Hydrogeochemical and environmental isotope characterization of the CO₂ springs along the Bongwana Fault, its impact on fresh water resources and implications for carbon capture and storage (CCS) in South Africa.
Natural CO²-rich springs at the Bongwana area in Eastern South Africa emanate from three sites along an 80 km long North-South trending Bongwana Fault. The geological unit that outcrops along the extent of the Fault are the Dwyka Group rocks that are made up of mainly tillites and subordinate sandstones, shales and conglomerates. The objectives of this M.Sc. study is to characterize these CO²-rich springs and assess their impacts on shallow groundwater and surface water chemistry and consequently to understand the implication of a failed CCS facility. Groundwater and surface samples were collected both at CO² emission and CO²-free springs, boreholes and streams around the length of the studied fault zone for the analyses of major ions, trace elements and environmental isotopes. Additionally, specific electrical conductivity (EC), total dissolved solids (TDS), pH, temperature, dissolved oxygen (DO), redox potential (Eh), total alkalinity, CO³²- and HCO³- concentrations were determined onsite. The results indicate that all the travertine cone springs located near Umtamvuna River are characterized by Na-Ca-Mg-HCO3 water types, while boreholes from shallow groundwater and river samples show Ca-Na-Mg-HCO³ types. Stable isotope (δ18O and δ2H) composition of the travertine cone springs shows a major negative shift from the meteoric water lines with δ18O and δ2H values ranging from -7.78 to -6.52 ‰ and -21.5 to -17.9 ‰, respectively. While, the stable isotopic composition of shallow groundwater reflects local and modern meteoric recharge. These observations indicate that the reservoir and source of recharge for the deep circulating groundwater are different from the shallow groundwater. Based on onsite hydrogeological, hydrogeochemical, and environmental isotope observations, a hydrogeological conceptual model is proposed, which states that the groundwater recharge for deep circulating groundwater is to the west of the Bongwana fault, located at a higher altitude. From these altitudes, groundwater percolates through deep fractures and faults to greater depths. As groundwater percolates through the rock, it interacts with minerals and the initial recharge chemistry and isotopic composition is altered along the groundwater flow paths. At depth, groundwater dissolves carbonate rocks and as a result CO² is generated. The dissolution of CO² in groundwater further drives the leaching of the formation minerals along the flow path. Near the surface, CO² exsolves and travertine mainly composed of calcite, amorphous silica and iron hydroxides is formed. Geochemical inverse modelling and bivariate correlation among groundwater hydrochemical parameters for travertine springs indicate that the major geochemical processes that are responsible for the observed chemical composition are the dissolution of calcite, dolomite, Pyrite, Goethite, K-feldspars, fluorite, albite and sylvite and the precipitation of calcite, amorphous silica, iron hydroxide, iron carbonates, kaolinite and CO² gas. The carbonate minerals are attributed to the dissolution of carbonate rocks at depth. Feldspars are common from the Dwyka Group Diamictites, whereas the plagioclase feldspar (albite) is probably originating from the recharge area outside of the Dwyka group or leached from the granitic and metamorphic fragments contained within the Dwyka tillites. These inverse modelling results are supported by the saturation indices (SI) for calcite and dolomite in these springs which range from 0.74 to 0.82 and from 0.24 to 1.35, respectively indicating oversaturation with respect to these minerals and subsequent precipitation out of the aqueous solution. The precipitation of calcite, amorphous silica and iron carbonates were confirmed by the XRF, XRD and thin section results of the travertine rock samples. Acidic pH conditions (5.5), elevated TDS (5937 ppm), EC (3271 mS/m) and high trace metals concentration were detected in all CO² emission sites compared to CO² free streams, springs and boreholes. These results clearly show the impacts of CO² on groundwater and surface water quality within the vicinity of emission points. Therefore, it appears that natural CO² emission along the Bongwana fault have impacted the ambient groundwater and surface water quality at the emission sites rendering it unfit for human consumption due to elevated concentration of dissolved constituents above safe drinking standards. The implication of this to CCS in South Africa is the fact that any unintended CO² leakage into fresh groundwater and surface water resources from a failed subsurface storage facility may impact freshwater resources. Thus, strict scientific site selection protocols and properly designed monitoring systems are required to minimise the risk.