Institute for Groundwater Studies (IGS)

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Open Access
Salinity in the Struisbaai aquifer
(University of the Free State, 1998-05) Weaver, John M. C.; Van Tonder, G.
English: The water-supply for Struisbaai has a historical reputation for its high salinity. Like most of the coastal holiday resorts along the Cape South Coast there are a number of permanent residents. For Struisbaai these comprise owners of small businesses servicing the holiday trade, fishermen and their families and retired folk. However, unlike other coastal resort-towns, Struisbaai is notable for the lack of houses with attractive gardens. This is a direct reflection of the relatively high salinity of the water-supply which in the past was not suitable for irrigation, having salinities of over 100 mS/m and sometimes over 500 mS/m. For example a sample of town water-supply was collected in June 1990 from a tap at Struisbaai Hotel and the EC was found to be 658 mS/m (4150 mg/L of dissolved salts). In 1990 a replacement wellfield was developed which has an EC of about 90 mS/m. With this relatively low salinity attractive gardens may become a feature of the future for Struisbaai. Three previous groundwater investigations have taken place, and in all of these the high salinities have been ascribed to over-pumping with resulting seawater intrusion. However, none of these authors gave any conclusive evidence for their theories. All of these reports gave field observations and then formulated theories for the origin of the salinity but in none of these reports were the two linked by logical arguments. This thesis thus considered the various possible sources of salinity and made conclusions as to the most likely source. The three sources of salinity that were considered are: HH. Salinity derived from sea-spray causing high salinity recharge. H'. Geological factors which yield high salinity groundwater. H'. Hydrogeological factors which result in sea-water intrusion. Field work consisted of drilling of four new boreholes, geophysical down-hole logging of six boreholes, establishing rainwater sampling points, collecting of groundwater samples and processing water samples for isotopic analysis. The methodology used to analyse the data was to closely consider the water quality of rainwater and groundwater samples as well as the isotopic composition of the groundwater. By considering ratios of the chemical composition, ratios of isotopic composition and using graphical plots the conclusion made was that the source of salinity is due to sea-spray which causes rain recharging to the aquifer to have a high salinity. This rain was measured to have a salinity of 10.5 and 12.5 mS/m respectively for the two rain collector stations. When a recharge to the aquifer of about 10% is allowed the resultant salinity is similar to that measured for groundwater at that site. Using the same methodology it was concluded, except for two individual boreholes, that geological and hydrogeological factors are not the source of salinity. For these two individual boreholes it was shown that overpumping and seawater intrusion was the probable cause of salinity. The decision to replace the old wellfield with the new wellfield has been a fortuitous decision. By doing so two positive effects have occurred. Firstly the wellfield is now in an area where the local recharge is less saline due to it being further away from the sea and thus receiving less sea-spray. Secondly the possibility of over-pumping and inducing seawater intrusion is minimal. This thesis has contributed to South African hydrogeology in that as far as can be ascertained this is the first documentation of the contribution that sea-spray makes towards causing salinity in South African coastal aquifers.
Open Access
Environmental impact of point pollution sources
(University of the Free State, 1999-07) De Lange, Stephanus Steyn; Van Tonder, G. J.
English: Due to the importance of groundwater as an alternative for supplementing existing sources of this precious fluid, any study that may help in understanding the complexity of the groundwater environment is an asset. Therefore this dissertation was written with the sincerest hope that the people and organisations involved with groundwater will gain a better understanding of it. The need to protect our groundwater sources has become very important and therefore a different, more practical approach to delineate borehole protection zones was considered based on the existing standards set by Germany and other European nations. As part of the study, emphasis was placed on the effect of the unsaturated (vadose) zone on the migration of pollutants before it reaches the groundwater environment. Field tests on the saturated as well as unsaturated zones were conducted with a conservative tracer (Fluorecein) as well as microbial tracers (bacteriophages), to assist in the delineation process. Thereafter it was decided to propose that three protection zones should be assigned depending on i) the size of the population and ii) the vulnerability and importance of the aquifer. The effect of a pollutant on human health and the possibility of fatalities were the criteria for deciding which protection zone is applicable under given circumstances. Protection zone I involves fencing off the immediate area around the borehole (a radius of 5 m is proposed) as well as the necessity of a well-constructed sanitary seal. Where a borehole supplies water to less than 20 people, a sanitary seal will be sufficient. To determine the extent of protection zone II, the idea was to protect the drinking water from microbial (bacteria and viruses) and nitrate pollution. Emphasis was placed on these two parameters because of their association with pitlatrines and septic tanks. Methods to estimate the fracture extent are proposed and, depending on the estimation of the nitrate and bacterial travel times and loads, a guideline to use i) half the fracture extent, ii) the whole extent of the fracture or iii) double the fracture extent, is proposed. Only if there is the possibility of a hazardous substance that may pollute the groundwater, it is proposed that protection zone III be assigned. The use of the word "hazardous" could make the decision more complex and therefore the issue of risk assessment and management is also addressed very generally in this dissertation. The proposed extent for protection zone III is the whole catchment area of the borehole. A programme called Borehole Protection Zone (BPZONE) was developed by making use of Microsoft Excel to assist the person(s) responsible in the decision making processes of delineating borehole protection zones. Information gathered during the field tests conducted at the Campus Test Site of the University of the Free State as well as the Meadhurst Test Site outside Bloemfontein, yielded very positive results, in terms of delineating protection zones.
Open Access
A geohydrological situation analysis for the construction of a groundwater management plan for the Sasolburg industrial and mining area
(University of the Free State, 1999-12) Cowley, Jennifer Anne; Van Tonder, Gerrit
English: The Department of Water Affairs in South Africa has recently moved towards the management of regions on a catchment and subcatchment scale. The Taaibos and Leeu Spruit catchments were identified as catchments where groundwater research was needed. The final objective of this work would be the implementation of a Catchment Management Plan These catchments house several influences on groundwater quality. These include the urban area of greater Sasolburg, several chemical industries (petrochemical, fertilizers, plastics ete), mining activities, power stations and agriculture. A groundwater situation analysis was therefore undertaken to highlight all relevant groundwater components. To achieve this, the study included a literature study, the creation of a database, fieldwork including sampling of 101 sites in the catchment for inorganic analysis, sampling of 10 sites for toxicity testing to indicate true toxicity of the water, aerial geophysics over a portion of the catchment and resistivity soundings at points surrounding the industries. Based on information gathered by this work it was possible to identify the principal aquifers in the area and classify them according to methods set out by Parsons (1995). The chief controls on groundwater movement and pollution sources on each site were identified. It was found that a dolerite sill of variable depth and thickness was the main control of groundwater movement underlying the industrial areas. The collected data and sampling highlighted the most important pollution sources. Use of toxicity testing showed the potential of these tests in indicating the true toxicity of samples. These results with, simplified numerical modelling in the area gave an indication of pollution migration and areas of concern. The area was subdivided into groundwater management areas based on the activities in each area. Three zones were defined representing the industrial and mining areas, a buffer zone and the areas to be protected respectively. In each zone different quality guidelines were proposed. A Threat Action Guide spreadsheet was developed to identify and prioritize areas of concern. It uses water quality data for different parameters, the aquifer classification, the water quality guideline for the zone and the distance to the nearest area of concern as inputs. A threat index is thereby calculated using various empirical equations and this index then suggests different generalized management options to be followed. The situation analysis has provided an overview of this heterogeneous area and put a basic framework in place on which the eventual Groundwater Management Plan will be based.
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
Estimation of the preliminary groundwater reserve using numerical models
(University of the Free State, 2000-06) Zhang, Jinhui; Chiang, Wen-Hsing
English: The Water Act requires the determination of the Groundwater Reserve to secure the basic human needs and to protect the environment (aquatic ecosystems in terms of the Act). In other words, it is necessary to determine the allocatable groundwater resources and its use may not cause unacceptable negative impacts. As numerical models have been greatly developed in assessing groundwater resources in recent years, it is used to estimate the Groundwater Reserve. An investigation into existing groundwater numerical models has been done in order to select the suitable numerical model for determining the Groundwater Reserve. The MODFLOW program is selected for the present work because of the widespread of applications and the existence of user-friendly graphical interfaces, e.g. Chiang and Kinzelbach (in press). The procedure of estimating the allocatable groundwater resources can generally be divided into two steps: The first step is to estimate the total accessible groundwater resources by means of the regional groundwater balance. Groundwater fluxes, recharge, abstraction, and leakage to/from surface water bodies are balanced over the whole catchment. The methodology is used for the Pienaars River catchment, where the groundwater recharge is the only inflow term to the aquifer and its determination will directly affect the result. The interaction (water exchange) between groundwater and surface water (rivers and wetland) cannot be quantified without additional data, especially river flow rate and evaporation measurements in proper positions. To date, most available computer models are unable to compute the dynamic interaction between river stages and the groundwater levels. As a result, in most MODFLOW (or other groundwater models) applications, the river stages are often assumed to be known. This is, however, not applicable in many cases, where the change of the surface water profiles (river stages) due to groundwater or surface water abstractions must be predicted. A computer program which is able to couple the groundwater and surface water flow models, will be useful for computing and predicting both the groundwater and surface water levels and their interactions. The second step is to estimate possible negative environmental impacts due to groundwater abstractions. The estimated impacts are scales for decision-makers to give allowances to future abstractions. There are many possibilities to perform the second step, for example post-auditing or by using numerical models. The former is often not appreciated, because most groundwater systems are difficult or impossible to restore once they are damaged or contaminated. The latter is superior due to its predictive capability and its flexibility at the price of the data requirement. Because of the complexity of the saltwater intrusion processes, the program SEAWATER is developed to solve the steady-state salt-/freshwater sharp interface for using with MODFLOW. SEAWATER calculates the interface by means of an iterative process, which uses the Ghyben-Herzberg approximation to determine the location of the interface. The results of the program are compared with several analytical solutions.
Open Access
Manual on pumping test analysis in fractured rock aquifers
(University of the Free State, 2000-11) Van Bosch, Izak Jacobus; Van Tonder, G. J.
English: With the depletion of surface water resources in South Africa the utilisation of groundwater as the only potable water resource for communities in some rural areas is quickly becoming a reality. Failure of this resource will cause serious problems and it may even lead to loss of life. To prevent failure and to ensure the sustainability of this new resource, pumping tests are performed to determine the long-term sustainable yield of bore hole. This is the main reason why pumping tests are performed in South Africa, but in some instances pumping tests are performed to determine parameter values for aquifers. This research focussed on presenting guidelines on the proper planning and execution of the different types of pumping tests. To perform a proper pumping test involves big financial costs and therefor it is important to ensure that the correct information is obtained during such a test. Recommendations regarding the lengths of pumping tests as well as the abstraction rates during pumping tests are also made in this research. Before the interpretation of pumping test data it is important to obtain a better understanding of the basic concepts and characteristics involved in groundwater hydraulics. These concepts, together with the interpretation of diagnostic and derivative data plots are discussed in detail and it will provide the reader with enough knowledge to identify and understand the different flow regimes present in an aquifer. This research looks at the different steps involved in the interpretation of pumping test data and these steps are illustrated by means of worked examples. The different analytical methods to determine parameter values for an aquifer are also discussed in this thesis and the assumptions made in developing these methods are pointed out to the reader. In most cases these assumptions result in incorrect parameter value estimates. Numerical models, developed for the unique aquifer systems of South Africa, yielding the correct parameter values, are also discussed. Well losses can be divided into linear and non-linear losses. Non-linear well losses are mainly caused by turbulent flow inside the aquifer and borehole. This research however shows a non-linear relationship between drawdown and abstraction rate while the flow inside the aquifer is still linear. The non-linearity between drawdown and abstraction rate is corroborated by means of several examples and this nonlinearity can only be the result of the unique composition of aquifers in this country. Different abstraction rates result in different fractures being dewatered and fracture extents that are reached, causing a smaller effective T-value. The consequence of this non-linearity is that it is very difficult to extrapolate the future water levels in an aquifer. To estimate a sustainable yield in the case of non-linearity this research recommends a revised step drawdown test. This l00-minute test, conducted without constant time intervals, will also point out the positions of the main water strikes. A method, developed in South Africa, with which the long-term sustainable yield can be determined, is also discussed in detail and it is explained by means of a step by step worked example. Included in this method is the effect of boundary conditions as well as a risk analysis.
Open Access
Assessment of the groundwater potential of the Middel Kop/Appleby aquifer, Stella District, Northwest Region
(University of the Free State, 2001-08) Nel, Jacobus Marthinus; Botha, J. F.
English: The importance of groundwater is strongly reflected in the new South African water policy and legislation. All water resources, including groundwater, are now seen as an indivisible asset, with the National Government as its custodian to ensure that the resources are protected, developed and managed properly. Groundwater is the sole source of potable water for the town of Stella in the Lower Vaal Water Management Area of the Northwest Province, but the existing well-field is barely able to supply the present demand. Previous attempts to supply the town with water from groundwater resources have failed, because of the low potential of the aquifers and the poor quality of the groundwater. The proposed resettlement of 2 000 people therefore has to be postponed until sufficient additional water resources can be found for the town. One possible source, not included in previous groundwater explorations of the area, is the granitic aquifer on the farm Middel Kop situated approximately 5 km south-east of Stella. Farmers have used this aquifer extensively since 1990 to irrigate 171 ha of agricultural land. It was consequently decided to investigate this aquifer as a possible additional source of water for Stella. However, farmers have already noticed a continuous decline in the water levels over the years. This meant that the focus of the investigation had to be changed from merely an exploration exercise to an evaluation of the aquifer and the development of a suitable management scheme for the aquifer, as described in this thesis. There are not enough data available to perform a detailed assessment of the aquifer at the moment, a deficiency that should be addressed in future investigations and management of the aquifer. The existing data, however, indicate that the aquifer may be able to sustain a yield of approximately 4 227 m3 d-I in the near future. This volume of water will be able to supply in the needs of the present population and the additional 2 000 resettled people, while allowing 73% of the current irrigated area approximately 8 000 m3 ha-I annually for irrigation. The latter figure can only be achieved if the farmers are allowed to produce one crop per year instead of the two, which is in some cases the practice. This reduction in the water allocated for irrigation may seem harsh, but the decline in the water levels over the last couple of years clearly indicate that the farmers cannot continue with the present practices for much longer. The groundwater quality of the Middel Kop aquifer can be rated as marginal for human consumption. This applies in particular to the relatively high concentrations of fluoride and nitrate in the water, which could affect the health of the population adversely if used untreated for long periods. The conclusion reached in the thesis is that the Middel Kop aquifer can only be used as a source of water for Stella on condition that a detailed monitoring scheme is introduced from the beginning and used to develop a suitable management scheme for the aquifer. This approach will ensure that any impacts on the resource can be properly evaluated and that the quality and quantity of the water can be preserved for future generations.
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
Evaluation of management options for intermine flow and associated impacts in the Central Witbank Coalfield
(University of the Free State, 2003-07) Hough, J. J. H.; Usher, B.
English: Research into the identification, quantification and impact assessment of the intermine flow on the groundwater and surface water quality of the Witbank and Highveld coalfields was undertaken as part of broader research initiatives The study area of this thesis included the coal collieries of the Central Witbank Coalfield. Intermine flow areas were previously identified in studies by Grobbelaar et al., 2001 and Grobbelaar, 2001. The aim of this thesis was to develop a quantitative prediction of long-term intermine flow in the central Witbank coalfield, using the available data collected in the project. From the quantification of these flows and different hydrochemical techniques, likely water quality profiles at these collieries were determined. The quantification on the intermine flow direction as well as the flow volume was predicted through numerical groundwater modelling using Modflow. Local and regional models were compared and it was found that regional models (relative to including the whole extend of the voids in the model area) yielded the most accurate answer of the two models in the study area. The quantification of the predicted impact on water qualities was done through the use of mass balance approximations and an empirical sulphate generation rate method (developed in this study for predicting future sulphate concentrations in underground workings) using available hydrochemical and acid base accounting data. It was calculated that the sulphate generation rate for the underground workings is in the order of 0.1 - 1.5 kg/ha/d depending on the degree to which the void is filled. An evaluation of water management strategies, which will minimise the long-term influence of intermine flow on the groundwater and surface water quality, was also done. A compartmentalization system to mining opencast pits has been suggested for opencast pits with low decant elevations, to inundate a maximum percentage of the spoils to limit AMD reactions. The implementation of artificially created, high recharge areas on opencast pits and underground workings also was investigated to reduce fill-up times of voids as to limit the exposure of oxygen to the acid generating materials. The research indicates that intermine flow can be potentially detrimental to the groundwater- and surface water systems depending on the flow volumes and quality of the intermine flow water in the Central Witbank Coalfield. Flow volumes predicted through numerical modelling was quantified to be considerable enough to be potentially detrimental to mining voids into which the intermine flow water flows. It was also found that the intermine flow could influence the fill up times of the mining voids (both opencast pits and underground workings). The evaluation of different management options identified viable alternatives to manage intermine flow in such a way as to yield future optimal water qualities for the Central Witbank Coalfield.
Open Access
Evaluation of groundwater resource potential of Pallisa district in eastern Uganda
(University of the Free State, 2003-09) Nyende, Jacob; Hodgson, F. D. I.
This study aimed at assessing the ground water resources potential in Pallisa district, which is found in the eastern part of Uganda. The aims of the investigation can be Iisted as follows: ~ To determine the reliability and applicability ofVES in Pallisa district; ~ To evaluate the potential for sustainable harvesting of groundwater resource; ~ To determine the hydrogeological properties of the aquifer and make recommendations; ~ To suggest improved management strategies of water resource in Pallisa district in order to provide basic water services to the rural population and; ~ To propose a monitoring programme in the area that will include boreholes to determine the variation of water quality and quantity with time. The majority of the population of Pallisa district mainly uses groundwater as a source of supply for potable water. Groundwater is increasingly on demand in Pallisa district due to: (i) Population growth, (ii) Modem agricultural practices (iii) Livestock demands. Though springs and shallow wells are a source of groundwater in Pallisa, many of them have not been developed. There is need to have many of them developed to provide clean water and increase in the number of deep wells also in case of consistent droughts. The geological environment covered by this study consists of a Gneissic Complex Formation or Gneiss and granitic formations of the Pre-Cambrian origins. The study shows that Gneiss complex forms the principal source of ground water supplies while fresh metamorphic rocks are impermeable. From the hydrogeological investigations, it is clear that among the three water bearing zones (the upper, the middle and the lower), the metamorphic formations are widely weathered as you go deeper the earth, meaning that the lower zone of fresh geological granitic material are of a fractured bedrock. The study further revealed that the fracture zones have higher hydraulic conductivity with low storage coefficients and are generally confined. The hydrogeological investigation shows that groundwater in Pallisa district lies right below 30 - 70 m from the surface topography. This was shown by the hydrogeological profiles constructed through the underlying aquifer of the wells around Pallisa town. Higher transmissivity values were observed in the crystalline rock aquifer which related to the western and north - western of Pallisa district, with an average of 13.5 m2/d, due to the presence of open waters of Lake Kyoga, Mpologoma and Dodoi rivers. Such transmissivity values typically corresponded to drawdowns in order of a few meters at pumping yields of about 1 m3/hr. The recharge rate of 110 mm/a, taken as uniform throughout Pallisa district, had the total amount of groundwater recharged being l.956*109*0.11 m3/a = 215.2 million m3/a. The amount of water demanded by both the rural and urban is 35.7-million m3/a, which represents 17% of the groundwater currently used. The rest 83% of groundwater is unutilised. The hydro-chemical characterisation of waters in the study area show that the groundwater from the bedrock aquifers of Pallisa district represent regimes where there are predominantly calcium enrichment, which is typical of lime dosing to neutralize acid waters and both the regolith and bedrock groundwaters are dominated by the carbonate ions. This diversity of bedrock types is as a result of weathering of the underlying material. However, groundwater from many of the boreholes in Pallisa district is generally acceptable for human and livestock consumption. Stable isotopes of oxygen and hydrogen has been extensively used to investigate the rainfall and seasonal patterns of deuterium and 180 content of rainfall in the study area i.e. Pallisa district. This was inferred from historical observations at Entebbe, approximately 180 km to the south. The average monthly rainfall and 018 clearly demonstrates a relationship between the amount of rainfall and depletion in 180. As. part of the present study, monitoring of the groundwater resource in terms of yield aquifers with respect to drawdowns and water quality, consumption at water using production boreholes and management of these boreholes play important roles that can never be underestimated as per the recommendations made.
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
Quantification of the impact of irrigation on the aquifer underlying the Vaalharts irrigation scheme
(University of the Free State, 2003-11) Ellington, R. G.; Usher, B. H.
Abstract not available
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
Site characterization and risk assessment of organic groundwater contaminants in South Africa
(University of the Free State, 2004-11) Hohne, Surina; Usher, B.
English: South Africa has only recently realized that organic groundwater contamination occurs in this country and that it can have a serious effect on the groundwater quality. The Water Research Commission (WRC) recently launched studies to investigate Non Aqueous Phase Liquid (NAPL) pollution, and Dense NAPL in specific. The understanding of NAPL pollution problems, is however, still very limited. Hence groundwater practitioners confronted with NAPL pollution problems have burning questions regarding amongst others the characterization of the pollution, which is much more sophisticated than in the case of inorganic pollution. While in this phase, groundwater practitioners can not even begin to consider remedial efforts for contaminated sites, which continue to pollute the groundwater. It is therefore of paramount importance to get up to speed with technologies and practices accepted worldwide for characterization. Much improvement is still needed on these characterization methods, but South Africa can learn from past mistakes made by other countries in addressing NAPL pollution. In order to begin contemplating addressing NAPL characterization, it is important to understand the nature of the problem, which is why Chapter 2 describes the current situation of organic groundwater pollution and the associated vulnerability of aquifers in South Africa. The general understanding of groundwater pollution by NAPL is distorted, not only in the eyes of the public, but also in the eyes of experts in the groundwater field. A general misconception is that NAPL pollution only occurs at heavy industries such as ISCOR and SASOL, but Chapter 2 clearly shows that organic pollution is much more widespread and sinister in nature than would have been thought before. Smaller urban activities and small industries have been identified to be just as large a contributor towards organic pollution as the heavy industries. Shortcomings in the current understanding of NAPL pollution have been highlighted in Chapter 2 and further studies can be focused on determining the current impact of various industries on groundwater in South Africa, as well as delineating towns in which leaking underground storage tanks may be a problem. In order to address the NAPL pollution problem, legislative tools have to be in place. Gaps in legislation have therefore also been highlighted, of which several are listed in Chapter 3. These concerns need to be addressed by making the applicable policies and regulations, and implementing these regulations. In order to shed light on how site assessment and characterization can be performed in South Africa, Chapters 4, 5, 6 and 7 address issues associated with site assessment and characterization. Risk assessment has also been addressed (Chapter 8) and several shortcomings, to be addressed by toxicologists and groundwater practitioners, have been highlighted. It was clear from the investigations performed throughout this thesis, that several shortcomings exist in association with site assessment, site characterization and risk assessment, which will need to be addressed in the near future.
Open Access
Characterisation of the dolomitic aquifer in the Copperbelt Province, Northern Zambia
(University of the Free State, 2005-02) Prinsloo, Martiens; Vermeulen, P. D.
English: External factors that influence the aquifer characteristics of, and sustainable yield from, the dolomitic aquifer include topography, rainfall, surface drainage, evapotranspiration, plant growth, geology, and soils. The topography is gently undulating with a gradient of between 1:400 and 1:800. The low topographical gradients inhibit surface run-off and promote high recharge rates. The annual rainfall will provide the volume of water present in the catchment area that is available for potential recharge to the aquifer. The average rainfall is 1 115mm/annum. Water that leaves the hydrological system of the study area as surface run-off impacts on the volume of water available for recharge to the aquifer in the area. Despite the high rainfall volume and high rainfall intensity only one perennial surface run-off structure exists in the study area. The Kafue River is on average 10 to 20m wide and 3m deep. Only one non-perennial surface run-off feature exists. This feature feeds into the Ipumbu Dam. The volume of water that evaporates from the soil and vegetation before it can reach the underlying aquifer will impact on the volume of water that can be abstracted from the aquifer without negatively impacting on the volume of water in storage in the aquifer. Very little evaporation data is available for the study area. Comparing the calculated potential total evaporation volume with the measured net evaporation and annual rainfall indicates a discrepancy between the evaporation and rainfall data. The rainfall data is considered to be accurate, and based on perceived inaccuracy of the evaporation data, it is recommended that the evaporation data not be used in water balance calculations. The natural plant growth influences the evapotranspiration and in some cases the surface run-off characteristics. Brachystegia Miombo woodlands cover the study area with areas of widespread grass cover between the trees. The main aquifer consists of Upper Roan Formation limestone and dolomite. The main joint set (160º strike direction) controls the strike direction of the solution cavities in the area. Schist and quartzite of the Lower Roan Formation border the limestone and dolomite. Extensive fracturing in the dolomite is found in the geological logs of the high yielding boreholes drilled in the area. The soil in the study area displays infiltration rates higher than the observed rainfall intensity. Combined with the low topographical gradients that retard run-off the high infiltration rate leads to high recharge percentages. The aquifer hydraulic characteristics were determined during the investigation. It is considered that Lake Nampamba forms part of an extensive, well-connected solution cavity network. No compartmentalisation of the dolomite is evident. Three prominent karstic features (Lake Nampamba, Lake Kashiba and the “Chibili Pavement”) occur in the area. Both Lake Nampamba and Lake Kashiba are sinkholes in the dolomite. The author has no scientific evidence to conclude that the solution cavity network stretches as far as the Kafue River. However, analysis of the water level, abstraction and water level data does confirm that Lake Nampamba in the east and the dambo areas in the west that feed Ipumbu dam are interconnected. The groundwater displays a calcium-magnesium dominant character, as is expected from a dolomitic aquifer. Plotting the chemistry data on a trilinear (Piper) diagram indicates that the groundwater recently recharged. This corresponds to the theory of high recharge due to the relatively high rainfall, flat topography, and high soil infiltration rate. Depth to groundwater varies on a seasonal basis due to fluctuations in rainfall and thus recharge, and abstraction volumes. The groundwater level shows an immediate response to recharge, but does indicate a one to two month delay between maximum rainfall and peak groundwater level. A total of 65 boreholes were drilled in the study area between 1978 and 2004. Preliminary aquifer tests were performed on 38 boreholes, and 14 boreholes were identified as high yielding boreholes. Constant rate aquifer tests were performed on these boreholes. The aquifer test data was analysed to obtain the aquifer transmissivity. The aquifer transmissivity ranges between 1 and 6 900m2/day. This wide range is expected in karstic aquifers where the high transmissivity is associated with solution cavities and fractured areas. The low transmissivities are associated with the fine crystalline, competent, unweathered rock. Sustainable yields from the borehole calculated from the aquifer test data range between 5 and 100l/s (432 and 8 640m3/day). Recharge calculations were performed using the Chloride, SVF, Equal Volume and CRD methods. The chloride method is used only as an indication of the recharge percentage. The SVF, Equal Volume, and CRD methods calculated recharge as 25% of the annual rainfall. Aquifer storativity is calculated using the SVF and CRD methods at 0.02 (2%). The total volume of water that can be abstracted from the combined eastern and western aquifers is calculated to be 136Mm3/annum. The current and proposed future annual abstraction volumes are 25.3Mm3 and 44.42Mm3 respectively. This indicates that the combined eastern and western aquifers are capable of sustaining the abstraction volumes. A numerical model was constructed to evaluate the assumptions made, and correlates the calculated values of the manual calculations. The model was initially constructed in steady state without taking the abstraction from the aquifer into account. Once the model was successfully calibrated, the abstraction volumes were incorporated into the numerical model and the model was further calibrated against time series observed rainfall, groundwater levels and abstraction volumes by comparing the groundwater levels with time against those calculated using the numerical model. Once the model was calibrated, the model was applied to evaluate the sustainability of the current and proposed abstraction programs. The numerical model confirms that the combined eastern and western aquifers are capable of sustaining the abstraction programs.
Open Access
Managing the impact of irrigation on the Tosca-Molopo groundwater resource
(University of the Free State, 2005-05) Van Dyk, Gabriel Stephanus Du Toit; Van Tonder, G. J.
From 1990 to 2000 rapid development of irrigation from groundwater resources in dolomite aquifers took place in the Tosca Molopo area. This abstraction lead to water levels declining 10 to 20m regionally and up to 60m proximate to intensive irrigation. The purpose of this study was to investigate the impact of irrigation on the resource and initiate actions to manage the resource. This thesis reports on the qualification and quantification of the impact, determination of water use and regulating use to ensure sustainable future use. The Tosca Molopo area is located in South Africa proximate to the Botswana border. The area of interest is characterized by a flat topography. From the watershed in the west at 1210 m the elevation gradually decline to 1070 m in the east over a distance of 60 km. A number of non-perennial rivers drain the area, and although insignificant as surface water resources they play a major role in groundwater recharge. A low annual rainfall, varying from 399 mm in the east to 385 mm in the west, characterizes the study area. Evaporation in the area is high at between 2050 – 2250 mm/a (WRC, 1994) with only a small percentage of rainwater available to recharge groundwater. Two distinctive aquifers namely a primary aquifer formed by fine-grained sediments of the Kalahari Group and fractured/ carstified dolomites of the Ghaap Plato formation contribute to the system. The general flow is from the SW to the NE with the Molopo River the base of drainage. From the observed water level reaction the sediments contribute largely towards the storage of the aquifer system with the fractures of the dolomite contributing to high yielding flow. The MODFLOW PMWIN 5.1.7 (Chiang 2000) software was used to construct a 2-layer finite difference flow model. The model covering 80 km east west and 50 km north south or 4000 km2 was divided into cells of 0.5 X 0.5 km generating 100 rows and 160 columns. Based on the conceptual model provision was made for 2 layers namely the unconsolidated primary aquifer and the underlying fractured dolomite with its aquifer characteristics. The first layer ranges from an elevation of 1160 mamsl at a depth of 10 m in the southwest. To the northeast it range from an elevation of 1080 mamsl to a depth 960 mamsl or a thickness exceeding 120 m. The base of the sediments is the top of the fractured dolomite aquifer with its base at 900 mamsl. Of the number of dolerite dykes intruded into the dolomite the Grassbank and Quarreefontein dykes (both 15 m thick) are the most influential on the groundwater flow. Both these dykes act as no-flow boundaries of the Neumann (impervious) type impeding flow from the south and west of the area. Towards the east the Quarreefontein dyke does not seem to be a no-flow boundary as the water level information indicate connection with the dolomite to the south. The surface and groundwater shed formed by the Banded Iron Formation of the Waterberge forms the boundary to the west. The combination of both a geological contact and watershed is a leaking boundary. The Molopo River forms the eastern boundary. Recharge to the aquifer was determined with the chloride mass balance method with groundwater sample analysis and the Cl rain content 0.8mg/l. Recharge zones as determined from this chloride analysis were used for the model. Recharge in each zone was based on seasonal recharge for the winter (ranging from 0.5% or 0.4 mm to 3% or 1.5 mm) and summer (ranging from 0.5% or 1.6 mm to 3% or 8.3 mm) depending on the precipitation. Groundwater is the sole source of water for both agricultural and domestic requirements. As irrigation use is responsible for 99.5 % of the total use no domestic and stock watering abstraction was considered. Irrigation abstraction was calculated from the registration areas, field observations and reports from users. The volume was then averaged over a six-month period (182.5 days) according to crop cultivated to obtain the daily abstraction from the aquifer. The calibrated model was used to test the following 10-year future scenarios of abstraction and recharge in order to assist in decisions regarding management of abstraction from the aquifer system. Scenario 1 was with average precipitation and recharge at the current high abstraction rate of 16.1 Mm3/a. This scenario was not acceptable due to the regional water level declines of 20 to 30m and 60 to 110 m water level declines proximate to irrigation. Scenario 2 was with was with average precipitation and recharge at the restricted abstraction at 11.1 Mm3/a. This scenario would result in regional water level declines of 10 to 20m and 30 to 60m proximate to irrigation. With strong abstraction control this scenario with controllable water level declines was acceptable. Scenario 3 was similar to scenario 2, but with 20 % less than normal precipitation. The water level declines that will result with this scenario were similar to scenario 1, but it was expected that below normal precipitation would be the exception. The 4th scenario tested was if normal precipitation prevailed and all irrigation abstraction was stopped. The regional water level would recover fully after 10 years with only 10 m still to recover proximate to heavily irrigated areas. The model demonstrated that rates as specified by scenario 2 can be sustainable abstracted from the system at average recharge and that these abstractions would still be sustainable at 20 % less than average recharge as in scenario 3. Management of abstraction of the aquifer was consequently structured to ensure that abstraction would not exceed the sustainable yield of 11.1 M m3/a. Based on the evaluation and modeling of the resource the regulating and management of abstraction was addressed within the legal framework provided by the National Water Act (NWA) to obtain sustainable, equitable and fare dispensation of water use. Only water use exercised before Oct 1998 is recognized as existing water use. Potentially unauthorized users were identified with the use of satellite images. These water users were given the opportunity to proof that they are authorized users through a communication process and to submit supporting evidence. Users who could not submit satisfactory evidence were directed to scale their use down to authorized use by a specific time (summer 2003). These water users appealed to the water tribunal against the ruling of the water use authority, but the tribunal ruled in favor of the water use authority. In line with equitable access, application from new users were still processed with only 60 m3/ha of property owned authorized in accordance with General Authorization as prescribed by regulations of the NWA. With these actions the resource was still over allocated with water use still not within the accepted sustainable abstraction. Therefore it was decided that regulations would be implemented to enforce users to restrict their water use to 60 % of authorized water rights. The NWA makes provision for local management structures to be established to manage their local water use. Such a Water User Association (WUA) was established in the Tosca area and would on the long term enhance the capabilities for water use management. The resource is currently over allocated. It is recommended that the irrigation water use be restricted with 40% of authorized water rights. The water rights are not fairly allocated. Although the above actions are aimed at normalizing the critical damage to the resource and eminent conflict in the area compulsory licensing would be the long-term solution in this area. Compulsory licensing is aimed at sustainable and equitable allocation of water rights. The WUA should ensure that all users comply to abstraction control measures and water level monitoring the boreholes in the monitoring network would indicate if the resource would stabilize and recover to within sustainable use.