Spatial distribution of heavy metals (Pb, Hg, As) in the estuary area of the Red river (Vietnam)

The features of spatial distribution of some heavy metals (Pb, Hg, As) in the system "suspended substance - bottom sediments" in the mouth area of the Red River (Vietnam) have been studied. The studies have been carried out during the main hydrological seasons of the years 2014-2016 (during the flood and low-water periods). Spatial analysis of the amount of heavy metals (HM) in the suspended form and in the bottom sediments has been carried out using the interpolation model in ArcGIS 10.2.2. Distribution of Pb, Hg, As concentration in the suspended form and bottom sediment phases in the estuary area of the Red River is characterized by a maximum in the mouths of the arms and a general decreasing gradient towards the sea. The maximum concentrations of Pb, Hg in suspended forms are observed in the water surface layer at the river - sea barrier. The content of Hg and As in the estuary region of the Red River has been observed in the following order: SSsurf - SSbott - BS; and content of Pb - SS - BS. By the values of the average enrichment coefficient of KSS/BS, heavy metals are located in the following decreasing series: Pb - Hg - As. Being on the river and sea boundaries of saline and fresh waters mixing zone, bottom sediments mark the local barriers within the mouth area at a distance of 20 to 30 km from the shore. Intense sedimentations and accumulations of HM in bottom sediments take place at these local barriers as a result of successive changes in the hydrodynamic and hydrochemical conditions in the mouth area of the Red River.