Characterization of confined and unconfined conditions in a dual-aquifer system using physical and chemical indicators: a case study of the Nakdong River delta region, Korea

The Nakdong River delta comprises an upper unconfined freshwater aquifer and a lower confined saline aquifer, where excessive pumping may induce saline migration, requiring systematic management.In this study, cross-correlation analysis, flow-direction logging, hydrochemical profiling, principal component analysis (PCA), hierarchical cluster analysis (HCA), vertical EC profiling, and pumping tests were used to differentiate the characteristics of the upper and lower aquifers.The PCA/HCA results indicate that the two aquifers form a dual-flow system with strong hydraulic isolation.The unconfined aquifer (mean EC = 1,853 S/cm) is characterized by oxidizing conditions, Na + /Cl - > 1, HCO 3 -/Cl -> 1, whereas the confined aquifer (mean EC = 19,822 S/cm) contains groundwater of post-Pleistocene seawater origin (Na + /Cl - 1, HCO 3 -/Cl -< 1, elevated Mg 2+ ), exhibits mildly reducing conditions, and maintains stable EC during pumping.Pumping tests revealed short 1 values and distinct initial drawdowns (0.21-1.0 m) in the confined aquifer due to elastic response, while the unconfined aquifer showed no clear elastic response.These findings confirm that the study area comprises a dual structure with an overlying unconfined aquifer and an underlying confined aquifer, highlighting the need for careful management of groundwater abstraction from the lower aquifer.