groundwater in 2004 are as high as 24,000µg/L. Furthermore, soil vapor samples have not been collected at the site and potential off-gassing of BTEX and TPHg from the groundwater to on-site buildings has not been assessed. Given the high concentrations detected in groundwater and the uncertainty associated with extent of the contamination, the discussion of contaminant migration potential in Section 3.3 should be revised to state the available soil and groundwater data indicates there is a potential health risk at the site. Also, Regional Water Board staff recommends adding collection of a few shallow soil vapor samples to the Final Plan in order to assess potential vapor intrusion hazards at the site. Soil vapor samples should be collected above soil and/or groundwater hotspots. Sampling procedures and assessment of potential vapor intrusion hazards should follow the California Department of Toxic Substances Control's Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (December 2004). 5. The Draft Plan indicates the concrete wash pad and oil-water separator have not been investigated previously as a potential source of petroleum contamination. Regional Water Board staff concurs with placing an exploratory boring adjacent to it, but CANG should also confirm that solvents were not released at this location. The Draft Plan should be revised to state that proposed investigation will verify whether a solvent release occurred at this location and ask the contract laboratory to report all EPA 8260 analytes for both soil and groundwater samples. 6. The proposed sampling program discussed in Section 6 and summarized in Table 6-1 is confusing and open to interpretation. This section and table need to be rewritten to clearly identify sampling rationale,target sample depths, and number of samples that will be collected at each boring location. 7. Regional Water Board staff does not have a copy of the CA ARNG Site Investigation Quality Assurance Project Plan(QAPP) (Radian International, 2000a) and CA ARNG Health and Safety Plan (Radian International, 2000b)that describes the field procedures that will be followed by CANG during implementation of the Draft Plan. Please forward copies of these documents to US. Specific Comments 1. Page 20, Section 6.0.3: The second sentence in this section is unclear. Is CANG proposing to begin soil sampling at 2 feet below the tank excavation fill/native soil interface or 2 feet above it at each boring location? The boundary of the tank excavation is not shown on any figures, but four of the borings are more than 50 feet from the location of the former tanks. Why is backfill expected at these locations? Revise the second sentence to clarify the sampling that CANG is proposing to do, explain why each boring is expected to encounter fill, and add the UST excavation boundary to the figures (if known). The Draft Plan indicates CANG is uncertain whether the soil used to backfill the tank excavation was clean fill or the contaminated soil excavated during removal of the USTs in 1996. If sampling of the fill is not included in the Draft Plan, then CANG needs to include it in the Final Plan.