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any construction or excavation activity is expected to be limited in <br /> duration For this reason, the exposure frequency was assumed to be 5 <br /> days per year, and the exposure duration was assumed to be 1 year <br /> In accordance with guidance developed by ASTM (1996), the development <br /> of risk-based screening levels for ground water required that acceptable or <br /> target levels be developed first for air (Table 10) under each exposure <br /> scenario (i e , commercial/industrial use and construction) Again, the <br />' calculations followed applicable guidance issued by USEPA (1996), <br /> USEPA Region 9 (1999), and ASTM (1996) Like the risk-based screening <br /> t levels for soil, the target levels for air incorporated current toxicity <br /> information and, where appropriate, generic exposure assumptions ' <br />' The target levels for air (Table 10) were then used to derive risk-based <br /> screening levels for ground water under both a commercial/industrial <br /> scenario (Table 11) and a construction scenario (Table 12) Development <br />' of the ground water screening levels followed guidance issued by ASTM <br /> (1996), and incorporated default assumptions for all parameters except <br />' depth to ground water Site-specific information was used for this <br /> parameter, as explained below <br /> • Under a commercial/industrial scenario, the depth to ground water <br /> was assumed to be 4 5 feet This represents the minimum (i e , the <br /> most conservative or most health-protective value) of the range of <br />' values observed during monitoring conducted at the former tank sites <br /> o Under a construction scenario, the depth to ground water was <br /> assumed to be 1 foot Note that, because of the shallow water table in <br />' the vicinity of the subject properties, any excavation will necessarily be <br /> quite limited in depth A value of 1 foot was selected as a conservative <br /> estimate of the minimum distance from the water table that any <br /> extensive excavation activity would likely occur Furthermore, if <br /> excavation below ground water were to occur, it would require the use <br /> of shoring and dewatering In this case, the health risk posed to a <br /> construction worker is reduced as the water and the volatile <br /> compounds it potentially contains are pumped away from the work <br /> area, thus reducing the potential for exposure <br /> ' Generic assumptions were used for all exposure parameters except the exposure <br /> Ifrequency and exposure duration terms under the construction scenario As noted in the <br /> discussion of the risk-based screening levels for soil,calculation of the target vaIues for <br /> air under a construction scenario incorporated an exposure frequency of 5 days per year <br /> and an exposure duration of Z year <br /> ERRE 22 HEINZ IJSA-3374 60-3/27/00 <br />