What timeframe should the initial storm water runoff be sampled for contaminants?

Sampling the initial storm water runoff within the first 30 minutes after a rain event is a crucial practice in storm water management. This timeframe is essential because early runoff is more likely to contain higher concentrations of contaminants. Pollutants from the surface, such as oils, sediments, heavy metals, and nutrients, tend to be washed away immediately during the first flush of the storm.

Capturing samples within the first 30 minutes allows for a more accurate assessment of the pollutants likely to be deposited on the site since they have less time to dilute. Additionally, focusing on this early runoff helps in identifying potential sources of contamination and assists in evaluating the effectiveness of storm water management practices in place. This approach is aligned with industry standards and regulations that emphasize the need to monitor the most concentrated portion of storm water runoff to effectively manage water quality.