Describe a scenario where a residential sprinkler system could cause backflow.

A residential sprinkler system can indeed cause backflow when it is connected without proper backflow protection. Backflow occurs when contaminated water flows back into the clean water supply, which can happen if there is a cross-connection.

In the case of a sprinkler system, if there is no backflow prevention device installed, pressure changes in the water system—due to usage elsewhere in the system, like a sudden demand for water—can create a negative pressure condition. This negative pressure could draw contaminated water from the sprinkler system (perhaps from fertilizers or pesticides used in the irrigation) back into the municipal water supply, posing a significant health risk.

Installing backflow prevention devices, such as a reduced pressure zone (RPZ) assembly or double check valve, is critical in preventing this scenario. These devices work by allowing water to flow in one direction only, thereby keeping the potable water supply safe from contamination.

The other options, while they illustrate scenarios that can lead to issues in water quality or safety, do not directly address the fundamental role of backflow prevention in safeguarding drinking water against contamination via a sprinkler system.