Abiotic Degradation of TCE by Naturally Occurring Magnetite can be Important in Large Plumes

Tuesday, August 8, 2017: 1:35 p.m.
John Wilson, Ph.D , Scissortail Environmental Solutions, Ada, OK
Todd Wiedemeier , T.H. Wiedemeier & Associates, Inc, Sedalia, CO
David Freedman, Ph.D. , Clemson University, Anderson, SC

Significant quantities of magnetite occur in many aquifers. In these aquifers, abiotic degradation of TCE by magnetite can be an important mechanism for natural attenuation. There is a direct correlation between the quantity of magnetite in aquifer sediments and their magnetic susceptibility. The magnetic susceptibility of aquifer sediment can be characterized with good sensitivity and at low cost using a downhole sonde in conventional monitoring wells at the site.

An empirical relationship has been established between the mass magnetic susceptibility of aquifer sediment and the rate constant for degradation of TCE and cis-DCE in the sediment. In a variety of aerobic unconsolidated sand aquifers, the magnetic susceptibility ranged from 2E-07 to 2E-06 m3 kg-1. Biological reductive dechlorination of TCE or cis-DCE would not be expected in these aquifers. However, the field-scale bulk rate constants for TCE or cis-DCE degradation varied from 0.1 per year to 0.7 per year with a median near 0.3 per year.

In large plumes, the rate constant for abiotic degradation of TCE by magnetite may be large enough that abiotic degradation is adequate to support monitored natural attenuation (MNA) as a remedy or part of a remedy at the site. If the concentration of TCE is 100 μg/L or less, and the degradation rate constant is 0.3 per year, the concentration can be brought to the MCL in 10 years of travel time along the flow path in the groundwater.

John Wilson, Ph.D, Scissortail Environmental Solutions, Ada, OK
John Wilson recently retired from the position Research Microbiologist for EPA’s Ground Water and Ecosystems Restoration Research (GWERD) Subsurface Remediation Branch. He holds a B.S. in Biology from Baylor University, an M.A. in Microbiology from the University of California at Berkeley, and a Ph.D. in Microbiology from Cornell University. Wilson led research dealing with biological and non-biological processes that destroy contaminants in groundwater and provided training and technical assistance to EPA’s regions and to state agencies on natural attenuation of hydrocarbon components in groundwater.

Todd Wiedemeier, T.H. Wiedemeier & Associates, Inc, Sedalia, CO

David Freedman, Ph.D., Clemson University, Anderson, SC
Dr. Freedman’s major teaching and research interests include: hazardous waste management, water and wastewater treatment, and biodegradation/bioremediation of recalcitrant organic compounds. Dr. Freedman's research focuses on the application of environmental microbiology to development of enhanced methods for biodegrading hazardous organic contaminants. Of particular interest is elucidation of biotransformation pathways and application of this knowledge to design of biological treatment processes.