Doctoral Degrees (Institute for Groundwater Studies (IGS))

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Open Access
Analysis of a post-closure safety assessment methodology for radioactive waste disposal systems in South Africa
(University of the Free State, 2000) Van Blerk, J. J.; Botha, J. F.
𝑬𝒏𝒈𝒍𝒊𝒔𝒉 Radioactive waste in South Africa is generated through the nuclear fuel cycle and the application of radioactive materials in industry, science and medicine. The radioactive waste is presently disposed at Vaalputs in Bushmanland and Thabana at Pelindaba in near-surface disposal facilities. No strategy exists at present for the disposal of high level waste. The objective of radioactive waste management and its underlying principles is to ensure that human health and the environment are protected at all times, without imposing an undue burden on future generations. This implies that, before any long-term management strategy of radioactive waste disposal can be implemented, the impact of the disposed waste must be determined as a function of time-a procedure referred to as post-closure safety assessment. In this thesis, a methodology to perform post-closure safety assessments of radioactive waste disposal systems in South Africa and other parts of Africa is described. Not only will it contribute significantly to reassess the suitability of current waste disposal practices, but also lays the foundation for future disposal practices. The proposed methodology-an integrated approach to radioactive waste management-is aimed at: (a) ensuring the safety of the present public and future generations, (b) enhancing the public acceptance of the methodology, (c) keeping the expenditure associated with the implementation of the methodology at a minimum. The methodology recognises the interdependence between operational phase activities and the post-closure behaviour of the disposal system. It is an iterative process that considers site-specific, prospective evaluations of the post-closure phase to ensure that the disposal system will comply with internationally accepted criteria, within reasonable limits. Provision is therefore made to identify the data, design and other needs that will contribute towards the achievement of this objective. The first step in this procedure is to identify those internal and external features, events and processes that can be used to predict how radioactive material may escape from the disposal facility, along which paths will it migrate and how it may impact humans. Various conceptual and mathematical models that can be used to develop appropriate scenarios of these processes and to compare the results with site specific data are discussed in the thesis. The cost to develop a waste disposal system, the disposal of the waste and the pre- and post closure assessments of the system, or so-called nuclear liabilities, can be astronomically high. Combining the post-closure assessment of the system with the decision analysis framework discussed in the thesis can reduce these costs considerably. Post-closure assessments of radioactive waste disposal systems have in the past often been interpreted as an exercise to predict the exact behaviour of the system far into the future. However, as pointed out in the thesis this is not possible, even with the technology available today. The more pragmatic approach, advanced in the thesis, is that modem technology is able to demonstrate to reasonable members of the public that such a system will be safe. Nevertheless it is recognized that the methodology cannot be implemented without the active participation of the public. It is therefore envisaged that the proposed methodology will be implemented with the close co-operation of the public, particularly those living near the site where the disposal system will be implemented.
Open Access
Application of electroseismic techniques to geohydrological investigations in Karoo rocks
(University of the Free State, 2003-11) Fourie, Francois Daniel; Botha, J. F.
English: The possibility of using surface electroseismic (ES) methods for groundwater exploration in fractured Karoo rocks is studied by investigating the criteria of vertical and lateral resolution of surface ES data and the ES thin bed response. The ES Fresnel zones for monochromatic excitation are found to be larger than their seismic equivalents and the lateral resolution of surface ES data is consequently poorer. Seismic velocity increases with depth result in larger ES Fresnel zones and poorer lateral resolution. As in seismics, only a single Fresnel zone can be identified for broadband excitation. Higher dominant frequencies and broader bandwidth result in higher lateral resolution. Rayleigh's criterion for vertical resolution applied to ES data requires that the imbedded layer has a thickness of at least λ/2 to be deemed resolvable, where λ is the wavelength of the seismic wave under consideration. There are, however, two wavelengths to consider for ES phenomena − those of the Biot fast pressure and slow pressure waves. Since the wavelength of the slow pressure wave in saturated Karoo rocks may be a couple of orders of magnitude smaller than the wavelength of the fast pressure wave, the theoretical limit of resolution is determined by the slow pressure wave. This wave is, however, strongly dissipative and the practical limit of resolution seems to depend more strongly on the wavelength of the fast pressure wave. A simplified approach to examine the ES thin bed response suggests that imbedded layers with thicknesses smaller than λ/2, where λ is the wavelength of the fast pressure wave, may be classified as electroseismically thin. Investigations by means of a full waveform approach that simultaneously takes the influence of the different wave types into consideration, supports the above observation. The results of an ES field survey on a site where a localised fracture is known to occur, supports the idea, but does not conclusively proof, that the lateral and vertical resolution of surface ES data is insufficient to detect fractures with apertures in the millimetre to centimetre range. The influence of porosity contrasts on ES reflection is studied by examining the change in the magnitude of the reflection coefficients when varying the porosity (and all porositydependent parameters, such as permeability) of a Karoo sandstone overlain by a Karoo mudstone. Increases in the sandstone porosity results in larger reflection coefficients, and consequently, stronger reflection. The possibility therefore exists to map porosity contrasts in a certain geological environment and thus identify zones more likely to be suitable as aquifers. The possibility of using ES techniques as a non-invasive means of obtaining information on the elastic properties of the rock matrix is investigated by examining the electroseismic and magnetoseismic transfer functions at positions in boreholes. The transfer functions are dependent on various physical and chemical parameters, including the elastic parameters and none of these parameters can be determined unambiguously from the evaluation of the different transfer functions. The shear modulus of the porous frame may be estimated from measurements of the shear wave velocity in a fluidsaturated porous system. Since an approximate linear relation exists between the shear and bulk moduli of the porous frame, the bulk modulus may be estimated once the shear modulus is known. Porosity changes due to aquifer deformation may lead to detectable changes in the transfer functions. The electroseismic transfer function of the fast pressure wave is insensitive to porosity changes in consolidated material. Although the electroseismic transfer function of the slow pressure wave is very sensitive to porosity changes, this wave is strongly dissipative and is notoriously difficult to measure. The magnetoseismic transfer function is sensitive to porosity changes and is likely to yield the most useful information on aquifer deformation.
Open Access
Aquifer parameter estimation in fractured-rock aquifers using a combination of hydraulic and tracer tests
(University of the Free State, 2002-08) Riemann, Kornelius; Van Tonder, G. J.
English: Water resources in South Africa are already being stressed and the country is slowly becoming a water-scarce country. This presents a challenge to all water resource managers to ensure that the basic water needs of every South African are met. A good estimation of the aquifer parameters is the basis of managing groundwater resources and understanding groundwater flow and transport processes. Because most of the suitable groundwater resources in Southern Africa occur in fractured rock aquifers, this thesis focuses on aquifer parameter estimation in fractured rock aquifers. A guideline for aquifer parameter estimation in fractured rocks is derived, based on a combination of hydraulic and tracer tests. The methods included account for the unknown flow geometry and the resulting uncertainty by introducing the non-integer flow dimension, which can be calculated from hydraulic and / or tracer test data. The guideline includes: o New methods for conducting and analysing tracer tests accounting for non-integer flow dimension prevailing during the tests in fractured aquifers. G A new method for estimating the kinematic porosity from single-well tracer tests. • Description and comparison of the use of a three-dimensional numerical model for aquifer parameter estimation . • The software TRACER-PLAN to enable the geohydrologist to conduct effective tracer tests. Depending on the type of test and the geological structure the test setup, such as discharge rates, amount of tracer and duration of the test, can be optimised. • To simplify and unify the analysing procedure the software TRACER enables the user to choose the correct analysing method depending on the test set-up and the conceptual model of groundwater flow. Most of the analysing procedures mentioned in this thesis are included. While the approaches for estimating the hydraulic parameters from pumping test data are commonly used methods, which are adapted to the situation in Southern Africa, new approaches to analyse tracer test data are developed, which account in a more general way for the unknown geometry. A combination of hydraulic and tracer tests including analysing procedure is proposed, which enables the hydrogeologist to estimate important hydraulic and transport parameters from the results of one test rather than conducting different tests. Depending on the purpose of the investigation, two types of combined tests are developed. As minimum requirement conducting and analysing a single-well test is suggested. If an observation borehole in the vicinity is available, a multiple-well test should be conducted. The proposed methods were used in case studies with different geological settings and compared to each other. From the results of the case studies and theoretical models it can be concluded that G analytical methods for estimating the hydraulic properties of fractured rock aquifers must be used with precaution, even when the correct conceptual model is chosen, I) for more accurate estimation numerical models, preferable a three-dimensional numerical model, should be used, Q the developed methods for analysing tracer test data, using the concept of fractional flow dimension, will yield more accurate and normally higher values for the flow velocity than the common methods, using the length of the tested section and a flow dimension of2, lil the developed methods for analysing tracer test data, using the concept of fractional flow dimension, are applicable in different geological settings, o the developed method for estimating the kinematic porosity from a single-well tracer test is generally applicable in both the fracture zone and the matrix, but the accuracy depends upon the correct conducting procedure, e the validity and accuracy of the results depends mainly on the quality of the conducting procedure and the correct conceptual model for the analysing procedure. The requirement for the analysing procedure can be summarised as: • From the hydraulic test data the conceptual model should be derived, using diagnostic plots and derivatives, as integrated in the software TRACER or in other software programs such as FC and TPA • The hydraulic data should be analysed applying the approach of non-integer flow dimension, as integrated in the software TRACER. • If necessary numerical models should be used for complete parameter estimation. lil The tracer test data should be analysed, applying the approach of non-integer flow dimension, as integrated in the software TRACER. The effect of matrix diffusion on solute transport in fractured aquifers is immanent, but no existing method is able to simulate it properly. Further research in estimating the impact of matrix diffusion on the plume movement and in developing a suitable method to simulate and quantify its effect is therefore strongly recommended.
Open Access
Aquifer test interpretation with special emphasis on the drawdown evaluation for wells within fracture networks smaller than the representative elementary volume (rev)
(University of the Free State, 2002-05) Bardenhagen, lngo; Van Tonder, G. J.
Fractured aquifers are characterized by the fact that most of the water flows along fractures, faults, open bedding planes, or other geological features. These features are embedded in a matrix that has either porous nature, like in sandstone, or is almost impermeable (inert), as in the case of granite. It is often observed that in fractured aquifers the measured air lift yield is a strong overestimation of the long-term sustainable yield of the well. The explanation for this effect is that the water extracted initially is provided by a geological feature that is high yielding but limited in its extension, while the long-term sustainable yield is the response of the matrix. Such a geological feature can be among others, a single vertical fracture or a fracture network, which usually acts as a preferential flow path. Pumping tests in primary and secondary aquifers are widely used by the ground water industry because they provide important information on the reservoir and the well performance. Various researches in the oil and ground water industries have found that the presence of single preferential flow paths results in characteristic drawdown curves. However, a lack of research is encountered, when it comes to more complex fracture networks. This work investigates the behavior of drawdown curves in fracture set ups below the representative elementary volume (REV), which is defined as the smallest volume of aquifer that can be considered as a homogeneous fractured unit. Emphasis is given to the importance of a thorough diagnosis of the data to be able to adequately estimate the aquifer properties. Chapter 2 of the present work summarizes the basic knowledge on ground water flow in fractured reservoirs, where the REV, fracture connectivity, and conductivity contrast between fracture and matrix are defined and explained. Thereafter, the flow behavior in fractured media (linear, radial, and spherical) are described. This chapter ends with the review of various well and reservoir boundary effects, such as well bore storage, well bore skin, partial penetration skin, fracture skin, pseudo-skin, fracture dewatering, and reservoir boundaries. Chapter 3 gives practical advice for the planning and performance of pumping tests and stresses the necessity of time correction in the case of variable discharge rate during the test. The importance of the pseudo-skin effect originated by the presence of a single vertical fracture is highlighted. It is shown that pseudo-skin effects are the reason for the apparent dependence of the storage coefficient (S) on the distance between the observation borehole and the single vertical feature, when the common evaluation methods are used for the estimation of S. Furthermore, the radial-acting flow phase and in relation to the REV is explained. This chapter ends with the description of various diagnosis tools, which allow, among others, the determination of the flow phases from pumping test data influenced by preferential flow paths. These tools are included in the computer program Test Pumping Analysis (TPA), which was compiled under the umbrella of this thesis. It is explained that data consistency can be rapidly analysed with the comparison between drawdown and recovery data and any discrepancy must be investigated additionally. The use of straight-lines, especial plots, and curves derivatives is described. Chapter 4 presents the most important analytical and semi-analytical available solutions for the analysis of pumping test data in fractured aquifers, which are included in TPA. For each case, the mathematical solution is first described. The influence of well bore and reservoir effects are explained using TPA, based on theoretical and field examples. Special emphasis is given to the various skin analyses and to the possible misinterpretation of drawdown curves. The solutions presented are: double porosity model of Moeneh (1984) • single vertical fracture with infinite conductivity and finite extent of Gringarten et al. (1974) • single vertical fracture with finite conductivity and finite extent of Cinco-Ley et al. (1978) • single vertical dike with finite conductivity and infinite extent of Boonstra & Boehmer (1986) • bedding plane fracture with infinite conductivity and finite extent of Gringarten & Ramey (1974) • generalized radial flow model for fractured reservoirs of Barker (1988) Chapter 5 investigates more complex fracture situations with help of numerical modelling based on the Darcian law. Synthetic pumping tests are simulated and their drawdown behavior is analysed. The single vertical fracture case is first computed to ensure that the model set up leads to the analytical and semi-analytical solutions of Gringarten et al. (1974) and Cinco-Ley et al. (1978), respectively. To investigate the influence of wider fault zones, which are assumed as a homogeneous fractured zone, faults with increasing width are modelled. It is found that: • for large storage capacities and finite conductivity, the drawdown at early time shows a radial-acting flow phase within the fault, which could be easily misinterpreted as double porosity. However, this effect occurs most likely under unconfined conditions The model is then modified to include parallel vertical fractures. It is found that: • parallel vertical structures with infinite conductivity have no influence on the drawdown at the well • parallel vertical structures with finite conductivity show minor influences at early time, if the dimensionless relative separation Sr (Sr = df/xf) is less than 0.125 Thereafter, the model is modified to represent a crossed fracture case and a bend fracture case, both vertical and with infinite conductivity. The computed drawdown differs significantly from the drawdown measured in the single straight fracture. It is found that: • this drawdown is comparable to that obtained with the uniform flux solution of Gringarten et al. (1974), although the influx along the fracture is not uniform. However, the authors mentioned that some field data from hydraulic fracturing fit better to the uniform flux solution. The results of this work give reasons to believe that such field data are attributed to more complex fracture networks similar to those studied here. The horizontal bedding plane case is also investigated. First, the model is run to compute the infinite and finite flux solutions from Valkó & Economides (1997). The modelled curves fit adequately the data for their solutions, although a labelling error in the published data is identified. Further, the influence of the fracture geometry is analysed. It is found that: • horizontal penny-shape fractures and square features with equivalent influx area have the same drawdown • rectangular horizontal features have a significant influence on the drawdown behavior The investigation of parallel bedding planes shows that: • the shape of the drawdown curve in parallel horizontal fractures is equivalent to that of the single horizontal bedding plane. Therefore, without additional on-site investigations (e.g. fluid logging or flow meter measurements) it is impossible to determine whether the drawdown belongs to a single fracture or to a series of parallel features • The analysis of drawdown curves produced by parallel horizontal fractures using type curves for single horizontal fractures leads to an over estimation of the fracture radius. This effect is important among others, for the design of protection zones Finally, intersections of a single vertical fracture and a single horizontal bedding plane are modelled. It is found that: • the obtained drawdown curves could be misinterpreted with drawdown curves of single cases. Therefore, it is concluded that additional information is necessary to correctly identify the geological set up. This issue is highly important for both the design of well protection zones and the estimation of the transport time
Open Access
Biobarrier formation for hydraulic control in groundwater remediation in South Africa
(University of the Free State, 2006-11) Van Wyk, A.; Usher, B. H.; Van Heerden, E.
Abstract not available
Open Access
Characteristics of fluid electrical conductivity (FEC) profiles associated with a contaminant plume in porous and weathered basement aquifer systems
(University of the Free State, 2023) Moleme, Malefa Florence; Gomo, Modreck
The fluid electrical conductivity (FEC) profiling method has been commonly applied to aid in the compilation of site-specific conceptual models and understanding of the subsurface environment. Although research has recently been conducted to help improve the knowledge and understanding of the evolution of FEC profiles under natural and saline contaminated environments within the fractured-rock aquifer system, a research gap still exists for such studies in other aquifer types. The type of aquifer system plays a significant role in determining the migration patterns and behaviour of contaminants. Therefore, it is expected that the evolution of FEC profiles in different aquifer systems will vary, and this needs to be understood. It is against this background that this research aimed to investigate the behaviour of FEC profiles associated with a saline contaminant plume in typical unconfined porous and weathered basement aquifer systems, using laboratory-based aquifer models. This was done to improve the conceptual understanding of contaminant migration within these aquifer systems, which will essentially improve the interpretation of their FEC profiles. To achieve this, two physical models were developed in the laboratory to represent an unconfined porous aquifer system and a weathered basement aquifer system. The performance of the models was evaluated and tested, and subsequently used to investigate the progression of FEC profiles associated with a saline contaminant plume. The outcome of the laboratory tests was also verified in the field. This study also explored the effects that the distance of a source from a monitoring point would have on the shape of FEC profiles. Unlike previous studies which conducted the FEC profiling technique under induced groundwater flow, this study investigated the efficiency of a non-invasive approach of applying the method under natural gradient conditions. From the analysis of profiles obtained within the two simulated aquifer systems conceptual profiles were developed. Within the unconfined porous aquifer system, FEC profiles recorded from the borehole located closer to the source were notably different from the FEC profiles recorded from a borehole positioned further away from the source, thus it was evident that the distance of a monitoring point from the source influenced the orientation of the plume, and ultimately the resulting FEC profile. This brought to light the phenomenon of “plume orientation”. The orientation of the plume is usually disregarded in groundwater models and assessments, however this research showed that it is an important aspect which can be used to assist with FEC data interpretation and contaminated site characterisation studies. The orientation of the plume was strongly influenced by the magnitude of the forces acting upon it, primarily the gravitational and advection force. Closer to the source, the plume took on a vertical to sub-vertical orientation, whereas as the plume continued to migrate further away from the source it aligned with the flow lines of the system which resulted in a horizontal orientation. From the analysis of the weathered basement aquifer system two distinct signatures were identified and conceptualised: the low FEC profile and the elevated FEC profile. The low FEC profile not only represented a profile captured under natural conditions in the absence of contamination, but also represented a profile that would be observed when the majority of the contaminant has passed the borehole and the system was in the process of re-establishing initial conditions. It had three distinct zones: the weathered zone, transition zone, and the impermeable zone. The elevated FEC conceptual profile was associated with contaminated groundwater conditions within the weathered basement aquifer system. It had two distinct zones: the weathered zone and the impermeable zone.
Open Access
A critical review of recharge estimation methods used in Southern Africa
(University of the Free State, 2003-12) Bean, John Alexander; Van Tonder, G. J.
English: A new stable isotope-based technique, the Modified Amount Effect (MAE) Method, was developed during this investigation. This technique provides insight into episodic recharge processes by estimating the proportion of preferential pathway-to-matrix-derived flow entering an aquifer, and the amount of rainfall required to initiate recharge via the respective flow paths. Significantly, the proportion of bypass flow can be determined without undertaking expensive and time consuming unsaturated zone studies, both factors often of primary concern when undertaking recharge investigations in developing countries. Four recharge thresholds can be identified using the MAE Method; the low and high recharge thresholds that must be exceeded before recharge occurs via preferential pathways or the matrix, respectively. These represent threshold limits, the low value only of importance following successive months of wet weather, the high value representing the rainfall that must be received to restore an aquifer system to equilibrium after prolonged dry spells. Once these thresholds are known, the recharge history of a site can be modelled using available rainfall data by adapting the Cumulative Rainfall Departure (CRD) Method. An important finding of modelling undertaken during this investigation is that in those semi-arid to arid areas where most recharge water enters the aquifer via the matrix, the period of time that elapses between successive rainfall events that exceed the matrix recharge threshold often extends to scores of years. This has significant resource management implications for much of the region as it indicates that the current approach of basing allocations on average recharge estimates is only justified if sufficient groundwater is available for use over the entire period between recharge events. In terms of recharge estimation, the Stable Isotope (SI) Method was found to return comparable results to the Chloride Mass Balance (CMB) Method in both wetter and drier inland areas of South Africa. However, both the SI and MAE Methods were found to be sensitive to the recharge history of the site, the returned recharge estimate significantly higher when calculated immediately after recharge via the matrix had occurred. This is not to say that these estimates were wrong (indeed they were representative of site recharge processes at the time of sampling), but that rainfall in the months prior to sampling should be considered. In general though, sampling should be undertaken near the end of the dry season, which in the summer-dominant rainfall areas of Southern Africa is between September and November (allowing for a 30 to 60 day lag time between rainfall and subsequent recharge). While the geological and geomorphological limitations of the CMB Method must be clearly understood before applying the technique, it does have application within many fractured rock terrains. On a regional scale, fractured rock aquifers are commonly regarded as equivalent porous mediums for modelling purposes, a necessity given the significant variations in porosity, hydraulic conductivity, and storage that occur between adjacent areas. Thus, even where longterm water level data is available, the hydraulic conditions that contribute to the observed water table response at a given site following recharge represent an average for the area surrounding a given borehole. The CMB Method negates the need for measuring or estimating these hydraulic parameters, as it already represents a long-term average of recharge. This is not to say that water levels should not be taken, but rather that recharge calculated using water balance methods be checked using the CMB Method in those areas completely overlain by a porous unsaturated zone of significant thickness. Indeed, the comparison of results obtained using multiple estimation techniques is recommended during all recharge-based investigations, whether conducted in fractured rock or porous environments.
Open Access
The development and evaluation of hydrogeochemical prediction techniques for South African coalmines
(University of the Free State, 2003-05) Usher, B. H.; Hodgson, F. D. I.
Abstract not available
Open Access
Development of a decision tool for groundwater management
(University of the Free State, 2007-05) Dennis, Stefanus Rainier; Van Tonder, G. J.
English: Water in South Africa is becoming a scarce and important resource and therefore has to be managed and protected in order to ensure sustainability, equity and efficiency. The SAGDT is designed to provide methods and tools to assist groundwater professionals and regulators in making informed decisions concerning groundwater use, management and protection, while taking into account that groundwater forms part of an integrated water resource. The SAGDT is spatially-based software, which includes: • A GIS interface to allow a user to import shape files, various CAD formats and georeferenced images. The GIS interface also provides for spatial queries to assist in the decision-making process. The GIS interface contains default data sets in the form of shape files and grid files depicting various hydrogeological parameters across South Africa. • A risk assessment interface introduces fuzzy logic based risk assessments to assist in decision making by systematically considering all possibilities. Included risk assessments relate to the sustainability of a groundwater resource, vulnerability of an aquifer, pollution of a groundwater resource (including seawater intrusion), human health risks associated with a polluted groundwater resource, impacts of changes in groundwater on aquatic ecosystems and waste site impact on an area. • Third-party software such as a shape file editor, an interpolator, a georeference tool, a unit converter and a groundwater dictionary. • A report generator, which automatically generates documentation concerning the results of the risk assessment performed and the input values for the risk assessment. • A scenario wizard for the novice to obtain step by step instructions in setting up a scenario. All case studies presented in this thesis is available in the scenario wizard. • The SAGDT allows problem solving at a regional scale or a local scale, depending on the problem at hand. This thesis discusses the origin, research, development and implementation of the SAGDT. Case studies are included to demonstrate the working of the SAGDT. They include: • Vunerability (Fish River Lighthouse) • Waste Site (Bloemfontein Suidstort) • Sustainability (De Hoop) • Mine (Van Tonder Opencast) The SAGDT relies heavily on the expertise of hydrogeologists, assumptions and approximations of real world conditions. Together with the heterogeneities present in groundwater systems it is impossible to guarantee the accuracy of the methodologies and this must be taken into consideration.
Open Access
Development of a numerical model for unsaturated/saturated hydraulics in ash/brine systems
(University of the Free State, 2010-01) Menghistu, Mehari Tewolde; Botha, J. F.
English: Vast quantities of coal combustion residues (ash) and effluents are produced simultaneously in the coal processing facilities of South African parastatals, Eskom and Sasol. The handling and disposal of saline effluents is a difficult and complex problem. The current practice used by these parastatals is to co-dispose the effluents with the ash in landfills; so-called ash dams. Although this practice provides a potentially elegant approach, at least from the viewpoint of the generator of both the ash and effluent, the co-disposal of ash and brine in a landfill could have dire consequences on the environment of the landfill site. This applies in particular to the release of environmentally deleterious and toxic constituents of the ash into the air, soil, surface and groundwater which can lead not only to environmental and land-use problems, but also jeopardize the health of organisms living in the surrounding ecosystem. The question therefore arises as to how Eskom and Sasol better manage their ash dams, to not only satisfy all legal requirements and possible pressure from social awareness groups, but also more importantly, prevent, or at least limit, pollution of the natural environment. The present investigation arose from a request by Eskom and Sasol to provide them with a detailed proposal for a framework with a view to increasing the competency of both organizations in the management of the co-disposal of ash and brine and the dissemination of knowledge with regard to the impacts of the sites. Two sites were selected by the organizations for this purpose: the Tutuka Power Station and the Secunda Synthetic Fuel Plant. The application of geohydrological models to assess the behaviour of a waste disposal site has historically often been viewed as an attempt to predict the future behaviour of the site. However, this would require information on relational parameters and known interactions whose behaviour far into the future cannot be determined with certainty. A geohydrological model should therefore never be viewed as an attempt to predict the future of a given waste site, but rather as an aid to assess how effectively the site is managed and controlled. The best way to achieve this is to investigate the waste site systematically, preferably utilizing wellestablished and accepted international methodology. Unfortunately, at this time, no documents exist that describe such a methodology, its implications and the steps necessary to implement it in practice in a way that can also be understood by interested members of the public. A two-dimensional transient model for flow through saturated/unsaturated porous ash dump media has been developed. This model numerically solves the governing partial differential equations, which are highly non-linear. The model code uses quadrilateral finite elements for the geometrical assembly: the bilinear Galerkin interpolation for the spatial integration, and the Gaussian elimination for the solution of the resulting matrix equations. In addition to the usual constant-flux and constanthead boundary conditions, the code is capable of applying pressure-dependent boundary conditions at the ground surface. Thus, infiltration into, or seepage from, this surface may be simulated. Each element may be assigned different material properties that allow the investigation of layered geologic formations. The results discussed in this work are all based on conclusions derived from a generic model for the Tutuka ash dumpsites. While such a model can provide valuable insight into the physical behaviour of such a study area, it can never replace field observations. Field observations and models must complement each other. An attempt was also made to provide guidelines for the development of site-specific data investigation methodology, based on the ISAM Safety Assessment Methodologies for Near Surface Disposal Facilities.
Open Access
Development of artificial neural network of mine dewatering
(University of the Free State, 2017) Ngoie, Sage; Fourie, Francois
Open pit mines often experience problems related to groundwater inflows. To perform mineral extraction in safe conditions with high productivity, it is essential to have dry working conditions. For this reason, the groundwater table is often lowered below the elevation of the floors of the pits by using various dewatering schemes. Numerical groundwater models are powerful tools that can be used to simulate the behaviour of aquifers during dewatering operations. However, these models typically require a lot of geohydrological data which are often expensive and time-consuming to collect. When geohydrological input data are limited, artificial neural networks (ANNs) provide an alternative way of predicting the behaviour of the groundwater system under dewatering conditions. ANNs can simulate complex systems, and have been used to provide simple and accurate solutions to problems encountered in many disciplines of the earth sciences.This study investigated the possibility of predicting the impacts of pit dewatering on the aquifer system in the vicinity of open pit mines where geohydrological inputs are limited, using ANNs. First, the performance of the ANNs in predicting hydraulic head responses was evaluated by using synthetic datasets generated by a numerical groundwater model developed for a fictional mine. The synthetic datasets were then used to both train and evaluate the performance of the ANNs. The ANN found to give the best predictions of the hydraulic heads had an architecture of 2-6-1 (input-hidden-output layers) and was based on the hyperbolic tangent transfer function. This network was selected for application to real open pit mines. The selected ANN was next used to predict hydraulic heads at a number of piezometers installed at two open pit mines in the Democratic Republic of the Congo. The only input to the ANN was the recorded hydraulics heads and the time of recording. A portion of the real dataset was used to train the ANN, while the remaining portion was used to evaluate the performance of the ANN in predicting the hydraulic heads. The results of the performance analyses indicated that the ANN successfully predicted the general behaviour of the aquifer system under dewatering conditions, using only limited input data. The results of this investigation illustrate the great potential of using ANNs to predict aquifer responses during dewatering operations in the absence of comprehensive geohydrological datasets. Since these networks recognise patterns in the training datasets without considering the underlying physical principles that govern the processes, the responses of complex systems that are dependent on numerous parameters may be predicted.
Open Access
DNAPLs in South African fractured aquifers : occurrence, fate and management
(University of the Free State, 2008-12-12) Pretorius, Jennifer Anne; Usher, Brent H.
English: The potential for the widespread contamination of groundwater by Dense Non-aqueous Liquids (DNAPLs) in South Africa is substantial, because of the extensive production, transport, utilisation, and disposal of large volumes of DNAPL chemicals. There are a great number of potential sites where DNAPLs may have been released to the subsurface in varying quantities. A basic understanding of the nature and occurrence of groundwater in South Africa aquifer systems is a prerequisite for assessment, monitoring and management of DNAPL contaminated sites. The physical properties of an aquifer that have the greatest impact on the fate and transport of DNAPL contaminants, are the flow rate and flow mechanism present, and the hydraulic conductivity. The major South African aquifer systems have been classified in relation to the dominant flow mechanisms and flow characteristics. The majority of the utilised South African aquifers can be classified as intergranular fractured aquifers. From the results of this study, which included laboratory experiments and the controlled injection of a surrogate DNAPL in the field, it is clear that preferential pathways in fractured rock will determine the flow path of any DNAPL phase contamination. Aqueous plumes of DNAPL contaminants will also be influenced by these pathways (dissolving and or diffusing from the NAPL into the water in fractures and matrix) which can result in spatially variable aqueous plumes in these aquifer systems. The local variations in fracture strike and dip play a far more important role in DNAPL flow than the regional fracture dip or groundwater flow directions. Natural attenuation processes are important consideration under South African conditions. Relatively high organic carbon in the shallow zones assists in retardation of the organic contaminants, while the large unsaturated zone, arid climate and high temperatures leads to significant loss of contaminant mass through volatilization. Although the National Water and Environmental Acts of South Africa are very clear on prevention of pollution to, and management of water resources, no guidelines exist on how to deal with DNAPL contaminated sites. Recommendations have been made relating to the regulations that are required for: · Site assessment · Sampling and monitoring · Implementation of monitored natural attenuation.
Open Access
Environmental geochemistry of the Waterberg coalfields
(University of the Free State, 2015-07) Deysel, Lore-Marie; Vermeulen, P. D.
English: The Waterberg Coalfields represent the last area in South Africa which contain large quantities of coal resources. The most important issue with economic growth and development is to take measures to minimise the impact on the environment. With the ever increasing demand for energy, the demand for continuous mining of coal increases and thus expands into areas not previously mined. The by-products of coal mining and the production of electricity from the coal leaves an environmental footprint. For this footprint to be minimised, all the risks associated with mining should be available and understood. Since there are still large coal deposits in the Waterberg which can be mined by opencast mining, the risk of the geological units still to be mined was assessed. Acid generation due the oxidation of mainly pyrite is a source of contamination that can impact the groundwater and to a lesser or no extend the surface water from the mining facilities. Another environmental risk is air pollution from mines, includingthat of Medupi Power Station which will commence shortly. It is a very dry area and thus the aquifers must be protected as far as possible. In this study area,coal is mined by the opencast method from the Karoo formation There currently only one active coal mine in the study area, namely the Grootegeluk mine which started operation in 1980. Samples were collected from exploration boreholes in all three areas in the region (partly weathered, Full succession and Middle Ecca). Acid-Base Accounting was done on all 800+ samples and it indicated that the interburden and discards contained the highest AMD potential while the overburden had the highest buffering capacity. The impact is clear on some of the monitoring boreholes on the site. Unlined facilities lead to the contamination of the aquifers to a point where the pH of the water is acidic. The monitoring boreholes close to the ash dump is affected by a raise in the total dissolved solids due to calcium and sulphate leaching from the ash. To get the total impact or toxicity potential of samples, it is necessary to identify the elements that are available in the different wastes that can leach into the environment. All the risks associated with the different geological units should be known so that the best decision and planning of waste facilities for future extensions can be made, and so that the negative footprint on the environment can be minimised.
Open Access
Establishing geobotanical-geophysical correlations in the north-eastern parts of South Africa for improving efficient borehole siting in difficult terrain
(University of the Free State, 2007-11) Meulenbeld, Paul Martin Peter Bernard; Van Tonder, G. J.
English: The determination whether certain botanic species can be associated and linked to the existence of groundwater resources by considering the relationship of these species with certain lithologies has been studied in the present research with the aid of geophysics, geology, soil quality analysis (geochemistry), aerial photographs and proven borehole records. Ordinary scientific groundwater exploration makes use of methods such as aerial photographic interpretation, geological modelling of the area under investigation by making use of published geological maps and field reconnaissance work, geohydrological concerns and geophysical data interpretation. The scientific approach can further be enhanced by incorporating other scientific fields such as botany and soil science. Any botanical species is a living organism that requires nutrients to function and live. Nutrients are derived from the soil wherein it grows, while soil is a weathering component of the original underlying rock. Differences in soil can be attributed to changes in the composition or type of the natural rock. By including soil and botany in groundwater exploration, the chances of success are increased. Numerous case studies are presented to illustrate that certain botanical species prefer to grow in nutritious places derived from weathered rock, where this rock acts as a groundwater resource. Soil samples taken from these places indicate different soil quality compositions when compared to the surrounding common veld. This phenomenon is representative throughout the entire study, representing various geological formations. The presence of alien rock formations in a fairly homogenous geological environment is indicated by means of geophysical profiles and soundings. Existing boreholes, whether at or removed from geobotanical communities, indicate the importance of such communities since they hint to the keen observer the likely presence of groundwater at such a community. The current study presents aquifer yields obtained at geobotanical communities and the average yield of aquifers in the same geological environment, where most boreholes are drilled without the incorporation of geobotanical indicators. Lastly, the value of soil quality samples is discussed with a preference for CEC-values to indicate geobotanical communities and/or the presence of groundwater. The statistical treatment of the data indicates distinctively that statistically significant regressions exist between geology, geophysics and groundwater as the explanatory variables and CEC and geobotany as the dependent variables.
Open Access
Estimation of episodic groundwater recharge in semi-arid fractured hard rock aquifers
(University of the Free State, 2010-08) Van Wyk, Ettienne; Van Tonder, G. J.; Vermeulen, P. D.
The semi-arid regions of southern Africa cover large portions of settled land where domestic and agricultural activities depends on isolated groundwater systems replenished by irregularly occurring rainfall events. Southern African rainfall patterns are regulated by the annual oscillation of winter-summer weather systems and most of all, abrupt changes in regional atmospheric patterns, which may result in either wet/dry cycles. Given the highly differential hydro-climatic conditions and hydrogeological environment in semi-arid regions, effective groundwater recharge events are episodic in nature and largely occur once in every five years. Sustainable, medium-term management of local groundwater resources requires dynamic hydrological information to ensure a healthy supply-demand balance; thus requiring dedicated hydrological monitoring. High-level monitoring programmes on a few experimental sites have produced localised hydrological data, which illustrate how erratic groundwater resources are replenished. For many years, it was postulated that groundwater resources were recharged every time the total annual rainfall peaks a certain threshold. This postulation may hold in humid regions, but surely not elsewhere in the drier parts of South Africa. Semi-arid regions portray a flattish regional landscape with occasionally elevated parent rock windows and mountainous regions. Soil cover is restricted to low-relief areas, and lacks the thick mature soils distinctive of the humid areas. Fractured hard rock windows with very little soil cover represent potential groundwater recharge terrains, allowing recharge-producing surplus rainfall to infiltrate directly into the underlying aquifer. The hydrogeological conditions of hard rock terrains in the same-arid environment do vary in terms of the rock types and their response on weathering processes. Nonetheless, an array of joints and fractures running from ground surface into the SZ represents fast and effective pathways when episodic high rainfall events occur. Mature soil/regolith profiles in plain areas enhances surface run-off and support local floods in rivulets where riparian vegetation and open-water evaporation intercepts most of the available bank storage and depression recharge. Atmospheric moisture is, in principal, generated by warm, evaporating maritime waters, and is therefore marked by its hydrochemical signature. This signature changed abruptly during its continental migration, and finally manifests as cloud water. Winter rainwater specifically demonstrates the impact of oceanic aerosols, hence characterized by a prominent NaCl composition. Summer rainwater is a diluted version due to continental rainfall/evapotranspiration events, and is transformed by anthropogenic airborne substances peaking during the late-winter months. The hydrogeochemical composition of rainwater is therefore quite diverse, and needs logic monitoring to understand its seasonal cyclic oscillation. Short-term hyetograph observations report episodic rainfall events, occurring mostly over the January-March period of wet hydrological cycles. These are spaced over a period of 4 to 8 days, of which at least one rain event exceeds ~45 mm, associated with a rain-rate intensity of >1.5 mm·h-1. Extraordinary depleted rainwater hydrochemistry and isotopic compositions are associated with these rain weeks, which are significantly different from normal seasonal concentrations. Wet Cl− concentrations during these high rainfall periods are almost an order of magnitude lower than the average annual values. Hyetograph-hydrograph sets confirm that extraordinary groundwater recharge occurs as the result of episodic rainfall events. Hydrogeochemical profiling in the upper section of the unsaturated zone verify the presence of different compositions which probably indicate different modes of recharged rainwater percolation in fractured, hard rock terrains. Hard rock profile sections below the rebound water table interface containing almost 50% less Cl− than country-wide background values of ~40 mg·H-1. Isotopic compositions in a typical rain week period report similar depleted concentrations and resemble a prominent amount effect. Such depleted rainwater is merely linked to specific seasons, for example the rainfalls of 2003-2004, 2005-2006 and 2007-2008 hydrological years can be clustered as high rainwater input periods with notable lighter isotopic compositions; around - 7.5‰ M18O, -41‰ M2H. The fact that most of the two (2) meter vertical profiles reported relatively negative isotopic compositions (-8‰ M18O, -44‰ M2H), indicates a high probability of preferential recharge with pristine rainwater with even more negative isotopic composition. Direct groundwater recharge estimations from local, short-term rainfall and groundwater rebound stage hydrochemical data proposes a recharge value <2% in most flat lying, semi-arid regions. Although fractured hard rock terrains are isolated, it allows in the order of 4%, where as local mountainous areas are high at 14%; obviously enhanced by orographic rainfall development. Recharge on dolomitic terrains are highly variable due to diverse ground surface conditions, and may vary between 6% in flat lying plains to 13% in mountainous regions (Kuruman Hills at Kuruman). Groundwater recharge varies significantly spatially. The control by prominent soil/regolith cappings is that high that establishing realistic recharge figures for a particular area, will require a dedicated soil mapping programme to identify direct recharge terrains.
Open Access
Estimation of representative transmissivities of heterogeneous aquifers
(University of the Free State, 2011-11) Steyl, Gideon; Van Tonder, G. J.
English: The study describes the effect of calculating a generalised mean transmissivity or hydraulic conductivity value for a region or aquifer system as it pertains to South Africa. Resource determination of an area is usually driven by the determination of the bulk flow parameters, such as hydraulic conductivity and storativity values. At this stage a decision is usually made on the basis of either maintaining the area under natural conditions (no pumping), or an abstraction (pumping) scenario is envisaged. In both instances water levels, hydraulic testing and distribution of the water resources (aquifer) are required. Since it is not possible to evaluate the total area for these parameters certain assumptions have to be made such as that an average bulk flow parameter for an area can be determined. In wide-ranging situations a simple average of observation points is assumed to be sufficient. A systematic research approach was followed in which a three-step process was used to evaluate methods of calculating these mean values. In the first instance a conceptual model approach was used, and all bulk flow parameters were generated by means of matrices to represent the natural system. Three typically employed mean values (arithmetic, geometric and harmonic) were calculated for two different dimensional matrices, i.e., N x N (N = 100 and 1000) with different hydraulic conductivity zones. In addition the relative difference between these hydraulic conductivity zones were steadily increased to mimic observed parameters in the field, i.e. typical hydraulic conductivity of shale (K = 0.01 m/d) versus a fracture zone (K = 100 m/d). In all instances the harmonic mean performed the best and as the number of sample sets were increased, a reduction in mean values were observed. As part of the conceptual model approach, two typically encountered scenarios were investigated, i.e. natural flow and forced gradient conditions. Under these two scenario conditions the harmonic mean performed the best to estimate the actual observed hydraulic conductivity value. Secondly, case studies were presented which highlighted the influence of sample size on observed parameters. Additionally, the effect of the differences between the low and high hydraulic conductivity zones on the calculated mean value as a function of sample size, was also reported. In all of these case studies the harmonic mean was the closest in approximating the observed hydraulic conductivity. It is evident from this section that the number of host rock (formation) hydraulic conductivity values plays a critical part in the mean value calculation since it is general practice in South Africa not to report low yielding borehole hydraulic test values. In the third step, the results were discussed in the context of a more general approach to the problem of calculating a regional mean hydraulic conductivity of transmissivity value. The estimation of representa-tive transmissivity values were discussed as seen from a stochastic modelling perspective as well as from the deterministic point of view. A comparison between main stream groundwater and oil industry specialists were noted in which both groups share the fundamental training but differ on the methodology of determining the observed transmissivity values. The impact of horizontal heterogeneities and different fracture networks was discussed and the influence these features have on the actual transmissivity value obtained, i.e. the influence of internal boundaries on hydraulic test data. Scale effects were also addressed from a regional perspective, with a focus on apparent scaling and the actual regional transmissivity value which should be obtained. The findings of this study are that in essence using geostatistical methods are not advised if regional transmissivity values are required from a South African perspective. The reason behind this statement is that the distribution of transmissivity values in an area does not follow the basic precepts that are required for these methods to work. In general the values are discontinuous in distribution and statistically skewed. Furthermore, the presence of transmissivity areas or points that differ significantly in magnitude, i.e. transmissivity values which differ by more than two orders, can be located within one meter from each other. The explanation of this phenomenon is the presence of dolerite dykes, which create baked-fractured zones with exceptionally large transmissivity values compared to the extremely low transmissivity ranges of the surrounding country rock (shales, mudstone and siltstone). In addition, the lack of data concerning low-yielding or “dry” boreholes is a major source of concern since it influences the calculated mean value to a high degree.
Open Access
Evaluation of the vulnerability of selected aquifer systems in the Eastern Dahomey basin, South Western Nigeria
(University of the Free State, 2015-01) Oke, Saheed Adeyinka; Vermeulen, Danie; Gomo, Modreck
This study aimed to evaluate the vulnerability of the shallow aquifer systems of the Dahomey Basin and formulate a simple vulnerability method with which data limited areas (which include the shallow unconfined aquifers in the Dahomey Basin) can be predicted. The Dahomey Basin is a transboundary aquifer which extends from Ghana to the western parts of Nigeria. The study covered the eastern section of the basin. The methodological approach involved a source–pathway–receptor vulnerability model. The Dahomey Basin was characterised through the geophysical, hydrological, litho-geochemical and hydrogeochemical approaches. The geology of the basin includes sedimentary rock types of sandstone, shale, limestone, alluvium conglomerate and the formations which are composed of sand, silt, clay, laterite and gravel. The geophysical study, which mainly aimed to estimate the depth-to-water table, identification of strata and vadose zone thickness, revealed topsoil, sandy clay, dry porous sandstone, conglomeratic sandstone, limestone and alluvium as the major lithological units in the basin. Geo-electrical curve types revealed an overlying multilayered rock. The vadose zone characterisation, which is the pathway through which contaminants infiltrate, aimed to determine the lithological properties which dictate the travel time of water. This was achieved by determining the hydraulic conductivity of the vadose lithology in the laboratory. Other important parameters such as grain size, porosity, shapes, textural classification and clay types were examined for their attenuation capacity. The hydrogeochemical investigation involving the collection and analysis of water samples from the hand-dug wells and shallow boreholes during the rainy and dry season was aimed at monitoring the groundwater quality of the basin. Ca-Mg-Cl water types and Na-K-Cl water types were delineated. Bacteriological examination of the shallow water reveals the presence of E.Coli, Heterotrophic bacteria and Salmonella/ Shigella. Precipitation which is a component of groundwater recharge ranged between 1 200–1 800 mm from the northern end to the southern end of the basin, respectively. Groundwater level were measured, monitored and average water level were delineated for the formations of the Dahomey Basin. The proposed RTt vulnerability method was applied to evaluate the groundwater vulnerability of the Dahomey Basin. The RTt method is an intrinsic physically based vulnerability method based on the concept of groundwater recharge from rainfall and travel time within the covering lithology over the aquifer. Travel time is the infiltration derived from multiplication of the slope and thickness of the vadose zone divided by fluid velocity. The fluid velocity is derived from the division of hydraulic conductivity by porosity. RTt method application results for the Dahomey Basin were presented on the RTt vulnerability map. The RTt vulnerability map was classified from very low vulnerability (12) to very high vulnerability (100). The RTt vulnerability results for the Dahomey Basin showed 18% of the areas classified as very high vulnerability, 7% of the areas classified as high vulnerability, 64% of the areas classified as moderate vulnerability and 10% of the areas classified as low vulnerability. The compared vulnerability maps of the RTt method and those of the DRASTIC, PI and AVI methods, showed similarities between the RTt method and the AVI and DRASTIC method, respectively. Areas classified as high vulnerability by these methods showed very shallow protective covers, high precipitation and porous aquifer materials, while areas classified as low vulnerability areas include thick protective cover, reduced rainfall, higher slope and higher depth-to-water. The RTt vulnerability map was validated with the hydrochemical tracer using chloride, DO and microbial loads as vulnerability indicators. This study has formulated an RTt method that can be used to predict the vulnerability of shallow unconfined aquifer systems, a key component in groundwater management. The major advantage of the RTt method is the use of less number of parameter to assess groundwater vulnerability. The method has been applied to investigate the regional aquifer of the Dahomey Basin and can be used to predict the aquifer vulnerability of similar basins across Africa with limited data.
Open Access
Experimental determination of rock hydrological properties using elastic parameters
(University of the Free State, 2007-11) Du Preez, Michael; Van Tonder, G. J.
English: As groundwater becomes increasingly vital as a viable source of fresh water in arid or remote areas, where surface water supplies are insufficient to sustain life, agriculture and industry, it has become important to accurately estimate, manage and monitor this valuable resource. Much has been done to improve the management of this precious resource by the development of numerical models that give a realistic estimate on how groundwater reserves will react to changing circumstances in groundwater conditions. The accuracy of these predictions is however limited to the effective accuracy of the predictive model, which in turn relies on accurate data for all the variables which will affect the flow of groundwater. This thesis presents a method to determine hydrological parameters of a rock sample by measuring its elastic parameters, using non-destructive ultrasound methods. This is done in two ways; • The first of these is call the time of flight method. This method measures the compressive and shear wave velocities of the rock, by inducing an ultrasonic pulse into one side of the sample and measuring the time it takes the pulse to travel through the sample. The travel times are then converted into compressive and shear wave velocities, which in turn are used to determine the bulk modulus and shear modulus of the sample. • The second method is to use resonant ultrasound spectrography, which measures the natural resonance frequencies of a rock sample induced by an ultrasonic frequency sweep. These resonance frequencies are then analytically verified against the bulk modulus and shear modulus of the rock sample determined by the time of flight method. Both of these methods use apparatus which clamp a cylindrical rock core sample between two sets of ultrasonic transducers. One set of transducers produce compressive ultrasonic waves and the other produce shear ultrasonic waves. An analogue to digital converter is used to read the changing voltage levels in the transducers, induced by the ultrasonic pulse travelling through the sample or the resonant vibrations induced by the ultrasonic frequency sweep in the sample. Once the rock samples elastic parameters are known they are applied to equations which related hydrological parameters to the samples elastic parameters. The resultant hydrological parameter values can then be determined.
Open Access
A generic assessment of waste disposal at Douala City practice, prinicipals and uncertainties
(University of the Free State, 2013-01) Atangana, Abdon; Botha, Joseph François
English: One reason why groundwater, so often constitutes the main source of drinking water in many cities and towns around the world, is because it is frequently present in sufficient quantities at the point of demand. However, this seemingly advantage may sometimes be its greatest disadvantage, especially in situations where the groundwater occurs at shallow depths and the area overlying the aquifer is populated densely. This problem is particularly relevant in the present technological age with its vast quantities of waste that is often disposed in an uncontrolled manner. Such a situation occurs at Douala the economic capital of Cameroon in central Africa. The city not only host more than 80% of industries in the country, but also has the largest urban population of approximately 3 000 000 with a population density of approximately 350 persons per square kilometre, which continue to increase at a rate of approximately 120 000 migrants per year from the rural areas, while the groundwater level is very shallow and may sometimes rise above the soil surface, especially during floods, which occur not too infrequently. Although the pollution problem is not restricted to groundwater as such, it is aggravated here, because of the ancient belief that wastes are safely disposed of, if buried below the earth’s surface. It took disasters like Love Canal and the Price Landfill to discover the detrimental effects that this practice may have on the population living on or near polluted aquifers. Extreme care therefore should be exercised to prevent the pollution of any aquifer that may pose problems to living organisms or to try and restore a polluted aquifer threatening the natural environment. Groundwater pollution should therefore receive urgent attention when discovered. This thesis describes an attempt to develop a set of guidelines for the restoration of the groundwater resources at Douala, based on the relatively new technique of permeable reactive barriers for groundwater remediation—a technique that is also increasingly applied in the restoration of the Superfund sites in the United States of America. Modern attempts to clean up contaminated aquifers, relies heavily on the use of suitable computational numerical models. Such models have in the past always been based on the classical hydrodynamic dispersion equation. However, an analysis of the equation in this thesis has shown that the equation cannot account for the long‐tail contamination plumes characteristic of fractured rock aquifers. Fortunately, it is not too difficult to develop a more suitable equation. For, as shown in the thesis, all that one has to do is to replace the ordinary derivatives in the classical equation with fractional derivatives. Mechanistic modeling of physical systems is often complicated by the presence of uncertainties, which was in the past usually neglected in the models used in the restoration of aquifers.While these uncertainties have regularly been neglected in the past, it is nowadays imperative that any groundwater model be accompanied by estimates of uncertainties associated with the model. Although a large number of approaches are available for this purpose, they often require exorbitant computing resources. The present investigation was consequently limited to the application of the Latin Hypercube Sampling method applied to an analytical solution of the hydrodynamic dispersion equation. It has been known for years that the hydrodynamic dispersion equation discussed in Chapter 5, is not able to account for the long‐tail plumes often observed in studies of contaminated fractured‐rock aquifers. An approach frequently used to account for this is to replace the ordinary spatial and temporal derivatives in the hydrodynamic equation with fractional derivatives—a procedure confirmed in this thesis.
Open Access
A groundwater-surface water interaction study of an alluvial channel aquifer
(University of the Free State, 2011-11) Gomo, Modreck; Van Tonder, G. J.
The study describes the application complimentary geohydrologic tools to investigate the geohydrological properties of an alluvial channel aquifer and its interaction with the river surface water resources. Primary field investigations were designed to determine the geologic, hydraulic, hydrogeochemical and solute transport properties of the alluvial channel aquifer as an important component of the groundwater‐surface water (GW‐SW) interaction system. The secondary investigations were then aimed at assessing groundwater discharge and recharge mechanisms of the alluvial channel aquifer at a local scale (< 1000 m). A water balance model was developed for the groundwater‐surface system as a tertiary level of investigation. Geological characterisation results show the spatial variation in the physical properties of unconsolidated aquifer materials between boreholes and at different depth. The drawdown derivative diagnostic analysis shows that the alluvial channel aquifer system response during pumping can be described by the following major groundwater flow characteristics; Typical Theis response; transition period from initial Theis response to radial acting flow (RAF); radial acting flow in the gravel‐sand layer and river single impermeable boundary effects. Detailed studies of the hydrogeochemical processes in the alluvial aquifer system have shown that dissolution of silicate weathering, dolomite and calcite minerals, and ion exchanges are the dominant hydrogeochemical processes that controls groundwater quality. Quantitative and qualitative investigations indicate that the alluvial channel aquifer is being recharged through preferential infiltration recharge as facilitated by cavities and holes created by the burrowing animals and dense tree rooting system. Tracer tests under natural gradient were successfully conducted in an alluvial channel aquifer, thus providing some advice on how to conduct tracer breakthrough tests under natural gradients in a typical alluvial channel aquifer. The findings of the study also highlights the value of developing a water balance model as a preliminary requirement before detailed GW‐SW interaction investigations can be conducted. Based on the theoretical conceptualizations and field evidence it is suggested that studies be conducted to determine if alluvial channel aquifers can be further classified based on the nature of the hosting river channel. The classification would split the alluvial channel aquifer into alluvial cover and fractured‐bedrock, or a combination of the two. The applications of the PhD thesis findings are not only limited to the case study site, but have important implications for GW‐SW interaction studies, groundwater resource development and protection in areas where groundwater occurs in alluvial channel deposits.