Flow-weighted mean concentration (FWMC) is used to describe what?

• A. The average concentration of a substance in water over a specified time period
• B. The peak concentration during a storm
• C. The minimum concentration during drought
• D. The total mass of contaminant per year

Answer: (A)

Flow-weighted mean concentration describes the average amount of a contaminant in water over a defined period, but it weights each moment by how much water is moving. Because the pollutant mass depends on both concentration and flow, periods with higher flow contribute more to the overall average. In practice, it’s calculated by dividing the time-integrated product of concentration and flow by the total flow over the period, giving a representative concentration that reflects how much pollutant is actually transported through the system, especially during variable conditions like storms. This is different from simply taking a time average of concentration (which ignores how much water is moving), and it’s not the peak concentration during a storm, the minimum during drought, or the total yearly mass (load).