Frequent Questions

During the first year of Stage 2 TTHM/HAA5 monitoring are systems required to continue conducting their Stage 1 TTHM/HAA5 monitoring until a full year of Stage 2 TTHM/HAA5 data has been collected and the first LRAA results can be calculated?

Once a system is required to begin Stage 2 quarterly compliance monitoring for TTHM and HAA5 under 40 C.F.R. 141.620(c), it will no longer be required to perform monitoring pursuant to its Stage 1 TTHM and HAA5 monitoring plan. Under Stage 2, systems will perform four consecutive quarters of monitoring and will calculate compliance at the end of the fourth calendar quarter that follows the compliance date. Systems will continue to conduct LRAA compliance calculations at the end of each subsequent quarter. (Please note a violation can be incurred earlier if the LRAA calculated based on fewer than four quarters of data would exceed the MCL.)
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