A trend line is only as sound as the records that feed it. When PFAS results are reported over several years, each point carries choices the chart does not show: how often the sample was drawn, which laboratory and method produced the number, and where the reporting limit sat on the day the sample was analyzed. The Circle of Blue report on PFAS underestimation, dated October 23, 2026, covers a new study suggesting that PFAS contamination in drinking water may be underestimated. That is a claim about totals. The sampling history behind any trend deserves its own reading before a slope is drawn.
Why a Sparse Record Looks Flatter Than It Is
Many utilities and laboratories do not sample PFAS on a fine schedule. A record built from quarterly or annual results has few points, and a few points can make a trend look smooth or stepped depending on where the detections fall. One detection between two non-detects produces a jump on the chart that may reflect the timing of the sample more than a change in the source water.
The non-detect is the harder part. The summary of the Circle of Blue story notes that "When PFAS concentrations in drinking water fall below laboratory reporting limits" the results become difficult to read at face value. A non-detect is not a zero. It is a statement that the method did not reach a stated floor on that day. Any trend that treats it as zero, as half the limit, or as the limit itself has adopted a convention, and each convention can produce a different slope when detections are rare.
Three Checks Before the Slope
First, confirm the reporting limit for every period. If the laboratory or method changed during the record, earlier non-detects may not be comparable with later ones. A series that appears to rise after a method change may reflect a lower floor rather than a higher concentration. Mark the break in the series instead of splicing it into one continuous line.
Second, record the substitution rule. Whatever convention was applied to non-detects should be written beside the chart, with the date it was adopted. A reader who sees a smooth line should be able to tell whether the smoothness came from the water or from the rule.
Third, check where the samples came from. A trend drawn from one entry point, or from a set of points that changed over time, describes that set. It does not describe the whole system. Sample locations, counts per period, and any changes to the location list belong with the data.
What an Undercount Question Changes
If the study's point holds, the early part of many PFAS records may understate what was present, and the apparent flatness at low levels could reflect what the method was able to see. That does not mean every historical result is wrong. It means a trend near the reporting limit carries less information than its line suggests. Operators should treat low-level trends as statements about detection, with that uncertainty written into the report.
The available summary does not describe the study's full method, sample design, or the concentration ranges involved. Anyone relying on its conclusions should obtain the full paper and read the methods section before carrying the finding into a board packet or planning document. Where the paper is silent, the uncertainty stays open.
Keep the Floor With the Series
A practical record is a set of columns kept alongside each result: sample date, analysis date, laboratory, method, reporting limit, qualifier flags, the non-detect convention applied, and the number of samples in each period. The chart should be generated from those columns, not the reverse. When a new laboratory or method enters the program, the file should carry the change forward, so the next person who draws a trend inherits the floor along with the number.
This is analysis of record keeping and interpretation, not advice on compliance obligations, which depend on the program and the regulator in each jurisdiction. The point for this desk is narrower: a PFAS trend is a sequence of measurement decisions. Read the decisions first, then the slope.