Characterisation of the geothermal waters of South Africa

Abstract

The lack of active volcanism in South Africa gives the impression that there are no geothermal energy sources. Conversely, there are 41 known geothermal water sites which are surface manifestations of geothermal energy in South Africa. By characterising the geothermal waters for their hydrochemical properties it is possible to investigate their potential as energy sources. Prior to the current investigation, there was insufficient regional data to properly characterise the geothermal waters as the South African energy nexus excluded geothermal energy. This research presents the hydrochemical characterisation of the 41 geothermal waters of South Africa against the backdrop of their geological settings. Their physical properties (temperature, pH, electrical conductivity, salinity) were measured at the source. Chemical analyses (Cl-, HCO3-, SO42-, Na+, Ca+, Mg2+, K+, and Fe2+, SiO2) were conducted using induced coupled plasma-optical emission spectrometer. Stable isotopes (δO18 and δD) and radiogenic isotope-tritium were analysed using an isotope ratio mass spectrometer and laser water isotope analyser, respectively. The results showed that the geothermal waters in South Africa are located in three main geological settings 1) basement/crystalline rock types, 2) orogenic belts and 3) intracratonic basins. Most of the geothermal waters display varying temperatures (25°C (Cradock) to 61°C (Brandvlei)), pH (3.5 (Badsfontein and Aliwal) to 9 (Warmbath1)) and electrical conductivity (2 μS/cm (Bricks) to 4930 μS/cm (Riemvasmaak). Four types of geothermal waters were identified in order of decreasing abundance, Na-Cl, mixed, Mg-HCO3 and Na-HCO3 waters. Most of these geothermal waters exhibit silicate weathering, partial equilibration and appear to be associated with a seawater signature. The isotope signatures confirm that the geothermal waters are of meteoric origin and are mostly older than 70 years. This study established that 20 out of the 41 geothermal water sites have low to medium enthalpy ranges of 432 to 624 kJ/kg, that could be harnessed for power generation. Furthermore, it was estimated that waters between 1.6 and 2.7 km deep are likely to reach temperatures ranging between 102°C and 148°C. Most of the favourable sites are associated with the basement/crystalline Archean granites and gneisses, and orogenic belts such as the Limpopo Mobile Belt (associated with Soutpansberg and Waterberg Groups), and the Namaqua Natal Province.