The identification and delineation of high-yielding wellfield areas in Karoo Aquifers as future water supply options to local authorities

English: This Masters thesis forms the capacity development component of the Water Research Commission (WRC) project number KS/1763, entitled "The identification and delineation of high-yielding well-field areas in Karoo aquifers as future water supply options to local authorities". The project was initiated due to the need to place the significant knowledge on groundwater of the Karoo Basin within the realms of water resource planning. The ever growing issues related to water resource planning include not only the challenge of finding groundwater resources, but quantifying the supply of this resource in terms that are readily understood by hydrogeologists and related professions. In an attempt to address these issues, a method by which groundwater resources can be identified as well as quantified is described in this thesis, which incorporates the concept of assurance of supply. This method involves the use of a number of tools, some of which are existing and are readily available to the public, others may be available in the specific area of interest (e.g. aeromagnetic imagery), and the remaining have been developed as part of the WRC project, and critically reviewed in this thesis. The development of the Transmissivity Map in this thesis took both existing borehole yields and geology into account, and provides a range of possible transmissivity values presented both in tables and maps. The ranges are provided for each hydrogeological domain (based on lithologies and in some cases, sub-divided lithologies), dole rite dykes and sills, fractured margins of sills and areas of thick alluvium. Woodford's method was used, which can be found in Dondo et al., 2010, which was then extrapolated across the Main Karoo Basin. This map is the most detailed map produced of the Main Karoo Basin and from the case studies presented appears to provide a reasonable estimate of transmissivity values. The Aquifer Assured Yield Model (AAYM) was run for a large number of quaternary catchments spread across the Karoo Basin to test the model's credibility, as well as to propose parameter values to be used per region or drainage basin. The AAYM compared well with other databases, namely the HP and GRAII AGEP. The work appears to be the first documented approach to quantifying groundwater with levels of assurance, and thus should be considered "work-in-progress", as is it requires an iterative process of development, testing, modifying and re-testing. The Wellfield Model was successfully developed on the basis of the Cooper-Jacob equation (Cooper & Jacob, 1946). Through the testing of the model, relationships of borehole spacing with transmissivity values were investigated in an attempt to provide a guideline on the design of a wellfield with certain borehole interference limitations. In addition to this, the distinct nature of groundwater flow in dykes was considered by referring to the Boonstra- Boehmer equation (Kruseman & de Ridder, 1992) whereby a certain increase in borehole spacing is required when a borehole is sited on a dyke. This model enables the designing and manipulation of a wellfield and the effect of groundwater abstraction on drawdown can be evaluated thereby aiding in the most optimum design. The methodology applied to case studies demonstrates the practical application of these tools and models described above. The purpose of the case studies was to apply the groundwater yield assessment methods in areas with known aquifer parameters and yields. The yield assessment methods were evaluated in terms of their accuracy and practicality by comparing the results with other existing yield assessment tools and with field data. The case studies showed that the newly produced geological maps and the Transmissivity Map can be easily used with satellite imagery to identify new potential borehole and wellfield areas. Overall, this thesis provides a step by step methodology to identify and delineate high groundwater potential areas in the Main Karoo Basin, and quantify the groundwater that is available in these areas. In order for groundwater resources to be accurately quantified, it must be presented with levels of assurance of supply and from these rates a wellfield can be developed whereby guidelines should be followed to obtain an optimum design in order to avoid over abstraction. Recommendations have been provided regarding further work and expansion to be undertaken in each of these tools and models.