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5 1 OBJECTIVES ' The objectives of the fate and transport modeling are to t + Utilize site specific data to assess the movement of naphthalene, the most mobile constituent of TPH/D, through vadose zone soils into the first encountered ' groundwater, + To model the movement of naphthalene at three groundwater levels which have occurred over the last 25 years based on regional hydrologic information, + To utilize conservative assumptions of the transport process, such as the highest P P p � g reported concentrations of TPHID at the deepest depth observed in the 1986 and 1988 ' soils data, and assuming no biodegradation of naphthalene in soils, and • To compare the model output with regional water quality goals, consistent with the Designated Level Methodology (RWQCB, June 1989), approach adopted by the RWQCB for classifying non-hazardous wastes in soils 5 2 OVERALL ASSUMPTIONS Three groundwater level scenarios were considered in the model VLEACH was run for depths to groundwater of 35 feet bgs, 54 feet bgs, and 70 feet bgs The 35 foot depth to groundwater was considered as a worst case, representing the shallowest groundwater levels which have been measured in monitoring wells at the site A depth of 70 feet depth was considered as representing the other extreme of the deepest groundwater levels measured at the site However, neither of these conditions are likely to persist for a significant length of time, as can be inferred from the groundwater hydrograph shown in Figure 8 Therefore, a third case of a depth of 54 feet was also modeled to represent a more realistic long term condition This depth to groundwater represents the estimated average depth to shallow groundwater beneath the site during the 25 year period from 1979 through 1994, based on-a correlation between water levels measured in site monitoring wells, and nearby California Water Service well 21-01 (Figure 8) All three modeling scenarios ignore biodegradation of naphthalene This assumption is conservative because naphthalene is biodegradable with a half life ranging from 17 to 48 days in the ' literature (Howard et al , 1991) For example, if the concentration of naphthalene present in the soil in 1987 was 10,000 mg/kg, the predicted concentration in 1994 considering biodegradation with a half life of 100 days would be approximately 0 0002 mg/kg This calculation assumes a 100 day half life, implying that degradation proceeds twice as slow as the slowest value in the literature Because SAC116 07 11 AUGUST 1994 DAMES &MOORE