Frequent Questions

How should the running annual average be calculated to determine compliance with the trihalomethane maximum contaminant level (MCL) when a system does not have monitoring data for one of the quarters?

The compliance calculations are based on a running annual average computed quarterly. If the system fails to perform all of the quarterly monitoring, compliance will be determined based on the available data (and the system will also have a monitoring violation) (Stage 1 Disinfectants / Disinfection Byproducts Rule Final Implementation Guidance, EPA816-R-01-012).
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