Water quality · San Marcos
What's in San Marcos water, by the numbers
San Marcos tap water is safe to drink and meets state and federal standards, with one recent exception the city reports it is correcting (a disinfection byproduct exceedance in late 2025). It's very hard, about 16 grains per gallon, and disinfected with free chlorine. Here are the numbers, with sources.
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The headline numbers
Very hard, chlorinated, and multi-source
These come from the city's own Consumer Confidence Reports, the annual water quality disclosure every utility publishes.
16gpg
Average hardness (14 to 18 gpg range)
2024 CCR
269mg/L
Same hardness, as labs report it (CaCO₃)
2024 CCR
1.26ppm
Average free chlorine residual at the tap
2025 CCR
3
Water sources feeding the city system
2025 CCR
The Water Quality Association calls anything over 10.5 gpg “very hard,” the top of its scale. San Marcos water sits comfortably past that, everywhere in town, all year.
Where the water comes from
Most homes in town are served by the City of San Marcos (SMTX Utilities). The city blends three sources: groundwater from the Edwards Aquifer, surface water from Canyon Lake pumped up the Guadalupe River system, and, since late 2024, Carrizo-Wilcox Aquifer water delivered through the Alliance Regional Water Authority pipeline.
Every one of those sources travels through or sits in limestone, which is why the hardness numbers above look the way they do. Blending also means the exact water at your tap can shift with the season and your neighborhood. The city's own reports show hardness ranging from 236 to 302 mg/L across samples.
One caveat: some addresses outside the city limits with San Marcos mailing addresses are actually served by other systems, like Crystal Clear Special Utility District. Your water bill says which utility is yours, and the free test works the same either way.
From the reports
Notable readings, next to their limits
A CCR lists everything detected. These are the entries homeowners ask about, from the city's 2025 report.
| Substance | Reported | Limit | Status |
|---|---|---|---|
| Nitrate | 1.77 ppm (highest) | 10 ppm (MCL) | Within limit |
| Fluoride | 0.21 ppm | 4 ppm (MCL) | Within limit |
| Total trihalomethanes (TTHM) | 81 ppb (highest running average) | 80 ppb (MCL) | Exceeded in late 2025; city correcting |
| Haloacetic acids (HAA5) | 25 ppb (highest running average) | 60 ppb (MCL) | Within limit |
| Lead (90th percentile, 2022–2024) | 1.63 ppb | 15 ppb (action level) | Within limit |
| Copper (90th percentile, 2022–2024) | 0.171 ppm | 1.3 ppm (action level) | Within limit |
About that TTHM line: trihalomethanes are byproducts that form when chlorine reacts with natural organic matter, and one monitoring location's running annual average came in at 81 parts per billion against a limit of 80 in the last quarter of 2025. The city's report describes its corrective work: targeted flushing, more sampling, and a root-cause study. In the 2024 report year, TTHM was within limits. We report this because it's in the public record, not to alarm you. It's a regulated item the utility is required to fix and report on.
Also worth knowing: the city stopped adding fluoride in 2015, so the small fluoride reading is what occurs naturally.
PFAS and nitrate
What the testing shows for PFAS and nitrate
These are the two contaminants homeowners ask us about most. Here's what San Marcos's own testing found, and what the numbers mean.
Nitrate is regulated because too much of it is genuinely dangerous, specifically for infants. The EPA's limit is 10 milligrams per liter, in place since 1992, and it's tied to a real health outcome: water above that level can cause blue baby syndrome in infants under six months who drink formula mixed with it. It isn't a concern for anyone else. San Marcos's highest recent reading, 1.77 ppm, is well under a fifth of the limit.
PFAS, often called forever chemicals, are a newer and less settled story. They're manufactured compounds used for decades in things like nonstick coatings and firefighting foam, and they break down very slowly. The EPA set its first enforceable drinking water limits for six of them in 2024, 4 parts per trillion each for PFOA and PFOS, the two studied longest. Those numbers have moved before: EPA's health guidance for PFOA and PFOS was 70 parts per trillion in 2016 and near zero by 2022, tightening as more research came in. San Marcos's required testing found no PFOA or PFOS at any sampling point, and its one detected compound with a federal limit, PFHxS, came back well under it. Nobody can promise trace amounts below today's limits are completely harmless, because the science on PFAS is still being written. That's a fair reason to want extra filtering even where the water passes every current test, and a fair reason not to. We wrote up what actually removes PFAS if you want the honest version.
Which numbers are about safety, and which aren't
The table above is the safety layer. It covers regulated substances with legal limits, monitored constantly and publicly reported. That system works, and San Marcos water passes it (with the one disclosed exception being corrected).
Hardness and chlorine taste live in a different category. The EPA doesn't set health limits for them because they aren't health problems. They're quality-of-life items. They're also the two things you actually notice every day, and the two things treatment exists to fix. Scale on your fixtures and pool-taste in your glass are optional. The safety layer stays the utility's job either way.
One question we hear a lot
Does San Marcos's water have PFAS?
San Marcos's required federal testing in 2023 and 2024 found no PFOA or PFOS, the two PFAS compounds with the strictest federal limits. One compound that does have a federal limit, PFHxS, came back around 3 parts per trillion, well under its 10 ppt limit. Two others, PFBA and PFPeA, showed up in trace amounts with no federal limit at all. The city's own water quality report and EPA's public testing data agree.
What a free test adds to the city's report
The CCR describes the system average. Your house is one point on that 236-to-302 range, at the end of your particular pipes. The free in-home test measures hardness, chlorine, iron, and TDS at your tap (TDS is a number the city's reports don't publish at all), and you keep the results.
Book the free testSources
- City of San Marcos 2025 Water Quality Report (CCR)
- City of San Marcos 2024 Water Quality Report (CCR)
- SMTX Utilities water quality page
- Water Quality Association: hardness and scale deposits
- EPA: PFAS national drinking water regulation
- EPA UCMR5 PFAS testing data finder
- EPA: Table of Regulated Drinking Water Contaminants (nitrate)
Figures reflect the most recent city reports at the time of writing. CCRs update annually.
Get your tap's own numbers
The free test takes about 30 minutes at your kitchen sink. We measure hardness, chlorine, iron, and TDS, and we explain every number in plain English. The city tests the system, and we test your house.
What you get
- Hardness, chlorine, iron, and TDS tested at your tap
- Results explained in plain English, yours to keep
- A recommendation sized to your home (only if you want one)