Seasonal characterization of the physical, chemical, biological and ecotoxicological variables in a stretch of the Paraíba do Sul River, SP, Brazil

Abstract The Paraíba do Sul River is located in a Brazilian region with high population density and great socio-economic importance. In the São Paulo stretch, the river is impacted and water quality is impaired due to the removal of riparian vegetation, increases in impermeable surfaces, industrial activity, the use of agricultural inputs, and the release of wastewater without proper treatment. This study characterized the physical, chemical, biological and ecotoxicological variables associated with the water quality of the Paraíba do Sul River in the upper-middle stretch during the dry and wet seasons. The sampling was carried out quarterly between August 2013 and August 2014. The variables evaluated were pH, temperature, conductivity, turbidity, dissolved oxygen (OD), biochemical oxygen demand (DBO), chemical oxygen demand (DQO), total phosphorus (PT), total solids (ST), fecal coliforms (CT), chlorophyll-a, acute toxic effect (ETA) and chronic toxic effect (ETC) and Trophic State Index (IET). The results showed that the variables OD, PT and CT were not in accordance with those established for a Class 2 aquatic environment, according to the National Counsel for the Environment 357/2005 (Portuguese acronym “CONAMA”). Some variables showed a reduction during the evaluated period, such as DBO (42.4%), ST (31.1%) and DQO (21.3%). Both ETA and ETC were observed in the rainy period. The statistical analysis showed that the rainfall has an influence on OD, DBO, ST, chlorophyll-a, ETA and ETC, acting as a possible intensifier agent of impacts on the river.