Waste discharge requirements are based on mass of pollutant per unit volume of which medium?

• A. air
• B. soil
• C. water
• D. sediment

Answer: (C)

Waste discharge requirements are set by how much pollutant is present in the liquid being released, expressed as mass per volume of the medium. For liquid effluents, this means the concentration in water (typically mg/L). This basis reflects how the pollutant will mix with and affect the receiving water body, influence treatment needs, and impact aquatic ecosystems as it dilutes downstream. Regulating concentration in water differs from regulating emissions to air (which would use mass per volume of air) or solids interactions with soil or sediment. Sometimes permits also specify total mass discharged per day, but the per-volume basis specifically points to water as the medium.