Explainer
San Luis Exceeded a Federal Drinking-Water Standard. Here’s What That Actually Means.
The City of San Luis reported two TTHM locational running annual averages above the federal standard, while saying the incident was not an emergency and that additional samples were below the limit.
Founder & Publisher
- Published
Full article
SAN LUIS EXCEEDED THE FEDERAL TTHM STANDARD
The City of San Luis reported a regulatory exceedance for the second quarter of 2026, according to its August 27 notice. The notice says second-quarter monitoring showed the water system went above the federal maximum contaminant level for total trihalomethanes, or TTHMs. The federal standard is 0.080 milligrams per liter (mg/L). The city reported locational running annual averages of 0.089 mg/L at one sampling location and 0.088 mg/L at another. The notice identifies the system as AZ04-14005. Both values are above 0.080 mg/L, so the exceedance is real and important.
WHAT THOSE NUMBERS ACTUALLY MEAN
The number that matters for compliance is a locational running annual average, or LRAA. It is based on samples from the previous four quarters and is calculated for specific monitoring locations. The reported 0.089 and 0.088 mg/L figures are averages at those locations, not single samples from one household tap.
WHAT ARE TTHMs?
TTHMs are disinfection byproducts. The EPA identifies them as byproducts of drinking-water disinfection. They can form when disinfectants used to treat water react with naturally occurring material in the water. The EPA sets the TTHM maximum contaminant level at 0.080 mg/L.
WAS THIS AN EMERGENCY?
The City explicitly said the incident was not an emergency. Its notice said residents did not need to boil their water or take other corrective action. A regulatory exceedance matters, but it does not by itself mean the City declared an acute emergency or issued a boil-water instruction.
WHAT HAPPENED AFTER THE EXCEEDANCE?
The City said it took additional samples and that results at all four sampling locations were below 0.080 mg/L. Those additional samples are relevant follow-up information, but they do not erase the earlier LRAA determination. The LRAA is a four-quarter compliance calculation; the City described the additional samples separately.
WHAT SAN LUIS SAYS IT IS DOING
The City said it planned additional water-main looping work intended to improve circulation in the system. It also said third-quarter samples had been collected as of August 13, 2026, and that it anticipated those results would be within compliance limits. That was the City’s expectation in the August notice, not a reported third-quarter result.
WHAT REMAINS UNKNOWN
The notice documents the second-quarter LRAA exceedance and the City’s separate follow-up sampling. It does not establish whether a later third-quarter LRAA returned the system to compliance. As of September 26, Yuma Informed did not find a newer official TTHM result on the City’s water or public-notice pages, so no later compliance result is stated here.
Sources: City of San Luis, August 27, 2026 notice; U.S. EPA, National Primary Drinking Water Regulations.
Sources 2
- Primary source · Government recordCity of San Luis TTHM Drinking-Water Notice, August 27, 2026
Official City of San Luis public notice describing the water system's second-quarter 2026 TTHM exceedance, the locational running annual averages, follow-up sampling, and planned water-main looping work.
- Primary source · Government recordEPA National Primary Drinking Water Regulations, Total Trihalomethanes
EPA's National Primary Drinking Water Regulations identify Total Trihalomethanes as a regulated disinfection byproduct, set the maximum contaminant level at 0.080 mg/L, and describe potential health effects associated with long-term exposure above the MCL.