Which of the following is true regarding monitoring schedules for contaminants?

Monitoring schedules for contaminants in water systems are often tailored to the specific circumstances and historical context of each system. This means that the sampling frequency can be adjusted based on past contamination levels. If a water system has previously experienced higher levels of certain contaminants, it may be subject to more frequent monitoring to ensure safety and compliance with health standards. Conversely, systems with a history of good water quality might be allowed to monitor less frequently. This approach allows for a more efficient allocation of resources while maintaining public health safety, recognizing that not all systems face the same level of risk or need for intensive monitoring.

The other options present statements that don't accurately reflect the flexible nature of contaminant monitoring schedules as they are mandated by regulatory agencies. Monitoring daily or requiring biannual sampling for all systems would not take into account the specific risk profile of different water systems, which is essential for effective management of water quality. Additionally, public notifications are an important part of transparency and regulatory compliance, required not just when violations occur but also under certain conditions even if water quality remains adequate.