Tracking Groundwater Remediation Efforts Using Rare Earth Elements
Published May 28, 2024
Contaminated sites can pose complex cleanup challenges, especially when there are multiple chemical contaminants and difficult geological or site conditions. Contaminated soils and groundwater are particularly tricky to remediate due to the challenge of gathering information about the underground environment from the surface.
Cleanup of groundwater and soil is often done in-situ (“in place”), meaning the contaminated material is treated where it lies, rather than removing it for treatment. In-situ remediation of groundwater is commonly done with a permeable reactive barrier (PRB), which is a wall created below the ground that allows groundwater to flow through it. The PRB must be placed in the path of the groundwater flow to ensure the contaminant plume is intercepted for treatment. The wall contains reactive materials, such as limestone, carbon, or iron metal, that can either trap the harmful contaminants or cause a chemical reaction to make them less harmful. The treated groundwater flows out the other side of the wall.
But when remediating groundwater – which can’t be readily observed – how can you know the cleanup effort is going as planned? How can you confirm that the contaminated groundwater is passing through an effective PRB remedy as intended?
To help address these questions, EPA researchers develop methodologies to evaluate and prevent remedy failures and to improve monitoring in the field. EPA scientists developed a new method to use patterns of rare earth elements (REE), which include metallic elements like Lanthanum, Cerium and Neodymium, found naturally underground throughout the world, to understand if groundwater has interacted with a PRB in the subsurface. Using these REEs as geochemical tracers can provide information about whether the PRB remedy was appropriately placed to intercept and treat groundwater contaminant plumes. REEs are commonly used as natural tracers in studies of groundwater migration and groundwater-surface water interactions, but this is the first use of REEs as tracers for understanding in-situ groundwater remediation.
“By measuring the concentrations of REEs in the groundwater before, during, and after its interaction with the PRB, site managers can confirm whether the contaminated groundwater has passed through the PRB as expected,” EPA geochemist Dr. Rick Wilkin explained. “For example, if REE concentrations do not decrease down-gradient or "downstream" of the treatment zone, that could indicate that the intended treatment isn't effective.”
EPA scientists demonstrated their method at several sites where a PRB is being used to treat contaminated groundwater. The method uses a High Resolution-Inductively Coupled Plasma-Mass Spectrometer to evaluate groundwater samples to understand groundwater chemistry and REE concentrations at the site.
“The method we developed takes advantage of the rare earth elements’ unique chemical and physical properties. As the REEs are carried by the groundwater and through the PRB, they react with the PRB due to their chemical properties,” Wilkin said.
In several examples of well-performing PRBs, REE concentrations were reduced to levels below detection as groundwater moved across the location of the installed PRBs. In one example of a non-functioning PRB (where there was no indicated contaminant reduction), REE levels and patterns were unchanged across the flow path, indicating that the PRB was not appropriately placed to intercept and treat the contaminated groundwater plume. This finding led to subsequent site investigations to better understand subsurface conditions. Field data on specific REE concentrations also led to new information about reaction conditions in the treatment zones of certain types of PRBs.
Using standard sampling methods and lab equipment to analyze elements found naturally in groundwater, EPA scientists have created a novel approach to further understand subsurface conditions, filling a need at contaminated sites nationwide. Through this groundbreaking work, EPA can provide its regional, program, state, and Tribal partners with state-of-the-science support on this methodology for groundwater studies, including at contaminated sites like Superfund sites.
Since publishing this methodology, EPA has also started applying it to characterize mining wastes, like coal ash and acid mine drainage, to understand the potential for extraction – and possible reuse – of rare earth elements and other critical minerals from these waste products. Additional research is underway to understand the full potential of mine waste as a reliable and economical critical mineral source. The successful recovery of critical rare earth metals could reduce the environmental impact of mine wastes, while providing economic benefits to affected communities.
Learn More About the Science
- Read the report: “Rare-Earth Elements as Natural Tracers for In-situ Remediation of Groundwater,” published in Environmental Science & Technology (2021).
- Research on the Management of Contaminated Sites
- Contaminated Site Clean-Up Information