Groundwater

Groundwater is found in sub-surface geological formations called aquifers. In southern Africa groundwater is a critical resource, due to the limited availability and variable quality of surface water resources (UNEP 2009). Groundwater accounts for approximately 30% of all freshwater on Earth and 94% of all available freshwater (freshwater not stored in glaciers and ice-sheets) (Ward and Robinson 2000). It is of major importance as a drinking water source due to its quality general being better than available surface water reserves. The table below illustrates the contribution of groundwater to freshwater systems and the hydrologic cycle.

Contribution of Groundwater to Freshwater Systems and the Hydrologic Cycle

As increased water demand has stressed or even exhausted surface water resources, national and regional authorities have increasingly turned to groundwater, a resource largely ignored and misunderstood in the past. Increased demand for water has lead to greater exploitation of groundwater, with aquifers often being explotied at an unsustainable rate.

Aquifers are usually defined as layers of hard rock or unconsolidated deposits that can hold significant amounts of water (Ward and Robinson 2000). An aquifer occurring in unconsolidated deposits is referred to as a primary aquifer (or having primary porosity), meaning that water is held in spaces between granules in the rock. A secondary aquifer is one occurring in hard rock, where the water is not absorbed into the rock, but held in spaces between bodies of impermeable stone (Spitz and Moreno 1996). These spaces are generally fractures, fissures or faults  created when the rock was formed or later by geological or tectonic processes.

The level of water in a body of rock is known as the water table. In a primary aquifer, this is a physical level in the rock, up to which water is present. In a secondary aquifer, the water table is an abstract representation of the quantity of water held in the spaces between the rocks.

The diagram below illustrates the role of groundwater in the hydrologic cycle.

The role of groundwater in the hydrologic cycle.

Source: CSIR 2004.

( click to enlarge )

In a confined aquifer, the upper boundary of the aquifer meets with a body of impermeable, unfractured rock that blocks both flow of water into the aquifer from above (recharge) and extraction of water from above. Confined aquifers must be recharged by groundwater flowing in laterally from an adjacent recharge zone. An unconfined aquifer is not restricted by an impermeable layer; it can be recharged by water percolating down from the soil and if it is under pressure, water can seep up to the surface, unrestricted.

Groundwater is most often a renewable resource; replenished or recharged when rainfall is absorbed through the soil and percolates down to add to the water already in an aquifer. Groundwater recharge occurs by:

• Infiltration of precipitation through the ground
• Lake and river water seepage through the beds and the banks
• Leakage from adjacent groundwater sources
• Artificial recharge from irrigation, pipeline leakage, direct injection, etc.

Groundwater discharge occurs by:

• Evaporation from a high water table near the surface
• Seepage into surface water bodies or spring flow
• Seepage into adjacent groundwater sources
• Groundwater abstraction

For groundwater to remain available for supply, it has to be continually replenished by recharge from surface water flows. These sources usually include rainfall and streamflow. The recharge process and rate depend on the nature of the overlying materia and on the nature of the aquifer itself as this affects its ability to receive and store groundwater and controls its movement within the aquifer. For instance, from fractured aquifers, rainfall can be a source of significant recharge that eventually re-emerges as base flow into streams during dry seasons.

However, if groundwater extraction (sometimes called abstraction) is higher than the rate of recharge, the resource will be depleted and can be exhausted. In some cases, unsustainable groundwater management can lead to the collapse of an aquifer, making it difficult to extract water.

Groundwater is normally exploited using wells or boreholes (Rebouças 2004). These wells are drilled through the soil layer, down into the rock until they meet the water table. Boreholes are usually drilled using a truck-mounted drilling rig. Groundwater can sometimes be held under pressure by a confining layer so that when a drill penetrates the aquifer, water is released under pressure. This is known as an artesian aquifer or well.

Proper location of boreholes (siting) depends on the geology of an area. A borehole target is usually chosen in order to intersect with a primary aquifer or to intersect a large fissure or fracture in the rock, allowing for water to be pumped out or to be released under pressure. Boreholes are sited using geological maps, aerial photography and satellite images which, combined with an understanding of the type of rock and the way it was formed, allows a hydrogeologist to estimate where water may best be found. In southern Africa, hydrogeologists identify zones of high probability of reaching groundwater using geological fractures and faults. In addition, knowledge of geomorphology and an understanding of the local and regional groundwater environment is used to identify potential drill locations (Hughes 2005). In drier environments, the presence of vegetation can also indicate relatively shallow groundwater availability.

SADC has recently produced a Hydrogeological Map showing the major aquifers of southern Africa.