Phosphorus adsorption/desorption processes in the tropical Saigon River estuary (Southern Vietnam) impacted by a megacity

Abstract

The Saigon River flows through one of the most rapidly growing megacities of Southeast Asia, Ho Chi Minh City (HCMC, > 8.4 million inhabitants). This tidal river is characterized by a tropical monsoon climate, alternating a wet and a dry season. In the last few decades, increased economic and urban developments of HCMC have led to harmful impacts on the water quality of this tidal river, with severe eutrophication events. This situation results from the conjunction of contrasting hydrological seasons and the lack of upgraded sanitation infrastructures: indeed, less than 10% of the domestic wastewater is collected and treated before being discharged directly into urban canals or rivers. This study focuses on P dynamics because this is considered the key nutrient factor controlling freshwater eutrophication. Based on field measurements and original laboratory experiments, we assessed the P levels in the river water and sediments, and investigated P adsorption/desorption capacity onto suspended sediment (SS) within the salinity gradient observed. Field surveys showed a clear impact of the HCMC megacity on the total P content in SS, which increased threefold at HCMC Center, as compared with the upstream values (0.3-0.8 gP kg −1). Downstream, in the mixed estuarine area, the Total P was lower than 0.5 gP kg −1. Laboratory experiments were carried out to characterize the influence of SS concentrations (SS = [0.25-0.9] g L −1), salinity (S = [2.6-9.3]) and turbulence (G = [22-44] s − 1) on the sorption capacity of P onto sediment. The size of sediment particles and their propensity to flocculate were also originally measured with a recently developed instrument: the System for the Characterization of Aggregates and Flocs (SCAF®). Under the experimental conditions considered, SS concentrations had the greatest effect on the adsorption of P onto sediment, e.g., P adsorption capacity increased when SS concentrations rose. In contrast, salinity and turbulence had a smaller effect on the adsorption properties of sediments. Among these observed variables, the SS concentration was shown to be the main driver for adsorption capacity of P onto SS within the salinity gradient. We discuss the implication of these findings on understanding P dynamics within a highly urbanized, tropical estuary.

The impact of tropical land-use change on downstream riverine and estuarine water properties and biogeochemical cycles: a review

“…Because P combines with certain soil minerals (e.g., calcium, iron, and aluminum), enhanced soil runoff due to logging would involve a drastic transportation of P from the surface land to the downstream aquatic ecosystem, decreasing the N:P ratio (Chappell et al 2005). In the downstream estuary, suspended particulate P desorbs from the soil particles, and PO 4 3− becomes available for primary producers (Zhang and Huang 2011;Lin et al 2012;Nguyen et al 2019a). The sorption/desorption of P is chemically affected by the water temperature and salinity; PO 4 3− desorption is enhanced with decreasing temperature or increasing salinity (Zhang and Huang 2011).…”

“…The sorption/desorption of P is chemically affected by the water temperature and salinity; PO 4 3− desorption is enhanced with decreasing temperature or increasing salinity (Zhang and Huang 2011). However, in a turbid tropical estuary of Saigon, Vietnam, the SS concentration was the greatest factor in determining the P content in the water because of the high flocculation of cohesive sediments (Nguyen et al 2019a). These properties of P indicate the importance of the management of soil discharge from logging sites to prevent P overloading in downstream aquatic ecosystems.…”

“…The remaining N percolates through soil into groundwater, discharges with surface water runoff, or it is released into the atmosphere through volatilization (e.g., NH 3 ) or denitrification (e.g., N 2 and N 2 O) (Downing et al 1999;Rivett et al 2008;Maranguit et al 2017). Conversely, P tends to remain in the surface soil or be discharged with sediment as surface runoff because of its ability to bind to certain soil minerals (Faithful and Finlayson 2005;Zhang and Huang 2011;Nguyen et al 2019a).…”

“…While N follows a pathway of release into the atmosphere through anammox or denitrification, P tends to remain in the aquatic system or to flow downstream by attaching to sediment particles, and this property of P sorption becomes stronger at higher temperatures (Zhang and Huang 2011;Nguyen et al 2019a). This difference between N and P gradually lowers the N:P ratio in water or sediments in tropical estuaries (Cotovicz Jr et al 2013).…”

Temporal dynamic of anthropogenic fibers in a tropical river-estuarine system

“…more than 1000 m 3 s -1 ; Camenen et al, 2017) with the lowest monthly discharges recorded in January 16 ± 42 m 3 s -1 and the highest in October 118 ± 80 m 3 s -1 (Nguyen et al, 2019a). Because of the tide dynamic and the intense navigation traffic, the Saigon River is a highly turbulent system with a well homogenized water column regarding temperature, salinity, TSS, pH, Chl-a, with a punctual and local stratification regarding dissolved oxygen concentrations (Nguyen et al, 2019b). HCMC is a dynamic megacity (8.4 million inhabitants) with a dense urban zone in the City Centre fluctuating from 10,000 to 40,000 inhabitants km -2 .…”