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 Box 11: Releases of mercury and pollution with heavy metals (Sweden)

Lake Mälaren located west of Stockholm is the third largest lake in Sweden. With an area of 1 140 km² and extending about 120 km across Sweden, it is the country’s third largest lake. There are 19 bathing sites on its shore. Until recently, mercury spread into Lake Mälaren in the municipality of Nykvarn. 

Between 1946 and 1966, a paper mill released fiber residues containing mercury into the Turingeån River and Lake Turingen with its outflow to Lake Mälaren. Although the use of mercury was banned in 1966, secondary releases continued, which resulted in high levels of mercury in water and fish tissues. When remediation management started in 1998, there was almost 400 kg of mercury in the river and lake bottom. In the first stage of the remediation project the mouth of the Turingeån River and an overgrown bay alongside the mouth of the river were dredged. The dredged materials were placed in the inner part of the bay and covered with a sheet and a sealing layer of sand. In the second stage 80- percent of the bottom of Lake Turingen was covered with an artificial bottom sediment to prevent the further leakage of mercury. In addition, 20% of the area outside the mouth of the river was dredged as well and capped with a strong, woven geotextile, fine sand and crushed rock. Water barriers were also built to control the exchange of water between lakes. As a result, about 95% of mercury was isolated. The mercury levels is steadily declining. The remediation could be also applied to cases when sediments are contaminated with other heavy metals or organic contaminants.