Changes in surface water and sediment quality at a complex tidal-influenced river: a case study of Ganh Hao river, Mekong delta, Vietnam

Abstract

The research was carried out to determine the extent of surface water and sediment pollution using the physicochemical parameters and heavy metal concentrations. The surface water and sediment samples were obtained from 25 sites along the river on March 22 and September 5, 2019 at the main river influenced by the complex tidal regime and aquaculture practices in Ca Mau peninsular in the Vietnamese Mekong Delta. The water quality was compared with international and national standards, while the sediment quality was compared with Vietnamese standards and pollution indices (geo-accumulation index, contamination factor, and pollution load index). Principal component analysis (PCA) was employed to explain the main factors responsible for the observed levels of water pollution. The dependence of water quality parameters and variations in water quality due to tidal regimes and seasonality were also evaluated by statistical comparisons. Based on the logistic regression models, the temporal variability of selected water quality parameters was visualized using QGIS. The Delft 3D model was used to evaluate changes in hydraulic characteristics based on actual fieldwork. The concentrations of nutrients and organic compounds exceeded 2-3 times greater than the standard for surface water quality. The contents of heavy metals were below the standard for sediment quality and consistent with the background levels. The influence of the tidal regime and seasonality caused significant changes in water quality and its association with various flow regimes (P<0.05). Three components based on the PCA accounted for approx. 79.84% of the total variance in water quality characteristics of which the first, second, third component explained for 53.64%, 18.43%, and 7.77% of total variance related to physicochemical properties and organic matter pollution, suspended solids and nutrients, and DO consumption, respectively. Water quality maps indicated pollution hotspots, and the extensive and improved – extensive shrimp culture practices were identified in connection to the changes in water quality. Changes in the river's hydraulic characteristics were influenced remarkably by the tidal regime as well as geomorphological changes. The results of the research addressed the gaps from the previous studies were to identify degradation of surface water quality was associated primarily with extensive and improved extensive shrimp culture. Seasonal factors and tidal regimes influenced significantly changes in water quality parameters. Pollution sources were specified by the principal component analysis. Spatiotemporal distribution maps of selected water quality parameters highlighted pollution hotspot. Changes in the river hydraulic characteristics were caused mainly by the tidal regimes as well as geomorphological changes. The selected water quality parameters responded as well as depended diversely to various flow regimes. The results could be useful not only to local policymakers in developing water management strategies but also to other rivers beyond the geographical regions.