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of site conditions, including mass balance calculations, a review of subsurface geology, groundwater levels, and the lateral and vertical distribution of hydrocarbons, Dames & Moore concluded that there was no significant risk to impact groundwater at the site, and recommended 5 years of groundwater monitoring If during this 5-year monitoring period no impacts to groundwater are reported, final closure would be recommended Comments from the PHS-EHD on the Draft Amended Closure Request were issued August 29, 1994, and concluded that the site remains a potential threat to groundwater The PHS-EHD requested that AFPC conduct a feasibility study to address reducing the contamination levels to background The PHS-EHD also requested construction of a monitoring well immediately down-gradient of the former waste oil tank, and additional bormgs to assess any changes in subsurface conditions since completion of the remedial actions in 1990 In November 1994, AFPC submitted a Technical Evaluation Report and work plan, prepared by Dames & Moore, to the RWQCB and the PHS-EHD The Technical work plan addressed installation of the requested monitoring well and additional soil borings The Technical Evaluation described and evaluated seven options for TPH-d-impacted soil at the site • Option 1 — No Action, • Option 6 — Excavation and Disposal to Background (6A) and 1,000 • Option 2 — Groundwater Monitoring, mg/kg TPH-d (6B) and • Option 3 — Asphalt Cap Maintenance and • Option 7 — Evaluation and On-Site Groundwater Monitoring; Treatment to Background (7A) and 1,000 mg/kg TPH-d • Option 4 — Bioventing, (7B) • Option 5 — In-Situ Bioremediation Additional options were considered, but screened out prior to detailed evaluation Option 1 was excluded from consideration because of administrative and regulatory concerns, Options 4 and 5 were technically unreliable and ineffective, and Options 6 and 7 were excessively costly (estimated capital cost of $1,900,000 to $3,700,000) Given the absence of impact or threat to groundwater at the site, Option 3 was considered to be the most technically sound and cost-effective and was recommended at the conclusion of the Technical Evaluation Report (Dames & Moore, 1994a) The PHS-EHD provided comments on the Technical Evaluation Report in a March 1, 1995 letter to AFPC The letter stated that "PHS-EHD has determined that the maximum allowable level of TPH as diesel (TPH-D) that can be left in the soil at this site using a depth to groundwater of 35 feet bgs is 100 parts per million " Options 1, 2 and 3, which provide for no action or limited groundwater monitoring, were refected as remedial options, by the PHS-EHD The March 1, 1995 letter from PHS-EHD also indicated that installation of the proposed additional monitoring well be SAC156 14 3 DAMES & MOORE