Operational Evaluations for Disinfection Byproducts in Drinking Water
When and how public water systems must complete operational evaluations for disinfection byproducts (DBP). Requirements for calculations, exceedances, and reporting.
On this page:
- Evaluations and Monitoring
- Calculating the Operational Evaluation Level
- Completing the Operational Evaluation Report
- Failure to Submit the Report
- Assistance and Tools for Public Water Systems
Evaluations and Monitoring
All community and nontransient noncommunity public water systems (PWS) must monitor for disinfection byproducts (DBP), total trihalomethanes and haloacetic acids. PWS personnel must complete the operational evaluation when the system has exceeded the Operational Evaluation Level (OEL), calculated using your DBP sample results. The Operational Evaluation Level Report documents and guides PWS personnel through the operational evaluation.
The goal of the operational evaluation is for PWS to do the following:
- Evaluate source water, treatment and distribution system processes.
- Identify issues and take action to decrease total trihalomethane and total haloacetic acid levels, also called disinfection byproducts (DBP).
- Avoid TTHM and HAA5 maximum contaminant level (MCL) violations.
Calculating the Operational Evaluation Level
The Operational Evaluation Level (OEL) is:
- Calculated using your sample results for total trihalomethanes (TTHM) and haloacetic acids (HAA5).
- Calculated for each TTHM and HAA5 sample location.
- Uses three quarters of results to calculate how much your average MAY BE in the future quarter, what we call the projected average.
- Different than the locational running annual average (LRAA) that is used for the maximum contaminant level (MCL).
The OEL formula uses both:
- Results for two previous quarters, (e.g. previous quarter one and previous quarter two).
- The current quarter's results twice.
Each sample location will have its own OEL. If the level for any sample location is more than 80 ug/L for TTHM or 60 ug/L for HAA5, your PWS has exceeded the OEL. The OEL exceedance is not a violation and does not require public notice. This exceedance triggers the requirement for an operational evaluation, and the submission of an Operational Evaluation Report to TCEQ.
Example Calculations for TTHM
Compare the locational running annual average for the TTHM MCL and the TTHM OEL, table below, at one sample location for 3rd quarter 2025 monitoring period (3Q2025). The TTHM sample results for 4th quarter 2024 are 56 ug/L, 1st quarter 2025 are 73 ug/L, 2nd quarter 2025 are 67 ug/L, and 3rd quarter 2025 are 103 ug/L. The TTHM LRAA is 74.75 ug/L which is less than the 80 ug/L MCL, thus the system did not receive a MCL violation. However, the TTHM OEL is 86.5 ug/L which is greater than the 80 ug/L OEL and is an OEL exceedance.
Completing the Operational Evaluation Report
In the event of an OEL exceedance, the system must conduct an operational evaluation and submit the Operational Evaluation Report to TCEQ. The report guides the system through an evaluation of treatment and distribution based on the type of drinking water source.
EPA Stage 2 Disinfectants and Disinfection Byproducts Rule Operational Evaluation Guidance Manual - a good resource for understanding the factors that can cause increased levels.
Texas Drinking Water Viewer - Use this database to determine the PWS’ Federal Source Type and choose the report that matches.
- TCEQ Surface Water (SW) and Groundwater Under the Influence of Surface Water (GUI) Operational Evaluation Report (TCEQ-20797a)
- TCEQ Groundwater Operational Evaluation Report (TCEQ-20797b)
- TCEQ Surface Water Purchased (SWP) and Groundwater Purchase (GWP) Operational Evaluation Report (TCEQ-20797c)
Submit the report to TCEQ within 90 days after the OEL exceedance has been identified by either the PWS when it receives DBP results from the lab or notification from TCEQ, whichever comes first. Submit the Operational Evaluation Report to TCEQ at DBP@tceq.texas.gov.
Failure to Submit an Operational Evaluation Report
An OEL exceedance is not a violation and does not require a public notification, but failure to submit a complete Operational Evaluation Report within 90 days of identification of the exceedance will result in a monitoring and reporting (M/R) violation and WILL require public notification.
A maximum contaminant level violation can also occur in the same quarter as an OEL exceedance.
Assistance and Tools for Public Water Systems
Contact the TCEQ DBP compliance coordinator at DBP@tceq.texas.gov or 512-239-1062 if you have questions about how to calculate the OEL, completing the operational evaluation or OEL Report, or for help finding DBP results on Drinking Water Viewer.
EPA Stage 2 Disinfectants and Disinfection Byproducts Rule Operational Evaluation Guidance Manual – Information about specific factors that influence DBP formation and ways to control formation can be found in this EPA guidance.
Financial, Managerial and Technical Assistance - TCEQ offers FREE financial, managerial, and technical (FMT) assistance to help PWSs comply with regulations. Contact FMT for help with issues such as, but not limited to preventing operational problems, training about how to comply with rule requirements and performing capacity assessments.
Technical Guidance for Public Water System Operators and Engineers - Information about technical questions, such as submitting plans, exploring treatment options, or requesting exceptions to design or capacity rules.
Disinfection Byproducts - Information about the formation of total trihalomethanes and total haloacetic acids, health effects, monitoring and sampling, and maximum contaminant levels.
Operating a Public Water System - Information about how to operate a public water system, drinking water quality requirements, and reporting requirements.