TTHM Locational Running Annual Average Exceedance After Rechlorination
You are the compliance manager for a 12,000-person community system that buys finished water from a regional wholesale supplier and rechlorinates at three booster stations to maintain residual. You are in the fourth quarter of the Stage 2 DBPR compliance cycle. At compliance site 3 (a residential tap on a long dead-end street), your quarterly TTHM results are 72, 78, 92, and 94 µg/L, and HAA5 results are 48, 52, 58, and 55 µg/L. Site 3 also has the lowest free chlorine residual in the system (0.25–0.4 mg/L). The other three sites are all below 70 µg/L TTHM. Your wholesale supplier's finished water TOC is 2.4 mg/L, and the state has approved your corrosion control at pH 7.6. System storage is 2.0 MG with an average turnover of 3.5 days. Booster 3 feeds the dead-end street.
1. Compute the locational running annual averages, determine compliance status, and state the required response.
Read the worked answer
TTHM LRAA at site 3 = (72 + 78 + 92 + 94) ÷ 4 = 336 ÷ 4 = 84.0 µg/L. HAA5 LRAA at site 3 = (48 + 52 + 58 + 55) ÷ 4 = 213 ÷ 4 = 53.25 µg/L, below the 60 µg/L MCL.
Under the Stage 2 DBPR, the LRAA is calculated at each compliance location from the last four quarters, and the system's compliance is judged by the highest location. Site 3 at 84.0 µg/L exceeds the 80 µg/L TTHM MCL, so the system has an MCL violation even though the other three sites are below 70 µg/L. Site 3 is not a statistical outlier to be explained away; it is the location that drives the violation.
Required response: notify MassDEP, issue a Tier 2 public notice within 30 days describing the violation, the health effects language for TTHM, and what consumers should do; continue quarterly monitoring; and report the violation and LRAA in the consumer confidence report. The rising trend (72 to 94 across four quarters) also means next quarter's LRAA will be worse if nothing changes, so operational correction has to begin now. The assessment is not just paperwork - the state will expect a documented evaluation of why site 3 forms more TTHM than the rest of the system.
2. Rank the corrective options you would evaluate, and explain the trade-offs among them.
Read the worked answer
Site 3 combines long water age, low residual, and elevated TTHM, so water age is the first lever.
- Reduce age at the dead end: loop the street, add an automatic flusher, or adjust valves so Booster 3 water does not sit. This lowers TTHM precursor contact time without touching disinfection. It costs capital or water, but it does not increase microbial risk.
- Targeted flushing is the interim measure. It wastes water and only temporarily resets age, but it is fast and it also helps the residual problem at the site.
- Ask the wholesale supplier about enhanced coagulation. At TOC 2.4 mg/L, better precursor removal upstream reduces formation everywhere and is the most durable fix, but it is outside your direct control and takes time to negotiate.
- Lower the chlorine residual at Booster 3 only within the state minimum and only with increased bacteriological monitoring. This trades DBP reduction for microbial risk on the weakest part of the system and could make the coliform problem worse.
- Convert to chloramines would cut TTHM substantially, but it requires state approval, conversion at the wholesale supplier, nitrification control, and public notification for dialysis patients and aquarium owners. It is a system-wide decision, not a site fix.
I would start with age reduction and flushing, pursue enhanced coagulation with the supplier, and keep residual reduction as a limited, monitored last step.
3. If the system eventually converts to chloramines, what monitoring and operational changes must be in place?
Read the worked answer
Chloramine changes the measurement and the biology, so the program has to change with it.
Residual monitoring switches from free chlorine to total chlorine, with free ammonia and monochloramine ratios checked to keep the blend in the monochloramine range rather than forming dichloramine, which causes taste complaints. Because chloramines are a weaker primary disinfectant and a nutrient for nitrifying bacteria, a nitrification monitoring program is mandatory: free ammonia, nitrite, and nitrate at representative sites, plus a written response plan with triggers for blending or breakpoint chlorination if nitrite rises.
Water quality also shifts. Chloramination can increase lead release at some utilities, so lead and copper monitoring and corrosion control at pH 7.6 need review, not assumption. Storage turnover matters more, because chloramine residual is stable but nitrification tends to occur in tanks and dead ends.
Public communication is required: dialysis providers and home dialysis patients, aquarium owners, and anyone using water for specialized processes need advance notice, and the state must approve the conversion. Finally, the DBP benefit should be verified with monitoring rather than assumed - TTHM and HAA5 can drop sharply, but the system then has to manage a different set of risks.