SITE INFORMATION AND CORRESPONDENCE

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• f Mr. Maurice Benson - 4 - 30 December 2011 Defense Logistics Agency and odor of soil in the zone of water table fluctuation. On the borings logs, the Munsell Soil Chart color described is very dark greenish gley (1 Y, 3/10) and a strong odor was observed. Gley soil and strong odor are common where diesel fuel is present. Regarding the assertion that turbidity in the samples may have contributed to elevated TPH concentrations, DLA presented no evidence that the samples were turbid. Furthermore, DLA has not presented data showing such turbidity would have affected the laboratory results. In the revised CAP, DLA must submit any original field notes and logs written by sampling personnel, the chain-of-custody for each sample shipped to the laboratory, and the full laboratory chemical analyses reports. 7) In the CAP, DLA estimates that the TPH mass remaining in groundwater is 0.40 pound. However, it is unknown how DLA estimated this amount because the CAP does not indicate the contaminated aquifer volume and average TPH concentration used for the calculation. Regardless, it is clear that DLA did not estimate the area under which saturated concentrations of TPH in groundwater are present. Using the groundwater concentration data that DLA collected from soil borings in 2009 and 2010 and presented on the TPH concentration contour map in the May 2010 UST 25 Technical Memorandum, Central Valley Water Board staff estimates the areal extent of TPH-saturated groundwater is approximately 4,000 square feet. Furthermore, by assuming that the saturated layer extends approximately '/2 foot below the water table, the mass of TPH remaining in groundwater is on the order of 25 to 35 pounds or more, depending on the average concentration selected. Please note that our estimates do not include the dissolved TPH within the aquifer that is outside of and below the TPH-saturated portion of the aquifer. Using similar data in the revised CAP, DLA must recalculate the TPH mass remaining in groundwater. 8) The CAP concludes that natural attenuation is occurring and therefore implementation of monitored natural attenuation is appropriate at Building 255. In support of this conclusion, DLA states that the subsurface TPH does not pose an immediate human health risk, the plume is generally defined, and petroleum hydrocarbon concentrations are stable or decreasing. Central Valley Water Board staff does not agree because 1) screening level vapor intrusion modeling resulted in a health hazard index of 1.47, indicating the need for further vapor intrusion study, 2) the plume of TPH-saturated groundwater has not been defined, and 3) the TPH concentration trend has not been established using groundwater monitoring well data collected by appropriate sampling procedures. 9) In the revised CAP, DLA must tabulate all of the soil matrix and groundwater sample laboratory chemical analyses data collected from all of the borings at the Building 255 site must be tabulated in the revised CAP. 10)Prior to completing the revised CAP, DLA must complete the soil vapor risk study and adequately characterize the extent and concentration of TPH in groundwater at the Building 255 site.