Post-mining neutralization of acidic surface mine lakes

Twenty core samples and 54 surface sediment samples were taken from surface mine lakes in Missouri, Illinois, and Indiana to determine the rates of neutralization of acid mine lakes. Sediment samples were analyzed for diatom microfossils, selected chemical elements, and the radionuclide, lead-210. The surface samples showed that there were strong differences in diatom microfossils between acidic and neutral lakes but there was no strong difference in sediment chemical composition between acidic and neutral lakes. Of 20 lakes from which cores were examined, 9 showed diatom evidence of neutralization and one of acidification over time. Cores from 5 of these were dated using lead-210 analysis. Diatom microfossils showed that neutralization took from less than 3 years to 30 years to occur. Comparisons between lake sediment and water column chemistry indicated that neither sulfide deposition nor H2S outgasing is likely to play a major role in the neutralization process. Chemical analyses of lake sediment showed that the sediment is a sink for heavy metals. These metals are held as sulfides. There is also a considerable fraction of metal ions strongly bound to clays. This research demonstrates that acid lake neutralization is common, that it occurs over moderate time spans and that the rate is controlled by rates of acid supply from the watershed.U.S. Department of the InteriorU.S. Geological SurveyOpe