Dump trucks carrying 32 tons each of freshly dug soil rolled through the dirt west of Hanford's D Reactor Tuesday.
By August, they should have carried away enough soil to leave a pit 85 feet deep, stretching over the size of seven and a half football fields.
Hanford officials believe the dirt, which once was beneath a system used to distribute sodium dichromate, is a major source of the contamination reaching the groundwater near the horn of the Columbia River as it crosses Hanford.
The chemical, which was added as a corrosion inhibitor to river water used to cool Hanford reactors, was brought in by railcar in massive quantities and then distributed through a system of piping and fill stations. Over time it leaked and spilled into the soil.
A new water treatment system there is removing chromium from the groundwater to prevent it from reaching the river, where it could harm young fish. Chromium is toxic to aquatic life at very low levels, including levels that meet drinking water standards.
But to keep the chromium in the soil from continually recontaminating the groundwater, the Department of Energy must remove it from the soil.
"Groundwater contamination is really expensive to treat compared to digging," said Cameron Hardy, DOE spokesman.
Washington Closure Hanford and subcontractor TerranearPMC are loading 300 to 450 truckloads a day, said Scott Myers, Washington Closure project manager for field remediation at the D, DR and H reactors. Terranear has a $12.3 million subcontractor to remove 2.5 million tons of material near the three reactors, including chromium-contaminated soil down to groundwater in three places.
Work began in December on the first deep hole near D Reactor and nearby DR Reactor after a 230 kilovolt power line was relocated and the infrastructure for three monitoring wells was removed.
An initial hole already had been dug to learn the extent of the contamination. It showed the green and yellow staining of chromium contamination at 15 feet, again at 30 feet when more digging was done and finally at 50 feet, when work stopped about 35 feet above groundwater with extensive contamination showing on the floor of the dig.
The hole being dug now will have a floor that's expected to be the size of a football field and to be large enough to have clean soil at its edges. But to safely dig a hole that deep, the hole must be much wider at the top.
"Essentially it is an open pit mine," Myers said.
Its engineered design calls for gently sloped sides at the top, with steeper slopes about halfway down, to prevent cave-ins, said Scott Parnell, Washington Closure deputy director of field remediation. It's built in lifts or layers of 15 to 18 feet, each with a safety shelf to catch any falling rocks.
Soil samples are collected and checked along the sides at each elevation, to confirm that soil at the pit's edges are clean before the hole gets deeper and sampling is more dangerous for workers.
Similar work was done to clean up soil contaminated with chromium down to 85 feet near Hanford's C Reactor, and workers picked up tips that will be used again. That includes using thick pads to link together and create roads to keep truck tires from becoming contaminated, said Clint Adamson, Washington Closure subcontracts technical representative for D Area.
About 750,000 tons of clean and contaminated soil will be removed at the pit being dug now. Initially the clean soil is being moved to a nearby shallower site excavated as part of earlier environmental cleanup work. That should save about $300,000 because soil will only have to be handled once.
But eventually, tall piles of clean soil will be made and held until it's time to back fill the pit, Adamson said. Contaminated soil will be hauled to the central Hanford landfill for radioactive or hazardous chemicals, the lined Environmental Restoration Disposal Facility. Soil with the heaviest contamination will have to be mixed with cement to contain the chromium before it is added to the landfill.
A second deep dig will start near the D and DR Reactors in May, where more chromium is expected to contaminate soil down to groundwater.
In addition, soil is expected to be dug up down to groundwater near H Reactor, the other reactor near the horn of the Columbia River, to remove suspected chromium. However, groundwater there is expected to be just 40 to 45 feet deep.
-- Annette Cary: 582-1533; email@example.com; Twitter: @HanfordNews