In the Ground Water Rule for systems that disinfect, large systems are required to monitor ______ and small systems are required to monitor ______.

• A. every 4 hours, daily
• B. continuously, daily
• C. continuously, once per shift
• D. every 4 hours, once per shift

Answer: (B)

Monitoring disinfectant residuals under the Ground Water Rule is driven by system size to balance safety with practicality. For large systems, continuous monitoring provides a real-time readout of the disinfectant level across the distribution system, with alarms if residual falls too low. This immediate feedback is crucial in larger, more complex networks where a loss of disinfection somewhere could affect many people. For smaller systems, daily monitoring still ensures the residual is checked at least once per day, offering a practical safeguard given their simpler networks and fewer resources for continuous online testing. The other timeframes would either miss real-time changes in a large system or impose more frequent sampling than necessary for a small system.