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Commingled Plume#35 Chevron 9-4275/ARCO#2133 S E C 0 R Site Conceptual Model September 1,2006 Page 4 In 1999, one 10,000-gallon fiberglass gasoline UST, product lines, vapor lines, and dispensers were removed from the site. Small cracks, that appeared to have been created during excavation activities, were observed in the tank. Soil samples collected from beneath the USTs contained maximum TPHg, benzene, and MtBE concentrations of 690 mg/kg, 0.692 mg/kg, and 0.156 mg/kg, respectively. Soil samples collected beneath the product lines and dispensers contained maximum TPHg, benzene, and MtBE concentrations of 102 mg/kg, 0.298 mg/kg, and 124 mg/kg, respectively. Oxygenates di-isopropyl ether (DIPE), tert-amyl-m ethyl ether (TAME), ethyl-tert-butyl ether (EtBE), tert-butyl alcohol (TBA), and ethanol were reported as nondetect in all soil samples (SECOR, 1999). One 550-gallon double-walled fiberglass waste oil UST was removed from the site in 2002. The tank appeared to be in good condition with no holes or cracks. One soil sample was collected from the center of the tank pit at a depth of approximately 11 feet bgs. Hexavalent chromium, chromium, nickel, zinc, and lead were reported at concentrations of 1.4 mg/kg, 35 mg/kg, 38 mg/kg, 49 mg/kg, and 12 mg/kg, respectively. All other constituents analyzed for were reported as nondetect (SECOR, 2002a). A groundwater extraction system was operated intermittently from 1984 to 1996. In 1995, an SVE/AS system was installed at the site. Currently nine groundwater monitoring wells are sampled quarterly, and another 11 wells are sampled semi-annually (Figure 4). Groundwater monitoring has been performed at the ARCO site since 1988. Tables 2 and 4 present the current and historical analytical data. Groundwater flow has generally been to the north-northeast at a hydraulic gradient of 0.003 ft/ft (Table 5). The second quarter 2006 groundwater elevation and hydrocarbon concentrations are shown on Figures 5 and 6, respectively. Well construction details are presented in Table 6. Historical soil analytical data for the ARCO 2133 facility are presented in Table 8. 2.3. Supply Well Survey A sensitive receptor survey has not been done for this site. However, a municipal supply well, City of Stockton Municipal Well #10, is located 100 feet east of the Chevron facility. The City of Stockton no longer operates this municipal supply well. Municipal Well #10 was sampled periodically from 1997 until June 2004. The City of Stockton has not allowed the well to be sampled since 2004 because of issues caused with activating the pump in the well. Other than minor detections of BTEX and MtBE in September 2003 samples collected from this well have been non detect. Analytical results for this well are presented in Table 1. 2.4. Summary of Remedial Actions SVE/AS systems have historically operated at both facilities. SVE systems at both sites have ceased operation. The only remediation currently operating at the site is the AS system at the ARCO facility. A summary of the historical remedial actions at both facilities is included below. 2.4.1 Chevron 9-4275 Facility An AS system was activated in April 1997 and a SVE system was activated in September 1997. The AS system consisted of a 1.5-horsepower (hp) rotary vane compressor, alternately injecting 94275-ARCO 2133 SCM 9-2006 4