Source identification of chromium in the sediments of the Xiaoqing River and Laizhou Bay: A chromium stable isotope perspective.

Hexavalent chromium, Cr(VI), is a heavy metal contaminant and the reduction of Cr(VI) is accompanied by large isotopic fractionation. In this study, the sources of Cr were explored using the Cr isotopic composition of sediments from the Xiaoqing River, a heavily polluted river located in the Shandong Province of China, which flows into Laizhou Bay. The results show that δ53Cr values of the sediments are the highest upstream near the pollution source, and gradually decrease along the river toward the range for igneous reservoirs observed near the estuary. Based on the calculation of authigenic Cr isotopic composition (δ53Crauth) using the detrital index and leaching experiments, we suggest that the authigenic Cr in the sample near the pollution source with the highest δ53Crauth value mainly comes from the reduction of Cr(VI) discharged by anthropogenic activity, and authigenic Cr in other samples in the midstream with δ53Crauth values slightly higher than the range of igneous reservoirs may come from natural oxidative Cr weathering products. By introducing a Rayleigh model, we calculate that at least 31%-55% of Cr(VI) in the river water had been reduced to Cr(III) near the pollution source. Due to the self-purification ability of the river, Cr(VI) was reduced; thus, there is no record of high δ53Crauth values in the downstream of the Xiaoqing River and Laizhou Bay, indicating no obvious Cr pollution in these locations. The limited variation of δ53Cr values for samples from a sediment core in Laizhou Bay is also indicative of no obvious Cr pollution in the history. The Cr isotopic compositions of the river sediments are useful for the identification of Cr sources and can be used to advise environmental remediation on Cr pollution.

[Distribution and Seasonal Variations of Chromophoric Dissolved Organic Matter(CDOM) in the Bohai Sea and the North Yellow Sea].

UV-Vis spectral data and a fluorescence excitation-emission matrix (EEM) was analyzed for the surface, middle, and bottom layer water samples from the Bohai Sea and North Yellow Sea in April, August, and December of 2016 and February of 2017. Distribution characteristics, influencing factors, and seasonal variation of CDOM in the study area were investigated. The results showed that the horizontal distributions of CDOM are similar in different seasons, showing a characteristic of high levels near shore and low levels in the offshore region. Strong correlations were observed between value a (355) and spectral slope S275-295, indicating that CDOM is significantly affected by land input. According to the vertical distribution, the CDOM level is the lowest in surface seawater in summer due to strong photo-degradation. In February, controlled by primary productivity, the lowest level of CDOM was found in the bottom seawater. From April 2016 to February 2017, the content of CDOM first increased and then decreased. Its concentration was highest in December. Seasonal variations are mainly controlled by land input and on-site production. According to analysis of the spectral slope S275-295, the average molecular weight of CDOM in winter was smallest in February, which was mainly related to the drastic reduction of land-based input at that time. The strong photo-degradation in summer resulted in a small average molecular weight of CDOM.