Evaluating the response of water quality to pollutant loading in the Saigon River system (Vietnam): modelling scenarios by C-GEM, an estuarine biogeochemical model

International audienceIn a context of densification of urban centers and climate change, coastal megacities in emerging countries have to face new issues about water management. With more than 9 million inhabitants, Ho Chi Minh City (HCMC), the economic capital of Vietnam, is in full demographic and economic expansion. The city does not have water treatment networks that meet the urgent needs. Indeed, in 2020, less than 15% of the population is connected to a sewerage network and treatment plant. The deterioration of water quality of Saigon River crossing the city is dramatic. Ho Chi Minh City authorities therefore plan to build new sanitation networks and ten wastewater treatment plants (WWTPs) over the next fifteen years, drawing inspiration from the centralized water management models of industrialized countries. However, the question arises of the relevance of this extremely expensive choice and not necessarily adapted to the particular conditions of this megacity in tropical monsoon zone and under the influence of tidal cycles. Using the estuarine biogeochemical model (C-GEM), this study aims at assessing the water quality of the Saigon River (oxygen depletion, nutrients concentration) in response to three scenarios of HCMC development (population growth and new water management infrastructures). In addition, projected new climatic conditions (temperature, hydrology) will be considered. The results of three scenarios show that the construction of new WWTPs is relevant. However, the rapid population growth in HCMC will make improvements support by WWTPs not sufficient to turn the water quality in Saigon River from a state of bad quality to good as expected