A Stable Isotope Sources and Sinks Model for Combined Source Apportionment and Quantification of Degradation of Environmental Pollutants
Abstract
Natural attenuation can represent a complementary or alternative approach to engineered remediation of polluted sites. In this context, compound specific stable isotope analysis (CSIA) has proven a useful tool, as it can provide evidence of natural attenuation and assess the extent of in-situ degradation based on changes in isotope ratios of pollutants. Moreover, CSIA can allow for source identification and apportionment, which might help to identify major emission sources in complex contamination scenarios. However, degradation and mixing processes in aquifers can lead to changes in isotopic compositions, such that their simultaneous occurrence might complicate combined source apportionment (SA) and assessment of the extent of degradation (ED). We developed a mathematical model (stable isotope sources and sinks model; SISS model) based on the linear stable isotope mixing model that allows for simultaneous SA and quantification of the ED in a scenario of two emission sources and degradation via one reaction pathway. It was shown that the SISS model with CSIA of at least two elements contained in the pollutant (e.g., C and H in benzene) allows for unequivocal SA even in the presence of degradation-induced isotope fractionation. This contrasts with the assumption that isotope fractionation due to transformation processes generally hampers the use of isotope data in source identification and apportionment. In addition, if degradation follows instantaneous mixing of the two contaminant pools, the model also allows for a precise determination of the ED. If mixing occurs after the two contaminant pools have degraded independently from each other, the SISS model can provide a conservative estimate of the overall ED. The SISS model was validated in reactive transport simulations of a two-dimensional flow system with two emission sources. Its results for SA and the quantification of the ED were in good agreement with the simulation results. The application of the SISS model to field data of benzene contamination was, however, challenged by large uncertainties in measured isotope data. Nonetheless, the use of the SISS model provided a better insight into the interplay of mixing and degradation processes at the field site, as it revealed the prevailing contribution of one emission source and a low overall ED. This suggests that the developed SISS model can help in the assessment of natural attenuation and in the planning of engineered remediation of contaminated aquifers. Relative contribution of source A to a sample in the (δ13C, δ2H)-space.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H34D..03L
- Keywords:
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- 1831 HYDROLOGY Groundwater quality;
- 1832 HYDROLOGY Groundwater transport;
- 1000 GEOCHEMISTRY