A claim that PFAS in drinking water may be underestimated raises an immediate question: underestimated against what? The answer depends on the measurement, and the measurement most utilities file is a targeted panel. A targeted method reports a fixed set of named compounds, each with its own reference standard, reporting limit, and recovery check. The summed result is a precise statement about those compounds. It is not a statement about every fluorinated substance in the sample.
Circle of Blue's report on a new study, headlined "PFAS Contamination in Drinking Water May Be Underestimated," is a useful case for this question. The report's summary points to results that fall below laboratory reporting limits, which is one place where the gap between what a panel reports and what a sample contains can open. The mechanisms below are general to PFAS analysis. They are not findings attributed to the study, and the summary available here does not describe its methods in enough detail to test them. Circle of Blue's report on the PFAS underestimation study is the starting point for any reader who wants the full text.
What a targeted panel measures
PFAS is a family that includes many thousands of fluorinated compounds, and drinking water methods are built around named lists. A list has clear advantages. Each compound has a calibration standard, a reporting limit, and a recovery check, so two laboratories can compare results on the same footing. Those controls are why a targeted sum is what compliance and regulatory files usually carry.
The tradeoff is that a list can only report what it was written to find. A compound outside the list contributes nothing to the total. A non-detect for that compound means only that the method did not see it under its conditions, not that the compound is absent. Recovery also matters. A compound that extracts poorly will read low even when it is on the list, and the reported sum will carry that shortfall without flagging it unless the file records recovery.
What total fluorine adds, and what it cannot say
Total organic fluorine and extractable organic fluorine methods approach the problem from the other side. They measure fluorine bound to organic material rather than individual molecules. Subtracting the sum of targeted PFAS from a total fluorine value produces a residual that represents organic fluorine the list did not account for. That residual is informative, but it is not the concentration of any specific substance, and it cannot tell an operator which compound to treat.
Total fluorine methods carry their own limits. They depend on extraction efficiency, they may not capture shorter-chain compounds that partition differently during extraction, and they are less standardized across laboratories than targeted methods. A large residual may reflect unidentified PFAS, non-PFAS organofluorine, or a method artifact. The number alone does not settle which. Treating the residual as a PFAS concentration would overstate what the measurement shows, and so would treating it as noise.
How to label the panel in your file
For a utility or laboratory file, the practical step is to record the panel alongside every result. Name the method, the number of analytes, the reporting limit for each one, and how the sum handles non-detects. Some sums treat non-detects as zero, some as half the reporting limit, and some as the limit itself. Those choices move the total, sometimes substantially at low concentrations, and a total that does not say which convention was used cannot be compared with another total.
When a study or advocacy group argues that results are underestimated, ask which comparison is being made. There are three common ones: the study's targeted sum against the utility's targeted sum, a total fluorine value against a targeted sum, or a result from one list against a different list. Each comparison answers a different question. Mixing them is the usual source of confusion in public debate, and it is avoidable if the file is labeled before the argument starts.
Where the gap matters for a decision
Treatment selection, source assessment, and public notices draw on specific compounds with specific health-based values, so a targeted panel remains necessary. It is the record that can be compared across systems and over time. But a board asked whether its numbers are complete should understand that a complete targeted list is not a complete accounting of organic fluorine.
Where the difference matters, the next step is a documented question to the laboratory about which analytes were run, what the reporting limits were, how non-detects entered the sum, and whether any total fluorine data exist for the same samples. That question is more useful than a conclusion about the study. Do not treat a residual as a compound, and do not treat a targeted sum as total PFAS. Write the uncertainty into the file. The gap may be small in one system and larger in another, and the data available today do not show which situation applies to any given source.