Which of the following scenarios would require using alternate water sources as a preventive measure?

Using alternate water sources as a preventive measure is essential during contamination events because these situations pose a direct threat to public health and safety. When any part of the water supply system becomes compromised due to contamination—whether from chemical spills, biological threats, or natural pollutants—it is crucial to ensure that safe, clean water remains available for the community.

During such events, relying solely on the compromised source could lead to severe health risks, making it necessary to draw from alternate sources to maintain water quality and supply. This not only mitigates immediate health hazards but also helps to maintain trust in public water systems.

The other scenarios, such as heavy rainfall, regular maintenance checks, and routine system updates, are important for overall system management, but they do not present the same direct risk that necessitates immediate use of alternate water sources. Heavy rainfall can affect supply levels but does not typically require an alternate source unless it contributes to contamination. Regular maintenance checks and routine updates are part of preventative practices that ensure the system functions well but do not indicate an emergency that would require immediate action regarding water safety.