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• • 25 November 2008 <br /> Mr. Maurice Benson - 3 - <br /> Defense Logistics Agency <br /> Comments <br /> Regional Water Board staff comments are presented below: <br /> 1. Concentrations of TPH in the diesel range indicative of separate-phase petroleum <br /> hydrocarbons were present in groundwater samples collected ten years after UST 25 was <br /> removed. The UST was removed in 1988, yet concentrations in excess of 180,000 µg/L <br /> were detected in 1998 samples collected 50 feet northeast (downgradient) of the former <br /> UST and 45 feet east (side-gradient) of the former UST. <br /> 2. Groundwater samples have not been collected beneath the former UST 25 location. The <br /> closest sample was collected in 1993 approximately 25 feet downgradient. <br /> 3. Monitoring wells have not been installed to monitor dissolved or separate-phase <br /> hydrocarbons released from UST 25 to groundwater. <br /> 4. Soil and groundwater samples have not been submitted to a laboratory for chemical <br /> analysis of naphthalene. <br /> In addition to any CPT borings that may be proposed in the Final Work Plan, DESJC must <br /> install at least three groundwater monitoring wells across the water table at the UST 25 site. <br /> One of the wells must be located in soil within ten feet of the north side of the former <br /> UST excavation. Another should be installed near the suspected toe of the plume in the <br /> vicinity of the north courtyard of Building 255. The third well should be placed side-gradient <br /> of the plume for verification of the local water table gradient. Soil and groundwater samples <br /> should be analyzed for naphthalene. <br /> If you have any questions, please contact me at (916) 464-4675, or by email at <br /> jbrownell@waterboards.ca.gov. <br /> James R. Brownell, P.G. <br /> Associate Engineering Geologist <br /> cc: Mr. Michael Work, USEPA Region 9, San Francisco <br /> Mr. Peter MacNicholl, DTSC Region 1, Sacramento <br /> Mr. Harlin Knoll, San Joaquin County Health Department, Stockton <br /> Mr. Rich Howard, TechLaw, Inc., Sacramento <br /> Mr. Brian Renehan, AFCEE, San Antonio, Texas <br /> Mr. Michael Thomas, URS, Sacramento <br /> Ms, Stacy Louie, URS, Sacramento <br /> Ms. Linda Bond, Noblis, Davis <br />