Page 218 - Soil and water contamination, 2nd edition

Substance transport 205

lakes. In groundwater, the typical flow velocities are much slower than in surface water and
there are no eddy effects.
Mixing, or dispersion , also occurs as a result of differences in travel time s of parcels of
water between adjacent water flow path s due to differences in flow velocities or in the length
of the flow paths. Although groundwater flow is laminar , at the microscale groundwater does
not flow straight on as it must take myriad detours around soil particles, as shown in Figure
11.4. Figure 11.4 also shows that the flow velocity varies within and between the pores. At
the macroscale, the same principle applies for flow around regions of less permeable soil.
Furthermore, in the unsaturated zone of the soil, inhomogeneities in effective permeability
(for instance due to lithological stratification or the presence of macropores and fractures)
lead to differences in travel times along parallel flow paths. Accordingly, heterogeneity at the
macroscopic scale contributes significantly to dispersion. In surface waters, velocity gradients
both laterally and vertically are caused by shear forces at the boundaries of the water body,
such as shear stress es at the water–air interface (due to wind) and at the water bottom and
the banks (Figure 11.5).
In addition, secondary currents can develop in rivers due to channel morphology: for
instance, back currents into stagnant backwaters, eddies induced by ripples, dunes, and
rocks on the river bed, and the corkscrew-like helicoidal flow in meandering river channels.
These variations from the average water flow velocity (because of the velocity gradients) and
travel distances (because of the random variability or tortuosity of flow paths) also cause

mixing of parcels of water. This mixing is called mechanical dispersion and likewise results

6642 6642 6642

Figure 11.4 Causes of mechanical dispersion in groundwater: flow variation around soil particles.

a

b

c 6642 6642 6642

u

Figure 11.5 Causes of mechanical dispersion in rivers: a. helicoidal flow; b. small-scale flow variations, swirls, and
eddies; c. horizontal and vertical flow variations.

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