Which option is NOT part of the standard infiltration/exfiltration limit expression?

• A. gallons
• B. per diameter inch
• C. per mile
• D. per year

Answer: (D)

Infiltration/exfiltration is treated as a flow rate that scales with pipe size and length, so the standard expression expresses water leakage as gallons per day per diameter-inch per mile. This means you include the volume unit (gallons), normalize for pipe size (per diameter inch), and account for how much pipe there is (per mile). Using per year would imply an annual total rather than a rate, and infiltration/exfiltration design focuses on a daily rate to reflect real operating conditions and variability. Therefore, per year is not part of the standard expression.