How do pathogens typically enter a water distribution system?

Pathogens typically enter a water distribution system primarily through cross-connections or main breaks. Cross-connections are points in the plumbing where potable water can be contaminated by non-potable sources, such as irrigation systems or industrial processes. This can lead to the backflow of contaminated water into the drinking water supply if appropriate backflow prevention measures are not in place.

Additionally, main breaks can create conditions where contaminated surface water or groundwater can infiltrate the distribution system. During a main break, reduced pressure in the system may allow contaminants to enter through cracks and openings, posing a risk to public health.

The other options are less relevant to how pathogens enter water distribution systems. Filtration systems are designed to remove pathogens, not allow them into the system. Evaporation does not facilitate the entry of pathogens into the water supply, and while chemical spills can pose risks, they are typically addressed at treatment facilities before reaching distribution systems. Thus, the movement of pathogens into a water distribution system is primarily through vulnerabilities like cross-connections and main breaks.