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

Abstract

The Ca River basin has an area of 27,200 km2 distributed across the territories of two countries: Vietnam (65.2%) and Lao (34.8%). Spatial and temporal variations in suspended sediment (SS) and dissolved nutrients (PO43-, NO3-, SiO2) were determined in two hydrological stations located along the Ca River 4–6 times per month in the rainy season and 1–4 times per month in the dry season, between the months of August 2017 and July 2018. A loading–discharge (L–Q) curve was used to analyze the correlation among water physicochemical parameters with seasonal river discharge. The results indicate that SS was higher in upstream flows compared to downstream flows, which is primarily due to erosion. Seasonal SS and dissolved phosphate have an inverse correlation trend to that of dissolved silica. Results revealed that the concentration of phosphate and SS was higher in the rainy season than in the dry season. This finding proves that rain washes particulate matter from the surface runoff into the Ca River. Significant correlations between discharge and dissolved nutrient load were observed. This study provides useful information regarding variations of SS and water physicochemical parameters with seasonal water discharge in the Ca River. HIGHLIGHTS Quartz is the predominant mineral in the SS samples.; SS concentrations were higher in upstream flows compared to downstream flows.; A positive correlation between discharge and the dissolved nutrient load was shown.; The strong influence of topography and agricultural land on nutrient levels in the Ca River.; Dilution or chemostatic behavior occurs at a discharge rate of more than 500 m3/s.;

Keywords

• agricultural catchment
• ca river
• c–q relationship
• dissolved nutrients
• suspended sediment
• water quality