Grantee Research Project Results
Final Report: Intrinsic Stable Isotopic Tracers of Environmental Contaminants
EPA Grant Number: R825420Title: Intrinsic Stable Isotopic Tracers of Environmental Contaminants
Investigators: Macko, Stephen A. , Kennicutt, Mahlon C.
Institution: University of Virginia , Texas A & M University
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1996 through September 30, 1999 (Extended to September 30, 2000)
Project Amount: $358,949
RFA: Exploratory Research - Water Engineering (1996) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
As human populations continue to grow and expand spatially, impacts on natural systems will also increase. One manifestation of this impact will be increased release of contaminants into the surrounding environment. There exists a need for reliable and versatile techniques for monitoring environmental contaminants and the development of new technologies aimed at tracing the sources and fates of these contaminants. The objective of this research was to further advance knowledge concerning current techniques for monitoring the sources and fates of two important contaminant classes, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs). In particular, this study focused upon advancing current knowledge surrounding the use of compound specific isotope analysis (CSIA) for contamination studies concerned with these two contaminant classes. Some prior work has been done using CSIA for PAH source apportionment, but no such work has been done on PCBs extracted from the environment. The goal of this project was to determine if the stable isotopic compositions of individual PAH and PCB pollutants can be used to uniquely identify the sources and to monitor the fates of organic chemical contaminants in the environment.Summary/Accomplishments (Outputs/Outcomes):
The results of this study indicate that there is a carbon isotopic enrichment at the compound level associated with biodegradation. Although this fractionation was not uniform from compound to compound, a 2-8 percent change over approximately 2 months was observed. These results have very important ramifications, however, for the source apportionment of PAHs using compound specific isotope analyses. Without taking into account the possible modifications of the original isotopic compositions through processes of biodegradation, source apportionment using CSIA may yield erroneous results. Additionally, contrary to expectations, sediment amended with nutrients and microbes known to attack PAHs showed very little enhancement of biodegradation compared to the control treatment; the nutrient amended samples showed isotopic changes similar to the non-nutrient amended samples at the compound specific level. Possible reasons for this are that the sediments were not nutrient limited, and that indigenous microbes capable of degrading aromatic hydrocarbons were already present.Although the trends of isotopic enrichment found in this study were predictable, the degrees of the enrichment were variable. Further work needs to be pursued in an effort to better quantify the effects of microbially-mediated isotopic fractionation, especially as a function of time. In addition, knowledge of the biodegrading capability of indigenous microbes on a site to site basis should be understood before applying CSIA techniques for contaminant source apportionment. Perhaps if the PAHs or other hydrocarbons were associated with considerable amounts of organic matter, they may be more resistant to biodegradation and would lead to less ambiguity associated with compound specific isotope analysis.
This was the first study aimed at using isotopic signatures of individual PCB congeners to trace their sources. Using CSIA, probable isotopic linkages between PCBs from environmental extracts and the Aroclor source material were observed. When isotopic differences between source and environmental sample PCBs were observed, the tendency was for the environmental sample values to be enriched in the heavy isotope relative to the Aroclor PCB values. This may have reflected an isotopic fractionation induced by aerobic and/or anaerobic biodegradation or a partitioning effect as the PCBs were distributed between a variety of matrices.
The results of this study have shown the potential for using compound specific isotope analysis for PCB source apportionment. This technique has shown utility for other contaminants, such as PAHs, but had never been applied to source apportionment questions related to PCBs. Owing to the complex histories of PCBs, tracing the sources and monitoring the fates of these widespread, persistent compounds has been an evolving task for researchers. The results of this study also have shown that for cases of known contamination, CSIA can be useful as a verification tool for linking a PCB source material to PCBs from environmental extracts and that compound specific isotope analysis can serve as an indicator of processes that are affecting individual compounds within the original PCB contaminant reservoir. In particular, assuming that the Aroclor 1260 sample is representative of material that had been used in the past near the Housatonic River, the enriched isotopic signatures of many of the tissue-extracted PCBs was thought to be indicative of microbially-mediated biodegradation in the area. One future direction of this research would be to test the possible effects of biodegradation on congener distributions, and more importantly, isotopic signatures of individual PCB compounds. Under controlled conditions, PCBs of known isotopic compositions could be applied to two sediment types, one known to contain microbes capable of aerobic breakdown of PCBs, and the other known to contain microbes capable of dechlorination. The isotopic signatures as well as congener appearance and disappearance could be monitored over time.
In addition to biodegradation, another factor that could affect the use of CSIA for PCB source apportionment is repartitioning. In particular, it is not known whether or not there is an isotope fractionation associated with the constant repartitioning of PCBs between various phases, such as the aqueous and vapor phases. An important future study would be to understand if isotope fractionation does indeed occur in this case. Although it has been shown that for smaller chlorinated molecules, biodegradation and evaporation induce opposite fractionations, no such analyses have been done for PCBs. It would be important to know the magnitude and direction of such an effect, and like this present study, if opposite trends were followed for the two processes. Compound specific isotope analysis would lend useful insights into the history of PCBs if the amount and trend of isotope fractionation could be measured.
The results of this study have shown that measuring the isotopic composition of individual congeners has the potential to lend insight into PAH/PCB source apportionment studies, but that biodegradation and/or phase partitioning can alter and confuse isotopic linkages. The fact that each molecule is identified independently was shown to be a great asset of the CSIA technique applied to PCBs, and potentially could be the key to source apportionment studies. There could be certain congeners, which after further study, prove to be more resistant than others and thus would serve as the keys to source apportionment using CSIA. Of great important of this compound specific approach is the ability to isotopically differentiate between the same PCBs from multiple Aroclor mixtures. That capability was shown for some congeners in this study, and would need to be proven to be true at the onset of every PCB source apportionment study utilizing isotope measurements. As technologies and techniques advance, compound specific hydrogen and chlorine isotope analysis would add a great deal of resolution to PCB apportionment and tracing studies. These two elements would act to reinforce theories that are based only on carbon isotope analysis, or would perhaps help in differentiating between the importance of two or more processes that may be affecting PCB contaminated systems.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
water, ground water, chemicals, PCBs, PAHs, identification of pollutants, isotopic composition, compound specific isotope analysis, CSIA, fate and transport of pollutants, model verification., Scientific Discipline, Toxics, Waste, Geographic Area, Water, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Physics, pesticides, State, Chemistry, Fate & Transport, Engineering, Chemistry, & Physics, fate and transport, mass spectrometry, Georgia (GA), PCBs, gas chromatography, PAH, environmental contaminants, groundwater contamination, water qualityProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.