Depending on how prevalent the contaminants are and at what levels they are found, EPA may conduct an evaluation to determine whether to begin regulating some or all of them.
In many cases, utilities will be testing for these contaminants at very low levels. That does not mean those contaminants have been determined to be harmful at those levels. EPA sets these testing levels based on the capabilities of current analytical methods and the agency's need to identify reference concentrations, so that they can offer context when they are making health-based regulatory decisions.