This section introduces some of the concepts that regularly feature in the technical vocabulary of hydrogeologists and which, if properly understood, will support a critical appreciation of the implications of large-scale groundwater abstraction.

In most areas of the world, over 90% of water stored in a catchment at any one time is found underground in aquifers. Groundwater could therefore represent a huge strategic water source and plays a vital role in the water cycle.

Groundwater is water that occupies pores, cavities, cracks and other spaces in the hard, outer rocks, weathered rocks, and soil that form the Earth's surface.

An aquifer is an underground geological formation capable of yielding sustainable amounts of water. Generally speaking, it is a layer of earth or rock that will absorb water and allow it to pass freely through. The term also refers to any layer of earth or gravel that is saturated with water and sufficiently porous and permeable to yield supplies of groundwater in the form of wells or springs. Simply put, an aquifer is rock or earth that holds and "leaks" water. An aquifer has a layer of impermeable material underneath which will prevent water from moving downwards. If the aquifer is bounded on its upper surface by an impermeable layer, it is termed a confined aquifer. It is as though the groundwater has a "lid" over it, through which it cannot directly pass. This "lid" that effectively seals the confined groundwater is known as an aquitard.

Groundwater captured in a confined aquifer is generally subject to pressure higher than that of the atmosphere. Artesian basins are examples of confined aquifers in which water would rise to the surface if a borehole were drilled through the confining layer. In an unconfined aquifer the upper level of the aquifer forms the water table or the upper level of groundwater saturation, i.e. there is no confining layer maintaining a pressure head in the aquifer.

Recharge refers to the total amount of water that is added to an aquifer by means of rain, snow or from wetlands or rivers. It is crucial to know the amount of recharge to an aquifer because this is the amount of water that is being replenished annually. Discharge refers to the amount of groundwater that leaves the aquifer system. Fracture flow describes groundwater movement through a variety of secondary structures in rock, i.e. joints, cracks, fractures and faults.

The two best-known aquifer types in the south-western parts of the country are porous (primary) aquifers and fracture (secondary) aquifers.

Structure of Porous and Fractured aquifers

Examples of porous or primary aquifers - in which water is contained in the spaces between the surrounding, loosely packed material - can be found in the coastal sands, gravels and other unconsolidated material along the coast, and sands and gravels along streambeds. Fracture or secondary aquifers refer to conditions where groundwater moves through a variety of joints, cracks, fractures and faults. The degree of fracturing of rocks in South Africa is a function of the tectonic history of the rocks, as well as the rock composition. Brittle rocks, such as the hard quartz sandstones of the TMG, fracture more readily than more pliable rocks such as dolomite and shale. Fracture aquifers can be either confined or unconfined structures.