| Summary: | The Paldang Reservoir (PDR) in South Korea is vital for supplying drinking water and maintaining ecosystems; thus, a comprehensive understanding of its water quality is necessary. Spatiotemporal changes in reservoir water quality were evaluated by applying Korean water quality indices and multivariate statistical techniques (MSTs). A dataset of 15 water quality parameters at five sites in the PDR were evaluated from 2017 to 2021. The organic matter, suspended matter, total phosphorus (TP), chlorophyll <i>a</i> (Chl-<i>a</i>), and total coliforms in the PDR exhibited a fair grade or higher. Chemical oxygen demand was found to correlate with biochemical oxygen demand, Chl-<i>a</i>, and TP. The average real-time water quality index (RTWQI) and average trophic state index (TSI<sub>KO</sub>) of the PDR were excellent and mesotrophic, respectively, and 46% of eutrophic conditions occurred during the monsoon season. For a hierarchical cluster analysis (HCA), the five sites were grouped into three polluted areas and 12 months were grouped into dry and wet seasons. Principal component analysis and factor analysis identified four potential pollution sources (domestic sewage, industrial wastewater, intensive agricultural activities, and livestock wastewater) in the PDR and explained 79.7% of the total changes. Thus, the RTWQI, TSI<sub>KO</sub>, and MSTs are useful tools for assessing freshwater quality in Korea, predicting potentially harmful conditions, and potentially assisting policymakers in PDR management.
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