Federal and state officials are alarmed that potential cancer-causing chemicals discharged by a pharmaceutical manufacturer are endangering the drinking water consumed by at least 10 percent of Iowa's population.

A new and previously undisclosed Environmental Protection Agency study reveals that 24 "priority pollutants" are entering the Cedar River watershed at Charles City, site of Salsbury Laboratoriess a manufacturer of veterinary pharmaceuticals.

Priority pollutants are described by EPA as either toxic, carcinogenic or mutagenic, meaning they cause cellular change. One of the chemicals - orthonitroaniline, or ONA - has been found 65 miles downstream in shallow wells that provide drinking water for the city of Waterloo, the report said.

EPA officials emphasized, however, that there is no immediate health threat.

Larry Crane, executive director of the Iowa Department of Environmental Quality, said the Cedar River watershed provide drinking water for some 300,000 to 500,000 Iowa residents. The state's population is approximately 3 million.

On the basis of the new finding, Crane said his agency will take action this week to require the cities of Plainfield and Janesville to install charcoal filters to purify their contaminated drinking water.

The city of Waterloo, he said, also will be required to charcoal-filter the water from its shallow wells.In addition, Crane said, owners of some private wells may be required to filter their drinking water.

No scientific study to date has linked the pollutants to cases of cancer involving Iowa residents, he said, but a preliminary study showed that people living in Blackhawk and Floyd counties, where Waterloo and Charles City are located respectively, have "elevated levels" of bladder cancer compared to persons living in similar regions.

"We can't make a definite link between the two," he said. "But there is some reason for concern."