An assessment of quality of stormwater in a subtropical urban watershed

The lack of a proper collection network combined with insufficient or ineffective treatment of the collected wastewater means that household wastewater greatly affects the quality of water resources in Brazil. This study aims to assess the presence of organic pollutants, nutrients and heavy metals in water, suspended river sediments and bottom sediments and determine the mean concentrations of pollutants in events in an urban watershed. The watershed investigated is located in Santa Maria, a city in the state of Rio Grande do Sul, Southern Brazil. Water quality parameters were analyzed using event mean concentrations (EMC), pollutant bioavailability in bottom sediments and Pearson's correlation coefficient. Soil use and occupation were shown to dramatically affect the water quality of rainwater runoff. The bioavailable fractions of heavy metals in bottom and suspended sediments did not exceed the threshold set by the applicable Brazilian National Council for the Environment Resolution (CONAMA 454/2012) nor the sediment guidelines of the Canadian Institute for Environmental Law and Policy. Stormwater samples showed high concentrations of TS, SS, SD, BOD, COD, Cu, Ni, Pb and NO2-. EMC tended to display higher values as rainfall and precipitation increased. The accumulation of pollutants on streets and urban drainage is also influenced by the antecedent dry period. The body of water receives a massive intake of pollutants during the flood wave.