5.2.1  Suspension of Surface Particles

 Particulate fugitive releases of contaminants are defined as emissions resulting from suspension of exposed surface contamination by wind and mechanical processes to the air.  If a site has only buried or immobilized wastes there will be no suspension emissions.  If the site has exposed surfaces with contamination or surfaces with contamination are exposed due to the erosion of cover materials, there will be emissions to the atmosphere caused by the suspension of respirable particulates.

 A surface cover with contaminants may be the result of either waste storage (e.g., mill tailings) or contamination by the operation of a facility.  The latter contamination may occur on natural surfaces (e.g., soil) or artificial surfaces (e.g., concrete pads, roadways).  The potential for suspension of contaminants varies greatly because of the wide variety of surface types and activities expected on the surfaces.

 The suspension of particles from the surface may occur as the result of wind action (Bagnold 1941; Sehmel and Lloyd 1976) or other physical action on the surface (Sehmel 1976).  Atmospheric turbulence plays a role in determining the extent to which the air movement over the surface can suspend surface particles.  Local mechanical activity on the surface, such as animal grazing, vehicular traffic, walking, and earth moving can greatly increase the fugitive particulate release rates compared to an undisturbed surface.

 The suspension of respirable particles (particles with diameters less than 10 mm) from contaminated areas at U.S. Department of Energy (DOE) sites is calculated using empirical relationships based on studies of wind erosion and surface disruption.  The source-term release module outputs for suspension from contaminated surface areas are expressed in terms of an airborne soil concentration normalized to a unit area of contaminated surface.  These soil concentration arrays are converted to arrays of contaminant concentrations using the fraction of surface contamination in the suspended soil.

Computing the suspension of contaminants from a surface into the atmosphere requires both contaminant and site data.  These data are used to define which formulations, if any, apply to the site.  If a computation is appropriate, these data are used to compute the suspension rates.

The source-term release module methodology for computing suspension rates is an adaptation of the methodology proposed by Cowherd et al. (1985) for rapid computation of potential long-term impacts from spills of hazardous materials.  This methodology, which includes formulations for contaminant suspension by winds, vehicular traffic, and other physical disturbances of the surface, is similar to, but not identical to, the U.S. Environmental Protection Agency's (EPA) AP-42 revision of the Cowherd et al. (1985) model for industrial wind erosion (EPA 1988b, 1995).