The U.S. Environmental Protection Agency has contaminant-specific fact sheets for many drinking water contaminants on their Webpage:
Drinking Water and Health – What you need to know!
Click on “What are the health effects of contaminants in drinking water?” This page also addresses the standards for levels of contaminants in drinking water under the heading Drinking Water Standards Program.
These drinking water standards were developed by the EPA after considering information on the occurrence and distribution of the contaminant, the health effects of the contaminant at various concentrations, and the economic costs for treatment to remove the contaminant.
More information can be obtained from the EPA Office of Ground Water and Drinking Water.