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
Risk based decision tool for managing and protecting groundwater resources
(University of the Free State, 2001-11) Van der Voort, Ingrid; Van Tonder, G. J.
English: The limited number of water resources in South Africa has resulted in increased emphasis being placed on groundwater. Groundwater supply of acceptable quality and quantity is a very important factor in the development of communities. The availability of water for various uses is directly related to the management of water quantity, quality and/or elimination of diseases. A risk can be defined as the probability that an adverse event will occur under specified circumstances. Effective decision-making involves the management of risks: the identification, evaluation, selection and implementation of actions to reduce risk. The aim of the research discussed in this thesis is to develop a decision tool to aid groundwater resource managers in the task of optimising the utilisation of groundwater. The decision tool will include: • Information concerning aquifer parameters: Pumping test analysis methods have been developed primarily to investigate and characterise flow within idealised confined radial flow systems. Unfortunately these assumptions are usually invalid with regard to the shallow fractured rock aquifers in South Africa. Notable attempts have been made to expand pumping test methodologies. A worthwhile method to consider when analysing a pumping test was developed by Barker (1988), where he generalised the Theis equation by including a term called the non-integer flow dimension, thereby making it applicable to arbitrary fractured confined aquifers. • Information concerning contaminant parameters: Dispersivity is a scaledependent property of an aquifer that determines the degree to which a dissolved constituent will spread in flowing groundwater. No in-depth investigation was conducted concerning this parameter, but as it plays an important role in the movement of contaminated groundwater, it is briefly discussed. Although matrix diffusion can influence groundwater contamination, very little research has been conducted in South Africa on this topic. The project therefore includes laboratory matrix diffusion experiments. The results of these experiments are included in the decision tool. • A framework for risk assessments: the project introduces tools based on fuzzy logic to assist in decision-making by systematically considering all possibilities. This tool takes into account the sustainability of a groundwater resource, the potential contamination of groundwater, human health risks and impacts of changes in groundwater (quantity and/or quality) on aquatic ecosystems. • Methods to make cost-effective decisions: Negative impacts can place heavy burdens on society and economics. Cost-benefit-risk assessments are therefore considered to define, compare and measure benefits and costs with regards to an impact. • Possibilities of remediation: Remediation forms an important component of many groundwater investigations and a few experiments were therefore conducted, the results, of which were included in the decision tool. The results provide the groundwater manager with an indication of the possible success of a remediation project.
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
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
Tracing water and nitrate movement through soils with bromide
(University of the Free State, 2003-01) Zeleke, Ketema Tilahun; Botha, J. F.; Bennie, A. T. P.
English: The pollution of surface and subsurface water is a serious problem worldwide. To clean up a once polluted groundwater source is very difficult and costly, if not impossible. Almost all the sources responsible for groundwater pollution originate in the vadose zone of the subsurface. A better understanding of the movement of chemicals in the vadose zone under different conditions can therefore contribute significantly to prevent the pollution of groundwater resources. One particularly important pollutant of groundwater sources in agricultural areas is nitrogen. However, it is very difficult to study the movement of this chemical in agricultural soils, because of its complex and interdependent transformations in the soil. The major aim of this thesis was to compare the movement of bromide and nitrogen in the Bainsvlei soil of South Africa and a soil of Ethiopia, under steady state, transient state and cropped soil conditions, with the view to use bromide as a substitute tracer for nitrogen. The hydraulic properties of the Bainsvlei soil were determined in situ. Different simplified models were used to determine the hydraulic conductivity of the soil, and the performance of the models was evaluated. A steady state experiment was conducted using bromide tracer and a rainfall simulator to determine solute transport parameters using deterministic and stochastic models. Parameters determined from the deterministic convective-dispersive equation and the stochastic stream tube model were similar. It was observed that this soil did not exhibit preferential flow. From the experiments conducted to determine the effect of intensity and continuity of water application on solute transport, it was observed that increasing the intensity and continuity of rainfall/irrigation increases leaching in this soil. The experiments conducted on a bare plot under natural rainfall conditions suggested the leaching of a fertilizer could be minimized if the fertiliser is not applied in batch mode at the beginning of the growing season of a crop, but split over the growing season of the crop. The study of the movement of bromide and nitrate was repeated with the alluvial sandy loam of Dire Dawa, Ethiopia. Two sets of plots were used for this purpose. One set was left bare and the other planted with maize. No clear conclusions could be reached from this study because of the high natural background nitrate concentration of the soil, which obscured the positions of the concentration peaks. Nevertheless, the experiment did show that the maize uptake attenuated the movement of both chemicals in the soil and that the rate at which maize plants take up nitrogen depends on the vegetative stage of the plants. When combined with the results of the leaching experiment on the bare plot, these results indicate that it would be more economical, and environmental friendly, if a farmer applies the fertilizer not in batch mode at the beginning of the growing season, but split it over the growing season, dependent on the vegetative phase of the crop and weather conditions.
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
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
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
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
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
Site characterisation methodologies for DNAPLs in fractured South African aquifers
(University of the Free State, 2007-05) Gebrekristos, Robel Amine; Usher, B. H.
English: The objective of this research is to characterise fractured aquifers contaminated with DNAPLs in South Africa. The site characterisation approach is conducted in two case studies by integrating a range of site assessment techniques. It started with the noninvasive methodologies that provide rapid and cost-effective ways for screening. After developing the initial conceptual model, the investigation proceeded to invasive techniques that provide more information on DNAPLs and fractures on a local scale. This research indicated that fracture “hunting” is extremely difficult with the current technology, making DNAPL characterization very challenging. It also indicated that no single technique provides an unequivocal indication of fracture position and orientation, but that best results were obtained when several invasive techniques were used in a toolbox approach. These tools were most appropriate when integrated with hydraulic testing methods. The site characterisation was done in a continuous and iterative process, and each phase of investigation was used to refine the conceptual model of the site. The non-invasive techniques applied included: • General assessment of Test Site 1 by site inspection, interviewing employees, documentation studies and aerial photo analysis. • Hydrocensus • Direct observing for DNAPL contamination using UV light and Sudan IV shake tests. • Soil gas surveys by using various models of PIDs to delineate the organic vapour plumes. • Surface and airborne geophysics for outlining major structures that may play an important role for DNAPL migration pathways. This did not yield useful results at Test Site 1 due to noise interference from industrial activities and infrastructure. The non-invasive techniques were not applicable for direct evaluation of DNAPLs migrating in fractured aquifers; however, they were essential in understanding DNAPL-release zones and vapour plumes. They were also found to be relatively cost and time efficient. The invasive techniques applied include: • Test pits (excavations on the top 3 m soil) • Drilling using augur, percussion and coring methods. (An outside-in approach was followed during the drilling to avoid unnecessary DNAPL mobilisation). • Several different borehole geophysical and geochemical logging methods. Results from this diverse range of activities were integrated to construct conceptual models of the preferential fracture pathways of the two test sites. Riemann (2002) found that early time drawdown data could be used to estimate the Tvalue of the fracture zone with the Cooper-Jacop 2 method. However, in this research, the method did not yield results that were consistent with other observations. The tracer experiment was extended to evaluate the effective matrix diffusion coefficient of the mudstone in the Campus Test Site. The values at depths of 27 and 36 m were estimated as 3.4 x 10-6 and 7.8 x 10-6 m2/day, respectively. This means that the rate of dissolved mass disappearance from the fracture to the matrix is relatively high compared to other geologic formations. An in situ method of effective porosity estimation using tracer tests was employed instead of the traditionally measurements on rock/soil samples from the aquifer. In this way, uncertainty in porosity estimation due to the inherent alteration during sampling was eliminated. Column experiments were conducted to evaluate the accuracy of PITT for residual DNAPL saturation estimation. The saturation was estimated with an accuracy of 95%. However, the experiment was conducted in unconsolidated, homogenous sand and the experiment may not be accurate as such when applied to heterogeneous aquifer systems usually encountered in South Africa. With the current technologies available, PITT is believed to have limited applicability in fractured rock aquifers. Finally, based on the findings, a methodology has been proposed for the characterisation strategy of DNAPL contaminated sites in fractured South African aquifers
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
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
Innovative methods for the characterisation of fractured rock aquifers
(University of the Free State, 2008-12) Akoachere, Richard Ayuk II; Van Tonder, Gerrit J.
Bulk flow is regional flow. The word region is used in two ways viz; i) A region may be a hydrogeologically and geographically distinct area. Ex: the Karoo basin. ii) A region maybe discontinuous but widespread encompassing related non adjacent aquifer systems such as surficial aquifers, coastal aquifers or as in our research project study case, some selected fractured rock aquifers in South Africa. In case (ii) regions, topical investigations are optimized for regional applications. In such investigations, focus is on processes rather than properties of specific aquifers (Groundwater science). Characterization tends towards common processes (drivers of the various processes) rather than geographical locations and particularities. Two new methods have been developed to determine inclined and horizontal fracture apertures b, in fractured rock aquifers. These methods are; i) The SLUG-TRACER (ST) TEST; ii) The NAPL ENTRY PRESSURE (NEP) TEST/ NAPL INJECTION PRESSURE (NIP) TEST. Mathematical formulations were developed from laboratory experimentation using transparent Perspex parallel plate physical models and 27 apertures of 0.008 mm to 6 mm, created by using aluminum foil and thickness gauges between 20 mm thick clamped Perspex plates. The ST test uses a slug in which is added NaCI as tracer (500mg-5000 mg/I) and an EC meter is used to detect breakthrough in the observation boreholes. The NEP test uses a NAPL (Sunflower oil) hydraulic head and transducers to get the entry pressure. Using these mathematical formulae, fracture apertures are then determined for horizontal and inclined apertures. The NIP test uses the entry pressure recorded by transducers, of a NAPL (Sunflower oil) by injection and its volume to determine the fracture aperture for horizontal and inclined fractures. Results from smooth and rough (Buffed to 10x20 microns) fracture surfaces gave accurate results for 96-98 % aperture determinations of twenty six (26) apertures from 0, 04 mm to 6.3 mm. The Phreatic Hydraulic Conductivity (PHC) apparatus was developed to measure the hydraulic head gradient of samples. The PHC apparatus was made of a solid body divided into three chambers, mounted on a ten liter capacity water reservoir, with a pump. Three types of samples can be used; Consolidated (in-situ), loose/friable (insitu), and unconsolidated samples (Drill/auger cuttings, Mine tailings/ash etc.). The apparatus was used to determine hydraulic conductivities of samples ranging from coarse gravel to very fine clayey dam tailings. The values ranged from 2.81 E-03 to 4.32E+03 (m/d). The results were reproducible and compared well with those of other methods. The PHC apparatus' advantages are: Can be used in the field and laboratory (compact); Simple to use and needs limited maintenance (Three components); Economical, needing small volumes of water (ten liters); Light (6kg) and compact (0.16 rrr'): Rapid results (Complete determination for a sample within tens of minutes); This apparatus is particularly suited to determine the hydraulic conductivity of clastic formations for non-confining flow under atmospheric conditions. Laboratory experiments on the small (cm) scale aimed at determining the effect of variable thickness of formations on the hydraulic conductivity, determine the effect of composition, layering ,spatial disposition and develop a tool for predicting bulk hydraulic conductivity in phreatic aquifers were carried out. From these, the partial hydraulic conductivity formulation was developed empirically, to determine the bulk hydraulic conductivity of the samples, irrespective of the spatial disposition. With geologic insight, the bulk hydraulic conductivities were determined using the partial hydraulic conductivity theory. When the thicknesses of the layered sequences varied, the laws of composition broke down. The Trigger-tube is an apparatus developed for mixing solutes and tracers for injection tests in boreholes. It is a simple cap-trigger tube segment and the technique mixes solutes in boreholes in two minutes. Solutes are introduced into the well and the trigger is released. The tube is withdrawn and the solute mixes instantaneously to give a homogeneous mixture of solute with the borehole groundwater. Field tests using this method and apparatus for point dilution tests gave a Darcy velocity of 4.06 m/day, Seepage velocity of 122.89 m/day and effective porosity of 0.33. Natural gradient tests gave a Darcy velocity of 4.06 m/day and natural velocity of 123 m/day using NaCI for the same fracture at 21m in borehole U05. This apparatus takes comparatively a shorter time to carry out SWIW tests than using the pump mixing method. Field tests gave 13 minutes for the trigger-tube method and 25 minutes for the pump mixing method for a point dilution test using NaCI. This apparatus can be used for any test that needs the introduction of a homogenous mixture in single well tests. The thermal dilution test is a test developed to determine the position, number and groundwater (Darcy) velocity of fractures found in a single borehole drilled into a fractured rock aquifer using temperature as a tracer. Using a trigger-tube apparatus, cold at 2 degrees Celsius is introduced into a single well. The rate at which warmer groundwater flows into the well is measured as the change in temperature and used to determine flow zones, the position of fractures, their depths and the Darcy velocity of the various lithologies and fractures with flow present, from top to bottom of the borehole the method was used in a single well test on borehole U05 to determine fractures at 14m, 15m, 16.8m, 18m, 19.4m, 21m, 22.4m, 24.2m,26m and 27.5m below the surface. These fractures had Darcy velocities ranging from 1.54m/day t04.17m/day, with the largest fracture contributing to flow in the borehole being that at 21 m. This was confirmed by acoustic scan and borehole camera images of the borehole. This method is very useful to determine the hydraulic properties of fractures and formations under natural conditions (Without pumping) using a single well.
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
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
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
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.
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
Study of flow and transport in fractured granitic rock
(University of the Free State, 2012-05) Ndiweni, Cliford; Van Tonder, G. J.
The hydrogeology of the Tono basin, Japan, is strongly influenced by the hydraulic properties of faults, especially the main Tsukiyoshi fault, which extends through the centre of the assessment area and has an E-W strike. According to the results of borehole investigations, the fault has N800W strike, 700 dip, 10 - 30m width and approximately 30m vertical off-set. Hydraulic head discontinuities over the main fault in the basin are about 40 m as a result of the low permeability of the fault acting as a barrier to flow perpendicular to it. The fracture data from the Tono basin was analysed in order to establish a correlation between geologic/geometric attributes of a fracture and associated permeability of the interval that contains the fracture, if any. Pressure response transients to excavation of two shafts that are monitored at various boreholes within the study site show that proximity to a fault is a key attribute that determines the ability of the fracture to conduct water. The responses in boreholes that are close to the fault are vertically invariant, indicating a large vertical permeability. This is not the case in boreholes that are further from the main fault, where there is depth dependence in the pressure responses. Near the fault, the damage zone seems to be equilibrating the heads between otherwise unconnected aquifers. The Tsukiyoshi fault therefore acts as barrier to flow perpendicular to it but also acts as conduit to vertical flow and flow parallel to the fault. A three-dimensional model that simulates groundwater flow in the Tono basin is constructed in order to study the dynamic fluid flow before and after it was disturbed by production and the excavation of the shafts. In the steady-state calculation, the model predicts that the hydraulic head at depth in boreholes near the fault is near the land surface. This condition indicates high vertical permeability in those boreholes. This thesis introduces a new approach of using pressure response data to do an inversion calculation for the effective porosity of the granite. Pressure response transients have been analysed using a numerical inversion procedure to estimate the specific storage of the granite. The specific storage was calculated using the pressure response data and ranged from 4:12 x 10-7 to 8:93 x 10-6m-1. These values of the specific storage were used to do a transport calculation in order to study the impact of the main fault on the transportation of hypothetical contaminants in the basin. Particle tracking was used to investigate and demonstrate the effects of the fault on path lines. The fault was found to have a strong influence on the transportation of contaminants. The general trend of the transportation of the contaminants follows groundwater flow from the northern high elevations toward the southern low elevation. This shows that the contaminants are transported mainly by advection. However, this trend is interrupted by the Tsukiyoshi fault that blocks horizontal flow and sends water toward the surface. An interesting feature demonstrated by the model is that, within the fault core, no contaminants were found. The contaminants rise through the high-permeability damage zone and cross over the fault through the weathered granite. However, at depths where the water changes direction slowly because of the fault barrier to horizontal flow, the contaminants seem to be able to cross the fault. The explanation is that diffusion becomes the dominant mode of transport at the point where the water moves at slow velocities.