Monitoring of CO2-rich waters with low pH and low EC: an analogue study of CO2 leakage into shallow aquifers

The geochemistry of CO2-rich springs and wells was monitored as a natural analogue study of CO2 leakage into shallow aquifers. In result, within the relatively small study area, diluted CO2-rich waters (DCWs), concentrated CW (CCW), and ordinary groundwaters were observed. DCWs showed an exceptionally low pH (mean 4.8) and low EC (mean 150 μS/cm). The low pH and EC as well as the time-invariant geochemistry of DCWs were probably due to continuous CO2 inputs into the open system, which had a short reaction time and rarely consisted of reactive minerals. DCWs showed the low concentrations of SiO2 (mean 20.4 mg/L) and the average tritium concentration of 3.4 TU, which indicates the low CO2–H2O–rock interaction. In addition, \(\delta^{13} {\text{C}}_{{{\text{CO}}_{ 2} {\text{g}}}}\) in equilibrium with water was estimated using the mass balance equation and δ13CDIC measured in water samples. The results of the stable carbon isotope analysis showed that the CO2 originated from a deep-seated source to the shallow DCW aquifer (<80 m deep below the surface), whereas the deeper CCW aquifer was affected by soil organic CO2 near the surface. To detect the CO2 leakage from CO2 storage sites, the geochemistry of shallow aquifers should be monitored. This study result suggests that at least pH, EC, DIC, and carbon isotopes should be monitored because the monitoring of pH is helpful in an aquifer with low buffering capacity; while EC can be low despite CO2 leakage and the subsequently low pH at early stages, depending on the subsurface environment. Above all, this present study indicates that understanding the characteristics of aquifer conditions is of great importance for CO2 leakage detection.