Retardation in the migration of contaminants in ground-water is due to the adsorption mechanism, which was described in Section 9.12 for both organic and inorganic constituents. The retardation coefficient can be calculated based on the distribution or adsorption coefficients of the contaminant and the characteristics of the porous medium as
where Kd is the distribution or adsorption coefficient described previously. The values pd and n are the bulk density and porosity of the soil. The velocity of the contaminant in groundwater can be calculated as follows:
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