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Most chemical elements have several isotopes: species with different numbers of isotopes and therefore different masses. Carbon has two stable isotopes – 13C (mass 13 Da) and 12C (mass 12 Da) – so has hydrogen (2H and 1H), nitrogen (15N and 14N) and chlorine (37Cl and 35Cl).
Isotopes of an element partake in the same reactions but at slightly different reaction rates. Molecules react slower if the reactive position is occupied by the” heavy” isotope (e.g. during dehalogenation where a 35Cl is removed faster than a 37Cl). This very slight difference in reaction rates (on the scale of ‰) is called the Kinetic Isotope Effect (KIE) and leads to isotope fractionation in the non-reacted pool of contaminant molecules (see Figure 1. Linearized Rayleigh Plot).

This means that during the course of contaminant degradation (Fig. 1 x-axis), the remaining primary contaminant molecules become enriched in the heavier isotope (Fig. 1 y-axis) whereas the products (secondary pollutants) may become depleted in heavier isotopes. The extent of a reaction can be estimated with CSIA (Compound-Specific Isotope Analysis) data from the field in combination with a laboratory-derived KIE. One particular strength of the CSIA assessment tool is that it provides quantitative diagnostics of key site characteristics while being independent of contaminant concentration, which may fluctuate due to e.g. dispersion, dilution, and volatilization.
Therefore CSIA provides direct evidence of the transformation processes acting on the contaminants at the site:
a)    The extent of the natural remediation (i.e. the percentage of remaining contaminant)
b)    The degradation rate (if combined with ancillary information)
c)    The type of reaction or process (e.g. microbial degradation pathway)
CSIA gives an edge compared to conventional techniques as it is unaffected by ‘known unknowns’ such as dilution and mass transport. Degradation rates can be obtained without extensive monitoring programs and are available early on in the decision making.

The research has received funding from the European Community's Seventh.
Framework Programme FP7/(2009-2012) under grant agreement no 212781.