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Rare-Earth Elements as Natural Tracers for In-situ Remediation of Groundwater
Citation:
Wilkin, Richard T., Tony R. Lee, Ralph D. Ludwig, C. Wadler, William Brandon, B. Mueller, E. Davis, D. Luce, AND T. Edwards. Rare-Earth Elements as Natural Tracers for In-situ Remediation of Groundwater. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 55:1251-1259, (2021). https://doi.org/10.1021/acs.est.0c06113
Impact/Purpose:
Much research has gone into developing in-situ remediation options for contaminated groundwater at hazardous waste sites. Technology development usually steps through theoretical evaluation, laboratory bench-testing, pilot-scale testing, and full-scale applications. Technologies that are proven at the full-scale, with time, become considered options during remedial investigations and feasibility studies. However, sometimes even the “well established” technologies fail to provide expected cleanup results. Such cases typically result in extensive follow-up study and analysis in order to uncover the cause(s) of the remedy failure. Our experience has indicated that new methodologies are needed to evaluate remedy failures and to improve monitoring in the field and here we provide a novel geochemical application for understanding in-situ remediation. The work is of interest to the international environmental community, including EPA’s regional, program, and local partners.
Description:
The utility of rare-earth elements (REE) as natural geochemical tracers for the analysis of groundwater remediation was examined in several example permeable reactive barriers (PRBs) that use zero-valent iron and organic-carbon plus limestone mixtures for contaminant treatment. Zero-valent iron removed REEs from groundwater to below detection levels (2 – 4 ng/L) and subsequent rebound of REE concentrations down-gradient of the treatment zones was not observed. In addition, REE concentrations within and down-gradient of an organic-carbon/limestone PRB were significantly reduced compared to levels in the influent groundwater. Thus, REEs are sensitive tracers for evaluating the interaction of groundwater with materials placed in the subsurface for contaminant remediation. Analysis of geochemical tracers for understanding in-situ remediation becomes important in situations where down-gradient contaminant concentrations fail to decrease within expected timeframes. The field data indicated that increased solid-phase partitioning of REEs occurred with increasing pH and heavy REEs were preferentially removed compared to light REEs in ZVI systems. In the organic-carbon PRB, unexpected negative europium anomalies were observed, revealing new information about redox conditions within the treatment zone. REE concentrations and shale-normalized profiles can be used as natural tracers to better understand in-situ technologies for groundwater remediation.
URLs/Downloads:
DOI: Rare-Earth Elements as Natural Tracers for In-situ Remediation of Groundwater
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