5 Saturated Steady Flow at the Field Scale

On a field scale, hydraulic head can be measured using various types of wells (either installed separately, in a spatially clustered arrangement, or at the same horizontal location but at varying depths). Groundwater wells are essentially large-scale versions of the laboratory-scale piezometers we have used to illustrate Darcy’s law in the preceding sections. The following sections present the relationship of hydraulic head and flow in three basic types of hydrogeologic formations (confined aquifer, aquitard, unconfined aquifer) in the saturated zone, as described in this section and shown in Figure 19.

A confined aquifer is bounded above and below by an aquitard, which is a semi-pervious formation that restricts flow between the overlying and/or underlying aquifers.

The water level elevation in a well screened in a confined aquifer will be higher than the base of the overlying aquitard. Interpolation of the water elevations between those measured in wells in a confined aquifer defines the potentiometric surface.

An unconfined aquifer is bounded only at its base by an aquitard. The top of the unconfined aquifer is the water table, which is the elevation of the water level in a screened well that is just deep enough to encounter standing water.

Figure 19 – Schematic cross section of an unconfined and confined aquifer separated by an aquitard. The unconfined aquifer, confined aquifer, and aquitard are all water-saturated and are therefore hydraulically connected (Cohen and Cherry, 2020).