Protecting Public Health and the Environment.

July 22, 2014

Operational Evaluation Level Report Required for Stage 2 Disinfection Byproducts Water Systems

Owners and operators of community and non-transient non-community public water systems that are on Stage 2 Disinfection Byproduct (DBP) monitoring and taking quarterly samples need to evaluate sample results regularly. The Operational Evaluation Level (OEL) requirement applies to all public water systems with quarterly monitoring schedules, which includes all public water systems that serve over 10,000 people, surface water systems serving over 500 people and on routine monitoring, and public water systems with quarterly monitoring schedules due to exceeding a DBP maximum contaminant level (MCL).

If a public water system’s DBP schedules are annual or less frequent, the OEL and OEL reporting requirement do not apply.

What is an OEL?

The OEL is an early warning to allow owners and operators to act and prevent violations. An OEL uses three quarters of total trihalomethanes (TTHM) and haloacetic acids (HAA5) results to predict the next quarter’s results. The OEL calculation is repeated each new quarter, using the most recent three quarters of results. OELs are determined by calculating the sum of the two previous quarters' results plus twice the current quarter's results divided four for each monitoring location.

Is an OEL exceedance a violation?

No. If the result of the OEL for TTHM is over the MCL of 0.080 mg/L and/or the OEL result is over the MCL of  0.060 mg/L for HAA5, it is an OEL exceedance, but not a violation. The OEL is an early warning to help prevent a violation. However, if the average of four quarters of actual results exceeds the MCL for that monitoring location, that would be an MCL violation.

What is an OEL report?

Although a water system is not in violation if OEL calculations exceed the MCL, an OEL report must be prepared. An OEL report includes an examination of operational and maintenance procedures and identification of issues that may be contributing to the exceedance(s), as well as an evaluation of ways to reduce those levels. A report that includes the evaluation and recommendations must be submitted to a DEQ drinking water representative or the health district no later than 90 days after being notified of the analytical results that caused the OEL exceedance. If you are able to identify the cause of the exceedances, the scope of the evaluation may be limited. A copy of the report is required to be kept on file for public review. Failure to submit the OEL report within 90 days is a violation. 

OEL Report Template

The OEL report template will help organize TTHM and HAA5 results, determine operational evaluation levels, review components of your system that could contribute to elevated levels of TTHM/HAA5, and identify the necessary steps to take to reduce future exceedances.

For more detailed information on operational evaluations, refer to the EPA Operational Evaluation Guidance Manual.

Staff Contacts

Drinking Water Field Services Lead
Curtis Stoehr
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0542
curtis.stoehr@deq.idaho.gov





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