Dynamics of nitrogen and phosphorus in reservoir in the semi-arid region using mass balance

Many reservoirs are subjected to impacts resulting from various anthropogenic activities, especially those located in metropolitan areas of the basins. The reservoir Gaviao is responsible for drinking water supply in the Metropolitan Region of Fortaleza and contains excessive presence of algae, a direct result of the enrichment of water by nutrients, which substantially hampers the treatment of water collected in the reservoir and increase the treatment costs. Therefore, it is essential to carry out studies of corrective and/ or preventative management plans so that future water quality is not impaired to the point that it culminates with the replacement or modification of the technology used in the treatment plant. The determination of nutrient loading and mass balances can evaluate the eutrophication process in the system, checking for export or retention of nutrients in the water body. Samples were collected monthly from January to May 2011, at five points in the reservoir in order to study the mass balance of macronutrients. During the study period the reservoir accumulated approximately 28.06 t of phosphorus and 121.42 t of nitrogen, and can be classified as eutrophic.