Distributions, sources and ecological risk assessment of arsenic and mercury in the surface sediments of the southwestern coastal Laizhou Bay, Bohai Sea

• As was mainly from natural sources in surface sediments of the coastal Laizhou Bay.
• Hg was identified to have extremely high ecological risk in some riverine sediment.
• Anthropogenic inputs might explain for high enrichment of Hg in riverine sediments.
• Sediment quality in the marine area on the whole was fine in terms of As and Hg levels.

The spatial distributions of As and Hg in riverine and marine surface sediments of the coastal southwestern Laizhou Bay were obtained, and multiple indices and guidelines were applied to assess their contamination and ecological risks. The As concentrations in riverine sediments were close to those in marine sediments, and on the whole the dominant proportion of As was identified to be from natural sources. The Hg concentrations in riverine sediments were much higher than those in marine sediments, so river transportation was likely the main way of Hg into the southwestern Laizhou Bay. In respect of As, the sediment quality was fine according to the risk assessment methods used; in contrast, Hg presented an extremely contaminated status with a very high ecological risk in some riverine sediments, while most of the marine sediments were relatively much less polluted by Hg and under a lower ecological risk from it.