Question 1

Is the 90th percentile an MCL?

Accepted Answer

No. It's an action level. Exceedance triggers public education and treatment changes, not an MCL violation in the legal sense — though some states treat it equivalently.

Question 2

What samples count?

Accepted Answer

Only first-draw, 1-L, after at least 6 hours of stagnation, from Tier 1 sites (homes with known lead service lines or lead solder).

Question 3

What if the system has only 5 samples?

Accepted Answer

0.9 × 5 = 4.5, rounds to 5. The 90th percentile is simply the highest of the five samples.

Question 4

What's changing under the 2024 LCRI?

Accepted Answer

The Lead and Copper Rule Improvements lower the lead action level to 10 µg/L (0.010 mg/L) starting in 2027 and require full lead service line replacement within 10 years.

Lead & Copper Rule 90th Percentile Calculator