TTHM and HAA5: the legal but high numbers hiding in your water report

Once a year your utility mails you a consumer confidence report, and once a year most people glance at the word "compliant" and throw it out. I don't blame anybody for that. The thing is buried in acronyms. But there are two acronyms on it worth finding, TTHM and HAA5, because they're the numbers that can sit near a federal ceiling while the report still says everything passed. Austin and I get asked about these constantly after somebody actually reads their mail. Here's what they are and where our territory actually lands.

How this was built: These figures come from Jones Air & Water's own 57-town water dataset, built from utility consumer confidence reports, DNR and EPA well data, and our own in-home testing, pulled in 2026. Towns are sorted by reported TTHM, high to low. Where a record gives a range, the table shows the top of that range, and where a town doesn't report a figure I've said so instead of filling in a guess.

What the two acronyms actually are

Both are disinfection byproducts. They are not something anybody adds to your water on purpose. They form when the chlorine or chloramine used to disinfect the supply reacts with natural organic material already in the source water, which is why river-fed systems around here tend to produce more of them than deep well systems.

TTHM is total trihalomethanes. The federal limit is 80 parts per billion. HAA5 is five haloacetic acids grouped together, and its federal limit is 60 parts per billion. Those two ceilings are the whole reason a report can say "compliant" while carrying a number that would still make you raise an eyebrow.

The twelve highest TTHM numbers in our service area

The two numbers that actually crossed a line

Almost everything in that table is legal. Two entries are not, and both of them are single-sample readings rather than the running averages compliance is judged on, which is exactly why the annual report can still say the system passed.

Herculaneum's record shows HAA5 up to 94.5 ppb with one tap above the 60 ppb federal limit. That is the highest byproduct figure anywhere in our dataset, and it's the only HAA5 number we have that exceeds its ceiling outright. Crystal City's record shows a 66 ppb TTHM running average, which our notes put at 83% of the legal limit, with a single sample peaking at 88.5 ppb, over the 80 ppb EPA limit.

Jerseyville is the highest average in the set at 77 ppb, which our own notes flag as 96% of the federal limit. It never crossed. It just doesn't leave much room.

The finding that should change how you read your grade

This is the part I'd want a customer to take away. Our dataset grades towns A through F, and that grade is a hardness grade. It is not a safety grade, and these two things do not track together at all.

We have exactly three B-graded towns, the softest public water in our whole territory: Festus at 5.3 grains, Herculaneum at 5.5, and High Ridge at 5.7. All three of them are in the top nine for trihalomethanes. Festus and Herculaneum are second and third worst in the entire dataset, and Herculaneum owns that 94.5 ppb HAA5 reading.

So the three towns where I'd be least worried about your water heater are among the towns where I'd be most interested in what's coming out of your drinking tap. If you live in one of them and you've been told your water is good because it's soft, that's true about the minerals and it says nothing about the byproducts.

The opposite holds too. Villa Ridge runs 18.1 grains, extremely hard, an F on our hardness scale, and its TTHM is 0.72 ppb, the lowest number in the set by a wide margin. Hard water and byproducts are two different problems with two different causes.

Why the health guidelines and the legal limits are so far apart

You'll see two very different yardsticks quoted for these compounds and it's worth knowing why the gap is so wide. The federal limits, 80 and 60 ppb, are what a utility has to meet. The independent health-research guidelines are set far lower, and against those the same numbers look dramatic. Our own records put Wood River's TTHM at roughly 370 times the EWG health guideline, High Ridge at about 350 times, Arnold at about 300 times, Innsbrook at about 240 times, Elsberry at about 175 times, Bethalto at 143 times, and Warrenton at 116 times.

Both things are true at once. Your utility is following the law, and the number is still high against a stricter benchmark. I'm not in the business of telling you your city is poisoning you. I am in the business of telling you what your options are once you know the number.

What I'd actually do about it

First, a softener will not touch this. Hardness and byproducts sit on separate lines of every town record we keep, and softening is ion exchange aimed at minerals. If somebody sells you a softener as the answer to a TTHM number, that's the wrong piece of equipment.

Second, I'm going to be straight about what our own equipment pages do and don't claim. The backwashing carbon filter we install is rated to remove up to 97% of chlorine and chloramines. That's a claim about the disinfectant itself, which is the precursor, not a published removal figure for TTHM and HAA5, and I'm not going to hand you a percentage our own documentation doesn't support. For drinking and cooking water specifically, reverse osmosis at the kitchen sink is the route we normally take, and our tankless R/O system's own description covers chlorine, lead and forever chemicals.

Third, and this is the honest answer: get the actual number for your address. The annual report is a system-wide average, and these compounds vary by how far you sit from the plant and how long the water has been in the pipe. Somebody at the end of a long distribution line can read very differently from somebody near the treatment plant in the same town.

Where the data runs out

Our file is uneven. Several towns report a TTHM figure in parts per billion with no HAA5 figure in the same units, Jerseyville and High Ridge among them, and a few report these compounds only as a multiple of a health guideline rather than as a concentration at all. Cedar Hill's record says plainly that TTHM and HAA5 were not reported and that assuming they are low is unconfirmed. I'd rather print that than pretend our file is complete. And if you're on a private well, none of this applies to you, because nobody is disinfecting your water. Your list is bacteria, nitrate, iron and radium, which I went through in well water vs. city water: what 30 years of testing taught me.

For where your town lands on the hardness side of the ledger, see the hardest water towns we serve, ranked by the numbers and what hard water actually costs you every year. For the two counties that dominate the table above, see Jefferson County water by the numbers.

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