The occurrence and location of groundwater in SWA

Abstract

Abstract provided by author:

(2) Because of the presence of excellent rock outcrops, geological methods for site location are predominantly used

Geophysical methods (electrical, magnetic, seismic and electromagnetic) have been used on a small scale where rock outcrops are not visible, and to determine basins of decomposition and thicknesses of river deposits

The electrical resistivity method is used successfully to determine potable or unpotable ground-water in the shales of the Dwyka and Stormberg Series of the Karroo System. Low apparent resistivities (less than 2500 ohm-cms) in these beds indicate the moisture-saturated, porous, but impervious shales (and therefore low yields to boreholes) of these Series, which should be avoided in ground-water location

(3) Regarding the occurrence of ground-water, the different-rock formations can be divided into (a) pervious rocks, including: (i) Calcareous sands and gravels of the Tertiary and Recent deposits (ii) Sandstones of the Nama System (iii) Sandstones of the Dwyka and Stormberg Series (b) Impervious rocks, including:- (i) Granites, gneisses, schists, amphibolites and lavas (ii) Dolomites, limestones and marbles (iii) Quartzites, slates, shales and clays

In the impermeable rocks, ground-water occurs and is located mainly - (i) In zones of jointing, fracturing and solution cavities (ii) Along contacts with intrusive rocks, like quartz-veins, dolerite and diabase dykes and sheets (iii) In pegmatite intrusions (iv) Along inclined bedding planes and contacts of dissimilar rocks (like gneiss and amphibolite)

The use of aerial photographs is often helpful in locating the above water-bearing structures, which are recognised by darker lines formed by a thicker concentration of plant growth. Certain types of vegetation are typical of certain rock formations, and these are described. Examples given of actual, site selection in the different pervious and impervious rocks, and the results obtained through drilling are tabulated in the Appendix

(4) Topography is a most important factor to be taken into account in the location of ground-water in the different rock formations

(5) In the granitic and carbonate rocks, the yields of boreholes decrease as the average annual rainfall decreases from the east to the west

In other rock formations the influence of rainfall variation is masked by surface cover, topographic changes and the presence or absence of geological structures (e. g. fractures in the mica schists). Yields therefore do not always decrease as the average annual rainfall decreases. Boreholes in low rainfall areas are, however, more likely to weaken with use than those in areas of higher rainfall. Maps indicating the distribution of the different geological formations (with isohyets), and the yields of boreholes in some of these formations are shown

However, all the water-bearing structures and strata in the different rock formations must be replenished by rain or storm-water, otherwise boreholes penetrating them become weak, or dry up completely

The sand-covered Epukiro region, east of Gobabis, is described, as a typical region where a Tertiary sand cover in excess of twenty feet thickness prevents infiltration to water-bearing strata underneath, with the consequent weakening and drying up of boreholes after a few years pumping

In the Recent deposits of the central coastal Namib, seasonal replenishment by flood-water from the higher rainfall inland plateau area is necessary to replenish the supply, and to lower the total dissolved solids in the ground-water, especially where rates of extraction are high

(6) From histograms drawn of those rock formations from which ground-water is extracted by means of boreholes and wells, the following become evident:- (a) A definite yield pattern can be associated with every one of the different rock formations. In the case of the three different age groups of the granitic rocks, the yields patterns are identical (b) The depth ranges of successful boreholes are different in the different geological formations (c) The depths at which water is struck in structures or strata, are different for the different geological formations

In only 1. 5 percent of successful boreholes in granites, gneisses, Dwyka shales and quartzites of the Nosib and Tsumis Formations is water struck in boreholes at depths exceeding 300 feet

In the case of the Stormberg lavas, mica schists, carbonate rocks of the Nama System and Stormberg sandstones and shales, the percentage boreholes in which ground-water is struck at depths exceeding 300 feet is respectively 3. 0, 4. 2, 7. 3, 16. 8 and 16. 8. (d) The water-level ranges in the successful boreholes are different in the different rock formations

In the Tsumis quartzites and shales south of Rehoboth, the mica schists of the Khomas Highland west and south of Windhoek, the Stormberg sandstones and shales north of Okahandja, the Tertiary deposits north of Gobabis (Epukiro) and north of Tsumeb, it is found that the water-level follows the surface topography. Sections, illustrating this are shown

(7) In those cases where comparisons can be made, geologists have higher percentage success than non-geologists

In the case of the Stormberg lavas, mica schists, carbonate rocks of the Nama System and Stormberg sandstones and shales, the percentage boreholes in which ground-water is struck at depths exceeding 300 feet is respectively 3. 0, 4. 2, 7. 3, 16. 8 and 16. 8. (d) The water-level ranges in the successful boreholes are different in the different rock formations