Stage 2 fixed the biggest loophole in Stage 1. Under Stage 1, a system could be in full compliance while some customers drank water with DBP levels well above the MCL. The system-wide average hid the problem. Stage 2 closed that gap by requiring every monitoring location to meet the MCL individually.
If you read the Stage 1 guide on this site, I used a specific example: a system with one site at 0.120 mg/L TTHM and four sites at 0.040 mg/L could average to 0.056 and stay compliant. Under Stage 2, that 0.120 site is a violation on its own. That is a meaningful change for the customers at the end of a dead-end main who were previously averaging away.
This guide covers the compliance math, the OEL early warning system, and the practical strategies for managing DBPs at the locations where they are hardest to control. These are the concepts the T-5 exam tests, and they are the concepts you will use when your distribution system has a site that keeps running hot.
What Is the Critical Difference Between Stage 1 and Stage 2?
Same MCLs. Different compliance math. That single change transformed how systems manage DBPs.
Under Stage 1, compliance for TTHM and HAA5 was based on a running annual average (RAA) of all monitoring locations system-wide. A site running consistently high could be offset by sites running consistently low, and the system would show compliance even though some customers were exposed to DBP levels above the MCL.
Under Stage 2, each monitoring site maintains its own average. If any single site exceeds the MCL based on its own four-quarter average, the system is in violation. The MCLs did not change. The accountability did.
Both Stage 1 and Stage 2 apply to community water systems and nontransient noncommunity systems that add a disinfectant or deliver disinfected water. The MCLs remain 0.080 mg/L for TTHM and 0.060 mg/L for HAA5.
The shift from RAA (system-wide) to LRAA (location-specific) is the single most testable concept from Stage 2 on the T-5 exam. If you understand nothing else about Stage 2, understand this: every site must individually comply.
How Does the Locational Running Annual Average Work?
The LRAA is calculated the same way as the RAA, but at each individual monitoring location rather than system-wide. You take the quarterly results from a single site over four consecutive quarters and average them. That site's LRAA must stay below the MCL.
For TTHM, each site's LRAA must be below 0.080 mg/L. For HAA5, each site's LRAA must be below 0.060 mg/L. If either value is exceeded at any site, the system is in violation for that parameter.
The practical implication is significant. Under Stage 1, operators primarily worried about the system-wide picture. Under Stage 2, you have to manage every monitoring location individually. A dead-end main that consistently produces high THM results cannot be averaged away by a well-flushed loop on the other side of the system. That dead end is its own compliance point, and it has to meet the MCL on its own.
What Is the Operational Evaluation Level and How Do You Calculate It?
The OEL is an early warning system built into Stage 2. It alerts you when a monitoring site is trending toward a violation so you can take corrective action before the fourth quarter pushes the LRAA over the MCL.
The formula is: (Q1 + Q2 + 2 times Q3) divided by 4.
That double-weighting on Q3 is intentional. It projects what would happen if Q4 is similar to Q3. If the OEL exceeds the MCL after three quarters of data, it means the site is on track for a violation unless something changes.
Let me walk through the math. Suppose a site's TTHM results are Q1 = 0.060, Q2 = 0.070, Q3 = 0.090. The OEL is (0.060 + 0.070 + 2 times 0.090) divided by 4 = 0.310 divided by 4 = 0.078. That is below 0.080, so no OEL trigger. However, if Q3 were 0.095, the OEL would be (0.060 + 0.070 + 0.190) divided by 4 = 0.080, which equals the MCL and triggers the evaluation.
When an OEL is triggered, the system must identify the likely cause of elevated DBPs at that location, implement measures to reduce them, and report findings to the State Board. The goal is to fix the problem before Q4 arrives, not to report a violation after the fact.
The OEL formula is a frequent exam question. Practice the math. Remember that Q3 is double-weighted, and the trigger threshold is the MCL itself (0.080 for TTHM, 0.060 for HAA5). The exam may give you three quarters of data and ask whether an OEL evaluation is required.
What Was the Initial Distribution System Evaluation?
The IDSE was a one-time monitoring effort required when Stage 2 was first implemented. Systems sampled TTHM and HAA5 at locations throughout the distribution system to identify where DBP formation was highest. The results were used to select the monitoring sites for ongoing Stage 2 compliance.
The logic is straightforward. If you are going to hold each monitoring location to the MCL individually, you need to make sure you are monitoring the locations where violations are most likely to occur. The IDSE identified those worst-case locations so that ongoing compliance monitoring would not miss the problem areas.
For the exam, know that the IDSE was the process that established Stage 2 monitoring locations, and that it was designed to identify high-DBP areas in the distribution system. You do not need to know the specific sampling protocols or timelines of the IDSE itself, as it was a one-time requirement that has been completed.
What Are the Monitoring and Reporting Requirements?
Systems must maintain a monitoring plan that identifies the number and location of monitoring sites. Sites must represent the high-DBP areas identified during the IDSE.
At some locations, dual sample sets are required, meaning paired TTHM and HAA5 samples collected from the same location at the same time. This ensures both parameters are tracked at the most critical points in the distribution system.
Reporting is quarterly. Results must be submitted to the State Board within 10 days after the quarter ends. Reports must include the LRAA calculation for each individual monitoring site, not just the raw results. This is different from Stage 1, where the system-wide RAA was the reported value.
How Do You Reduce DBPs at Problem Locations?
This is where the exam meets real-world operations. The single most important factor driving DBP levels in the distribution system is water age. The longer water sits in contact with residual disinfectant and organic matter, the more DBPs form. TTHM concentrations in particular increase with time.
That means the highest DBP levels are almost always found in the same types of locations: dead-end mains, oversized storage tanks with long turnover times, and long transmission mains with low demand. These are the locations the IDSE was designed to identify, and they are the locations that drive Stage 2 compliance challenges.
Practical strategies for reducing DBPs at problem locations include flushing programs that reduce water age by moving stagnant water out of dead ends, looping dead-end mains to create circulation where none existed, reducing storage tank residence time by adjusting fill and draw cycles, and optimizing disinfectant dose and application point to minimize the residual that reaches high-age areas.
High water age equals high DBPs. If the exam presents a scenario where a specific monitoring site consistently exceeds the TTHM MCL, the first question to ask is whether that site has high water age. Dead ends, oversized tanks, and long mains with low demand are the usual suspects.
Stage 2 made DBP compliance personal. Every monitoring location stands on its own, and the OEL gives you a warning before a problem becomes a violation. If you operate a system with dead-end mains, long transmission lines, or oversized storage, you are managing Stage 2 compliance every day whether you think about it that way or not.
I will continue building T-5 study guides on H2oCareerPro.com. If this guide helped clarify the Stage 1 to Stage 2 progression, share it with a colleague who is also preparing. We are all in this together.
This guide is for educational purposes and reflects federal and California regulations as of April 2026. Always verify current requirements with your state regulatory agency.
Frequently Asked Questions
What is the difference between RAA and LRAA?
The RAA (Running Annual Average) averages all monitoring locations system-wide over four quarters, used for Stage 1 compliance. The LRAA (Locational Running Annual Average) calculates a separate four-quarter average at each individual monitoring site, used for Stage 2 compliance. The shift from RAA to LRAA means every monitoring location must independently stay below the MCL, preventing high-DBP sites from being masked by low-DBP sites elsewhere in the system.
How do you calculate the Operational Evaluation Level?
The OEL formula is (Q1 + Q2 + 2 times Q3) divided by 4. Q3 is double-weighted to project what the LRAA would be if Q4 is similar. If the OEL exceeds the MCL (0.080 for TTHM, 0.060 for HAA5) after three quarters, the system must conduct an operational evaluation to identify the cause of elevated DBPs and implement corrective measures before the fourth quarter.
Why do dead ends and storage tanks have the highest DBP levels?
Dead ends and storage tanks have the highest water age in the distribution system. DBP formation is a time-dependent reaction: the longer water remains in contact with residual disinfectant and organic matter, the more trihalomethanes and haloacetic acids form. Flushing programs, looping dead ends, and reducing tank turnover times are the primary strategies for addressing high-age locations.
What is an Initial Distribution System Evaluation?
The IDSE was a one-time monitoring effort required under Stage 2 where systems sampled TTHM and HAA5 throughout the distribution system to find the locations with the highest DBP formation. Those results were used to select monitoring sites for ongoing Stage 2 compliance, ensuring the highest-risk locations are tracked rather than representative or convenient ones.