Impacts of urban wastewater and hydrogeochemistry of the São Mateus River, Espírito Santo, Brazil

The waters of the River Sao Mateus and its tributaries, Abissinia and Preto do Sul, were analyzed to evaluate the impacts caused by the direct disposal of untreated domestic effluents generated on the city of Sao Mateus-ES, Brazil. Between the evaluated parameters, the most affected one was the dissolved oxygen (DO) in Abissinia River, which is completely consumed. Regarding Sao Mateus River, the situation is worrisome because its self-depurative capacity is exceeded and by the elevated levels of phosphorus nutrient, which may lead to eutrophication. A prototype was built to carry out the respirometric characterization of the effluents, aiming to determine the DO consumption rate (K 1). This constant was used in the self-depuration model, which shows the need of effluent treatment with a minimum efficiency of 60% in removing the organic load. The chemical characterization revealed similar sodium chloride composition for these waters with contrasting patterns of lanthanides. Despite the low levels of toxic trace elements registered at Sao Mateus River, the concentrations of Pb and As were higher in the Preto do Sul River. This finding requires further investigation to track the source of these elements.