Total Trihalomethanes in Drinking Water: The Public 0.080 mg/L Line

By SubcontractorHub Editorial Team·Published October 2026

Total trihalomethanes in drinking water show up when a lab, or a city report, prints TTHM, chloroform, or a sum. The national primary rules apply to public water systems. They are not a private-well statute. The four-row byproduct block, including haloacetic acids, bromate, and chlorite, is disinfection byproducts in drinking water. The five-acid row is haloacetic acids in drinking water. This page is only the trihalomethane sum.

Total Trihalomethanes in Drinking Water Are One Public Sum

On the disinfection byproducts table, total trihalomethanes (TTHMs) sit on their own line. The MCL column is 0.080. The unit note says milligrams per liter unless otherwise noted, and that milligrams per liter are equivalent to parts per million. The health-effects cell says liver, kidney, or central nervous system problems, and an increased risk of cancer. The source cell says byproduct of drinking water disinfection.

The opening sentence of the national primary drinking water regulations says those rules are legally enforceable standards and treatment techniques that apply to public water systems. A dealer who pastes 0.080 onto a private-well proposal as if it were a county pass line is using the wrong statute. Keep the number, and label where it came from.

Four Names in the Footnote, One Group MCL

Footnote 6 says there is no collective MCLG for this contaminant group, and then it lists individual MCLGs. Those goals are not a second set of MCLs. The enforceable group figure on the table remains 0.080 mg/L.

If the lab prints only chloroform, write chloroform. Do not relabel that single line as the group sum. If the lab prints a TTHM total, write the total, and keep any individual lines beside it. The household should be able to match your note to the PDF.

A Locational Running Annual Average Is Not One Jug

40 CFR Part 141 Subpart V says its requirements are national primary drinking water regulations. It establishes monitoring for maximum contaminant levels based on locational running annual averages (LRAA) for total trihalomethanes and for haloacetic acids (five). For systems required to monitor quarterly, compliance uses LRAAs, and each LRAA is not to exceed the MCL. If more than one sample is taken in a quarter at a monitoring location, those samples are averaged for that quarter before they enter the LRAA.

Systems that monitor yearly or less often determine that each sample is less than the MCL. If no sample exceeds the MCL, that sample is considered the LRAA for that monitoring location. None of those sentences is a method for a single bottle drawn at a kitchen tap on a private well. Write “one sample” when that is what you hold. Do not write “LRAA” unless the utility report is the document in your hand.

0.080 mg/L and 80 µg/L Are the Same Mass Label

Some sheets print micrograms per liter. The scale on milligrams and micrograms is one thousand micrograms per liter to 1 milligram per liter. 0.080 times 1,000 is 80. So 0.080 mg/L and 80 µg/L are the same mass on that scale. The conversion does not create a tighter public line, and it does not say a device worked.

Footnote 2 also says milligrams per liter are equivalent to parts per million. 0.080 mg/L and 0.080 ppm are the same digits on that note. Parts per trillion are a different step. They belong on parts per trillion in drinking water, not on this row.

What to Copy Before You Talk About Carbon

Copy the laboratory name, the sample date, the sample point, the unit, and whether the figure is a single result or an average the utility already computed. Copy each of the four names if the sheet printed them, even when one cell is blank. A blank is not a zero you invented.

Chlorine and chloramine are disinfectant rows, not this sum. Their public residual lines are on chloramine in drinking water. A residual of disinfectant is not a TTHM result. Ozone's bromate note is a different byproduct. It stays on ozone water treatment and on the byproduct hub. Do not fold bromate into the 0.080 line.

The well water testing guide is where a private-well panel is planned. This page does not add TTHM to that panel as a required federal test. If the customer is on a public system, ask for the consumer report before you treat a single home sample as the system's compliance math.

What This Page Will Not Put on the Proposal

No removal percentage. No claim that a carbon bed, a pitcher, or a reverse-osmosis membrane hits a stated percent on this contaminant. No empty-bed contact time. No installed price. No statement that boiling the water changes the sum. Those sentences are absent because this fetch did not supply them.

Primary standards, as a class, are explained in primary and secondary drinking water standards. An MCL and an MCLG are different columns. That difference is MCL and MCLG. On this group the collective MCLG cell is “n/a,” and the footnote supplies individual goals instead. Say that out loud if a customer asks why the goal cell looks empty.

A Blank Cell Is Not a Zero You Supply

If the laboratory prints three of the four names and leaves one cell empty, copy the empty cell as empty. The footnote gives each chemical an MCLG. It does not tell you to fill a missing lab result with that goal.

Zero in the footnote is a goal, not a measurement. A measurement of zero, when the sheet actually printed one, is the lab's digit. Those are different sentences. The proposal should not swap them, and it should not treat a blank as proof the chemical was absent.

The Group Line and the Individual Lines Travel Together

When the sheet prints a group total and also prints chloroform, keep both. The total is the row the 0.080 mg/L MCL belongs to. Chloroform is one name inside the footnote. Replacing the total with the chloroform line, or replacing chloroform with the total, makes the next reader compare the wrong cell to the wrong figure.

If only one of them was printed, say which one. A city report that shows the group and a home sample that shows a single chemical are not the same document. File them as two results, with two dates, even when the household thinks they are one test.

A City Report and a Kitchen Jug Are Different Documents

Public systems on this rule monitor at locations. When they sample quarterly, they build a locational running annual average, and more than one sample in a quarter at the same location is averaged before it enters that figure. The sentence that defines the average is the Subpart V text linked above.

A jug drawn at a private kitchen is one sample. Write the address, the date, and the laboratory. Do not write the average's name on a document that is not the utility's compliance calculation. If the household buys water from a public system, ask for that system's report before you treat the home sample as the system's math.

What You Can Say Without a Treatment Claim

You can say the public group MCL is 0.080 mg/L, that the footnote lists four MCLGs, and that milligrams per liter match parts per million on the EPA unit note. You can point at the milligrams page for the 1,000-step into micrograms per liter. 0.080 mg/L and 80 µg/L stay one mass label on that scale.

You cannot say a carbon filter, a pitcher, or a membrane removes a stated share of the sum. That percentage was not in the pages fetched for this article. Leave the device claim off until a source you actually open states it, and keep the lab unit next to the digits either way.

How Should a Dealer Read Total Trihalomethanes in Drinking Water?

Read the group MCL as 0.080 mg/L on a public system, then read the four footnote MCLGs as goals rather than as four extra MCLs. Ask whether the number in front of you is one sample or a locational running annual average the utility computed. Keep chloroform, if that is all the sheet shows, labeled as chloroform. Leave removal claims off the proposal until a source you fetched actually states them.

Frequently Asked Questions