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Mr. Maurice Benson - 2 - 3 November 2008 <br /> Defense Logistics Agency <br /> petroleum hydrocarbons. Contaminant fate and transport modeling indicated that these <br /> CDCs in soil posed a potential future threat to groundwater. <br /> The ROD selected the following remedy for cleanup of SWMU 20: soil vapor extraction <br /> (SVE), excavation/disposal, and natural attenuation. The ROD-specified cleanup standards <br /> for TCE in soil, soil vapor, and groundwater are 5 micrograms per kilogram (pg/kg), 350 parts <br /> per billion volume (ppbv), and 5 micrograms per liter (Ng/L), respectively. <br /> Between September 1997 and June 1999 two phases of sump removal work were <br /> conducted adjacent to the northeast corner of Warehouse 10 (Attachment A). Before the <br /> work was conducted, a concrete slab foundation for a former underground storage tank was <br /> discovered at 9 feet below surface grade in soil boring SB204 at this location. This tank <br /> location had not been identified during Remedial Investigation and Feasibility Study activities <br /> at the Depot. Laboratory chemical analyses of soil samples collected beneath the slab <br /> detected total petroleum hydrocarbons (TPH) at concentrations between 15,000 and <br /> 100,000 pg/kg. During the two phases of sump removal work, 332 cubic yards of soil were <br /> excavated. Concentrations of TPH and TCE greater than the ROD-specified cleanup <br /> standards remained in place after the work was completed. The concentration of TPH <br /> immediately adjacent to Warehouse 10 was reported at 48,000 pg/kg. <br /> The 2004 Explanation of Significant Differences to the Site-Wide Comprehensive Record of <br /> Decision (ESD), finalized in September 2004, added institutional controls to the remedy for <br /> SWMU 20. This was due to the inaccessibility of residual contamination underneath <br /> Warehouse 10, which was still in use. Also, the ESD deleted SVE from the remedy because <br /> TPH was also present in the soil. The ESD deemed that, while SVE may be a viable remedy <br /> for TCE, it was not for TPH because it's constituents were determined to be in the diesel <br /> range. <br /> REGIONAL WATER BOARD OBSERVATIONS AND COMMENTS <br /> 1. The Draft Report presents chemical laboratory analyses data for TCE <br /> concentrations in soil and groundwater samples collected from beneath and <br /> adjacent to the northeast corner of Warehouse 10. As explained below, DESJC <br /> should implement soil remediation beneath Warehouse 10 because it would effect a <br /> relatively rapid improvement of groundwater quality beneath the warehouse. <br /> Relatively rapid improvement was observed after the soil excavation work conducted <br /> in June 1999 adjacent to the northeast side of Warehouse 10. <br /> Pre- and Post-1999 TCE Concentration Data. Regional Water Board staff prepared <br /> concentration trend graphs for TCE in groundwater samples collected from <br /> monitoring wells LM093AU (Attachment B) and LM175AU (Attachment C). <br /> LM093AU was destroyed because it was within the area excavated in June 1999. <br /> After the excavation was completed and backfilled, replacement well LM175AU was <br /> installed within a few feet of the former well. As is evident by the trend graphs, TCE <br /> concentrations in groundwater were almost always above the ROD-specified <br /> cleanup concentration before the 1999 remediation work and, with only one <br />