EWG's Tap Water Database



Ark Park

Selenium is an essential element in diets. But too much selenium can decrease thyroid hormone production and cause hair loss, skin lesions and brittle fingernails.





Samples exceeding legal limit (MCL)


Samples exceeding
health guidelines

Testing results - average by year

YearAverage resultSamples takenDetectionsRange of results
201227.0 ppb1127.0 ppb
201358.7 ppb1158.7 ppb
20155.30 ppb115.30 ppb

ppb = parts per billion.

State and national drinking water standards and health guidelines

Health Guideline 30 ppb

The health guideline of 30 ppb for selenium was defined by the California Office of Environmental Health Hazard Assessment as a public health goal, the level of a drinking water contaminant that does not pose a significant health risk. This health guideline protects against hair loss and nail damage.

ppb = parts per billion.

EPA Maximum Contaminant
Level (MCL) 50 ppb

The enforceable federal standard that defines the highest level of a contaminant allowed in drinking water.

ppb = parts per billion.

EPA Maximum Contaminant
Level Goal (MCLG)50.0 ppb

A non-enforceable federal health guideline. For cancer-causing chemicals, these health goals are typically set to zero.

ppb = parts per billion.

All test results

Date Lab ID Result
2012-06-1212062124-00127.0 ppb
2013-06-1013061299-00158.7 ppb
2015-06-08S5F1182-015.30 ppb