Characterization of the Co2 Springs Associated With the Bongwana Fault, Its Impact on Fresh Surface Water and Groundwater Quality and Implications on Carbon Capture and Storage (Ccs) In South Africa

The natural CO2 springs at the Bongwana area emanates from a number of sites along an 80 km long N-S trending fault known as Bongwana fault. CO2 rich groundwater is observed at a shallow depth in a borehole drilled for water supply parallel to the main fault. The geology along the entire fault length is characterized mainly by Dwyka Group sediments. The objectives of this study are to characterize the CO2 springs and assess their impacts on groundwater and surface water quality. To that end, existing literature and data were reviewed followed by the collection of groundwater and surface water samples at both CO2 emission and CO2 free streams, springs and boreholes around the length of the Fault zone. Major ions, trace elements and environmental isotopes analyses were carried out on the samples collected. EC, TDS, pH, Temperature, DO, Eh, ORP, total alkalinity, CO3 2- , HCO3 - ) were measured onsite. Acidic pH conditions, elevated TDS, EC and 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. 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. The correlation among the deep CO2 rich groundwater parameters indicates that the major geochemical processes that could be responsible for the observed chemical composition are the precipitation of calcite and dolomite where their saturation indices (SI) range from 0.74 to 0.82 and from 0.24 to 1.35, respectively and the weathering of feldspars. 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. 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. 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 subsurface storage site can impact this already scarce resource. Therefore, strict scientific site selection protocols and CO2 leakage detections through properly designed monitoring systems are required to minimise the risk.