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, (Formula presented.) in equilibrium with water was estimated using the mass balance equation and δ¹³C_DIC 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.