Changes and ecological risks of surface sediment heavy metal pollution in the shallow sea wetlands of the Liaodong Bay: A comparative analysis based on the survey data from 2005 and 2013

This paper is the result of environmental geological survey engineering.

Objectives To clarify the distribution characteristics and sources of heavy metals, so as to provide new quantitative data and deep understanding for the historical changes of heavy metal pollution in Liaodong Bay, and to provide historical cases for the government to trace the evolution trend of near sea pollution based on the changes of economic development indicators.

Methods By analyzing surface sediment heavy metals (As, Cd, Pb, Zn, Cu, Cr, Hg), organic carbon (Corg), and particle size in the shallow sea wetlands (SSW) of Liaodong Bay from 2005 and 2013, this study investigated the spatial distribution characteristics, pollution sources, and ecological risk contributions of surface sediment heavy metals and Corg in the SSW of the Liaodong Bay.

Results The results indicate consistent spatial distribution patterns of heavy metals in both 2005 and 2013, with significant enrichment observed in the southwest of the Xiaoling River, controlled by grain size. The overall spatial distribution of metal concentrations shows a pattern of south > central > north and west > east > central. In 2005 (2013), industrial, agricultural, and bedrock derived heavy metal accounted for 40.68% (29.03%), 36.69% (48.03%), and 22.63% (22.94%), respectively. In comparison to 2005, significant changes in heavy metal sources were observed due to adjustments in coastal urban industrial structures, including that agricultural production contributed the most to the overall ecological risk (RI) that revealed by geographical detector analysis, and a slight increase in Cr, Cu, Pb, and Zn pollution, while As and Hg pollution significantly intensified that tested by the analysis of the geo-accumulation index (I_geo) and positive matrix factorization (PMF) in 2013. Conversely, Cd pollution showed a notable decrease, suggesting the crucial role of industrial wastewater control efforts. Other heavy metal pollutants were mainly attributed to agricultural activities, with potential contributions from maritime transport to Pb pollution.

Conclusions Despite the 2 to 3 times growth in the economy of cities near Liaodong Bay, the government's focus on controlling the treatment and discharge of industrial waste water has led to a relatively gentle increase in heavy metal pollution. However, the pollution of heavy metal elements such as Hg, Cu, Pb, and Zn cannot be ignored, especially in areas of sediment with finer grain size, where the ecological risks of heavy metals As and Hg are more severe.