Quantitative and qualitative hydrologic balance for a suburban watershed with a separate sewer system (Nantes, France)

A qualitative and quantitative budget at the outlet of the storm-water runoff system of a small suburban watershed is presented together with some data regarding waste-water. 445,000 m³ (34% of the rain-water volume) were drained by the storm-water runoff system and 40,879 m³ by the waste-water system from September 2002 to March 2004. Storm-water runoff is generally not heavily polluted with regard to trace metals but concentrations occasionally exceed the standards for surface water of good quality. On the contrary, pesticides (diuron and glyphosate) have very high concentrations especially in spring and autumn when their use is maximum. As the St Joseph storm-water runoff is finally discharged into the Erdre River, measures to reduce the use of these pollutants should be considered.