Frequent Questions

Can a public water system composite monitoring samples to determine compliance with the maximum contaminant levels (MCLs)?

States may allow the use of compositing to reduce the total number of samples that must be analyzed for both inorganic and organic chemicals. Composite samples from a maximum of five samples are allowed, provided that the detection limit of the method used for analysis is less than one-fifth of the MCL. Compositing of samples must be done in the laboratory (40 CFR 141.23(a)(4) and 40 CFR 141.24(f)(14)). For organic chemicals, compositing of samples must be analyzed within fourteen days of sample collection (40 CFR 141.24(f)(14)).  Federal regulations are available by visiting the Government Printing Office Web site ( or by visiting
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