'Not Detected' Isn't Zero: TCU-Led Study Says Standard PFAS Reporting May Capture Only 44% of Estimated Levels in U.S. Drinking Water

When a drinking water test result for PFAS comes back below the lab’s reporting limit, it is often counted as zero. A new study led by a Texas Christian University researcher says that habit may significantly understate how much of these “forever chemicals” is in U.S. drinking water. The paper was published October 23, 2026, in the Cell Press journal Cell Reports Sustainability, according to TCU.

What the researchers found

• Most results are below the limit. The team examined more than 1.8 million PFAS measurements from a national EPA dataset, and about 98% fell below minimum reporting levels, TCU said. Lead author Gehendra Kharel told Inside Climate News the data were roughly 1.86 million results from EPA’s January 2026 dataset.
• Zero captured 44%. Using statistical models to estimate the below-limit results, the researchers found that treating them as zero captured only 44% of the estimated average concentration of 25 PFAS across 9,991 community water systems, under the study’s primary model. The averages give more weight to systems that serve more people.
• More locations near the 4 ppt line. Against the study’s comparison threshold of 4.05 parts per trillion, just above EPA’s 4 ppt limit, an additional 16 to 33 sampling locations crossed it for PFOA and 4 to 25 for PFOS, depending on the model, Kharel told Inside Climate News.

“When a water quality report says ‘not detected,’ that means below the reporting limit, not zero,” said Kharel, an associate professor of environmental and geological sciences at TCU. His coauthors are Omar Harvey of TCU, Andrei P. Kirilenko of the University of Florida and Chris B. Zou of Oklahoma State University.

What the study does not say

TCU is clear that the findings do not mean every result below a reporting limit is a health risk, or that any water system is unsafe or out of compliance with drinking water standards. The study is about how PFAS measurements are summarized. “We studied water entering distribution systems, not the water people actually drank,” Kharel told Inside Climate News.

Laurel Schaider of the Silent Spring Institute, who was not involved, compared it to weighing moving boxes: assuming the light ones weigh nothing underestimates the whole shipment. “Simply substituting zero is not the best way to handle a non-detect,” she said.

What could change

Kharel said agencies could clearly report the limits they used and how often results fell below them, and show estimates that account for below-limit results alongside zero-substitution averages. Under EPA’s Fifth Unregulated Contaminant Monitoring Rule (UCMR 5), public water systems test for 29 PFAS.

Where the federal limits stand

The EPA’s 2024 drinking water rule set enforceable limits of 4.0 parts per trillion each for PFOA and PFOS. On May 18, 2026, the agency proposed keeping those two limits while letting eligible systems request two more years to comply, moving the deadline from 2029 to 2031, the EPA says. That proposal has not been finalized.

Why WaterVO™ follows this

Every number in this study comes from testing water drawn from wells, rivers and lakes. Atmospheric water generation, which we call vapor optimizing (VO), starts from a different source: it filters incoming air and condenses drinking water from humidity instead. We believe VO is likely the best option for drinking water. It uses energy, but we think cleaner drinking water is worth it.

Sources

• Texas Christian University, “TCU Research Finds Drinking Water May Contain More ‘Forever Chemicals’ Than Standard Reporting Suggests” (read Oct. 24, 2026): cse.tcu.edu
• Inside Climate News, “PFAS Contamination in Drinking Water May Be Underestimated, New Study Suggests,” October 23, 2026: insideclimatenews.org
• Kharel et al., “Censoring-Aware Models Quantify Systematic Underestimation of PFAS Occurrence in U.S. Drinking-Water Monitoring,” Cell Reports Sustainability, 2026: cell.com
• U.S. EPA, “Per- and Polyfluoroalkyl Substances (PFAS)” drinking water rule page (read Oct. 24, 2026): epa.gov

Photo: the Texas Christian University campus in Fort Worth, by Michael Barera via Wikimedia Commons (CC BY-SA 4.0); cropped, headline added in the featured image. Illustrative.

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