EWG's Tap Water Database — 2021 UPDATE


Total trihalomethanes (TTHMs)

Rehoboth District

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.

Click here to read more about disinfection byproducts.





Samples exceeding legal limit (MCL)


Samples exceeding
health guidelines

Testing results - average by year

YearAverage resultSamples takenDetectionsRange of results
20143.21 ppb16160.710 ppb - 7.80 ppb
20152.77 ppb16160.580 ppb - 6.46 ppb
20162.56 ppb16160.500 ppb - 5.29 ppb
20173.36 ppb16160.630 ppb - 9.77 ppb
20184.11 ppb1615ND - 9.88 ppb
20193.43 ppb16160.540 ppb - 6.70 ppb

ppb = parts per billion

State and national drinking water standards and health guidelines

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
2014-03-05S4790543.81 ppb
2014-03-05S4790535.15 ppb
2014-03-05S4790521.84 ppb
2014-03-05S4790511.68 ppb
2014-06-02S4791264.65 ppb
2014-06-02S4791253.35 ppb
2014-06-02S4791243.04 ppb
2014-06-02S4791230.710 ppb
2014-09-03S4791924.83 ppb
2014-09-03S4791913.60 ppb
2014-09-03S5369642.54 ppb
2014-09-03S5369657.80 ppb
2014-12-01S3164494.27 ppb
2014-12-01S3164482.01 ppb
2014-12-01S3164470.730 ppb
2014-12-01S3164461.29 ppb
2015-03-02S5805052.28 ppb
2015-03-02S5805043.21 ppb
2015-03-02S5805030.650 ppb
2015-03-02S5805020.690 ppb
2015-06-01S6124141.40 ppb
2015-06-01S6124150.700 ppb
2015-06-01S6124174.70 ppb
2015-06-01S6124165.63 ppb
2015-09-09S6020384.80 ppb
2015-09-09S6020372.61 ppb
2015-09-09S6020366.46 ppb
2015-09-09S6020351.44 ppb
2015-12-07S6250123.69 ppb
2015-12-07S6250110.580 ppb
2015-12-07S6250102.58 ppb
2015-12-07S6250092.83 ppb
2016-03-07S6021041.36 ppb
2016-03-07S6021032.37 ppb
2016-03-07S6021050.500 ppb
2016-03-07S6021061.96 ppb
2016-06-06S6290493.01 ppb
2016-06-06S6290483.74 ppb
2016-06-06S6290473.00 ppb
2016-06-06S6290460.660 ppb
2016-09-07S6703824.84 ppb
2016-09-07S6703811.51 ppb
2016-09-07S6703804.10 ppb
2016-09-07S6703795.29 ppb
2016-12-05S6704003.51 ppb
2016-12-05S6704033.10 ppb
2016-12-05S6704020.530 ppb
2016-12-05S6704011.54 ppb
2017-03-06S6793663.02 ppb
2017-03-06S6793650.630 ppb
2017-03-06S6793641.36 ppb
2017-03-06S6793630.670 ppb
2017-06-06S7308092.88 ppb
2017-06-06S7308082.69 ppb
2017-06-06S7308073.16 ppb
2017-06-06S7308064.47 ppb
2017-09-05S7371379.77 ppb
2017-09-05S7371417.20 ppb
2017-09-05S7371453.80 ppb
2017-09-05S7371495.54 ppb
2017-12-04S6793793.84 ppb
2017-12-04S6793750.700 ppb
2017-12-04S6703961.79 ppb
2017-12-04S6590902.30 ppb
2018-03-06S7706246.68 ppb
2018-03-06S7706161.34 ppb
2018-03-06S7706121.56 ppb
2018-06-04S7708333.30 ppb
2018-06-04S7708060.700 ppb
2018-06-04S7707541.60 ppb
2018-06-20S7705754.10 ppb
2018-09-05S8123018.20 ppb
2018-09-05S8122971.65 ppb
2018-09-05S8122938.80 ppb
2018-09-05S8122899.88 ppb
2018-12-05S8170293.33 ppb
2018-12-05S8170217.28 ppb
2018-12-05S8170176.80 ppb
2018-12-05S8170130.580 ppb
2019-03-04S8479333.25 ppb
2019-03-04S8479373.59 ppb
2019-03-04S8479410.540 ppb
2019-03-04S8479453.81 ppb
2019-06-03S8480715.54 ppb
2019-06-03S8480671.83 ppb
2019-06-03S8480634.40 ppb
2019-06-03S8480596.70 ppb
2019-09-03S8647092.30 ppb
2019-09-03S8647050.790 ppb
2019-09-03S8647014.12 ppb
2019-09-03S8646975.60 ppb
2019-12-03S9199073.38 ppb
2019-12-03S9199195.12 ppb
2019-12-03S9199150.670 ppb
2019-12-03S9199113.22 ppb