Final answer:
Backflow in a water supply system is caused by a partial vacuum or low pressure, which can occur during times of heavy use or when there is a break or leak in the water pipe. This can lead to the flow of contaminated water back into the water supply system.
Step-by-step explanation:
The cause of backflow in a water supply system is a partial vacuum or low pressure.
Backflow can occur when there is a drop in pressure within the water main, causing the water to flow in the opposite direction. This can happen when there is a high demand for water, such as during times of heavy use or when there is a break or leak in the water pipe.
When the pressure drops, it creates a suction effect, which can draw contaminated water back into the water supply system. Therefore, backflow can also be a result of contaminated water.