The federal Safe Drinking Water Act (SDWA) sets limits on allowable levels for bacteriological and chemical contaminants in drinking water. These limits are called Maximum Contaminant Levels or MCLs. In order to protect the water quality of each aquifer and in turn the health and safety of the public, the DNR has established certain standards applicable to all new and existing wells. The City of Glendale has adopted those standards in Ordinance Section 8.1.14 which requires periodic inspections and testing of every well in operation in the City.
While there are several contaminants and conditions that may affect water quality including bacteria, nitrates, and minerals to name a few, at minimum, an annual bacteriological test is recommended.
Other tests may be prudent depending on your geographical location and geological and other environmental factors. Wells should also be tested whenever you notice a change in taste, odor, color, or clarity and especially after a flood.