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ii 1 Mr. Harlin Knoll Page 2 of 3 Based on the results of the investigations, source removal excavation and supplemental groundwater removal and disposal were proposed in an Interim Corrective Action Workplan dated February 15, 2000. I� Approximately 3,700 cubic yards of soil were excavated from the area immediately surrounding the former USTs in March 2000. The excavation proceeded to a depth of approximately 15 feet bgs where subjective analysis and field PID readings suggested the vertical -xtent of contamination had been reached. Approximately 1,100 cubic yards of soil was classified as uncontaminated and was utilized as backfihl material. Approximately 2,600 cubic yards of low to moderately impacted soil remained stockpiled on site: Confirmation soil samples collected from the sidewalls and floor of the excavation confirmed that virtually, all contaminated soil was excavated. r Approximately 60,000 gallons of contaminated groundwa er was pumped from the excavation, treated through ex-situ air-sparging to reduce contaminant conce trations, and subsequently transported to the Sacramento County Regional Wastewater Treatment Plant for disposal under a temporary discharge permit', Analytical results from groundwater samples collected on the first day of excavating and after interim remedial measures were completed indicated that petroleum hydrocarbon constituents were reduced via interim remedial measures by approximately 97-99.9%, except for MTBE, which was reduced by approximately 80%. Monitoring wells MW1 through MW4 were installed at the si e on May 17, 2001. Soil samples collected from boring MW through MW4 contained no detectable concentr ions of TPHd, BTEX, 1,2-DCA, or EDB. Low levels of MTBE were detected in boring MW3. MTBE was the only constituent of concern detected in the initial groundwater samples collected from wells MW1, MIA3 and MW4. No MTBE was detected in well MW2. 11 is Monitoring wells MW5 through MW7 were installed at the s to on October 30, 2002. Soil samples collected from the borings contained no detectable concentrations of TPHd, BTEX, 1,2-DCA, or EDB. Low levels of MTBE were detected in boring MW3. MTBE was the onl constituent of concern detected in the initial groundwater samples collected from wells MW5. No MTBE was detected in wells MW6 or MW7. �I Ground Zero has monitored and sampled groundwater at the site on a quarterly basis since the wells were installed in May 2001. MTBE concentrations in site wells o January 6, 2004 were below both the Primary Maximum Contaminant Level (MCL) of 13 ppb and the Seco Bary.MCL of 5 ppb in wells MW 1, MW2, and MW4 through MW7. MTBE remained above the MCL only iii well MW3 (88 ppb). The location of the site is shown on Figure 1. The monitoring well locations are depicted on Figure 2. WORKPLAN V&W Drilling (C57 License #720904) will destroy wells IV.W1 through MW7 under the supervision of a Ground Zero geologist. Figure 2 shows the locations of sit wells. Attachment 1 contains copies of well logs. Monitoring wells MWl and MW3, which were installed in or near the former excavation, will be destroyed by overdrilling with 8-inch diameter hollow stem ugers to total depth of 24 feet. The wells will then be backfilled to the surface with neat cement grout empliced with a tremie system. The remaining wells (MW2, MW4, MW5, MW6, MW7), which have contaminant levels below both the Primary and Secondary MCLS, will be destroyed by pressure grouting. Neat cement grout will be pumped through a tremie pipe until returned to the surface. A compression fitting will then be attached to the well collar and pressurized to approximately 35 pounds per square inch (psi) for at least S minutes. H G:IGROUNDZE18amyard-Manser\ReportslWKPLAN_DESTROY MWl-MW7. oc it