SITE INFORMATION AND CORRESPONDENCE

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I'A. Scott Mansholt - 3 - 8 July 2008 Chevron ' Tracy Byron Road Based on review of the current and historical findings, the majority of TPH impacted soil occurs between depths of 4 and 20 feet. The vertical extent of impacted soil was previously delineated between depths of 24 to 36 feet, where TPHc concentrations ranging from ND to Iless than 100 milligrams per kilogram (mg/kg) were reported. Groundwater ' Groundwater grab samples were obtained from each soil boring and submitted for analysis. Concentrations of TPH as diesel (TPHd) were reported in groundwater at 100, 140, 160, 330, ' and 330 micrograms per liter (pg/L), respectively in borings SB-37 to 41. BTEX compounds were ND; with one exception, PAHs were ND: sample SB-39 reported a concentration of 0.40 pg/L benzo(g,h,i)perylene, which exceeds the Environmental Screening Level (ESL) of 0.13 ' pg/L. The ESLs are described in: Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final 2007 (San Francisco Bay RWQCB, November). The vertical extent of impacted groundwater was defined at a depth of approximately 27 feet by earlier borings and groundwater sample results. ' Two sam samples were sumitted for total dissolved solids (TDS) and one for general minerals ' analysis. TDS values were 1,980 and 2,180 mg/L; general minerals analysis of chloride, sodium, boron, sulfate, and manganese exceeded respective Water Quality Objectives (WQOs) described in the Central Valley RWQCB -Water Quality Control Plan for the Sacramento River Basin and the San Joaquin River Basin, 4«' ed., revised August 2006 (Basin Plan). According to the Basin Plan, an aquifer is considered a potential drinking water source provided: a) TDS in groundwater is less than 3,000 mg/L; and b) groundwater yield is greater than 200 gallons per day. The reported TDS values exceed the U.S. Environmental Protection ' Agency (EPA) Secondary Maximum Contaminant Level (MCL) for TDS (500 mg/L), which is based on aesthetic (taste and odor) and technical (staining and corrosion) characteristics (U.S. ' EPA, 2008). The City of Tracy Municipal Code, for administrative reasons unrelated to petroleum impacts, prohibits the use of shallow groundwater wells for domestic or municipal use. Domestic and 1 municipal water services are provided by the City of Tracy. SAIC Recommendations ' Based on the recent findings, SAIC recommended the following: ' • Soil and groundwater has been adequately delineated at those areas investigated by the five soil borings and that additional characterization was not needed. • Step-out borings SB-42, 43, and 45 (originally approved by the 2004 workplan) should be relocated to nearby properties. ' • The boring proposed on the Calderon Store property was not needed since TPHc compounds were not identified in borings advanced during an earlier underground tank removal investigation at that location.