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REVISED SOIL AND GROUNDWATER INVESTIGATION WORK PLAN MOUNTAIN HOUSE DEVELOPMENT SITE #6 1. INTRODUCTION At the request of Chevron Environmental Management Company(CEMC), Science Applications International Corporation(SAIC)presents this work plan for additional subsurface investigation at Mountain House Site#6 (the site; see Figure 1). Former crude oil facilities of Chevron's Old Valley Pipeline (OVP)and the Tidewater Associated Oil Company(TAOC)pipelines are located on the site. Past site investigations documented the presence of total petroleum hydrocarbons(TPH) and polynuclear aromatic hydrocarbons(PAHs). Although benzene,toluene, ethylbenzene, and total xylenes(BTEX) were minor constituents of crude oil,they are also constituents of concern(COCs), as they are sometimes encountered at TAOC and OVP facilities. The objectives of the investigation are to evaluate the extent of soil and/or groundwater affected by the COCs and,if necessary,recommend future assessment activities. In 2001, Condor Earth Technologies,Inc. (Condor)investigated 14,000 feet of the Byron Road corridor adjacent to the OVP right of way(ROW) and documented the presence of petroleum hydrocarbons in the subsurface (Condor,2001). Condor identified this site as one of six where COCs had affected soil and groundwater and indicated that more investigation was required. On March 31, 2005, SAIC submitted a work plan to the Central Valley Regional Water Quality Control Board(RWQCB)to advance 30 primary and 12 contingency soil borings to assess the extent of affected media at the site(SAIC, 2005). On February 1,2006,a work plan addendum was submitted, after it had been noticed that two borings had been omitted from the work plan's table of boring objectives(SAIC, 2006a). On March 6,2006,the RWQCB accepted the amended work plan(RWQCB, 2006). However, since the RWQCB accepted the work plan over a year ago, significant new methods have been developed for Chevron's Historical Pipeline Portfolio(HPP)investigations (e.g., groundwater sampling using pre-packed well screens, comparison of analytical data to risk-based screening levels,testing for general minerals to evaluate groundwater potability). Also,based on further review of the accepted work plan, it was determined that project objectives could be achieved with fewer borings advanced in more effective locations. In order to complete a more efficient investigation using current methods, SAIC has prepared this new work plan describing how the extent of soil and/or groundwater at the site will be investigated. This investigation will be guided by the Technical Approach to Site Evaluation and Decision-Making, Historical Pipeline Portfolio, Central Valley Region(referred to as the Consistent Technical Approach [CTA]; Geomatrix Consultants, Inc. [Geomatrix],2005). The CTA specifies Central Valley RWQCB Water Quality Objectives(WQOs) as the health-based and aesthetic criteria for determining whether groundwater is affected, delineating the extent of any affected groundwater, and assessing whether further groundwater characterization is appropriate. Although the CTA does not prescribe criteria by which affected soil is defined,U.S. Environmental Protection Agency(EPA)Region 9 Preliminary Remediation Goals (PRGs; EPA, 2004)and San Francisco Bay RWQCB Environmental Screening Levels(ESLs; RWQCB,2005)will be used. Soil and groundwater analytical results will also be compared to ESLs,PRGs, and WQOs as a screening step to streamline the risk assessment process. The presence of a constituent at a concentration above a screening level does not necessarily indicate an impact to human health or the environment. It simply indicates that potential risk may exist and that additional evaluation may be warranted. Based on comparison of analytical results to the screening levels,the site may qualify for No Further Action status. 1 r!nZs of MMFr11`® From Science to SWullons