Monument Valley and Rifle: Monitoring Plant-Based Approaches to Milling Site Remediation

The Department of Energy's Office of Legacy Management (DOE-LM) is using cutting-edge technology to give its scientists a virtual look underground at its Monument Valley processing site in Arizona, where a uranium mill once operated.

By tracking the size and movements of a nitrate plume below the site, DOE-LM is assessing the effectiveness of a plant-based treatment plan.

Plants are also key to environmental remediation at a former uranium mill site in Rifle, Colo. There, DOE-LM is planting and seeding vegetation with the aim of reducing the amount of groundwater that reaches a buried disposal cell.

The Monument Valley mill was constructed and operated from 1955 to 1968 by the Vanadium Corporation of America. Before and during the milling operations, the site was leased from the Navajo Nation. When the lease expired in 1968, full control of the site reverted to the Navajo Nation. Even though the site's tailings piles were removed in 1992, contamination lingers in the groundwater, especially in the form of nitrates.

The main source of the nitrates was thought to be the soil under the former mill site. Between 1998 and 2016, DOE teams cleaned these subpile soils using plants in a process called phytoremediation.

The idea was that the plants would cut off the contamination source and the nitrate plume in the groundwater would eventually shrink. The phytoremediation method was highly effective, reducing the nitrogen in those subpile soils by 80 percent.

However, even with the subpile soils mostly cleaned up, the nitrate plume in the groundwater kept growing. It was spreading over a larger area and getting deeper, and LM scientists were surprised to learn that the total nitrate in the plume doubled over a 22-year period.

"This told our team that our original understanding of the site-what we call the conceptual site model, or CSM-needed an update," said LM Monument Valley Site Manager Joni Tallbull. "There had to be another, previously unrecognized, source of nitrate."

Going to the source

To dig into the data, LM used 3D visualization software called Earth Volumetric Studio to create detailed models of the site. This allowed them to "see" the underground layers, the water table, and how the nitrate plume changed over time. This helped them see where the data was lacking and where they needed to investigate further.

They then conducted targeted field investigations, collecting sediment samples from various spots. They analyzed these samples for different chemicals, analyzed the soil constituents, and investigated how quickly contaminants could move through the soil. What they found was a game changer.

"We found discontinuous layers of clay deep in the ground, within the saturated zone where the groundwater is," Tallbull said. "These clay layers were holding a lot of ammonia, which was slowly but surely converting into nitrate, continuously feeding the expanding plume."

LM's analysis showed that through the "nitrification" process, the ammonia stored in the clay layers could actually double the amount of nitrogen in the nitrate plume over several decades.

"This was a huge discovery," Tallbull said. "It shifted the focus from the old subpile soils to these saturated zone clays as the main ongoing source of nitrate."

This project proved how using advanced 3D tools and targeted investigations can help LM better understand complex environmental problems and come up with more effective cleanup plans.

New growth at Colorado's Rifle disposal site

While nitrates were the target of the phytoremediation campaign at Monument Valley, revegetation at another DOE-LM site is focused on keeping water from seeping into a mill tailings disposal cell. Vanadium processing began near the western Colorado town of Rifle back in 1924. Work stopped in 1932, but resumed in 1942, when the mill began processing uranium ore as well as vanadium.

When the Old Rifle mill closed in 1958, the New Rifle mill began uranium processing operations nearby that continued until 1973. In 1988, the state of Colorado acquired both Rifle milling sites from the Union Carbide Corporation; ownership rights are now held by the city of Rifle, and DOE-LM manages the site.

After installing a water extraction system at the mill tailings disposal cell in Rifle in 2024 (Radwaste Solutions, Fall 2025, p. 54), DOE-LM started a project called the Rifle Enhanced Cover Application Pilot Test (RECAP).

The main goal of RECAP is to test more efficient ways to manage water using evapotranspiration (ET)-enhanced covers. Such covers use plants and soil to naturally absorb and release water into the atmosphere, reducing the amount of water that seeps into the disposal cell. This is a more proactive and long-term solution than continuously pumping water out.

RECAP has three key objectives:

Understanding water sources: This involves tracking where the water comes from, such as rain, drainage patterns, and subsurface water flows.

Tailoring solutions: The project will incorporate the specific engineering and environmental conditions of the Rifle site.

Provide efficient strategies: The team is looking for enhancements that can use existing facilities, keep costs down, and follow current regulations.

This approach builds on previous successes at another LM site, in Grand Junction, Colo., where similar revegetation efforts reduced water percolation by 93 percent. This was achieved by boosting natural processes like soil storing water, plants growing, and water evaporating and transpiring.

The pilot test is comparing two main cover enhancement methods applied on individual 2.5-acre plots:

Revegetation alone: Planting vegetation directly is a simpler method, but it takes four to 10 years for the plants to fully establish and perform optimally.

Revegetation combined with void-filled riprap: This method uses mixtures of gravel, sand, and soil applied to the existing rock cover along with revegetation. It's more complex, but is expected to show good results within two to five years.

LM is also using a control plot with the existing cover conditions, which is expected to have more water seep through and require ongoing water extraction.

Watch and learn

To keep tabs on how well these methods work, the Rifle project uses a variety of monitoring tools, including soil moisture sensors, plant surveys, and advanced remote sensing with machine learning.

The desired outcomes from RECAP include a better understanding of the sources of pore water and how water moves through the cover, a better understanding of how the different methods improve water management, and identification of the most effective areas of the current disposal cell cover for large-scale application.

By significantly reducing the amount of water entering the disposal cell, these enhanced covers offer a resilient and sustainable alternative to the traditional method of pumping water out and could change how similar disposal sites are managed in the future.

Ultimately, RECAP aims to shift toward a more hands-off, self-sustaining way of managing pore water. Unlike energy-intensive pump-and-treat systems, these ET-enhanced covers rely on natural processes that become more effective as the vegetation matures.

This could potentially eliminate the need for constant mechanical intervention, saving costs and reducing the burden of maintenance while still ensuring the site performs reliably.

Article courtesy of the Department of Energy's Office of Legacy Management.