CCR Units of Measurement

A Consumer Confidence Report expresses contaminant levels in several measurement units — mg/L, µg/L, ng/L, pCi/L, NTU, and MFL — each matched to the concentration range where a given MCL or action level applies.

Where you'll see them on a CCR

Every CCR includes a water quality data table listing detected contaminants, their measured levels, and the regulatory limit. The unit column tells you the scale of that measurement. Reading it correctly matters: 0.004 ng/L and 0.004 mg/L differ by a factor of one million. Regulators assign units deliberately — they pick the scale that puts most results between 0 and a few hundred rather than reporting 0.000004 of anything.

Contaminant pages on this site show units alongside each limit so you can compare your CCR's numbers directly to the federal standard.

The unit equivalence table

| Abbreviation | Full name | Equivalent | Common use on a CCR | |---|---|---|---| | mg/L | milligrams per liter | = ppm (parts per million) | Chlorine residual, nitrate (MCL 10 mg/L), total trihalomethanes | | µg/L | micrograms per liter | = ppb (parts per billion); 1 mg/L = 1,000 µg/L | Lead action level (15 µg/L LCR; 10 µg/L LCRI effective Nov 1, 2027), arsenic MCL (10 µg/L) | | ng/L | nanograms per liter | = ppt (parts per trillion); 1 µg/L = 1,000 ng/L | PFAS MCLs (PFOA 4 ng/L, PFOS 4 ng/L) | | pg/L | picograms per liter | = ppq (parts per quadrillion); 1 ng/L = 1,000 pg/L | Not currently used in federal CCR reporting; listed for completeness | | pCi/L | picocuries per liter | Radioactivity unit, not a mass ratio | Radionuclides: gross alpha MCL 15 pCi/L; beta particle and photon emitters 4 mrem/yr equivalent | | NTU | Nephelometric Turbidity Unit | Light-scattering intensity, not a mass ratio | Turbidity; surface water treatment rule limits vary by system type | | MFL | Million Fibers per Liter | Fiber count, not a mass ratio | Asbestos MCL 7 MFL |

The ppm/ppb/ppt labels are informal equivalents — they appear in consumer-facing explanations but the regulatory text in 40 CFR Part 141 uses mg/L and µg/L. Either form is technically correct; the mg/L and µg/L forms are what you'll see in your CCR's compliance table.

Why PFAS uses ng/L (parts per trillion)

PFOA and PFOS are toxic at concentrations so low that mg/L or even µg/L would produce numbers like 0.000004 — too small to read clearly in a table. EPA set the MCLs at 4 ng/L for both PFOA and PFOS under the April 2024 PFAS National Primary Drinking Water Regulation (40 CFR § 141.61; 89 FR 32532, effective June 25, 2024). Expressing that as 0.000004 mg/L would be accurate but unreadable, so ng/L is the correct unit for PFAS results on a CCR's PFAS section.

This is a general pattern: as EPA regulates contaminants at progressively lower thresholds, the unit shifts down the scale. Radionuclides use pCi/L because radioactivity is measured by decay events per second (a curie is 3.7 × 10¹⁰ disintegrations per second; a picocurie is one trillionth of that), not by mass.

Related terms

Citations