The Hydrogeochemical Characteristics of Groundwater Subjected to Seawater Intrusion in the Archipelago, Korea

The effect of seawater on the groundwater in archipelago of South Korea where it has rarely been investigated was analyzed by examining the hydrogeochemical characteristics. A total of 74 groundwater samples were classified by water quality type and Cl-/HCO(3)(-)molar ratio. First, 36 samples of the Ca-Cl type and 32 samples of the Na-Cl type (accounting for 91.9% of the total) were considered to have been influenced by seawater. When the samples had been classified based on the Cl-/HCO(3)(-)molar ratio, the samples with a Cl-/HCO(3)(-)molar ratio of 2.8 or higher (indicating that seawater had highly influenced the groundwater) accounted for 40 out of 74 samples. This confirms that the groundwater in the study area had been affected by seawater. When quantitatively determining the influence of seawater on the groundwater, the seawater mixing ratios using either Cl or Br ion were found to be almost the same. In the case of Cl ion, the mixing ratio was in the range of 0-10.4% (average of 1.0%), while when using Br ion, the mixing ratio was in the range of 0-7.6% (average of 0.6%). From a principal component analysis, it can be seen that the influence of seawater occupied the first component of 54.1% and it is evident that the samples with a large mixing ratio of seawater were from regions where seawater has a large influence. The ion-exchange reaction was proceeded by calculating the ionic delta value to indicate the seawater intrusion and cation exchange, and specific trends of the ions participating in the geochemical reaction related to the seawater mixing ratio are reported herein. It was found that the ionic delta value of each ion had a mixing ratio and specific tendency according to the change in mixing ratio before the constant value of the seawater mixing ratio saturated with Na2+. Our results show that it can be possible to grasp the contribution of the geochemical reactions of each ion to the seawater mixing ratio.