Exelon Corp. is distributing bottled water to about 420 residences as it tests drinking-water wells at homes near an underground pipeline that leaked radioactive tritium several times dating to 1996.

Exelon will test private wells in the far southwest suburb of Godley and at other residences near the Braidwood Generating Station in Will County. That's being done at the request of Illinois' attorney general and the Illinois Environmental Protection Agency.

The testing will take about 12 weeks, and the Illinois Department of Public Health will verify the results, the company said.

Exelon has conducted extensive groundwater testing near the Braidwood station to gauge the level of tritium contamination, and company officials said they do not expect the new tests to show additional contamination.

"We have no reason to believe any detectable tritium exists in those residence wells, nor do we suspect it," said Keith Polson, spokesman for the Braidwood station.

Tritium is a radioactive substance commonly found in small concentrations in surface water. Studies have shown long-term exposure can lead to cancer and birth defects.

Since testing programs began in late 2005, Exelon said one private well near the Braidwood station was found to have levels of tritium above the amount that occurs naturally in the environment.

Last week, Attorney General Lisa Madigan and Will County State's Attorney James Glasgow filed a lawsuit accusing Exelon and its ComEd and Exelon Generation Co. subsidiaries of failing to properly maintain the underground pipeline that leaked. Exelon officials maintain that the leaks posed no danger to the surrounding communities.