Levels, fractionation and mobility of potentially toxic elements in sediments of Quang Ninh-Hai Phong coastal region, Vietnam

In this project, three estuarine and river-mouth areas, namely Cua Ong Estuary, Cua Luc Estuary and Cam River-mouth that belong to the Quang Ninh- Hai Phong coastal region (Vietnam) were investigated for the status as well as history and origin of the heavy metal and arsenic contamination of sediments. In addition, the speciation and mobility of heavy metals and arsenic under changing environmental conditions (Eh and pH) in vertical profiles or upon simulated on-land disposal of dredged sediments in the Cam River-mouth area (Hai Phong Province) were also investigated. The project included many different works including the collection of surficial bed-sediment and core-sediment samples, the performance of ripening experiments, porewater collection, single (ammonium-EDTA) and sequential (BCR 3-step) extractions, and pHstat-leaching tests as well as analyses of chemical composition of major and trace elements (Al, Ca, Fe, K, Mg, Na, S, P, As, Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn), anions (F-, Cl-, Br-, NO2-, NO3-, SO42- and PO43-, etc .), grain-size, mineralogy, organic matter content (OM), cation exchange capacity (CEC), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and 137Cs isotope dating.In Cua Ong and Cua Luc estuaries, heavy metals (Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn) and As mainly display a natural origin (from the weathering and erosion processes of rocks). In contrast, in the Cam River-mouth, heavy metals and arsenic are considered to be minorly to moderately enriched as a result of anthropogenic activities in Hai Phong City and Northern Vietnam. Especially, Cu, Mn, Pb and Zn in sediments increased rapidly by a factor of approximately 2 from 1954 to 1975 and then remained nearly unchanged from 1975 to 2008.In the Cam River-mouth, heavy metals (Cu, Mn, Pb and Zn) and As show overall decreasing trends of concentrations in porewater or sediment from top to bottom of a sediment core. Moreover, non-silicate fractions of these elements (exchangeable and carbonate-bound, Fe and Mn hydr/oxide-bound, and organic matter and sulphide-bound) also decrease from top to bottom as a result of their anthropogenic origin in surficial sediment layers and decreasing Eh and increasing pH values with depth. The pHstat-leaching tests on the original suboxic and oxidized sediments indicate that the pH-dependent leaching behavior of Cu, Mn, Pb, Zn and As reflects a V-shaped pattern in which they are strongly leached at highly-acid pH (2) or highly-alkaline pH