First flush pollution load of urban stormwater runoff

In a storm event, a first flush (FF) phenomenon occurs when most of the pollution load is transported in the initial part of the event discharged volume. The objectives of the study were to consider its severity for a number of pollutants as well as to recognize rainfall–runoff characteristics that influence it. The study was conducted in a semi-arid region of a mixed residential and commercial urban catchment in Iran. Ten major rainfall events were monitored for total solids (TS), total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), Zn, and Pb. Total suspended solids and COD were usually above the permitted discharge. Power and polynomial functions fit well to the normalized curve of the distribution of pollutant load with volume. Discharge loads of TN, Pb, and Zn were approximately uniform. There was a relatively weak FF for TS, TSS, and COD. No correlation between FF load of TS and COD and rainfall–runoff characteristics was observed, but the amount of the FF load of TSS increases when the intensity and duration of a rainfall event increase. Key words: first flush, urban runoff quality, pollution load distribution, event mean concentration, urban stormwater runoff.