Reaction path modeling of hydrogeochemical evolution of groundwater in granitic bedrocks, South Korea

Groundwaters from areas of granitic bedrock in Daejeon, Eonyang and Yeongcheon in South Korea mainly belong to Ca-HCO ₃ and Na-HCO ₃ groundwater types according to their chemical composition, with fewer examples of the Ca-(Cl+SO ₄) and Na-(Cl+SO ₄) types. To explain the hydrogeochemical evolution of these groundwaters during deep circulation in granitic rocks, we performed reaction path modeling. The results show that the hydrochemical type of groundwater progressively evolves from an initial Ca-Cl type (rainwater) to a final Na-HCO ₃ type (of deep groundwater), through an intermediate Ca-HCO ₃ type. The reaction path model agrees well with the clay mineralogy of the granites. As the reaction between rainwater and granitic rock progresses, the activity of hydrogen ion decreases (i.e., pH increases) and the concentrations of cations are controlled by the dissolution of major rock-forming minerals, followed by the precipitation and dissolution of secondary minerals, according to the pH. The continuous reaction with granite causes the formation of secondary minerals in the following sequence: gibbsite plus hematite, Mn-oxide, kaolinite, chalcedony, chlorite, muscovite (a proxy for illite here), calcite, laumontite, prehnite, and finally analcime. Silica minerals are predominant in abundance among the minerals formed through the granite-water reactions. The results of this study can provide geochemical information for the preliminary site characterization for high-level nuclear waste disposal at depth in granitic rocks in South Korea and elsewhere.