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Chevron Environmental Management Company Soil and Groundwater Investigation Report Tracy Byron Road Project 6.5 WATER SUPPLY WELLS SAIL and RWQCB collected split grab groundwater samples on October 23 and 30, 2003, from the water supply wells identified at several rural properties along West Byron Road. SAIC's laboratory results did not indicate concentrations of petroleum hydrocarbons at or above the laboratory detection limits in any of the tested samples. A split sample collected by the RWQCB from a water supply well located at 3190 West Byron Road contained TPHd at a concentration of 58 µg/L. The laboratory indicated that the sample chromatogram pattern exhibited atypical weathered hydrocarbon pattern. (The sample collected by SAIC did not contain TPHd.) The RWQCB also submitted the water supply well samples for analysis of TDS. Each sample contained concentrations of TDS above the recommended secondary MCL range of 500 mg/L California Department of Health Services Drinking Water Source Assessment and Protection Program Section 7 (CA DHS DWSAP Section 7). SAIC reviewed Water Well Driller Reports from the water supply wells located within the TBRP investigation area. SAIC identified a total of 37 water supply wells within the 2,000-foot search area. Based on the DWR file search, 29 of the 37 water supply wells identified were listed as domestic wells, three were listed as irrigation wells, and two were listed as unknown. Of the 29 domestic wells listed,three wells were listed as abandoned. Several domestic wells identified are adjacent to Byron Road and are pumping water from a depth considerably deeper than known impacted soil. Based on well driller reports reviewed,the domestic wells identified were constructed with sanitary seals 50 feet or greater, and appear to be separated from known affected-soil intervals. Water supply well samples collected by SAIC did not contain concentrations of petroleum constituents at or above the laboratory detection limits. 6.6 CROSS SECTIONS AND WEST BYRON ROAD BISECTS SAIC prepared four cross sections and four bisects to better illustrate the soil and groundwater conditions encountered during recent subsurface investigations. Cross-sections A-B,B-C, C-D, and D-E illustrate the subsurface conditions along the West Byron Road corridor, from Interstate 205 to the west,to Corral Hollow Road to the east(Appendix F, Insert 1). Each of these cross sections is approximately 1,600 to 1,700 feet long and all four present summarized data for approximately 6,600 feet of the West Byron Road corridor. All cross sections present the combined data collected from dewatering well installation (Geomatrix and Viking Drillers boring logs), trench excavation observations(Summary Report Tracy Westside Channel Outfall Project [WSCO], SAIC, 2003) and soil boring log data from SB-1 through SB-36 (SAIC). Explored subsurface depths for these wells and borings varied from 20 to 50 feet bgs with the majority extending from the 20-to 30-feet bgs range. SAIC grouped the soils into two main categories exhibiting similar characteristics that included fine-grained soils (clays, silts, and their mixtures), and coarse-grained soils (sands, gravels, and their mixtures). These features included particle or grain size,porosity, and permeability of the soil. The four bisects illustrate simplified and generalized subsurface conditions across the sewer line installation trench(Appendix G; Bisects A through D). Bisect A shows main features and dimensions of the utility corridor along West Byron Road,bisect B illustrates conditions with contamination present on the south side of West Byron Road, bisect C illustrates conditions on the north side of West Byron Road, and bisect D illustrates the combined northern and southern conditions along West Byron Road. 7. SUMMARY/CONCLUSIONS The data collected during former and current subsurface investigations indicate that residual petroleum has affected soil from various depths bgs to approximately 30 feet bgs, and that the majority of the affected soil is located between 4 and 20 feet bgs. Upper clay and silt layers appear to be the most 11