Different starting material and production mechanisms give rise to source diagnostic isotopic composition of the contaminants (Fig. 2). For instance, the relative contribution to the environmental load of PAHs may be deciphered with natural abundance radiocarbon measurements (14C/12C) as fossil fuel combustion and petroleum spills are 14C depleted (“radiocarbon dead”) whereas biofuel combustion has a modern 14C signal (“radiocarbon alive”). Similarly, different petroleum feedstocks in industrial synthesis may give rise to source unique CSIA patterns. To illustrate, natural gas and petroleum distillates have very different (2H/1H) composition. While the primary sources at a heavily contaminated brownfield may be clear, the more diffuse regional contamination of e.g. agricultural soils through atmospheric deposition and groundwater infiltration may benefit enormously from CSIA-based source apportionment to guide society regarding which sources to target for effectful mitigation.

CSIA is an intensive concept – the isotope signature or fingerprint of a chemical at a specific point in space and time reflects only isotope-fractionating processes such as degradation reactions. The isotope signature can be used to calculate the extent of degradation (see Figure 1) or trace sources (see Figure 2). The isotopic information is accumulated and retained in any selection of the molecular population. Hence, the isotopic information is decoupled from nonfractionating processes such as dilution, migration and mixing.
Isotopic fingerprinting is the only quantitative method to distinguish between structurally identical pollutants from different sources, in contrast to qualitative alternatives (e.g. based on source patterns of isomers or enantiomers).
This technique can be used to differentiate sources at a local scale (e.g. mixing plumes of solvents in soil or groundwater), regional scale (e.g. release of PAHs from biomass vs. fossil-fuel combustion) or where diffuse contamination may interfere with characterization of local pollution for a certain compound.

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