Modeling the Trophic State of Subtropical Reservoir in Southern Brazil

Abstract

Eutrophication is an environmental imbalance arising from excess nutrients and causing a decrease of water quality. This study assessed the water quality of the Vacacaí Mirim reservoir in the city of Santa Maria, Brazil, according to Brazilian Environmental Standard from 21–211, using a numerical model to simulate temperature, dissolved oxygen, 5-day biochemical oxygen demand, ammonium, nitrite-nitrate, ortho-phosphate, total phosphorus, and chlorophyll-a. Calibration was performed manually and was found to be satisfactory when the accuracy was checked using mean absolute error and root-mean square error. Extreme case scenarios were proposed to monitor the response of the reservoir to droughts, heavy rainfall, population increases, and wastewater treatment. Simulations show that a 3-day period of dry weather causes the reservoir to become hypereutrophic. Increased precipitation simulation could not translate the interactions that occurred in the environment. Increased pollution load scenarios show that high concentrations of total phosphorus and chlorophyll-a change the trophic state of the reservoir to hypertrophic. Simulations of reduced wastewater dumping show that a 66% decrease in dumping volumes considerably improve water quality and the reservoir becomes mesotrophic.