Groundwater – Sustaining life in arid Namibia

Large expanses of Namibia have no surface water sources and an estimated 80% of the surface area of the country depends entirely on groundwater.

The occurrence, sustainability and protection of this underground resource is challenging as there are no direct ways to “observe” groundwater.

The science of hydrogeology applies geology, physics, and chemistry to understand the occurrence and flow of groundwater, and to manage and protect the resources.


Groundwater in Namibia

The availability of groundwater and its long term sustainable use depends largely on the rate of groundwater replenishment or recharge rates. Groundwater originates from rain, a small part of which infiltrates deep underground into aquifers (a body of porous rock that can store and transmit water, en.wikipedia.org). Groundwater is therefore part of the water or hydrological cycle.

The hydrological cycle begins with evaporation from oceans and continents followed by condensation and precipitation. In the desert environment such as in Namibia, precipitation is highly variable being dependent on complex climatological factors. The amount of water that infiltrates depends on many aspects including the nature of soil or exposed rock, ground slope, drainage, and vegetation cover. Water is mainly lost through runoff, evaporation and transpiration. Evaporation and transpiration returns water to the atmosphere.

Transpiration occurs when vegetation takes up water through their roots that is released to the atmosphere through the leaves. The amount of rain infiltrating deep to form groundwater could be as little as 1% or less of the total rainfall. Rainfall has to exceed losses that limit its downward flow (a threshold value) before infiltration occurs. This condition is met only in a few years in a decade or decades depending on the location.

The nature of recharge in Namibia is therefore episodic rather than annual as in humid regular rainfall areas. On average, recharge is progressively lower towards the west and south of the country as mean rainfall is less. Groundwater, like surface water, flows downstream (from higher to lower elevation and pressure heads), from recharge areas to discharge areas which could be local or regional in scale. In a regional scale, groundwater discharge is either to the sea or to inland depressions like the Okavango swamps or Etosha Pan.

The water from the inland discharge areas mostly evaporates. Groundwater flows slowly, a rule of thumbs says that in unconsolidated sands and gravel, velocity is in the order of meter per day while in hard rock it may be in the order of meter per annum. Using radiocarbon dating techniques of dissolved carbon dioxide in groundwater it was estimated that it takes 20 000 years for groundwater in the Windhoek aquifer to travel from the Auas Mountains to the centre of Windhoek City.

Deep circulation while flowing underground along the Pahl fault to depths of two to three kilometres causes the water to heat up to 80°C. In nature two types of aquifers exist: porous rock aquifers where the water is stored and moves in pore spaces between grains (such as in sand or sandstone); or fractured rock aquifers where water flows along a system of fractures or faults in an otherwise impermeable rock such as granite. Solution cavities along fractures and joints in limestone or dolomite are considered another type of fractured rock aquifer called karst.

The Hydrogeological Map of Namibia (van Wyk, Strub et al. 2001) indicates fractured aquifers in brown shades (higher potential in lighter brown) and porous aquifers in blue shades (high potential in darker blue). Groundwater storage properties of porous rocks are much superior and therefore better aquifers while the large areas under fractured rock aquifers have variable groundwater occurrence and sustainability.

Water quality

With long residence time groundwater quality changes with dissolution of minerals in the rocks, ion exchange with aquifer material and concentration of salts through evapotranspiration. Highly mineralised groundwater is encountered close to discharge areas, for example close to the Etosha Pan.

In addition, pollution from natural or manmade contaminants can drastically alter groundwater chemistry such as high nitrate concentrations in boreholes close to kraals that have been dissolved from farm animal waste by infiltrating water. The Water Quality Map (Huyser 1982) shows the total dissolved solids distribution in Namibia from low (yellow) to high (purple). Water with high total dissolved solids or salinity and contaminants is unsuitable for consumption.

Managing groundwater resources

Groundwater potential is evaluated from data collected during drilling and pump testing of boreholes that helps to define the extent of the aquifer and its water transmitting and storage properties. Borehole siting or locating favourable drilling sites is only an initial part of the groundwater investigation. For larger water supply schemes the data from several boreholes in the aquifer is incorporated into a conceptual model and substantiated with information collected during production pumping (e.g. water level, abstraction and water quality).

With enough data, computer based numerical flow models are constructed that help to manage aquifers and optimise water abstraction. Isolated small scale supply boreholes such as in a farm have little information other than that collected during drilling and pumping of the borehole itself. But, provision for collection of drilling and testing data exists mainly through submission of such data to databases maintained at the Geohydrology Division at the Department of Water Affairs and Forestry and aids groundwater exploration and management.

Unfortunately, not all data from drilling programs in the country are submitted to the Division. Understanding of groundwater resource potential in Namibia requires continued research as demand on groundwater resources grows. Inadequate management results in overexploitation and pollution of aquifers as general level of understanding of groundwater systems is low. Depending largely on the replenishment rate, shape and the permeability of aquifers in arid regions the quantity that can be abstracted is much smaller – often less than 10% of the stored volume.

Impact of pumping on the environment (e.g. water bodies and dependent vegetation) has to be taken into consideration. The country is dependent on groundwater for supply of potable water at low cost, particularly in the rural areas. The groundwater resources belong to the nation and it is in everybody’s interest to protect it and utilise it sustainably.


References

  1. Huyser, D. J. (1982). Water quality map 1 : 1 000 000 (TDS, SO4, F, NO3). Pretoria, National Institute for Water Research/Council for Scientific and Industrial Research.
  2. Mendelsohn, J.M., Jarvis, A.M., Roberts, C.S. & Robertson, T. (2002). Atlas of Namibia
  3. van Wyk, A., H. Strub, et al. (2001). Hydrogeological Map of Namibia 1 : 1 000 000. Windhoek, BGR, DWA, GSN, NamWater.

By Jürgen Kirchner & Diganta Sarma

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