History of Mine Drainage Impact on Lake Orijärvi Algal Communities, SW Finland

The Cu (Pb, Zn) mine of Orijärvi (1757–1956) was the first mining operation in Finland where flotation techniques (1911–1955) were used to enrich ore. Large quantities of tailings were produced. The impacts of past mining activities on the aquatic ecosystem of nearby Lake Orijärvi were studied using a combination of paleolimnological methods (analysis of sedimentary diatom frustules, chrysophycean cysts, metal concentrations and radiometric datings). The acid mine drainage (AMD) – derived metal impact to the lake was found to be the strongest thus far recorded in Finland. Concentrations of Cu, Pb and Zn in sediments are two to three orders of magnitude higher than background values. During the most severe loading phase, there were practically no algae in the lake. Achnanthes  minutissima was the hardiest species able to tolerate increased metal contents. The metal load has changed the properties of sediments in such a way that chrysophycean cysts were impossible to identify because of coating and corrosion. Lake water still has elevated heavy metal concentrations, indicating that the impact from the tailings area continues to affect the lake. It has low productivity, and the planktic diatom community is still not developed. The study demonstrates that unremediated mining areas form a major risk to the environment. The damage to aquatic ecosystem can remain severe for decades after the mining activities have ceased.