Deoxygenation rate, reaeration and potential for self-purification of a small tropical urban stream

Mathematical models used to simulate the water quality and the processes of self-purification of water bodies are important tools to support effective water resource management. The objective of this study was to evaluate the impact of organic pollution on water quality and the assimilative capacity of organic matter in the Ribeirao Vermelho stream situated in the city of Lavras in Minas Gerais. The deoxygenation rate (K1) and reaeration (K2) were determined and tested in three segments of the Ribeirao Vermelho over two seasons (summer and winter). The purification process simulation was performed using the Streeter- Phelps model. The highest K1 and K2 values occurred during the winter period which was characterized by lower flows. Simulation of the dissolved oxygen profile and biological oxygen demand fitted the model well, indicating the prevalence of carbonaceous deoxygenation due to nitrification and also demonstrating the importance of natural reaeration as part of the purification process. Different response was observed during the winter, in which the high primary productivity values stood in the reaeration process stream. Biological oxygen demand values did not meet the limits recommended by the Resolution of the Ministry of the Environment 357/2005 throughout the river system and the entire study period. In conclusion, the Ribeirao Vermelho is unable to self-purify along its entire length, and water quality is influenced by effluent discharge which intensifies degradation.