Doctoral Degrees (Institute for Groundwater Studies (IGS))

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Browsing Doctoral Degrees (Institute for Groundwater Studies (IGS)) by Subject "Aquifers"

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    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.
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    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.
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    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.
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    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.
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    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.
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    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.