Your Water Passes on Average. September Is the Part That Averages Out.

Your city's water passed. It says so in the annual report.

That's true. It's also a statistical statement, not a daily one. And September is the month where the difference actually matters.

I sell bottleless water systems for a living, so take my angle for what it is. But the math here isn't mine. It's the EPA's, and it's public.

Compliance is an average, on purpose

The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules cap total trihalomethanes at 0.080 mg/L and five haloacetic acids at 0.060 mg/L. Stage 2 requires compliance to be calculated as a locational running annual average — meaning each sampling site gets judged on its own rolling twelve-month number.

That structure exists for a reason. It was intended to better capture high-byproduct formation at warm, high-organic-carbon sites instead of letting cold-season results wash out summer peaks. That's a genuine improvement over system-wide averaging. I'm not here to dunk on it.

But read what it is. A rolling annual average at a monitoring location. Not a guarantee about Tuesday. Not a guarantee about your building, which may sit well downstream of anywhere your utility actually samples.

The turbidity rule works the same way

Same pattern in the Surface Water Treatment Rules. For conventional and direct filtration systems, combined filter effluent generally has to stay at or below 0.3 NTU in at least 95% of monthly measurements, and never exceed 1 NTU.

Ninety-five percent. That other 5% is within the standard. It's engineered that way because no plant runs perfectly through snowmelt, runoff, and storm season. The rule is honest about that.

So the plant can have a rough week during a September downpour, stay fully compliant, and your coffee still tastes like a pond.

Late summer is when the source water fights back

Warm water, sunlight, and nutrient loading promote cyanobacterial harmful algal blooms in lakes and reservoirs. Those blooms can produce cyanotoxins and taste-and-odor compounds that utilities have to manage seasonally — at the source and inside treatment.

Meanwhile, plenty of systems use chloramine as a residual disinfectant because EPA notes it provides a longer-lasting residual and forms fewer regulated byproducts than free chlorine. Many of those systems periodically switch back to free chlorine to clean out the distribution network, and they typically notify customers about temporary taste and odor changes when they do.

Warm source water. Higher organic load. Disinfectant changes. All landing in the same stretch of calendar. Every one of those events can be legal, documented, and averaged into a number that will read fine next spring.

What the average can't tell you

I hear the same thing in breakrooms every fall. "The water's been off lately." Then someone pulls up the consumer confidence report, sees a passing grade, and decides everyone's imagining it.

Nobody's imagining it. They're just comparing a single sensory experience — this cup, today, at this building — to a twelve-month rolling number from a sampling point somewhere else. Those two things were never measuring the same thing.

Compliance means the utility met a federal performance standard across time and across locations. That's a real accomplishment and I don't want to minimize it. Municipal water in this country is generally very good.

It's also the beginning of the answer, not the end of it. The last few hundred feet of pipe, the age of the building, the season, and the point where water actually leaves a tap are outside the average entirely.

If your team says the water tastes different in September, the regulatory framework already explains why that can be true and compliant at the same time. Believe them and check your own building. The annual report was never going to.