Yongxin Xu

The role of ground water in sub-Saharan Africa.

Although water resources managers speak of a water crisis in Africa, the management of ground water has to date not featured strongly in national and regional African water agendas. Examination of the physical environment of the continent and, in particular, the water resources in relation to the socioeconomic landscape and regional development challenges makes it clear that widely occurring, albeit largely low-yielding, ground water resources will be crucial in the achievement of water security and development. Ground water is important primarily in domestic water and sanitation services, but also for other local productive needs like community gardens, stock watering, and brick-making, all essential to secure a basic livelihood and thus to alleviate poverty. Despite the importance of small-scale farming in Africa, there is little information on the present and potential role of ground water in agriculture. In contrast to its socioeconomic and ecological importance, ground water has remained a poorly understood and managed resource. Widespread contamination of ground water resources is occurring, and the important environmental services of ground water are neglected. There appear to be critical shortcomings in the organizational framework and the building of institutional capacity for ground water. Addressing this challenge will require a much clearer understanding and articulation of ground waters role and contribution to national and regional development objectives and an integrated management framework, with top-down facilitation of local actions.

Groundwater pollution in Africa.

Introduction: Issues of groundwater pollution in Africa Overview: Nitrate pollution of groundwater in Southern Africa Nitrate pollution of groundwater in Nigeria Groundwater quality in Cameroon and its vulnerability to pollution Groundwater contamination in Ghana Characterization of diffuse pollution of shallow groundwater in the Harare urban area, Zimbabwe Groundwater contamination from pesticides in Africa Degradation of groundwater resources under African cities Technical and socio-economic challenges Groundwater Pollution Status and Aquifer Vulnerability: A new cartographic approach to determine the groundwater vulnerability of the Abidjan Aquifer Vulnerability assessment of the Abidjan Quaternary Aquifer using the DRASTIC method Groundwater pollution from urban development in Cotonou City, Benin Assessment of water pollution and risks to surface and groundwater resources in Bamako, Mali Statistical assessment of groundwater quality in Bamako City, Mali Spatial and temporal variations of groundwater pollution in Ougadougou City, Burkina Faso Groundwater contamination in the Niamey urban area, Niger Management of nitrate pollution of groundwater in African cities: The case of Dakar, Senegal The application of electrical resistivity techniques in the delineation of saltwater-freshwater in the Keta Basin, Ghana Degree of groundwater vulnerability to pollution in Addis Ababa, Ethiopia Vulnerability and pollution of groundwater in Kisauni, Mombasa, Kenya A chemical study of surface and groundwater in the Lake Chilwa Basin, Malawi Groundwater quality assessments in the John Laing and Misisi areas of Lusaka Groundwater quality case studies in Botswana Contamination and protection in the Cape Flats Aquifer, South Africa An evaluation of groundwater pollution from pesticides in South Africa using the VLEACH model. Special Topics: Contamination of the Abidjan aquifer by sewage: An assessment of extent and strategies for protection The impacts of coal and gold mining on the associated water resources in South Africa Monitored natural attenuation of petroleum hydrocarbons in a fractured environment - A case study Management strategies to mitigate clogging and biofouling in production boreholes Way Forward: A South African perspective on the protection of groundwater sources

Estimation of the sustainable yields of boreholes in fractured rock formations

Abstract The simplest way to derive an estimate for the sustainable yield of a borehole is to study the behaviour of drawdowns observed during a hydraulic (also known as a pumping test) of the borehole, through an appropriate conceptual model. The choice of this model is probably the most difficult choice that the analyst of such a hydraulic test has to make, since a wrong model can only lead to the wrong conclusions and failure of the borehole. This paper discusses a semi-analytical and two numerical methods that can be used to simplify the analyses of hydraulic tests in fractured rock formations. The first method, called the Method of Derivative Fitting (MDF), uses a new approach to identify the conceptual model needed in such analyses. This is achieved by characterizing the various flow periods in fractured rock aquifers with numerical approximations of the first logarithmic derivative of the observed drawdown (the derivative of the drawdown with respect to the logarithm of the time). Semi-analytical expressions are used to estimate the influence that boundaries may have on the observed drawdown and the sustainable yield of a borehole — the rate at which a borehole can be pumped without lowering the water level below a prescribed limit. An effort has also been made to quantify errors in the estimates introduced by uncertainties in the parameters, such as the transmissivity and storativity, through a Gaussian error propagation analysis. These approximations and the MDF, called the Flow Characteristics Method (FCM) have been implemented in a user-friendly EXCEL notebook, and used to estimate the sustainable yield of a borehole on the Campus Test Site at the University of the Orange Free State. The first numerical method, a two-dimensional radial flow model, is included here because it allows the user more freedom than the FCM, although it requires more information. One particular advantage of the method is that it allows one to obtain realistic estimates of the storativity and transmissivity of Karoo aquifers in particular, which is required in the estimation of the sustainable yield of a borehole. There is no doubt that a three-dimensional numerical model, the second numerical method discussed here, is the best method with which to analyse a hydraulic test in a fractured aquifer. The method was consequently used to evaluate the accuracy of the implementation of the MDF in the Excel notebook and its application to the borehole on the Campus Test Site. The good agreement between the sustainable yield estimated with the three-dimensional numerical model and the FCM indicates that the FCM can be used with confidence to estimate the sustainable yields of boreholes in fractured media.

Conservation status of large branchiopods in the western Cape, South Africa

Temporary wetlands are an ecologically and economically important habitat in South Africa. They harbor large branchiopods, known to be flagship species of nonpermanent aquatic habitats, and sensitive to land use changes. In this study we review the current status of large branchiopods in the Western Cape, a South African province subject to increasing agriculture and urbanization. We studied the species diversity and distribution of large branchiopods by sampling 58 temporary wetlands in an area covering about 30% of the Western Cape. Information obtained from field samples was supplemented by incubating resting egg banks from the sampled wetlands. Our data were compared with all known distribution records for large branchiopods in the target region. Based on this combined information, the International Union for the Conservation of Nature and Natural Resources (IUCN) Red List category was assessed for each species. Four of the eight large branchiopod species known to occur in the sampling area were collected. Of all wetlands sampled, 40% harbored large branchiopods. Most anostracan populations were small, and species co-occurred in only one wetland. From the entire Western Cape, 14 species have been recorded in the past. Two of these are already included in the IUCN Red List. Insufficient data are available to determine the IUCN Red Data Category of six other species. A large variation in the telsonic appendages of S. dendyi was found across the studied area. In view of possible ongoing speciation and subsequent radiation, individual populations need protection. Since little information is available, it is difficult to evaluate recent changes in the conservation status of large branchiopods. Their populations are currently very low and have probably diminished in the last few decades. More knowledge about the functioning of temporary systems is needed to manage these vulnerable habitats and conserve their threatened species.

Review: Groundwater management and groundwater/surface-water interaction in the context of South African water policy

Groundwater/surface-water interaction is receiving increasing focus in Africa due to its importance to ecologic systems and sustainability. In South Africa’s 1998 National Water Act (NWA), water-use licenses, including groundwater, are granted only after defining the Reserve, the amount of water needed to supply basic human needs and preserve some ecological integrity. Accurate quantification of groundwater contributions to ecosystems for successful implementation of the NWA proves challenging; many of South Africa’s aquifers are in heterogeneous and anisotropic fractured-rock settings. This paper reviews the current conceptualizations and investigative approaches regarding groundwater/surface-water interactions in the context of South African policies. Some selected pitfall experiences are emphasized. The most common approach in South Africa is estimation of average annual fluxes at the scale of fourth-order catchments (∼500 km2) with baseflow separation techniques and then subtracting the groundwater discharge rate from the recharge rate. This approach might be a good start, but it ignores spatial and temporal variability, potentially missing local impacts associated with production-well placement. As South Africa’s NWA has already been emulated in many countries including Zambia, Zimbabwe and Kenya, the successes and failures of the South African experience dealing with the groundwater/surface-water interaction will be analyzed to guide future policy directions.RésuméL’interaction eau souterraine/eau de surface est un centre d’intérêt croissant en Afrique étant donné son importance pour les systèmes écologiques et leur pérennité. Dans le National Water Act sud-africain de 1998 (NWA), des autorisations d’utilisation de l’eau, incluant l’eau souterraine, sont accordées seulement après évaluation de la Réserve, de la quantité d’eau requise pour satisfaire les besoins humains essentiels et préserver une certaine intégrité écologique. Une évaluation précise des contributions de l’eau souterraine aux écosystèmes pour la mise en œuvre du NWA s’avère être un défi; beaucoup d’aquifères d’Afrique du Sud sont dans des encaissants rocheux fracturés hétérogènes et anisotropes. Cet article passe en revue les concepts et approches actuels des interactions eau souterraine/eau de surface dans le contexte des politiques d’Afrique du Sud. Certaines expériences piège sélectionnées sont mises en évidence. L’approche la plus commune en Afrique du Sud est l’estimation des flux annuels moyens à l’échelle des bassins versants du quatrième ordre (∼500 km2) avec des techniques de séparation du débit de base, puis de soustraction des taux de décharge et de recharge. Cette approche pourrait être un bon début, mais elle ignore la variabilité spatiale et temporelle, éludant potentiellement les impacts locaux associés à la localisation des puits de production. Comme le NWA d’Afrique du Sud a déjà fait des émules dans de nombreux pays dont la Zambie, le Zimbabwe et le Kenya, les succès et les échecs de l’expérience sud-africaine concernant l’interaction eau souterraine/eau de surface seront analysés pour guider les orientations de la politique future.ResumenLa interacción agua subterránea/agua superficial está recibiendo una creciente atención en África debido a su importancia en los sistema ecológicos y su sustentabilidad. En Sudáfrica, según la Ley Nacional de Agua (NWA) de 1998, las licencias de uso del agua, incluidas las aguas subterráneas, sólo se conceden después de definir la Reserva, que es la cantidad de agua necesaria para abastecer las necesidades humanas básicas y para preservar la integridad ecológica. La cuantificación precisa de la contribución del agua subterránea a los ecosistemas para la implementación exitosa de la NWA resulta difícil, ya que muchos de los acuíferos de Sudáfrica presentan la configuración heterogénea y anisotrópica en rocas fracturadas. En este documento se realiza un revisión de los conceptos y enfoques actuales de la investigación de la interacción aguas subterráneas/aguas superficiales en el contexto de las políticas de Sudáfrica. Se destacan algunas experiencias seleccionadas con las dificultades encontradas. El enfoque más común en Sudáfrica es la estimación de los flujos anuales promedio en cuencas de drenaje de cuarto orden (∼500 km2) utilizando las técnicas de separación del flujo de base y luego restando la tasa de descarga del agua subterránea a la tasa de recarga. Este enfoque podría ser un buen comienzo, pero no tiene en cuenta la variabilidad espacial y temporal, faltando los impactos locales asociados a la ubicación de los pozos de producción. Como la NWA de Sudáfrica ha sido emulada en muchos países, incluyendo Zambia, Zimbabwe, Kenia, los éxitos y fracasos de la experiencia de Sudáfrica sobre la interacción aguas subterráneas/aguas superficiales deberán ser analizados para guiar las futuras orientaciones políticas.ResumoA interacção água subterrânea/água superficial recebe cada vez maior atenção em África, devido à sua importância para os sistemas ecológicos e para a sustentabilidade. Na África do Sul, no Acto Nacional da Água (NWA) de 1998, as licenças de uso da água, incluindo a água subterrânea, são garantidas apenas depois de definidas as Reservas, a quantidade de água necessária para suprimir as necessidades humanas básicas e a preservação de alguma integridade ecológica. A quantificação acurada das contribuições da água subterrânea para os ecossistemas para uma aplicação com sucesso da implementação do NWA provou-se ser desafiadora; muitos dos aquíferos situam-se em meios heterogéneos e anisotrópicos, em rochas fracturadas. Este documento revê a conceptualização corrente e os processos de investigação que têm sido aplicados às interacções águas subterrâneas/águas superficiais no contexto das políticas Sul-Africanas. Algumas experiências seleccionadas são enfatizadas. A aproximação mais comum na Áfria do Sul é a estimação dos fluxos anuais médios à escala da bacia de quarta ordem (∼500 km2), utilizando técnicas de separação do fluxo de base e subtraindo então a descarga subterrânea da taxa de recarga. Esta aproximação pode ser um bom começo, mas ignora a variabilidade espacial e temporal, negligenciando potencialmente os impactes locais associados à exploração de campos de captações. Como o NWA Sul-Africano já foi emulado por muitos outros países, incluindo a Zâmbia, o Zimbabué e o Quénia, os sucessos e insucessos da experiência Sul-Africana ao lidar com as interacções águas subterrâneas/águas superficiais serão analisadas, para guiar as direcções políticas futuras.

Sustainable groundwater resources in Africa: water supply and sanitation environment.

I - Best practice guidelines 1 Water supply and sanitation issues in Africa *1.1 Contribution of water supply and sanitation to African development *1.2 Millennium development goals and water *1.3 Water supply and sanitation coverage *1.4 Challenges and strategies to achieve the MDGs for WSS *1.4.1 Rural water supply *1.4.2 Urban water supply *1.4.3 Sanitation *1.4.4 General *1.5 Various programmes to achieve goal *1.6 From water for domestic use to a livelihoods focus *1.7 Conclusions 2 Groundwater resources in Africa *2.1 Characteristics of groundwater resources *2.1.1 Major types of aquifers *2.1.2 Groundwater resources in Africa *2.1.3 Roles and functions *2.2 The sustainable groundwater utilization and management in Africa *2.2.1 Major groundwater-related issues and problems on the African continent *2.2.2 The data and information management relating to groundwater resources management *2.2.3 Implication of climate change *2.3 Outlook for the sustainable utilization of groundwater in Africa 3 Framework of best practices for groundwater supply and sanitation provision *3.1 Introduction *3.2 Integrated water resources management *3.3 Groundwater and IWRM *3.4 Integrated water, sanitation and hygiene delivery *3.5 Integrated service delivery in the urban environment *3.6 Integrated service delivery in the rural environment *3.7 A framework to facilitate groundwater management 4 Best practice for groundwater quality protection *4.1 General introduction *4.2 South African borehole guidelines *4.2.1 Introduction *4.2.2 Relevant literature *4.2.3 Contracts and agreements *4.2.4 Supervision of activities *4.2.5 Materials and equipment *4.2.6 Aanomalous circumstances *4.2.7 Straightness and verticality *4.2.8 Protecting the resources *4.2.9 Safety *4.2.10 Conclusions *4.3 Groundwater protection guidelines *4.3.1 Basic approaches *4.3.2 Concept of safe distance *4.3.3 Stepwise procedure from case studies *4.3.4 Zoning approach *4.3.5 Protection approach in fractured rock aquifers *4.3.6 Spring and shallow well protection *4.4 Drought proofing *4.4.1 Basic concepts *4.4.2 Types of drought *4.4.3 Livelihood approach *4.5 On-site sanitation and groundwater *4.6 Ecosan approach for effective groundwater resources management *4.6.1 Historical context of Ecosan *4.6.2 Main objective of Ecosan projects *4.6.3 The philosophy about Ecosan *4.6.4 Ecosan and groundwater quality *4.6.5 The challenge for Ecosan *4.7 Community participation best practice *4.8 Towards a comprehensive approach 5 Summary and recommendations *5.1 Summary *5.2 Recommendations II - Case studies 6 Groundwater dynamics in the East African Rift System *6.1 Introduction *6.2 Tectonic framework *6.3 Hydrology and climate *6.4 Hydrogeological framework *6.5 Hydrogeochemical feature *6.6 Implication on water supply and sanitation *6.7 Summary and recommendations 7 Aquifer vulnerability and its implication for community water supply of Porto-Novo region (South - East of Benin) *7.1 Introduction *7.2 Site description *7.3 Data requirements *7.3.1 DRASTIC and GOD methods *7.3.2 Evaluation of DRASTIC and GOD parameters *7.3.3 Validation of vunerability maps and determination of aquifer protection *7.4 Results and discussion *7.4.1 Vulnerability maps *7.4.2 Discussion *7.5 Conclusion 8 Vulnerability of dolomite aquifers in South Africa *8.1 Introduction *8.2 Geological description *8.3 Methodology *8.3.1 Overlying layers map (O-Map) *8.3.2 Concentration of flow map (C-Map) *8.3.3 Precipitation map (P-Map) *8.4 VUKA index map *8.5 Summary 9 A low tech approach to evaluating vulnerability to pollution of basement aquifers in sub-saharan aquifer *9.1 Introduction *9.2 The weathered basement aquifer *9.3 The groundwater vulnerability drivers *9.4 Simple field approach to vulnerability assessment *9.5 Scorecard validity *9.6 Conclusions 10 Preserving groundwater quality against microbiological contamination through differentiated aquifer management in Africa *10.1 Introduction *10.2 Microbiological transport within secondary aquifers *10.3 Extent of microbiological impacts on some africa aquifers *10.4 State of microbiological monitoring networks in Africa *10.5 Groundwater protection zoning *10.6 Way forward *10.7 Conclusion 11 Fluoride in African groundwater: Occurrence and mitigation *11.1 Introduction *11.2 Sources of fluoride in groundwater *11.3 Formation of fluoride-rich groundwater *11.4 Determination of fluoride *11.5 Fluoride occurrence in African groundwater *11.6 Removal of fluoride from drinking water supplies *11.7 Alternative mitigation *11.8 Implication for rural water supplies 12 Practical methods to reduce iron in groundwater with a case study *12.1 Introduction *12.2 Project area *12.3 Methodology *12.4 Iron removal *12.5 Results *12.6 Iron filter construction *12.7 Conclusion 13 Investigation of borehole failures - experience from Botswana *13.1 Introduction *13.2 Hydrogeology of the study area *13.3 Methodology and data uesd *13.3.1 Methodology *13.3.2 Data used *13.4 Results and discussion *13.4.1 Evidence of possible recent decline in groundwater yield *13.4.2 Correlation analysis *13.4.3 Reliability analysis of groundwater monitoring data *13.5 Summary, conclusion and recommendation 14 Cost-effective boreholes in sub-saharan Africa *14.1 Introduction *14.2 Assertions, information and evidence of high drilling prices *14.3 Conceptual framework *14.3.1 Borehole costs and quality *14.3.2 Borehole price *14.3.3 The core factors *14.3.4 Key elements *14.4 Analysis of the thirteen elements of cost-effective boreholes *14.4.1 Operation and maintenance procedures *14.4.2 Who drills water wells? *14.4.3 Borehole standards and designs *14.4.4 Drilling equipment - smaller and less costly rigs *14.4.5 Procurement process *14.4.6 Contract packaging *14.4.7 Contract packaging *14.4.8 Siting practices *14.4.9 Supervision *14.4.10 Pumping test *14.4.11 Groundwater resources monitoring and evaluation *14.4.12 Hydrogeological data *14.4.13 Regulation and professionalism of the private sector *14.5 Conclusions and recommendations 15 Water supply and sanitation in the democratic republic of the Congo *15.1 Introduction *15.2 Overview of the study area *15.2.1 Location and climate *15.2.2 Geology and tectonics *15.2.3 Soils and land use *15.2.4 Hydrography *15.3 Socio-demography *15.4 Water resources *15.4.1 Surface water resources *15.4.2 Groundwater resources *15.5 Water supply *15.5.1 Water supply sources and access to drinking water *15.6 Sanitation *15.6.1 Current situation in the DRC *15.6.2 Water and health issues *15.6.3 Financial requirements *15.7 Institutions and legislation *15.7.1 Laws and regulations *15.7.2 Institutions *15.8 Government policies and strategies to improve WatSan sector *15.9 Constraints to the WatSan developent In DRC *15.10 WatSan donors *15.11 Conclusion 16 Rural water supply and sanitation in Malawi: Groundwater context *16.1 Introduction *16.2 Study area charaterization *16.3 Methods *16.4 Results and discussions *16.4.1 Water sources and water point coverage *16.4.2 Functionality status of rural water points *16.4.3 Groundwater availability and its associated quality pattern *16.4.4 Water quality monitoring *16.4.5 Current sanitation service provision *16.4.6 Current initiatives within the rural water supply and sanitation sub-sector *16.5 Conclusions 17 Community-based groundwater quality monitoring: A field example *17.1 Introduction *17.2 Background *17.3 Methods *17.4 Results from the Initial Implementation *17.5 Analysis of the Initial Implementation *17.6 Brief comparison with another recent study on use of simple measures *17.7 Conclusions 18 Charitable endowments as an institute for sustainable groundwater development and management *18.1 Introduction *18.2 Practice in the Middle East *18.3 Socio economic issues *18.4 Technical issues *18.5 Conclusion Subject index Colour plate

Determining discharges from the Table Mountain Group (TMG) aquifer to wetlands in the Southern Cape, South Africa

The focus of this study was to determine whether coastal wetlands in lowland settings could be dependent on groundwater from the deep circulating confined Table Mountain Group (TMG) aquifer. Groundwater interactions with wetlands are normally perceived to be limited to primary aquifers. A comparative study was done between two endorheic coastal wetlands in the southern Cape. Earlier reports stated that these groundwater dependent wetlands were fed by discharges from the fixed dunes surrounding them. On the basis of a three-dimensional electrical conductivity (EC) interpolation for Groenvlei, a hydrological link between the TMG aquifer and Groenvlei and Van Kervelsvlei was investigated by measuring water level and quality of groundwater and surface water. Water quality parameters used were EC, pH, Na+, Fe2+ and Cl−. The results from this, and an accompanying study, on the basis of water quality and plant nutrient cycling assessments, indicated direct groundwater discharges from the TMG to at least Van Kervelsvlei, with Groenvlei receiving secondary discharges from the TMG via Van Kervelslvlei. These findings significantly affect the current knowledge on which water balance models are based for the determination of groundwater availability for the area.

Diagnostic analysis of pumping tests using derivative of dlgs/dlgt with case study.

Diagnostic derivative interpretation of drawdown data from pumping tests is discussed in this paper. An emphasis is placed on the conceptualization of derivative term of dlgs/dlgt. Use of a combined plot of dlgs/dlgt and ds/dlgt is made to identify various flow patterns from variable discharge tests with infinite conditions, constant rate tests in bounded aquifers, and tests involving double porosity, which can be used to further characterize the aquifer. Compared with the standard derivative method (ds/dlgt), the combined derivative analysis of dlgs/dlgt and ds/dlgt of drawdown behaviours possesses certain advantages identified as follows: (1) the plot of dlgs/dlgt is strikingly sensitive for use in unveiling differences between pumping and recovery periods in a variable discharge test; (2) storativity (S) of pumped aquifers can be accurately evaluated by the combined plot; and (3) a quantitative assessment of double porosity behaviour can also be achieved. Based on applied case studies, Lagrange Interpolation Regression (LIR) is recommended for numerical derivative calculation. Advantages and disadvantages of LIR and traditional methods are demonstrated via a selected case study. The result of the case study confirms that LIR is a preferred method for numerical derivative calculation as it can be used to effectively minimise noisy effect during derivative processes. The proposed derivative approach provides the hydrologists with an additional tool for characterizing pumped aquifers.

The diagnostic plot analysis of artesian aquifers with case studies in Table Mountain Group of South Africa

Parameter estimates of artesian aquifers where piezometric head is above ground level are largely made through free-flowing and recovery tests. The straight-line method proposed by Jacob-Lohman is often used for interpretation of flow rate measured at flowing artesian boreholes. However, the approach fails to interpret the free-flowing test data from two artesian boreholes in the fractured-rock aquifer in Table Mountain Group (TMG) of South Africa. The diagnostic plot method using the reciprocal rate derivative is adapted to evaluate the artesian aquifer properties. The variation of the derivative helps not only identify flow regimes and discern the boundary conditions, but also facilitates conceptualization of the aquifer system and selection of an appropriate model for data interpretation later on. Test data from two free-flowing tests conducted in different sites in TMG are analysed using the diagnostic plot method. Based on the results, conceptual models and appropriate approaches are developed to evaluate the aquifer properties. The advantages and limitations of using the diagnostic plot method on free-flowing test data are discussed.RésuméLes estimations des paramètres des aquifères artésiens, où la charge hydraulique est au-dessus du niveau du sol, sont largement réalisées au moyen d’essais de pompage en écoulement libre et de suivi de la remontée. La méthode de la droite proposée par Jacob-Lohman est souvent utilisée pour l’interprétation du débit mesuré au niveau de forages artésiens en écoulement. Cependant, l’approche ne parvient pas à interpréter les données d’un essai en écoulement libre réalisé au sein de deux forages artésiens de l’aquifère fracturé du groupe de la Montagne de la Table (TMG) en Afrique du Sud. La méthode du graphe de diagnostic utilisant la dérivée de l’inverse du débit est adaptée pour évaluer les propriétés de l’aquifère artésien. La variation de la dérivée aide non seulement à identifier les régimes d’écoulement et discerner les conditions aux limites, mais elle facilite aussi la conceptualisation du système aquifère et la sélection d’un modèle approprié pour l’interprétation ultérieure des données. Les données de deux essais en écoulement libre conduits en différents sites du TMG sont analysées en utilisant la méthode du graphe de diagnostic. Des modèles conceptuels et des approches appropriées sont développés sur la base de ces résultats, afin d’évaluer les propriétés des aquifères. Les avantages et limites de l’utilisation de la méthode du graphe de diagnostic pour les données d’essais en écoulement libre sont discutés.ResumenLa estimación de los parámetros de acuíferos artesianos donde la carga piezométrica está por encima del nivel del terreno, en gran medida, son realizados a través de ensayos de flujo libre y recuperación. El método de la línea recta propuesto por Jacob-Lohman se usa a menudo para la interpretación de las medidas de caudal en las perforaciones artesianas. Sin embargo, el enfoque es equivocada para la interpretación de los datos de ensayos de flujo libre a partir de dos perforaciones artesianas en un acuífero de rocas fracturadas en el Table Mountain Group (TMG) de Sudáfrica. El diagnóstico por el método gráfico usando la derivada de la tasa recíproca se adapta para evaluar las propiedades del acuífero artesiano. La variación de la derivada ayuda no solamente a identificar el régimen de flujo y discernir las condiciones de los límites, sino también que facilita la conceptualización del sistema acuífero y la selección de un modelo apropiado para la interpretación posterior de los datos. Se analizan los datos de dos ensayos de flujo libre realizados en diferentes sitios en TMG usando el método gráfico de diagnóstico. En base a los resultados se desarrollan modelos conceptuales y enfoques apropiados para evaluar las propiedades del acuífero. Se discuten las ventajas y limitaciones de usar el método gráfico de diagnóstico en datos de ensayos de flujo libre.摘要通过自流和恢复试验主要对压水头高于地面的自流含水层的参数进行了估算。Jacob-Lohman提出的直线法经常用于解译自流钻孔的流量。然而,该方法不能解译南非桌山山脉组断裂岩含水层中的自流试验数据。利用倒数率导数的诊断标绘方法适合评估自流含水层特性。导数变化不仅有助于确定水流动态、识别边界条件,而且还可以促进含水层的概念化及以后用于数据解译的合适模型的选择。对桌山山脉组不同地点进行的自流试验数据采用诊断标绘方法进行了分析。根据研究结果,开发了概念模型和适当方法用来评估含水层特性。探讨了分析自流试验数据所用的诊断标绘方法的优点和局限。ResumoA estimativa de parâmetros dos aquíferos artesianos onde os níveis piezométricos se encontram acima do nível do solo é, na maior parte das vezes, baseada nos caudais livres e em ensaios de recuperação. O método da linha reta proposto por Jacob-Lohman é muitas vezes usado para a interpretação da descarga medida em poços artesianos repuxantes. No entanto, a abordagem falha na interpretação dos dados do ensaio de caudal livre de dois poços artesianos no aquífero fraturado do Grupo de Table Mountain (GTM), na África do Sul. O método da projeção diagnóstica, que usa a derivada do recíproco do caudal, é adaptado para avaliar as propriedades do aquífero artesiano. A variação da derivada ajuda não só na identificação dos regimes de fluxo e no discernimento das condições de fronteira, como também facilita a concetualização do sistema aquífero e a seleção de um modelo adequado para a interpretação dos dados numa fase posterior. São analisados os dados de dois ensaios de caudal livre realizados em diferentes locais no GTM, usando o método da projeção diagnóstica. Com base nos resultados, são desenvolvidos modelos conceituais e abordagens adequadas para avaliar as propriedades do aquífero. São discutidas as vantagens e as limitações do uso do método da projeção diagnóstica com dados de ensaios de caudal livre.

Foreword to Special Section: Groundwater in Africa

The vast continent of Africa is a land of contrasts—from the sophisticated urban and industrial centers of Cape Town and Johannesburg to the rural bushlands and mountains of Zimbabwe and Kenya. As in the rest of the world, groundwater resources in Africa are critical to human livelihood, agricultural and industrial development, and maintenance of aquatic ecosystems. Widely available scientific works on hydrogeology in Africa have been relatively few and far between compared with the mass of publications from other parts of the world, from North America and Europe in particular. Over 900 million people live on the continent of Africa, and the range of current groundwater issues is tremendous, from subsistence water supply for basic human needs to geotechnical problems associated with mine dewatering and the infrastructure of urban construction. Likewise, the knowledge and professional skill base for studying and managing groundwater varies from crude outdated resource mapping in some areas to state-of-the-art simulation modeling in others. In assembling this collection of papers devoted to groundwater in Africa, we had two main goals. The first was to bring the technical and scientific issues present in African hydrogeology to a wider scientific audience, and possibly to encourage groundwater scientists elsewhere in the world to tackle projects and research topics relevant to the African situation. The second was to encourage African scientists and their colleagues and students to disseminate their work more broadly. Excellent, innovative