RICHLAND — The federal government has made the first decision on how to clean up contaminated soil in the heart of Hanford, settling on a combination of methods planned to protect the environment.

The soil, contaminated with plutonium, is within the central 10 square miles of Hanford that will be permanently used for radioactive and hazardous waste disposal. Land use plans call for no one to ever live there and the area will be cleaned up only to industrial standards.

"This decision balances public sentiment for digging deeper with a science-based cleanup action that will protect human health and the environment," said Matt McCormick, manager of the Department of Energy Hanford Richland Operations Office, in a statement. "This decision was based on the anticipated future use of this area of the Hanford Site."

An initial proposal relied heavily on capping waste sites to prevent precipitation from driving contamination deeper underground toward ground water. That plan was modified to dig up more of the waste in response to concerns raised by the Hanford Advisory Board, the state of Oregon and the tribes during initial discussions in 2008.

The cleanup decision made by DOE and the Environmental Protection Agency largely follows the revised proposal they favored this summer before holding a series of public meetings this summer.

However, after hearing comments this summer the agencies did agree to tighten up some of the standards being used.

They agreed to use a more conservative standard to decide what soil required cleanup based on the amount of plutonium it contained. In addition, more plutonium-tainted soil may be removed in areas where the plutonium was mixed with nitric acid, depending on what is found during excavation.

Plutonium typically clings to the soil rather than easily spreading deeper. But some of the plutonium is mixed with nitric acid that can cause it to travel deep into the soil, possibly 100 feet underground.

The final decision calls for a combination of techniques. Some of the contaminated soil will be dug up and treated. More heavily contaminated soil would be sent to the Waste Isolation Pilot Plant in New Mexico. Other soil would be taken to the Environmental Restoration Disposal Facility, a lined landfill for low level radioactive waste near the contaminated soil sites in central Hanford.

In some places, a soil cover still will be maintained over the contaminated soil. In addition, a current vapor extraction system will continue to be used to clean up carbon tetrachloride in the soil.

"Finalizing this decision was challenging because remediating plutonium-contaminated waste sites is a highly charged topic," said Emerald Laija, an EPA scientist.

Plutonium has a longer half life, a measure of how long it takes for half the radioactivity to decay, than some of the other radionuclides found in Hanford waste. Plutonium 239 has a half-life of 24,000 years.

Large volumes of liquid waste were generated when plutonium was produced at Hanford during World War II and the Cold War. The liquids came from plutonium processing plants within the 10 square miles in the center of Hanford called the Central Plateau Inner Area. It is about seven miles from the Columbia River.

Liquids containing low levels of plutonium and other contaminants were disposed directly to underground structures in the soil, such as ditches, contaminating the underlying soil.

The new cleanup decision covers four soil waste sites, the first of which has soil contaminated from cooling water and steam condensate from the Plutonium Finishing Plant complex, where plutonium was shaped into metal buttons the size of hockey pucks for the nation's nuclear weapons program.

The cooling water was sent through a series of shallow open ditches from 1944-95 to a pond. When one ditch would become too contaminated, another one would be dug alongside it. The trenches were about 4 to 6 feet deep, although the system includes a deeper tile field.

That soil will be dug up and likely sent to the central Hanford landfill. Cost is estimated at $58 million.

The remaining three soil waste sites were all used for disposing of processing waste water, including from the Plutonium and Uranium Extraction facility, or PUREX, which was used to extract plutonium from irradiated fuel rods.

For the plutonium contamination there not mixed with nitric acid, workers will dig down as much as 33 feet to remove a significant portion of the contamination. It likely will be taken to New Mexico. An earthen cap to prevent infiltration of water will be constructed over the top. The cost is estimated at $81 million.

For areas with plutonium mixed with nitric acid, Hanford workers will dig down deep enough to get to the disposal structures and then cover the excavated site with about 22 feet of clean soil and a cap. Cost should be about $107 million.

An area with primarily cesium 137, which has a half life of 30 years, will not be dug up, but soil will be added to make sure contamination is covered by at least 15 feet of clean soil. Cost will be $11 million.

In addition, some settling tanks will have sludge and liquid containing plutonium and americium removed and sent to the national repository. The tanks will be grouted in place for a project cost of $40 million. An additional $5 million will be spent to excavate pipelines.

* More Hanford news at hanfordnews.com.