The geochemical evolution of very dilute CO2‐rich water in Chungcheong Province,Korea: processes and pathways

A geochemical study was carried out on the CO₂‐rich water occurring in granite areas of Chungcheong Province, Korea. In this area, very dilute and acidic CO₂‐rich waters 62–242 mg l^?1 in total dissolved solid (TDS), 4.0–5.3 in pH; group I) occur together with normal CO₂‐rich waters (317–988 mg l^?1 in TDS, 5.5–6.0 in pH; group II). The concentration levels and ages of group I water are similar to those of recently recharged and low‐mineralized groundwater (group III). Calculation of reaction pathways suggests that group I waters are produced by direct influx of CO₂ gas into group III type waters. When the groundwater is injected with CO₂, it develops the capacity to accept dissolved solids and it can evolve into water with very high solute concentrations. Whether the water is open or closed to the CO₂ gases becomes less important in controlling the reaction pathway of the CO₂‐rich groundwater when the initial pco ₂ is high. Our data show that most of the solutes are dissolved in the CO₂‐rich groundwater at pH > 5 where the weathering rates of silicates are very slow or independent of pH. Thus, groundwater age is likely more important in developing high solute concentrations in the CO₂‐rich groundwaters than accelerated weathering kinetics because of acidic pH caused by high pco ₂.