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Supplemental Data for Commingled Plume Evaluation 2 ARCO#2133 and Chevron 99-4275,2905&2908 Benjamin Holt Drive,Stockton,California July 30,2001 groundwater monitoring wells, 14 soil vapor extraction (SVE) wells, and nine air-sparge (AS) wells have been installed at the site in response to these releases, , (Figure 2). A groundwater extraction system was operated intermittently from 1984 to 1996. In 1995, a SVE/AS system was installed at the site. To date, the SVE system has removed 3,980 pounds of total petroleum hydrocarbons as gasoline (TPHg) from the soil and groundwater beneath the site. The system has been deactivated due to high groundwater water levels, rendering operation unfeasible. Groundwater flows generally toward the northeast with a hydraulic gradient of 0.003 feet/foot. The current groundwater monitoring and reporting program includes nine groundwater wells monitored quarterly and an additional i 1 wells monitored semi- annually. Table 1 presents current and historic groundwater monitoring data and analytical results. Chevron 49-4275 Four single walled, fiberglass underground storage tanks (USTs) were installed at this service station was constructed in 1973. A gasoline release from a product line was discovered in 1985. In January 1989, these four USTs (three gasoline and one waste oil) were removed and a new tank UST pit was excavated and five double-walled, fiberglass USTs were installed (4 gasoline and 1 methanol). An SVE system was activated in September 1997 and air sparge (AS) system was activated in April 1997 at the site. The SVE system is comprised of ten SVE wells (VW-1 and GM-1 through GM-9), a 5-horsepower (hp) positive-displacement blower, and a100-cubic feet per minute (cfm) catalytic oxidizer. The AS system consists of a 1.5-hp rotary vane compressor, alternately injecting air into nine wells (GM-1 through GM-9). The SVE system has been deactivated due to high groundwater levels. Prior to shutdown, the SVE system has extracted approximately 70 pounds of gasoline from the soil and groundwater beneath the site. The AS system continues to operate. Groundwater flows generally toward the north with a hydraulic gradient of 0.003 feet/foot. The current monitoring and reporting program includes nine groundwater wells monitored (Figure 2). Table 2 presents current and historic groundwater monitoring data and analytical results. DATA REVIEW AND EVALUATION SECOR reviewed current and historic groundwater level, flow, and analytical data for the ARCO and Chevron sites to evaluate time, concentration and distance trends of TPHg, benzene, and MtBE concentrations in groundwater at both sites. Data used in this evaluation is included in Tables 1 and 2. Data was evaluated through construction and inspection of isoconcentration maps of each chemical in groundwater. I:\ARCM21331Misc Letters\Commingled letter Revision 0.doc