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Hydrogeology Journal

, Volume 27, Issue 3, pp 915–928 | Cite as

Review: Approaches to groundwater exploration and resource evaluation in the crystalline basement aquifers of Zimbabwe

  • Innocent MuchingamiEmail author
  • Constant Chuma
  • Mervyn Gumbo
  • Dumisani Hlatywayo
  • Robin Mashingaidze
Paper
  • 461 Downloads

Abstract

Assessment of the groundwater potential of crystalline basement aquifers is challenging. These systems can be highly spatially variable, as indicated by the drilling of numerous dry boreholes and seasonal variation in discharge rates. This paper reviews methodologies applied for the evaluation of groundwater occurrence and yield estimation in the crystalline basement aquifers of Zimbabwe. These aquifers underlie much of the country and are described in terms of low yield owing to low recharge potential in the semiarid climate. In such regions, exploitable groundwater forms a strategic supply of potable water used to meet the socio-economic needs of the local population. Case studies are used to show how remote sensing and geophysical methods are integrated to improve borehole success rates in the basement aquifers of Zimbabwe. Potential threats to groundwater resources and quality within crystalline basement aquifers are discussed. It can be concluded that major issues remain to be addressed if sustainable use of the water resources of crystalline basement aquifers in Zimbabwe is to be achieved, especially with respect to borehole-siting approaches and prevention of groundwater contamination. The key recommendation is to address the paucity of primary groundwater monitoring data within the crystalline basement aquifers at the national level, thereby creating a technical groundwater management framework.

Keywords

Zimbabwe Crystalline rocks Groundwater potential Hydro-geophysical Vulnerability Sub-Saharan Africa 

Revue: Approches pour l’exploration des eaux souterraines et l’évaluation de la ressource des aquifères de socle du Zimbabwe

Résumé

L’évaluation des potentialités en eau souterraine des aquifères de socle reste difficile. Ces systèmes peuvent être très variables spatialement, tel qu’indiqué par l’existence de nombreux forages secs et les variations saisonnières des débits de pompage. Cet article examine les méthodologies mises en œuvre pour l’évaluation de la présence d’eau souterraine et l’estimation de la productivité dans les aquifères de socle du Zimbabwe. Ces aquifères sont présents sur une large partie du pays et sont décrits comme peu productifs à cause d’un potentiel de recharge limité en contexte de climat semi-aride. Dans ces régions, l’eau souterraine exploitable constitue une source d’approvisionnement en eau potable stratégique pour satisfaire les besoins socio-économiques des populations locales. Les cas d’étude sont utilisés pour montrer comment la télédétection et les méthodes géophysiques sont intégrées pour améliorer les taux de succès des forages dans les aquifères du socle du Zimbabwe. Les risques potentiels sur les ressources en eau souterraine et leur qualité sont discutés. En conclusion, des questions importantes restent à aborder si une utilisation durable des ressources en eau des aquifères de socle du Zimbabwe doit être atteinte, en particulier pour ce qui concerne les approches d’implantation des forages et la prévention de la contamination des eaux souterraines. La principale recommandation est de répondre à l’insuffisance de données de suivi des eaux souterraines dans les aquifères de socle à l’échelle nationale, en mettant en place un cadre technique de gestion des ressources en eau souterraine.

Revisión: Enfoques para la exploración y evaluación de los recursos de aguas subterráneas en acuíferos del basamento cristalino de Zimbabwe

Resumen

La evaluación del potencial de agua subterránea de los acuíferos del basamento cristalino es un desafío. Estos sistemas pueden ser altamente variables espacialmente, como lo indica la perforación de numerosos pozos secos y la variación estacional en las tasas de descarga. Este artículo revisa las metodologías aplicadas para la evaluación de la presencia de agua subterránea y la estimación del rendimiento en los acuíferos del basamento cristalino en Zimbabwe. Estos acuíferos subyacen en gran parte del país y se describen en términos de bajo rendimiento debido al bajo potencial de recarga en el clima semiárido. En tales regiones, el agua subterránea explotable se constituye en un suministro estratégico de agua potable que se utiliza para satisfacer las necesidades socioeconómicas de la población local. Los estudios de casos se utilizan para mostrar cómo se integran los métodos de percepción remota y geofísicos para mejorar las tasas de éxito de los pozos en los acuíferos del basamento en Zimbabwe. Se discuten las amenazas potenciales a los recursos y la calidad del agua subterránea dentro de los acuíferos del basamento cristalino. Se puede concluir que aún quedan por resolver los principales problemas para lograr el uso sostenible de los recursos hídricos de dichos acuíferos en Zimbabwe, especialmente con respecto a los enfoques de la ubicación de pozos y la prevención de la contaminación del agua subterránea. La recomendación clave es abordar la escasez de datos de monitoreo en los acuíferos del basamento cristalino a nivel nacional, creando así un marco técnico para la gestión del agua subterránea.

综述:津巴布韦结晶基岩含水层地下水勘查和资源评价方法

摘要

结晶基岩含水层地下水潜力评价具有挑战性。这些系统空间上可能变化无常,正如众多干井眼钻探及排泄量季节性变化表明的那样。本文综述了津巴布韦结晶基岩含水层地下水分布评估及出水量估算中应用的方法。这些含水层分布于整个国家大部分地区,由于半干旱气候中补给潜力很低,因此,出水量不大。在这样的地区,可开采的地下水形成了饮用水的战略供给,以满足当地居民的社会-经济需求。本文用研究案例展示了遥感和地球物理方法是怎样结合在一起提高津巴布韦基岩钻孔成功率的。论述了对结晶基岩内地下水资源和水质的威胁。结论是,如果要实现津巴布韦结晶基岩水资源的可持续利用,特别是针对钻孔选址方法以及地下水污染的预防,主要问题依然需要解决。关键的建议是解决国家层面上结晶基岩含水层基本的地下水监测数据。从而,搭建起技术上的地下水管理框架。

Revisão: Abordagens para a exploração de águas subterrâneas e avaliação de recursos nos aquíferos do embasamento cristalino do Zimbábue

Resumo

A avaliação do potencial das águas subterrâneas de aquíferos de embasamento cristalino é um desafio. Estes sistemas podem ser altamente variáveis espacialmente, como indicado pela perfuração de numerosos furos secos e variações sazonais nas taxas de descarga. Este artigo revisa metodologias aplicadas para a avaliação da ocorrência e estimativa da produção de água subterrânea nos aquíferos do embasamento cristalino do Zimbábue. Estes aquíferos cobrem grande parte do país e são descritos em termos de baixo rendimento devido ao baixo potencial de recarga no clima semiárido. Em tais regiões, as águas subterrâneas exploráveis formam um suprimento estratégico de água potável usado para atender às necessidades socioeconômicas da população local. Estudos de caso são usados para mostrar como métodos de sensoriamento remoto e geofísicos são integrados para melhorar as taxas de sucesso de poços nos aquíferos de embasamento do Zimbábue. Ameaças potenciais aos recursos de água subterrânea e qualidade dentro de aquíferos de embasamento cristalino são discutidas. Pode-se concluir que as principais questões que ainda precisam ser abordadas para alcançar o uso sustentável dos recursos hídricos dos aquíferos de embasamento cristalino no Zimbábue, especialmente no que diz respeito a abordagens de localização de poços e prevenção da contaminação das águas subterrâneas. A principal recomendação é abordar a escassez de dados primários de monitoramento de águas subterrâneas dentro dos aquíferos do embasamento cristalino em nível nacional, criando assim um arcabouço técnico de gerenciamento de águas subterrâneas.

Introduction

Aquifers of the crystalline basement, developed within crystalline rocks of igneous or metamorphic origin (Clark 1985; Wright 1992), are mostly developed within the weathered overburden and fractured bedrock of these crystalline rocks. Such aquifers typically have a fractured-weathered layer that has a fracture density that decreases with depth and controls most of the aquifer hydraulic parameters, including the aquifer storage properties. The crystalline basement aquifers of Zimbabwe, along with such aquifers in most other semi-arid countries in the Sub-Saharan Africa region, are of utmost importance for water supply. As there are few alternate sources of year-round water supply, such aquifers are more often the main source of potable water supply to many rural communities in Zimbabwe. In their technical report, PHHEP 2013 established that 73% of Zimbabwe’s 12.5 million population is dependent on accessing safe drinking water and small-scale garden irrigation through low-yielding groundwater sources from crystalline basement aquifers that are susceptible to seasonal variation, yield decline and failure during prolonged droughts. Groundwater distribution in Zimbabwean basement aquifers is dependent upon the interaction of geology and weathering processes controlling recharge to the unconfined aquifers in accordance with the theoretical framework for basement aquifers presented in Chilton and Foster (1995). Geologically, Zimbabwe occupies a tectonically stable plateau underlain by ancient Precambrian crystalline basement rocks that form a central craton bounded by east–west trending mobile belts, the Zambezi mobile belt to the north and the Limpopo mobile belt to the south. The Zimbabwean Craton is an Archaean complex of greenstones, mafic and ultramafic rocks, gneisses and magmatites, and late intrusive granites (Schluter 2006; Fig. 1). The widespread ‘older’ gneisses and magmatites, of variable composition, have a metamorphic texture and evidence of shearing and recrystallisation. Like many Sub-Saharan Africa countries, most groundwater well drilling sites in Zimbabwe have often been controlled by the demand rather than indications that groundwater was likely to be available in the vicinity. As such, this has often resulted in a low borehole success rate in crystalline basement terrain in Zimbabwe. The high number of cases of dry holes and seasonal wells have thus created a need to implement a management framework with regard to assessment of groundwater resource potential, and protection within Zimbabwean crystalline basement aquifers. In this regard, a review on the understanding of the distribution, exploration and nature of groundwater resources in crystalline basement aquifers in Zimbabwe is made through a review of the geological assessment of basement aquifer distribution, exploring the common methods used for groundwater prospecting and investigating potential contaminant factors affecting the resource potential (Fig. 2).
Fig. 1

Geological map of Zimbabwe showing the general occurrence of crystalline basement terrain in the country (adapted from Schluter 2006)

Fig. 2

Rock structure, permeability and porosity controlling groundwater occurrence in basement aquifers in Southern Africa (from Chilton and Foster 1995)

Groundwater flow in such hard-rock aquifers is governed by the hydraulic potential gradient and the hydraulic conductivities in the regolith and underlying fractured bedrock. Wright (1992) described the long-term borehole productivity for crystalline basement aquifers as being controlled by the presence of weathered material overlying the fractured rock or an alternative source of recharge such as groundwater/surface-water interaction in rivers. Therefore, in more arid crystalline terrain with a lack of surface-water resources and where a thinner weathered overburden is present, the need for appropriate and adequate groundwater exploration techniques is increased. Theoretical studies of two-dimensional (2D) groundwater flow in vertical sections by Tóth (1963) indicated that local, intermediate, and regional flow systems could be superimposed on one another within a groundwater basin with fractures exposed to the surface.

Where good hydraulic interconnection is present between the basement rock and regolith, it is likely that excessive exploitation of the basement rock aquifer would induce vertical leakage from the regolith, thereby effectively utilising the regolith resource (Howard and Karandu 1992). As such, groundwater abstraction in crystalline basement aquifers needs to be done in a sustainable manner in order to avoid over-abstraction of the aquifer system.

Approach to groundwater exploration in basement aquifers

Basement aquifers are distinctive in that their occurrence and characteristics are largely a consequence of the interaction of weathering processes related to recharge and groundwater through flow. A close relationship exists, therefore, between groundwater occurrence and relief, geology, surface-water hydrology, soil and vegetation cover. Improvements in the understanding of these relationships are fundamental to the planning and management of groundwater resources in crystalline basement terrain and reduction of development costs. The problems facing any groundwater investigation programme are the zones of occurrence and recharge (Raghunath 1987). The investigations should attempt to determine the thickness of the overburden, the role of geological structures on yield, the depth to which boreholes should be drilled and the suitability of water quality for the desired usage. Sami (2009) formulated a process for exploration of groundwater in basement aquifers (Figs. 3 and 4).
Fig. 3

Photographic descriptions of the typical fracturing controlling groundwater occurrence in gneiss crystalline basement aquifers forming part of the Limpopo mobile belt in the Limpopo river basin, shared between the southern part of Zimbabwe and northern part of South Africa

Fig. 4

Flow chart of the recommended groundwater exploration process

Groundwater exploration is significantly more cost effective when structural controls on groundwater occurrence are considered so that only potentially significant targets are considered for field investigation (Sami 2009). Hydrogeological evaluations done through estimation of expected groundwater availability and variability is of utmost importance in terms of the socio-economic planning on the utilisation of available water resources. Some of the main groundwater exploration stages and methods applied in Zimbabwean basement aquifers are discussed herein.

Remote sensing

Through the use of remote sensing and geographic information systems (GIS), suitable areas for recharging basement aquifers can be delineated which have porous lithologies, maximum fractures, highly weathered regions and regions of null slope. For instance, in Chuma et al. (2013a), exploration of surface features by integrating the remote sensing data and GIS techniques proved to be essential in identifying aquiferous zones for potential groundwater exploitation in Bulawayo crystalline basement rock in south-western Zimbabwe (Fig. 5). Their study provided preliminary information that enhanced the success of the quantitative subsurface geophysical investigations. The procedure described in Chuma et al. (2013b) encompasses (1) production of a regional structural lineament map of Bulawayo Metropolitan area from remotely sensed data and topographic, geological inference and structural data, and (2) preparation of thematic maps of the area such as lithology, lineaments, drainage density, slope and land use/land cover from remotely sensed data and other data sources. This was then followed by an assessment of the hydrogeological implication of the lineaments by integrating them with the available ancillary data, especially digital elevation models (DEM; Fig. 6). High groundwater potential zones were then delineated through integration of various thematic maps within a GIS environment.
Fig. 5

The methodology used to determine the groundwater potential zones in the crystalline basement aquifer in south-western Zimbabwe (with permission from Chuma et al. 2013a)

Fig. 6

a Integration of the thematic maps used to give the potential in urban Bulawayo aquifer. b Groundwater potential zones delineated using remote sensing and GIS in the crystalline basement aquifer in Bulawayo, south-western Zimbabwe (with permission from Chuma et al. 2013a)

Chuma et al. (2013a) suggested that there is large spatial variability of groundwater potential while the most promising potential zone in the Bulawayo Metropolitan area is related to volcanic rock aquifers. The volcanic rock is affected by secondary structures and has interconnected pore spaces, with gentle slope and less drainage density. It can also be inferred that most of the zones with low to poor groundwater potential lie in the massive basement unit which is far from lineaments.

Electrical resistivity geophysical methods

Electrical resistivity geophysical methods can be effectively used as a tool for prospecting for suitable groundwater abstraction sites and to monitor the average depth to the bedrock as well as for mapping in basement aquifers (e.g. in Bhattacharya and Patra 1968; Zohdy 1969; Koefeod 1989; Muchingami et al. 2012; Chuma et al. 2013a, b) and estimation of aquifer parameters (Kosinski and Kelly 1981; Sri and Singhal 1981 and 1985; Mazac et al. 1985; Yadav and Abolfazli 1998). The electrical resistivity method is based on the relative distribution of impressed current in the earth controlled by subsurface resistivity distribution. Logically, the resistivity distribution in a vertically inhomogeneous earth can be derived from distribution of electrical potential at the surface (Fig. 7). Electrical resistivity of basement rocks is greatly dependent on the degree of fracturing and the percentage of fractures filled with groundwater (Archie 1942).
Fig. 7

Theoretical framework of the electrical resistivity geophysical method used as a tool in groundwater exploration for basement aquifers. a Variation in electrical resistivity of different earth materials; b basic concept of electrical resistivity subsurface measurement (Muchingami et al. 2012); c field set up of a typical electrical resistivity meter and; d an illustration of the framework of a 2D model development from measured resistivity values (with permission from Chuma et al. 2013b)

In basement terrains, groundwater is generally believed to occur within the overlying unconsolidated material derived from the in-situ weathering of rocks, and the fractured/faulted bedrock (Clark 1985; Jones 1985; Bala and Ike 2001). Since the intrinsic resistivity of the unconsolidated overburden and that of the crystalline basement differs by orders of magnitude, geoelectrical methods are suitable to map the thickness and extent of the overburden (Koefeod 1989). A generally accepted correlation of potential and electrical resistivity for crystalline aquifers is summarised in Bernardi et al. (1988) wherein the range of optimal to good groundwater potential was given to be between 20 and 150 Ωm. The correlations are relatively insensitive, and inherent sources of error can include over-riding influences of narrow high-permeability zones, both sub-vertical or sub-horizontal (basal saprolite) which are not detectable in the overall electrical resistivity response.

The electrical resistivity depth sounding is useful in locating areas of maximum aquifer thickness and serves as a good predictive tool for estimation of borehole depth. The direct- current (DC) electrical resistivity method for conducting a vertical electrical sounding (VES) has proved very popular with groundwater exploration for basement aquifers in Sub-Saharan Africa because of the direct relation between groundwater potential and electrical resistivity, e.g. in Chuma et al. (2013b) who carried out groundwater exploration in basement aquifers around the Bulawayo Metropolitan Province (Figs. 8 and 9). The geophysical results presented therein show a shallow low-yielding unconfined aquifer, which is heterogeneous in terms of borehole yield, with a good yielding potential in areas that have thick overburden deposits of weathered metabasalt. The thickness of the regolith varies between 10 and 100 m. In such a case, a 2D-subsurface-resistivity-distribution model constructed from the VES results showed that the average overburden layer in the study area is about 23 m, which is less than the 25-m-average thickness of the regolith of Matsheumhlope aquifer as reported by Martinelli and Hubert (1985).
Fig. 8

Vertical electrical sounding (VES) curves and data for stations in basement aquifers around the Bulawayo Metropolitan Province. Most the curves show three-layer models consisting of the subdivided regolith layer and the basement rock that are a characteristic of basement aquifers (with permission from Chuma et al. 2013b)

Fig. 9

Typical 2D hydro-geophysical model showing distribution of electrical resistivities using results from Fig. 8. Areas with a potentially thick weathered regolith can be identified. Such areas are potential targets for yielding wells (with permission from Chuma et al. 2013b)

An overburden thickness of more than 25 m coupled with a resistivity range between 100 and 150 Ωm gives the highest groundwater potential in crystalline basement aquifers. Porosity is the major control of resistivity of rocks and resistivity increases as porosity decreases; however, crystalline rocks are conductive along cracks and fissures. Since the resistivity of the soil or a rock is controlled by pore-water conditions, there are wide ranges in resistivity for any particular aquifer types.

Electromagnetic geophysical method

Despite the fact that their effectiveness is subject to the absence of cultural interferences such as power lines and buried metal pieces, the electromagnetic (EM) geophysical method is amongst some of the most widely used and useful hydro-geophysical techniques in groundwater resource exploration. The EM method has resulted in a wide range of applications in mapping groundwater potential for Zimbabwean crystalline basement aquifers including identification of groundwater bearing fracture zones (Ndlovu et al. 2010; Ranganai et al. 2003; Greenbaum 1995 and Carruthers et al. 1991). In the EM method, measured changes in electrical conductivity are often associated with differences between lithological sequences and over disturbed ground such as faulted or mineralised zones. A detailed description of electromagnetic techniques for reconnaissance groundwater mapping has been given by Richards et al. (1995). The equipment used in the electromagnetic survey method includes an electromagnetic wave transmitter, a receiver, two coils and a cable that connected the transmitter to the receiver (Fig. 10).
Fig. 10

a Photographic image of the equipment, and b the principle of operation for the electromagnetic exploration method as applied to groundwater investigations

In the EM method, electromagnetic waves of several frequencies are transmitted concurrently. Different frequencies probe different depths of the subsurface, hence producing a good picture of the anomalies, how deep-seated they are, and how wide they are stretched. Frequencies that are usually transmitted by most EM units are 37.5, 75, 150, 300, 600, 1,200, 2,400 and 4,800 Hz with multiple frequencies being employed to allow for simultaneous multiple depth investigations. The resolution is always high for shallow depths of investigations.

Separate graphs of amplitude (conductivity of the sub-surface at that station) against station spacing are then plotted for each respective frequencies, and amplitude analysis is used for interpretation and locating the area of highest groundwater potential (e.g. in Fig. 11). Low frequencies like 37.5 and 75 Hz penetrate deeper into the subsurface, while higher frequencies like 150 Hz, 300 Hz and 600 Hz are able to penetrate the shallow subsurface with sufficient resolution.
Fig. 11

Example for application of EM methods for groundwater exploration in Zimbabwean basement aquifers around Bulawayo. Deep seated anomalies shown by the rising amplitudes of the graphs indicate that the increase in conductivity of the subsurface are observed in stations at 0, 50, 80, 100 and 130 m respectively. The anomaly from stations 80 to 110 was picked by all the eight frequencies and a shear zone was visible on the surface of the ground with indications that the anomaly was stretching deeper into the ground and hence was a site for a yielding borehole

Magnetic method

Magnetic surveys, (both ground and airborne) have been successfully used to obtain information pertinent to the groundwater availability such as the presence of faults, lineaments and dykes that form aquifer boundaries or control subsurface flow in crystalline basement aquifers. The regional total magnetic field analysis used for such purposes in Zimbabwe are based on the 1:50,000 map obtained from airborne magnetic surveys done at a flight line spacing of 1 km with a terrain clearance of 305 m during the period 1983–1992 by the Zimbabwean Government. Lineament analysis using the regional total magnetic field maps provides an insight into the regional occurrence and the possible direction of groundwater flow. Studies by Greenbaum (1992) revealed that there is no dominant lineament bearing associated with groundwater occurrence in south-eastern Zimbabwe. As such, depending on the availability of equipment and expertise, the magnetic method can be applied at local scale to aid in borehole siting (e.g. Fig 12 shows groundwater potential for the andesitic terrain in southern Zimbabwe).
Fig. 12

Use of the magnetic method in groundwater potential assessment in andesite basement aquifers in the southern part of Zimbabwe. The borehole was sited on a geological contact inferred from varying total magnetic field (TMF) intensity. A fracture was intercepted at a depth of 22 m during drilling and the borehole has a consistently low yield of 0.02 L/s

Groundwater quality and contaminant transport considerations in basement aquifers

Because of the shallow depth to the static water table in most cases, crystalline basement aquifers are vulnerable to pollution of the groundwater, particularly where the regolith is thin, since groundwater movement through fractures is rapid and the fractured rock matrix provides little attenuation of contaminants. It is, therefore, necessary to test groundwater for natural quality, and groundwater development in a new area should always take water quality into account. Numerous village water supply boreholes in basement areas have been sited close to pit latrines, and microbial contamination has occurred, since both latrine and borehole penetrate to below the zone of lower permeability regolith. In Zimbabwe, the risk of faecal pollution in groundwater sources was highlighted by the 2008 cholera outbreak around Harare City which claimed thousands of lives (Love et al. 2006). However, in as much as the spread of microbial contamination could be limited to the most vulnerable hydrogeological conditions, it currently remains a much more widespread problem because of continual burst of municipal sewer systems which result in contamination of surface water and hence groundwater resources through groundwater/surface-water interaction (Fig. 13).
Fig. 13

Photographs of sources of potential contamination to Zimbabwe’s basement aquifers through groundwater/surface-water interaction. Untreated faecal sewage flowing through a residential area, which may in turn get into contact with unprotected water wells or surface-water bodies

Studies by Mangore and Taigbenu (2004) suggested that the quality of groundwater in some basaltic aquifers of Bulawayo City is deteriorating due to industrial, urbanisation and agricultural activities. Their work sampled 32 boreholes that are located in the Matsheumhlope Wellfield, a basement aquifer that underlies the city of Bulawayo. The results showed that the majority of the parameters (iron, manganese, copper, nitrate, fluoride, sulphate and cyanide) at most sampling stations are within the recommended and permissible limits specified in Zimbabwe drinking-water guidelines. Microbiological analysis indicated that 27% of the samples showed positive total coliform and 8% positive faecal coliform with their occurrences being randomly distributed spatially and temporally. Their study revealed that leaks from industrial and domestic sewers, commonly being experienced due to the age of the sewer lines, are increasingly compromising the quality of the groundwater.

Discussion

The thickness of the fractured zone in the Zimbabwean basement aquifer system varies sharply in terms of spatial distribution and often some sections are characterised by shallow surface fractured zone. Most large fractures at around 15 m to 20 m below surface have been identified and comprise the weathered regolith unconfined aquifer, with a second water strike from a fractured rock aquifer at depths of 45–55 m. Hydrogeophysical models obtained from basement aquifers have shown that groundwater resource potential is controlled by an interactive function of the type of hard-rock formation present and its structural condition, defining primary jointing and fracturing.

In addition to this, using geophysical methods in groundwater exploration for such basement aquifers would also need to consider application of numerical models that can consider the geomorphological evolution determining the depth of regolith weathering, degree of saprock fracturing and the present-day groundwater recharge and discharge regime controlling regolith saturated thickness and available drawdown to the bedrock. Jones (1985) and Martinelli and Hubert (1985) used electrical resistivity sounding to study the crystalline basement aquifer of Zimbabwe and came up with a correlation between thickness of the overburden and borehole success rates, with an overburden thickness of 10–20 m giving an average success rate of 25%, while those for the 20–25 m range and >25 m range are 45 and 70%, respectively.

Experience in prospecting and developing in the weathered crystalline basement in certain parts of Zimbabwe confirms the suggestions by Chilton and Foster (1995) that the associated yields vary from 0.86–8.6 m3/day, with generally low transmissivity in the range of 2–5 m2/day and only locally exceeding 10 m2/day. An analysis of borehole yields (Greenbaum 1992) for crystalline basement aquifers in the Masvingo province, southern Zimbabwe, showed that although failure rates can be high, fractures of all trends are capable of providing an adequate hand-pump yield (0.25 L/s). It also concluded that within the near-surface zone of most importance to basement rock aquifers (40–80 m), fractures of all origins are in a state of tension probably as a result of recent uplift and erosional unloading. As such the development of a sound groundwater monitoring regime is of paramount importance in the promotion of sustainable groundwater utilisation in most rural areas of Zimbabwe that are underlain by crystalline basement terrain. In addition, there is a need to delineate restricted zones where favourable lithology, structural features and deep weathering combine to provide higher transmissivity and available drawdown, and/or to design and construct water wells of large effective radius, and to come up with a groundwater yield regime that is applicable to different basement aquifers in Zimbabwe. Fissure permeability is assumed to correlate to some degree with frequency of fracture occurrence, with a further assumption that both parameters will decrease with depth. Data from boreholes in southern Zimbabwe (Houston and Lewis 1988) showed a greater frequency of fractures in the upper 20 m of bedrock below the regolith interface. Such a distribution probably reflects fracture systems mainly of decompression type. Fracture sealing in the weathered bedrock profile is known to occur, probably by clay illuviation, and may result in an increasing permeability with depth.

In most instances, nitrate, phosphates, organic compounds and microbiological contamination, as well as trace elements from industrial effluent disposal, are the main contaminants that frequently percolate into groundwater in urban areas through mechanisms of groundwater/surface-water interaction, resulting in recurrent cases of water-borne diseases including cholera and typhoid. Thus, priority must also be put on improving groundwater resource and vulnerability appraisal, efficient well design, and aquifer recharge and wellhead protection, together with improved monitoring, as a sound basis for future expansion.

Conclusion and recommendations

Crystalline basement aquifers in Zimbabwe have generally shallow depths, making them more susceptible to contamination and low groundwater storage capacity. The number of dry wells within these aquifer systems is also reduced when proper borehole siting techniques and groundwater management policies are not implemented. Therefore, intensive geophysical methods, remote sensing and geological maps should be used in siting boreholes to identify the weathered saturated fracture zone and to map its extent, and to eliminate negative-potential sites that have hard rock at shallow depth. In addition, an integrated water resource management framework has to be implemented within the aquifer systems in order to reduce the aquifer vulnerability by contamination through groundwater/surface-water interaction. Furthermore, the understanding of groundwater resource occurrence in crystalline aquifers in Zimbabwe needs to be further enhanced by addressing the paucity of primary groundwater monitoring data within the respective basins, mainly caused by lack of updated comprehensive national monitoring and academic research programmes.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Innocent Muchingami
    • 1
    Email author
  • Constant Chuma
    • 1
  • Mervyn Gumbo
    • 1
  • Dumisani Hlatywayo
    • 1
  • Robin Mashingaidze
    • 1
  1. 1.Geophysics Research Group, Department of Applied PhysicsNational University of Science and TechnologyBulawayoZimbabwe

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