You open a water test report and find a row that reads "not detected" beside a chemical you have been worried about. That feels like good news, and often it is. But the phrase hides a number you should read before you relax: the reporting limit.
Recent coverage of atrazine shows why this matters. The Guardian reported on a lawsuit that asks a federal judge to order action on surface-water pollution from the "hormone-disrupting pesticide atrazine." According to Tom Perkins of The Guardian, the suit alleges that the EPA has for decades failed under the Clean Water Act to address that pollution. Read The Guardian's report on the atrazine lawsuit for the full allegations. A court case does not change what is in your glass, but it does mean a contaminant is still in the news, which is exactly when a clear test result matters most.
What "not detected" actually means
Every lab sets a reporting limit, the smallest amount its method can measure with reliable confidence. When a result falls below that line, the lab reports a nondetect. A nondetect does not mean the water contains zero of the chemical. It means any amount present is below what that lab can count.
Why the number decides the answer
Two labs can both report a nondetect on the same sample while using different limits. A lab with a higher limit can miss amounts that a lower-limit lab would catch. For a contaminant with a regulatory benchmark, the question is simple: is the reporting limit below that benchmark? If it sits above, a nondetect tells you almost nothing about whether your water is under the line. For atrazine in public water, the EPA sets a legal limit, so compare the lab's reporting limit to the figure in that regulation, or ask the lab to confirm the comparison in writing.
Questions to ask before you order
Before you pay for a test, ask the lab or kit maker four things. Which method does the lab use for each contaminant, and what is the reporting limit for that method? Is the lab certified for drinking water testing in your state? Does the report show the reporting limit beside every result, or only the word "nondetect"? And does the test cover the chemical you care about at all? A low limit on the wrong panel still tells you nothing.
Read the utility report alongside your test
If you are on a public water system, the annual Consumer Confidence Report lists the contaminants the utility found and the levels it measured. Your own test and the utility's report are different samples taken at different points, so compare them rather than letting one replace the other. If the utility lists a detected level and your test says nondetect at a high limit, that gap is a question to bring to your lab, not a reason to trust the nondetect.
What to do with a nondetect
If your water source is near farmland or you have specific questions, ask for a test with a reporting limit below the relevant benchmark, and keep the report in your water file. Write the date, the lab, the method, and the limit next to each result so you can compare future tests. If you or someone in your home has health concerns, bring the report to your doctor or local health department rather than interpreting the numbers alone.
A clear report is only useful when you can read the limits behind it. Get the number first, then decide what the nondetect actually tells you.