Agriculture Reference
In-Depth Information
but finer, loamy or clayey textures cannot well be sparged in view of their air entry values.
Engineers were particularly interested in the so called radius of influence (ROI), which is the
radius of the supposedly circular cross section of the cone through which air flows at the
phreatic groundwater level. In addition, air saturations in this cone are of interest.
A mathematical analysis has been worked out by Van Dijke et al. (1995) and was later
refined by Philip (1998). In both cases, relatively simple solutions were obtained, but in both
cases for a single cone and for a homogeneous aquifer. In a refinement, Van Dijke & Van der
Zee (1998) considered a two layered medium, i.e., air was injected below the groundwater
level, in a coarse layer that is overlain by a slightly finer textured layer. The contrast in
physical properties of the two layers was very small (hydraulic conductivity differed only
by a factor √2, i.e., hardly measurable). Even with such a small contrast in properties, the
effect on the sparged area was major, as can be seen from Figure 8. It appears that at the
interface between the two layers, the cone rapidly expands to larger radial distances away in
a horizontal direction from the injection point, and air saturations rapidly decrease.
Important is to note, that these calculations were done for an axially symmetric situation.
The practical impact of these results is the following: if the air flow pattern is so sensitive to
slight spatial (vertical) variability of the subsoil, for real subsoils with intricate spatial
patterns in 3D, this must also be expected and (in view of variability) in an enhanced way.
The air flow will be along a path of minimal resistance, and along this path, almost all flow
will concentrate. Therefore, although injected air will oxygenate the subsoil, this will be a
very inefficient way to do so, as flow is excessively preferential. Moreover, it cannot be well
steered, to get the air to flow to a particular part of the subsoil, e.g. the contaminated part. In
reality, the flow may follow many possible paths around a contaminated volume.
Fig. 8. Iso-concentration lines of air saturation for a two-layered medium with layer interface
at dimensionless depth Z. Dimensionless radial distance is r (Van Dijke and Van der Zee,
1998).
In more general terms, spatiotemporal variability leads to uncertainty and therefore some
suggestions can be given regarding soil/water contamination decision making under
uncertainty, focused at the de-icing chemical contamination context.
6. Guidelines for decision making
Many contaminated sites and polluting surface activities rely on natural or manipulated
remediation to remove pollutants in the soil. In order for this approach to be acceptable it is
