The US national Primary Drinking Water Regulations establish standards for water purity that apply to public water systems.*
The standards define a permitted "maximum contaminant level" (MCL) for various minerals, chemicals and other pollutants that has been arrived at by weighing health risks, expected exposure, technical feasibility of treatment, and other cost-benefit analyses. The standards are reviewed periodically.
The private wells of homeowners are not legally subject to these standards. However, they do offer homeowners a reference for deciding if their water is safe. (Want a different reference? Here are the European Union's Standards for Drinking Water)
In addition to the primary standards, the EPA has established national Secondary Drinking Water Regulations that set non-mandatory water quality standards for 15 contaminants. The EPA does not enforce these "secondary maximum contaminant levels" (SMCLs). They are established as guidelines to assist public water systems in managing their drinking water for aesthetic considerations, such as taste, color, and odor. They are sometimes referred to as "nuisance" impurities.
*Defined here: What is a public water system? --Maine Drinking Water Program