Environmental Engineering Reference
In-Depth Information
Equation (
11.10
) is a simple formula connecting piezometric head
h
and
velocity
v
.
11.3 Darcy's Law for Flow in Porous Media
Not only the visible or sensible flow of water or air, which we observe above the
ground surface, is relevant for the distribution of potentially harmful substances
within the environment. A multitude of pathways below the ground surface con-
tribute substantially to the migration of pollutants. The sketch of Fig.
11.6
visua-
lizes some of those.
Chemical substances are transported from a contaminated site with the seepage
flow into the unsaturated soil as first compartment in the subsurface. Even in arid
regions such transport can be observed, although only scarce precipitation events
produce a transient flow field. Modern landfills are equipped with a confined bottom
to prevent the downward movement of components. Thus, the described migration
by seepage is reduced significantly on these sites. However, the confinement is not
complete and may last only temporarily. Toxic, aggressive waste may diminish the
sealing function over long time scales. Old landfills or contaminated sites can be
sealed at the top by a cover which is more or less impermeable to water. This helps
to reduce flow and advective transport effectively.
Contaminants can pass the unsaturated zone and enter the groundwater compart-
ment. While in the vadose zone the dominant flow direction is vertically downward,
in groundwater layers the horizontal velocity component usually dominates. The
substances may thus be transported to more vulnerable regions. Where water is
Fig. 11.6 Crucial sub-surface flow paths at waste disposal sites
