SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt Mountain House Site #3 , 16 April 2008 Based on the findings of the historical documentation and the recent investigation, SAIC concluded the following: The greater delineated area of affected soil is approximately 300 by 400-feet and the vertical extent is at 54-feet in boring MH3-1; a smaller area of separate phase crude oil in soil approximately 100-feet by 35-feet deep is in the northern section, generally centered around boring MH3-1; the smaller area occurs along the southern edge of Byron Road extending to the adjacent agricultural field; • Within the remainder of the larger area, affected soil was noted from approximately 15 to 35-feet; TPH as crude oil (TPHc) concentrations range from 2,600 mg/kg at 23-feet to 1,900 mg/kg at 33-feet; elevated TPH concentrations in soil below the water table occur with affected groundwater; • The extent of affected groundwater has been delineated at approximately 500-feet across; approximately one-third of the affected groundwater lies within the adjacent agricultural field to the south and the remainder downgradient (north) beneath Byron Road, the former OVP/TAOC pipelines, the UPRR tracks, and two existing active pipelines; • TPH as diesel (TPHd) in groundwater was reported at concentrations up to 500,000 pg/L; elevated concentrations of PAHs (polynuclear aromatic hydrocarbons) correspond to samples with high levels of TPH; and, • Analysis of a water sample from the nearby private Pombo supply well reported non detect for TPH constituents; the Fish family supply well reported 170 pg/L TPHd; the private wells are greater than 1,000-feet crossgradient, east of the site; SAiC stated that the wells are for agricultural use, not for domestic needs; the Fish family well will be re-sampled. Regional Water Board staff has the following comments: • Regional Water Board staff concur that the lateral and vertical extent of affected soil and groundwater has been adequately defined and that additional characterization is not needed; • An area approximately 100-foot wide in the north part of the site contains visually affected soil where crude was noted between two and 35-feet; this area of affected soil occurs at the north end of an agricultural field near the property line and along the edge of Byron Road, next to the approximate location of the former OVP pipelines; • Removal of impacted soil containing elevated concentrations of separate phase Petroleum hydrocarbons to approximately 10 to 12-feet, in the vicinity of boring MH3-1 would reduce the volume of contaminants in soil. The removal would reduce possible exposure to TPH constituents by future construction/trenching workers and/or utility maintenance crews and further reduce groundwater impacts;