Total trihalomethanes (TTHMs)
Watertown Water & Sewer Authority
Trihalomethanes are cancer-causing contaminants that form during water treatment with chlorine and other disinfectants. The total trihalomethanes group includes four chemicals: chloroform, bromodichloromethane, dibromochloromethane and bromoform. Read More.
Studies conducted in the U.S. and Europe have found that drinking tap water that carries disinfection byproducts increases the risk of developing bladder cancer. In animal studies, all trihalomethanes cause liver, kidney and intestinal tumors. Some human epidemiological studies also reported an association between disinfection byproducts and an increased risk of problems during pregnancy, including spontaneous miscarriage, cardiovascular defects, neural tube defects and low birth weight.
The health guideline of 0.15 parts per billion, or ppb, for the group of four trihalomethanes, or THM4/TTHM, was defined in a peer-reviewed scientific study by EWG and represents a one-in-one-million lifetime cancer risk level.
Samples
Samples exceeding legal limit (MCL)
Samples exceeding
health guidelines
Testing results - average by year
| Year | Average result | Samples taken | Detections | Range of results |
|---|---|---|---|---|
| 2018 | 64.6 ppb | 16 | 16 | 30.5 ppb - 99.8 ppb |
| 2019 | 61.2 ppb | 16 | 16 | 29.6 ppb - 91.8 ppb |
| 2020 | 61.1 ppb | 16 | 16 | 30.8 ppb - 109.0 ppb |
| 2021 | 63.4 ppb | 16 | 16 | 27.6 ppb - 99.0 ppb |
| 2022 | 53.7 ppb | 16 | 16 | 38.8 ppb - 79.9 ppb |
| 2023 | 51.4 ppb | 16 | 16 | 27.3 ppb - 92.2 ppb |
ppb = parts per billion
State, National, and Health Guidelines for Drinking Water
EWG Health Guideline: 0.15 ppb
The health guideline of 0.15 parts per billion, or ppb, for the group of four trihalomethanes, or THM4/TTHM, was defined in a peer-reviewed scientific study by EWG and represents a one-in-one-million lifetime cancer risk level.
EPA Maximum Contaminant
Level (MCL): 80 ppb
The legal limit for the group of four trihalomethanes, established in 1998, was based on the need for residual disinfectant levels in water served to customers and the cost of treatment. This limit does not fully protect against the risk of cancer due to exposure to trihalomethanes.
ppb = parts per billion
All test results
| Date | Lab ID | Result |
|---|---|---|
| 2018-03-07 | DBP 200489534 | 48.0 ppb |
| 2018-03-07 | DBP 200489535 | 53.5 ppb |
| 2018-03-07 | DBP 200489536 | 54.7 ppb |
| 2018-03-07 | DBP 200489533 | 63.3 ppb |
| 2018-06-06 | DBP 200496328 | 79.1 ppb |
| 2018-06-06 | DBP 200496329 | 72.8 ppb |
| 2018-06-06 | DBP 200496330 | 68.7 ppb |
| 2018-06-06 | DBP 200496331 | 73.2 ppb |
| 2018-09-05 | DBP 200504061 | 87.8 ppb |
| 2018-09-05 | DBP 200504062 | 99.8 ppb |
| 2018-09-05 | DBP 200504063 | 85.4 ppb |
| 2018-09-05 | DBP 200504064 | 69.3 ppb |
| 2018-12-05 | DBP 200514980 | 51.2 ppb |
| 2018-12-05 | DBP 200514982 | 49.4 ppb |
| 2018-12-05 | DBP 200514981 | 46.4 ppb |
| 2018-12-05 | DBP 200514978 | 30.5 ppb |
| 2019-03-06 | DBP 200523708 | 57.6 ppb |
| 2019-03-06 | DBP 200523709 | 42.2 ppb |
| 2019-03-06 | DBP 200523710 | 45.4 ppb |
| 2019-03-06 | DBP 200523711 | 34.7 ppb |
| 2019-06-05 | DBP 200535068 | 67.1 ppb |
| 2019-06-05 | DBP 200535069 | 79.2 ppb |
| 2019-06-05 | DBP 200535070 | 60.2 ppb |
| 2019-06-05 | DBP 200535071 | 67.7 ppb |
| 2019-09-05 | DBP 200547637 | 91.6 ppb |
| 2019-09-05 | DBP 200547640 | 78.0 ppb |
| 2019-09-05 | DBP 200547639 | 86.8 ppb |
| 2019-09-05 | DBP 200547636 | 91.8 ppb |
| 2019-12-05 | DBP 200559147 | 29.6 ppb |
| 2019-12-05 | DBP 200559148 | 48.1 ppb |
| 2019-12-05 | DBP 200559149 | 37.8 ppb |
| 2019-12-05 | DBP 200559146 | 62.0 ppb |
| 2020-03-04 | DBP 200568197 | 57.3 ppb |
| 2020-03-04 | DBP 200568198 | 30.8 ppb |
| 2020-03-04 | DBP 200568199 | 40.9 ppb |
| 2020-03-04 | DBP 200568200 | 31.3 ppb |
| 2020-06-03 | DBP 200577855 | 56.2 ppb |
| 2020-06-03 | DBP 200577856 | 58.0 ppb |
| 2020-06-03 | DBP 200577854 | 52.8 ppb |
| 2020-06-03 | DBP 200577853 | 73.6 ppb |
| 2020-09-02 | DBP 200588679 | 109.0 ppb |
| 2020-09-02 | DBP 200588680 | 88.7 ppb |
| 2020-09-02 | DBP 200588681 | 76.3 ppb |
| 2020-09-02 | DBP 200588682 | 85.7 ppb |
| 2020-12-02 | DBP 200599791 | 64.8 ppb |
| 2020-12-02 | DBP 200599792 | 42.3 ppb |
| 2020-12-02 | DBP 200599793 | 55.3 ppb |
| 2020-12-02 | DBP 200599794 | 53.9 ppb |
| 2021-03-10 | DBP 200609349 | 43.2 ppb |
| 2021-03-10 | DBP 200609346 | 55.6 ppb |
| 2021-03-10 | DBP 200609347 | 27.6 ppb |
| 2021-03-10 | DBP 200609348 | 40.7 ppb |
| 2021-06-02 | DBP 200619137 | 55.7 ppb |
| 2021-06-02 | DBP 200619139 | 60.8 ppb |
| 2021-06-02 | DBP 200619140 | 71.0 ppb |
| 2021-06-02 | DBP 200619136 | 83.0 ppb |
| 2021-09-08 | 300053894 300053896 | 78.5 ppb |
| 2021-09-08 | 300053900 300053902 | 83.7 ppb |
| 2021-09-08 | 300053903 300053905 | 72.2 ppb |
| 2021-09-22 | 300065698 300065700 | 99.0 ppb |
| 2021-12-01 | D300110687 300110689 | 60.5 ppb |
| 2021-12-01 | D300110684 300110686 | 62.3 ppb |
| 2021-12-01 | D300110681 300110683 | 51.2 ppb |
| 2021-12-01 | D300110678 300110680 | 69.8 ppb |
| 2022-03-02 | D300151783 300151785 | 46.4 ppb |
| 2022-03-02 | D300151786 300151788 | 44.1 ppb |
| 2022-03-02 | D300151789 300151791 | 38.8 ppb |
| 2022-03-02 | D300151792 300151794 | 44.6 ppb |
| 2022-06-01 | D300192098 300192100 | 63.4 ppb |
| 2022-06-01 | D300192101 300192103 | 55.1 ppb |
| 2022-06-01 | D300192104 300192106 | 55.2 ppb |
| 2022-06-01 | D300192107 300192109 | 58.1 ppb |
| 2022-09-14 | D300244033 300244035 | 74.8 ppb |
| 2022-09-14 | D300244027 300244029 | 55.8 ppb |
| 2022-09-14 | D300244024 300244026 | 79.9 ppb |
| 2022-09-14 | D300244030 300244032 | 71.1 ppb |
| 2022-12-07 | D300282718 300282720 | 44.7 ppb |
| 2022-12-07 | D300282721 300282723 | 39.0 ppb |
| 2022-12-07 | D300282724 300282726 | 43.9 ppb |
| 2022-12-07 | D300282727 300282729 | 43.9 ppb |
| 2023-03-16 | D300321892 300321894 | 44.1 ppb |
| 2023-03-16 | D300321895 300321897 | 27.3 ppb |
| 2023-03-16 | D300321898 300321900 | 40.8 ppb |
| 2023-03-16 | D300321901 300321903 | 31.3 ppb |
| 2023-06-14 | D300365450 300365452 | 36.4 ppb |
| 2023-06-14 | D300365456 300365458 | 48.3 ppb |
| 2023-06-14 | D300365453 300365455 | 49.9 ppb |
| 2023-06-14 | D300365447 300365449 | 56.0 ppb |
| 2023-09-06 | D300410877 300410879 | 88.9 ppb |
| 2023-09-06 | D300410880 300410882 | 82.3 ppb |
| 2023-09-06 | D300410883 300410885 | 85.5 ppb |
| 2023-09-06 | D300410886 300410888 | 92.2 ppb |
| 2023-12-07 | D300456855 300456857 | 35.7 ppb |
| 2023-12-07 | D300456858 300456860 | 29.1 ppb |
| 2023-12-07 | D300456861 300456863 | 37.7 ppb |
| 2023-12-07 | D300456864 300456866 | 36.4 ppb |