SITE INFORMATION AND CORRESPONDENCE

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M. Scott Mansholt - 3 - 3 April 2008 Mountain House Site #1 The TPHc ESL for a construction/trench worker is 15,000 milligrams per kilogram (mglkg) and the calculated 95- percent UCL for the highest TPHc concentrations exceeding the ESL (2 of 45 results reported 20,000 mg/kg at 9-feet and 29,000 mg/kg at 14-feet). The calculated 95-percent UCL was 9,971 mg/kg, which is well below the 15,000 mg/kg ESL for TPHc. Thus, SAIC concluded the TPHc at the site does not pose a threat to human health for a construction/trench maintenance worker scenario. We previously concurred with Chevron that removal of impacted soil was not feasible since the bulk of the source occurs within and beneath the railroad and Byron Road easement, which also contains two existing active pipelines in addition to the former OVP and TAOC lines. SAIC previously assessed available technologies and review of various remedial alternatives for crude oil at the OVP/TAOC projects. The feasibility study determined that the residual crude oil is viscous and immobile, has been delineated, and with time natural attenuation will reduce petroleum hydrocarbons. Low permeability soil in the project area has limited the movement and migration of crude oil in the subsurface. Based on the technological feasibility evaluation, SAIC concluded there is no feasible cost effective means or easily implemented process to actively and/or effectively remove the petroleum hydrocarbons identified at the site. SAIC's report demonstrated that consistent with State Water Resources Control Board Resolution No. 92-49, review of the readily available cleanup technologies found that all options would not be effective or feasible and those options considered would be very costly given that the affected area lies beneath Byron Road, the railroad tracks and existing active pipelines. According to the findings provided in site characterization reports and in conjunction with the results of SAIC's environmental risk evaluation, Regional Water Board staff concur that the extent of impacted soil and groundwater has been delineated and is limited to within the Byron Road/UPRR easement ROW's. Review of the data indicates that residual petroleum hydrocarbons in soil at the site do not result in a significant source of TPH constituents to groundwater; and, the groundwater plume is limited in extent and slightly larger than the area of affected soil. Regional Water Board staff concurs with the summary provided in the Request for No Further Action at the Mountain House Site #1 project and that it appears the site is a candidate for closure based on the following: The extent of affected soil and groundwater have been adequately delineated and residual petroleum hydrocarbons remaining at the site will continue to decrease over time by natural attenuation, • Mountain House Site #1 lies within a public roadway, the UPRR easement ROW, and beneath existing active pipelines; remedial alternative studies by SAIC indicated that, since the majority of soil and groundwater lie beneath the road, railroad, and pipelines, it is not feasible to conduct site excavation activities;