Exploration of Deep Aquifer Systems in the Northern Kalahari Basin

Country / Region: Namibia + Angola / Africa

Begin of project: October 1, 2014

End of project: March 31, 2018

Status of project: March 31, 2018

Background:
Unlike fossil groundwater reserves, semi-fossil aquifers are still integrated in the hydrological cycle and hence, partially renewable. In Africa, semi-fossil aquifers provide an important freshwater resource that is not yet fully explored. Groundwater in deep aquifers is often of considerable age and both the stored volume and the recharge processes of such deep lying groundwater bodies are usually poorly understood. Two recently discovered, deep porous aquifers in the northern parts of the Kalahari Basin are currently investigated, namely the Ohangwena II (KOH-2) aquifer in the border region of Namibia and Angola and the Lower Kalahari Aquifer (LKA) in northeastern Namibia.

Location of study areas in the Kalahari Basin

The research project aims at developing or improving methodologies to explore complex layered aquifer systems and hence, to develop techniques for sustainable usage of such resources. A key question is to what extent the deep aquifers are recharged under present-day climatic conditions. The exploration for deep groundwater should focus on the identification of the main geological, tectonic and sedimentological structures, similar to what is applied in the oil and gas exploration.

Study areas:
The study areas form part of the northern Kalahari Basin that covers parts of southern Angola, western Zambia, northern Namibia and northern Botswana. The KOH aquifer system is located within the endorheic Cuvelai-Etosha (Sub-) Basin (CEB) between 16°0' and 18°0' southern latitude and 16°15' and 18°15' eastern longitude. The transboundary sub-basin is shared between Angola and Namibia. The LKA is situated in the eastern part of the Zambezi Region, Namibia, between 17°30' and 18°30' southern latitude and 23°15' and 24°15' eastern longitude and is bordered to the east and south by the Kwando and Linyanti rivers, respectively.

The climate in the northern Kalahari Basin is referred to as a hot arid steppe climate (BSh) by the Köppen-Geiger climate classification system. It is characterized by a unimodal austral summer rainfall regime. A cool and dry winter and minimum temperatures during June and July is followed by a pre-rainy season from September to November, characterized by the highest temperatures, and a hot rainy season during the austral summer months from mid-November to March. Total mean annual rainfall decreases in northeast – southwesterly direction from 600 – 700 mm in the Zambezi Region to below 400 mm/a in the CEB. Due to the semi-arid conditions over most of the Kalahari Basin, present-day recharge rates are generally very low.

Simplified, conceptual model of the study areas

The Kalahari sedimentary basin forms part of the Southern African intracontinental plateau. The Kalahari Group consists of terrestrial sediments that were deposited in a tectonically active basin since the Late Cretaceous. The investigated deep aquifers are developed in areas where the sedimentary cover reaches extraordinarily large thicknesses of well over 300 m. The Ohangwena aquifer system is a multi-layered, porous aquifer system. It is developed within a paleo-megafan of the Cubango River with a total size of about 55,000 km². The KOH-2 aquifer occurs at depths of 150 – 300 m. The proto-Cubango formed a drainage line that provided sediment to the CEB until - probably in the Late Tertiary - the river took its present-day westerly course towards the Okavango Delta. Groundwater flow is from Angola to the South. The main recharge area for the deep aquifer is assumed to be in the Angolan highlands where mean annual rainfall is considerably higher (1200 – 1400 mm) and the aquifer layers crop out.

Tectonic setting and hydrology of the study area in Eastern Zambezi Region

The Zambezi Region belongs to an incipient rift system extending in NE-SW direction from the Machile Graben in Zambia across the Okavango Delta to the Eiseb Graben NE Namibia. The on-going tectonic activity is linked to the propagation of the East African Rift Zone and had a major impact on the hydrology over the entire area. The capture of the Upper Zambezi and Kwando rivers is evident from abrupt elbow-shaped changes in the direction of flow. The redirection of river courses, initiated by tectonic subsidence and uplift and subsequent headward erosion, were - together with climatic changes - responsible for the formation and desiccation of vast but shallow (paleo-) lakes. The LKA is graben-bound: to the North by the inferred Sibbinda Fault and to the south by the Linyanti Fault. The depression was filled with predominantly fluviatile-lacustrine deposits of the Kalahari Supergroup and is underlain by Mesozoic basalt or clastic sediments of the Karoo Supergroup. The LKA aquifer occurs at depths between 130 and 250 m. Hydrochemical and hydroisotopical results for the LKA indicate that since the Late Pleistocene, a presumably brackish groundwater body is undergoing “freshening” in southward direction.

Methods:
An interdisciplinary approach is applied including the following individual methods:

Literature review with a specific focus on paleoclimatic conditions in the region
In-depth analysis of tectonic structures and sedimentological processes
Geophysical soundings, in particular Transient-Electromagnetics (TEM)
Hydrochemical analysis and modelling
Stable isotope techniques and radiometric dating of groundwater
Conceptual and numerical hydrogeological modelling

Groundwater sampling and in situ testing