The overarching objectives of this project were to (1) Provide a method to readily and inexpensively acquire the magnetic susceptibility data required to evaluate the abiotic degradation of trichloroethene (TCE) by magnetite in aquifer materials using existing non-metallic groundwater monitoring wells; and (2) Provide a method to readily and inexpensively acquire the data required to evaluate and quantify the rate constant for aerobic biological co-oxidation of TCE.

Using mass magnetic susceptibility to predict abiotic degradation of chlorinated alkenes by magnetite in the aquifer matrix has been shown to be viable, but previously, such evaluation required that a core sample of the aquifer material be submitted for laboratory analysis. This project demonstrates that an inexpensive downhole sonde (probe) can be used in existing 2- and 4-inch Polyvinyl Chloride (PVC) groundwater monitoring wells to quantify magnetic susceptibility of aquifer material.

Bacteria that degrade natural organic matter in groundwater contain enzymes (oxygenases) that can aerobically degrade TCE through a process of biological co-oxidation. Bacteria that contain active oxygenase enzymes can be recognized using fluorescent Enzyme Activity Probes (EAPs), and the bacteria can be counted under a microscope. There are primers that can be used in the Quantitative Real-Time Polymerase Chain Reaction (qPCR) to amplify Deoxyribonucleic Acid (DNA) that codes for selected oxygenase enzymes. A qPCR assay can be used to determine the number of gene copies for these enzymes in a sample of groundwater.

Aerobic co-oxidation is a promising risk management strategy for large dilute plumes, but its application has been limited because the co-oxidation of TCE in the environment is difficult to quantify by simply measuring changes in the concentration of TCE in the field, and the number of bacteria in groundwater that have the oxygenase enzymes has not been directly correlated to field-scale rates of degradation.

Because determining field scale rates for co-oxidation of TCE using concentration data is problematic, a Radioactive Carbon 14 (¹⁴C) labelled TCE assay was developed to measure rate constants. The utility of EAPs and qPCR assays to evaluate co-oxidation of TCE was determined by comparing the rate constant developed using the ¹⁴C-labelled TCE assay to the abundance of cells that react with EAPs or the abundance of gene copies for oxygenase enzymes.

Values for volume magnetic susceptibility were determined in 26 PVC wells using a downhole sonde. The values were converted to mass magnetic susceptibility and compared to values for mass magnetic susceptibility from laboratory analyses of samples from boreholes that were adjacent to the wells. There was good agreement between the two measurements.

Out of 19 groundwater samples evaluated using the Trichloroethene labelled with Carbon 14 (¹⁴C-TCE) assay, TCE co-oxidation could be documented in 8 samples, with first order rate constants ranging from 0.00658 to 2.65 yr-1. 

In a particular water sample, the abundance of gene copies of the most common oxygenase was similar to the abundance of cells reacting to the EAPs. Some oxygenase enzymes were more abundant in groundwater from some wells and other enzymes were more abundant in other wells. Co-oxidation of TCE could not be attributed to any one oxygenase enzyme. To further complicate interpretation of the abundance of DNA gene copies, not all the DNA in bacteria is actively transcribed to make enzymes at any one time. If the messenger RNA (mRNA) transcript for an enzyme is present in a sample, that is evidence of the gene being transcribed to make the active enzyme. The total abundance of active DNA gene copies was calculated as the sum of the individual gene copies of oxygenase enzymes for which the mRNA transcript was detected.

There was a useful relationship between the total abundance of active DNA gene copies and the rate constants for TCE co-oxidation. The 80% prediction interval of a regression of the rate constants on the total abundance of active DNA gene copies is only one order of magnitude wide.

Laboratory microcosm studies have shown that some aquifer sediments have appreciable values for mass magnetic susceptibility but no evidence for abiotic degradation of TCE. Values of mass magnetic susceptibility should only be used as a secondary line of evidence to support a rate constant for TCE degradation that is extracted from site characterization data, as is illustrated in the decision logic of Lebrón et al. (2015). Mass magnetic susceptibility should not be used as a primary line of evidence to extract a rate constant.

Similarly, the abundance of cells that react to an EAP or the abundance of DNA amplified by a qPCR marker for an oxygenase enzyme should be used as a secondary line evidence to support a rate constant for TCE degradation that is extracted from site characterization data. They should not be used as a primary line of evidence to extract a rate constant.

Other significant implementation issues identified are the cost of the EAP analyses, which is considerable, and can only be completed by the Pacific Northwest National Laboratory (PNNL) as well as the requirement that the ¹⁴C-TCE assay be done in a certified and permitted laboratory. Another implementation issue was identified in the integrity of the PVC monitoring wells; specifically, 2-inch groundwater monitoring wells. If these wells are not sufficiently straight, or if the joints are not flush, then the magnetic susceptibility sonde cannot be lowered into the well, and obtaining mass magnetic susceptibility readings in such wells becomes impossible.

Wiedemeier, T.H., B.H. Wilson, M. Ferrey, and J.T. Wilson. 2017. Efficacy of an In-Well Sonde to Determine Magnetic Susceptibility of Aquifer Sediment. Groundwater Monitoring & Remediation, 37(2):25-34.