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4; RESPONSE TO COMMENTS (December 2010) DRAFT WORK PLAN FOR GROUNDWATER MONITORING WELL INSTALLATION, SOIL GAS AND GROUNDWATER INVESTIGATION,AND GROUNDWATER MONITORING, October 2010 Army Aviation Support Facility, 2000 Stimson Road, Stockton, CA OTIE Contract Number:W91238-09-D-0017, DO 0004,Amendment 01 8 Doc. Source Section/Page Comment Response(Contractor) 4 WP CVRWQCB — The proposed soil vapor sampling program in the Draft Per CVRWQCB recommendation,the depths of all soil Work Plan targets a zone from 20 to 35 feet below ground vapor samples will be based upon the lihtologies surface(bgs). This depth range was selected because encountered (logged in the field), photoionization groundwater is anticipated at approximately 35 to 40 feet detector(PID)readings, and the field experience of the bgs and the upper 20 feet of vadose zone is expected to site geologist. Encountered soils(i.e. borings)will be be relatively impermeable based on previous borings screened with a PID for VOCs. Boring depths with advanced at the site. This proposed soil vapor sampling elevated levels of VOCs(as compared to all boring program may provide data on vapors off-gassing from the intervals screened)will be noted.These values will local water table, but it may not adequately characterize contribute to the decision of associated soil vapor areas where solvents were potentially released into the sample depths. subsurface(source areas). The more permeable soils should be targeted, but the upper vadose zone should be In the case of soil vapor samples planned within a targeted too if the boring location is adjacent to a suspected source area(inferred by location or elevated suspected solvent release area such as the former or PID readings)possessing what appear to be existing wash rack or associated clarifiers. Central Valley impermeable soils, one sample attempt per discrete Water Board staff recommends selecting the proposed location will be made, prior to attempting a sample at a soil vapor sample depths based on the actual soils more feasible(i.e. more permeable)sample depth. encountered at each proposed boring locations rather than assume the upper 20 feet of the vadose zone is too The text of the Work Plan has been revised to reflect impermeable to yield viable soil vapor samples across the that the aforementioned response. site. Furthermore, CANG should make a few attempts to sample the shallow vadose zone in the suspected source areas even if the soils appear to be impermeable. 5 WP CVRWQCB — CANG should consider using a mobile lab or portable See response to Comment#4. equipment that can provide quantitative or semi- quantitative analysis of VOCs during advancement of the 40 borings, so subsequent borings can be adjusted to characterize any release areas that may be identified during investigation. Alternatively, if CANG suspects VOCs have been encountered based on field screening readings,the sample could be analyzed on an accelerated turnaround at a fixed-base laboratory. 2