Part 2
In Part 1 we covered the basics of the Pacific Northwest National Laboratory report, Adaptive Site Management Strategies for the Hanford Central Plateau Groundwater, that outlines an innovative strategy to tackle the challenge of groundwater cleanup. In Part 2 we’ll cover the history of Hanford's soil and groundwater contamination, current cleanup strategies, and the various challenges to cleaning up the soil and groundwater.
Hanford's history of soil contamination
The Hanford Site has a history of dumping radioactive and chemical waste directly into the ground on site. About 450 billion gallons of nuclear and chemical waste were dumped directly into the soil during the plutonium production years—the equivalent of more than 680,000 Olympic-size swimming pools. Manhattan Project workers dumped waste in unlined cribs, ponds, ditches, and trenches—four different types of holes in the ground used for disposing of waste. Injection wells pumped the toxic waste directly into the soil to dispose of it.
Workers constructed 177 underground tanks (149 single-shell tanks and 28 double-shell tanks) to hold the most dangerous, high-level waste. However, the soil contamination didn't stop there. These enormous underground tanks were connected in a row of three or four tanks. The Manhattan Project workers used a process called cascading—which allowed them to fill up one tank with waste, and while the waste solids settled to the bottom, the liquids would flow from one tank to another. If too much waste was added to the final tank, it would overflow to the soil. "From 1944 through the late 1980s, Hanford generated nearly 2 million cubic meters (525 million gallons) of high-level tank waste. Liquid evaporation, discharge to the ground, chemical treatment, and tank leakage reduced this volume by 90%—to 204,000 cubic meters (54 million gallons)."[1]
Cascading wasn't the only way that waste reached the soil from the tanks. The tank farms were backfilled under an 8-to-10-foot layer of soil before waste was added. Workers built the single-shell tanks between 1943 and 1964. As their name suggests, they only have one liner of carbon steel to contain the waste. Sixty-seven single-shell tanks are known or suspected to have leaked 1 million gallons of waste into the surrounding soil. Two single-shell tanks—B-109 and T-111—are currently leaking. The single-shell tanks were designed to contain the waste for 20-25 years, and they are now more than 40 years past their design life. As these tanks get older and older, they are more likely to fail—causing the waste to leak out into the soil. Once the waste gets into the soil it may remain there—making it very hard to remove—or it may travel with water through the soil and reach the groundwater.
Diagram showing the different sources of contamination and how the contaminants flow underground, Photo Credit: Columbia Institute
Current cleanup of the groundwater
Today, the soil at the Hanford Site (particularly in the Central Plateau) remains heavily contaminated. Some radioactive and chemical contaminants are more mobile in water, which means a rainstorm may cause those contaminants to move with the water through the soil—reaching the groundwater and ultimately the Columbia River.
One of the cleanup methods to prevent contaminants from spreading and reaching the groundwater is to remove contaminated soil by digging it up and disposing of it in the Environmental Restoration Disposal Facility. Hanford Challenge is concerned that USDOE will decide that it doesn't need to dig up all of the contaminated soil and will leave it in place—which would increase the risk of harm to future generations.
USDOE implements specific strategies for cleaning up the groundwater. One of those strategies is pump and treat. Pump and treat is the process of pumping contaminated water to the surface, filtering out some of the contaminants, and injecting the water back into the ground. Monitoring wells, extraction wells, and injection wells are interspersed throughout the Hanford Site to implement the pump and treat process. There are six pump and treat facilities on site.
Soil flushing is one strategy used to enhance the pump and treat process. Some contaminants remain in the soil and may take a long time to reach the groundwater. Until the contaminants hit the groundwater, they are impossible to capture with the pump and treat system. Soil flushing speeds up the process by using 225 gallons of water per minute to force—or flush—these hard-to-reach contaminants down to the groundwater where they can be brought up to the surface with the pump and treat system. USDOE has found success using soil flushing to push hexavalent chromium to the groundwater to treat it.
An additional strategy for meeting water quality standards is monitored natural attenuation. Contamination is left to naturally attenuate, which means letting the radiation decay over time. It sounds like a do-nothing approach, and it basically is.
Challenges to groundwater cleanup
USDOE faces many challenges when pursuing groundwater cleanup. As previously mentioned, there are hundreds of contaminated soil sites at Hanford due to past dumping practices and leaking underground tanks. The extent of groundwater contamination is vast.
There are significant data gaps regarding the number of contaminants in the vadose zone (the area of soil between the ground surface and the water table), the depth and location of the contamination, and the risk the contamination poses to groundwater.
Some hard-to-control, persistent contaminants, such as technetium-99, iodine-129, uranium, nitrate, and chromium, are located in the deep vadose zone and pose a long-term risk to the groundwater.
There are extensive groundwater plumes with intermixed contaminants (or contaminants located together), making it difficult to accurately measure the total amount in the aquifer and the contaminant distribution.
Depending on the contaminant, one specific treatment may work better than another. When contaminants are intermixed, the treatment process becomes more complex. Multiple technologies used in tandem or various treatment methods may need to be used to effectively treat intermixed contaminants.
The soil underneath the tank farms is contaminated by tank leaks, accidental spills, and intentional releases, which creates an additional pathway for contaminants in the soil to reach groundwater. As tanks leak—potentially more frequently—they become an additional complexity in groundwater cleanup.
A borehole is a circular hole drilled into soil or rock that draws samples from deep below ground. USDOE uses boreholes to characterize, or identify, the physical and chemical properties of the contaminants in the vadose zone. Unfortunately, deep borehole characterization is limited in certain areas due to the high price of drilling—contributing to the lack of information regarding the amount, location, and strength of contaminants in the soil.
Geological challenges to groundwater cleanup
Hanford's geology poses unique challenges to groundwater cleanup. Manhattan Project managers chose the site partially for its geology and proximity to the Columbia River. The reprocessing facilities were sited in certain areas at Hanford because the gravelly soil allowed them to dump waste into the ground, where it percolated down and vanished without a trace. It was a handy way of disposing of the waste—it just disappeared—but the dumped waste now requires a complicated cleanup strategy.
The 200 Area in the Central Plateau contains a high hydraulic conductivity zone that consists of porous soils and rocks that allow contaminants to quickly move through the soil to groundwater and eventually to the Columbia River. USDOE doesn't know the exact size and location of the hydraulic conductivity zone in the 200 Area, which means that the underground movement of liquids between the Central Plateau and the Columbia River is still an area of considerable uncertainty. On the other hand, some places at Hanford's Central Plateau have less permeable soils that trap specific contaminants, making it difficult to separate the contaminants from the soil and treat them using the most common cleanup strategies.
Ancient lake beds are hidden underneath the surface and cause contaminants to move laterally (horizontally) instead of vertically down to the groundwater. Lake beds cause contaminants to take longer to reach the groundwater because they aren't taking the most direct route straight down, and are instead moving sideways. USDOE uses models to predict when specific contaminants will reach groundwater. USDOE bases its models on the assumption that contaminants move vertically to the groundwater. However, ancient lake beds and the lateral flow of contaminants challenge that assumption and highlight the need for USDOE to update its models to better account for the geologic conditions underneath the site.
Perched water also complicates groundwater cleanup. Imagine a bird's nest that is perched or sitting in a tree. Now, imagine that bird's nest perched in a tree underground and filled with water. As contaminants move through the soil they can get caught and trapped in that underground bird's nest. The underground nest creates a pocket of contaminants that is hidden and hard to reach. USDOE is aware of several contaminated perched water areas at Hanford, but lacks information about the size, what contaminants they hold, and how full the perched water areas are. USDOE must incorporate perched water areas into its strategies to ensure a comprehensive cleanup plan for groundwater.
Groundwater cleanup at Hanford is incredibly complex due to the history of waste disposal, the inherently dangerous nature of the contaminants, and the challenges created by the site's geology. Hanford Challenge urges USDOE to update its groundwater models to include the intricacies of Hanford's geology, such as ancient lake beds and perched water. Hanford Challenge also encourages USDOE to recognize, investigate, and resolve the uncertainties present in groundwater cleanup.
If you are interested in learning more about Hanford's geology, check out Tim Connor's presentation on the cataclysmic floods that shaped the Hanford Site and Vince Panesko's presentation on the ancient lake beds that impact cleanup.
This blog post is funded through a Public Participation Grant from the Washington State Department of Ecology. The content was reviewed for grant consistency, but is not necessarily endorsed by the agency.
[1] Gephart, Roy. E. (2003). A Short History of Hanford Waste Generation, Storage, and Release. Pacific Northwest National Laboratory, PNNL-13605.