Analysis of Changes in the Quality of Surface Water after Filling of Hydroelectric Reservoirs in the Amazon, Brazil

Abstract

We employed statistical techniques to assess the quality of surface water measured before and after filling the reservoirs of four important hydroelectric power plants (HPPs) established in the Brazilian Amazon (Belo Monte, Jirau, Teles Pires, and Santo Antonio do Jari). A cluster analysis efficiently identified distinctions between the dendrograms obtained for the pre- and post-filling phases in the four projects and indicated changes in water quality after the reservoirs were filled. The Mann-Whitney U test (at the 5% significance level) allowed for the identification of parameters that exhibited significant changes after filling the reservoirs. Across the four HPPs, a total of 45 monitoring points and 54 water quality parameters were analyzed. The results revealed 14 parameters that changed significantly at a greater number of monitoring points in more than one HPP, namely water temperature, pH, dissolved oxygen, electrical conductivity, oxidation-reduction potential, transparency, total dissolved solids, chlorophyll-α, thermotolerant coliforms, alkalinity, total phosphorus, and calcium, sulfate, and sulfide ion concentrations. These parameters were identified as the most important for explaining the changes in water quality after filling the reservoirs. All monitoring points located within the reservoirs (41 points) and immediately downstream of the dams (4 points) revealed significant changes in water quality. Thus, changes in the quality of surface water were observed along the entire lengths of the reservoirs, including the initial stretches. This study contributes to the understanding of the temporal and spatial effects of the formation of reservoirs on surface water quality parameters.