Chapter 5.1.3. The Intermediate Zone

Most of the dynamics of soil moisture movement is in the root zone or top one to three meters of the unsaturated zone. The intermediate zone is much less dynamic and tends to dampen events that occur at the soil surface or water table. For thick intermediate zones water contents are more or less constant with time. However, for prolonged surface flooding conditions, wetting fronts will move throughout the intermediate zone and moisture contents will exhibit a dynamic behavior.

Usually, water contents are low and gradients are near unity. Consequently, hydraulic conductivity is low and as a result velocity flux is low. For thick unsaturated zones of 10’s of meters, water travel times from the soil surface to the water table may be on the order of decades. This damped behavior of the intermediate zone has profound consequences when contaminants are being transported in the moving water. It may take many years before a problem shows up at the water table after an event such as a contaminant spill on the land surface.