Multi-index evaluation of surface water quality at the transboundary section of the Mekong River: A case study of Nakhon Phanom Province, Thailand

A seasonal, multi-index examination of surface water quality was performed for the Mekong River at the Thailand-Lao transboundary section in Nakhon Phanom Province, utilizing six internationally recognized Water Quality Indices such as MRC-WQI al , PCD-WQI, NSF-WQI, CCME-WQI, BIS-WQI, and EU-WQI. The monitoring period showed that the Mekong River always had good oxygen levels (DO = 7.48–8.77 mg/L), which shows that it has a strong ability to clean itself. However, pronounced nutrient amplification during the dry season became the primary ecological control, with ammonium levels rising from 0.03–0.04 to 0.27–0.39 mg/L, nitrate levels from 0.23–0.32 to 1.86–1.95 mg/L, and total phosphorus levels from 0.04–0.06 to 0.43–0.48 mg/L, surpassing eutrophication thresholds by nearly threefold. Microbial contamination exhibited an inverse seasonal trend, with total coliforms reaching their zenith in the wet season (1.69–1.90 × 10⁴ MPN 100 mL⁻¹) because of storm runoff and diffuse fecal loading, subsequently experiencing a 5–6-fold reduction in the dry season. The assessment of heavy metals revealed that most elements had low concentrations. However, cadmium consistently exceeded international safety limits, resulting in hazard quotients as high as 6.67 and dominating the overall ecological risk index. There was a big difference between the WQI frameworks. Ecological indices (MRC-WQIal and CCME-WQI) showed a clear change from good to very good quality in the wet season to bad to marginal quality in the dry season. On the other hand, general-use indices (PCD-WQI, NSF-WQI, BIS-WQI) still rated the river as good to very good, showing that single-index interpretation can hide a lot of risk. The Mekong River has two types of seasonal risks right now: wet-season microbial contamination and dry-season phosphorus-driven eutrophication. There is also a constant risk of cadmium. These results show reliable transboundary river management and early warning of ecological degradation in the Lower Mekong Basin, which need integrated, multi-index, seasonally resolved frameworks.