Evaluation of nitrogen and phosphorus in surface reservoirs of the semi-arid region of Brazil using mass balance

Abstract

Water reservoirs were built in the semi-arid region to mitigate the effects of drought and water scarcity. However, these reservoirs are subject to variables that impact the volume and quality of the stored water, with the contribution of such nutrients as Total Nitrogen (TN) and Total Phosphorus (TP) being one of the main factors in intensifying the eutrophication process. The aim of this study was to quantify the principal inputs and outputs of nutrients (N and P) and calculate the nutrient balance, in order to verify the influence of seasonality on the import and export of these nutrients in surface reservoirs of the semi-arid region of Brazil. Seven campaigns were carried out between 2015 and 2016, with water samples collected at five points in the Pereira de Miranda reservoir and four in the General Sampaio reservoir. The inflow of nutrients was greater when the inflow rate of the water was higher. The balance showed the accumulation of 22.8 tons of TN and 2.8 tons of TP in the Pereira de Miranda reservoir, and 13 tons of TN and 1.5 tons of TP in the General Sampaio. These reservoirs are therefore retaining nutrients, acting as a storage site for nutrients transported by surface runoff in the basin of the Curu River. This increase in storage occurred mainly during the rainy season, when nutrient retention by the reservoirs impaired water quality, resulting in eutrophication, and making it difficult to meet the uses for which the reservoirs were intended.