Which term is not considered a "discharge" under U.S. pollution regulations?

The term "sampling a harmful substance" is not considered a discharge under U.S. pollution regulations because sampling typically involves taking a small, controlled amount of a substance for testing or analysis purposes. This action is part of efforts to monitor compliance and ensure safety and environmental protection. Sampling does not involve the release or escape of a harmful substance into the environment.

In contrast, pumping, spilling, and leaking all refer to the actual release of harmful substances into the environment, which can lead to pollution and ecological harm. These actions can trigger regulatory responses due to their potential negative impacts on waterways and surrounding ecosystems. Thus, sampling is distinct in purpose and outcome from the other listed actions, which are considered discharges as they involve uncontrolled emissions of harmful materials.