1.2 Hydraulic Schematization of the Subsurface

If initial conditions, boundary conditions and stresses are specified, a three-dimensional description of the subsurface (in terms of hydraulic parameters) is adequate for simulating the behavior of groundwater variables such as pressures, water levels and flow rates. Nevertheless, hydrogeologists and other groundwater practitioners prefer, for various reasons, an additional step in conceptualization: hydraulic schematization. This is based on observed or assumed contrasts in the capacities of adjoining subsurface layers or other volumetric domains to store and transmit groundwater. The schematization subdivides the subsurface into volumetric bodies, classified in principle into the following categories:

• unsaturated zone: upper part of the subsurface in which the pores or fissures contain air as well as water;
• aquifers: domains with a high capacity to store and transmit groundwater;
• aquitards: domains with low capacity to transmit groundwater (storage capacity may vary); and,
• impervious beds or rock masses: domains unable to transmit groundwater (barriers to groundwater flow).

There are no absolute criteria to distinguish between ‘high’ and ‘low’ storage and transmission capacities; while lateral or vertical contrasts in these properties are in many cases not along sharp and undisputed divides. Furthermore, the frequently used term ‘confining bed’ refers in some cases to an aquitard, in other cases to a virtually impervious lithological unit. Therefore, the hydraulic schematization of the subsurface in any particular area is not only approximative but usually also somewhat subjective. Figure 2 shows a hypothetical cross-section of the subsurface in which the mentioned categories of hydraulic units are shown. Note that the aquifer covered by an aquitard (semi-pervious confining bed) is called a ‘leaky confined aquifer’.

Figure 2 – Hypothetical vertical cross-section through the subsurface, showing the different categories of hydraulic units (vertical scale exaggerated).

The unsaturated zone may be absent at some locations (e.g., below a lake or river) and elsewhere be tens to hundreds of meters thick. The groundwater table forms its lower limit. Due to matrix forces, the water pressure in the unsaturated zone is less than atmospheric pressure. For this reason, water in the unsaturated zone is not included in the term ‘groundwater’; it is called ‘vadose water’ or (if the unsaturated zone is shallow) ‘soil moisture’.

Aquifers are both accessible reservoirs of groundwater and ‘highways’ for local or regional groundwater flow. Unconfined aquifers have a water table, while an aquifer bounded at its top and bottom by confining beds is called ‘confined’, or ‘leaky confined’ (semi-confined) in the case where at least one of these boundary layers is an aquitard. The potentiometric water level of confined and semi-confined aquifers is located above the aquifer’s top; if it is above the land surface, and thus allows free outflow of groundwater from wells, then the term ‘artesian aquifer’ is used.

Like aquifers, aquitards may also contain very significant volumes of groundwater, but their low hydraulic conductivity does not allow substantial quantities of groundwater to be abstracted using conventional wells. Aquitards may provide shortcuts for groundwater flow from one aquifer to another (in a vertically stacked position); those of large areal extent may transmit considerable quantities of water to or from the aquifers they cover, markedly influencing regional groundwater flow.