Groundwater
Author: Christine Colvin ~ CSIR
( Article Type: Explanation )
Aquifers are permeable rock strata that store and transmit water underground.
This groundwater transmission occurs in the saturated zone of aquifers beneath the water table. Permeable rocks and sediments include unconsolidated sands (e.g. the Kalahari, along the coast and dry riverbeds), fracture zones in hard rocks and dissolved fissures in dolomites (North West province). Groundwater is recharged by rainwater that infiltrates the soil and overlying unsaturated zone. In semi-arid areas, approximately 3–5% of rainfall percolates through the unsaturated zone to the water table to add to the groundwater store. Ancient aquifers that were previously recharged during different climatic regimes thousands of years ago contain fossil groundwater. The Nubian sandstone in North Africa is an example.
Typically, over 90% of the liquid, available water stored in a catchment at any one time is stored in aquifers. Aquifers act as nature’s reservoirs, slowly releasing water back to the surface as baseflow to rivers or wetlands, or supplying the deeper tap roots of trees and shrubs, or discharging to the sea. Ecosystems linked to these discharge zones are often dependent on aquifers and rely on the extra water supplied by them all year. People have depended on groundwater since civilization began and many cultures were built around hand constructed wells.
Groundwater is generally a more reliable supply than surface water because of the larger stored volumes; it is also a cleaner supply as it is protected from pollution at the surface; and, in semiarid areas, more widely accessible than the limited extent of rivers. People access groundwater by shallow wells or deeper drilled boreholes.
Groundwater is threatened by over-exploitation, when more is abstracted than is recharged – particularly by farmers as most groundwater (about 80%) is used by farmers for irrigation. Aquifers are also threatened by pollution, especially from underground petrol storage tanks, septic tanks, mines and agri-chemicals. Aquifers represent a strategic source of water in many areas that require integrated pollution control and integrated water resource management to protect.
About 4.8% square kilometres of groundwater is produced per year, of which an estimated three square kilometres is, in turn, drained by rivers. Although groundwater is limited due to the geology of the country and large porous aquifers occur only in a few areas, it is extensively utilised in the rural and more arid areas. Available yield from these resources are estimated at one square kilometre per year. It is foreseen that groundwater use for human consumption will increase, especially in the western part of the country, which lacks perennial rivers.
