Summary

Vurdering af naturlig nedbrydning af PCE i grundvandsmagasin ved isotopfraktionering

Isotopic fractionation is a method which can be used for the documentation of natural degradation of organic contaminants in groundwater at contaminated sites. Degradation of perchloroethylene (PCE) and other organic compounds results in a change in the ratio between stable carbon isotopes in the molecule. A change in the isotope ratio (isotope fractionation) of a compound along a flowline from the source of contamination can be used as documentation for degradation of the compound in the plume.

PCE contamination at the former central drycleaning facility in Rødekro has resulted in the spreading of a long and deep contaminant plume of PCE and its degradation products trichloroethylene (TCE), dichloroethylene (DCE) and vinylchloride (VC) in the groundwater aquifer. The plume originates from a single source and is well delineated. Hence, the conditions for use of isotopic fractionation for documentation of the degradation of PCE are good.

The scope of the project is: (1) to evaluate the potential for use of isotopic fractionation for documentation of natural degradation under Danish conditions, (2) to evaluate the potential for use of isotopic fractionation for determination of degradation rates, and (3) to evaluate the natural degradation in the plume by use of isotopic fractionation.

The investigations at the site have included sampling of existing wells placed centrally along the plume (following a flowline) for analysis for chlorinated ethenes (incl. ethene in selected wells), redox parameters (in selected wells) and isotopic fractionation. In the first sampling round, carbon isotopic fractionation of PCE, TCE and cis-DCE was analyzed. The concentrations of VC were too small for isotope analysis. Supplementary sampling has been conducted to facilitate analysis of VC carbon isotopic fractionation and cis-DCE chlorine isotopic fractionation, and to analyze for specific degraders and the specific degradation products ethene, ethane and acetylene (with low detection limits) to further elucidate the degradation pathway/-type in the plume.

A literature survey focusing on enrichment factors for isotopic fractionation of PCE and its natural degradation products has been conducted as part of the project in order to evaluate the extend of natural degradation and degradation rates in the plume. Finally a M.Sc. project was carried out alongside this project at the University of Neuchatel, in which degradation rates for PCE and its degradation products were modeled on the basis of isotopic fractionation results. A summary of the project is given in this report.

The investigation shows that degradation of PCE and its degradation products result in significant isotopic fractionation. The results for isotopic fractionation document that PCE is degraded to VC via TCE and cis-DCE, and that VC is also degraded in the aquifer.

The method has proved to be very sensitive with respect to analysis of the degradation of the chlorinated ethenes. This has facilitated correlation of the degradation of the individual compounds with changes in the redox conditions along the plume flowline and particularly over depth in the aquifer. The degradation of PCE to TCE starts under nitrate-reducing conditions and is quickly completed in the transition zone to manganese- to iron-reducing conditions, where simultaneous degradation of TCE to cis-DCE takes place. Cis-DCE accumulates in this zone. However, at the interface to stricter iron-reducing conditions, isotopic fractionation reveals the production of VC from initial degradation of cis-DCE. In the strictly iron-reducing zone and further along the plume, where conditions become less reducing again, isotopic fractionation documents that VC is degraded.

Chlorine isotopic fractionation shows the same picture with respect to cis-DCE degradation as the carbon isotopic fractionation. The good correlation between chlorine and carbon isotope data is a strong indication that cis-DCE is degraded by reductive dechlorination, which corresponds with the production of VC. Results of analysis for the specific degraders Dehalococcoides (Dhc) revealed indications (below quantification level) of their presence in 2 samples from the part of the plume where degradation of cis-DCE was most clearly documented by the isotopic fractionation, only. The indicated presence of specific degraders further strengthens the assumption that cis-DCE is degraded by reductive dechlorination, as these specific bacteria are only found where compounds undergoing reductive dehalogenation are present.

In the outermost part of the plume, where VC is degraded, the average isotopic fractionation for all chlorinated ethenes starts to deviate from the fractionation at the source. As no ethene or ethane is detected in the plume, this may, , indicate that VC is degraded by a different process than reductive dechlorination. Anaerobic oxidation of VC is a possibility.

The fraction of each compound which has been degraded and the degradation rates in the aquifer have been estimated based on published enrichment factors and the measured isotopic fractionation. The estimates have been based on the Rayleigh equation, which describes the correlation between concentration and isotopic composition during degradation of a single compound in a closed system. For sequential degradation of e.g. chlorinated ethenes, the model describes the degradation of the mother compound only and not the degradation of daughter products, as these are influenced by production as well as degradation. As PCE is degraded via TCE almost completely to cis-DCE before further significant degradation of cis-DCE occurs, cis-DCE can by approximation be viewed as a mother compound in the part of the plume where cis-DCE and VC dominate. For the compounds which are produced and degraded simultaneously, the fraction degraded and the degradation rates are underestimated by the approximation.

The calculations confirm that PCE and TCE are degraded completely, whereas only about 20% of the DCE has been degraded in the front of the plume and about 35% of the cis-DCE has been degraded in the deepest point sampled in the outer part of the plume, and that more than about 35% of the VC has been degraded in the front of the plume, provided the VC is degraded by reductive dechlorination. If the VC is degraded by anaerobic oxidation, the fraction of VC degraded is expected to be larger.

As mentioned earlier, degradation rates can also be estimated based on published enrichment factors and measured isotopic fractionation. For the degradation products from PCE, the published enrichment factors fall within a fairly short interval for degradation by a given process (e.g. reductive dechlorination). Hence, the uncertainty lies not so much in the determination of enrichment factors and whether these are site specific, as in whether the degradation pathway/process is known. The degradation and thereby the degradation rates are, as mentioned above, very dependent on the redox conditions. Therefore, determination of degradation rates requires zonation of redox conditions in the aquifer, and the estimated rates will be very sensitive to the distance from a redox border (transition zone) to a monitoring point and the flowrate between these. The degradation rates calculated from the published enrichment factors and the isotopic fractionation are of a reasonable order of magnitude. The strong dependence on redox conditions means that the uncertainty of the redox zonation is of significance for the estimation of degradation rates. In the same way, uncertainty related to the hydrogeological conditions, including flowpaths and flowrates, is of significance for estimation of rates. As the estimated degradation rates are reasonable, they can be used for improved risk assessment using modeling or JAGG-calculations, if a sufficiently discrete level of knowledge is established for the evaluation of flow conditions for redox zonation and for isotopic fractionation.

For this location, isotopic fractionation has provided a much greater insight into the natural degradation of chlorinated ethenes in the groundwater aquifer, of great significance for risk assessment. Isotopic fractionation has proven to be a very strong and sensitive tool for evaluation of degradation and degradation pathways for chlorinated ethenes. The use of isotopic fractionation has documented that cis-DCE and VC are degraded in the aquifer under predominantly iron-reducing conditions without significant accumulation of VC. This project has further shown that it is possible to estimate the fraction of a compound which has been degraded, and degradation rates for chlorinated ethenes in the groundwater aquifer, based on the isotopic fractionation and values for enrichment factors from literature.

Similarly, isotopic fractionation is believed to constitute a very strong and sensitive tool for the evaluation of degradation and degradation pathways for chlorinated ethenes at other sites in Denmark. As observed for this location, it is expected to be of great importance that VC isotopic fractionation can be measured at a low concentration level. Furthermore, the possibility for performance of the analysis in Danish laboratories is of importance if more widespread use of the method is to be achieved.