WASHINGTON — Millions of gallons of potentially hazardous chemicals and known carcinogens were injected into wells by leading oil and gas service companies from 2005 to 2009, a report by three House Democrats said Saturday. The report said 29 of the chemicals injected were known or suspected human carcinogens. They either were regulated under the Safe Drinking Water Act as risks to human health or listed as hazardous air pollutants under the Clean Air Act.