TTHM Locational Running Annual Average Exceedance
A community water system serving 3,400 people treats surface water with conventional filtration and free chlorine. The system collects quarterly TTHM and HAA5 samples at four distribution locations. At the highest TTHM location, the last four quarterly results are 78, 84, 86, and 80 µg/L. At that same location, HAA5 results are 55, 58, 54, and 57 µg/L. The TTHM MCL is 80 µg/L and the HAA5 MCL is 60 µg/L. The system has no previous Stage 2 DBPR violation. The operator believes the system is in compliance because the individual quarterly TTHM results are mostly near 80 µg/L and the HAA5 results are below 60 µg/L. The distribution system has a long dead-end main with low demand, and finished water pH is 8.3. The system has not changed its coagulation practice in several years.
1. Calculate the locational running annual average and determine compliance.
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The TTHM locational running annual average at the highest location is (78 + 84 + 86 + 80) / 4 = 328 / 4 = 82.0 µg/L. Because 82.0 µg/L is greater than the 80 µg/L TTHM MCL, this is a TTHM MCL violation at that location, even though some individual quarterly results were below 80 µg/L. The HAA5 locational running annual average is (55 + 58 + 54 + 57) / 4 = 224 / 4 = 56.0 µg/L, which is below the 60 µg/L HAA5 MCL. The compliance determination must be made for each monitoring location and each contaminant, not by averaging all locations together. The operator's reasoning is incorrect because Stage 2 DBPR uses locational running annual averages, and a single location above the MCL is a violation.
2. What public notice tier and timing apply, and what must the notice say?
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A TTHM MCL violation is a Tier 2 public notice because it does not present an immediate acute health risk. The system must provide public notice within 30 days of the violation, using KDHE-approved language. The notice should identify the system, the contaminant, the MCL, the measured locational running annual average, the violation period, and the health effects language for TTHM. It should explain what customers should do, such as using an alternative water source if they have specific health concerns, and what the system is doing to correct the problem. It should give a contact name and phone number. The system should also notify KDHE promptly and follow its direction on the exact notice content and method. Tier 2 does not mean optional: failure to issue the notice on time is a separate violation.
3. What operational changes would you evaluate to reduce TTHM formation?
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The system should focus on precursor removal and distribution water age. Review coagulation performance and jar-test enhanced coagulation to improve natural organic matter removal; this is usually the most durable control. Lower finished-water pH where corrosion control allows, because TTHM formation increases at higher pH. Reduce water age by flushing the long dead-end main, looping or improving circulation, and managing storage tank turnover. Evaluate chlorine dose and contact time, and consider switching to chloramines if the system can manage nitrification and maintain adequate secondary disinfection. Consider source blending or granular activated carbon if precursor removal is insufficient. Any change must be checked against disinfection requirements and corrosion control. The system should also verify its sampling locations and continue quarterly monitoring, because the running annual average will not drop until new lower results replace the high quarters.