SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt - 4 - i 4 May 2007 Mountain House Site #2 (Interim Final, 2005) document by the Regional Water Quality Control Board-San Francisco Bay Region; the document is referred to as "Environmental Screening Levels" or ESLs document. The analytical results of the petroleum hydrocarbon constituents at the site, i.e., TPHc, PAHs, and BTEX compounds, were compared to ESLs Table K-3 for construction/trench workers. The screening levels are based primarily on Preliminary Remedial Goals (PRGs) developed by the U. S. Environmental Protection Agency Region IX. ESLs Table K-3 - Direct Exposure Screening Levels - Construction/Trench Worker Exposure Scenario, lists 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 delineate between shallow, "surface" soils where regular exposure to residents and/or workers is assumed and deeper, "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 should be evaluated on a site-by-site basis. SAIC concluded that based on the analytical results for the site compared to the screening levels for construction/trench workers, the PAH and BTEX compounds were below their respective ESL values. Additionally, the highest TPHc reported, 12,000 mg/kg in boring GP-175 at 20-feet, did not exceed the construction/trench worker ESL of 15,000 mg/kg, provided in Table K-3. Based on the U.S. EPA program, ProUCL, SAIC further evaluated the TPHc concentrations relative to the site ESLs, using a 95-percent upper confidence limit (UCL) for the TPHc data. 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 concentration. U.S. EPA risk assessment guidance also stipulates that the exposure point concentration estimates should be based on the 95-percent UCL of the arithmetic mean to estimate a reasonable maximum exposure scenario. The TPHc data and ProUCL were used to calculate the 95-percent UCL for TPHc, which was determined to be 6,123 mg/kg; this value is well below the ESL of 15,000 mg/kg for TPHc. Thus, the report concluded that TPHc at the site does not pose a threat to human health for the construction/trench worker scenario. SAIC proposed a no further action for the site. SAIC also recommended that a soil management plan be developed for the site to use in the event affected soil and/or groundwater are encountered during future site development activities.