What does "first flush" refer to in stormwater runoff?

The term "first flush" specifically refers to the initial runoff that occurs at the beginning of a rain event. This first portion of stormwater is significant because it often contains the highest concentration of pollutants. During dry periods, contaminants such as oils, heavy metals, and sediments accumulate on surfaces like roads, parking lots, and rooftops. When the rain first begins, the initial runoff washes away these pollutants. As the rain continues, the runoff becomes more diluted as it mixes with additional water, resulting in lower concentrations of contaminants. Therefore, understanding the concept of first flush is crucial in stormwater management as it informs strategies for treatment and mitigation to reduce pollution entering waterways.