SITE INFORMATION AND CORRESPONDENCE

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C. Scott Mansholt - Chevro* - 4 - 19 June 2008 Mountain House #4 SAIC stated that an environmental risk evaluation should be conducted to determine the potential risks to human health and the environment. The findings for SAIC's environmental risk evaluation are summarized below. ENVIRONMENTAL RISK EVALUATION SAIC stated that the land use for Byron Road and the associated ROW will remain unchanged and continue as a roadway with occasional roadway and pipeline maintenance. Future residential and/or commercial use of this site is improbable, thus residential and/or commercial use exposure to affected soil or groundwater is unlikely. Based on this, SAIC concluded the applicable human health risk scenario consideration will be for construction/trench worker exposure to affected soil and/or groundwater. Groundwater use does not exist at or near the site and it is unlikely domestic/commercial use of shallow groundwater will occur in the future. Consequently, exposure pathways from groundwater to potential receptors should not occur. SAIC stated that no surface water habitat is present within the area of affected soil and/or groundwater, so surface water receptor scenarios are not applicable. The chemicals of potential concern at the site - TPHc, BTEX, and PAHs were identified during the site investigations. SAIC compared the analytical results to the Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater (Interim Final, 2007) document by the Regional Water Quality Control Board-San Francisco Bay Region. The document is referred to as the "Environmental Screening Levels" or ESLs document. The screening levels are generally based on Preliminary Remediation Goals (PBGs) developed by the U. S. Environmental Protection Agency, Region IX. The analytical results for TPHc, PAHs, and BTEX compounds were compared to ESLs Table K-3, entitled - Direct Exposure Screening Levels - Construction/Trench Worker Exposure Scenario. This Table provides screening levels that are protective of workers who may be exposed to chemicals in shallow soils on a regular basis via incidental ingestion, dermal absorption, and inhalation of vapors and particulate matter. Information provided in the ESL document indicates that a depth of three meters (approximately 10 feet) is used to differentiate between shallow, "surface" soils where regular exposure to residents and/or workers is assumed and deeper (> three meters) "subsurface" soils where only periodic contact during construction and utility maintenance work is assumed. According to the ESLs, this is regarded as the maximum likely depth that impacted soil could at some point in the future be excavated and left exposed at the surface. The potential for deeper soils to be brought to the surface during large-scale redevelopment activities would need to be evaluated on a site-by-site basis. SAIC concluded that based on the analytical results discussed above, when compared to screening levels for construction/trench workers, the PAH and BTEX results were below their respective ESL values. However, the highest TPHc reported, 19,000 mg/kg in boring MH4-1 at 16 feet exceeded the construction/trench worker ESL of 15,000 mg/kg. SAIC reevaluated the TPHc concentrations using a 95 percent upper confidence limit (UCL) based on the U.S. EPA program, ProUCL. In cases where a site has been adequately characterized, the 95 percent UCL of the arithmetic mean may be used to represent the exposure point