Groundwater recharge of perched aquifers in the Cuvelai-Etosha basin, Namibia

A dissertation submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy in Science (Geology)

The United Nation predicted that by 2025, 1.8 billion people will be living in countries with absolute water scarcity and two-thirds of the world population could be under stress condition. In semi-arid regions, most communities depend on groundwater as the source of drinking water and thus with changes in global climatic conditions and increase in population, groundwater resources are facing challenges of both over-exploitation and contamination. Therefore, there is an urgent need to improve the understanding of existing groundwater resources in terms of aquifer distributions and interactions and process that control the groundwater dynamics, recharge and chemistry groundwater for an effective strategy to reduce the pressure on the hydrologic system. The main aim of this PhD is to contribute to knowledge about shallow groundwater in semi-arid environments by estimating groundwater recharge of perched aquifers in the Cuvelai-Etosha Basin (CEB). Four specific objectives were examined in this study; first, the study characterizes the groundwater chemistry and isotopic composition of oxygen (δ18O) and hydrogen (δ2H) in order to understand mechanisms of groundwater dynamics and quality of groundwater in perched aquifers, secondly, it analyses and understand the spatial and temporal variations of hydrochemical data and isotopic compositions of hand-dug wells in the CEB, with particular focus on water origin and recharge processes, thirdly, the study evaluates the relationship between the shallow (perched) aquifer and the deeper seated aquifers and finally develops a conceptual model for the perched aquifers. Methods employed in this research are based on isotopic and hydrochemical data to understand groundwater recharge mechanisms. Integrated isotopic and hydrochemical tracers along with standard hydrological data are used to understand complex dry land hydrological processes on different spatial and temporal scales. Different spatial and temporal scales are particularly important for arid environments due to high heterogeneity that are associated with these environments. Therefore in this study, water samples were collected from rain collectors, hand-dug wells and boreholes and analysed for major ions and stable isotopes (18O and 2H) for three years (2014-2017) in a total of 12 sampling campaigns. Chemical analyses were performed at the Analytical Laboratory Services in Windhoek, Namibia and at the hydrochemistry laboratory of BGR in Hanover, Germany using Titration, Ion Chromatography and ICP-OES. The reliability of the analyses was checked by an ion charge balance error on all samples. Stable isotopes were measured at the University of Namibia (UNAM) and BGR laboratories using an off-axis integrated cavity output spectroscope (OA-ICOS, Los Gatos DLT-100) and a cavity ring down spectrometer (CRDS, model L2120-i, Picarro Inc.) respectively. Results show that groundwater chemistry of perched aquifers is controlled mainly by strong evaporation, dissolution of carbonate minerals (calcite and dolomite) and evaporitic minerals (gypsum and halite) and silicate weathering and cation exchange. Stable isotope composition suggests that deep groundwater is recharged by high intensity/large rainfall events, whereas the shallow wells can be recharged by less intense/small rainfall events. Water in deep wells reflect mixture of water influenced by evaporation during or before infiltration and water that infiltrated through fast preferential pathways whereas shallow wells are strongly influenced by evaporation. The mean parent isotopic composition for shallow wells in the ephemeral river is -7.8 for δ18O and -51.8 for δ2H, for deep wells in the pans and depressions is -8.7 and -58.2 for δ18O and δ2H as well as - 8.6 and -57.5 for δ18O and δ2H for wells in Omusati region. Hydrochemical and isotopic data reflect spatial variability between samples from Omusati and Ohangwena regions. The spatial heterogeneity as shown by TDS can be attributed to lithological, climatic and anthropogenic factors. Furthermore, temporal variations indicate the timing of the groundwater recharge. Results also imply interaction between perched aquifer and regional aquifer in the pans and depressions while in the ephemeral river no relation could be established. High recharge rates are estimated for the pans and depressions (7.3 % to 25.5%) in comparison to the ephemeral river (7.9% to 17.8%). Therefore, it is recommended that groundwater management practices should be designed taking into account differences in perched aquifer characteristics. For example designing abstraction infrastructures which include treatment for natural contaminants i.e. fluoride and TDS. On the other hand contaminants from anthropogenic sources in the wells can be reduced or prevented by introducing protection zones. Education on basic water usage and protection will also be of an advantage. Furthermore, it could be shown that it is indeed essential to unravel the hydrogeological complexities of heterogeneous perched aquifers using isotopic and hydrochemical tracers at different spatial and temporal scales and thus more research is needed in this regard.