Urbanized tropical estuaries : hot spots of greenhouse gas emissions

IGE and CARE International Joint Laboratory (Asian Center for Water Research) have been conducting research for several years on the pollution flows of organic matter and nutrients (nitrogen and phosphorus) from the Ho Chi Minh Megacity in southern Vietnam. This large Southeast Asian city is undergoing rapid economic and demographic expansion (10 millions inhabitants in 2020). With less than 15% of domestic urban water collected and treated, the city has a lasting impact on the quality of the Saigon River and its estuary. The level of eutrophication is extreme with regular phytoplankton blooms in the dry season and oxygenation rates at their lowest, close to anoxia (< 1 mgO2 L-1).

This new study published in Environmental Pollution explores the link between the eutrophication level of the Saigon River estuary and the concentrations and fluxes of 3 greenhouse gases (CO2, CH4 et N2O). This original study presents a complete set of data in wet and dry season to estimate the emissions of a tropical estuary and to compare them with those of temperate estuaries, better documented in the scientific literature. Concentrations of CO2 (3174 ± 1725 μgC-CO2 L−1), CH4 (5.9 ± 16,8 μgC-CH4 L−1), and N2O (3,0 ± 4,8 μgN-N2O L−1) are 13-18-fold, 52-332-fold, and 9-37-fold higher, respectively, than the average concentrations of estuaries on a global scale. This results in average CO2 fluxes of 36.6 gCO2/m2/d, CH4 fluxes of 0.6 gCO2Eq/m2/d, and N2O fluxes of 8.8 gCO2Eq/m2/d. The increase in eutrophication status along the dense urban area is linearly correlated with the increase in gas concentrations. In particular, the intense nitrification and denitrification of nitrogen and mineralization of organic carbon orginated from urban wastewater leads to high concentrations of CO2, CH4 and N2O. A more drastic treatment of urban wastewater discharges would allow to reduce the emissions of these gases with high stakes for the global warming.

Reference :

Nguyen AT, Némery J, Gratiot N, Dao TS, Le TTM, Baduel C, Garnier J (2022) Does eutrophication enhance greenhouse gas emissions in urbanized tropical estuaries ? Environmental Pollution 303 : 119105

https://authors.elsevier.com/c/1enWBzLNSc73s

This publication is part of the Phd work of Nguyen Truong An :
Biogeochemical modeling of nutrients in an urbanized tropical estuary and eutrophication management scenario (2021). Phd of Université Grenoble Alpes, 210p

http://www.theses.fr/2021GRALU038#

Scientific contacts :
Nguyen Truong An : doctorant IGE/CARE
Julien Némery : Enseignant Chercheur IGE UGA/Grenoble-INP