MCL and MCLG: What the Two Public Water Columns Mean

By SubcontractorHub Editorial Team·Published October 2026

Dealers meet MCL and MCLG as a pair of numbers and then quote only one of them. The household hears a law. The table had a goal and a standard, and they are not the same kind of sentence. This page is the legend for that table.

The mercury numbers are in mercury in well water. The cadmium numbers are in cadmium in well water. SubcontractorHub is an all-in-one sales and project management platform for solar, roofing, and HVAC contractors. The habit is the same: name the column before you compare.

Where MCL and MCLG Show Up

The national primary drinking water regulations are legally enforceable primary standards and treatment techniques that apply to public water systems. The inorganic table prints a contaminant, then an MCLG, then an MCL or a treatment technique, then a health-effect phrase, then sources.

If you only photograph the MCL cell, you have thrown away the label that says whether the number is a goal. If you only photograph the MCLG, you have thrown away the cell EPA calls enforceable. Keep the row.

What Each Column Is Allowed to Mean

These are EPA's definitions from the notes under the table, shortened only by leaving out the disinfectant goals, which are a different pair of acronyms.

“As close as feasible” is why a goal and a standard can differ on some contaminants. Cost and treatment technology are part of that sentence. This page does not pick a technology, and it does not decide what is feasible for a house.

MCL and MCLG Can Match on a Row

On the inorganic table fetched with these definitions, three rows show the same figure in both number columns. Mercury (inorganic) is 0.002 mg/L and 0.002 mg/L. Cadmium is 0.005 mg/L and 0.005 mg/L. Chromium (total) is 0.1 mg/L and 0.1 mg/L.

A match means you cannot shrug and say the goal was stricter. It does not mean the row named a filter. Mercury's effect line says kidney damage. Cadmium's effect line also says kidney damage, at a different concentration.

Chromium's effect line says allergic dermatitis. Read the row you were asked about. The chromium article is chromium in well water.

Nitrate on that same table is its own row, measured as nitrogen, and it already has a page: nitrates in well water. Do not paste nitrate's 10 into a metal note because both pages mention an MCL.

Who the Columns Govern

Public water systems are the subject of these primary standards. A private well that serves one house is not a public system because a dealer wishes the number were binding. The map of that difference is private well water regulations.

You can still write the public MCL next to a private-well lab. Label it. “Public MCL 0.005 mg/L” is a comparison. “You are over the legal limit” is a claim about a statute you have not opened. State and county rules, when they exist, are their own documents.

Units Sit Under the Columns

EPA's note 2 says units are in milligrams per liter unless otherwise noted, and that milligrams per liter are equivalent to parts per million. The inorganic header repeats mg/L on the MCLG and MCL columns.

USGS, in the Circular 1209 glossary, says one thousand micrograms per liter equals 1 milligram per liter, and it describes milligrams per liter as equivalent to one part per million in most stream water and ground water. The “most” is USGS's word. Use it. The arithmetic page is milligrams and micrograms, and the keypad is the microgram milligram converter.

A picocurie is not a milligram. Activity units convert with 0.037 on the picocurie becquerel converter. That tool does not produce a mass, and it does not fill an MCL cell.

How to Write an MCL and an MCLG on the Quote

Six lines are enough. More than that usually means a second contaminant got mixed in.

1. Name the contaminant the way the table names it, parentheses included.
2. Write the MCLG and the MCL as two figures, even when they match.
3. Write the unit. mg/L, or µg/L with the 1,000 factor shown.
4. Say public water systems. Do not let the number stand as a private-well statute.
5. If the cell says TT, copy TT. Do not replace it with a milligram from memory.
6. Leave treatment, removal percent, and price off this note unless a listing you actually have says them.

The order of tests, before any of these columns matter, is the well water testing guide. A column legend does not tell you which bottle to fill.

What an MCL and an MCLG Will Not Do

They will not diagnose a person. The effect phrases on the metal rows are EPA's list for exposure above the MCL. They are not a chart you use in a kitchen. This page is not medical advice.

They will not size a filter. Best available technology is part of the MCL definition as a reason the enforceable number can sit near the goal. It is not a parts list. No removal percentage is stated here. No installed price is stated here.

Who Should Ignore a Guess From This Page

A utility operator following a state implementation of these rules should use the state's text. A household that wants a medical reading should call a clinician. A dealer comparing a private-well PDF to the public table can use this legend so the goal and the standard do not collapse into one unmarked number.

How MCL and MCLG Sit on One Row

Read left to right. The contaminant name comes first, including any parentheses. The MCLG cell comes next. The MCL or the treatment technique comes after that. Then the effect phrase, then the sources. If you skip a cell, the note on the job is a different row than the one EPA printed.

When the two number cells match, write both anyway. On this fetch, inorganic mercury is 0.002 mg/L and 0.002 mg/L. Cadmium is 0.005 mg/L and 0.005 mg/L. Chromium (total) is 0.1 mg/L and 0.1 mg/L. A match means the goal and the enforceable public standard landed on the same figure. It does not mean the row named a filter.

A Treatment Technique Is Not a Blank You Fill

EPA defines a treatment technique as a required process intended to reduce the level of a contaminant in drinking water. That sentence is the whole cell when the row says TT. It is not an invitation to type a milligram from a neighboring metal.

Mercury's 0.002 does not fill a TT cell. Cadmium's 0.005 does not fill one either. If the row you were asked about uses TT, the quote says TT. Best available technology shows up in the MCL definition as a reason the enforceable number can sit near the goal. It is still not a parts list for a house.

A Lab Floor Is Not MCL and MCLG

A lab may print a less-than sign or a reporting floor. That mark is the lab's note about what it could measure on that bottle. It is not the MCL, and it is not the MCLG. The public table fetched for this page does not print a detection floor for the inorganic rows used as examples.

Copy the lab's symbol as printed. Then write the public columns beside it, with the unit. Do not replace a less-than sign with zero, and do not replace it with the MCL. Those are three different sentences.

ppm and ppb Under MCL and MCLG

EPA's note says milligrams per liter are equivalent to parts per million. The inorganic header still says mg/L. If a metal row was printed in mg/L, a ppm figure with the same digits matches that cell under the note. It does not need the 1,000 factor.

The 1,000 factor is the USGS step from milligrams per liter to micrograms per liter. The same glossary describes micrograms per liter as one part per billion in most stream water and ground water. Use most when you say ppb. MCL and MCLG on these inorganic rows stay in mg/L until the lab's own unit is written next to them.

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