Trihalomethanes and Haloacetic Acids in Cary, NC Water

Triangle Waterworks · informational guide

Direct answer: Yes — the Town of Cary's 2025 water quality report measured trihalomethanes as high as 43 ppb and haloacetic acids up to 17 ppb, both compliant with EPA's 80 and 60 ppb limits. These form when Cary's chloramine disinfectant reacts with organic matter in its Jordan Lake source water. A carbon filtration stage removes them; Morrisville, on Cary's utility, receives this same finished water.

What are trihalomethanes and haloacetic acids in Cary's water?

Trihalomethanes (TTHM) and haloacetic acids (HAA5) are disinfection byproducts formed when Cary's chloramine treatment reacts with organic material carried in the raw water it draws from B. Everett Jordan Lake. Every chlorinated or chloraminated water system produces some amount of them — they're a consequence of the disinfection process, not an outside contaminant.

The Safe Drinking Water Act sets a Maximum Contaminant Level of 80 ppb for TTHM and 60 ppb for HAA5, both measured as running annual averages. Cary's own testing falls under both limits.

How high are TTHM and HAA5 levels in Cary, NC water?

Cary's 2025 water quality report lists TTHM at an average of 37 ppb, with site-level readings running as high as 43 ppb. HAA5 averaged 17 ppb, ranging from 9 to 17 ppb. Both figures are compliant, well under the 80 and 60 ppb limits, and Cary reports no violations.

43 ppb, the top of Cary's reported TTHM range, is a better number to plan around than the 37 ppb average. It reflects what testing actually found at some sites in Cary's distribution system, not a system-wide blend.

Why does Cary's water carry these byproducts?

Cary operates the Cary/Apex Water Treatment Facility, a 56-million-gallon-per-day plant it co-owns with the Town of Apex, drawing raw water from B. Everett Jordan Lake in the Cape Fear River basin. As the lead agency, Cary runs the plant that also supplies Apex, RDU, and RTP South.

Treatment starts with powdered activated carbon dosed into the raw lake water, followed by ozone to break down organics and manage taste and odor, coagulation with alum and polymer, and filtration through sand-and-carbon media, with fluoride and corrosion-control chemicals added along the way. Only at the very end, in the clearwell, does the plant combine chlorine and ammonia — added as two separate doses — to produce the chloramine that disinfects water on its way out to customers.

Chloramine is a stable residual, which is part of why Cary uses it, but that stability means it keeps reacting slowly with organic matter still present in the water as it sits in pipes and storage tanks. That slow, ongoing reaction across a surface-water system is the source of the TTHM and HAA5 Cary reports, not a sign of a treatment failure.

Does Morrisville's water carry the same TTHM and HAA5 levels as Cary's?

Largely, yes. Cary has operated Morrisville's water and sewer utility since April 2006, and Morrisville customers receive Cary's finished water rather than a separately treated supply. Morrisville doesn't publish its own water quality figures, so Cary's 2025 numbers — TTHM up to 43 ppb, HAA5 up to 17 ppb — are the best available picture for a Morrisville tap, not just a Cary one.

That also means anything that affects Cary's finished water, including its annual chloramine-to-chlorine switch, shows up on the Morrisville side of the system too.

What health effects does EPA link to TTHM and HAA5?

EPA's National Primary Drinking Water Regulations table states the potential health effects from long-term exposure above the MCL for TTHM as "liver, kidney or central nervous system problems; increased risk of cancer." HAA5's listed effect is "increased risk of cancer."

Cary's water sits under both legal limits, so those effects describe exposure above the MCL, not Cary's current compliant supply. Still, "compliant" means under a ceiling EPA set using its own risk tolerance — it isn't a claim that the water is risk-free. TTHM and HAA5 are precisely the compounds behind that ceiling, and a carbon filtration stage removes them rather than counting on staying under the number.

How hard is Cary's water, and does that matter here?

Cary's own reporting puts finished water hardness at 25 to 30 ppm, roughly 1.5 to 1.75 grains per gallon. Triangle Waterworks measures a wider range across the homes we test in the area, 1.6 to 3.7 grains per gallon by ZIP code.

On the WQA/Hach scale, the scale used by the Hach Total Hardness Test Kit Model 5-B, both figures land in or near the slightly hard band, with our top measured reading crossing into moderately hard. Hardness isn't the main story in Cary the way TTHM and HAA5 are, but it's a second reason an address-level test is worth doing rather than relying on a town-wide average.

How do I remove TTHM and HAA5 from Cary tap water?

Activated carbon is what removes TTHM and HAA5 from tap water, adsorbing the compounds as water moves through the media. It's the carbon side of the all-in-one whole-home system, which pairs that carbon stage with softening resin — one install handles disinfection byproducts, chlorine taste, and PFAS on the carbon side, while the softening side protects your water heater, dishwasher, and plumbing from scale.

Because Cary's site-level readings vary and Morrisville shares the same supply, a free in-home water analysis is the way to know what's actually at your tap rather than relying on the town's system-wide figures.

Take the 60-second water quiz to see which all-in-one system fits your Cary or Morrisville home, or book a free water analysis for your own address's numbers. For more on what that annual disinfectant switch can taste like, see Cary water smells like a chlorinated pool, and for the broader safety picture, see is Cary, NC water safe to drink?

FAQ

Is Cary's tap water safe to drink with these TTHM and HAA5 levels? Yes — Cary's 2025 report shows both compounds under their EPA limits, with no reported violations. Compliance is a legal ceiling based on EPA's own risk tolerance, not a claim of zero risk, which is why a carbon stage that removes the compounds outright is worth having even on compliant water.

I just moved to Cary and my water sometimes smells like a swimming pool — is that connected to TTHM and HAA5? It can be related, though not directly. Cary disinfects primarily with chloramine, which the town describes as low in taste and odor, but Cary also switches to free chlorine for a period each year to flush its system, and that switch is when a pool-like smell tends to show up. TTHM and HAA5 themselves usually aren't what you're smelling — a free water analysis can pin down the actual cause at your address.

My house is in Morrisville — does this article even apply to me? Yes. Morrisville has been on Cary's water and sewer utility since April 2006 and receives Cary's finished water, so Cary's TTHM and HAA5 figures are the relevant numbers for a Morrisville tap too, since Morrisville doesn't publish its own.

Is Cary's HAA5 number the same thing as the HAA9 figure I've seen elsewhere? No. HAA9 is a nine-compound composite some organizations track with no EPA limit attached to it. Cary's compliance figure is HAA5, the five-compound group EPA actually regulates with a 60 ppb limit, and Cary's reported 17 ppb sits well under that.

Does boiling water get rid of TTHM and HAA5 before I drink it? No — boiling isn't a reliable way to remove disinfection byproducts and can concentrate some compounds while driving off others. Removing TTHM and HAA5 takes activated carbon filtration, not heat.

I have a private well outside Cary's service area — is any of this relevant to me? Not directly. TTHM and HAA5 come specifically from disinfecting treated municipal water, so they don't apply to an untreated private well. Wells carry their own set of concerns, like iron, sulfur, or low pH, which call for a different kind of system.

Sources

• Town of Cary — Water Quality Report
• Town of Apex — 2024 Annual Drinking Water Quality Report (shared Cary/Apex Water Treatment Facility background)
• EPA — National Primary Drinking Water Regulations

Related reading: Triangle NC Disinfection Byproducts Explainer | Trihalomethanes in Tap Water | Haloacetic Acids in Drinking Water