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Draft AAFS Work Plan -2 - 23 November 2010 <br /> t <br /> Also, provide a table with the Final Work Plan that includes all individual analytes that <br /> CANG proposes to test each soil, groundwater, and soil vapor sample for. The new table <br /> should include the proposed test method for each analyte. <br /> 3. The proposed soil vapor sampling program in the Draft Work Plan targets a zone from 20 <br /> to 35 feet below ground surface (bgs). This depth range was selected because <br /> groundwater is anticipated at approximately 35 to 40 feet bgs and the upper 20 feet of <br /> vadose zone is expected to be relatively impermeable based on previous borings <br /> advanced at the site. This proposed soil vapor sampling program may provide data on <br /> vapors off-gassing from the local water table, but it may not adequately characterize <br /> areas where solvents were potentially released into the subsurface (source areas). The <br /> more permeable soils should be targeted, but the upper vadose zone should be targeted <br /> too if the boring location is adjacent to a suspected solvent release area such as the <br /> former or existing wash rack or associated clarifiers. Central Valley Water Board staff <br /> recommends selecting the proposed soil vapor sample depths based on the actual soils <br /> encountered at each proposed boring location rather than assume the upper 20 feet of <br /> the vadose zone is too impermeable to yield viable soil vapor samples across the site. <br /> Furthermore, CANG should make a few attempts to sample the shallow vadose zone in <br /> the suspected source areas even if the soils appear to be impermeable. <br /> 4. CANG should consider using a mobile lab or portable equipment that can provide <br /> quantitative or semi-quantitative analysis of VOCs during advancement of the 40 borings, <br /> so subsequent borings can be adjusted to characterize any release areas that may be <br /> identified during investigation. Alternatively, if CANG suspects VOCs have been <br /> encountered based on field screening readings, the sample could be analyzed on an <br /> accelerated turnaround at a fixed-base laboratory. <br /> 5. The Draft Work Plan implies that CANG intends to install two permanent groundwater <br /> monitoring wells prior to advancement of the 40 investigation borings where both soil <br /> vapor and groundwater grab samples are proposed for collection. Central Valley Water <br /> Board staff recommends installing these permanent monitoring points after advancement <br /> of the 40 borings. Data collected from the 40 borings should be used to optimize <br /> placement of the permanent monitoring wells. <br /> Addtionally, CANG should consider installing additional monitoring well(s) downgradient <br /> of the former USTs and suspected solvent release areas. Currently, the Draft Work Plan <br /> proposes installation of two groundwater monitoring wells in or upgradient of the former <br /> USTs and fueling areas. Based on the site conceptual model, releases from the UST <br /> and washracks would have occurred many years ago and only investigating the potential <br /> release areas may not fully characterize potential groundwater contamination nor provide <br /> an adequate monitoring network for the future. Central Valley Water Board staff <br /> understands that securing funds for investigations at this site has always been <br /> problematic, so funding for the additional wells may not be available. However, the <br /> proposed density of investigation borings appears to be more than what is necessary to <br /> meet project objectives, so the additional wells may not actually require additional <br /> funding. CANG could potentially reduce the number of investigation borings near the <br /> existing wash rack and along the surficial drainage system on the southern site boundary <br />