Name the most common cause for public water supply contamination.

• A. Cross-Connection
• B. Contamination by corrosion
• C. Sedimentation
• D. Backflow

Answer: (D)

Backflow is the active process that introduces contaminants into the public water supply. It happens when water in the distribution system flows backward into the pipes, bringing along pollutants from downstream sources or from a customer’s plumbing. This reverse flow can occur in two main ways: backsiphonage, when a drop in system pressure draws water back, and backpressure, when higher downstream pressure pushes water back into the supply. A cross-connection is a risky setup that allows backflow to occur, but contamination itself happens because backflow takes water in the wrong direction. Sedimentation and corrosion describe other water quality issues, not the mechanism by which contaminants enter the system.