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Operational Evaluations & Operational Evaluation Levels

Requirements for Monitoring

A system may still be in full compliance with the Stage 2 DBPR even when individual DBP sample results are above 0.080 mg/L for TTHM and 0.060 mg/L for HAA5 since compliance is based on a running annual average. Because of concerns that high levels of DBPs can occur even when the system is in compliance, there is an additional requirement in the Stage 2 DBPR for operational evaluations.

As a part of Stage 2 DBPR compliance monitoring, water systems are required to monitor operational evaluation levels (OELs). The OELs are determined by calculating the sum of the two previous quarter's results plus twice the current quarter's results divided by 4. This locational running annual average (LRAA) threshold should help water systems identify if they are likely to exceed the MCL in the next quarter, and gives them a chance to make operational changes.

If OELs are higher than the MCL for TTHM or HAA5 at any location in the distribution system, you must conduct an operational evaluation. Your operational evaluation must include:

  • An examination of system treatment and distribution operational practices, including:
    • Storage tank operations
    • Excess storage capacity
    • Distribution system flushing
    • Changes in sources or source water quality
    • Treatment changes
    • Any problems that may contribute to TTHM and HAA5 formation
  • What steps could be considered to minimize future exceedances

Information gathered from the operational evaluation must be compiled into a report and sent to Drinking Water Services for review within 90 days after being notified of the high TTHM or HAA5 sample result that causes the OEL to exceed the MCL. Systems can use the OEL report template which refers to sections of the EPA Operational Evaluation Guidance Manual.

Note: If you know the cause of the OEL, you can ask Drinking Water Services to limit the scope of your evaluation. Requesting approval to limit the scope of the operational evaluation does not extend the schedule for submitting the operational evaluation report, which is still due 90 days after notification of the high analytical result.