Exploration of deep groundwater systems in the Cuvelai-Etosha Basin (Western Kalahari Basin in Angola and Namibia)

Country / Region: Namibia + Angola / Southern Africa

Begin of project: January 1, 2018

End of project: December 31, 2022

Status of project: December 31, 2022

Project background and objectives:
Given the increasing demand for water linked to population growth and economic development, as well as the rising frequency of extreme events such as droughts and floods due to climate change, the exploration of groundwater aquifers in Southern Africa is of very high importance. The focus is on not yet fully developed freshwater resources that are found at greater depths. In most cases, little is known about recharge, spatial extent and flow directions of these aquifers. Their groundwater age can range from several thousand to more than one hundred thousand years, implying that these fossil or semi-fossil aquifers formed under paleo-climatic conditions and therefore are not or only partially renewable.

The international research project has two specific objectives:

• First, methods for the exploration of complex groundwater systems shall be advanced, including remote sensing and geophysical methods, conceptual and numerical hydrogeological models, geochemical modelling and groundwater dating using noble gas isotopes (helium-4, krypton-81). New developments are tested and applied at field scale in the Cuvelai-Etosha Basin (CEB).
• Secondly, groundwater potential for exploitation and risks associated with future abstraction are estimated in cooperation with partners from Angola and Namibia. The focus is on the deep-seated transboundary Ohangwena-2 aquifer (KOH-2) shared between Angola and Namibia.

The project is linked to the IAEA coordinated research project "Use of Long-lived Radionuclides for Dating Very Old Groundwater", which addresses the use of new tools for the estimation of very old groundwater ages (> 50,000 years) in various aquifers around the world. The cooperation particularly aims at evaluating the reliability of helium-4 and krypton-81 as age tracers for old groundwater.

Location of the study area Source: BGR

Study Area:
The endorheic CEB is situated between 15°30'S and 20°0'S and between 14°0'E and 18°0'E. It covers parts of southern Angola and northern Namibia and belongs to the Kalahari Basin, which stretches over parts of Angola, Zambia, Namibia, Botswana, South Africa, the DR Congo and Zimbabwe.

According to the climate classification by Köppen and Geiger the northern part of the Kalahari Basin, including the CEB, belongs to the hot arid steppe climate (BSh). It is characterised by average annual temperatures between 22 °C and 24 °C, high potential evaporation rates (> 2,500 mm/a) and predominately, a unimodal austral summer rainfall regime. The long-term mean annual precipitation is highest at the northern and southern boundaries of the basin (Angolan highlands and Otavi Mountainland, respectively) and decreases to 400 – 500 mm/a towards the basin centre which lies at 1,100 m asl. Limited precipitation in combination with extremely high rates of evapotranspiration results in low groundwater recharge.

Geology of the Cuvelai-Etosha Basin Source: BGR

The Kalahari Basin is part of the intracontinental African Plateau. The sedimentary Kalahari Sequence was deposited into a tectonically active basin since the beginning of the Upper Cretaceous and exceeds a thickness of 300 m in the study area.

The deep-seated KOH-2 aquifer has high potential for regional groundwater development and is therefore a focus of interest. It is a multi-layered aquifer which has been formed in a paleo-megafan of the Cubango River and has been identified at depths of 150 – 300 mbgl. The recharge zone is assumed to be in the Angolan Highlands, where the average annual precipitation of up to 900 mm/a is considerably higher than in the central areas of the CEB. The presumed flow direction is roughly southwards, from Angola to Namibia.

Gas-extraction device Source: BGR

Methods:
Successful exploration and development of deep groundwater aquifers requires a good understanding of the tectonic and sedimentological processes governing the system. We develop and implement a multi-disciplinary approach, that comprises the following methods:

• Detailed analysis of tectonic structures and prevailing sedimentation processes
• Geophysical exploration, particularly electro-magnetic methods
• Assessment of groundwater quality and geochemical modelling
• Investigation of paleoclimate conditions during groundwater recharge using stable isotopes (as climate proxy) and noble gases (noble gas thermometry)
• Groundwater age-dating using helium-4, carbon-14, krypton-81 and chlorine-36
• Development of conceptual and numerical groundwater models

Gas composition of groundwater was sampled using a newly developed gas-extraction device. The instrument consist of a hydrophobic semi-permeable membrane, a membrane-vacuum pump, a compressor and a memograph which records gas pressure and water discharge.

Results:
In March 2019 an extensive sampling campaign was conducted which comprised 8 deep boreholes tapping the KOH-2 aquifer. The following samples were collected:

Groundwater sampling in the CEB Source: BGR

• Water samples to determine the chemical groundwater composition based on major and minor ions, trace elements, stable isotopes, total inorganic carbon (TIC) / total organic carbon (TOC), and carbon isotopes (Delta-C-13 and C-14)
• Water samples in copper tubes to measure noble gas concentrations (helium, neon, argon, krypton, xenon)
• Water samples in headspace vials to determine gas composition (noble gases and nitrogen)
• Gas samples to measure noble gas isotopes krypton-85 and krypton-81

At each borehole, bulk gas samples were collected in a 12.3 litre stainless steel cylinder and compressed to approx. 1.5 bar. Extraction of krypton from the bulk gas is done at the University Bern in Switzerland. Krypton abundances are measured using the laser-based atom trap trace analysis (ATTA) at the National Laboratory Argonne, USA.

Sampling bottles Source: BGR

Copper tubes to measure noble gas concentrations Source: BGR

The chemical composition of groundwater from KOH-2 changes along the presumed flow path from calcium-carbonate water type to sodium-carbonate water type, revealing a freshening front and processes of cation-exchange taking place. Evaporation on the other hand – a process that may influence groundwater chemistry and is typically observed through stable isotopes values – plays a minor role in KOH-2.

Values of stable isotopes in KOH-2 are relatively low compared to values in modern precipitation, implying that groundwater recharge took place during cooler than present-day temperatures, i.e. paleoclimate conditions. This finding is in good agreement with results from the noble gas measurements. Simulations yielded a noble gas temperature (NGT) of about 18 °C during recharge, which is considerably lower than recent average annual temperatures in the Angolan highlands, the recharge zone.

Determination of noble gas temperature Source: BGR

Values of stable isotopes in groundwater Source: BGR

Preliminary results for groundwater dating of KOH-2 indicate that groundwater ages usually exceed the limit considered to be the maximum for the radiocarbon dating method (approx. 30,000 years).

Literature:

• HIMMELSBACH, T. (2017): Tiefe, semi-fossile Grundwasserleiter im südlichen Afrika: Hydrogeologische Untersuchungen im Norden von Namibia. - GMIT – Geowiss. Mitt., 67: 7 - 18. (PDF, 4 MB)
• HOUBEN, G.J., KAUFHOLD, S., MILLER, R.M., LOHE, C., HINDERER, M., NOLL, M., HORNUNG, J., JOSEPH, R., GERDES, A., SITNIKOVA, M. & QUINGER, M. (2020): Stacked megafans of the Kalahari Basin as archives of paleogeography, river capture, and Cenozoic paleoclimate of southwestern Africa. - Journal of Sedimentary Research, 90(9), 980-1010; doi: 10.2110/jsr.2020.46
• HOUBEN, G., KAUFHOLD, S., LOHE, C., MILLER, R.M., SITNIKOVA, M., JOSEPH, R., NOLL, M., HORNUNG, J., HINDERER, M. & QUINGER, M. (2019): Core drilling WW203302, Report on mineralogical, granulometric and hydraulic analysis of core material. - Technical report of the TC project "Groundwater management in the north of Namibia", 117 pp, Federal Institute for Geosciences and Natural Resources (BGR), Hannover.
• LINDENMAIER, F., MILLER, R., FENNER, J., CHRISTELIS, G., DILL, H.G., HIMMELSBACH, T., KAUFHOLD, S., LOHE, C., QUINGER, M., SCHILDKNECHT, F., SYMONS, G., WALZER, A. & WYK, van B. (2014): Structure and genesis of the Cubango Megafan in northern Namibia: implications for its hydrogeology. - Hydrogeol J 22(6): 1307–1328; doi: 10.1007/s10040-014-1141-1
• SCHILDKNECHT, F. (2012): Groundwater Exploration with TEM-soundings in the Cuvelai-Etosha Basin. - Technical Report Vol. I-d of Technical Cooperation Project “Groundwater for the North of Namibia”, prepared by Department of Water Affairs and Forestry (DWAF), Namibia & Federal Institute for Geosciences and Natural Resources (BGR), Germany.
• WALLNER, M., HOUBEN, G., LOHE, C., QUINGER, M. & HIMMELSBACH, T. (2017): Inverse modeling and uncertainty analysis of potential groundwater recharge to the confined semi-fossil Ohangwena II Aqufer, Namibia. - Hydrogeol J 25(8): 2303-2321; doi: 10.1007/s10040-017-1615-z