Frequent Questions

I am looking at a water report for my public water system. Under the contaminant heading for lead and copper there is a column heading titled 90th percentile. What does this column mean?

Under the Lead and Copper Rule, lead and copper are regulated by action levels. The lead action level is exceeded if the concentration of lead in more than 10 percent of tap water samples is greater than 0.015 mg/L (i.e. if the "90th percentile" lead level is greater than 0.015 mg/L). The copper action level is exceeded if the concentration of copper in more than 10 percent of tap water samples is greater than 1.3 mg/L (i.e. if the "90th percentile" copper level is greater than 1.3 mg/L) (40 CFR Section 141.80(c)).
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