What aspect of storm water discharges is typically sampled to evaluate pollution levels?

Sampling the first flush of storm water is crucial for evaluating pollution levels because this initial runoff often contains the highest concentration of pollutants. During a rain event, the first flush refers to the very beginning of storm water discharge, which collects contaminants that have accumulated on surfaces during dry periods. This may include metals, nutrients, oils, and other hazardous substances.

Analyzing the first flush allows for an accurate assessment of the pollutants that enter water bodies as a result of storm water runoff. This is especially important for meeting regulatory requirements and implementing effective mitigation strategies. The subsequent waters flowing during the storm may dilute these pollutants, leading to an underestimation of the potential impact on the environment if the sampling were conducted later in the discharge event.

In contrast, sampling approaches such as collecting from the last 15 minutes of discharge or during a mid-discharge interval may not capture the peak contamination levels that occur initially. The concept of final discharge output typically encompasses all the water that has flowed out at the end of a storm event, which again may not provide an accurate representation of pollution levels at their highest concentration. Therefore, focusing on the first flush ensures that the sampling targets the most critical period for pollutant evaluation.