Natural CO2-rich springs emanate along an 80 km long North-South trending Fault in Eastern, South Africa. The geological units that outcrops along the Fault are the Dwyka Group rocks that are made up of mainly tillites and subordinate sandstone, shales and conglomerates. Hydrogeological, hydrochemical and environmental isotopes (δ2H, δ18O, 3H, δ13C and 14C) were used to characterise deep and shallow circulating groundwater along the fault. Groundwater and surface water samples at both CO2 emission and CO2 free streams, springs and boreholes around the Fault zone were collected and analyzed. Onsite physicochemical parameters including electrical conductivity (EC), total dissolved solids (TDS), pH, Temperature, DO, Eh, ORP, total alkalinity, CO32-, HCO3- at various surface and groundwater points were measured onsite. The results indicate that all deep circulating travertine cone springs along the fault zone are characterized by high salinity (EC > 6000 µS/cm), Na-Ca-Mg-HCO3 water type, depleted heavy stable isotopic signal with > 30‰ d-excess values, detectable tritium and dead 14C values. The δ18O versus δ2H stable isotope plot 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 shallow circulating groundwater and river samples show freshwater, Ca-Na-Mg-HCO3 water type, stable isotopic composition that reflect local and modern precipitation and < 18 d-excess values. These observations indicate that the reservoir and source of recharge for the deep circulating groundwater are different from the shallow groundwater and surface water.
A conceptual hydrogeological including inverse hydrogeochemical model is proposed to explain the deep circulation of groundwater and generation of CO2 along the fault zone and formation of travertine springs. The hydrogeochemical inverse model indicates that the major geochemical processes that are responsible for the observed hydrochemistry are the dissolution of calcite, dolomite, Pyrite, Goethite, K-feldspars, fluorite, albite and sylvite and the formation of calcite, amorphous silica, iron hydroxide, iron carbonates, kaolinite and CO2 gas. The inverse modelling results are supported by calculation of saturation indices (SI) for various mineral phases. The precipitation of calcite, amorphous silica and iron carbonates were validated by XRF, XRD and thin section analyses results of samples taken from the travertine cones.
Key Words/Phrases: CO2-rich springs, deep and shallow groundwater, Environmental isotopes, Hydrogeochemical characterization, eastern South Africa.