Date of Award
Master of Science (MS)
Environmental Engineering and Earth Science
Dr. David L. Freedman
Dr. Kevin Finneran
Dr. Elizabeth Carraway
Among the most challenging sites to remediate are those where the groundwater is contaminated with large, dilute plumes of tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene isomers (cDCE, tDCE, 1,1-DCE) and vinyl chloride (VC). Monitored Natural Attenuation (MNA) by abiotic processes may be a viable strategy, but it can be difficult to document. The overall objective of this project is to develop a deeper understanding of the role of magnetic materials in mediating abiotic degradation of low concentrations of chlorinated ethenes in aquifers. Authentic core aquifer sediment has been collected from Site A at the former Twin Cities Army Ammunition Plant (TCAAP) near Arden Hills, MN, where degradation of cDCE was detected but not PCE and TCE. The overall objective of this research was to evaluate the role of magnetic materials in mediating the abiotic degradation of chlorinated ethenes in TCAAP-A sediment. The specific objectives were to characterize the sediment, utilize a 14C-assay to measure the rate of abiotic transformation of TCE and cDCE in groundwater in contact with core samples from the TCAAP-A aquifer, and assess the biotic contributions to the degradation of chlorinated ethenes observed in the 14C-assays.
Through the use of a simple bar magnet, it was possible to separate the TCAAP-A sediment into fractions that are highly enriched and depleted in magnetic materials. The magnetic susceptibility of the enriched fraction was an order of magnitude higher than in the unprocessed sediment, and three orders of magnitude higher than in the depleted sediment. In the enriched and unprocessed sediment, the percentage of magnetic material was highest in the smallest size fraction (<150 μm), which constituted 13-28% of the total mass. Of the 21 elements evaluated by ICPMS, the concentration of Fe, Al, Mn, Zn, Cr, Ni, Co, and Ga in the enriched fraction was at least 30% higher compared to the depleted fraction.
The 14C-assay results indicate that abiotic degradation of TCE and cDCE is occurring, with rate constants that may be meaningful for considering monitored natural attenuation. However, the hypothesis that magnetically enriched sediment would have higher rate constants was not supported. For the TCE anaerobic treatment, the rate constants were statistically equivalent for the original wet sediment, enriched sediment, and depleted sediment. For the TCE aerobic treatment, the rate constants for the enriched, depleted, and unprocessed sediment were statistically equivalent, followed by the original wet sediment. For the cDCE anaerobic treatment, the rate constants for the enriched and depleted sediment were statistically equivalent and higher than for the unprocessed sediment. For the cDCE aerobic treatment, the rate constant for the unprocessed sediment was highest, followed by the enriched and depleted sediments. In all instances, the aerobic treatments resulted in higher rate constants than the corresponding anaerobic treatments. The anaerobic treatments with a radical scavenger added were consistently lower than the corresponding anaerobic treatments, suggesting that a radical is involved in the degradation pathway.
The accumulation of acetylene, ethene, and/or ethane during the incubation period in the anaerobic, anaerobic with scavenger, and aerobic treatments suggests that at least a portion of the degradation pathway occurs via an abiotic reductive pathway. Accumulation of the reduced gases was highest in the anaerobic treatments with enriched sediment, for TCE and cDCE. Nevertheless, reduced gas formation also occurred in the aerobic treatments, especially with enriched sediment. This indicated that even with oxygen in the headspace of the aerobic microcosms, microenvironments existed that permitted reductive reactions to occur.
The contribution of biotic activity to the degradation rate constants was assessed by the addition of acetylene to selected aerobic treatments, with the expectation that acetylene would inhibit monooxygenase activity if it was a factor. With TCE and the original wet sediment, the presence of acetylene did not decrease the rate constant. However, in the unprocessed sediment with TCE and the enriched sediment with cDCE, adding acetylene did result in statistically lower rate constants. It was noted, however, that there was a high level of accumulation of ethene in these microcosms, suggesting the acetylene may have interfered with the pathway for abiotic degradation. To further investigate the contribution of biotic activity, experiments were performed with the addition of a methanotrophic enrichment culture that cometabolizes TCE and cDCE. The culture was desiccated in a similar manner to the sediment. Rate constants with the culture added were statistically equivalent to the rate constant without the culture added. Taken together, these results indicate that the TCE and cDCE rate constants reported in this study are attributable to abiotic activity.
Dunn, Olivia, "Evaluating the Role of Magnetic Materials in Mediating the Abiotic Degradation of Chlorinated Ethenes at TCAAP-A Using a C14-Assay" (2023). All Theses. 3972.