The confining beds of a shallow confined aquifer are never truly impermeable; we indicate by the term “confined” that the leakage through the confining beds is negligibly small. If the leakage cannot be neglected, the aquifer is referred to as a semiconfined aquifer and the leaking confining bed as a leaky layer, a semipermeable layer, or an aquitard. We use the term shallow leaky aquifer flow whenever the aquifer is sufficiently shallow that the resistance to flow in the vertical direction may be neglected.
An important property of leaky systems is that there exists an equilibrium condition if there are no features in the system, wells, for example, that induce leakage. We discuss in this chapter how to describe the leakage induced by features in systems of two aquifers, separated by a single leaky layer.
Shallow Semiconfined Flow
The simplest case of shallow leaky aquifer flow occurs if the head is constant above the upper semipermeable layer, and the lower boundary of the aquifer is impermeable. Such a case applies if there is little or no flow in the aquifer above the semipermeable layer. We refer to this type of flow as semiconfined flow. We derive the basic equations for this type of flow and consider several examples. The equilibrium condition in a semiconfined aquifer, i.e., in a system without features that induce leakage, is that the head in the lower aquifer equals the constant head in the upper aquifer.
A vertical section through a semiconfined aquifer is shown in Figure 5.1; the hydraulic conductivity and thickness of the aquifer are kand H. We label all quantities associated with the semipermeable layer with an asterisk: the hydraulic conductivity and thickness of the semipermeable layer are k * and H *. The constant head in the aquifer above the leaky layer is ϕ *. All heads are measured with respect to the impermeable base of the aquifer.