In line with the Safe Drinking Water Act (SDWA), the Environmental Protection Agency publishes a contaminants list. Also the SDWA directs the EPA to choose five or more  contaminants from their current list to determine whether or not they will regulate these contaminants with a National Primary Drinking Water Regulation.
In order for the EPA to regulate a contaminant the SDWA requires that they determine if this contaminant meets three criterion. For this current contaminant list:
EPA is announcing the final determinations not to regulate four of the 116 CCL 3 contaminants – dimethoate, 1,3-dinitrobenzene, terbufos, and terbufos sulfone. The agency is delaying the final regulatory determination on strontium to consider additional data and decide whether there is a meaningful opportunity for health risk reduction by regulating strontium in drinking water.

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