The Fate of Hg Contaminated Sediments of the Ravenna Lagoon (Italy): Final Burial or Potential Remobilization

Abstract

In the period 1957-77 a part of 100-200 tons of mercury as industrial waste was discharged into a 10 km² lagoon (“Pialassa Baiona”) connected to the Ravenna harbor and the Northern Adriatic Sea (Italy). Extremely high levels of Hg (up to 160 µg/g) were found in surface sediments, but the maximum values (up to 1114 µg/g) were detected in subsurface layers, down to 80 cm, close to the industrial source, and values of 1-2 µg/g were recorded in the fore-harbor, outside the lagoon. The calculation of total Hg budget in sediments gave 38 tons of Hg being stored within a contaminated area of 15 km². The redistribution of polluted sediments would seem to be due mainly to the tidal action; more than 80% of the total Hg discharged is confined in 10% of the area, i.e. in the southern ponds close to the input point. The time necessary for a restoration of the area was estimated by using different approaches of accumulation rate calculations (²¹⁰Pb, Hg peak). It appears that 40-50 years would be needed to clean the surface sediments of the Ravenna lagoon from the mercury contamination. We suggest that remedial plans should be focused on the southern ponds.