How to read drinking water test results
Understand water test units, reporting limits, nondetects, sample dates, PFAS results, and lead and copper summaries in plain language.
Read the sample date
A sample describes a particular place and time. Check the monitoring period and the source report before comparing results.
Check the units and qualifiers
Compare values only when their units and methods are compatible. A less-than symbol indicates a result below a reporting limit; it is not a measurement of zero.
Understand the record type
Lead and copper 90th-percentile summaries describe a monitoring period, not an individual household. Missing records are not evidence that no testing occurred.
Reporting limits: a worked example
Illustrative example, not a local result: < 0.004 µg/L means the result was below a reporting limit of 0.004 µg/L. The exact amount is not reported. It does not mean zero, and the reporting limit is not a health standard.
Reading a lead or copper summary
Illustrative example, not a local result: a published lead 90th percentile of 0.005 mg/L describes a set of samples for a monitoring period. It is not the maximum result, a household result, or a claim that every tap had that concentration. Read the period and source notes with the number.