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AFAR R ASSOCIATES <br /> Mr. Donald T. Bradshaw, R.G. <br /> October 16, 2000 <br /> Page 3 <br /> Specific Comments <br /> 6 Page 1, bottom - Cited reference to "(DTSC, 1992)" should be changed to "(DTSC, <br /> 1996)" to reflect the most current document version available, and to match the HRA <br /> References listing (page 30). <br /> 7 Page 2, bottom - Cited reference to "(DTSC, 1992)" should be changed to "(DTSC, <br /> 1996)." <br /> 8 Page A-1 - Johnson and Ettinger state that their model (ES&T, Vol. 25, pp. 1445-1452, <br /> 1991) is a"heuristic model for screening-level calculations." Because of this, and because <br /> reliable field data exist for simultaneous VOC concentrations in indoor air and soil gas, the <br /> modeling results should not be used in lieu of the field data; rather, the field data should be <br /> used to calibrate the model and validate model results. The modeling analysis should be <br /> reviewed and the model revised to produce results consistent with existing field data prior <br /> to any further use of the modeling approach to estimate exposure-point concentrations and <br /> associated health risks. <br /> 9. Page A-4, Equation 5 - This equation is based on the assumption that vapor flux into the <br /> building is through cracks in the floor. However, the results of flux-chamber <br /> measurements at Aaron Brothers show that vapor flux through the slab floor was similar <br /> in measurements over cracked and non-cracked floor areas. Therefore, the model (i.e., <br /> Equation 5) is inappropriate for the given site conditions. Johnson and Ettinger's <br /> Equation 26, which models diffusion-driven vapor transport through the entire floor, <br /> should be used instead of the selected model. <br /> 10. Page A-9, 1' paragraph - The depth to the water table has decreased recently, indicating <br /> that VOC flux from groundwater could be greater than calculated using LFR's assumed <br /> 40-ft depth to water table. In addition, the depth of the vadose zone (over which VOCs <br /> from groundwater diffuse to the land surface) should be defined to the top of the capillary <br /> fringe rather than the water table. Based on the presence of finer-grained soils (producing <br /> a substantial capillary fringe) and the current water table depth, a depth of between 26 to <br /> 30 feet below ground surface would seem appropriate. <br /> lolud'affIoMyFile.\LPL\Letters\lei 10 1 600HRA.wpd <br />