Doctoral Degrees (Institute for Groundwater Studies (IGS))

Permanent URI for this collection

http://hdl.handle.net/11660/434

Browse

Now showing 1 - 20 of 31

Open Access
Mathematical modelling of pressure build-in due to geological carbon dioxide storage in deep Saline Aquifers, using non-local operators: the context of groundwater protection in the climate change mitigation era
(University of the Free State, 2023) Mbah, Hans Tah; Atangana, Abdon
Man-caused Green House Gas (GHG) emissions have perturbed the earth’s energy balance, and the need to achieve deep emission reduction is a pressing challenge, faced by humankind. Carbon Capture and Storage (CCS) in deep geological saline aquifers is a viable option for Green House Gas (GHG) mitigation. Industrial-scale scenarios may induce large-scale reservoir pressurization and displacement of native fluids. Especially in closed systems, the pressure buildup can quickly elevate beyond the reservoir fracture threshold and potentially fracture and or reactivate existing faults in the cap rock. This can create conduits for focused leakage and mobilization of heavy metals and harmful trace elements, into capture zones of freshwater wells. Risk assessment in CCS requires that careful safety and environmental impact evaluations be considered. Through sustainable pressure management, the reservoir's hydraulic integrity is maintained. This is theoretically achievable with the help of a numerical simulation, using robust mathematical models that provide consistent and effective ways to understand and predict pressure buildup in such complex systems. This thesis focuses on modelling two aspects: (𝑖) the pressure buildup and (𝑖𝑖) CO₂ saturation evolution in the two-phase flow zone, by extending the classical pressure diffusivity and the Buckley-Leverette fractional flow models to the framework of non-local differential and integral operators. The extended models describe the pressure behavior during CO₂ injection through a vertical well, open in a saline aquifer. To include in the mathematical models the effect long-range, fading memory and weak crossover from stretched exponential to power-law, several differential operators were considered. For each extended model, different numerical schemes were adapted to derive numerical solutions. The presented numerical results provide an overview of subsurface transient pressure response and are useful tools for accurate risk assessment and sustainable reservoir management and operational safety during geo-sequestration in basins with shallow freshwater capture zones.
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
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
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 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
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
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
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
Surface water - groundwater interactions: development of methodologies suitable for South African conditions
(University of the Free State, 2013) Moseki, Motlole Christopher; Vermeulen, Danie; Dennis, Ingrid
English: Winter (1999) succinctly, made a profound statement to the effect that understanding the basic principles of the interaction between surface water and groundwater is needed for effective management of water resources. Hence, the research investigation was aimed at identifying appropriate methodologies for assessment of surface water - groundwater interaction, thus enhancing the understanding thereof. The methodology used entailed a review of national and international literature on related previous and current models, systems and methods used in assessment and quantification of water exchange between groundwater and surface water. This was then followed by relevant case study analyses where distinct areas where chosen based on availability of relevant data and information by previous investigators. The findings were that various methods and classification systems are widely available but the applicability thereof under the South African conditions depends on the conceptual understanding of the area or system under investigation, availability of data and the basic assumptions associated with the particular model or method. The surface water - groundwater interaction cannot easily be quantified with confidence without requisite data available. The other finding is that use of multiple techniques to reduce uncertainties and to confirm or verify the existence or non-existence of the interaction is essential. Preferably, at least one method should be utilised to trace flow or qualitatively establish the water exchange while the alternative method is used for quantitative estimation of the interaction between surface water and groundwater. Some of the products emerging from this research investigation include decision tables for choosing applicable analytical method, applicable numerical method and the framework for guiding the selection of appropriate methodologies for assessing or quantifying the interaction between surface water and groundwater. Knowledge generated is applicable to water resource management, resource protection, water allocation and monitoring.
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
Quantification and modelling of heterogeneities in aquifers
(University of the Free State, 2017-01) Ahokpossi, Dehouegnon Pacome; Atangana, Abdon; Vermeulen, Danie
English: The future of modelling of heterogeneity in aquifers is definitively in the designing of new in situ testing (hydraulic and mass transport) procedures with new corresponding mathematical models. New trends in mathematical differentiation offer opportunity to explore more flexible and practical mathematical model solutions. This applies to both analytical and numerical modelling. Only a sound understanding of rock structures can clearly pose the problem which will then be used to define hydraulic equations to be solved by mathematical models, and numerical software. The most recent concept of differentiation based on the non-local and non-singular kernel called the generalized Mittag-Leffler function, was employed to reshape the model of fractured aquifer fractal flow. The solution was successfully applied to experimental data collected from four different constant discharge tests. Additionally, a new analytical solution to the fractal flow in a dual media was proposed, where the media could be elastic; heterogeneous; and visco-elastic. The existing dual media fractal flow model was modified by replacing the local derivative with the nonlocal operator (operator with Mittag-Leffler kernel, and Mittag-Leffler-Power law kerne)l. The more accurate numerical scheme known as Upwind was used to numerically solve each model. Heterogeneity in a typical South African crystalline rock aquifer was assessed. From this, a methodical level for quantifying and modelling heterogeneity in an aquifer was deduced. It was demonstrated how spatial heterogeneity in aquifers can be modelled based on the most commonly available tools and data in mining environment. The capability of selected numerical geohydrological softwares were assessed using spatial variability of hydraulic parameters (hydraulic conductivity and recharge). Geostatistical tools were specifically applied. Focus was also given to hydro-geochemical characterization by using bivariate scatter plots, Piper and Expanded Durov diagrams, and PHREEQC hydro-geochemical model as complimentary tools to analyse the groundwater chemistry data to describe different hydro-geochemical process which prevail in the monitored groundwater system. Three manuscripts have been submitted out this thesis, in top tier journals of the Natural and Applied Sciences.
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
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
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
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
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 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
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.