Friday, January 31, 2014

Hanford’s Big Dig

By Ginger Wireman, community outreach & environmental education specialist, Nuclear Waste Program

Hexavalent chromium leaves a trail on the surface of the dig.
These pictures(above) show the green trail of hexavalent chromium in the bottom of the excavation. The soil and gravel are saturated. This is the top of the water table. (Photo taken Dec. 18, 2014)

This photo was taken January 27. The white dot is a barrel that blew into the pit during a recent windstorm. For scale it is about 10’ in diameter. Materials at the bottom of the pit are being sampled to see how much chromium remains and to help establish a path forward.
Sometimes chasing waste at Hanford is like digging to China. One big challenge for us is trying to trace historic hexavalent chromium contamination. It’s not just child’s play. This is serious business.

Chromium-6, or hexavalent chromium, was discovered in the soil around Hanford’s D Reactor in April 2008. Chromium was used as a corrosion inhibitor for eight of the nine reactors along the Columbia River that produced nuclear weapons materials at Hanford. Hexavalent chromium is a carcinogen, and can be easily transported into human cells. It also may cause damage to aquatic ecosystems. Limiting the amount that enters the Columbia River is critical. The telltale sign of yellowish-green-stained soil that identifies chromium contamination luckily makes chasing this plume much easier than following other soil contamination. It leaves its own trail.

Hanford’s “pump and treat” system

Across Hanford’s 100-D Area, a pump and treat system is being used to remove contamination from groundwater. In pump and treat systems, contaminated groundwater is extracted and run through chemical resins to remove contaminants, and clean water comes out the other side. In this case, the clean water is then injected back into the ground outside the contaminated area. It then flows back through the chromium-contaminated portion of the aquifer, and the contaminated water is again extracted and sent to the pump and treat facility. This is known as active treatment, because water is actively directed through the contaminated zone to push the contamination towards the well and shrink the area of contamination.

In addition to pump and treat, workers dig and remove contaminated soil at Hanford. But at 85 feet deep, the three-acre excavation at 100-D hit groundwater. Groundwater in this spot is directly connected to the Columbia River. In fact, there is so much movement between the Columbia River channel and the adjacent groundwater, that monitoring results show wide variations in near shore wells throughout the year. Less contamination is detected when the river is high because of the dilution from the river mixing with groundwater. As the river drops in fall and winter, the recharge from the river to groundwater decreases, and water quality in near shore wells worsens because it’s not going out to the river. This gives us an opportunity to look at cleanup progress in this area.

New testing begins

Recently, the U.S. Department of Energy (USDOE) contractor shut off the injection wells and halted digging to see if the groundwater that has come into the excavated area drops. If the water drops, excavation will potentially continue deeper into the aquifer. If it doesn’t, the plan is to take 40 soil samples from the saturated bottom of the excavated area to try to locate the highest source of contamination. The soil sampling study will help us better understand the high concentrations of hexavalent chromium in the aquifer and its soils. Hopefully we can determine if it is a persistent source or not and its future impacts on the current pump and treat system.

Overall, Ecology is pleased with the efforts of USDOE and its contractors to reduce the high concentrations of hexavalent chromium at Hanford. The work being done at the 100-D area is helping ensure that progress continues to protect the Columbia River from this toxic threat.

Note the proximity to the Columbia River, which helps to explain the influence river flow has on the groundwater here. The circle shows the excavation pit. Note the tiny white dot – the barrel of the previous picture.

The gray pads are the clean materials that came from the sides of the excavation. Chrome contaminated soil was packaged and moved to the Environmental Restoration Disposal Facility, a lined landfill in Central Hanford.

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