In arid areas, underground reserves of water from deep sedimentary aquifers containing old groundwater, probably recharged thousands of years ago, have been recognized as a valuable additional source of drinking water. The recently discovered, deep-seated groundwater reservoir of the Ohangwena (KOH-2) aquifer forms part of the north-south trending Cubango Megafan deposited in the border area between Angola and Namibia. It has been explored at depths between about 250 to 350 meters in northern Namibia in an area between 17.3 and 18 degrees southern latitude and 16 and 17.3 degrees eastern longitude. The amount of stored groundwater in the KOH-2 aquifer on Namibian territory is conservatively estimated at 20 billion cubic meters. A proper understanding of the origin, history and dynamics of the presumably very old groundwaters found in the KOH-2 aquifer is a key for the assessment of the reliability and potential of this aquifer as a strategic resource for supply of drinking water into the future.
The objective of the study is to determine the apparent age of the groundwater in the KOH-2 and to improve the understanding of the hydrogeological setup of the multilayered aquifer system developed within the Cubango Megafan. An overall aim is to investigate the use of long-lived radionuclides, particularly 4He and 81Kr, to assess their reliability as age tracers of old groundwaters. The use of the radioactive 81Kr and 36Cl isotopes, with a half-life of 229,000 and 301,000 years, respectively, allows an estimation of recharge over timescales ranging from 50,000 to several hundred thousand years, a range beyond the reach of radiocarbon. Concentrations of radiogenic 4He derived from the decay of uranium and thorium in deep groundwater may be used as a qualitative or semi-quantitative age proxy.
A comprehensive sampling campaign was carried out during March 2019, which comprised eight deep boreholes of the KOH-2 aquifer. The apparent groundwater ages will be determined from gas and water samples using radionuclide dating techniques comprising 85Kr/81Kr, 4He, 14C and 36Cl. In order to extract bulk gas from groundwater, a new field-gas extraction device was developed comprising a commercially available hydrophobic semipermeable membrane contactor, a membrane vacuum pump, a compressor and a memograph to record gas pressure and water flows. For each well, the gas was collected in a 12.3 L evacuated stainless steel cylinder and compressed to 1.5 bar. Krypton separation from the bulk gas acquired in the field is currently performed at the University of Bern, Switzerland. Krypton abundances are determined using atom trap trace analyses (ATTA) by the National Laboratory Argonne, U.S.
The research is carried out as under the international Coordinated Reseach Programme “Use of Long-lived Radionuclides for Dating Very Old Groundwaters” (F33023) initiated and coordinated by the International Atomic Energy Agency (IAEA).