Hardrock mining companies and electric power plants are the worst toxic-chemical polluters in the U.S., according to EPA Toxics Release Inventory data unveiled yesterday. In all, the EPA found that 7.3 billion pounds of toxic chemicals were released by eight industries in 1998, the most recent year for which data are available. Metal mining accounted for nearly half of the discharges and power plants for about 15 percent. The heaviest polluting power plants were 27 coal-burning facilities in Ohio, which have long been blamed for pollution that drifts into the Northeast and contributes to acid rain and health problems. EPA Administrator Carol Browner talked up the fact that citizens can access the data on the Internet and in other places to learn about toxic pollutants being emitted into their communities.