Groundwater and urbanisation, risks and mitigation: The case for the city of Windhoek, Namibia

The City of Windhoek is underlain by the Kuiseb Schist, locally known as the “Windhoek Schist” and amphibolites. In the low-lying parts of the Windhoek valley, gravels and sands are present. The Windhoek schist has several lithologies, dominated by garnet-muscovite-chlorite-biotite schist, with distinctive cleavage. This pervasive cleavage renders the underlying lithology permeable to fluids percolating from the surface into the aquifer. Other minor lithologies are trachytes, metarhyolites and quartzites found to the east of the city. The amphibolite is part of the Matchless belt, and traverses the city in a NE-SW fashion. When weathered, it forms a perfect aquiclude. North-south and northeast-southwest trending faults with a few splays cut across the Kuiseb Schist. The faults play a significant role in increasing the fracture density of the fissile schist. The faults are the major links that form channels between the surface and the aquifer below. The city of Windhoek uses the aquifer both as a source of fresh water and as a storage facility. The recharge areas of the aquifer lie to the east and south- to southeast of the city in the vicinity of the suburb of Kleine Kuppe. The soil horizon over the Windhoek schist is very shallow and most buildings are built directly on bedrock. The thin soil horizon makes the aquifer prone to pollution, caused either by accidents such as spills or by carelessness due to unsupervised dumping. The fissility and fracture density of the schist imply that leakage of surface waters, phenols, septic-tank spills, sewer-bursts, chemical and industrial contaminants and other such materials can reach the aquifer in unusually high rainfall years. The effects of fuels and oils are much more adverse, as they may remain in soils for long periods. The rapid urbanization and building of informal settlements without sewage reticulation has increased the risk of pollution to the Windhoek aquifer. The close monitoring of sewage pipes, filling stations and dump sites (including cemeteries), preferably using a GIS-based model, is the best way to prevent future pollution.