What hazard is primarily associated with back-siphonage?

Back-siphonage occurs when there is a negative pressure in the distribution system of potable water, allowing contaminants from non-potable sources to be drawn into the drinking water supply. This can happen when there is a sudden drop in water pressure, such as during a main break or high water demand, which creates a vacuum effect that pulls water from an unsafe source into the safe drinking water supply.

The correct choice reflects the significant concern for public health and safety—namely, the potential for contamination through a direct connection between potable and non-potable water sources. Protecting against this hazard is crucial in sanitary engineering, as it helps ensure that drinking water remains free from pathogens and chemicals that can arise from cross-connections with contaminated water systems.

Addressing other options, unequal pressure in siphons is a broader concept and does not accurately represent the specific risk associated with back-siphonage. Pressure buildup in sewer lines relates more to sewage management and does not directly pertain to the risks posed by back-siphonage in potable water systems. The siphoning out of water seals in traps can occur under different conditions, such as negative pressure scenarios, but it does not specifically address back-siphonage in terms of cross-contamination risk.