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