Characterization of water quality in a small hydropower plant reservoir in southern Brazil

The construction of large reservoirs can cause profound environmental changes. Reduced water flow, increased water residence time, thermal stratification, increased sedimentation rates and decreased dissolved oxygen concentrations are examples of such changes. These changes can affect water quality and the biota in the environments adapted to the natural conditions of a river. Small reservoirs developed in conjunction with hydropower plants, however, could reduce the degraded water quality. This study focuses on characterizing water quality in a small hydroelectric reservoir. The study reservoir has an area of 1.4 km² and a short water retention time. The Monte Claro Hydroelectric Power Plant is part of a complex consisting of three plants on the Antas River in the north‐west of Rio Grande do Sul state, Brazil. The reservoirs associated with these plants are operated as run‐of‐the‐river facilities. Monitoring results obtained by CERAN, the Energetic Company of Antas River (Companhia Energética Rio das Antas), were used to evaluate the reservoir water quality. Three samples were collected seasonally (spring, summer, autumn and winter) in the area of influence of this plant following the filling of the reservoir (2005–2008). The examined water quality parameters were electrical conductivity, colour, turbidity, alkalinity, pH, biological oxygen demand (BOD), chemical oxygen demand (COD), total phosphorus, dissolved oxygen, sulphate, nitrate, nitrite, ammonia, suspended and dissolved solids, chlorophyll‐a, total and faecal coliforms, water temperature and Secchi depth transparency. The results were interpreted using an index of water quality, Trophic State Index, reservoir water quality and CONAMA Regulation 357/05 (Brazilian legislation). Based on these analyses, no significant changes were exhibited in the water quality of the reservoir from the hydroelectric plant operation.