State law requires public water systems to test for many contaminants, including nitrate, on a regular basis. Public water systems with nitrate levels over 10 mg/L must notify people who receive water from them.
If you own a private well, you are responsible for testing your own water. Certified water labs can provide information and instructions for getting your well water tested for nitrate. For a list of certified labs in Yakima County, visit: http://www.wukongcoo1.com/344/Drinking-Water-Testing.