Characteristics of First Flush Effects in Stormwater Runoff via Retention-Detention System

Typically, the peak concentrations of contaminants in storm water runoff occur at the initial stage of the event known as the 'first flush'. Reducing first flush effects can lessen adverse impacts on receiving waters. Various retention- detention designs, such as wetlands, detention ponds and proprietary devices, have been utilized to treat stormwater runoff. Frequently, performance is evaluated in terms of overall pollutant removal and runoff quantity equalization. The runoff quality equalization, in terms of first flush reduction, has had little investigation. In this study, characteristics of first flush in and out of the retention-detention devices were analyzed using a first order modeling method. The significance of first flush reduction, as a function of detention time, flow-through removal and static removal, and diffusion factor, was investigated. Pollutant mitigation data were obtained from a wetland receiving runoff from a section of an asphalt highway in Ohio. Individual aliquot samples of runoff were collected during each rainfall-runoff event at both the wetland inlet and outlet and analyzed for total metals, total suspended solids (TSS), and chemical oxygen demand (COD). First flush effects at both the inlet and outlet of the wetland during thirteen rainfall/runoff events were investigated and compared. First flush reduction was observed for TSS, COD and most total metals analyzed (Ca, Cu, Mg, Ni, Pb and Zn). Average first flush coefficients were reduced from 0.75 to 0.46 or 38%.