Frequent Questions

A public water system (PWS) supplied by a surface water source or a ground water source under the direct influence of surface water must monitor the residual disinfectant level in the distribution system (40 CFR 141.74). At what points in the distribution

The residual disinfectant concentration must be measured at least at the same points in the distribution system and at the same time as total coliforms are sampled, as specified in 40 CFR 141.21 (141.74(b)(6)(i), 141.74(c)(3)(i)). Monitoring frequency is based on the population served by the water system. Samples must be collected at regular time intervals throughout the month (141.21). A state may specify alternate sampling points if it determines that those points are more representative of treated (i.e., disinfected) water quality in the distribution system (141.74(b)(6)(i), 141.74(c)(3)(i)).
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