Counting 'not detected' as zero may halve US drinking water PFAS figures, model finds

US drinking water may hold about twice the PFAS that national summaries suggest, according to a modeling study led by Gehendra Kharel of Texas Christian University and published October 23 in Cell Reports Sustainability. The gap comes from a bookkeeping habit: results below the lab reporting limit are usually treated as zero.

For the six PFAS the EPA regulates, including PFOA and PFOS, the reported national average is about 2.6 nanograms per liter. The team's model puts it at about 5.2 ng/L.

Almost every result was a blank

The team analyzed over 1.8 million PFAS test results from a national EPA dataset that was made public in January 2026. Roughly 98% of those results sat below the minimum reporting level, the lowest concentration a lab is expected to report reliably.

That does not mean the water held none. A value under the limit could be just under it or close to zero, and the usual summaries cannot tell the difference. When the team counted those results as zero, the totals captured only 44% of the average concentration their model estimated across 9,991 community water systems.

"When a water quality report says 'not detected,' that means below the reporting limit, not zero," Kharel said in the Cell Press release.

How the model fills the gaps

Instead of a single substitute value, the models generated 500 plausible estimates for each below-limit result and averaged them into a best guess. The team ran this for the 25 PFAS that showed up at least once in the data. The Safe Drinking Water Act monitoring program covers 29.

The authors frame the method as a tool utilities can use to see where their numbers are most uncertain and where extra testing would pay off. The study was supported by seed funding from TCU's Natural Resource Research Initiative.

What the estimate does not say

The concentrations below the limit are predictions from the model, not measurements. The paper looks at how PFAS data are summarized, and it does not claim that every below-limit result is a health risk, according to the release.

The flip side also holds: a result under the reporting limit is not proof that the water is PFAS free. How much of the modeled extra exposure sits in specific systems, and whether utilities adopt this kind of correction, is still open.