The Firstflush of Pollutants in Surface Runoff

One of the factors impacting quality of water resources is pollution due to urban storm runoff during first stage of storm runoff and is commonly called "firsflush". At this stage the pollution load is rather high. However, if this pollution is properly controlled and managed, the size of the required treatment facilities will be considerably reduced. The surface runoff pollution in the city of Isfahan is high and necessitates the implementation of some control system. For this purpose, ten rainfall events were studied during fall and winter between 1999 and 2000. The data were gathered through grab sampling from Sio-se-pol bridge watershed. Statistical methods were used for analyzing data. The pollution load distribution vs. runoff volume was investigated.The results indicate that total solids (TS), suspended solids (SS), organic compounds and lead concentration exceeded discharge standards. TS, SS and organic compounds were more significant during the firstflush. SPSS (a statistical software package) analyses showed that there is a correlation between TS in the firstflush and runoff flow rate.Statistical analysis of the results indicated that rainfall depth and duration increase SS washoff during first stage of runoff. Also it was found SS washoff increases with increasing average discharge during firstflush.The results also indicated a correlation exists between the SS in the firstflush and the rainfall depth, event maximum intensity, average intensity, rainfall compounds and rainfall characteristics. Finally, an equation was suggested to predict pollution loading of runoff during firstflush.