During an exfiltration test, how high should the lower manhole be filled relative to the pipe, and how often should measurements be taken?

• A. One foot, every minute
• B. Two feet, every four minutes
• C. Three feet, every six minutes
• D. Four feet, every eight minutes

Answer: (B)

In an exfiltration test, you establish a known hydrostatic head in the lower manhole and monitor how the water level changes over time to detect leakage. Using two feet of water above the pipe provides a consistent, moderate pressure that reveals leaks without overdriving the system. Recording measurements every four minutes gives enough data points to accurately calculate the exfiltration rate while keeping the test practical. Too little head or too long between readings can miss or underestimate leakage, while too much head or overly frequent measurements can distort results or be inefficient. Therefore, two feet of head with readings every four minutes is the reliable approach.