BGR Bundesanstalt für Geowissenschaften und Rohstoffe

TC Namibia: Groundwater Management in the North of Namibia

Report of the project:

Background:
Access to safe freshwater is the main limiting factor for the economic and social development of Namibia. Surface water is mainly restricted to four perennial rivers at the Northern and Southern borders. Therefore groundwater in Namibia, as it is valid for most arid countries, plays a vital role for the supply of wide areas.

The Cuvelai-Etosha Basin is situated in the central-northern area of the country. It extends northwards into Southern Angola and represents the largest aquifer system in the northern part of Namibia, covering an area of approx. 100,000 km². Due to the immediate proximity of this area to Angola and further favourable conditions, the CEB is developing rapidly. While the country in total currently has a population growth rate of 1.57 % per annum, the annual growth rate in the CEB runs to 2.8 % per annum. In fact, half of the country’s population is living in the CEB. Currently most of the population in the CEB is supplied with drinking water by a pipeline system conveying water into the basin from the Kunene River, more precise the Calueque Dam in Angola. Intermittent droughts and floods as well as population growth lead to an increasing demand and put pressure on the existing water resources.

Between 2007 and 2014 the project Groundwater Investigations in the North of Namibia could identify and delineate a vast multi-layered groundwater system, the Ohangwena Groundwater System (OGS), with its main aquifers KOH-1 and KOH-2. The OGS is a high yielding aquifer system with an enormous volume of stored potable water. The amount of stored groundwater in the KOH-2 Aquifer on the Namibian side is very conservatively calculated to 20 billion m³.

Project:
As part of the technical cooperation between Namibia and Germany the Government of the Federal Republic of Germany provided financial and technical support through the Federal Institute for Geosciences and Natural Resources (BGR) on the project Groundwater Management in the North of Namibia. The project started in 2014 as a follow-up of the groundwater investigation project which was conducted between 2007 and 2014.

The Department of Water and Forestry (DWAF) should be enabled to provide fundamental information on the groundwater potential, protection and sustainable utilization of the Ohangwena Aquifer System (OGS) to Namibian decision makers. Based on this objective three main outputs need to be achieved:

  • An improved monitoring system for the Ohangwena Aquifer System is established.
  • The potential of the Ohangwena Aquifer System is determined.
  • Capacities of the institutions involved in sustainable management and protection of the OGS are developed. This includes the further development and implementation of the national groundwater information system GROWAS II to enhance and support management procedures and decision processes.

Geology and geological evolution of the Cuvelai-Etosha Basin (CEB):
The Cuvelai-Etosha Basin is an extensive sedimentary basin which is part of the much larger Kalahari Basin covering parts of Angola, Namibia, Zambia, Botswana and South Africa.

Basement: The platform carbonates of the Neoproterozoic Otavi Group are the deepest rocks encountered in the CEB. These are overlain by the foreland molasse of the Mulden Group deposited during continental collision in the Neoproterozoic orogenic Damara Belt to the south and the Kaoko Belt to the west – part of the continental amalgamation that formed the Gondwana supercontinent. This orogenesis folded the Otavi and Mulden rocks along the southern and western margins of the CEB. The CEB architecture, thus, is a remnant of this late Neoproterozoic orogenesis. The folded platform carbonates form the elevated, weather-resistant margins of the basin.

Extent of the Kalahari deposits and location of the Cuvelai-Etosha Basin and Core Drilling WW203302Extent of the Kalahari deposits Source: BGR

Some 200 million years of Palaeozoic erosion was followed by deposition of the flay-lying, end Carboniferous to Jurassic Karoo Supergroup on the basin floor. The base of the Karoo succession is 800 to 900 m below the present average elevation of the basin margin carbonates indicating that the CEB had become a deep basinal feature by the end of the Carboniferous. The maximum preserved thickness of the Karoo succession in the basin is 357 m. Thin Karoo basalts cap the succession in the east (MILLER, 2008).

Cretaceous erosion under extremely humid conditions produced the African surface bevel over the whole of Africa. High rainfall produced deeply incised river systems in the interior of southern Africa that are now covered by the Kalahari succession. Kalahari fluvial erosion is considered to have contributed to deepening the floor of the CEB once again. Thus, by the end of the Cretaceous, the CEB was a depression some 400 to 500 m or more deep with elevated pre-Karoo margins to the south, west and north.

The Kalahari Group: Within the CEB, the Kalahari Group consists of four superposed formations, namely, from the base upwards, the Ombalantu, Beiseb, Olukonda and Andoni Formations. A fifth unit, the Etosha Calcrete Formation forms an inner, approx. 80 km wide apron to the elevated basin margins and is a lateral equivalent of all the above four formations. Deposited as a groundwater calcrete from innumerable springs sourced in the karsted Otavi carbonates of the elevated basin margins, its deposition began at the same time as the earliest Kalahari clastic sediments were being deposited. Deposition continued throughout the Tertiary and still continues today at the water table (MILLER, 2008).

The Cubango Megafan and Ohangwena Groundwater System (OGS):
The symmetrical, north-south-trending Cubango Megafan straddles the Namibia/Angola border and forms the upper part of the Tertiary Kalahari Group fill of the eastern part of the deep CEB. This megafan, 350 km long and 300 km wide, was formed by the palaeo Cubango-Okavango River.

The Cubango Megafan is made up of the two upper formations of the Kalahari Group, the Olukonda and Andoni Formations. The most recent of the boreholes drilled into the aquifers during the present groundwater exploration programme, WW 203302, recovered 400 m of core and reveals new information on these upper two formations.

Three aquifers, referred to as the Kalahari Ohangwena Aquifers (KOH), occur in the Cubango Megafan. The uppermost discontinuous perched aquifer occurs in the aeolian sheets sands, is found throughout the CEB and is named KOH-0. It is an important source for potable water for the local population but yields are limited and little is known about the aquifer; thus, it is not a target for large-scale exploitation. The upper Ohangwena Aquifer (KOH-1) is located between 60 and 160 mbgl (meters below ground level) in unconsolidated and partially cemented sands of the Andoni Formation. The KOH-1 is a confined to semi-confined aquifer system that becomes progressively more brackish as it extends toward the Etosha Depression. The KOH-1 extent is related to the shape of the Cubango Megafan (LINDENMAIER et al., 2014).

Indications of a deep-seated freshwater aquifer -- the lower Ohangwena Aquifer (KOH-2) -- were first described by BITTNER, who drilled a deep borehole (WW 37070) for DWAF that reached freshwater at a depth of 190 m. Transient electromagnetic soundings (TEM) followed by exploratory drilling have resulted in the successive recognition of the size of this previously unknown aquifer (LINDENMAIER et al., 2014).

The potential of the Ohangwena Groundwater System (OGS): Drilling campaigns between 2009 and 2014 could verify the existence and spatial extent of the KOH-2 Aquifer in the Ohangwena region. Additional observation boreholes were drilled 2011 till 2014 in the Ohangwena Region to delineate the freshwater extent and to set-up a groundwater monitoring network.

Geological cross section and positioning of the aquifer units KOH-0, KOH-1 and KOH-2Geological cross section Source: BGR

The geological setting of the Ohangwena 1 and 2 Aquifer was translated into a conceptual hydrogeological model. Extensive drillings, hydraulic tests, recharge calculations, water level and water quality monitoring provide information to parameterize the conceptual model and to develop a numerical groundwater model. The model will be used as a decision tool for the planning of groundwater abstraction schemes. Several abstraction scenarios will be calculated and the impact on the resource visualized.

The results reveal a huge potential of the Ohangwena 2 Aquifer (KOH-2) for regional water supply. The amount of stored groundwater in the KOH-2 Aquifer on the Namibian side is very conservatively calculated to 20 billion m³. Recharge to the KOH-2 is highly dependend on the rainfall in the Angolan highlands of the CEB. Generally recharge is low with rates of 0.5 to 5 million m³ per year.

In cooperation with the bulk water supplier NamWater a scheme for water supply tapping the KOH-2 resource was established and is currently extended by three additional boreholes.

Hydrochemistry and Water Quality: Although water quality of the KOH-2 Aquifer is generally good groundwater samples revealed that treatment in some areas is still necessary. Fluoride content is the main limiting factor to supply households directly from the water resource. Fluoride exceeds the Namibian limit of 3 mg/l with a factor of 1.5 to 3 in around 50 % of the samples.

Usability of groundwater for livestockUsability of groundwater for livestock Source: BGR

The cation and the anion triangles of the KOH-1 and KOH-2 groundwater samples show a gradual transition between Earthalcalic-Carbonate (CaHCO3) type and Alcalic-Carbonate (NaHCO3) type which indicate cation exchange as one of the dominating processes in both aquifers. With proximity to the adjacent saline aquifers in the centre of the CEB and south of the OGS water becomes brackish to saline and is in that area not suitable for drinking water purposes.

Groundwater Monitoring: The exploration boreholes drilled into the OGS are equipped with water-level and temperature recording devices. Currently thirty boreholes are continuously logging these parameters in all three aquifers. The water level in an aquifer does not necessarily always stay the same. Droughts, seasonal variations in rainfall, and pumping affect the height of the underground water levels. Knowing the change in water level fluctuation inform about change in stored water and gives an indication if the system is recharged.

For the future management of the OGS resources the information will be transmitted in real-time via a telemetric monitoring network to the Department of Water and Forestry (DWAF) in Windhoek. It is expected that the system is operational by mid of 2017.

The National Groundwater Information System GROWAS II:
Another major activity of the project is the finalization and implementation of the national Groundwater Information System GROWAS II.

Screenshot of the Map Visualization Module of GROWAS IIScreenshot of the Map Visualization Module of GROWAS II Source: BGR

A detailed workflow analysis revealed 2010, that the current database lacks essential functionalities for the management of groundwater relevant data. As the analysis of the old system indicated unclear data operation procedures as a major source of errors, improved user friendliness was high on the agenda for the new database. Developed closely to the needs of Namibian Water Authorities, GROWAS II features a GIS based graphical user interface (GUI) with a vast range of query functions, a modular system including time series tools, hydrochemistry, licenses for abstraction application and groundwater status reporting functions among others.

Quality control is secured through different measures like the "fosterage" option which allows the input of data into a temporary status with restricted access until released by senior experts, the quick and direct interaction with Google Earth to verify locations and the extensive use of look- up tables and descriptive keys in alignment with other regional geo-databases. Furthermore, data entries can be marked according to their estimated reliability with traffic light coding. These measures should ensure that only good quality data will be added in the future. Upcoming development steps are the practical tests of the single modules in day-to-day use, the integration into or exchange with other information systems and the improvement of older existing data as far as possible.

Awareness:
Awareness rising in regard to groundwater resources management is one main task of the project. Therefore materials were developed to increase the understanding of the project objectives and the importance of groundwater as an essential resource for the livelihood in the north of Namibia.

The educational comic book and animation "The incredible adventures of H2O and his friends" are explaining the hydrological cycle, groundwater exploration strategies and groundwater protection to the young community in the southern African context.

"OMEVA" is a documentary collaboration between professional filmmakers and five young people from Ohangwena, one of the most drought-ridden areas of Namibia and the main project area. The film interweaves the young filmmakers’ personal experiences with stories of water from their community.

Partner:

Literature:

Project Reports

Paper

Presentations/Posters

  • LOHE, C. et al. (2012): Combination of user friendly Decision Support Systems with numerical solutions for groundwater flow and water balance for effective water resource management on basin scale in the Cuvelai-Etosha Basin, Namibia, 34th International Geological Congress, Brisbane, Australia.
  • LOHE, C. et al. (2012): Hydro-chemical methods for the delineation of freshwater bodies and groundwater flow patterns in multilayered aquifer-systems. An example from Groundwater Systems in the Cuvelai-Etosha Basin, Namibia, 34th International Geological Congress, Brisbane, Australia.

Further Literature

Comic

Films

Contact 1:

    
Dipl.-Ing. Christoph Lohe
Phone: +264-(0)61-208-7150
Fax: +264-(0)61-208-7149

Contact 2:

    
Dr. Georg Houben
Phone: +49-(0)511-643-2373

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