Define the comparison
State the treatment process, water source, target analytes and the question being evaluated. A comparison of selected analytes is not a performance certification for every treatment claim.
Match collection conditions
Identify the upstream and downstream locations, treatment status and operating conditions. Samples collected under different source or operating conditions can be difficult to compare directly.
Keep the analytical basis aligned
Use comparable fractions, methods, reporting limits and units. Record any dilution, preservation or other handling differences.
Interpret change carefully
A measured decrease or increase applies to the targets and conditions tested. Consider sampling variability and reporting limits before extending the result to a broader performance claim.
Related testing & guidance
Technical & service references
Vanguard water-testing menuEPA: Clean Water Act method questionsEPA: Drinking-water analytical methodsSource review: November 8, 2026. The agreed method and project scope govern the testing.