A fire has been smoldering underground at an old Pasco garbage dump since November 2013. The place is now a federal Superfund site, and the state’s going to try a more aggressive firefighting plan.

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Officials are ready to try a more aggressive plan to extinguish a fire that’s been smoldering underground at the Pasco Sanitary Landfill since November 2013.

The fire will be circled with underground walls and then the smoldering trash will be mixed with grout, according to the Washington State Department of Ecology.

The 250-acre landfill just east of the city limits is a federal Superfund site. It no longer accepts waste.

The fire is burning in an area of the landfill where bales of household and other municipal waste and construction debris were buried until 1989. The debris has been found to also include some smoldering shredded tires, making the fire more difficult to put out.

That area of the landfill is adjacent to an area of greater concern, where an estimated 35,000 drums of solvent and paint sludges, cleaners and other waste were buried.

Water has not been considered to extinguish the fire because it could carry contamination down through the soil and pollute the groundwater.

Instead, in an attempt to smother the fire, contractors have three times placed soil on areas where the ground had settled, and cracks and smoke had appeared. The work done in December 2013, April 2014 and May 2014 did not extinguish the fire.

Next, the coalition of businesses and governments potentially liable for cleanup were required by the state Department of Ecology to inject liquid carbon dioxide under pressure into the ground to cool the waste and displace the oxygen that has been feeding the fire.

The approach had some initial success, said Chuck Gruenenfelder, Department of Ecology site manager for the project. Probes in the ground showed the temperature declined.

But the positive results were not uniform. Obstructions in the ground kept the liquid carbon dioxide from spreading throughout the smoldering trash, and the decline in temperature did not last.

The fire remains fairly small, covering an underground area about 25 by 25 feet.

Some temperature monitoring has been done recently, and a more extensive effort is planned over the next month or so to identify where the waste is smoldering.

The information will be used to dig a trench around the perimeter of the fire, Gruenenfelder said. It will be dug in a rectangular shape.

A grout or concrete mix will be poured in the trenches to form walls about 3 feet wide and 35 feet deep, to limit the fire’s movement.

The walls will form a pit where waste can be dug up to 30 to 35 feet and mixed in the ground with concrete and clay to form a slurry with a consistency almost like toothpaste. It should not burn.

A layer of concrete or grout will be used to form a cap over the treated waste, and soil will be placed over that.

“It will be well entombed,” Gruenenfelder said.

Temperature monitoring devices will be installed after the waste is treated, but once the fire is extinguished, it is not expected to reignite.

Because of the more-hazardous waste nearby, additional trenches will be dug and filled with a concrete slurry to better isolate the area from the adjoining area holding hazardous industrial waste.

The potentially liable parties include companies such as du Pont, Boeing, Union Oil of California and Basin Disposal and government entities such as Franklin County and the Air Force.

Work is expected to wrap up before the end of the year.

A study to evaluate cleanup options for the entire landfill is expected to be available for public comment later this year.