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?

Answer:

The Consumer Confidence Report must include the 90^th percentile concentration of the most recent round(s) of sampling for lead and copper. If the “90^th percentile” level is above the action level, this is called an action level exceedance. The action level is the concentration of a contaminant which, if exceeded, triggers treatment actions or other requirements which a water system must follow. 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"90^th 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"90^th percentile" copper level is greater than 1.3 mg/L) (40 CFR Section 141.80(c)). 

Question (23002-32408)

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