Trihalomethanes and Haloacetic Acids in Durham, NC Water — What the Numbers Show
Direct answer: Durham's 2025 Annual Drinking Water Quality Report shows trihalomethanes (TTHM) at a highest locational running annual average of 56 ppb against an 80 ppb legal limit, and haloacetic acids (HAA5) at 49 ppb against a 60 ppb limit — the highest verified readings of either compound anywhere in our service area. Both are legally compliant. A carbon filtration stage removes both, in the same all-in-one system built for city water here.
What Do Durham's Own Water Quality Numbers Show for TTHM and HAA5?
Durham's 2025 Annual Drinking Water Quality Report lists a highest locational running annual average (LRAA) of 56 ppb for total trihalomethanes (TTHM), against an 80 ppb EPA limit, and 49 ppb for haloacetic acids (HAA5), against a 60 ppb limit. Both sit under their respective ceilings, and Durham reports no violation for either compound.
Those two figures are the highest verified TTHM and HAA5 readings anywhere in the Triangle Waterworks facts corpus — higher than what Raleigh, Cary, or Apex report in their own current reports. That is a genuinely notable data point, not an exaggeration: it comes straight from Durham's own published report.
What Does "Locational Running Annual Average" Actually Mean?
It means Durham measures TTHM and HAA5 at multiple points across its distribution system, then reports the highest of those location-specific annual averages — not a single system-wide number. An LRAA can run higher than a plant-average figure because it captures the worst location in the network, typically the point furthest from treatment where water has had the most time to react and disinfectant has had longest to work on organic matter in the pipes.
That distinction matters for comparison. A city that reports a single system-wide average and a city that reports its highest LRAA aren't reporting the same kind of number, even when both figures come from EPA-compliant testing. Comparing Durham's 56 ppb LRAA directly against another utility's system average is indicative of relative water quality, not a strict apples-to-apples league table. The honest read is that Durham's own worst-case location is real and compliant — both things are true at once.
Where Does Durham's Water Come From, and Why Does That Affect These Numbers?
Durham draws from two surface reservoirs — Lake Michie and the Little River Reservoir — treated at the Williams and Brown treatment plants. Surface water carries more dissolved organic matter than groundwater, and that organic matter is exactly what reacts with chlorine-based disinfectants to form TTHM and HAA5 in the first place. Durham's report also discloses that the city purchased water from the Town of Cary during the reporting year, meaning some Durham addresses may have received a blend rather than water sourced purely from the two reservoirs.
Durham disinfects with chloramines, which carry a chloramine residual reported at 1.5 ppm and a chlorine residual running annual average of 0.63 ppm. Chloramines tend to produce lower disinfection byproduct levels than free chlorine over time, but the LRAA still reflects real chemistry happening in the pipes between the treatment plant and the tap.
Why Does the EPA Regulate TTHM and HAA5 at All?
Because both are linked to real long-term health effects, which is the actual reason they carry legal limits instead of being left unregulated. EPA's National Primary Drinking Water Regulations table lists the potential health effects from long-term exposure above the MCL for TTHMs as "Liver, kidney or central nervous system problems; increased risk of cancer," and for HAA5 as "Increased risk of cancer."
Those are EPA's own words, and they describe risk from exposure above the legal limit sustained over years — not a claim about what Durham's current, compliant water is doing to anyone drinking it today. Durham is legally compliant on both compounds. But "legally compliant" is a ceiling set by EPA's own risk tolerance, not a guarantee of zero risk. The 56 ppb and 49 ppb figures are the exact compounds and exact effects EPA had in mind when it drew that line — and a carbon filtration stage removes them from the water rather than asking a household to rely on staying under a legal limit that has room to spare but isn't zero.
Is HAA5 the Same Thing as HAA9?
No, and this distinction matters if you're comparing numbers across sources. HAA5 is the five-compound sum EPA actually regulates, with the 60 ppb MCL used throughout this article. HAA9 is a nine-compound sum tracked by some independent databases — it carries no federal Maximum Contaminant Level at all. If you see an HAA9 figure elsewhere for Durham or any Triangle utility, it is not being measured against the same legal standard as the HAA5 number here, and it should never be described as "compliant with EPA limits," because there is no EPA limit for it to comply with.
How Does Durham Compare to Other Triangle Utilities on TTHM and HAA5?
| Utility | TTHM (highest verified) | MCL | HAA5 (highest verified) | MCL | |---|---|---|---| | Durham (2025, highest LRAA) | 56 ppb | 80 ppb | 49 ppb | 60 ppb | | Apex (2024 CCR) | 53 ppb | 80 ppb | 19 ppb | 60 ppb | | Cary (2025 CCR) | 43 ppb | 80 ppb | 17 ppb | 60 ppb | | Raleigh (2025 CCR) | 42.8 ppb | 80 ppb | 33.6 ppb | 60 ppb |
Every utility in that table is fully compliant. Durham's own reported figures are the highest of the group on both compounds, which is a meaningful data point for a Durham household weighing whether a carbon stage is worth adding — but remember the LRAA caveat above before treating this table as a precise ranking rather than a directional comparison.
What About PFAS in Durham's Water?
Durham's 2025 report also shows PFOA up to 5.7 ppt and PFOS up to 7.2 ppt, both figures disclosed in the same report as the TTHM and HAA5 data and both reported as compliant. PFAS removal is a separate mechanism from disinfection byproduct removal — it needs carbon specifically certified for PFAS (NSF/ANSI 53 or P473) or reverse osmosis (NSF/ANSI 58) — but the same all-in-one system's carbon stage is the piece of equipment doing that work too.
Does Durham Publish a Water Hardness Number?
No. Durham's water quality report contains no hardness figure at all — it is simply not something Durham publishes, unlike several neighboring towns. If hardness matters to your decision, the only reliable path is a free in-home water analysis at your own address, or testing it yourself with a Hach Total Hardness Test Kit, Model 5-B, which reads out in grains per gallon on the same industry scale Triangle Waterworks uses. Never assume a Durham hardness number from a source that isn't the address itself.
Does a Water Softener Remove TTHM or HAA5?
No. A water softener is ion exchange built specifically for calcium and magnesium — it has no mechanism for removing disinfection byproducts. TTHM and HAA5 are organic compounds formed by a chemical reaction between disinfectant and organic matter, and removing them requires activated carbon, not ion exchange. That's exactly why an all-in-one system pairs a carbon filtration stage with softening resin in one unit: the carbon side handles TTHM, HAA5, and PFAS, while the softening side handles whatever hardness your own address turns out to have.
FAQ
I just read Durham's water quality report and saw the 56 ppb TTHM number — should I be worried? It's the highest verified TTHM reading in our regional data, and it's still under the 80 ppb legal limit, so Durham is compliant. Compliant doesn't mean zero risk, though — EPA sets that limit at the edge of its own acceptable risk tolerance, and a carbon filtration stage removes TTHM from your water rather than relying on staying under a number that has some room in it.
What's the difference between a "highest LRAA" and an "average" for these compounds? An LRAA is the highest of several location-specific annual averages across a distribution system, so it reflects the worst-case monitoring point rather than a single citywide blend. A straight average can be lower simply because it's averaging across better and worse locations together — both are legitimate ways to report data, but they're not directly comparable numbers.
Does chloramine disinfection mean Durham has lower TTHM and HAA5 than a chlorine-only system? Chloramines generally produce lower disinfection byproducts than free chlorine over time, and Durham does use chloramines. But Durham's reported LRAA figures are still the highest in our regional comparison, which shows that source water and distribution system factors matter alongside the choice of disinfectant.
Is Durham's water safe to drink as-is? Yes, by the legal standard — Durham reports full compliance with both the TTHM and HAA5 limits, along with PFAS levels below EPA's limits. Whether to add a carbon filtration stage on top of that legal compliance is a separate, personal decision about reducing exposure to compounds EPA itself flags as having real long-term health effects above the limit.
Why does Durham's report mention buying water from Cary? Because Durham disclosed purchasing water from the Town of Cary during the reporting year, which means the blend reaching some Durham addresses may include water from more than one source. That's one more reason an address-level test matters more than any single citywide figure, including the ones in this article.
Does a whole-home system handle both the disinfection byproducts and whatever hardness my house has? Yes — that's the point of the all-in-one design. Softening resin and a carbon filtration stage sit in the same unit, so the carbon side removes TTHM, HAA5, and PFAS while the softening side handles calcium and magnesium at whatever level your own address tests at.
See Where Your Durham Address Stands
Durham's own 2025 report puts TTHM and HAA5 at the highest verified levels in our regional data, and a carbon filtration stage is what actually removes both. Take the 60-second water quiz or book a free water analysis to see what an all-in-one system looks like for your Durham home. For the fuller regional picture, see the Triangle NC Disinfection Byproducts Explainer.
Sources
- City of Durham — 2025 Annual Drinking Water Quality Report
- EPA — National Primary Drinking Water Regulations
Related reading: Triangle NC Disinfection Byproducts Explainer | What Are Trihalomethanes in Tap Water? | What Are Haloacetic Acids in Drinking Water? | Does Durham, NC Water Have PFAS? | Water Softener vs. Whole-House Filter
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