Spatial and temporal variabilities of suspended sediment and dissolved nutrients in the Ca River basin, North Central Vietnam

Suspended sediment (SS) and dissolved nutrients (i.e. silicate, phosphate, and nitrate) play an important role in sustaining riverine, coastal, and oceanic ecosystems (Meire et al. 2016). Suspended sediment concentration (SSC), which is defined as the total value of both mineral and organic material carried in suspension by a river (Fryirs & Brierley 2013), is variable and depends on both natural and anthropogenic factors. SSC data in various rivers around the world range quite widely both during individual hydrological years and during multi-year periods (Walling & Fang 2003). Analyses of the SSC in several major rivers in Poland (Brański & Banasik 1996) indicate that the highest SSC has been reported for rivers flowing through mining areas, where the SSC on some days reaches up to 1,000–2,000 mg/L as a result of mine water contamination. Moreover, a high SSC is reached in highlands (200–300 mg/L) and upland rivers (values close to 100 mg/L). Significantly, lower SSCs are reported for lowland rivers (20–40 mg/L). In terms of dissolved nutrients, the silica content of natural waters is commonly in the 5–25 mg/L range (ASTM 2016). The dissolved silica (DSi) of the global average is 13.1 mg/L (Wallington et al. 2022) and among Asian rivers is 11.7 mg/L (Livingstone 1963). The presence of most silica in natural waters comes from the gradual degradation of silica-containing minerals. Compared to other major ions, DSi was relatively independent of lithology; however, DSi could be influenced by natural phenomena (e.g. geochemistry, precipitation) or human activities (e.g. irrigation) resulting in the increase or decrease of DSi concentration (Paudel et al. 2015; Venugopalan et al. 2020). Phosphate and nitrate are major nutrients needed by living microorganisms for their physiological processes. However, the excessive input of these nutrients may accelerate the eutrophication process. Agricultural lands are responsible for increased sediment and nutrient loads into waterways (Ekholm et al. 2000), which has been associated with the degradation of water quality in freshwater and estuarine ecosystems (Sharpley 2013).

Concentrations of SS and dissolved nutrients in water bodies are affected by the topography of a landscape, the intensity of a rainfall event, and the rate of movement of water over the surface or through the soil profile (McDowell et al. 2001). During rain events, the nutrients accumulated in surface soil are flushed out by runoff to rivers and then transported into estuaries. Paudel et al. (2019) revealed that water quality parameters (SS, dissolved nutrients) changed with seasonal and river discharge. SS increases are associated with higher precipitation (Zhang et al. 2018). Nitrogen and phosphate respond quite differently to rainfall. Nitrogen is much more mobile, and therefore movement is primarily via subsurface pathways, while phosphate transport generally occurs as surface runoff and tends to be dependent on the phosphate status of the soil (Dougherty et al. 2008). However, subsurface transport of phosphate is also possible through preferential flow and in sandy soils (Nash & Halliwell 1999). Silica in estuaries is primarily carried by sediments transported by freshwater inflow to estuaries (Humborg et al. 2000). Silica minerals in estuary sediments can control DSi concentration (Rickert et al. 2001). The above studies have investigated nutrient transport in undisturbed streams and rivers, but information on tropical rivers' nutrient load in an agricultural catchment area is limited.

The Ca River basin has an area of 27,200 km² distributed across the territory of two countries: Vietnam (65.2%) and Lao PDR (34.8%). Agricultural land comprises more than 75.8% of the watershed in this study. In recent years, the Ca River basin is severely affected by the significant impact of global climate change. Studies by Hoang (2010) and Pham et al. (2014) predict that floods, droughts, and saline intrusion become irregular and increasingly serious in the Ca River. In addition, water quality degradation greatly threatens the living environment and the use of surface water from the Ca River basin (MONRE 2018). This study aims to investigate the characteristics and temporal variation of SS and dissolved nutrients in the Ca River basin. Understanding the correlation of nutrient transport change with flow regimes and their responses during hydrological episodes may become essential in determining the current site status and making short-term predictions concerning water availability and potential related risks in the Ca River basin.

The Ca River basin is located in a monsoon climate, and rainfall is distributed over the year, which has two distinct seasons: a rainy season (from May to October) and dry a season (from November to April). The average annual precipitation in the basin is from 1,100 to 2,500 mm. On average, the Ca River is located at an altitude of 294 m and with an 18.3% slope. The river and stream densities in this catchment area are 0.6 km/km². The topography of the Ca River basin is relatively complex, diverse, and strongly fragmented. The Ca River, which is the third largest river in North-Central Vietnam, plays a particularly important role in the socio-economic development of the provinces in the basin.

Water samples were collected 4–6 times per month during the rainy season and 1–4 times per month during the dry season. A total of 48 water samples were collected at a depth of 10 cm, stored in a cool box, and then delivered immediately to the laboratory of Vinh University. Each sample was stored in two polyethylene bottles. One bottle of the sample was filtered by a 0.45-μm filter and concentrations of phosphate (⁠⁠), nitrate (⁠⁠), and silica (SiO₂) were determined by using a colorimeter (DR900, HACH). Another sample was used to determine the levels of SS. Instantaneous discharge was obtained from the Dua and Yen Thuong hydrological stations.

Meanwhile, the qualitative mineralogy and surface morphology of sediment samples were delivered to the laboratory of Okayama University, Japan for further analysis. The qualitative mineralogy of the sediment samples was determined by the standard interpretation procedures of X-ray diffraction (XRD) (Rigaku RINT2100). The surface morphology and microstructure of the suspended sediments were visualized using scanning electron microscopy (SEM) (Hitachi/S-3500N). The SEM images of sediment particles were obtained at 9,000, 3,500, 1,000, 800, 470, and 320 times magnification. The size distribution of microplastics was determined with a Coulter counter (Multisizer 3) equipped with a 2- to 62-μm aperture.

The correlation coefficient was calculated using Microsoft^® Excel 2019. Critical levels of the correlation coefficient serve as the foundation for the statistical test for significance. Spearman's correlation coefficients were calculated between SS and nutrient concentration with discharge, and between SS and nutrient loading with discharge.

The concentrations of ⁠, ⁠, SiO₂, and SS from the monitoring stations was used to demonstrate the relation of nutrient concentration (C) and nutrient load (L) with discharge, known as C–Q and L–Q rating curves (Tsushima et al. 2009).

The weather of North-Central Vietnam is characterized by two seasons: the dry season and the rainy season. The characteristics of the early stage of the rainy season and the end stage of the rainy season differ. Therefore, in this study, all data were divided into three periods: Period 1 from August to October of 2017 (the end stage of the rainy season), Period 2 from November 2017 to April 2018 (the stage of the dry season), and Period 3 from May to July of 2018 (the early stage of the rainy season).

The SS size distribution of the sediment particles ranged from 2 to 29.2 μm and from 2 to 24.4 μm at Dua and Yen Thuong stations, respectively. At the Dua station, particles with a size less than 3 μm account for a large proportion of the total number of the particles at 56.6%. In the rest of the samples, particles with a diameter from 3 μm to less than 5 μm, from 5 μm to less than 10 μm, from 10 μm to less than 20 μm, and from 20 to 29.2 μm account for 30.5, 11.3, 1.5, and 0.1%, respectively. There was a similar SS particle size distribution obtained at the Yen Thuong station, where the proportion of particles with sizes less than 3 μm, from 3 μm to less than 5 μm, from 5 μm to less than 10 μm, from 10 μm to less 20 μm, and from 20 to 29.2 μm account for 56.1, 30.2, 11.8, 1.8, and 0.1%, respectively. SS act as contaminants and pathogens are carried on the surface of the particles. A higher proportion of small particle size of SS (less than 3 μm) was observed at both stations, and this is noteworthy because small particles can carry higher amounts of contaminants (nutrients, heavy metals, organic contaminants).

The results of the mineral content in the sediment deposited obtained by the XRD shows that quartz (SiO₂) is the predominant SS in the samples. Quartz (SiO₂) is a common material and a ubiquitous mineral, the second most abundant mineral in the earth's crust after feldspar. It is present in many types of rocks. Consequently, quartz has a major influence on geochemical processes, including the formation of mineralized deposits of economic value. More importantly, the dissolution and crystallization of quartz may play a part in controlling the concentration of silica in water and hydrothermal systems, providing controls on natural waters (Crundwell 2017).

The results of DSi indicate a similar trend of temporal variation at both stations. DSi concentration range from 8 to 14 mg/L for a year, with an average of 11.3 mg/L for Period 1, 12.4 mg/L for Period 2, and 11.6 mg/L for Period 3 (Figure 3(b)). The results show that DSi was diluted in the rainy season causing a lower concentration than in the dry season.

In contrast to DSi, phosphate concentrations in the rainy season were higher than in the dry season. An average concentration was 0.18, 0.11, and 0.19 mg/L for Period 1, Period 2, and Period 3, respectively (Figure 3(c)). Such nutrients come from different sources, including surface runoff, infiltration runoff, and erosion. In the dry season, from the infiltration source is significant. At the early stage of the rainy season (May 1–June 3), increases quickly, indicating a significant contribution of from the surface runoff and erosion. The surface runoff washed away deposits stored from urban, agricultural, and forestry land during the dry season into the catchment basin. In addition, erosion accompanied by the transport of high SS causes an increase in (Zhang et al. 2017; Paudel et al. 2019).

Finally, nitrate concentrations (⁠⁠) have a similar trend at both stations, fluctuating between 1.05 and 5.97 mg/L (Figure 3(d)). There was no remarkable seasonal variation of nitrate concentrations in the Ca River. The previous studies revealed that in agricultural catchments, nitrate concentration is high during the rainy period, and low during the dry period (Reynolds et al. 1992; Chen et al. 2010). Nitrate produced in the soil by biogeochemical processes such as summer mineralization is flushed into the streams and rivers during rainy periods (Creed & Band 1998). Another hypothesis cited nitrate is stored temporarily in the groundwater. During the rainy season, high nitrate concentrations are the result of high nitrate-rich shallow hillslope groundwater (Altman & Parizek 1995; Hill 1996). The conceptual model for determining stream water seasonality showed concentrations increased not only during the high groundwater season but also during the low groundwater season (Ohte et al. 2003). is mobile and therefore can enter the catchment via subsurface pathways.

The concentrations of and in the Ca River basin were higher compared to other main river catchments in Southern (Saigon and Dongnai Rivers, Lower Mekong River) and Northern Vietnam (Nhue River) (Table 1) (Nguyen et al. 2016, 2019; Trinh et al. 2016). Nutrient concentration in natural water depends on some catchment characteristics such as land use, fertilizer application rate, soil type, and hydrological pathways between the land and the stream (Heathwaite & Johnes 1996). Rivers in the Central Region of Vietnam often have narrow riverbeds, steep slopes, and small catchment areas. In the rainy season, the flow is often concentrated quickly with high discharge rates (MONRE 2018). Due to the region's topography, high concentrations of nutrients in surface soil are flushed out by erosion and runoff to rivers and then transported into estuaries.

Table 1

Comparison of nutrient concentrations among main river catchments in Vietnam

River basin .	Concentration (mg/L) .	Sources .
Nhue River	(0.0–0.1)	Trinh et al. (2016)
Ca River	(1.05–5.97); (0.18)	This study
Saigon and Dongnai rivers	(<2.5)	Nguyen et al. (2016, 2019)
	(0.002–0.395)
	(0.1); (0.05 − 0.2)

River basin .	Concentration (mg/L) .	Sources .
Nhue River	(0.0–0.1)	Trinh et al. (2016)
Ca River	(1.05–5.97); (0.18)	This study
Saigon and Dongnai rivers	(<2.5)	Nguyen et al. (2016, 2019)
	(0.002–0.395)
	(0.1); (0.05 − 0.2)

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The relationship between SS, nutrient concentration, and water discharge was obtained using the C–Q equation (C = aQ^b). Particles and solutes C–Q relationships in various catchments have been explored in past decades and can represent the integration of hydrological and biogeochemical responses of catchments to understand the riverine solute source, transport, and reaction (Rose et al. 2018). The slope of regression b was used to identify nutrient concentration behaviors as ‘chemostatic’ or ‘non-chemostatic’. When b is between −0.1 and <0, nutrient concentration showed only minimal effects of dilution by meteoric water and was defined as chemostatic (Godsey et al. 2009). In contrast, non-chemostatic elements were defined by exhibiting dilution behavior (b < −0.1) when concentrations decreased with an increasing Q or exhibiting enrichment behavior (b > 0) when concentrations increased with an increasing Q.

It has been reported that the contents of biological solutes are often linked to anthropogenic sources, influencing solute-flushing behaviors in forested and agricultural catchments, especially in extreme climate events (Rose et al. 2018). However, as a result of weathering, SiO₂ can accumulate in the weathered zone. During intensive rainfall, it can be washed out from this zone into surface water. According to Johnson et al. (1969) dilution can result during rainfall events as water stored in a catchment is diluted by less concentrated meteoric water. Enrichment can result if a more concentrated source (e.g. surface runoff, infiltration runoff, erosion) mixes with river water during high-flow discharges due to rainfall. In contrast, chemostasis cannot be explained by the simple mixing of multiple sources and therefore has been attributed to processes such as chemical reactions with the solid phase along the pathway of water flow (Godsey et al. 2009). Nutrient concentration responses to seasonal Q variations revealed a large dispersion in C–Q plots. This supports the observations from previous studies (Duncan et al. 2017; Li et al. 2019) showing that C–Q relationships may vary across different time scales because the variety of processes shaping C–Q curves has different temporalities.

SS particles sized less than 3 μm account for a large proportion of particles in the Ca River basin. Quartz (SiO₂) is the predominant mineral in the SS samples. SS was higher upstream compared to downstream, which is found to mainly be due to erosion. The trend in the correlation between seasonal SS and dissolved phosphate was opposite to that of DSi. Results reveal the concentrations of dissolved phosphate and SS were higher in the rainy season than those in the dry season. The strongest relationship of SS, SiO₂, ⁠, and concentrations with discharge (R² > 0.2) was observed in Period 1, followed by Periods 2 and 3, respectively (R² < 0.11, except for SS). Significant correlations between discharge and dissolved nutrient loads were observed. In different seasons, the SiO₂ load has a strong correlation with discharge (R² > 0.96). The and load have the strongest relationships in Periods 1 and 3 (R² > 0.94), but only moderate relationships in Period 2 (0.8 > R² > 0.5). Compared to the other main river catchments in Vietnam, the Ca River basin shows a higher concentration of phosphate and nitrate, which may be due to the strong influence of the topography and agricultural land surrounding the Ca River. This study provides pertinent and useful information for understanding the current state and status of the site, while also providing short-term predictions concerning water availability and risks in the Ca River basin.

The first author is grateful for financial support provided by the Science and Technology Project of the Ministry of Education and Training of Vietnam (B2021-TDV-09).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.