What is the “first flush” phenomenon in stormwater runoff?

The "first flush" phenomenon refers to the initial stormwater runoff that occurs at the beginning of a rain event. This runoff is significant because it typically contains the highest concentration of pollutants. During the dry period leading up to a rain event, contaminants such as oils, heavy metals, and other debris accumulate on impervious surfaces like roads and rooftops. When the rain begins, this initial flow of water washes those accumulated pollutants into the stormwater system, resulting in a spike in pollutant concentrations. Recognizing the importance of the first flush is crucial for effective stormwater management, as implementing control measures such as detention basins or treatment systems can help reduce the impact of these pollutants on receiving waters.

The other options do not accurately capture the essence of the first flush. For example, the concept does not pertain to runoff after a specific duration of rain or evaporation processes, nor does it relate to water that is absorbed by the soil before generating any runoff. Each of these alternatives misses the critical "initial" aspect and its connection to pollutant concentration associated with the first flows of stormwater.