CARLSBAD -- Small concentrations of tritium found last week in groundwater under the decommissioned Unit 1 reactor at San Onofre pose no threat to human safety or sea life, according to the plant's owners and the federal Nuclear Regulatory Commission.

"We do not see a threat to public health and safety because of the low level of concentration, but our concern is that it's in a place where it shouldn't be," said Victor Dricks, a spokesman for the commission, which regulates and monitors the nation's nuclear power plants.

Workers discovered higher-than-normal concentrations of tritium, a radioactive hydrogen isotope, on Aug. 7 while dismantling the containment building that housed the reactor, said Ray Golden, a spokesman with Southern California Edison. The reactor operated from 1968 to 1992. The utility owns 80 percent of the reactor and SDG&E owns the remaining 20 percent.

"They're looking for sources, but we're pretty sure it's from the reactor since tritium is a byproduct of nuclear fission," said Drick, adding that federal inspectors stationed at the plant are monitoring the demolition process.

Tritium occurs naturally in the environment and is a byproduct of nuclear reactions. It is used commercially in products such as luminous dials and exit signs.

As part of decommissioning work, workers drilled holes through the concrete floor of the building to test for radioactivity under the foundation, Golden said.

Two water samples taken on different days last week from two different locations under the plant showed concentrations of 50,000 picocuries and 330,000 picocuries per liter, Golden said. Radiation exposure during a chest X-ray is about 50,000 picocuries. Humans are exposed each year to about 1.8 million picocuries from man-made and naturally occurring radioactive sources, according to Golden's figures.

The Environmental Protection Agency, which monitors water quality, allows water with anything less than 20,000 picocuries per liter to be called drinking water, Golden said. California has a drinking water public health goal for less than 400 picocuries per liter, but the state regulation is 20,000 picocuries per liter.

"It's not a surprise that as we dismantle this plant that we've found radioactive materials," Golden said. "Our job is to measure those levels and have programs and take actions to remove them to licensed disposal sites."

About 65 percent of the plant has been dismantled, including the reactor itself, as well as all piping and some concrete.

"It's not a leak," Golden said, "because there's nothing left to leak from."

The water under the reactor is naturally occurring groundwater, Golden said, and may have been under the plant for years.

The low-level concentrations fall below the reporting and disposal requirements of the Nuclear Regulatory Commission. However, the company has released about 10,000 gallons of the tritium-laced groundwater into the Pacific Ocean via a 1.5-mile outflow pipe, according to Golden. He said the tritium water was diluted with 1.6 million gallons of piped-in seawater before being released for further dilution in the open ocean. The plant sits about 200 yards from the ocean.

Similar releases from the two remaining reactors are common and are noted in reports to the commission, Dricks said.

Golden said there is no chance the radioactive groundwater might migrate into the drinking supply. The nearest residential development is two miles away uphill.

- Contact staff writer Philip K. Ireland at (760) 901-4043 or pireland@nctimes.com.