The hydrogeochemical study on the $CO_2$-rich water in the Chojeong area was carried out. The $CO_2$-rich water of Ca-$HCO_3$type is characterized by low pH (5.0~5.8). high $CO_2$concentration ($Pco_2$＜$10^{0.31}$atm) and high TDS. The water chemistry indicates that the $CO_2$-rich water was probably evolved by the local suppy of deep seated $CO_2$gas resulting in the enhanced water/rock (granite) interaction under low pH conditions. High $NO_3$concentration indicates that the $CO_2$water was mixed and diluted with low $CO_2$groundwater in the vicinity of the area, in which the extensive groundwater abstraction occurred during the past years. The evoiution of the $CO_2$-rich water in the Chojeong area for the process of $CO_2$injection water/rock interaction and mixing processes was thermodynamically simulated by PHREEQC. Although the simulation was limited to water/plagioclase interaction, the results show the feasible explanation about the observed trend of pH and Ca and Na concentrations of the $CO_2$-rich water.