Jari Mäkinen, Geological Survey of Finland (Finland)
Tommi Kauppila, Geological Survey of Finland (Finland)
Kirsti Loukola-Ruskeeniemi, Helsinki University of Tehcnology, Geoenvironmental Technology Section (Finland)
Jukka Mattila, Water and Environment Association of the River Kymi (Finland)
Juha Miettinen, Univeristy of Joensuu (Finland)
During the last 400 years, three lakes in the Sotkamo area (Jormasjärvi, Nuasjärvi, Kolmisoppi) have been a focus of various forms of anthropogenic loading. The first significant event in the Nuasjärvi area was the beginning of cultivation in the 17th and 19th centuries, which caused soil erosion and increased the accumulation of mineral matter in the lake. At the same time the grain size of the particles decreased, reflected in an increase in the concentrations of several acid-soluble elements in the sediment (Al, Ca, Cr, K, Mg, Na, Ni and Sr; US EPA method 3051). However, the concentrations of C, N, and elements bound to Fe-Mn oxides decreased. According to diatom-based nutrient modelling, the ecological status of the lake slightly altered, but no significant eutrophication was observed.
In the 20th century, anthropogenic loading from the watersheds significantly diversified. In terms of geochemistry, aerial fallout was common to all the lakes, with increases especially in Cd, Pb and S concentrations (1.9-, 7.2- and 3.0-fold, respectively, compared to the natural background concentration). This fallout peaked in the 1980s, after which the Cd and Pb concentrations decreased to 1.4- and 4.1-fold enrichment levels. Furthermore, the concentration of S has recovered to the natural level in Jormasjärvi and Kolmisoppi. Significant drainage has been performed in all three catchments, a feature that is seen most clearly in the Lake Kolmisoppi sediment sequence for the 1980s. The geochemical signature in Lake Kolmisoppi was similar to that in Lake Nuasjärvi, but the microfossil record indicated distinct eutrophication of the former lake. Eutrophication also occurred during the 1980s in Lakes Jormasjärvi and Nuasjärvi. Water level regulation in Lake Nuasjärvi since the end of the 1940s has caused no significant shifts in the geochemical or ecological status of the lake.
Since the end of the 1960s, Lake Nuasjärvi has received acidic mine drainage (AMD) from the black shale tailings of the Lahnaslampi talc mine. These black shales contain elevated concentrations of Cd, Ni and S, and the oxidation of sulphides has caused metal leaching and emanation to Lake Nuasjärvi. The maximum period of this loading was dated to the 1980-1990s, the enrichment factors for Cd, Ni and S being 12, 5 and 34 compared to the natural concentration level.