Kalahari: Hydrogeology and Sedimentology of the Cuvelai-Etosha Basin

Country / Region: Namibia + Angola / Africa

Begin of project: August 1, 2009

End of project: December 31, 2015

Status of project: December 31, 2015

Fig. 1: Location of the Cuvelai-Etosha Basin Source: BGR

Background:
The research project "Kalahari Sediments" aims to investigate the paleoclimatological influence of the genesis of sediments that were deposited in the Cuvelai-Etosha Basin (CEB), a large endorheic hydrological catchment located in Angola and Namibia. An interdisciplinary team of geoscientists of the Federal Institute for Geosciences and Natural Resources (BGR) and its Namibian counterpart work together within this project. Paleontologists, geophysicists, sedimentologists and hydrogeologists are involved to improve the geological understanding of the CEB.

Forming the north-eastern part of the CEB, the so called Cubango Megafan is an ancient delta of the Okavango River that now dewaters into the Okavango Delta 700 km further east. It has both a unique sedimentological and hydrogeological setting, closely related to each other.

In the frame of a technical cooperation project “Groundwater for the North of Namibia“ executed together with the Namibian Department of Water Affairs and Forestry (DWAF), boreholes and cored boreholes were drilled to explore a new groundwater resource. With the courtesy of DWAF, samples are being investigated by the Federal Institute for Geosciences and Natural Resources (BGR) funded research project.

Fig. 2: Position of exploration drill holes within the Cubango Megafan Source: BGR

Quantification of groundwater resources:
The current water supply is based on a canal and pipeline system that pumps water from the Angolan Calueque Dam of the Kunene River towards the densely populated North of Namibia. A rather large portion of the basin is supplied by sparsely distributed groundwater wells that tap low yielding, medium deep aquifers. High fluoride and salt contents pose a significant problem for using these aquifers. To overcome water quality problems and potential water shortages in periods of draught that might last several years, an additional groundwater source of high yield is of great value. For a sustainable use, the exploration of the deeply seated groundwater needs to provide quantifiable data. To better understand the distribution and quality of the groundwater resource, more detailed understanding of the whole sedimentary system of the Kalahari Sediments is required. The research is aimed at understanding the development of the thick blocking layer that separates saline groundwater located on top of the fresh groundwater. Interestingly, the confining layers are not clays but rather sands which pores are filled with recently formed smectites and smectite gels (so called authigenic minerals). Smectites are a type of clay minerals that swell under certain conditions. The sediments tell a story of a sub-aerial delta system very much like contemporary Okavango Delta. Microfossils and root channels show that lakes, mudflats and delta systems existed throughout the Cenozoic. Heavy mineral analyses give clues of the origin of the deposited sediments.

Fig. 5: 3D model of sediments of the Cubango Megafan Source: BGR

3D model of the underground:
The results of the research are used to define sediment layers of similar conditions, so called lithofacies units. These can be used together with digitized maps, cross sections and results of geoelectric investigations (Transient Electro Magnetic Sounding: TEM) as well as borehole geophysical logs to develop a three-dimensional understanding of the subsurface. Lithofacies units that bear freshwater layers are displayed in such a model; here the software GSI3D was used, to help the experts, decision makers as well as public users to understand the complex groundwater systems.

Literature:

• DILL, H.G., KAUFHOLD, S., LINDENMAIER, F., DOHRMANN, R., LUDWIG, R. & BOTZ, R. (2012): Joint clay–heavy–light mineral analysis: a tool to investigate the hydrographic–hydraulic regime of Late Cenozoic deltaic inland fans under changing climatic conditions (Cuvelai-Etosha Basin, Namibia). International Journal of Earth Sciences 102(1): 1-40; doi: 10.1007/s00531-012-0770-7
• LINDENMAIER, F., DILL, H.G., DOHRMANN, R., FENNER, J., GERSDORF, U., KAUFHOLD, S., KRINGEL, R., LUDWIG, R., MILLER, R.G., NICK, A., NOELL, U. & WALZER, A. (2012): Results of Analysis from Drill Holes on the Cubango Megafan. - Technical report prepared for the Kalahari Research Project "Development and Dating of Duricrusts as Base for a Paleogeographic and Hydrogeological Model of the Cuvelai-Etosha Basin in North Namibia". 50 p.; Hannover. (PDF, 2 MB)

Partner:

• Division of Water Affairs, Ministry of Agriculture, Water and Forestry, Namibia