Ask the Lab for the Floor: Reading a Rural Well Report After a PFAS Undercount Warning

Most well owners know a lab report has a column for results. Fewer know that the column has a floor. A recent piece from Circle of Blue, dated October 23, 2026, takes up a question that matters for anyone reading a PFAS result from a private well: what a reported low value or a "not detected" entry actually means. The summary of the story begins with the condition "When PFAS concentrations in drinking water fall below laboratory reporting limits," and that condition is the right place to start reading. The full report is at Circle of Blue's report on whether PFAS contamination in drinking water may be underestimated.

What a reporting limit is

A reporting limit is the smallest amount a laboratory will report as a number under its own method and quality controls. Below that line, the lab is saying it cannot give a dependable figure. It is not saying the water is free of the compound. That difference is easy to lose on a one-page sheet, where a "not detected" entry and a numeric value sit in the same column and look equally final.

The limit can also change. Labs update methods, instruments, and calibration practices, so the floor on one year's report may not match the floor on the next. A household comparing two years of results side by side may be comparing two different measuring tools without realizing it.

Reading the sheet line by line

Start with the header rather than the result. Look for the sample date, the collection point, the list of compounds tested, and the reporting limit printed for each one. A sample drawn at the kitchen tap after a filter is not the same as a sample drawn from the raw water at the wellhead, and the sheet should say which one it is. If the reporting limit is missing from the page, that gap is a question to put to the lab.

Next, read the qualifier codes. Many reports place a letter or symbol beside a value to flag results between the detection limit and the reporting limit. Those flags are the lab describing its own uncertainty. A bare number with no flag is not automatically more certain than a flagged one, so the flag deserves the same attention as the number.

Questions to ask before the next sample

Ask the lab, in writing, for the reporting limit and the detection limit it used for each PFAS compound on your report.

Ask which analytical method was used, and whether your state's drinking water program has a position on that method. The answer tells you whether this year's result can be fairly set beside last year's.

Ask whether a repeat sample can be collected under the same conditions as the first, including the same sampling point and the same treatment in place, so the two results can be compared.

Ask the lab to explain, in plain words, what a result below its reporting limit means on the report you are holding.

Keep your own record

A household water record works best when it stays with the house, not with any single report. Keep each result with its sample date, the lab name, the reporting limit, and a note of any treatment in place at the time, such as a filter change or a new cartridge. Over several years, that file shows whether a reading moved or whether the floor under it moved.

The record cannot answer health questions. Whether a given result matters for the people who drink from a well is a conversation for your doctor, and for a local health office if your area has one. This article is about the paperwork, which is the part a reader can control before the next sample comes back.