Geochemical consequences of acid mine drainage into a natural reservoir: inorganic precipitation and effects on plankton activity

Results of a field experiment demonstrating the response of a reservoir (Ob reservoir) to the inflow of a restricted portion of an acid mine drainage solution (AMD) derived from the Berikul sulfide tailings is presented. Drainage water samples from Berikul tailings are acidic (pH 2.0-2.4), with high levels of Fe (up to 30 mg/l), As (up to 4.5 mg/l) and other metals (e.g. Pb, Zn, Cu, Cd). High metal concentration and low pH of these solutions are the result of the intensive interaction of rainwater and soluble efflorescent phases on the Berikul tailings surface. The experiment was carried out in 3 mesocosms (bounded water volume), located in the small gulf of the Ob reservoir without any industrial activity. The discharge of an AMD solution to the mesocosms caused the development of an abundant reddish suspension. The latter relates to the neutralization of the acid solution and formation of gypsum, jarosite and Fe-oxides. The uptake of dissolved metals resulted in fast mortality of phytoplankton due to the strong toxic effect. Compared with background phytoplankton numbers, the population decreased with a factor 2-5. pH values decreased sharply both due to acidification by drainage fluids and the effect on plankton activity. A decrease in zooplankton quantity took place after two days as a consequence of a loss in food. The subsequent decrease in metal concentrations in the reservoir is the result of both co-precipitation with new-forming mineral phases and the adsorption by phytoplankton. Fe and As occurred dominantly in the suspended phase, while Zn and Cd remained in dissolution. Two mechanisms of metal precipitation have been traced, namely precipitation as sorbed ions onto surfaces of newly formed Fe compounds and bioaccumulation, whereby physico-chemical reactions played the dominant role during the first 40 h. The latter are very effective (with settling rate is about 0.5 cm/s) but single factor of metal removal. Biotic mechanism became detectable from the 40th hour, till the end of the experiment (i.e. 234th hour). Metal settling occurred rather slow (rate is similar to0.01 cm/s), but it is a continuous cyclic process, owing to renascent of new phytoplankton cells. The restoration of original reservoir conditions is attained after 10 days. (C) 2001 Elsevier Science B.V. All rights reserved.