Abiotic degradation can be a substantial or even the primary attenuation process for TCE and other chlorinated hydrocarbons in long dilute plumes and at sites undergoing or transitioning to monitored natural attenuation (MNA). MI offers an Abiotic Panel that includes Magnetic Susceptibility, X-Ray Diffraction (XRD), and Percent Clay to identify aquifers where abiotic degradation is a plausible mechanism for observed trends in contaminant concentrations or mass.
Abiotic degradation of PCE, TCE, cis-DCE, and carbon tetrachloride by magnetite can be an important attenuation mechanism. While no direct chemical test is available, magnetite is the most abundant mineral in natural sediments that exhibits magnetic behavior. Therefore, magnetic susceptibility provides an inexpensive and valuable estimate of the quantity of magnetite in environmental samples.
X-RAY DIFFRACTION (XRD)
XRD can provide relative abundances of reactive iron bearing minerals including pyrite and the crystalline form of mackinawite. Both minerals will transform PCE, TCE, and carbon tetrachloride.
While less well studied than the other iron-bearing minerals, various phyllosilicate clays have been shown to be capable of degradation of PCE, TCE, cis-DCE, vinyl chloride, and carbon tetrachloride.
14C TCE RATE CONSTANT
Prof. David Freedman from Clemson University has developed an innovative 14C assay to determine TCE degradation rate constants from environmental samples as part of a recent ESTCP project.
HOW TO USE THE ABIOTIC PANEL:
As part of a multiple lines of evidence approach, use magnetic susceptibility and the abiotic panel to identify aquifers where it is plausible that chlorinated compounds may be degraded through an abiotic mechanism.
Use the Abiotic Panel to help answer…
- Are iron bearing minerals present that can degrade site contaminants?
- Is abiotic degradation of chlorinated compounds plausible at my site?
- Is abiotic degradation a reasonable explanation for decreasing contaminant concentrations and lack of daughter products?
- Is the 14C TCE degradation rate consistent with degradation rates observed in the field?
QuantArray®-Chlor or CENSUS® qPCR
In addition to abiotic degradation, aerobic cometabolism (co-oxidation) of TCE and other chlorinated solvents can be an important component of MNA. QuantArray®Chlor and CENSUS®qPCR are used to quantify functional genes encoding enzymes such as soluble methane monooxygenase that are capable of cometabolism of TCE.
Compound Specific Isotope Analysis (CSIA)
While contaminant and pathway dependent, aerobic cometabolism and abiotic degradation can result in significant isotopic fractionation and CSIA can be a strong supporting line of evidence in evaluating degradation mechanisms that contribute to MNA.