Nutrient dynamics and eutrophication assessment in the tropical river system of Saigon – Dongnai (southern Vietnam)

Abstract

Saigon-Dongnai Rivers in Southern Vietnam is a complex lowland hydrological network of tributaries that is strongly influenced by the tidal cycles. The increasing economic, industrial and domestic developments in and around Ho Chi Minh City (HCMC) have led to serious impacts on water quality due to lack of appropriate wastewaters treatment. Drinking water production is impacted and the large aquaculture production areas may also be affected. We analyzed spatial and seasonal variability of nutrient concentrations (Phosphorus, Nitrogen and Silica) and eutrophication indicators (Organic Carbon, 2 Chlorophyll-a and Dissolved Oxygen) based on bimonthly monitoring during two hydrological cycles (July 2015-December 2017). Four monitoring sites were selected to assess the impact of HCMC: two upstream stations on the Saigon River and Dongnai River branches to provide the reference water quality status before reaching the urbanized area of HCMC; one monitoring station in the city center to highlight Saigon River water quality within the heart of the megacity; the fourth station downstream of the confluence to evaluate the impact of HCMC on the estuarine waters. This study points to excess nutrients in HCMC's water body with concentrations of NH4 + and PO4 3averaging to 0.7 ± 0.6 mgN L-1 and 0.07 ± 0.06 mgP L-1 , respectively in mean over the monitored period and rising up to 3 mgN L-1 and 0.2 mgP L-1 , in extreme conditions. During the dry season, we evidenced that untreated domestic discharges leads to degradation of the Saigon River's water quality with extreme values of algal biomass (up 150 µChl-a L-1) and hypoxic conditions occurring episodically (DO < 2 mg L-1) in the heart of the megacity. Until now, eutrophication in the urban center has had no clear effect downstream because eutrophic water mass from the Saigon River is efficiently mixed with the Dongnai River and sea water masses during the successive semi-diurnal tidal cycles.

Promotion of maize growth using endophytic bacteria under greenhouse and field conditions

“…To maintain a high maize crop yield, fertilizers rich in nitrogen and phosphorus have considerable economic importance due to their direct relationship with production costs. Under certain conditions, fertilizers can have environmental impacts such as soil salinization, underground water contamination and river eutrophication (Nguyen et al, 2019). Considering these factors, it becomes crucial to search for strategies to reduce the use and improve the absorption efficiency of fertilizers without reducing yield or promoting environmental damage.…”

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

“…The population density varied from 2,000 inhabitants per square kilometre in rural districts up to 50,000 inhabitants per square kilometre in urban districts (Figure 1). In recent years, less than 10% of domestic wastewater was collected and treated before release to urban canals and rivers (Nguyen et al, 2019).…”

“…Because of the asymmetric semidiurnal tides, the estimation of the residual river discharges requires hourly measurement of discharge over a minimum period of 24 hr (two tidal cycles). The mean water discharge is then deduced from the integrative flux between the tide inflow and outflow (Nguyen et al, 2019).…”

“…The residual discharge corresponds to the net positive flow of the river from land to sea. Even with the low frequency measurement of the discharge (i.e., once per month), the hydrological seasonality is clearly observed (Nguyen et al, 2019). For water quality analysis, surface (20-30 cm deep) riverine and urban canal waters were collected manually during low and high tides, 4 m from the riverbank, and were filtered on preweighted filters.…”

Phosphate ion adsorption properties of PAN-based activated carbon fibers prepared with K2CO3 activation

“…However, according to the Environmental White Paper of FY2017, the TP concentrations in both lakes obviously exceeded the standard values (0.14 and 0.15 mg/L, respectively), and they were reported to be the first and third worst eutrophic lake in Japan in 2016 by the Ministry of the Environment, Government of Japan [4]. The World Health Organization (WHO) also recommends phosphate ion concentration of less than 5 mg/L in natural water [6], though water blooms have broken out in China and Southeast Asia frequently [1,7,8]. In order not to occur water environmental problems, it is important to not only reduce releasing nutrients into natural water but also remove phosphate from wastewater [9][10][11].…”