5.4.3 Wind Suspension or Water Erosion

     Contaminant mass can also be lost from the source zone by removal of soil (which contains contamination) from the upper surface of the zone by natural processes. Particles of contaminated soil can be suspended in the air and moved away from the source zone by wind action, or they can be dislodged from the soil surface by water erosion and moved away from the source zone suspended in overland runoff. While both of these processes are governed by unique mechanisms, the ultimate mass loss flux equations for both processes have similar mathematical forms. Therefore, both of these contaminant loss pathways will be discussed in this one section, even though the resulting equations presented in this section are strictly for mass loss when only one of these processes is occurring.

     First, consider scenarios where the soil is contaminated all the way up to the soil surface. These surface removal processes then strip off bulk soil from the top of the contaminant source zone (which is exposed to the wind or the overland runoff water). Consequently, these scenarios can be characterized by surface soil removal rates due to wind suspension, S, and water erosion, E. These rates are defined as the depth of surface soil lost per time (which is equivalent to the volume of surface soil lost per time per area of surface). Because these processes strip off a depth of the soil itself, they remove all contaminant that is contained in the bulk soil. Therefore, the mass loss flux is not affected by how the contaminant is partitioned between aqueous, solid-sorbent, vapor, or NAPL phases. The contaminant mass flux to a surface soil removal pathway is then given by



The volume of the source zone is given by



where h is the thickness of the contaminant source zone (cm).

 Substituting Equation 5.69 into Equation 2.2, and then substituting the resulting equation into Equation 5.68, we obtain



     Note that the thickness, h, of the source zone is not a constant in these cases, but instead decreases linearly with time because the surface soil removal process is constantly stripping away soil from the top of the zone. When this time dependence is explicitly written, the mass flux equations for loss from the source zone by either wind suspension alone or water erosion alone are given by



and



     On the other hand, there can also be scenarios where the soil is not contaminated all the way up to the soil surface. If a clean uncontaminated soil layer exists above the contaminant source zone, these surface soil removal processes will be stripping away clean soil. Therefore, the contaminant mass flux is zero for wind suspension and water erosion as long as a clean layer remains. However, as the clean layer is gradually removed, the contaminant source zone may eventually become exposed. When the clean layer is completely eroded away, mass fluxes to these two pathways will become non-zero according to Equations 5.71 and 5.72.

     Note, that deposition of clean soil particles onto the top of the site (by either wind or water) can also be simulated by the module by entering negative values of S or E. In these cases, the module sets the contaminant mass flux for wind suspension and water erosion to zero.