SITE INFORMATION AND CORRESPONDENCE

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SOIL AND GROUNDWATER INVESTIGATION VERNALIS OFFSITE 1. INTRODUCTION Science Applications International Corporation(SAIC)prepared this report of a soil and groundwater investigation at the off-site portion of the former Vernalis Pump Station in Vernalis, California(Vernalis Offsite; see Figure 1) at the request of Chevron Environmental Management Company(CEMC). The previous site investigation along with semi-annual groundwater monitoring and sampling documented total petroleum hydrocarbons(TPH) and polynuclear aromatic hydrocarbons(PAHs)in groundwater extending onto Vernalis Offsite. The Vernalis Pump Station was part of the former Old Valley Pipeline (OVP) system that transported heated crude oil. Benzene,toluene, ethylbenzene, and total xylenes (BTEX) are minor crude oil constituents;however,they have been encountered in soil and groundwater at the Vernalis Pump Station and therefore are also constituents of concern(COCs). The purpose of this site investigation was to further evaluate the extent of soil and groundwater affected by the COCs and recommend future assessment activities based on the results. SAIC documented the presence of the COCl in subsurface media in the Soil and Groundwater Investigation Report, Vernalis Pump Station, Vernalis, California(the SGIR; SAIC,2006a). The SGIR concluded that the extent of affected soil and groundwater is delineated at Vernalis Pump Station but not off site. The Regional Water Quality Control Board(RWQCB) accepted the conclusions from the SGIR and made recommendations for additional off-site investigation in a letter dated September 20,2006 (Appendix A). In a letter dated February 14,2007,the RWQCB also agreed to divide the project into two sites to facilitate future investigations and reporting: Vernalis Pump Station and Vernalis Offsite. 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)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,the United States Environmental Protection Agency(EPA)Region 9 Preliminary Remediation Goals(PRGs; EPA,2004) and San Francisco Bay RWQCB Environmental Screening Levels(ESLs;RWQCB, 2007)were used to evaluate laboratory analytical data. Soil and groundwater analytical results were also compared to ESLs, PRGs (soil), and WQOs (groundwater; RWQCB, 2004) 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. 2. BACKGROUND 2.1 LOCATION AND SITE DESCRIPTION The former Vernalis Pump Station is located at 35500 Welty Road,Vernalis, San Joaquin and Stanislaus counties, California(Figure 1). The former pump station primarily consists of two properties: the Bellini property(formerly known as the Roberts property), and the Arcos property(Figure 2). The Bellini property(San Joaquin Valley Assessor's Parcel Number[APN] 255-260-03)is a triangular-shaped, 5.73- acre vacant lot located on the northern part of the pump station site. The Arcos property(Stanislaus County APN 016-038-003)is an irregularly shaped parcel on the southern part of the former pump station site, and is currently used as a storage facility for equipment and materials; a residence and an on-site office are also on the property. from science roSolurions