Monitoring of CO 2 -rich waters with low pH and low EC: an analogue study of CO 2 leakage into shallow aquifers

The geochemistry of CO 2 -rich springs and wells was monitored as a natural analogue study of CO 2 leakage into shallow aquifers. In result, within the relatively small study area, diluted CO 2 -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 CO 2 inputs into the open system, which had a short reaction time and rarely consisted of reactive minerals. DCWs showed the low concentrations of SiO 2 (mean 20.4 mg/L) and the average tritium concentration of 3.4 TU, which indicates the low CO 2 –H 2 O–rock interaction. In addition, δ^13C_CO_ 2g in equilibrium with water was estimated using the mass balance equation and δ 13 C DIC measured in water samples. The results of the stable carbon isotope analysis showed that the CO 2 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 CO 2 near the surface. To detect the CO 2 leakage from CO 2 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 CO 2 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 CO 2 leakage detection.