“Abiotic Natural Attenuation by Reactive Minerals: Core Concepts” by Dr. Paul Tratnyek (SERDP Project Webpage)

It is now widely recognized that abiotic processes play an important role in the natural attenuation of groundwater contaminants. This development has created demand for new and improved methods of measurement and/or enhancement of abiotic natural attenuation (ANA) under in situ conditions. This SERDP-funded project helped meet this need by developing three “core concepts” that should provide an improved foundation not only for ANA, but also for in situ chemical reduction (ISCR) more broadly. This presentation emphasized two of the core concepts, (1) performance metrics for quantitative comparison among outcomes, and (2) enhancement of remedial performance by mediation or activation. To begin performance metric evaluation, we compiled an extensive database of kinetic data on contaminant reduction under conditions relevant to ANA or ISCR and used these data to test various normalization methods for increased generality. This and new experimental results from several SERDP projects suggest that dissolved iron, Fe(II), must be considered in addition to the availability of iron oxide or sulfide minerals. One effect of dissolved Fe(II) appears to be formation of authigenic, secondary mineral phases that serve as reactive mineral intermediates, which appear to be a major cause of contaminant reduction by ANA.

“Abiotic Natural Attenuation of Tetrachloroethene and Trichloroethene by Reactive Minerals” by Dr. Michelle M. Scherer     (SERDP Project Webpage)

Chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE), are some of the most prevalent contaminants at DoD sites. While biological degradation has been studied in some detail, it remains unclear when and to what extent abiotic reactions with iron minerals contribute to plume attenuation. The goal of this SERDP effort was to evaluate the factors controlling abiotic degradation of PCE and TCE by magnetite and clay minerals. We found that magnetite and reduced iron-containing clay minerals alone did not reduce PCE and TCE under anoxic conditions. We did, however, find that both magnetite and iron-containing clay minerals reduced PCE and TCE in the presence of high concentrations of dissolved iron, Fe(II). Our findings suggest that the presence of either magnetite or iron-containing clay minerals in aquifer sediments is not sufficient to suggest that ANA is likely. Rather, it appears that conditions that favor active precipitation of reactive mineral intermediates may be necessary. Our work suggests that zones of active Fe(II) precipitation in anoxic aquifers could result in PCE and TCE reduction that is sufficiently fast to help attenuate PCE and TCE plumes.

Dr. Paul G. Tratnyek is a professor in the School of Public Health at the Oregon Health & Science University (OHSU) in Portland, Oregon, where his research focuses on zerovalent iron (ZVI) for the remediation of contaminated groundwater, as well as more broadly aspects of in situ chemical reduction (ISCR) and in situ chemical oxidation (ISCO). He has published some of the earliest work on abiotic reduction of contaminants and the largest body of high-impact research on zerovalent iron. Most of his recent and on-going SERDP projects relate to the characterization of in situ redox processes involved in ISCR and abiotic natural attentuation. Prior to joining OHSU, e served as a National Research Council Postdoctoral Fellow at the U.S. Environmental Protection Agency laboratory in Athens, Georgia, and as a Research Associate at the Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG). Dr. Tratnyek has a doctoral degree in applied chemistry from the Colorado School of Mines.

Dr. Michelle Scherer is the Donald E. Bently Professor of Engineering in the Department of Civil and Environmental Engineering at the University of Iowa. Dr. Scherer’s expertise is in environmental geochemistry including redox reactions at mineral-water interfaces and contaminant fate in soils and sediments. She has published over 70 journal articles, and her work has been cited over 4,000 times. Her research on the abiotic attenuation of chlorinated ethenes was recently recognized with the 2018 SERDP Project of the Year Award. In addition, she is the recipient of the May Brodbeck Distinguished Achievement Award (2016) and the Malcolm Pirnie/AEESP Frontier in Research Award (2010). Dr. Scherer has served as a member of EPA’s Environmental Engineering Scientific Advisory Board and as associate editor of Environmental Science and Technology. She has a bachelor’s degree in systems engineering from the University of Virginia, a master’s degree in civil and environmental engineering from the University of Connecticut, and a doctoral degree in environmental science and engineering from the Oregon Graduate Institute of Science and Technology.