The most serious example of water changing in the distribution system from the water that entered the system at the water plant is which of the following?

• A. Bacterial growth
• B. Algae growth
• C. Cross connection
• D. Corrosion by products

Answer: (C)

This question is about the risk of backflow in the distribution system and how a cross-connection enables contamination. A cross-connection is any physical link between the potable water system and a potential contaminant source (like hoses, irrigation lines, or other equipment) that could allow water to flow backward into the system if pressure drops. That backflow can introduce substances that were never intended to be in the drinking water, making it the most serious way water quality can be altered after leaving the treatment plant. Other phenomena, such as bacterial growth, algae, or corrosion by-products, stem from conditions within the system itself (stagnation, light, pipe materials, water chemistry) and are important, but they don’t inherently create a backflow path that could bring external contaminants into the entire distribution system.