TTHM (Total Trihalomethanes) in drinking water
6,250 active public water systems in 50 states had a TTHM (Total Trihalomethanes) violation in the past three years, and 784 had one in the latest quarter of EPA's data.
You are on Tapcensus, an independent guide to the EPA compliance records of every public water system in the United States. This page explains TTHM (Total Trihalomethanes) and lists the water systems whose records include a related violation. It does not show measured levels.
Is TTHM (Total Trihalomethanes) in the records for your water system?
Enter a ZIP code, town, or street address to see the water systems that serve your area, their status, and which rules and contaminants appear in their violations.
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What it is and where it comes from
TTHMs are four compounds — chloroform, bromodichloromethane, dibromochloromethane, bromoform — formed when chlorine reacts with organic matter in water. They are among the most common disinfection byproducts and form more readily in surface water systems with high organic loads.
Health effects
Long-term exposure is associated with increased bladder cancer risk. Some studies link high TTHM exposure to adverse reproductive outcomes. Bromodichloromethane is classified as a possible human carcinogen.
The standard and what a violation means
EPA limit: 0.080 mg/L (80 ppb) locational running annual average (LRAA).
Water systems with a TTHM (Total Trihalomethanes) violation, by state
Active public water systems that reported at least one related violation to EPA in the past three years. Each state links to the list of its affected systems.
| State | Systems with violations, 3 yr | Share of active systems | In violation, latest quarter |
|---|---|---|---|
| Pennsylvania | 535 | 7% | 39 |
| Texas | 514 | 7% | 157 |
| Mississippi | 501 | 42% | 11 |
| Florida | 471 | 9% | 6 |
| Illinois | 355 | 7% | 50 |
| Oklahoma | 349 | 27% | 123 |
| North Carolina | 234 | 5% | 34 |
| Arizona | 219 | 13% | 23 |
| New York | 217 | 3% | 1 |
| Washington | 212 | 5% | 22 |
| Colorado | 181 | 8% | 13 |
| West Virginia | 143 | 19% | 32 |
| Kansas | 134 | 14% | 13 |
| Oregon | 131 | 5% | 6 |
| Michigan | 122 | 1% | 11 |
| California | 120 | 2% | 4 |
| Alaska | 119 | 9% | 23 |
| Virginia | 116 | 4% | 17 |
| Missouri | 107 | 4% | 18 |
| Louisiana | 102 | 8% | 48 |
| Ohio | 102 | 2% | 6 |
| Indiana | 94 | 2% | 6 |
| Montana | 87 | 4% | 5 |
| New Jersey | 75 | 2% | 11 |
| Iowa | 71 | 4% | 1 |
| Georgia | 66 | 3% | 11 |
| Alabama | 65 | 12% | 8 |
| Vermont | 65 | 5% | 9 |
| Maryland | 63 | 2% | 2 |
| Tennessee | 61 | 8% | 6 |
| Utah | 59 | 5% | 1 |
| Wisconsin | 59 | 1% | 3 |
| Idaho | 53 | 3% | 1 |
| Kentucky | 46 | 11% | 17 |
| New Mexico | 46 | 4% | 6 |
| Massachusetts | 43 | 3% | 7 |
| Arkansas | 42 | 4% | 3 |
| New Hampshire | 42 | 2% | 10 |
| Maine | 38 | 2% | 2 |
| Nevada | 34 | 6% | 1 |
| Wyoming | 34 | 4% | 2 |
| South Dakota | 31 | 4% | 0 |
| Connecticut | 23 | 1% | 3 |
| South Carolina | 18 | 1% | 5 |
| North Dakota | 16 | 4% | 2 |
| Rhode Island | 15 | 3% | 2 |
| Minnesota | 11 | 0% | 3 |
| District of Columbia | 5 | 36% | 0 |
| Nebraska | 3 | 0% | 0 |
| Hawaii | 1 | 1% | 0 |
Largest water systems with a TTHM (Total Trihalomethanes) violation
The ten biggest systems, by people served, whose records include a related violation in the past three years.
| System | Serves | Status | Rules violated, 3 yr | Unresolved |
|---|---|---|---|---|
|
Mdwasa Main System Miami · Miami-Dade County |
2,377,460 | 2 | 0 | |
|
City of Baltimore Baltimore city County |
1,600,000 | 3 | 0 | |
|
Central Alabama Water Birmingham · Jefferson County |
653,322 | 2 | 1 | |
|
Pittsburgh Water & Sewer Auth Allegheny County |
520,000 | 3 | 0 | |
|
Pinellas County Utilities Clearwater · Pinellas County |
506,353 | 4 | 0 | |
|
ST Petersburg, City of Odessa · Pinellas County |
349,979 | 4 | 0 | |
|
Manatee County Utilities Dept Bradenton · Manatee County |
347,800 | 2 | 0 | |
|
Ocud/eastern Water System (2 Wp) Orlando · Orange County |
293,374 | 1 | 0 | |
|
Lee County Utilities Fort Myers · Lee County |
282,556 | 1 | 0 | |
|
Mobile, Bd. of W&s Comm. of the City of Mobile · Mobile County |
279,000 | 2 | 0 |
About this data
A system is counted once if any of its violations in the past three years involved TTHM (Total Trihalomethanes), including monitoring and reporting lapses. Rules violated and status in the systems table are for the whole system, not TTHM (Total Trihalomethanes) alone. Counts cover active systems in all 50 states and the District of Columbia.
Source: the U.S. EPA's Enforcement and Compliance History Online system (ECHO), which publishes the federal Safe Drinking Water Information System. EPA's terms and values are shown as published. The records are from EPA's data refresh of Oct 7, 2026; EPA refreshes this data quarterly, and states report to EPA with a lag of three to six months, so the newest violations may not appear yet.