Groundwater must not be more than ____ below the surface if groundwater monitoring is used as the only method of release detection.

• A. 10 feet
• B. 15 feet
• C. 20 feet
• D. 25 feet

Answer: (C)

When groundwater is the only method used to detect a release, the depth to groundwater directly affects how quickly a leak can be found. The limit is twenty feet, because keeping groundwater within this distance allows monitoring wells to intercept a plume and detect contaminants in a timely manner. If groundwater were deeper than twenty feet, a leak could go undetected longer, increasing the risk of spreading contamination before detection. So twenty feet is the maximum depth that still enables effective single-method monitoring; depths shallower than that are also acceptable, while deeper groundwater would require additional detection methods.