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P A G E 7 1 �3 1 -I ' duration. For this reason, the exposure frequency was assumed to be 5 days per year, and the exposure duration was assumed t'o be I"Gyear. In accordance with guidance developed by ASTM (1996)`the development j of risk-based screening levels for ground water required,that acceptable or target levels be developed first for air (Table 2) under each exposure scenario (i.e., industrial use and construction). Again, the calculations followed applicable guidance issued by USEPA (1996), USEPA!rReg.on 9 (1999), and ASTM (1996). Like the risk-based screening Ievels for soil, the target levels for air incorporated current toxicity information and, where appropriate, generic exposure assumptions.' i. 3� The target levels for air (Table 2) were then used to derive risk-based �3: screening levels for ground water under both an industrial scenario (Table 3) and a construction scenario (Table 4). 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 an industrial scenario, the depth to ground wafer was assumed to be 4 feet. This represents the minimum (i.e., the most conservative or most health protective value) of the range of values!cited!lin the Closure Report (ERM, 1999). • Under a construction scenario, the depth to ground water was assumed to be 1 foot. Note that, because of the shallow water table in i 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 3 ' s Generic assumptions were used for all exposure parameters except the exposure frequency and exposure duration terms under the construction scenario. As l I4 noted in the discussion of the risk-based screening levels for soil,calculation of the target values for air under a construction scenario incorporated an exposure frequency of 5 days per year and an exposure duration of 1 year. s �1