Abstract
This study assessed surface water quality changes in Tien Giang province, Vietnam, during 2019–2023 using the water quality index (WQI), principal component analysis (PCA), and cluster analysis (CA). Researchers measured eleven parameters (temperature, pH, TSS, DO, BOD, COD, NH4+-N, NO3--N, PO43--P, Cl-, and coliform) at 34 monitoring stations. Results indicated a marked decline in water quality over time, especially severe organic and microbial pollution during 2022–2023. During 2019–2020, water quality was mainly good to very good, with over 85% of stations stable; this temporarily improved in 2021 (44.12% very good). From 2022, VN_WQI ranged from 9–94, with poor, heavily polluted, and very heavily polluted stations increasing sharply; by 2023, 91.18% of stations recorded poor water quality. This decline was driven primarily by a more than fourteen-fold increase in mean coliform density between 2021 and 2023, together with persistently elevated BOD and COD, despite a concurrent improvement in dissolved oxygen. CA identified four groups with distinct pollution levels: Group I (S17, S19, S20) was most severely polluted; Groups II and IV showed large fluctuations; Group III (S34, Ly Quan canal) had the best water quality. PCA extracted four principal components explaining 77% of the total variance. The main factors affecting water quality included agricultural activities (cultivation, livestock, aquaculture), domestic and industrial wastewater, and saltwater intrusion. These findings provide a scientific basis for spatially prioritized pollution control and for rationalizing the provincial monitoring network, which is particularly relevant following the 2025 merger of Tien Giang into Dong Thap province.