A simpler statistical approach to estimate the allowable effluent discharge into a low monitored river network

The goal of the study was to illustrate how the variation in streamflow patterns affects the magnitude of pollutants that can be loaded into the hydrography in a more straightforward manner. For that, two basins in the Doce River basin (Brazil) were evaluated in order to estimate the variation in the allowed wastewater in streamflow units (m³s^-1) into the hydrography without impairing the rivers' water quality. The main water quality parameters of the basin were considered for the analysis: E. coli, total phosphorus (P), nitrate‑nitrogen (N-NO₃^-), and biochemical oxygen demand (BOD). Because the already existing E. coli in the system is too high, the allowable streamflow, considering this pollutant, was null. For N-NO₃^-_, the model was not valid because the concentration of it the wastewater is smaller than the allowed in the legislation. For last, considering P and BOD, it was observed that, for most gauges, it was possible to load more wastewater in the hydrography during wet months, especially when the effluent goes through some treatment. For example, considering P, a significant difference between the allowable pollutants in the wet season in comparison to the dry season, indicating that for one gauge, more wastewater is allowed in the dryer season. For BOD, contrarily, the allowable wastewater in the hydrography increased by about 50% in both seasons, without much variation. With that, we conclude that the dilution capacity in the waters is variable, so should be the allowed amount of pollutants into the river network throughout the year.