SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

SOIL AND GROUNDWATER INVESTIGATION REPORT MOUNTAIN HOUSE DEVELOPMENT SITE #6 1. INTRODUCTION Science Applications International Corporation(SAIC)prepared this report of a soil and groundwater investigation at Mountain House Development Site#6 (the site; see Figure 1) at the request of Chevron Environmental Management Company(CEMC). CEMC wanted to further assess subsurface conditions due to the former crude-oil facilities of Chevron's Old Valley Pipeline(OVP) and Tidewater Associated Oil Company(TAOC)pipelines that were located on the site. Past investigations documented the presence of total petroleum hydrocarbons(TPH)and polynuclear aromatic hydrocarbons(PAHs). Other constituents of concern(COCs)include benzene,toluene, ethylbenzene, and total xylenes (BTEX),which are minor constituents of crude oil sometimes encountered at TAOC and OVP facilities. The objectives of the investigation were to evaluate the extent of soil and/or groundwater affected by the COCs and recommend future assessment activities. 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). A work plan addendum was submitted on February 1,2006, after the discovery that two borings had been omitted from the work plan's table of boring objectives (SAIC, 2006). The RWQCB reviewed the amended work plan and accepted it in a letter dated March 6,2006 (RWQCB, 2006). 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, and testing for general minerals to evaluate groundwater potability) since the work plan addendum was submitted in 2006. Further review of the work plan determined that project objectives could be achieved with fewer borings advanced in more effective locations. To complete a more efficient investigation using current methods, SAIC prepared a revised work plan(SAIC,2007b) describing how the extent of soil and/or groundwater at the site would be investigated. The RWQCB accepted the revised work plan on July 31, 2007. A copy of all regulatory correspondence is included in Appendix A. This investigation was also 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])prepared by Geomatrix Consultants,Inc. (Geomatrix,2005). The CTA specifies RWQCB Water Quality Objectives (WQOs;RWQCB, 2004) 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. The CTA does not prescribe the criteria by which affected soil is delineated; therefore,U.S. Environmental Protection Agency(EPA)Region 9 Preliminary Remediation Goals(PRGs; EPA,2004) and San Francisco Bay RWQCB Environmental Screening Levels (ESLs;RWQCB,2008)were used to evaluate laboratory analytical data. Soil and groundwater analytical results were also compared to ESLs, PRGs (soil), and WQOs(groundwater) as a screening step to streamline the risk assessment process. The presence of a COC at a concentration above a screening level does not necessarily indicate a risk to human health or the environment; it simply indicates that potential risk may exist, and additional evaluation may be warranted. COC concentrations below screening levels may qualify the site for No Further Action Required(NFAR) status. 1 A :. From Science to Solutions