Trace metals in the suspended particulate matter of the Yellow River (Huanghe) Estuary: Concentrations, potential mobility, contamination assessment and the fluxes into the Bohai Sea

• The contents and fractionations of six trace metals in SPM of the YRE were studied.
• The proportions of all studied metals except Cd in the most labile fraction were low.
• The SPM of the YRE might bring slight trace metal pollution threat to the Bohai Sea.
• The previously reported particulate metal fluxes through YRE might be overestimated.
• SPM and YRE stand for suspended particulate matter and Yellow River Estuary.

Excessive input of trace metals contributes much to the degradation of many coastal ecosystems. As one of the most turbid large rivers in the world, the Yellow River, also called Huanghe, transports huge amounts of freshwater and suspended particulate matter (SPM) each year from its catchment as far as thousands of kilometers inland into the Bohai Sea, and has the most profound influence on the ecosystem of the Bohai Sea compared with other rivers that empty into it. In this research, the SPM were collected from the Yellow River Estuary twice in April and August, 2013. Six environmental quality assessment-related trace metals, namely Cd, Cr, Cu, Ni, Pb and Zn, were measured for their total concentrations and fractionations to understand their spatial and seasonal variations, potential mobility, pollution status and fluxes. The total concentrations of Cd, Cu and Pb in the wet season were very similar to their corresponding values in the dry season, while the total concentrations of Cr, Ni and Zn in the wet season were slightly higher than those in the dry season. The distributions of the studied metals in the geochemical fractions showed no notable spatial and seasonal variations. Except for Cd and Ni, the metals in the most labile fraction having the highest potential risk to biota on average accounted for <2% of their respective total concentrations; the percentages of Cd and Ni in this fraction were 54.5% and 10.1%, respectively. Anthropogenic influence on the spatial and seasonal variations of particulate Cr, Cu, Ni and Zn in total concentrations seemed weak and they were dominantly of natural sources. Results indicated that the Yellow River emptied about 0.0081×103, 2.11×103, 0.98×103, 1.10×103, 1.01×103 and 2.64×103 t a−1 of particulate Cd, Cr, Cu, Ni, Pb and Zn into the Sea, 86% of which happened in the wet season starting from June through October. These numbers were only 2.5–5.3% of the annual trace metals fluxes in the 1980s. One of the reasons for this was the reduction of SPM load; the fact that previous chemical fluxes data were measured at sites inland far from the mouth of the Yellow River was also responsible for this because a major proportion of SPM recorded at that site were deposited into the river bed during the transport between that site and the mouth of the Yellow River. Such a deposit actually did not reach the Bohai Sea, and probably had no direct influence on the environment and ecosystem of the Bohai Sea beyond the mouth of the Yellow River.