The Middle Iskar cascade is situated along the middle course of the Iskar River (Bulgaria) after the capital city Sofia and has three small hydroelectric power plants that were put into operation by the end of 2012. The aim of this study was to evaluate the self-purification potential of water in the reservoirs of these plants as an important and necessary condition for their ecological functioning. The assessment was made by hydrochemical parameters (dissolved suspended solids, insoluble suspended solids, total suspended solids, nitrites, nitrates, ammonium, phosphates, chemical oxygen demand, dissolved oxygen) and microbiological parameters (aerobic heterotrophic bacteria and bacteria growing in an Endo medium) and covered a period of three years (2010, 2011 and 2012). Standard methods were applied, mainly colorimetric and microbiological cultivation methods. The obtained results showed high levels of some of the tested indicators during 2012. In the section of the Middle Iskar cascade a high self-purification potential was observed in the reservoirs which maintain good water quality.
Keywords: hydrochemical and microbiological parameters; reservoirs; water quality.