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Notes on the Harare groundwater situation
TJ Broderick
October 2012

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Underground water in Harare, and everywhere, is a finite resource. Its presence is dependent on the hydrological cycle, which is climate related. It depends on rainfall for its recharge to groundwater storage. The means to store and transmit underground water is controlled by the geological formations that prevail. Figure 1 is a simplified geological map of Harare based on the mapping of Tyndale-Biscoe (1957). Baldock (1991) with others remapped the geology around Harare, and it is this work that has provided an increased understanding of the city's lithological variation and its structural deformation. The geological map and its explanation are available from the Department of Geological Survey, Maufe Building, Fifth Street and Selous Avenue.

As can be seen the geological expression across the city is highly variable, but what is certain is that all rocks are of either igneous or metamorphic origin, which makes most of them massive and crystalline in nature. That is we class them in the category of 'hardrock', which effectively means that in their fresh and unaltered state the rocks are impervious and are unable to store water in a primary sense ie. between the mineral grains that make up the rock. Therefore, any aquifer that we have in the city is referred to as being 'unconfined' as it is open to the elements, and in consequence the available groundwater is superficially stored. This storage is in what we refer to as 'secondary porosity'. That is the groundwater storage depends on the degree of fracturing and weathering that the bedrock has been subjected to. Some rock-types are more susceptible to weathering than others due to their mineral make-up, the degree of imposed shear, and the intensity of jointing and fracturing, all of which may provide open space to facilitate the ingress of recharge water and its subsequent storage. Therefore the main area for groundwater flux is at the highly fractured interface between completely weathered, and often silty, bedrock and the highly weathered to weathered fracture zone. This interface profile, which also responds to topographic gradient, is hugely irregular in shape, and it is the more deeply weathered and fractured pockets that sustain the more effective boreholes. Stress and strain on the bedrock mass has introduced fracturing, which tends to describe linear traces relating to the attitude of the introduced planes of weakness in the ground. These features may be evident due the presence of say a dolerite dyke or a quartz reef infilling. They effectively create the targets for groundwater development, but are by no means ubiquitous in their distribution.

Much of the southern portion of Harare is underlain by massive granite, extending from Amby and Msasa through Hatfield and Waterfalls to Highfield and the western suburbs. Most of this area is notoriously poor for the development of groundwater from boreholes due to the massive and resistant nature of the bedrock. The available groundwater tends to be perched at high level, and is often only exploitable by means of hand-dug wells. It is over the granites that most of Zimbabwe's classic dambo or vlei features develop across our regional watershed. These have recently been defined by Government Notice as the Cleveland, Mabvuku, Greengrove, Prospect, Budiriro and Manyame wetlands in Harare, all of which play an essential function in the control of rainfall runoff to the Mukuvizi, Ruwa and Manyame rivers together with countless others in the catchment that feeds to Lake Chivero, our prime water source for which the function of the seasonal wetlands is to attenuate the base flow of our rivers beyond the cutoff of the rains. The granite terrain may not be the most appropriate for groundwater development from boreholes, but it is important that the defined wetlands are protected and managed for the sake of our extended surface water supply. That management includes the control of inappropriate agricultural practices, the restriction of imposed drainage to make way for construction development, and encouragement in the construction of artificial wetlands in order to rehabilitate the function of these natural features. The water table over granite generally remains perched and high due to annual direct precipitation, but the ravishes of the 1987 and 1992 droughts were telling, judging from the frequency of stressed complaints from those people dependent on shallow wells, notably in the Ruwa area, when the available groundwater then became severely depleted.

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