Surface-to-Air Particle Suspension Formulations: Computed Source Term Release Model, Multimedia Environmental Pollutant Assessment System (MEPAS)
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Preface
Summary
Table of Contents
1.0 Introduction
2.0 Surface-to-Air Particle Suspension Models
3.0 Revised Formulations for Surface-to-Air Mass Flux Models
2.0 Surface-to-Air Particle Suspension Models
This section provides an overview of the various surface-to-air suspension models required in the source and suspension components of a multimedia modeling system such as MEPAS.
Contaminated surfaces in the environment can result from operations such as waste management (mine tailings, mill tailings, etc.), accidents (spills, leaking storage, etc.) and production/manufacturing activities (liquid discharges to the surface, deposition from airborne plumes, etc.). Such surface contamination can also be the result of material stored/released directly on the surface or of migration of buried material to the surface. The potential for suspension of contaminants varies greatly because of the many surface types and activities expected on the surfaces. If a site has only buried or immobilized wastes, there will be no potential for suspension of particles. If a site has, or in the future may have, exposed surfaces with contamination, the potential exists for suspension of contaminated particles to the atmosphere.
The primary source refers to the starting point for the environmental analysis and a secondary source refers to the surface contamination that results from deposition of material suspended from the primary source. The contaminated surface for both primary and secondary sources can be a variety of surfaces forms - soil, vegetation, buildings, roadways, etc.
The soil-to-air emission of surface particles may occur from suspension of exposed surface contamination by combinations of mechanical processes. The suspension of particles from the surface may occur as the result of wind action (Bagnold 1941; Sehmel and Lloyd 1976) or other physical actions 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, sweeping, and earth moving can greatly increase the fugitive particulate release rates compared to those for an undisturbed surface.
Figure 2.1 illustrates the models used in the MEPAS formulations to estimate air concentrations resulting from surface suspension processes from both the primary and secondary source areas. Air concentrations are needed at locations both over the source and downwind of the source areas. The "Mass Loading Model" provides a means of estimating exposures directly over primary source contaminated areas and a "Surface Mass Flux Model" provides estimates of the amount of material going into the air from that area. For a secondary source area, a "Suspension Factor Model" is used to estimate the concentrations over the contaminated area. The "Suspension Factor Model" for suspension from secondary sources relies on empirical relationships relating the surface contamination and resultant air concentrations to estimate exposures. The mass loading and suspension factor models account for the emission and dispersion processes in a single model using empirical relationships. These mass loading and suspension factor models are used mainly for convenience in estimating the magnitude of exposures in what are often complex situations. These models could be replaced with more detailed physically-based models for specific situations with special modeling requirements.
Figure 2.1 Particle Suspension Models for Primary and Secondary Source Areas
Table 2.1 provides characteristics of the various suspension models. The following sections of this report document formulations for the "Surface Mass Flux Model" for estimating soil suspension rates for a primary source from wind suspension, mechanical disturbances, and vehicles (on roadways). That model connects with an "Air Pathway Model" to estimate air concentration downwind of the source area. Other suspension models are documented in the exposure formulations.
| Model Type | Suspension Mechanism | What Model Estimates | Model Application | Formulation Documentation |
|---|---|---|---|---|
| Surface Mass Flux | Wind and mechanical actions result in particle suspension | Mass flux from area or roadway to air(mass/time) | Define rate of release as input to the airborne transport and dispersion computation | This document |
| Suspension Factor | Disturbances (wind, walking, running, etc.) suspend particles. Uses an empirical "Resuspension Factor." | Air concentration over contaminated surface (mass/volume) | Local air concentration encountered by an individual located on an area with deposited materials | Exposure formulations |
| Mass Loading | Assumes a typical mass loading in the air over the site is all from suspension of soil from the site | Air concentration over contaminated surface (mass/volume) | Maximum local air concentration in the air over a contaminated surface | Exposure formulations |
Although the local exposures resulting from suspension are considered, the suspension and transport of the secondary material to downwind locations are not directly considered in these current models. To evaluate a secondary source that may potentially represent a significant transport pathway, the secondary source can be considered a new primary source area in a separate scenario.
In summary, depending on the model requirements, different approaches are used to characterize the particle suspension. For input to an air transport model, a detailed mass flux model is used to evaluate the suspension from the primary source. To estimate local air concentrations over both primary and secondary sources, empirical relationships are used.
