Water season quality variation brings pollutant load into the largest hydropower reservoir in Vietnam after the dam impoundment (2014-2024)

Abstract

Hydropower reservoirs have been essential for electricity generation, water regulation, and water supply. The possibility of water quality and pollutant load variation may pose the potential risks to exposed ecosystems and livelihoods in the downstream areas. This study was conducted to determine the water quality change dissolved Oxygen (DO), total suspended solids (TSS), biological oxygen demand (BOD₅), chemical oxygen demand (COD), ammonium (NH₄⁺), nitrite (NO₂^-), nitrate (NO₃^-), phosphate (PO₄^3-), and total iron (Fe) and pollution load of the mainstream and three tributaries of the Son La hydropower reservoir in Vietnam during 2014 - 2024. The results of water flow revealed the increase trend during 2014 - 2024 and flow in rainy season (1,562 - 3,678 m³/s) higher water that in dry season (741 - 2,086 m³/s). Water quality parameters showed no significant variation between dry and rainy season with the exception of DO, TSS, NH₄⁺, NO₂^-, NO₃^-, and PO₄^3-, total Fe demonstrated an increase trend during 2014 - 2023; however, TSS, BOD₅, and COD slightly decreased. The pollutant loads of four rivers entering the reservoir were 1,581,848 to 9,714,484 tones/year, average: 3,403,604 tones/year, respectively. Pollutant load in the rainy season accounted for 63.1 - 92.4% of the yearly pollutant load. The average pollutant loads ratio (tones/year) in the following order: TSS (75%) > COD (15%) > BOD₅ (8%) > NO₃^- (0.7%) > total Fe (0.7%) > NH₄⁺ (0.5%) > PO₄^3- (0.4%) > NO₂^- (0.2%) of the yearly pollutant loads. The quantification of water quality variation and pollutant load may provide valuable and scientific basis for monitoring water quality and controlling pollution in the hydropower reservoir basins.