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TC Namibia: Groundwater Investigations in Oshivelo, Eastern Caprivi and Omaheke

Report of the project:

Outline map of Namibia with position of investigation areasOutline map of Namibia Source: BGR

Background:
Covering a land surface of approx. 825 000 km², Namibia is 2.5 times bigger than Germany and lies in one of the driest regions of southern Africa. With only 2 million inhabitants the country is one of the least populated of the African continent. Half of Namibia is considered arid and because of scarce precipitation falls unpredictable in time and space, periods of water shortage are commonplace. By request of the Namibian government, a German-Namibian cooperation project was carried out between 2002 and 2005 in the northeast of Namibia. The Department of Water Affairs (DWA) within the Ministry of Agriculture, Water and Forestry (MAWF) of Namibia and the German Federal Institute for Geosciences and Natural Resources (BGR) were partners in the exploration project.

The main objective of the project was the investigation of groundwater resources in three areas in the north and northeast of Namibia (Figure 1), with a high water demand:

Helicopter D-HBGR with bird starting for groundwater exploration in Northeast-NamibiaBGR-Helicopter with bird Source: BGR

Investigation Concept:
Already in 1999 a desk study had been conducted by an external consultant to improve the level of knowledge about the proposed investigation areas (BIWAC, 1999). The final definition of areas of activity strongly based upon an analysis of the regional tectonics and geologic structures using satellite images. Helicopter geophysical measurements were conducted in the investigation areas Oshivelo and Eastern Caprivi, whereas ground-geophysical measurements were done in all three investigation areas. While the airborne measurements resulted in densely spaced detailed information down to depths of approx. 80 m, the ground based survey contributed with resistivity values at selected single points down to a depth of approx. 400 m. For the field work of the ground based resistivity soundings a private company was engaged. The airborne measurements were carried out from October 2002 until March 2003 by the BGR-working group for aerogeophysics.

The survey results and their interpretation were controlled and confirmed with test drillings, carried out from April to November 2004. Pump tests revealed information concerning the productivity of the explored groundwater resources and with hydrochemical and isotope analyses of the water its quality, origin and age was examined. Finally the information was combined in a so-called hydrogeological conceptual model. These concepts about genesis and hydraulics of the water occurrences present the base of a future responsible and sustainable groundwater resources management.

Results:
Investigation Area 1 – Oshivelo:
Oshivelo is located east of the Etosha Pan, an endorheic depression at the southern margin of the Cuvelai Basin (Figure 2). The Otavi Mountains, located to the south of the project area, are considered the most important groundwater recharge area in northern Namibia. Since it is assumed that a large share of this groundwater recharge flows towards the Etosha pan, the project had to evaluate where the conditions for exploitation of the aquifer system are optimal. The main investigation target was the Oshivelo Artesian Aquifer (KOV2). If sufficient amounts of fresh groundwater would be available, even a transfer to the north of Namibia (Owamboland), where water demand is strong, might be possible. There, water supply currently relies on surface water conveyed from the Kunene River, which constitutes the border line with Angola in northwestern Namibia. The Namibian Government, however, intends to become more independent of this resource, since it constitutes an internationally shared resource and water supply may not be reliable in a situation of political crisis.

Bird's eye view of the Otavi MountainsBird's eye view of the Otavi Mountains Source: BGR

During previouis investigations, conducted since the mid 1980s, the KOV2 aquifer had been discovered as a freshwater bearing aquifer with a high yield in the surrounding of the village of Oshivelo. However, the extent of this aquifer was only insufficiently known (BIWAC, 1999). Moreover, its exploitation potential could until now not be determined with sufficient certainty due to the weak information basis. The geophysical measurements performed by the project had led to the assumption that the aquifer might extend much farther to the east than was initially thought. Based on the interpretation of these measurements (FIELITZ et al., 2004; SIEMON et al., 2005a) exploration boreholes were sited.

However, the above-mentioned hypothesis could not be confirmed by the exploration boreholes. The main reason for this failure is the complicated geological structure in the area, which is located in the transition zone between different depositional environments: aeolian (evidenced by the longitudinal dunes in the northern part, in the Kalahari sequence), fluvial (near the ephemeral creek Oruamba Owambo) and chemical (calcretes, in the southern part). The interfingering of calcretes with silty-sandy formations has led to a misinterpretation of the geophysical measurements.

According to the drilling results the extent of the KOV2 aquifer reaches only to about 25 km east of the village of Oshivelo. High transmissivities have only been found in a narrow stretch, indicating that the KOV2 aquifer receives its inflow in this area from deeper layers in the groundwater system (Figure 3). The distribution of piezometric heads (Figure 4) leads to the assumption that the KOV2 aquifer is fed by the Otavi Dolomite Aquifer (DO). This aquifer receives its recharge in the Otavi Mountains, located to the south of the project area, where this unit is at outcrop. Presumably, this more than 1000 m thick aquifer unit is folded and rises near to the surface in an anticline near the creek Omuramba Owambo, so that it may be in hydraulic contact with the KOV2 aquifer (Figure 5).

The Karoo Sandstone Aquifer (KR), which has been encountered underneath the KOV2 aquifer in the project area contains saltwater north of the Omuramba Owambo, whereas it is of drinking water quality south of it. Since there is most likely a hydraulic connection between the KOV2 and the KR, the KOV2 aquifer may be affected by saltwater intrusion, if developed. Optimal conditions for the development of the KOV2 aquifer are probably given in the area south of the Omuramba Owambo. It was recommended to evaluate the chances for a direct development of the Otavi Dolomite Aquifer (DO) by conducting seismic profiles and drilling a deep exploration borehole targeting the structural high position of the DO. The use of groundwater resources for domestic water supply is constrained by elevated fluoride contents, especially in the northern part, so that treatment is often required (Figure 6). The exploitation potential of the KOV2 aquifer is around 5 million m³ per year (MCM/a). Due to the high risk of saltwater intrusion, it is recommended presently not to exceed an abstraction of 2.5 MCM/a. The basis for the estimation of the exploitation potential is still rather weak and should be improved before starting to develop this aquifer.

Investigation Area 2 – Eastern Caprivi:
This investigation area is located in the extreme northeast of Namibia, bounded by the Kwando and Zambezi Rivers to the west and east (Figure 7) and has approx. 100,000 inhabitants. Water supply currently is difficult in the central and southern part of this area because only brackish or saltwater has been encountered so far in boreholes drilled in this area. The aim of the investigation was to find out whether it might be possible to exploit groundwater resources of sufficient quantity and acceptable quality at greater depths.

Evaporation pans in Eastern-CapriviEvaporation pans in Eastern-Caprivi Source: BGR

The interpretation of the satellite image and the digital elevation model (based on SRTM data) reveals that the Eastern Caprivi area constitutes a geological graben, named Caprivi Graben (Figure 8a Figure 8b). The location of the geophysical survey areas and boreholes is shown in Figure 9. The upper part of the aquifer system contains brackish or saltwater in large parts of the Eastern Caprivi. Therefore water supply in the east and along the Kongola-Katima Mulilo road (Golden Highway) is based on surface water from the Zambezi and Kwando Rivers, which is distributed by a pipeline network (Figure 10).

The helicopter electromagnetic investigations had confirmed that there were no chances in the southern part of the Eastern Caprivi to exploit fresh groundwater resources in the upper part of the aquifer system. Therefore transient electromagnetic (TEM) measurements were conducted in order to determine the hydrogeological setup of the deeper part of the aquifer system. The interpretation of these measurements led to the assumption that fresh groundwater resources may be found at greater depths in the northern and central part of the Eastern Caprivi. Six boreholes were drilled here and confirmed this hypothesis. They all showed a similar geological sequence (Figure 11): under a 100 to 115 m thick upper aquifer (Upper Kalahari Aquifer), which consists mainly of fine to medium grained sand, a 15 to 25 m thick clay aquitard was found (Upper Kalahari Aquitard); underneath, a 60 to > 125 m thick lower aquifer (Lower Kalahari Aquifer) was encountered, also chiefly composed of fine to medium grained sand; under the Lower Kalahari Aquifer Karoo basalt was found in three of the four deep boreholes. For the first time it could be proven that Karoo basalt probably occurs in large parts of the Eastern Caprivi.

In the southwest of the project area, however, basalt may not be present or may be found at much greater depth, so that a hydraulic window may exist here between the Lower Kalahari Aquifer and the Karoo Sandstone Aquifer, which is assumed to underlie the basalt (Figure 12). At two borehole sites the hydraulic head of the lower aquifer is 14 m higher compared to that of the upper aquifer, i.e. there is an upward hydraulic gradient. The groundwater flow pattern in the lower aquifer is similar to that in the upper aquifer (Figure 13), i.e. flow comes from the Kwando River and from the north. Comparing the streamflow at the gaging stations Kongola and Lianschulu, the Kwando River loses about 175 MCM/a. This fact and the distribution of δ18O and δD values (Figure 14) indicate that considerable amounts of surface water from the Kwando must infiltrate by river bank infiltration into the Upper Kalahari Aquifer and possibly the Lower Kalahari Aquifer. Such infiltration of surface water must also occur along the Zambezi River in the vicinity of the city Katima Mulilo. By means of a deep borehole near Kongola it will be verified whether a hydraulic connection exists between Upper and Lower Kalahari Aquifer. In the central part of the Eastern Caprivi groundwater recharge in the upper aquifer is negligible. This is indicated by the high mean residence times (14C) of the groundwater and the tritium contents. The exploitation potential of the Lower Kalahari Aquifer can presently not be determined.

Investigation Area 3 – Omaheke:
In Omaheke in northeastern Namibia in the border region to Botswana a tectonic Graben, the so-called Eiseb-Graben was chosen for a detailed research because good chances for the development of fresh groundwater resources were expected to be given (Figure 15). This area is part of a national repatriation program for the Herero tribe, which the German side has promised to support.

The Eiseb-Graben is the westernmost extension of the Linyanti-Gomare Fault. Its existence has been assumed since the mid 1970s (SCHOLZ et al., 1976) but its horizontal and vertical dimensions have only now been determined by the project. With surface based resistivity soundings the area of the supposed graben was systematically examined. The measurements confirmed the interpretation results of the satellite pictures. The graben shoulder, already delineated with the satellite images, was accurately displayed by the distribution of the resistivities of the subsurface formations (Figure 16). The interpretation of these data point to the fact that the graben is bounded by rocks of high resistivity (gneiss) to the north and south (Figure 17) and that outside the graben area the chances of finding groundwater are slim. The yields of previous boreholes drilled in the graben area have only been relatively low, i.e. between 1 and 3 m³/h.

Presentation of the project outcomes to local authorities in GobabisPresentation of the project outcomes Source: BGR

In a first drilling campaign conducted by this project 4 boreholes were drilled. Two were targeted at the northern margin of the graben, one at the central part and another at the southern margin. It could be proven by the boreholes that at the northern margin a narrow part of the graben seems to be deeply downlifted (trench with a Kalahari sequence filling of supposedly more than 400 m). One borehole with a recommended yield of around 120 m³/h exceeds all expectations. This borehole encountered coarse, freshwater bearing horizons at greater depths. The borehole in the central part penetrated clay at greater depths, as was predicted by the geophysical measurements. The borehole at the southern margin could not confirm the assumption of another freshwater filled trench there. During a second TEM- and drilling campaign the position of the trench at the northern margin of the graben was confirmed. Packer tests, conducted in two boreholes, showed that inflow in the wells comes mainly from depths below 200-250 m. The exploitation potential in der Eiseb Graben is currently estimated at 0.7 MCM/a. The mechanism of groundwater recharge, however, is still insufficiently known.

The groundwater found in the Eiseb Graben can be used to improve the water supply of the local population whose life is mainly based on cattle farming. Because the groundwater recharge and the cattle carrying capacity of the Omaheke region are very low, a careful management of the aquifer is of utmost importance. The number of stock which now could be supplied with water exceeds by far the carrying capacity of the area; hence the danger of desertification by overgrazing is immense. A series of information road shows was accomplished in order to explain the complex interrelations and to raise awareness for the problem of the local authorities and the stakeholders.

Literature:


Contact:

    
Dr. Georg Houben
Phone: +49-(0)511-643-2373
homepage: www.bgr.bund.de/houben

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