Forty-eight rail cars carrying contaminated soil from the former Maine Yankee nuclear power plant to a landfill in Utah are on their way back to Wiscasset, following the discovery that previous shipments contained water and, in a few cases, were leaking, officials said.

The low-level radioactive waste is not considered a significant environmental threat by regulators who are monitoring the cleanup. But the landfill's owner, Envirocare of Utah, cannot bury soil that is too wet. The disruption could delay further Maine Yankee's removal and cleanup process.

"Envirocare has placed a stop shipment order on Maine Yankee until we resolve the issue," said Eric Howes, a spokesman for Maine Yankee.

The rail cars are expected to arrive back in Maine from various locations over the next several weeks, Howes said. Workers will check to make sure the soil is dry and the containers are sealed before reshipping them to Envirocare, one of the nation's only landfills for low-level radioactive waste.

Maine Yankee hopes to resume shipments soon, but the problem could affect the schedule for finishing the cleanup, Howes said. Maine Yankee had planned to complete the $500 million job in February, and now is working to finish the cleanup by the end of May because of unrelated delays. All of the buildings have been removed. The cleanup is now focused primarily on soils on the property.

The soil being loaded into rail cars contains small amounts of radioactivity, and any water leaking out could be slightly contaminated, too, officials said. The material is considered far less dangerous to the environment and public health than the nuclear fuel rods stored at the site or the building materials and other high-level wastes that have already been removed, according to officials monitoring the cleanup process.

"It would clearly be of a very low magnitude that would be very difficult to measure," said Ron Bellamy, regional chief of decommissionings for the U.S. Nuclear Regulatory Commission.

He said there are no federal rules for transporting low-level waste, but shippers show an abundance of caution.

The moisture appears to be the result of condensation created as the soil warms up inside the rail cars, Bellamy said. An NRC inspector has been aware of the problem and will return next week to check on the rail shipments and other activities at Maine Yankee.

"They are in the process, as we speak, of confirming why it's occurring and determining whether any steps are warranted," Bellamy said.

After loaded containers were found to be leaking at Maine Yankee last month, the company resealed many of its rail cars, Howes said.

Other cars, however, were dripping last week when they arrived at the landfill in Clive, Utah, and one load was wet enough to prompt the stop order, Howes said.

Tim Barney of Envirocare said Maine Yankee's shipments don't pose a significant risk and have generally been secure. "It's been one of our more successful projects," he said.

Barney said soil accepted at the facility must be dry to avoid draining that could cause radioactivity to concentrate in one area. Envirocare periodically stops shipments that might be too wet because it runs out of space in an area used for drying soil, he said.

It's unclear how soon Maine Yankee can resume shipments. The company hopes that the soil in the returning cars is not too moist, or that the addition of material that absorbs moisture will make it acceptable to Envirocare.

The decommissioning project is now a few months behind schedule because of the wet winter and because workers have found traces of radioactivity in more soil than expected, according to Howes. "Until you start digging, you don't know how deep it goes."

Ray Shadis, a longtime Maine Yankee watchdog who has kept a close eye on the cleanup, said leaking shipments of soil are a potential problem, if not a significant one.

"It is the least contaminated of all the materials they are shipping off-site," he said. "Just because it's in low concentration doesn't mean it isn't a matter for environmental concern."

Shadis is more frustrated with the cleanup overall, and said Maine Yankee is now cutting corners to speed up the process. He criticized a decision to leave behind radioactive piping that will be buried in place, for example.

"At the end of the day, the question is whether they are leaving excess radioactive material," he said.

Howes said that Maine Yankee is removing everything that exceeds contamination levels agreed to as part of its decommissioning license, and that those levels are below federal requirements.

Bellamy, of the NRC, said the residual radioactivity at the site will amount to about one dental X-ray per year.

"All of the material left on site has been evaluated . . . and it has been determined that what they are going to leave in site is acceptable."

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