EWG's Tap Water Database — 2021 UPDATE



Roslyn WD

Chlorate forms in drinking water as a byproduct of disinfection. Chlorate impairs thyroid function, making chlorate exposure most harmful during pregnancy and childhood. Read More.

In 2002, California scientists proposed an advisory value, called an action level, for chlorate at 200 parts per billion. Between 2013 and 2015, the EPA collected data through the Unregulated Contaminant Monitoring Rule program and found that over 15 percent of water tests from 38 percent of utilities had chlorate levels above a health-protective concentration, which federal scientists defined as 210 parts per billion, nearly equivalent to the California advisory value.

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
201416.6 ppb83ND - 63.0 ppb
201529.2 ppb64ND - 63.0 ppb

ppb = parts per billion

State and national drinking water standards and health guidelines

EWG Health Guideline 210 ppb

The EWG Health Guideline of 210 ppb for chlorate was defined by the Environmental Protection Agency as a benchmark for testing under the Unregulated Contaminant Monitoring Rule program. This health guideline protects against hormone disruption.

ppb = parts per billion

All test results

Date Lab ID Result
2014-08-1408010 08201463.0 ppb
2014-08-1502400 082014ND
2014-08-1807104 082014ND
2014-08-2007873 082014ND
2014-08-2004265 082014ND
2014-08-2159398 08201448.0 ppb
2014-08-2501870 082014ND
2014-10-0105852 10201421.9 ppb
2015-02-1007104 022015ND
2015-02-1059398 022015ND
2015-02-1008010 02201563.0 ppb
2015-02-1805852 02201537.0 ppb
2015-02-1804265 02201529.0 ppb
2015-02-2302400 02201546.0 ppb