Weapons Complex Monitor - Workers Dig Deep in C Reactor Cleanup
Weapons Complex Monitor - April 6, 2012
Workers have dug up contaminated soil down to just shy of groundwater near Hanford's C Reactor in a new approach to cleanup. It's common to dig up contaminated soil to about 15 feet deep and occasionally as deep as about 35 feet in the river corridor. But the deep dig sites where hexavalent chromium was spilled near C Reactor go down to 85 feet, which is about one foot above groundwater, and they cover the area of about 15 football fields. They will be back filled with clean soil, but in the meantime they look like pit mines. Their tiered sides, which safety benches about every 20 feet to catch loose rocks, and road-sized service ramps to get equipment to the bottom of the holes were designed with the help of mining engineers. The goal was to clean up soil contaminatet with hexavalent chromium before more the the chromium could be spread to the groundwater beneath it and then travel to the Columbia River less than one mile away. "It's a much better alternative than pump and treat," said Cameron Hardy, Department of Energy spokesman. It should be less expensive and more efficient than removing chromium contamination from groundwater with a long-running treatment system, he said.
Work has been completed to excavate two dig sites, collectively called 100-C-7, as planned when a $5.3 million subcontract was awareded in late 2010 for the excavation to Sage Tec, which teamed with Federal Engineers and Constructors. The plan was to dig as deep as groundwarter, if needed, but the hope was that workers would get to the bottom of the chromium contamination before that. "We got to groundwater," 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 plume of contamination extending from the digs to the west was also discovered.
More Than 2M Tons of Material Removed
Before work started near C Reactor, bore holes were used to map the contamination. Then clean concrete rubble from facility foundations in the area was hauled off to be used as fill material at U Plant, a former processing canyon in central Hanford. About 630 tons of scrap metal including piping, rebar, grating and structural steel, also were recovered. Since 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. About 650,000 tons of excavated soil were found to be contaminated and were shipped to the Environmental Restoration Disposal Facility in central Hanford. About a tenth of that had enough contamination that it is required to be mixed with chemicals and contained in grout before it can be disposed of there.
During peak operations, about 5,000 banked cubic meters of soil a day were dug up and stockpiled. Work to haul contaminated soil to the landfill will continue until mid-summer. The soil that is not contaminated is heaped in massive piles around the dig sites. Work should state this fall to fill in the holes with that soil and additional soil brought from a borrow pit to the west to meet requirements that the waste sites will be returned to a natural condition. Native vegetation also will be planted on the sites.
Chromium Not as High as at Other Reactors
The 100-C-7 waste site was picked for a deep dig in part because chromium contamination levels near C Reactor are not nearly has high as those near some other reactors, including D, H and K reactors. Groundwater near those reactors already is being treated to remove chromium. But based on the amount of chrumium contamination found in the soil at the deep digs, it's surprisng that groundwater near C Reactor hasn't had higher levels of contamination, Buelow said. "We hope we had really good timing and were able to dig out the mass of hexavalent chromium before it became a bigger problem for groundwater," she said.
A decision still must be made on how to clean up groundwater that is contaminated there. The groundwater has about 140 parts per billion at the highest concentration, said Geoff Tyree, DOE spokesman. The federal drinking water standard is 100 parts per billion. But because chromium is particularly toxic to young salmon and other aquatic life, the goal is to reduce groundwater contamination to about 20 parts per billion, which would dilute to about half that by the time it reaches the river. With the main excavation near C Reactor complete, Washington Closure is looking at the potential to use similar deep digs elsewhere near the Columbia River, possibly near the D and DR reactors.