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• All corrective actions regarding tanks,piping and impacted soils have been conducted under a work plan requested by either PHS-EHD or RWQCB ' 61 3 Modeling • Three model simulations of vadose zone leaching without biodegradation for a period ' of 25 years were used to predict what impact the residual soil TPH/D and its most mobile constituent (naphthalene) would have on groundwater at 35 feet, 54 feet, and ' 70 feet below ground surface (bgs) • The first model run suggests that concentrations of the indicator parameter ' naphthalene would reach a maximum of 36 5 ug/L in groundwater with water level at 35 feet bgs (a 25-year high) • The second model scenario used an estimate of 54 feet as a long term average depth ' to groundwater resulting in a maximum naphthalene concentration of 2 98 ug/L in groundwater This scenario is considered conservative since it does not reflect long term downward trends resulting from groundwater basin overdraft in the past 50 years • The third model scenario used an estimate of 70 feet as the water level which was observed at the site in 1991 and 1992 If this groundwater level persisted for 25 ' years, the predicted impact on groundwater would be approximately 0 172 ug/L • All three scenarios are conservative because they ignore biodegradation of naphthalene Twenty five years represents approximately 250 to 500 half lives for naphthalene ' • The Water ate Quality Goal proposed for this site is the EPA SNARL for naphthalene ' of 20 µg/L If this goal is appropriate, the soils remaining at the site do not constitute a "Designated Waste" so long as groundwater level does not rise above 54 feet bgs i • If groundwater levels would rise to 35 feet bgs some impact of TPH/D constituents may be observed in groundwater, if this water level persists and if no biodegradation were to occur SAC 116 07 16 AUGUST 1994 DAMES & MOORE