Federal/EPA
Do you want a VERY comprehensive overview? See the EPA's Ground Water and Drinking Water Website.
http://www.epa.gov/safewater/

Municipal Water
Do you have municipal water? Review the EPA's Safe Drinking Water reports for your local water company.
http://www.epa.gov/enviro/html/sdwis/sdwis_query.html#geography

Ground Water
Do you want in-depth and up-to-the minute information about Ground Water? See the National Ground Water Association Web Site.
http://www.ngwa.org/

EPA Radon Facts
Get the complete facts about Radon in water and air from the EPA.
http://www.epa.gov/iaq/radon/