<p>For a source of water supply, it is crucial to understand the hydrological system and identify the anthropogenic stress factors for sustainable use of water resources. This study employs hydrochemical and isotopic data to evaluate the groundwater flow system of the Gwanin Water Intake Plant (GWIP), where Quaternary basaltic rocks have erupted over granite bedrock. Groundwater geochemistry is distinctly categorized by chemical weathering of the bedrocks based on Ca + Mg vs. HCO₃ + 2SO₄, which is supported by a hierarchical cluster analysis of measured parameters. The correlation between Cl and SO₄ showed that groundwater dominated by basalt weathering was more susceptible to contamination. Meanwhile, the correlation between NO₃ and Cl suggests differences in the relative contributions of various contamination sources depending on aquifer lithology. Evaporation signatures in in δ¹⁸O and δ²H indicate that local recharge from impounded water in paddy fields is the primary driver of these differences. ⁸⁷Sr/⁸⁶Sr ratios in groundwater indicate that the differentiation in chemical weathering is due to the distinct aquifers associated with different bedrock types. The clear difference in δ¹³C-DIC between surface water and groundwater suggests that their interaction is largely restricted. Based on the ⁸⁷Sr/⁸⁶Sr results, an end-member mixing analysis using SiO₂ and δ¹⁸O reveals that the contributions of impounded water from paddy fields and the Naengjeong Reservoir on GWIP range from 19% to 26%. These results underscore the need for managing contamination sources originating from the reservoir and paddy fields to ensure the sustainable use of GWIP.</p>