Dispersion

The dispersion of contaminants in the groundwater is due to a combination of molecular diffusion and hydrodynamic mixing. The net result is that the concentration of the material is less, but the zone of contact is greater at downgradient locations. Dispersion occurs in a longitudinal direction (Dx) and transverse to the flow path (Dy). Dye studies in homogeneous and isotropic granular media have indicated that dispersion occurs in the shape of a cone about 6° from the application point (Danel, 1953). Stratification and other areal differences in the field will typically result in much greater lateral and longitudinal dispersion. For example, the divergence of the cone could be 20° or more in fractured rock (Bouwer, 1978). The dispersion coefficient is related to the seepage velocity as described by Equation 3.6:

where

D = Dispersion coefficient: Dx longitudinal, Dy transverse (ft2/d; m2/d).

a = Dispersivity: ax longitudinal, ay transverse (ft; m).

v = Seepage velocity of groundwater system (ft/d; m/d) = V/n, where V is the Darcy's velocity from Equation 3.5, and n is the porosity (see Figure 2.4 for typical values for in situ soils).

The dispersivity is difficult to measure in the field or to determine in the laboratory. Dispersivity is usually measured in the field by adding a tracer at the source and then observing the concentration in surrounding monitoring wells. An average value of 10 m2/d resulted from field experiments at the Fort Devens, Massachusetts, rapid infiltration system (Bedient et al., 1983), but predicted levels of contaminant transport changed very little after increasing the assumed disper-sivity by 100% or more. Many of the values reported in the literature are site-specific, "fitted" values and cannot be used reliably for projects elsewhere.

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