Can carbon and chlorine stable isotope (δ13C – δ37Cl) act 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 of Alice Badin, a Marie 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 the bacterial genus Sulfurospirillum.

scheme 2

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

These lab 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.

After a painstaking and extensive study, Alice recently presented her results at the Isotopes 2013 conference in Sopot (Poland) under the heading “Carbon and chlorine isotopic trend in fingerprinting and anaerobic dechlorination of tetrachloroethene”

Badin A, Buttet G, Maillard J, Holliger C, & Hunkeler D (2014). Multiple dual C-Cl isotope patterns associated with reductive dechlorination of tetrachloroethene. Environmental science & technology, 48 (16), 9179-86 PMID: 25000152

The installation of the Vadose Monitoring System (VMS) was carried out successfully in Belgium last June

The installation of the Vadose Monitoring System (VMS) was carried out successfully in Belgium last June

I am pleased to announce that Natalia Fernandez together with her research group, HGeo³-Hydrogéologie et Géologie de l’Environnement, proceeded with the installation of the Vadose Monitoring System (VMS) in Belgium in June. The objective is to develop a methodology that is able to quantify contaminant fluxes, identify their sources and pathways and understand the various reactive processes in soil and groundwater.

The combined experiment consisted of a tracer test performed directly in the vadose zone via infiltration rings, located within an infiltration pond. To do this, a flexible sleeve was installed in a slanted borehole with the aim of capturing a tracer infiltrated throughout undisturbed material above the borehole. To measure water content, Flexible Time Domain Reflectometry probes (FTDR), which contain stainless steel waveguides, were installed in the outer wall of the flexible sleeve. As well, Vadose Sampling Ports (VSP) were placed in the inner wall of the flexible sleeve for sampling pore water in the vadose zone. Finally, additional boreholes were installed in the unsaturated zone to conduct cross-hole geophysics with the aim of monitoring contaminants and tracers as they move into the saturated zone (see it in the pictures).

The outlook of this experiment is to use the advantages of the combination of the Vadose Monitoring System and geophysical techniques with the aim of developing a conceptual model that better characterizes the transport of pollutants in the vadose zone of industrial sites. The objective is to use such a methodology as an approach to improve risk assessment and remediation measures for the vadose zone.

Natalia experiment 0 Natalia experiment 1Natalia experiment 2  Natalia experiment 2b Natalia experiment 3aNatalia experiment 3bNatalia experiment 4aNatalia experiment 4b   Natalia experiment 5

How much impact has climate change on contaminated land and pollutants?

How much impact has climate change on contaminated land and pollutants? 

How much impact has climate change on contaminated land and pollutants is an excellent question with a blank answer currently. The impact of climate change factors on the risk assessment, design of future remediation systems and management of current and future contaminated sites will be likely a key point that we should take into account or consider.

On the one hand, sustainability indicators in terms of environmental, economic and social are the basis for the sustainable remediation assessment of contaminated soils and groundwater. In this way, the UK Sustainable Remediation Forum (SuRF-UK) has developed a framework for assessing their sustainability, and for incorporating sustainable development criteria in land contamination management strategies, setting up in this sense a series of sustainability indicators for their remediation. These indicators are indicative of the range of issues that may be relevant, and are provided to help assessors identify the most critical issues to evaluate further in a project. As well, they highlight which are the challenges at global, national or local level.

On the other hand, in readiness for our future climate and its changes, there is a need to evaluate the risks of climate change and to predict how it is going to affect our future. In this way, some European projects have tried to give a response since 2000. PRUDENCE, Prediction of Regional scenarios and Uncertainties for Defining EuropeaN Climate change risks and Effects, was a European scale investigation project which aimed to quantify the confidence and the uncertainties in predictions of future climate and its impacts, using an array of climate models and impact models and expert judgement on their performance. Continuing the theme of this investigation, the project ENSEMBLES was carried out, based on Predictions of Climate Changes and their Impacts. This project aimed to build a common ensemble climate forecast system which would be developed for use across a range of timescales (seasonal, decadal and longer) and spatial scales (global, regional and local). So, this model system would be used to construct integrated scenarios of future climate change, including both non-intervention and stabilisation scenarios. ENSEMBLES ended in 2009 and immediately a new major project was started up, CORDEX, COordinated Regional climate Downscaling Experiment, which is an international project to produce an improved generation of regional climate change projections world-wide for input into impact and adaptation studies.

All this effort provides us a quantitative risk assessment of climate change and climate variability. On this basis, the next step would be to quantify the impact of climate change on contaminated land and to examine technical evidence of this impact and potential technical adaptation strategies that should be followed.

Although, all projects identified in this document have done extremely respectable and useful work, there is currently very little published work providing experimental evidence of potential direct impacts of climate change on contaminated land and remediation systems. The closest work is that which investigates and compares the impacts of different climatic regions on biological and chemical properties of contaminated soils and contaminant behaviour. Consequently there is a need for effort in this area to ensure that remediation choices being made now are the right ones by future land use, climatic conditions and societal demographics. 

So, in the United Kingdom, in 2007, a multi-institutional and multi-disciplinary research consortium was involved in a project called SUBR:IM, Sustainable Urban Brownfield Regeneration: Integrated Management, whose aim was producing integrated and sustainable solutions for the development of brownfield land in urban areas. They concluded that from the evidence available in the literature and collected as part of the study, it is clear that certain climate change scenarios are expected to have significant impacts on current and future contaminated land and remediation systems. These impacts will have major effects on the future management of contaminated and remediated sites and are expected to influence the way risk is managed on those sites and the design of future remediation strategies. 

However, this project is only the beginning of an emerging area of research. We still have a long way to go. It is important to set up a good correlation between the climate change and the current soil remediation technologies so that their implementation will not be a complete waste of time in the future when the environmental conditions change, in particular, those systems that required long time scales.