The environmental situation in the state of Espírito Santo is serious and major rivers that supply the metropolitan region of the state are below the critical threshold. The worst drought in 80 years has caused problems in supply, as well as losses in agriculture and farming. Water quality can also be affected by environmental and human factors. An agraves to the quality of surface water is the seasonality of environmental variables, such as precipitation. Several towns are in critical situation caused by the lack of water, including the Vitória capital. This is the first time that the capital goes through water rationing. Thus, the aim of this study was to evaluate the seasonal effect on the quality of surface waters of an important river in southeastern Brazil, to ensure water quantity and satisfactory quality for future generations. Water samples were collected at 6 sites during 1 year, comprising the dry and rainy periods, evaluated 10 physicochemical and microbiological variables. Statistical analysis indicated variables related to demand for oxygen and nitrogen compounds as important to evaluate the influence of seasonality. There was no significant difference between the dry and rainy periods for the WQI, but many parameters were outside the limits set by CONAMA. The seasonality factor and its impact on diffuse sources contributed to decline in water quality.
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