Groundwater in Namibia - an Explanation to the ... - BGR

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32 This chapter describes some of the general features of the hydro - geological environments in which ground water occurs. This should help the reader to understand the chapters that provide a more detailed account of the geology related to water bearing formations, the occurrence of groundwater, and the water supply potential of the aquifers. The occurrence of groundwater is closely associated with the rock formations making up the crust of the earth. When these three-dimensional bodies of rock contain underground water, they are called “aquifers”. These aquifers have unique properties dependent on internal and external factors controlling their size, capacity, the groundwater flow regime, the quality of the groundwater and their long-term sustainable safe yield potential. A groundwater flow system with its input (recharge) and output (discharge) Groundwater potential The potential of an aquifer to yield a certain quantity of water with a certain chemical quality at a certain rate depends on its size, the volume of water that can be stored, (called the storage capacity) the chemical composition of the rocks that the water comes into contact with, the volume of water moving through the aquifer system per time unit (called the flux), the water available to replenish or recharge the aquifer and the water flowing out of the s ystem (called the discharge). The recharge is normally from rainfall and runoff seeping into the aquifer while the discharge can be natural or man-made. Natural discharge takes place at springs or seeps. Man-made discharge is caused by collecting water from wells that have been dug by hand or by pumping the water out through boreholes drilled deep into an aquifer. The nature of the rocks underground determines if they are water bearing, the quality of the accumulated water, and how much groundwater there is. The storage capacity of an aquifer in a rock formation is determined by the percentage of open spaces in the rock that can collect water in comparison to the total volume of the solid rock. This is called the porosity of the aquifer. In sand, the porosity may be as high as 20 % because the number of spaces between the sand grains are high. The unconsolidated or poorly consolidated sand and gravel layers in sedimentary rocks generally form excellent aquifers, but the stored volume of groundwater depends on the thickness of the rock saturated with water. The quantity of groundwater stored in fractured aquifers is usually much less, because the space is confined to cracks, fissures and fractures in an otherwise solid mass of rock. An important variation in fractured hard rock aquifers are the carbonaceous rocks in which the fractures have been enlarged by the chemical solution of the rock in the water percolating through the aquifer system. These aquifers are called karstified aquifers and the aquifers in the Grootfontein- Tsumeb-Otavi Mountain Land are typical examples. The rate at which water can flow through an aquifer is called the permeability of the aquifer. In some deposits such as clay, silt and fine sand the porosity may be high, but the permeability is low because the capillary forces hold the water confined in the rock mass. Aquifers with low perm - eability are called aquitards or aquicludes. In such cases it is difficult or impossible to abstract the groundwater economically because a large number of large dia meter boreholes must be drilled and installed to obtain enough
The principal aquifer formations of Namibia and their representation on the Hydrogeological Map water to meet the required demand. Certain favourable geological features enhance the use of aquifers with low perm eability. When fault zones or valleys following zones of crustal weakness cut through aquifers with low permeability, the water collects over a large area and can be abstracted at an exceptionally high rate by boreholes drilled into these favourable water collecting structures. As shown in the map above, the following types of aquifers are widespread in Namibia: • Porous aquifers Source: HYMNAM Project © GSN, DWA • Fractured aquifers • Aquitards or aquicludes The rate at which groundwater can be abstracted from an aquifer is determined by the porosity and the permeability. The quantity of groundwater that can be abstracted over time is determined by the rate at which the water can be abstracted and the volume of water that is available for abstraction is determined by the storage capacity of the aquifer. The long-term sustainable safe yield from an aquifer also depends on the recharge of the water abstracted or discharged from an aquifer. From this it is clear that a number of hydrogeological 33
Page 1: Groundwater in Namibia an explanati
Page 4 and 5: 2 Federal Institute for Geosciences
Page 6 and 7: 4 Acknowledgements The publication
Page 8 and 9: 6 Executive Summary Water is life,
Page 10 and 11: 8 Introduction 3.0
Page 12 and 13: 10 In arid areas, where rainfall is
Page 14 and 15: 12 ited to the perennial rivers on
Page 16 and 17: 14 Natural& Socio-Economic Environm
Page 18 and 19: 16 Geology Namibia’s varied geolo
Page 20 and 21: 18 mouth, where dunes are locally d
Page 22 and 23: 20 Calcrete The deposition of calcr
Page 24 and 25: 22 The landscape of Namibia Geomorp
Page 26 and 27: 24 an extremely high evaporation ra
Page 28 and 29: 26 rial. The saprolite material can
Page 30 and 31: 28 To a lesser extent, cash crops a
Page 32 and 33: 30 Essentials of Groundwater 5.0
Page 36 and 37: 34 factors must be considered when
Page 38 and 39: 36 creates surface runoff, and only
Page 40 and 41: 38 Source: C Wessels, NamWater Corr
Page 42 and 43: 40 Groundwater and waste disposal W
Page 44 and 45: 42 Hydrogeological Framework
Page 46 and 47: 44 Groundwater basins and their hyd
Page 48 and 49: 46 Lacustrine deposits are found in
Page 50 and 51: 48 north-eastern Namibia. The south
Page 52 and 53: 50 The eleven schemes still operati
Page 54 and 55: 52 of this occur in the Kamanjab In
Page 56 and 57: 54 “Omifima” tapping the Discon
Page 58 and 59: 56 boreholes penetrating the VDAOL
Page 60 and 61: 58 Utilisation of ground - water Si
Page 62 and 63: 60 Otavi Mountain Land The hydrogeo
Page 64 and 65: 62 Source: HYMNAM Project D Plöthn
Page 66 and 67: 64 The numerical groundwater models
Page 68 and 69: major lower aquifer with transmissi
Page 70 and 71: 68 eighth of the total spring disch
Page 72 and 73: 70 The Eastern National Water Carri
Page 74 and 75: 72 Northern Namib and Kaokoveld The
Page 76 and 77: 74 for the schools at Anker and Erw
Page 78 and 79: 76 were subsequently folded and met
Page 80 and 81: 78 Damara Sequence. This sequence u
Page 82 and 83: 80 The success of water demand mana
Page 84 and 85:
82 a comparatively good yield poten
Page 86 and 87:
84 of the barrier on 10-20m thick a
Page 88 and 89:
86 Hochfeld-Dordabis-Gobabis Area T
Page 90 and 91:
88 Conellan and a portion of the fa
Page 92 and 93:
90 dune fields cut off the Molopo f
Page 94 and 95:
92 water of a good quality. Very fi
Page 96 and 97:
94 Source: TS Kock Drillsite placed
Page 98 and 99:
96 Southern Namib and Naukluft The
Page 100 and 101:
98 Namib sand sea supplies groundwa
Page 102 and 103:
100 Karas Basement The greater port
Page 104 and 105:
102 Paleoproterozoic rocks at the O
Page 106 and 107:
104 The Groundwater Map
Page 108 and 109:
106 Data and use of the Map This ch
Page 110 and 111:
108 tion of the thematic data layer
Page 112 and 113:
110 ern Africa”, at a scale of 1:
Page 114 and 115:
112 Bibliography Anhaeusser CR & Ma
Page 116 and 117:
114 Facts and problems. Palaeoecol.
Page 118 and 119:
116 Annex 1: Groundwater supply sch
Page 120 and 121:
118 Annex 2: Selected representativ
Page 122 and 123:
120 Annex 3: Irrigation schemes usi
Page 124 and 125:
122 Annex 4: Comparison of selected
Page 126 and 127:
124 Glossary a - Annum (lat.), year
Page 128 and 129:
126 Group A - No risk, excellent wa
Page 130:
128 a product of putrefaction. Ofte
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