v>EPA United States Environmental Protection Agency epa.gov/research Rare Earth Elements (REEs) as Natural Tracers for Evaluating In-situ Groundwater Remedies Innovative Science for a Sustainable Future EPA developed and tested a methodology for using Rare Earth Element (REE) patterns as tracers to understand whether groundwater has interacted with reactive materials in the subsurface used for site remediation. EPA evaluated this approach at several real-world sites where the Permeable Reactive Barrier (PRB) technology is being used to treat contaminated groundwater. EPA's Office of Research and Development (ORD) is now able to provide regional, program, state, and tribal partners with state-of-the-science support on the application of this REE methodology for ground- water studies, including its use for tracking the efficiency of groundwater remediation efforts. What are Rare Earth Elements? The Rare Earth Elements (REEs) are a group of metallic elements with unique fluorescent, conductive, and magnetic properties. REEs include the elements Lanthanum (La) to Lutetium (Lu), or atomic numbers ranging from 57 to 71, excluding Promethium (Pm) which is unstable in nature (see Figure 1). REE's unique properties and natural presence in the subsurface make them well-suited as natural tracers in studies of groundwater movement and groundwater-surface water interactions. These elements are commonly present in groundwater at concentrations in the nanogram per liter to microgram per liter range. How Can REEs Act as Tracers? While REEs are commonly used as natural tracers in studies of regional groundwater migration and groundwater-surface water interactions, this research documents the first use of REEs as tracers for understanding in-situ groundwater remediation. This research showed that REEs are sensitive tracers for evaluating how groundwater interacts with reactive materials, such as zero-valent iron (ZVI) and organic carbon, that are placed in the subsurface for contaminant remediation. Reactive materials used for remediation lower contaminant concentrations in groundwater by changing redox (reduction-oxidation) conditions and contaminant solubility in water, or by providing 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu H a He Li Be B C N 0 F Ne Na Mg Al Si P S CI Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Q Hf Ta W Re Os Ir Pt Au Hg TI Pb Bi Po At Rn Fr Ra Ac Rf Db sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts og Figure 1. The Rare Earth Elements are part of the Lanthanide group inthe periodic table of elements. mineral or organic surfaces that bind contaminants and remove them from water. Analysis of geochemical tracers for understanding in- situ remediation is an important tool in cases where contaminant concentrations in areas downgradient from treatment zones fail to decrease within expected timeframes, i.e., where the in-situ remedy fails to meet cleanup objectives. How Can REEs Inform Site Cleanup? Over the past several decades, in-situ remediation technologies for cleaning up contaminated groundwater at hazardous waste sites have emerged through research and applied testing. Technology development has typically progressed from (Continued on page 2) @ EPA re search Connect with us Online @EPAgov @EPAresearch /usepagov # •• You >mm /USEPAgov ------- v>EPA theoretical conceptualization, to laboratory bench- and pilot-scale testing to full-scale application for site cleanup. However, even the "well established" technologies sometimes fail to provide expected cleanup results. Such cases typically result in follow-up analysis in order to uncover the cause(s) of the remedy failure. EPA research is developing needed methodologies to evaluate and prevent remedy failures and to improve monitoring in the field. What Research Has EPA Done? EPA's Office of Research and Development (ORD) developed a method for measuring REEs in groundwater using a High Resolution-lnductively Coupled Plasma-Mass Spectrometer (HR-ICP-MS). The method uses a standard "metals" sample (filtered or unfiltered) collected in a plastic bottle and preserved with nitric acid for sample analysis and determination of REE concentrations in groundwater. ORD, working with scientists from EPA Regions 1 and 6, evaluated groundwater geochemistry and REE concentrations at several sites where the PRB (Figure 2) technology is being used to treat contaminated groundwater. The PRBs utilize zero- valent iron (ZVI) and organic-carbon plus limestone mixtures for contaminant (inorganics and chlorinated solvents) treatment. The research showed that REE concentrations & patterns can be used as tracers to probe the key question of whether groundwater has interacted with reactive materials emplaced using subsurface injection ortrench-and-fill methods. In several examples of well-performing PRBs, REE concentrations were reduced to levels below detection (<4 ng/L) as groundwater moved across the location of the installed PRBs. In one example of a non-functioning ZVI PRB (i.e., no indicated contaminant reduction), REE levels and patterns were unchanged across the flow path presumed to intercept the treatment medium. The implication at Figure i. Conceptualization of a groundwater contaminant plume interaticing with a Permeable Reactive Barrier, Source: https://www.enviro.wiki/index.php? title=Zerovalent Iron Permeable Reactive Barriers this site is that the ZVI was not appropriately placed in order to intercept and treat the contaminated groundwater plume. This finding has led to subsequent site investigations to better understand subsurface conditions. More information on this collaborative research can be found in the paper titled "Rare-Earth Elements as Natural Tracers for In-situ Remediation of Groundwater," published in Environmental Science & Technology (2021, volume 55, pages 1251-1259; https://pubs.acs.orq/doi/10.1021/acs.est.0c06113). Contacts Technical Contact/Principal Investigator: Richard Wilkin, Ph.D. EPA Office of Research and Development 580-436-8874 wilkin.rick@epa.gov Communications/Media Contact: Charlena Bowling EPA Office of Research and Development 513-569-7648 bowlinq.charlena@epa.gov EPA/615/F-22/001 June 2022 @ EPA re search Connect with us Online @EPAgov @EPAresearch /usepagov # u You imrci /USEPAgov ------- |