Geophysical investigations in the Khakhea-Bray Transboundary Aquifer
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Date
2022
Authors
Ngobe, Thandeka Fortunate
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
A limited number of transboundary aquifers (TBAs) in the Southern African Development Community (SADC) region have been subjected to investigations that will improve the hydrogeological understanding of the shared groundwater resource. The Khakhea-Bray transboundary aquifer, shared between Botswana and South Africa, is one of the TBAs that lacks a conceptual understanding of the groundwater systems. Therefore, this study aims to contribute to filling this research gap by conducting a geophysical investigation in the Khakhea-Bray TBA. The study applied the magnetotelluric approach to understanding the factors influencing groundwater occurrence in the dolomite of the Khakhea-Bray TBA.
An ADMT-300S groundwater detectors of the ADMT series products was used to conduct the geophysical survey. This equipment measures the electric potential difference of the subsurface geology for 300 m below the surface measured at a 5 m depth interval. The magnetotelluric geophysical survey was carried out by targeting existing boreholes with accessible drilling data within the TBA. The survey stations were spaced at 3 to 5 Km intervals in areas with no boreholes. The integrated analysis approach used lithology data and water strike information from seven boreholes. The lithology data provided insight into the subsurface geology of the study area. The water strikes information of boreholes was used to calibrate the geophysical data of the dolomite aquifer system. This was done to identify the electrical properties of the water strike zones within the aquifer system, establishing the factors influencing groundwater occurrence. The geophysical data from survey stations on the same transverse line was processed into electric potential difference cross section models using RockWorks 2021 software. The distance weighting anisotropic method was applied to interpolate the data between the survey stations.
The results show that the weathered-fractured zone is the main factor influencing the groundwater occurrence in the dolomite rock. The weathered-fractured zone was characterized by electric potential difference values varying between 0.02 mV to 0.065 mV. The weathered fractured zones in the dolomite were observed at shallow depths ranging between 7.5 m and 90 m. These dolomite aquifers were confirmed by the water strikes of the existing boreholes that were drilled targeting the dolomite aquifer zones. Another zone exhibiting the electric potential difference values ranging between 0.02 mV and 0.065 mV was observed at depths between 165 m and 300 m. The zone below 165 m showing the electric potential difference values between 0.02 mV and 0.065 mV was regarded as an unconfirmed potential weathered-fractured zone that needs to be validated with lithology and water strike data. Since the weathered-fractured zone was identified as the factor for groundwater occurrence in the dolomite of the study area, this suggests that during groundwater exploration for borehole drilling in the study area, the zones showing electric properties of a weathered-fractured aquifer zone must be targeted to increase the success rate of the borehole.
The developed models showed the subsurface stratigraphy and the potential zones for groundwater occurrence within the transboundary dolomitic aquifer system of Khakhea-Bray TBA. This implies that the MT has the ability to map the aquifers and delineate the subsurface stratigraphic layers in dolomite settings. The models also revealed that the dolomite aquifers are confined, suggesting that these aquifers are less vulnerable to pollution from surface sources. The confined aquifer also suggests that the aquifer is not recharged directly from the top but through preferential flow paths on the dolomite rock surface.
Description
Dissertation (M.Sc.(Geohydrology))--University of the Free State, 2022
Keywords
Conceptual models, dolomite aquifer, electric potential difference, magnetotelluric geophysics, transboundary aquifer