Disinfectant residual monitoring is applicable to all community and non-transient non-community water systems that use chlorine or chloramines as a disinfectant. Chlorine is a disinfectant that is used to control microbes. Some people who use
water containing chlorine well in excess of the Maximum Residual Disinfectant
Level (MRDL) could experience irritating effects to their eyes and nose and
could experience stomach discomfort and anemia.
Systems must monitor for chlorine or chloramines every time they sample for total coliform - this includes repeat sampling. Samples must be taken at the same location in the distribution system as those locations used for total coliform sampling. This is a separate requirement from the minimum twice weekly monitoring and
recording of the disinfectant residual.
Results may be submitted on the coliform sampling form or the Maximum
Residual Disinfectant Level Report (MRDL) Form (doc)
Maximum Residual Disinfectant Level (MRDL)
The MRDL for chlorine is 4.0 mg/L (measured as free chlorine).
The MRDL for chloramines is 4.0 mg/L (measured as total chlorine).
Compliance with the MRDL is based on a running annual average of monthly averages. Any running annual average that exceeds the MRDL is a violation.