How often you should test your water depends on your water source. The importance of testing water is greater if your water comes from a groundwater source, such as a well. You should also test more often if you drink from a private source. Public water companies have to send out notices if their water contains potentially harmful contaminants beyond national standards. However, if you drink from a private source, the responsibility to measure contaminants is yours alone. As a rule of thumb, you should test your home’s water at least once a year. Testing should be more frequent if you use a private water source. In addition, you should test your water for nitrates if you are expecting a child and while your infant is young.