A mercury model used for assessment of dredging impacts

The effects of dredging of contaminated sediments on the mercury (Hg) concentrations of prey and predatory fish were calculated for the Kokemäenjoki River and its estuary in Western Finland. The accumulation of Hg in fish is controlled by the Hg concentrations in water, zooplankton, zoobenthos and by suspended solids. Hg is accumulated into fish mainly through food web, eg. from perch (Perca fluviatilis) as prey and to pike (Esox lucius) as predator. In addition to dredging, temperature and flood situations have also increased the Hg accumulation and release from the bottom sediments. The validity of the model has been tested with data recorded from earlier dredgings. Thereafter the model has been used to predict the Hg levels caused by dredging planned upstream in the river. The predictions are supported by the concentrations of total mercury (Tot.Hg) and methyl mercury (MeHg) measured in water and in sediments under several flow conditions. As a result, 30 % increase of Hg in pike — from 0.8 to 1.05 mg/kg —was expected. This was too high, and therefore dredging was not included in the final plan for flood protection.