Characterization of first flush phenomenon in an urban stormwater runoff: A case study of Flamingo Tropicana watershed in Las Vegas valley

This case study deals with the characterization of pollutants in runoff for major storm events in an urban watershed in Las Vegas, NV. The occurrence of first flush was checked based on several first flush definitions. Grab samples were collected from 6 major storm events at the Flamingo Tropicana Wash during a period from January 2008 to December 2009. Correlation of non point pollutants such as total phosphorous (TP), total nitrogen (TN) and total suspended solids (TSS) with runoff was derived through the use of hydrographs and pollutographs. The concentration of grab samples at different stages of the same storm event varied considerably. Based on concentration, the higher strength of first flush was seen in case of TSS and lower in TN and TP. The analysis of cumulative runoff volume vs. pollutant load showed that the first 30% of the intercepted runoff volume transported 44% of TN, 34% of TP and 35% of TSS respectively. The first 20%, 30%, 50% and 80% of runoff volume transported 28%, 38%, 58% and 85% mass of the pollutants respectively. Based on first flush definitions, the storm events showed the presence of moderate first flush. The ADD was found to have a direct impact on build up and wash up of TSS. The rainfall events with higher water depths were more polluted for TSS than the less intense events. The findings will be helpful in retrofitting or designing BMPs, developing other treatment works and control strategies to enhance the water quality standards, and reducing the pollutant loads in surface water bodies