Using water isotopes, geochemistry, and unmanned aerial vehicles to investigate the origin of the Shu-Shu thermal springs and their role in affecting surface water/groundwater heat and chemical exchange: uThukela River.

Abstract

The Shu-Shu thermal springs are located in Nkandla, KwaZulu-Natal, emerging underneath and at the margin of the uThukela River. This work aimed to determine the geothermal energy potential of the Shu-Shu thermal springs, their origin, and their role in heat and chemical exchange with the uThukela River. The specific objectives of the investigation included determining the origin, the extent of discharge, the depth of circulation of the thermal springs, and assessing the impact the Shu-Shu thermal springs have on the surrounding surface water of the uThukela River. The Shu-Shu thermal springs were investigated using; isotopic, physiochemical, and Unmanned Aerial Vehicles (UAV) methods. Water samples were collected from the thermal springs, surrounding surface waters, and rainfalls across different altitudes during wet and dry seasons. The stable isotope (δ 18O, δ 2H) analysis confirms the meteoric origin of the thermal waters with a positive 18O shift. The Shu-Shu thermal springs are regionally sourced as they have a similar isotopic signature to the rainfall in the Drakensberg Mountains. A hydrochemistry analysis performed indicates that the Shu-Shu thermal springs are Na-Cl waters that are dominated by ions such as SO4 2- , Na+ , Cl- , Ca+, and SiO2. The major ions signify a rock-water interaction and the leaching of amphibolites, gneiss, and pyrite. From the numerous geothermometers used, only the Silica (quartz and chalcedony) geothermometers provide a reliable estimation (76-80 °C) of the reservoir temperature, and the estimated depth of 1.8-2 km. Mineral saturation states calculated from GWB 11 software indicate that thermal waters are supersaturated (SI>0) with respect to quartz, tridymite, chalcedony, cristobalite, and undersaturated (SI<0) with respect to amorphous silica, anhydrite, gypsum, bassanite, halite and many more. The thermal map produced from the UAV images shows that the Shu-Shu thermal springs discharge zone extends to an area of 3100 m2 along the faultline that strikes at N 63° E. The metamorphic rocks underlying the thermal springs have a low hydraulic conductivity, causing a low discharge rate. Based on the reservoir temperature estimations using geothermometry, it can be concluded that the Shu-Shu thermal springs have a low-enthalpy (temperature) energy potential. The origin of the springs is controlled by faults and fractures. The use of geophysics is recommended to further investigate the subsurface extent of the springs. The association of the springs with dykes/fractures indicates the need to investigate the geothermal energy potential associated with the other dense network of fractures and dykes in South Africa.