Nutrient budgets in the Saigon–Dongnai River basin: Past to future inputs from the developing Ho Chi Minh megacity (Vietnam)

Abstract

Ho Chi Minh City (HCMC, Vietnam) is one of the fastest growing megacities in the world. In this paper, we attempt to analyse the dynamics of nutrients, suspended sediments, and water discharges in its aquatic systems today and in the future. The work is based on nine sampling sites along the Saigon River and one on the Dongnai River to identify the reference water status upstream from the urban area and the increase in fluxes that occur within the city and its surroundings. For the first time, the calculated fluxes allow drawing up sediment and nutrient budgets at the basin scale and the quantification of total nutrient loading to the estuarine and coastal zones (2012-2016 period). Based on both national Vietnamese and supplementary monitoring programs, we estimated the water, total suspended sediment, and nutrients (Total N, Total P, and dissolved silica: DSi) fluxes at 137 m 3 year −1 , tonSS year −1 , 5,323 tonN year −1 , 450 tonP year −1 , and m 3 year −1 for the Saigon River and 1,693 year −1 , 3,292 2,734 1,175 × 10 3 tonSi × 10 3 tonSS year −1 , 31,030 tonN year −1 , 1,653 tonP year −1 , and 31,138 tonSi year −1 for the Dongnai River, respectively. Nutrient fluxes provide an indicator of coastal eutrophication potential (indicator of coastal eutrophication potential), using nutrient stoichiometry ratios. Despite an excess of nitrogen and phosphorus over silica, estuarine waters downstream of the megacity are not heavily impacted by HCMC. Finally, we analysed scenarios of future trends (2025-2050) for the nutrient inputs on the basis of expected population growth in H C M C and improvement of wastewater treatment capacity. We observed that without the construction of a large number of additional wastewater treatment plants, the eutrophication problem is likely to worsen. The results are discussed in the context of the wastewater management policy.

Plastic Plants: The Role of Water Hyacinths in Plastic Transport in Tropical Rivers

“…The Saigon river is controlled upstream by a reservoir and is subjected to diurnal asymmetric tides. Its net annual mean water discharge was estimated of 50 m 3 s −1 , fluctuated monthly from 4 to 222 m 3 s −1 (Nguyen et al, 2020). Plastic pollution in the Saigon river has already been monitored by several studies, thus enabling investigation of transport dynamics (van Emmerik et al, 2018;van Emmerik et al, 2019a).…”

Biogeochemical functioning of an urbanized tropical estuary: Implementing the generic C-GEM (reactive transport) model

“…1). In addition, the Saigon River is connected to an urban river (Vam Thuat River, 4 m 3 s −1 ) and three urban canals of HCMC (combined discharge of 5.5 m 3 s −1 ) before the confluence with Dongnai River (Nguyen et al, 2020a).…”

“…The water quality status at the upstream and downstream ends of the estuary are considered good, based on Vietnamese regulation on surface water quality . The water quality status of Saigon River becomes moderate to bad condition after connecting with the Vam Thuat River (an urban river, km 114) and urban canals of HCMC (km 130, Nguyen et al, 2020a). While nitrate (NO 3 − ) concentrations in canals are similar or lower than in the upstream stretch of Saigon River (Strady et al, 2017), the ammonium (NH 4 + ) and total organic carbon (TOC) concentrations in those canals are many times higher than the threshold of 0.3 mg NH 4 + L −1 , 4 mg TOC L −1 provided by the Vietnamese regulation for surface water -QCVN 08:2015/BTNMT (Nguyen et al, 2020a).…”

“…This estuary is considered as a natural collector for domestic wastewaters of about 9 million people living in Ho Chi Minh City (HCMC). Urban canals of HCMC and the Saigon River receive about 90% of untreated domestic wastewater from HCMC (Nguyen et al, 2020a). The high polluted loads regularly lead to an excess of nutrients and intense periods of anoxia, especially during the dry season .…”

“…Simultaneously, the risk of estuarine pollution is expected to increase in the coming years, under the impact of megacity's development. The population of HCMC is indeed expected to reach 23 million by 2050, the Saigon River would then receive three times more pollution than what is currently released by the city (Nguyen et al, 2020a). Understanding the self-purification capacity of this tropical estuary is important for HCMC's development plans related to water resources safety.…”