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I I Mr. Harlin Knoll 'C Page 2 of 4 ,C In July 1999, five direct push soil borings were adN arced along the perimeter of the UST excavation to investigate the vertical and lateral extent of soil and groundwater contamination, and four hand augured soil borings were advanced to investigate the lateral extent of shallow soil contamination. Total petroleum hydrocarbons s gasoline (TPHg) and total petroleum hydrocarbons as diesel (TPHd) were detected in the soil at low to moderate levels. Benzene and methyl Cert butyl ether (MTBE) were detected only in one boring in the immediate vicinity of the former USTs. Elevated levels of benzene, loluene, ethylbenzene xylenes (BTEX) TPHg, TPHd, and MTBE were detected in grab groundwater samples collected during drilling. The data suggested that the vertical extent of soil contamination did not exceed 15 feet below ground surface (bgs) and occurred within a relatively limited area near the former USTs. 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. Approximately 3,700 cubic yards of soil were excavated from the area immediately r 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 extent of contamination had been reached. Approximately 1,100 cubic yards of soil was classified as uncontaminated and was utilized as backfill material. Approximately 2,600 4 cubic yards of low to moderately impacted soil remained stockpiled on site. Confirmation soil j samples collected from the sidewalls and floor of the excavation confirmed that virtually all contaminated soil was excavated. I Approximately 60,000 gallons of contaminated groundwater was pumped from the excavation, treated through ex-situ air-sparging to reduce contaminant concentrations, and subsequently i. 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 i I 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 MWl through MW4 were installed at the site on May 17, 2001. Soil samples collected from boring MMI through MW4 contained no detectable concentrations 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 3amples collected from wells MWl, MW3 and MW4. No MTBE was detected in well MW2. Ground Zero has monitored and sampled groundwater at the site on a quarterly basis since the wells j were installed in May 2001. MTBE concentrations in site wells in June 2002 were below the Primary Maximum Contaminant Level (MCL) of 13 ppb in wells MW 1, MW2, and MW4. , MTBE remained above the MCL only in well MW3 (290 ppb). j G:1Data1GROUNDZ,E\Bamyard-Mamer\Reports\WKPLAN MW5-MW7.doc ' I�