Theme: 01. Remediation Technology Innovations

Topic Area: 01a. Abiotic and In Situ Biogeochemical Processes: Applications and Lessons Learned

Formatted Title:

Quantification of Abiotic Transformation Rates for cDCE Using a ¹⁴C Assay

Background/Objectives:

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. 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 for the role of magnetic materials in mediating abiotic degradation of low concentrations of chlorinated ethenes in aquifers. For this presentation, the focus is on application of a ¹⁴C assay that was used to measure the rate of abiotic transformation of cDCE in contact with sediment samples of aquifer materials. The intent of the assay is to provide a tool to estimate abiotic degradation rate constants that can be used as part of the third line of evidence in the USEPA protocol to assess MNA.

Approach/Activities:

Sediment samples were collected from five sites spanning a variety of aquifer characteristics. The sediment was dried and sieved (850 µm), and the fraction less than 850 µm was subjected to enrichment in magnetic material using a strong magnet, along with collection of the magnetically depleted fraction. The magnetic susceptibility of the enriched fraction ranges from 1.2E-06 to 8.4E-05 m³/kg, while the magnetic material ranges from 1.4% to 20%. The ratio of enriched to depleted magnetic material is 20:1 to 16,200:1, indicating significant variability across the sites. The microcosms contain approximately 70 g of sediment and 70 mL of filter-sterilized groundwater. Five treatments were evaluated in triplicate: magnetically enriched sediment, magnetically depleted sediment, unaltered sediment, the original wet sediment, and no sediment. Within each treatment, the assay was performed in the absence of oxygen, in the absence of oxygen with a hydroxyl radical scavenger (2-propanol) added, and in the presence of oxygen. After injecting purified ¹⁴C-cDCE and adding cDCE saturated water to an initial concentration of approximately 1500 µg/L, the microcosms were placed on a tumbling device to ensure gentle agitation during incubation. At weekly intervals over 42 days, 5 mL samples were withdrawn, filtered, and sparged to remove the unreacted ¹⁴C-cDCE. The amounts of ¹⁴C products in the form of ¹⁴CO₂ and ¹⁴C-labeled soluble products were quantified. Based on accumulation of the ¹⁴C products, first order rate constants were determined.

Results/Lessons Learned:

Rate constants for cDCE have been determined for four of the sites thus far. The rate constants range from 0.0067 ± 0.0026 yr^-1 to 1.1 ± 0.12 yr^-1, with corresponding half-lives ranging from 0.63 (0.56, 0.72) to 103 (74, 169) years. Anaerobic treatments with the radical scavenger (2-propanol) added had the lowest rate constants, suggesting a role for a radical in degradation under anaerobic conditions. In some cases, the magnetically enriched sediment exhibited higher rate constants in comparison to the magnetically depleted sediment, but not uniformly. Rate constants from the original wet sediment can be related to field-scale activity by adjusting for the ratio of water to sediment in the microcosms versus in situ.

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Anangelica Rivera-Cruz