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1 • • Mr. Maurice Benson - 3 - 30 December 2011 Defense Logistics Agency 2) In both the Modeling Report and the previous draft, DLA estimations of soil vapor concentrations utilizing soil matrix TPH constituent concentration data did not include results on soil samples collected in 2009. Even though the highest soil matrix concentrations of naphthalene and 2-methyl-naphthalene were detected in the 2009 samples, DLA chose not to utilize the data because the laboratory chemical analyses were performed by USEPA Method 5035A. While this soil matrix analysis method should not be used as the soul method for estimating risk from soil vapor concentrations, it can be used, provided other lines of evidence are also used, or DLA could sample and analyze soil vapor. DLA should refer to the Department of Toxic Substances Control (DTSC) document dated October 2011, and titled Final Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air. The section titled Use of Soil Matrix Data on page 17 clarifies the intent of the same section presented in DTSCs December 2004 Interim Final guidance document. DLA must utilize the 2009 soil matrix laboratory results to estimate soil vapor concentrations. Alternatively, DLA could resample the 2009 locations, analyze these samples utilizing non-Method 5035A analyses, and complete the soil vapor concentration estimations. Another alternative open to DLA would be to sample and analyze soil vapor to establish soil vapor concentrations for model input. 3) The sampling method for collecting groundwater samples from monitoring wells described in the Modeling Report is not acceptable for characterization at a petroleum hydrocarbon- contaminated site such as the Building 255 site. During each monitoring event, DLA's contractor carefully lowers a tube approximately 5 feet below the water table in each well then slowly pumps a groundwater sample through the tube. This method is not appropriate for petroleum hydrocarbons because, between sampling events, they can volatize within the well casing and dissipate through the screen into the unsaturated zone; also, the highest concentrations in wells are usually at or near the water table. DLA must propose, and Central Valley Water Board staff must approve, alternate methods for collecting representative samples from groundwater. 4) In the revised Modeling Report, DLA must tabulate all of the soil matrix and groundwater sample laboratory chemical analyses data collected from all of the borings at Building 255. CAP Comments 5) In the CAP, DLA proposes reducing the groundwater monitoring sampling frequency from quarterly to semi-annual. Given the need to revise the sampling methods at Building 255 site monitoring wells and the effect that the fluctuating water table has on petroleum hydrocarbon concentrations in soil and groundwater within this zone of fluctuation, it is premature to reduce the sampling frequency. DLA must continue to collect and analyze groundwater samples every quarter. 6) The CAP concludes that the CPT samples collected in 2009 "cannot be relied upon as a quantitative indicator of ambient, dissolved groundwater quality' because the laboratory analyses results were estimated and the "samples were collected from a soil boring and likely contained a high degree of turbidity'. Although the concentrations were estimated, they did contain saturated-levels of TPH, as observed by the sampling personnel (telephone conversation between James Brownell of the Central Valley Water Board staff and Marshall Cloud of DLA, January 28, 2009). Also, the soil boring logs for the two down- gradient monitoring wells located closest to the former UST 25 location describe the color