Carbon and chlorine stable isotope as indicators of treatment performance for groundwater remediation

First of all, we need to understand what an isotope is. Easy answer? Let us give it a try… the atoms of a particular element must have the same number of protons and electrons, but they can have a different number of neutrons. When atoms differ only in the number of neutrons, they are referred to as isotopes of each other. In addition, if a particular isotope is not radioactive, it is called a stable isotope.

The key issue that we need to provide an answer for is how the isotopes may act as indicators of treatment efficiency and performance for natural biological processes such as bioremediation or natural attenuation, which can remove organic contaminants in the environment. What is helpful is that when organic contaminants are degraded in the environment, the ratio of stable isotopes will change, and the extent of degradation can be recognized and predicted from the change in the ratio of the stable isotopes. Recent advances in analytical chemistry make it possible to perform Compound Specific Isotope Analysis (CSIA) on dissolved organic contaminants such as chlorinated solvents, aromatic petroleum hydrocarbons, fuel oxygenates and many other organic chemicals, at concentrations in water that are near their regulatory standards.

Once we understood this, we can go one step further, and approach the research topic ofAlice Badin, aMarie Curie Fellow in the ADVOCATE network. Alice is working at the University of Neuchatel in Switzerland. Her research looks at the variability of carbon and stables isotope ratios in chlorinated ethenes, which are common groundwater contaminants, for various applications such as source identification and characterisation of biodegradation. The isotopic signature measurement of such solvents might be a great help in providing a rigorous basis to identify the source, timing and fate of chemicals released to soil and groundwater.

According to a previous research, the isotopic signatures (i.e. combination of isotopic ratios of chlorine, noted δ37Cl and carbon, noted δ13C in the solvent molecule) of pure compounds from different manufacturers were measured, it could be observed that the signatures varied depending on the manufacturer. Hence, in the field, neighbour spills might have different signatures, so when we don't know which spill is responsible for further downstream contamination, a comparison between the downstream signature and the suspected sources signatures might help delineating the responsible source (see drawing). However, there are few detailed case studies on the potential application, and the lack of signature variability at a country scale might be a brake to its use. This is the key reason why Alice's research is partly evaluating the variability in stable isotopic signature of these organic chemicals in Switzerland.

Scheme 1

Based on this, Alice first completed field studies where she measured the isotopic signature of tetrachloroethene (PCE)at 10 different contaminated sites in Switzerland.


Tetrachloroethene (PCE)

The question that Alice had to contend with was: "Do sites contaminated with PCE in Switzerland have similar stable isotopic signatures?" Although the sites were distributed throughout the country and represented different industrial activities, the PCE examined had very similar isotopic signatures. This thus limits the use of isotopic signature measurement for PCE source delineation in Switzerland. On the other hand, an average value of the stable isotopic signatures determined in these sites could represent a starting point for the assessment of PCE biodegradation at contaminated sites in Switzerland.

The next step in Alice's research was to assess the relationship between the δ13C and δ37Cl composition of chlorinated ethenes during PCE biodegradation, as this can further help assessing the extent of biodegradation in the field (see multistep biodegradation chain) Currently,the interpretation of this compound specific isotope data set is challenged by a shortage of experimental Cl isotope enrichment factors. Here, isotope enrichments factors for C and Cl were determined in the lab for biodegradation of PCE to TCE,using microbial enrichment cultures originating from an aquifer contaminated with chlorinated ethenes, which contains members of thebacterial genus Sulfurospirillum.

scheme 2

Multistep biodegradation: the most toxic compound vinyl chloride can eventually be degraded into not harmful ethene or inorganic carbon

Theselab experiments are also intended to help understanding better the mechanisms involved during degradation by looking at trends in the stable isotopic ratios. The aim is to relate these changes to some possible degradation pathways or mechanisms, but this part is still under discussion.

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