The largest source of chromium contamination near the Columbia River at Hanford has been removed after workers dug up contaminated soil down to groundwater 85 feet deep.
Over the years Hanford officials had talked about finding the mother lode of chromium in the soil that was contaminating groundwater near the former D and DR Reactors and then the river, said Mark French, Department of Energy project director for the work.
"Once we started digging, it became obvious this was it," he said. It was the largest source of chromium contamination near the Columbia River, he said.
Two new groundwater treatment systems have been built near the horn of the Columbia River as it passes through Hanford, one near the D and DR Reactors and another nearby close to the H Reactor.
Together they can treat 50 million gallons of contaminated water a month, replacing a smaller system that began operating in 1997. But the level of contamination was not dropping because the groundwater was being recontaminated with chromium in the soil, said Dwayne Crumpler, a hydrogeologist for the Washington State Department of Ecology.
Now some initial tests show promise that the level of contamination in the groundwater is starting to go down, he said.
"We've got the source," he said.
Sodium dichromate, which was added as a corrosion inhibitor to river waster used to cool Hanford reactors that produced weapons plutonium, was brought in by railcar in large quantities and then diluted for use in the reactors. It leaked from pipes or spilled to contaminate the soil.
The form of chromium contaminating Hanford groundwater can cause cancer in humans and is particularly toxic to fish and other aquatic life, including salmon fry from spawning areas in the river near the D and H reactors.
To remove the contaminated soil, a Washington Closure Hanford team dug down to groundwater at three places near the D and DR Reactors. Two of the dig sites would merge into one near the D Reactor, but the other dig site is the largest.
Enough soil was excavated there to create a hole covering the area of about seven and a half football fields at the ground's surface and about one football field at the bottom.
Because of its size, the hole had to be engineered like an open pit mine.
It was designed with gently sloped sides at the top to prevent cave ins, giving way to steeper slopes about halfway down its 85 foot depth. It was built in lifts or layers of 15 to 18 feet, each with a safety shelf to catch any falling rocks.
This is the second time that Washington Closure has dug up chromium-contaminated soil down to groundwater 85 feet deep near the river. The strategy was used successfully near Hanford's C Reactor in a dig completed in 2012.
Among the tips picked up from the first deep dig was the value of collecting samples of soil as the dig was in progress, French said.
In some places the chromium contamination is obvious, marked by a bright green-yellow stain. But soil contaminated with lower levels of chromium may not be discolored.
In addition, workers learned to put down thick pads to be linked together to keep truck tires from becoming contaminated.
Work was carefully managed, with trucks coming and going up and down ramps into the hole, to do the work safely, French said.
About 785,000 cubic yards of soil was removed at that largest dig site and about half as much was removed from the smaller site.
About a third of that soil was contaminated and was disposed of at the Environmental Restoration Disposal Facility, a lined landfill in central Hanford. The most heavily contaminated soil was mixed with cement to contain the chromium before it was added to the landfill.
Some of the clean soil removed from the holes was used to backfill a nearby area excavated as part of earlier environmental cleanup work. But the rest of the clean soil has been piled around the holes, said Dean Strom, the Washington Closure project manager overseeing the work.
Digging has stopped, but Hanford officials are waiting for testing of samples to be completed to confirm that chromium contamination is gone.
Then work to backfill the holes will begin, likely in October, Strom said. Refilling the holes is expected to take about eight months. Then the surface of the ground will be replanted, likely starting in late 2015.
Removing the chromium contamination from the soil was significant, in part, because contamination was already reaching the river, French said. Longer term it will reduce the time that groundwater treatment systems need to be operated, he said.
-- Annette Cary: 509-582-1533; email@example.com; Twitter: @HanfordNews