Santa Gilla lagoon (Italy): a mercury sediment pollution case study. Contamination assessment and restoration of the site

The Santa Gilla lagoon, on the southern coast of the island of Sardinia (Italy), has received industrial discharge of mercury, lead and zinc compounds, as well as municipal untreated sewage for several decades from the urban area of Cagliari (about 400 000 inhabitants). The lagoon, which covers an area of 15 km2 of shallow waters, has always represented an important source of quality fish and shellfish for the island. Pollution sources were brought under control only in the mid-1980s, when a costly restoration program, which is still in progress, was started. About 26 tons of mercury, discharged from a chlor-alkali plant, have been estimated to have been deposited in the lagoon since the mid-1960ʹs, mostly confined in an area of about 2 km2 in front of the industrial area. According to the restoration project this area has been isolated with proper dykes and used to discharge sediments dredged from the most polluted sectors of the lagoon. It was finally covered up with sediments dredged from cleaner areas of the lagoon with the aim of using it for aquaculture. This work reviews several investigations carried out on the sediments of the S. Gilla lagoon to monitor and control the real extent and hazard of the mercury pollution. Recent data on the mercury concentration in the sediments of the lagoon after remediation were compared with previous data on the same area and the effectiveness of the clean-up plan was evaluated.