On a TPDES permit, concentrations are reported in what unit, and loadings are reported in what unit?

• A. ppm - lbs/day
• B. g/L - lbs/day
• C. mg/L - lbs/day
• D. mg/L - kg/day

Answer: (C)

In TPDES permits, you measure two related aspects separately: how concentrated the pollutant is in the water, and how much total pollutant is discharged per day. The concentration is reported as milligrams per liter (mg/L), which shows the pollutant’s strength in the water. The loading is reported as pounds per day (lbs/day), which reflects the total mass discharged over time and depends on both the concentration and the volume of water flowing out. So, concentrations use mg/L and loadings use lbs/day. (Note: concentration in mg/L is closely related to ppm for dilute aqueous streams, but mg/L is the standard reporting unit in permits.)