2.13 MASS BUDGET FOR VOLATILES

        Source depletion by the loss of volatile constituents to the atmosphere is incorporated in the MEPAS atmospheric component for computing volatilization. The overall source depletion mass budget is discussed in Section 4.10. For all cases, this module assures that the total computed emissions never exceed the inventory of available material. Also for the land treatment/old spill case, no emissions occur after the computed dry-out period.

        The total long-term mass losses of volatile materials may be limited by either the emission rates or the total mass of material contained in the landfill, spill, or pond. The atmospheric pathway component computes average lifetime exposures of environmental concentrations based on a 70-year time period. For materials with a relatively rapid volatilization rate (e.g., benzene), a significant reduction in volatile emissions will often occur over the 70-year period, and these computed concentrations will be mainly a function of the total amount of material released rather than initial emission rates (e.g., the values of computed environmental concentrations are a function of the initial site inventory rather than factors controlling the emission rate). At sites where the exact inventory is not known, an upper-limit inventory can be used to avoid an evaluation based on a release of an unreasonably large mass of material. Without input of an estimate of the total mass, MEPAS will assume the same emission rate over the entire 70-year period.

        For materials that volatilize relatively slowly (e.g., semivolatiles), the environmental concentrations will normally be a direct function of the emission rates computed with the above volatilization models. However, even for many slowly volatilizing materials, the mass budget limitation can reduce the emission rate significantly over a 70-year time period.