Surface-water quality assessment across a river–lagoon continuum in central Vietnam using parameter-based analysis, PCA and VN_WQI

This study assessed surface-water quality across a river–lagoon continuum in Central Viet Nam using a framework of three modules: individual-parameter assessment, principal component analysis (PCA) and the Viet Nam Water Quality Index (VN_WQI). Twelve monitoring campaigns were conducted from June 2022 to December 2023 at 20 sites covering the entire river–lagoon surface-water system, generating a dataset of 240 observations for 18 physicochemical and microbiological parameters. PCA retained five principal components explaining 74.8 per cent of the total variance and identified nine priority parameters controlling spatiotemporal variability: E. coli, total coliforms, water temperature, BOD5,N– NO3 −,N–NH4 +, turbidity, Mn, and total dissolved Fe. Two-way ANOVA revealed significant seasonal differences, with higher rainy-season values of E. coli,N–NO3 −,N–NH4 +, turbidity, Fe, and water temperature. Spatially, water quality was generally better in the upstream reaches of the Huong and Bo rivers, whereas the Phu Bai river showed the most pronounced deterioration and recurrent pollution hotspots. For the 16 riverine monitoring sites, VN_WQI classified 42 per cent of observations as excellent, 31 per cent as good, 23 per cent as average, and 4.2 per cent as poor; both monitoring location and season had significant effects on VN_WQI values. Overall, the results indicate that water-quality deterioration in the study area was hotspot-driven and seasonally amplified, rather than occurring uniformly across the system. The proposed framework provides a practical basis for resource-efficient adaptive monitoring and future water governance.