175 - Source tracking of mercury in a brackish fjord system

"Gunneklevfjorden is a small (0.8 km2) brackish fjord in south Norway with a long pollution history, including decades of local industry releases and emissions. Determining the origin of mercury (Hg) in Gunneklevfjorden biota is challenging considering the range of possible sources. However, an accurate estimate for the contribution of ongoing and historic sources is critical to evaluate the potential success of sediment remediation strategies.

A high-resolution analytical method was used to assess whether potential Hg sources to Gunneklevfjorden have distinct Hg isotope fingerprints, and whether these signatures resemble the signatures of surface sediment, suspended particles and samples from the food chain, including invertebrates and fish (perch, Perca fluviatilis). Samples were collected as close as possible to known previous and potential ongoing Hg sources surrounding Gunneklevfjorden. Additional samples from reference areas (nearby lakes and open sea) were collected to establish background depositional sources to the region.

Gunneklevfjorden sediments have enriched 202Hg signatures characteristic of Hg isotope signatures found at contaminated sites near chlor-alkali facilities and other industrial processes that use elemental Hg. In perch and invertebrates from the fjord, tissue was enriched in 202Hg relative to sediments but did not contain an accompanying increase in 199Hg that is classically associated with photo-demethylation processes in food webs with strong pelagic influences. The lack of 199Hg enrichment in biota of Gunneklevfjorden suggests that Hg accumulating in the food web is not tied to pelagic feeding, but rather a benthic bioaccumulation pathway connected to the sediment.

In conclusion, there is strong evidence that the Hg burdens in fish and invertebrates are closely connected to the industrial sources in the sediment. The evidence pointing to a closely connected benthic food web structure further indicates that remediation targeting the food web entry points could be an effective way to rapidly reduce biological Hg burdens."

Authors: Hans Fredrik Veiteberg Braaten, Nathan Johnson, Sarah Janssen, Marianne Olsen