Counting 'non-detects' as zero may halve true PFAS levels in US tap water

A modeling study suggests average PFAS in US drinking water is roughly double what standard summaries show, because tiny readings get rounded to zero.

A study in Cell Reports Sustainability estimates that PFAS concentrations in US drinking water may be about twice what standard reporting suggests. The lead researcher quoted is Gehendra Kharel, an environmental scientist at Texas Christian University.

The gap comes from a routine habit. When a lab result falls below its minimum reporting level (MRL), standard calculations usually treat it as zero. The team did not. They took 1.86 million drinking water data points supplied by the US Environmental Protection Agency, 98 percent of which sat below the MRL, and used computer models to predict those hidden values from measured data on 25 different PFAS. By their estimate, standard reports give an average PFAS level 56 percent lower than the modeled figure. For the six PFAS that carry EPA legal limits, the modeled average was 5.2 nanograms per liter, against 2.6 ng/L when the low results are counted as zero.

For access to clean water, the point is that the baseline may be understated. If exposure is higher than regulators' summaries show, treatment and cleanup needs are larger than planned, and water that looks compliant is less reassuring. That cuts against the shared resource of safe tap water. The practical effect on compliance looks modest, though: including the hidden levels pushed only a few dozen more sampling locations over EPA benchmarks for PFOA and PFOS.

The researchers want more sensitive testing, so results aren't rounded down, and broader monitoring of water systems. They are releasing their tool, the PFAS Censoring-Aware Screening Tool (PFAS-CAST), for other teams and agencies to use. Caveats matter here. Readings below the MRL can't be reliably measured, so these values are model estimates, not direct measurements. Kharel says the study looks only at how measurements are summarized and doesn't show that every below-limit result is a health risk. PFAS have been linked to faster aging, cancer spread, weaker bones and multiple sclerosis, but cause and effect are hard to prove.

Source: ScienceAlert