4.0 TRANSPORT, DISPERSION, AND DEPOSITION

        Once a contaminant material is airborne, it is transported and dispersed by air movement. The contaminant will be carried by the winds, and the atmospheric contaminant concentration will be reduced by dispersion and deposition processes. The near-surface atmospheric concentrations computed in the transport, dispersion, and deposition module of MEPAS provide the basis for evaluating inhalation exposures.

        The relative importance of the atmospheric pathway at various sites is controlled by a combination of topographic and climatological influences. Controlling parameters include the distance and direction from the inactive waste site and the local wind conditions and stability. Because dispersion is a strong function of the downwind distance a contaminant travels, the physical distances between the contaminant site and population centers are of prime importance. The local frequencies of wind occurrence by direction, particularly in areas with topographic channeling of winds, are important in calculating exposure and risk associated with contaminants in the atmospheric pathway. The relative rates of atmospheric dilution between the sites are mainly a function of local wind speeds and dilution (stability) parameters.

        As a result of surface-induced mechanical mixing, the local surface roughness influences local dispersion rates. The MEPAS formulation for local dispersion rates accounts for the effect of local surface roughness. All other factors being equal, a site with a smoother surface will have slower dilution rates than a site with a rougher surface.