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{ Mr. Maurice Benson - 2 - 30 December 2011 Defense Logistics Agency samples from soil borings located at the south, east, and north sides of the UST excavation area and performed laboratory chemical analyses. The maximum concentration of TPH was 17,000 mg/kg in soil and 650,000 µg/L in groundwater. In October 2003, DLA submitted the Final DDJC-Tracy UST Program Preferred Alternatives Report(PAR). The PAR presented soil and groundwater sampling results for 13 UST sites at the Tracy supply depot. The PAR recommended closure for 12 of the sites, and monitored natural attenuation for the Building 255 UST site. The PAR presented chemical analyses results on a soil sample collected from 15 feet below the surface at the former UST location and a groundwater sample collected approximately 30 feet north. The TPH concentration in the soil sample was 16,000 mg/kg and in the water sample was 20,000 µg/L. At that time, DLA estimated that approximately 400 cubic yards of soil contaminated with approximately 200 kilograms of TPH remained in soil beneath the former UST. Also, DLA estimated that the plume of contamination was over 24,000 square feet with a portion of the plume beneath the building. The PAR determined that the Building 255 site was a low risk site, but due to the TPH concentrations in soil and groundwater, the site should be monitored. The PAR recommended that the site be monitored for an additional year; soil borings would be advanced, and soil and groundwater samples collected and analyzed to verify decreasing petroleum hydrocarbon concentrations before the site is recommended for closure. In January 2009, DLA again collected soil and groundwater samples from borings at and near the former UST location. The maximum concentration of TPH was 7,200 mg/kg in soil and 720,000 µg/L in groundwater. In February 2010, DLA collected groundwater samples from soil borings. The maximum concentration of TPH was 619,000 µg/L. In May 2010, DLA installed five groundwater monitoring wells LM198AU through LM202AU, screened across the water table. Laboratory chemical analyses results on groundwater samples collected during the second, third, and fourth quarters of 2010 using low-flow techniques detected TPH at concentrations up to 1,150 µg/L. In August 2010, DLA performed vapor intrusion modeling using the Johnson and Ettinger Model to estimate health risks resulting from potential migration of TPH volatile organic constituent (VOC) vapor into Building 255. On 16 August 2010, DLA submitted a draft modeling report, and following the 5 April 2011 Central Valley Water Board staff review letter, submitted the Modeling Report dated 4 June 2011. Both reports indicated that the excess cancer risk from volatilization and intrusion of TPH VOCs into Building 255 indoor air was less than 1x10-6. However, the hazard quotient reported in the Draft 16 August 2010 Modeling Report was 1.47, whereas the hazard quotient in the Draft/Final 4 June 2011 Modeling Report was less than 1.0. Modeling Report Comments 1) In the Modeling Report, DLA changed the model input parameters, and resulting excess cancer risk and hazard quotient modeling results that were presented in the draft modeling report. The reasons for these changes were not presented in the Modeling Report, and were not requested in the 5 April 2011 Central Valley Water Board staff review letter. Changes to the modeling must be justified in a revised Modeling Report, which must also include changes made to comply with other requirements described below. If changes are not justified, another phase of data collection and risk evaluation may be necessary.