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1 I Site Assessment Report,Site Conceptual Model,and Site Investigation Work Plan R&L Diesel Services, 2417 West Lane, Stockton,California,95205 4� 3.0 SITE INVESTIGATION F 3.1 Scope of Work FUThe UST is 4 ft. diameter by 6 ft. 6 in. high, and is installed under two concrete floors inside the Transmission Shop. in March 26, 2008, a Geoprobe (push-drill)was used'to secure the soil samples; two borings were advanced with the drill vertical (B1 and B2), one.boring was F!' advanced with the drill angled 24` from vertical (133) and one.boring was advanced with the drill angled 18° from vertical (134). Borehole locations are displayed in Figure 3. The soil samples were capped, labeled, placed in Ziploe sealable bags, packaged in an iced cooler, and transported with chain-of-custody documentation to a state of California-certified hazardous waste testing laboratory for analysis (Precision Enviro-Tech of Stockton, California). Each of I the soil samples was analyzed on a standard turn-around time for TPH-D and TPH-G by EPA Method 8015M and for volatile organic compounds, including BTEX and PCE, by EPA Method 82608. Boreholes were abandoned by filling with cement grout for their entire depth. The push-drill did not produce any cuttings. F1 3.2 Analytical Results ' Total Petroleum Hydrocarbons, as Diesel (TPH-D)were detected in all borings with a maximum concentration of 7,810 milligrams per kilogram (mg/kg) i,n B-1 at 14 feet bgs and a minimum concentration of 399 mg/kg in B-1 at 24 feet bgs. The Highest concentrations of benzene, toluene, ethylbenzene, and total xylenes (BTEX) were detected.in B-1 at 14 feet bgs at concentrations of 2.55 micrograms per kilogram (uglkg)� 200 ug/kg, 97.3 ug/kg, and 407 ug/kg, respectively. Total Petroleum Hydrocarbons as Gasoline (TPH-G)was not detected in any of the borings, and no analytes were detected at depths of 44 feet bgs in B-1. The chlorinated y hydrocarbon (solvent) tetrachloroethylene (PCE)was detected in B-1, B-3 and B-4, with a maximum concentration of 85.5 ug/kg in B-1 at 14 feet bgs. F. Analytical results are summarized in Table 1 and Figure 4. Laboratory reports with chain of # custody form are included in Appendix A. 3.3 Discussion A review of the data available shows that contamination levels generally decrease with depth, as illustrated in the accompanying graphs of TPH-D vs.IDepth for borings B-1 and B-2 and for the tank center. The contamination is relatively immobile (tank closure being 20 years ago) and primarily concentrated near the bottom of the tank. Additionally, there is a very dense clay layer y encountered at a depth of 45 ft. to 48 ft., which could not be penetrated by the geoprobe. Concentrations of the constituents of concern were below laboratory detection limits at this clay layer indicating that it may act as a potential aquiclude and preclude the downward migration of contaminants. WRM recommends the advancement of additional borings to further define both the vertical and lateral extent of contamination. Soil types should be logged in all borings, along with PID readings and other lithogical characteristics necessary for geologic interpretation of the chemical and lithologic data. Earlier borings completed without boring logs should be twinned and logged to allow for a complete geologic interpretation of the earlier soil analytical results. �1 5 i Fi