In the arid and semi-arid region in northern Namibia, there are three main catchment areas the Kunene, the Kavango, and the Cuvelai-Etosha Basin (CEB). The Kunene and the Kavango Rivers are perennial and provide the northwestern and northeastern regions with water. In central northern Namibia, the transboundary CEB consists of an ephemeral and endorheic drainage system with channels and depressions, known as the Iishana system. High temperatures, high evaporation rates, and low rainfall increase the risk of polluting these waters. Groundwater resources near the surface are heavily salinized and can hardly serve as an adequate water source. Deeper groundwater reservoirs are difficult to access. The high population density in the Iishana system area increases the demand upon the surface waters. The geomorphology and surface hydrology of the Iishana system favors very slow surface runoff and low infiltration rates. An extreme change in dry and rainy seasons results in an erratic freshwater supply, which can decrease water quantity and water quality and is thus a major challenge. Since there were no data on the quality of the surface waters in this area, the state of the ecosystem and the health risk for the population are completely unclear; therefore, the regional surface waters were examined to determine their suitability as potable water. The aim is to better understand the hydrological systems in northern Namibia. In four field campaigns between 2017 and 2021, the surface waters, suspended solids, and sediments of the three systems (Kunene, Kavango, and CEB) and the local water supply system, the Calueque-Oshakati Canal, were sampled at more than 30 sites. Relevant hydrochemical parameters were analyzed, microplastics were investigated, and bioassays were conducted to gain insights into the status of aquatic ecosystems and the ecological impacts of pollutants. Spatial differences in the water quality relative to the decreasing precipitation gradient from east to west were noticed. Furthermore, metals such as aluminum and iron accumulate around the densely populated region in the eastern part of the study area. These waters were more polluted during the drought events in 2018 and 2019 than in 2017. Microplastics were found in different quantities, in particular PE and PP fragments. Three different trophic levels (algae, daphnia, and zebrafish embryos) were tested for acute toxicity. Mechanism-specific effects, such as estrogenicity (YES), cytochrome potential (micro-EROD), and mutagenicity (AMES) were investigated using in vitro assays. Acute toxicity could be detected for all three systems; in particular, the fish embryos showed several effects. Estrogenic and mutagenic effects were identified for several sampling sites. The different methodological approaches facilitate a holistic monitoring. This work is the first study to provide a comprehensive statement on water quality. The ecosystems of the Iishana, the Kunene, and Kavango Rivers are severely stressed and differ significantly. They show several signs of anthropogenic pollution that can affect human health. The use of water as potable water is not possible without prior treatment. Further investigation of the exact influencing factors, such as pesticides, is necessary to find suitable treatment measures for water use.
