Since a major pathway for food contamination involves uptake from contaminated soil by plants, determination of soil concentrations is often a first step in a food chain transport assessment. The major deposition and removal processes are illustrated in Figure 8.4. Deposition pathways include dry or wet deposition from the atmosphere, direct deposition from a leak or spill, and irrigation with contaminated water. Removal can occur by degradation, decay, resuspension to the atmosphere, runoff from the surface, and leaching to deeper soil. For modeling purposes, contaminants are considered to be deposited on the soil surface and to become mixed uniformly with the underlying soil. The level of contamination on the surface is expressed as the areal concentration (or areal contamination density), %[M(c)/L2], which is contaminant mass per unit area. Conversion of areal concentration to concentration in the underlying soil is accomplished by considering the contaminant to become uniformly mixed to the depth of the root zone, which is typically taken to be 0.15 m in the absence of site-specific information. With reference to Figure 8.4, the concentration in the soil is the total amount of contaminant on the surface, which is %A, divided by the mass of soil, which is pBAZR. This yields
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