Influence of acidification and liming on metals in lake sediments

Sammanfattning: Emission and precipitation of acidifying substances and metals has caused severe acidification and increased metal concentrations in lakes and streams in Sweden as well as many other countries during the last decades. To counteract the negative effects of acidification, liming has been used extensively in Sweden since the 1970’s, a treatment that has been called one of the largest man-maid manipulations of aquatic ecosystems ever.The aim of this thesis is to investigate the influence of acidification and liming on deposition of metals to lake sediments, and to look at the fate of metals in sediments during a possible reacidification process because of terminated liming.Studies of both surface sediments and dated sediment profiles sampled from limed lakes and non-limed reference lakes indicated that liming causes increased sedimentation of many metals. Increased deposition to the sediments was found for Al, Cd, Co, Ni, Fe, Mn and Zn, probably as a direct result of the increased pH. Liming was also found to increase the deposition of As and Cr, but that was probably due to the secondary effect of association to Fe and Mn complexes. No influence of lime treatment on the deposition of Hg, Pb and V to lake sediments could be demonstrated. The studies also indicated that acidification can decrease the deposition of Cd, Fe, Mn and possibly also Co and Zn to the sediments.The contribution from the used lime products to the metal load in sediments of lakes limed directly on the lake surface was also evaluated. The lime products were found to be an important source for Al, Cu and Ni and a relatively important source for Co, Cr, Hg and V, but did not seem to contribute much to the content of As, Cd, Pb or Zn in lake sediments.An experimental study of reacidification showed that (re)mobilisation of metals, e.g Al, Cd, Mn and Zn, from sediments may occur and that reacidification of formerly lime treated lakes may contribute to increased concentrations of these metals in the lake water. Cu, Fe and Pb are less affected by pH changes and the concentrations of these metals will probably not increase in lake water during reacidification.Altogether, the thesis shows that lime treatment causes increased deposition of many metals to the lake sediments, whereas reacidification may lead to remobilisation of some metals from the sediments with increased concentrations in the water phase as a result.