NGUYEN Truong An

soutenance en 2021

Modélisation biogéochimique des nutriments dans une rivière tidale et scénario de gestion de l’eutrophisation

supervisors : Julien Némery, Nicolas Gratiot, Thanh-Son Dao

Date de début et de fin de la thèse : 11/2018 – 11/2021
Thèse financée par une bourse ministérielle

Context

The work undertaken at CARE (Centre Asiatique de Recherche sur l’Eau) for the past 4 years on the study area in southern Vietnam has resulted in the acquisition of a large database that now allows us to better understand the biogeochemical and sedimentary dynamics of the Saigon River. This tidal river is subject to strong anthropogenic pressures with the presence of the megacity of Ho Chi Minh (> 8 million inhabitants). In particular, the river suffers from an excess of untreated urban waste waters that increases nutrient and organic matter concentrations (Strady et al., 2017). Phytoplankton blooms are observed in the dry season and are symptomatic of a high level of eutrophication (Nguyen et al., 2019). If this diagnosis is now established, it seems essential to explore remediation solutions, particularly through the implementation of a complete hydrodynamic and biogeochemical modeling. This type of tool does not exist in the study area and is clearly a strong demand from the Vietnamese watershed managers (Department of Natural Resources and Environment).

Objective

The objective of this thesis project is to set up a complete modeling of the Saigon River system capable of reproducing the sedimentary dynamics, nutrients and organic matter and the development of algae. The field database acquired so far through several research projects will allow calibration and validation of the model. Once validated, the model will be used to explore different scenarios for reducing the risk of eutrophication (proposals for installation of treatment plant, reduction of agricultural inputs, etc). The C-GEM model was chosen and developed for this study in view of its capacity to take into account the specificities of the site (tidal river, under tropical monsoon regime).

The project

Understanding the processes which control the nutrient dynamics along the salinity gradient of estuary is essential for evaluating the risk of eutrophication response to the nutrient supplies from anthropogenic activities. There exists various numerical models of nutrient dynamics such as "Box models" which simulated the estuary as a single, vertically and horizontally well-mixed box with steady residual hydrodynamic characteristics ; "Reactive transport models" (RTMs) which were the quantitative tools for disentangling the complex biogeochemical transformations and fluxes in coastal environments. Recently, Carbon-Generic Estuary Model (C-GEM), one-dimensional, generic reactive-transport model for the biogeochemical dynamics of carbon and associated bio-elements (N, P, Si) was developed for the tidal estuaries. C-GEM uses an idealized representation of the estuarine geometry to support hydrodynamic calculations in order to reduce the data requirement, however it can provide accurate description of the transport of nutrients and the biogeochemical reaction processes (Volta et al., 2014, 2016).
Saigon river estuary (202 km from the reservoir to the estuarine mouth), a tidal estuary in the southern Vietnam, is chosen to apply the C-GEM to reproduce nutrient dynamics within two zones (i) Tidal freshwater zone : receive nutrient supplies from more than 8 million inhabitants and the intensive agriculture and industries (especially agro-food processes). (ii) Brackish to saline zone : the major aquaculture production and Can Gio mangrove (20000 ha, classified as a ’biosphere reserve’ by UNESCO). The scientific objective of this thesis is to understand (/quantify) the fate of nutrients in the river to external forcings and tidal estuarine dynamics : (i) Setup a model in Saigon River, capable of reproducing nutrients, organic matter, phytoplankton and sedimentary dynamics and an indicator of potential eutrophication. (ii) Model application to explore different scenarios for reducing risk of eutrophication (WWTP, reduction of fertilizer uses, climate change effect).

References

T.T.N. Nguyen, J. Némery, N. Gratiot, E. Strady, V.Q. Tran, T.A. Nguyen, Joanne Aimé (2019). Nutrient dynamics and eutrophication risk assessment in a tropical river : case study of Saigon - Dongnai river system (Southern Vietnam). Science of the Total Environment. 653, 370-383
Strady, E., Dang, V. B.H., Nemery, J., Guedron, S., Dinh, Q.T., Denis,H., & Nguyen, P. D. (2017). Baseline seasonal investigation of nutrients and trace metals in surface waters and sediments along the Saigon River basin impacted by the megacity ofHo Chi Minh (Vietnam). Environmental Science and Pollution Research, 24(4), 3226-3243
Volta, C., Arndt, S., Savenije, H. H., Laruelle, G. G., & Regnier, P. (2014). C-GEM (v 1.0) : a new, cost-efficient biogeochemical model for estuaries and its application to a funnel-shaped system. Geoscientific Model Development, 7(4), 1271-1295
Volta, C., Laruelle, G. G., Arndt, S., & Regnier, P. (2016). Linking biogeochemistry to hydro-geometrical variability in tidal estuaries : a generic modeling approach. Hydrology and Earth System Sciences, 20(3), 991-1030