Workers have dug so deeply into contaminated soil near Hanford's former C Reactor that they have stopped just shy of ground water.
The deep dig is a new approach to contaminant cleanup in the area along the Columbia River, where it's more common to dig up soil to about 15 feet deep and occasionally as deep as about 35 feet.
But the dig sites where hexavalent chromium was spilled near C Reactor go down 85 feet, which is about one foot above ground water, and they cover the area of about 15 football fields.
They will be backfilled with clean soil, but in the meantime they look like pit mines. Their tiered sides, with safety benches about every 20 feet to catch loose rocks and service ramps to get equipment to the bottom of the holes, were designed with the help of mining engineers.
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The goal was to clean up soil contaminated with hexavalent chromium before more of the chromium could be spread to the ground water beneath it and then travel to the Columbia River less than one mile away.
"It's a much better alternative than pump and treat," the system Hanford uses to pump up and clean contaminated ground water, said Cameron Hardy, Department of Energy spokesman. It should be less expensive than removing chromium contamination from ground water with a long-running treatment system, he said.
Work has been completed to excavate two dig sites planned when a $5.3 million subcontract was awarded in late 2010 for the excavation to Sage Tec, which teamed with another Richland business, Federal Engineers and Constructors.
The plan was to dig as deep as ground water, if needed, but the hope was that workers would get to the bottom of the chromium contamination before that.
"We got to ground water," said Mark Buckmaster, Washington Closure Hanford project manager.
Workers also uncovered larger quantities and higher concentrations of hexavalent chromium than expected, said Laura Buelow, a scientist with the Environmental Protection Agency, a regulator on the project.
A plume of contamination extending from the digs to the west also was discovered. Work is under way to relocate a high-voltage Bonneville Power Administration line to allow the additional plume to be dug up, Buckmaster said.
The contamination from the largest of the two dig sites is believed to have come from a 1966 spill of sodium dichromate when it was transferred from one tank to another and overflowed.
The source of the chromium at the other contamination site is uncertain.
But chromium contamination is common near many of the reactors that lined the Columbia River at Hanford to produce weapons plutonium during World War II and the Cold War. The chromium was added to water in the reactors to prevent corrosion.
Before work started near C Reactor, which is near Hanford's historic B Reactor, bore holes were used to map the contamination. Then clean concrete rubble from facility foundations in the area were hauled off to be used as fill material at U Plant, a former processing facility in central Hanford.
About 630 tons of scrap metal, including piping, rebar, grating and structural steal, also were recovered. Because there was no radiological contamination in the area, the metal was surveyed and recycled.
In total, 2.3 million tons of soil, concrete and scrap metal were removed. That included significant amounts of clean soil as wide holes were dug to get to the contaminated soil.