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S E C 0 R - - - Cleanup Plan October 27, 2003 ' Page 6 wells have screen intervals of approximately 15 to 30 feet bgs and 5 to 30 feet bgs, ' respectively. The ISO-GEN TM treatment system appeared to have limited impact on the increase of dissolved oxygen (DO) concentrations and the reduction of methyl tertiary butyl ether (MtBE) concentrations. However, the ISO-GEN TM system did reduce TPHd ' concentrations in well MW-26A and MtBE concentrations in wells MW-28A and MW-32A (SECOR, 2003e). The results of the ISO-GEN TM pilot study indicated the following biological activity is occurring at the Banta Fuel Terminal: ' 1 Nitrate (NO3) and sulfate (SO4) are present throughout the site, with the exception of the area around MW-26A. Nitrates and sulfate in this area are depleted due to natural biodegradation. ' 2. Surface water infiltration from the agricultural drainage trench is high in nitrates and sulfates. This situation creates a biodegradation-barrier to contaminant migration. ' 3. The mass flux of TPHd and MtBE along the down-gradient margin of the Banta Fuel Terminal is three orders in magnitude lower than the concentrations of available electron acceptors. This situation indicates natural attenuation should be sufficient ' to remediate the downgradient margin of the plume. ■ In July of 2001, SECOR submitted an assessment report and workplan for additional 10 assessment as part of the ongoing investigation of hydrocarbon-impacted groundwater and soil at the site (SECOR, 2001). Data collected in April of 2001 and May of 2001 indicated the extent of the MtBE within shallow groundwater (northeast of the site and on private t property) was not defined. Off-site groundwater analysis results in May of 2001 reported MtBE concentrations ranging from 6.1 pg/L to 95 pg/I. Microbiological analysis of soil and groundwater samples collected on April 12, 2001 suggested a healthy population of ' hydrocarbon degraders exists with good natural attenuation potential at the offsite sample locations. ■ On December 28, 2001, a flange on a piping manifold near MW-15A failed and released less ' than 10 gallons of red dye diesel at the site. On January 9, 2002, 15 soil samples were collected in and around the spill area. Borings were advanced to approximately 0.5 to 1.5 feet bgs (SECOR, 2002a). ■ In November of 2002, SECOR submitted an assessment report and work plan for additional site assessment, which summarized the results of the installation of five offsite monitoring ' wells (MW-54A through MW-59A) (SECOR, 2002b). The objective of the off-site assessment activities was to delineate the lateral extent of petroleum hydrocarbons (including MtBE) in offsite groundwater. MtBE concentrations were detected in five of the five borings, ranging from 0.6 µg/I (MW-55, MW-57 and MW-59) to 1.0 Pg/I (MW-54). TPHd concentrations were detected in three of the five samples at concentrations ranging from 84 µg/I (MW-55) to 160 µg/I (MW-57). ' In August of 2002, SECOR submitted a construction support activities report (SECOR, 0 2002c). This report documented the following activities: ' I:\Chevron\1001621\REPORTS\2003\2003-Cleanup Plan\Cleanup Plan.doc