Risk assessment of sedimentary mercury at Snäckviken, Södertälje channel

The sediments in the northern Södertälje channel are highly polluted with mercury (Hg). Although point source emissions stopped during the mid 1960ies, considerable amounts of Hg are still present in the upper sediments. Because Hg is inherently toxic and may biomagnify, these sediments compose a potential hazard to the local ecosystem. There is also a risk that Hg is transported downstream into the Baltic coast. This report provides a risk assessment based on all pertinent data from the region. The report is intended to provide a basis for discussing whether remediation should be considered or not. The ultimate goal is to identify whether Hg is causing adverse biological effects and, if this is the case, to suggest how exposure of Hg to biota can be limited. In such cases the exposure pathway must be identified. The major overall conclusions of this risk assessment are: Sedimentary mercury in Snäckviken causes no significant ecological impact in situ. Dredging in the area leads to elevated risk in the short term. Intensified vessel traffic may bring about a permanently elevated risk.