PFAS in San Marcos Water: What Has Been Detected and What It Means

PFAS — per- and polyfluoroalkyl substances — are a family of synthetic chemicals used since the 1940s to make products resist water, grease and stains. They are often called “forever chemicals” because their molecular structure makes them extremely slow to break down, in the environment and in the body.

They have been detected in San Marcos water. That sentence deserves context rather than alarm, so here is the detail.

Four PFAS compounds have shown up in testing of the SMTX Utilities supply:

“ppt” is parts per trillion. For a sense of scale, one part per trillion is roughly a single drop in twenty Olympic swimming pools.

The detail worth noticing: none of the four is PFOA or PFOS, the two compounds that carry enforceable federal drinking water limits. That is a meaningful distinction, and it is the opposite of what most national coverage of PFAS would lead you to expect.

San Marcos draws on a genuinely mixed supply — the Edwards Aquifer, Canyon Lake via the Guadalupe-Blanco River Authority, the Carrizo-Wilcox Aquifer via the Alliance Regional Water Authority, and the San Marcos River via the Canyon Regional Water Authority. You can see the full picture, including disinfection byproducts and hardness, in the San Marcos water quality report.

This area has changed substantially, and older articles on PFAS are often badly out of date. For years there was only a non-binding EPA health advisory of 70 parts per trillion, with no enforceable requirement on utilities at all. That is no longer the situation.

In April 2024 the EPA finalised the first national primary drinking water regulation for PFAS, setting legally enforceable maximum contaminant levels — including 4 parts per trillion for PFOA and 4 parts per trillion for PFOS. The agency has since confirmed it intends to keep those two limits, while proposing to extend the compliance deadline for water systems from 2029 to 2031, and to rescind and reconsider the limits it had set for several other PFAS compounds.

In short: the rules are real now, they are still moving, and the specifics are worth checking against the EPA’s own PFAS drinking water pages rather than any article, including this one. EPA cites potential health effects from long-term exposure including developmental effects, certain cancers and immune system effects, which is why the agency regulates them at such low concentrations.

PFAS were used to make products that repel dirt, grease, water and stains: non-stick cookware, stain-treated carpet and upholstery, water-resistant clothing, food packaging, some firefighting foams, sealants and floor wax. Because they do not readily break down, they persist in soil and water and can move into groundwater and surface water long after use stopped.

That persistence is why they turn up in supplies with no industrial source nearby, and why treatment at the tap is the practical answer for a household rather than waiting for the source to clear.

Not everything marketed as a water filter does anything for PFAS. A refrigerator or pitcher filter is activated carbon, which is built for chlorine taste and odor and is not an effective barrier to PFAS at these concentrations.

The two technologies that do work are reverse osmosis and specialised activated carbon or ion exchange media designed for PFAS. A reverse osmosis drinking water system pushes water through a semi-permeable membrane that rejects contaminants a carbon filter passes straight through. Our PFAS page goes into what each approach handles.

The figures above are for the municipal supply. If you are on a private well around Wimberley, Dripping Springs, Blanco or Driftwood, nobody is testing your water but you.

A free in-home water test takes about twenty minutes, costs nothing and carries no obligation. It is worth doing sooner rather than later if your water tastes off, is not clear, has an unpleasant odor, or if your water bills have risen unexpectedly.

Culligan of San Marcos serves San Marcos, Buda, Kyle, Dripping Springs, Wimberley, Woodcreek, Driftwood, Blanco, Johnson City and Lockhart.