Organic Pollution Dynamics and WQI Masking Effects in Surface Waters of Lai Chau Province

Abstract

Composite water quality indices such as the Vietnam Water Quality Index (VN_WQI) are widely used to synthesize complex monitoring data; however, their aggregated structure may obscure environmentally relevant pollution signals. This study evaluates surface water quality in Lai Chau Province, Vietnam, using VN_WQI in combination with component-based, spatial, and scenario-based analyses to examine the representation of organic pollution dynamics. Water quality data from 59 monitoring sites collected during the rainy and dry seasons in 2024 were analyzed using three VN_WQI calculation scenarios, hierarchical cluster analysis (HCA) based on dissolved oxygen (DO), chemical oxygen demand (COD), and biochemical oxygen demand (BOD₅), and non-parametric statistical tests. VN_WQI consistently classified surface waters as “Good–Excellent” (95.5–97.6 rainy; 79.4–95.5 dry depending on scenario), yet component-level analysis revealed substantial spatial and seasonal variability. HCA identified three statistically distinct clusters representing different organic pollution regimes that were not apparent from aggregated VN_WQI results. Seasonal analysis showed significant increases in COD and BOD₅ during the dry season (p < 0.001), while DO exhibited no significant change. Notably, BOD₅ exceedances reached 86.4% of sites in the dry season despite uniformly high index values. Scenario comparison indicates a trade-off between comprehensiveness and sensitivity. The priority-weighted formulation enhances the detection of organic pollution while retaining the full parameter set, providing a practical approach for improving VN_WQI application. These findings support a flexible, context-dependent use of VN_WQI to better identify spatially and seasonally sensitive water-quality risks.