Testing Potential Reservoir Connectivity Using Isotopes

Anticipated Shale Gas Development could intensify possible natural hydraulic connectivity between deep groundwater reservoirs and shallow aquifers in the Karoo. This project attempts to test geochemical evidence of natural mixing between old groundwater from deep aquifers and young groundwater from shallow aquifers using selected isotopic signatures in conjunction with borehole yields. Borehole yields were determined using slug tests. All isotopes (δ18O, δ2H, δ13C, 3H and 14C) were analysed in the laboratory of Environmental Isotope Group of iThemba Laboratories in Gauteng. To date, results from four water samples collected in Jansenville reveal these average isotope signatures: δ18O = -3.02 ‰, δ 2H = -21.17 ‰, δ 13C = 12.46 ‰ 3H = 0.45 TU and 14C = 65.38 pMC. The δ18O-δ 2H relationship for the groundwater has a gradient of 4.48. This demonstrates that the groundwater has experienced evaporation before or during recharge. The unevaporated isotopic signature of the water is -5.86 ‰ and -33.89 ‰ for δ18O and δ2H, respectively. The enriched δ13CDIC signature suggests that methanogenesis has influenced the groundwater. The unstable isotopes (3H and 14C) suggest that there is groundwater mixing in Jansenville between younger water from shallow aquifers and older water likely from deeper aquifers. Borehole yields increase with decreasing radioactive isotope concentrations. This suggests that high yielding boreholes are areas of potential contamination because they are associated with mixed groundwater.