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University of the Free State (2004)

A critical review of recharge estimation methods used in South Africa

Bean JA

Titre : A critical review of recharge estimation methods used in South Africa

Auteur : Bean JA

Université de soutenance : University of the Free State

Grade : Doctor of Philosophy (PhD) 2004

Résumé
This investigation provides an overview of recharge-related research undertaken in South Africa and its neighbouring countries to date, and where possible, integrates new findings such that regional recharge processes can be better understood. Particular emphasis was given to the use of environmental tracers, specifically chloride and the stable isotopes δ2H and δ180, due to the relatively low cost associated with applying these techniques. Episodic recharge processes were also considered, a field of study that appears to have received minimal attention in research undertaken to date in Southern Africa During the course of this investigation, refereed articles from local publications and international journals were widely consulted, with preference given to those relating to research undertaken on the African continent, or regions with a climate similar to semi-arid to arid Southern Africa. Wherever possible, published material was compared to field data collected during this study from various inland locations in South Africa, and differences and similarities between the respective datasets discussed. Case studies from different geohydrological environments in South Africa were also undertaken with a view to determining the applicability of environmental tracer methods for quantifying recharge processes in the region A new stable isotope-based technique, the Modified Amount Effect (MAE) Method, was developed during this study. 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. Indeed, in many instances it may be more realistic to base groundwater allocations on the proportion of bypass flow-derived recharge entering site aquifers initially, the allocations increasing once aquifer storage, recharge threshold, and recharge event return period characteristics are better understood Modelling of recharge processes could be significantly refined if long-term Static Water Level (SWL) and stable isotope data was available for a given aquifer. Indeed, the most important recommendation in this report is to encourage the collection of monthly rainfall, SWL, stable isotope and chemistry (rain, surface, and groundwater) data at selected sites in Southern Africa. Sites should be selected on the basis of land use, climatic zone, and aquifer type, with a view to extrapolating findings made there to similar locations elsewhere, and data stored and managed using centralized databases. This can be best achieved with government funds, although given the recent changes in legislation requiring industry to ensure site monitoring is undertaken in some countries in the region, there is considerable scope for private money to contribute to data collection. The development of a standardized monitoring code of practice for industries operating in the region, which outlines minimum monitoring frequencies for input parameters necessary for recharge estimation, should therefore be a priority 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 preceding months should be considered prior to sampling. 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 Given the observed sensitivity between SI and CMB Method estimates and site recharge history, there is a potential for those estimates based on unsaturated zone moisture concentrations to be a reflection of the last recharge event and not long-term recharge to the aquifer. As such, it is recommended that, in Southern Africa, estimates be based on chloride and stable isotope concentrations determined for saturated zone (i.e. groundwater) samples only While the CMB Method is an attractive recharge estimation option in Southern Africa, geomorphological and geological controls were found to significantly influence the techniques application, particularly at sites where recharge via preferential pathways occurs. Of concern, however, is that the method represents a long-term average condition, which dependent on the volume of groundwater stored in a given aquifer, could extend to thousands of years or more. Thus, the validity of applying the method could be questioned at some sites because of past land use and climatic changes, and indeed those that may be currently occurring as a result of global warming. To restore confidence in the method, steps should be taken to assess what influence these changes may have had on aquifer chloride concentrations using an inverse modelling approach. Further, the impact of future changes on the chemical and isotopic composition of recharge water should also be considered Given the limitations of the CMB Method, it may seem paradoxical that its use be recommended within some 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 long-term 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 at fractured rock terrains, but rather that recharge calculated using water balance methods is 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 technique’s is recommended during all recharge-based

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