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Using Isotopes to Identify Sources of TCEBy Brian Murphy, Ph.D and Farrukh Mohsen, Ph.D., P.E.
All trichloroethylene (TCE) is composed of atoms of carbon, chlorine, and hydrogen in the same arrangement. TCE in the environment can either be due to a spill of TCE or to a spill of perchloroethylene (PCE) that has biodegraded. This poses a challenge in identifying sources and allocating responsibility when there are multiple spills of TCE and/or PCE in an industrial or commercial area. In this situation isotopic analysis can be useful, not only for TCE and PCE but also for many other compounds.
Many elements exist in nature in several isotopic forms. The difference between these isotopic forms is in the number of neutrons in the nucleus of the atom. Isotopes can be either radioactive or stable (nonradioactive). For example, carbon exists mostly with six protons and six neutrons in the nucleus. However, about 1% of carbon naturally has six protons and seven neutrons in the nucleus. An even smaller amount, with eight neutrons in the nucleus, is the well-known carbon-14 used for age dating organic materials. These latter isotopes are referred to as 13C and 14C to distinguish them from the more common 12C. 13C is stable, while 14C is radioactive and hence decays over time. Useful stable isotopes of chlorine and hydrogen with their natural abundances are 35Cl (76%), 37Cl (24%), 1H(>99%), and 2H or deuterium (<1%). Various laboratories specialize in analysis of some or all these elements.
There are three main uses of isotopes for forensic analysis of TCE and PCE releases which are as follows:
- Identifying different sources by “fingerprinting” based on different isotope ratios
- Identifying additional sources based on changes in isotopic ratios along a groundwater flow path
- Distinguishing a release of TCE from TCE due to biodegradation of PCE.
Isotopic composition of fresh TCE/PCE is determined by the process and the feedstock used in its manufacture. Thus spills of solvent from different manufacturers or purchased at different times can have different isotopic “fingerprints.” Cross plots of carbon and chlorine isotopes ratios, such as shown in the example below, can be useful in spotting these differences.
Changes along a Groundwater Flow Path
Isotopes with fewer neutrons are lighter and are more favored in chemical reactions, including biodegradation. Thus, when a parent compound, say PCE, biodegrades to the daughter compound TCE, the parent compound becomes heavier (i.e., enriched in the heavier isotope). In situations where biodegradation proceeds along a groundwater flow path, the heavier isotope becomes increasingly enriched in the parent compound. The relationship between the fraction biodegraded and the shift in isotopic composition of the parent is described by what is known as Rayleigh’s Law. Sometimes the parent compound has degraded to very low groundwater concentrations. In such cases, isotopic analysis of daughter and granddaughter compounds may be of interest.
Figure 1. Isotopic shift for parent (PCE) and daughter (TCE) compounds
Figure 1 shows the consequences of Rayleigh’s Law for d13C when PCE, with an initial value of -30, per mil is degraded to TCE. The fraction of the initial PCE remaining is f and the enrichment factor is -5.51. Figure 1 also shows how d13C changes for the daughter compound, TCE, for two cases: 1) no degradation of TCE, and 2) TCE degradation at a rate half that of PCE and with an enrichment factor of -13.81. In both cases TCE starts (at f=1) with a ratio that is less than the parent PCE and becomes increasingly enriched with the heavier isotope.
Carbon atoms are conserved in PCE/TCE biodegradation. Hence, the ratio of 13C to 12C is a constant when the number of carbon atoms is summed over parent, daughter, granddaughter, etc. compounds. This is illustrated in Figure 1 for the no-degradation case. Note that the value of d13C for TCE at f=0, when all the PCE has degraded, is the same as the value of d13C for PCE at f=1 before degradation.
Deviations from this behavior—a) increasing isotope enrichment along a groundwater flow path or b) conservation of carbon isotope numbers—can indicate that more than one groundwater plume is present.
1 Slater, G.F., B.S. Lollar, B.E. Sleep, and E.A. Edwards. 2001. Variability in carbon isotopic fractionation during biodegradation of chlorinated ethenes: Implications for field applications. Environ. Sci. Technol. 35(5):901−907.
Distinguishing TCE Spills from PCE Biodegradation
Manufactured TCE obtains its hydrogen from a petroleum source, whereas TCE from PCE biodegradation obtains its hydrogen from groundwater. The isotope ratios of these two hydrogen sources can be very different. This provides a way of distinguishing TCE due to PCE biodegradation from a spill of TCE.
An equipment manufacturing facility with known historical usage of PCE was characterized by two disjoint plumes—one centrally located near suspected PCE usage and the other at the downgradient boundary near a former dry cleaning facility. Groundwater samples collected from wells in the central plume and in the boundary plume were analyzed for 37Cl and 13C for PCE, TCE, and the TCE biodegradation product cis-1,2-dichloroethene(c-1,2-DCE). While differences in either 37Cl and 13C can point to different sources, isotopic cross-plots involving both are even more useful. Figure 2 presents isotopic cross plots for PCE, TCE, and c-1,2-DCE, shown in ‰, that suggest different origins of the central and boundary plumes.
However, when the molar-weighted averages for carbon are calculated, the differences are less significant than for the individual plots, about -22 for Boundary 1 versus -23 to -29 for the other locations. Finally, the chlorine isotope value proves conclusively that there is a separate source, since Boundary 1 is downgradient, yet has a lighter isotopic value for PCE, the opposite of what is expected from Rayleigh’s Law.
When there are commingled TCE groundwater plumes, chemical analysis methods cannot distinguish them. However, isotopic methods often can. These isotopic methods rely on differing fingerprints in fresh TCE, on the known isotopic shifts that occur due to biodegradation, and on isotopic differences between TCE spills and TCE formed from PCE biodegradation and represent powerful environmental forensic tools.
Central to Exponent’s environmental expertise is a deep capability in environmental forensics. We have applied our expertise and experience to a wide variety of situations: refineries, former manufactured gas plants, mines, smelters, foundries, pulp and paper mills, wood treatment facilities, oil spills, fuel terminals, and many manufacturing facilities with contaminants in air, groundwater, surface water, sediment, and soil. We have more than 30 scientists and engineers with a variety of experience in environmental forensics.
For more information on our Environmental Forensics services, please visit our website.