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ATC Associates Inc. 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 209-579-2221 - -.R S S O C t A T.r;'S 4 N C. Fax:209-579-2225 continuous operation. Results of a rebound test conducted on July 14, 2009, after 151 days of non- operation, indicated the extracted VOC concentrations were similar to those at the termination of the S VE. system in February of 2009. Based on the data associated with the SVE effort, it appears the majority of petroleum hydrocarbon vapors in the vadose zone have been removed from within the horizontal vacuum ROI of approximately 30 feet associated with MW2. An estimated 6,260.15 pounds of vapor phase hydrocarbons were removed and treated. The total mass extracted exceeded the calculated mass indicating that the initial calculation was indeed low. This suggests the majority of hydrocarbons contained in the targeted area have been removed as well as additional hydrocarbons that existed outside the most heavily impacted area, such as underneath the building. To date, monitored natural attenuation is the only remedial action conducted with respect to groundwater at the site. Monitored natural attenuation processes'are typically used to achieve site-specific remediation objectives within a reasonable time frame when compared to costs and timeframes offered by other more active methods. The natural attenuation processes that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants. Based on recent groundwater analytical data,the groundwater quality beneath the study area has achieved acceptable cleanup levels,as none of the analytes of interest exceed applicable MCLS. 6.0 SENSITIVE RECEPTOR SURVEY ATC conducted a sensitive receptor survey (SRS) within a 2,000-foot radius of the site in 1998. Information regarding the SRS was submitted to the SJCEHD in ATC's Workplan for Additional Subsurface Investigation at Diesel Performance,dated August 24, 1998. A summary of the SRS is provided below. Groundwater well construction logs of wells located within 2,000 .feet of the site were obtained from the Department of Water Resources (DWR) and reviewed. Seven wells were identified within 2,000 feet of the site. The two closest identified wells are located approximately 700 feet west and approximately 900 feet south of the site. According to the information obtained from the DWR,the well located to the west is approximately 176 feet deep and is screened from 165 to 176 feet bgs. The well located to the south is approximately 578 feet deep and is screened from 188 to 578 feet bgs. The five other wells are located at least 1,200 feet northeast of the site. The regional groundwater gradient is to the northeast according to the San Joaquin County Flood Control and Water Conservation District's Spring 1997 Groundwater Report. Historical groundwater elevation data collected at the site indicate the hydraulic gradient beneath the site is very flat with variable directional groundwater flow. The predominant groundwater flow direction appears to be south, southeasterly. Based on distance, topography, groundwater gradient; and subsurface geology, it appears to be unlikely that the seven wells identified by the SRS are threatened by the contaminant plume at the site. The nearest surface water to the site is the Stockton Diverting Canal located approximately one mile northeast of the site. The Fillmore Elementary School and Fremont Elementary School are located approximately 1,000 feet north and 3,000 feet west of the site, respectively. SaEnviroamental1625771Rcports12010closure summary.doc 9