Initial tests showed levels of tritium in groundwater under the retired reactor exceeds amounts recommended by the Environmental Protection Agency for drinking water, but were lower than the maximum annual leakage the Nuclear Regulatory Commission allows for nuclear power plants.

Tritium is a byproduct of the nuclear fission that produces electricity. In high levels it can cause cancer or birth defects.

"There is no danger to public safety," said Victor Dricks, the commission's spokesman.

San Onofre's spokesman, Ray Golde, said the leak could have started years ago, but noted that the nearest well for drinking water is two miles uphill at Camp Pendleton.

Golde said that if the tritium washes into the ocean, it would be diluted to levels safe for ocean users and aquatic life.

The reactor was retired in 1992. Crews demolishing it discovered the leak last week, and the commission was notified Monday.

San Onofre officials said if additional tests confirm the concentration of leaked tritium meets the commission's standards, they would release the contaminated groundwater 11/2 miles offshore.