Natural attenuation processes in two water reservoirs receiving acid mine drainage.

Characteristics of water profiles and sulphide formation processes in sediments were studied in two water reservoirs affected by acid mine drainage in order to investigate the mechanisms controlling the physical and chemical processes that, under favourable conditions, act to reduce the toxicity, mobility and concentration of metals and metalloids in the water column. Water columns and pore-waters from sediments were analysed for Fe species, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn, Cr), sulphide, sulphate and bicarbonate. Inorganic reduced sulphur compounds (acid volatile sulphur, pyrite sulphur and elemental sulphur) and reactive Fe were determined in the sediments. A sequential extraction was also performed. Both reservoirs behave like holomictic and monomictic lakes, with a summer thermal stratification that disappears during winter. pH values between 4 and 7 can be observed along the water columns. Pore-water concentrations of up to 25 mg/l of Fe, 4 mg/l of Al, 1.3 mg/l of Zn, 170 microg/l of Pb, 11 microg/l of As, etc. have been found. The results suggest that toxic elements such as Cu, Zn, Co, Pb, Cr, As, etc. are mainly found in the bioavailable fraction which is the most hazardous for the environment. The calculated degree of sulphidization (DOS) and degree of pyritization (DOP) values indicates that removal of trace elements from anoxic pore-waters occurs by coprecipitation and/or adsorption on newly formed Fe sulphides (framboidal pyrite), attenuating the contamination. However oxidation of the sediments during turnover periods also occurs, which releases toxic elements back into the water column.