In the preceding sections, the various experimental setups used to illustrate concepts of hydraulic head and flow are similar to those in a confined aquifer, because a confined aquifer is bounded by layers of low hydraulic conductivity relative to the aquifer. When the confining layers have a much lower hydraulic conductivity than the aquifer, the confining layers act in a manner similar to no-flow boundaries, because they restrict nearly all flow across the aquitard, which was the case for all of the experimental scenarios presented earlier. Under most natural conditions, some water can migrate across a water-saturated aquitard in accordance with Darcy’s law if there is a hydraulic gradient across the aquitard.

Fundamental to flow in confined aquifers is the concept of the potentiometric surface. Figure 20 provides insight into why the phrase is applicable to confined conditions; the potentiometric surface is actually an imaginary surface (Figure 20a) that can be defined using equipotential contours viewed in the x-y plane (Figure 20b). We are already familiar with its representation in cross section (Figure 20c). The effects of aquitards on confined flow are presented in Section 5.3.

Figure 20 – Potentiometric contours for a confined aquifer shown in various views: a) perspective; b) plan: and, c) cross section. In the plan view, the potentiometric surface can be represented using equipotential lines in a manner similar to elevation contour lines used to represent ground surface topography (Cohen and Cherry, 2020).

Example Problem 4

The figures for this problem show a potentiometric contour map of a confined aquifer and a cross section along transect A-A′. What is the depth of the water level in the well (relative to ground surface)? Explain.

a) 3 m
b) 5 m
c) 7.5 m
d) 10 m

Click here for solution to Example Problem 4