Long-term lake sediment records and factors affecting the evolution of metal(loid) drainage from two mine sites (SW Finland)

Geochemical profiles were analyzed from 137Cs-dated lake sediment cores near two sulfide mines (Ylöjärvi and Haveri) in SW Finland to obtain long-term records of elemental discharge to surface waters from the mine sites before, during and after mining. The results showed that both the intensity and chemical composition of mine water loading changed over time, and that pre-mining land uses (e.g. agriculture) also mobilized mining-associated elements in an area with mine potential. At Ylöjärvi the sulfide-derived metal concentrations peaked during mining, but the lack of post-closure peaks probably resulted from the diffuse nature of the loading from the tailings facility. Major post-mining acid mine drainage (AMD) impact was detected at Haveri, where the relative amounts of metals accumulated in sediments corresponded to the amounts of metals leached from the oxidized layer of the tailings. However, the temporal sequence of metal loading (Ni>Cu, S>Ag, As, Cd, Fe, Zn, V>Co, Mo) did not follow the classical sequence of sulfide weathering due to geochemical processes acting along the pathway to the receiving lake. The metal concentrations gradually decreased in recent sediments because of the progressive depletion of sulfides in the oxidized surface tailings, attenuating mechanisms such as adsorption by secondary Fe(III) minerals and increasing percolation of the contaminant plume into the groundwater. Nevertheless, the recent recorded values remain above the pre-anthropogenic levels, especially for copper. The examination of lake sediment cores allows assessment of the long-term evolution of metal and metalloid drainage from mining areas including pre-mining conditions. ► Geochemistry of age-dated lake sediments cores was investigated at two mine sites. ► Lake sediment records allow assessment of long-term evolution of elemental drainage. ► Pre-mining sediments revealed that other land uses may mobilize mining-related metals. ► Mining impact could clearly be detected in contemporary and post-mining sediments. ► Recent accumulations indicate continuing mine drainage, despite attenuating processes.