Natural Attenuation - Determination of biological degradation by means of the 13C method:
Microorganisms cause a modification of the natural isotope ratio (e.g. 13C/12C) of a substrate due to their degradation of the pollutants. Preferably, the microorganisms dispose of the pollutants which are build by the "light" isotopomeres (compounds with relatively high 12C proportion). Thus, the "heavy" isotopomeres (compounds with relatively high 13C proportion) will be enriched in the residual pollutant fraction (see Fig. 1).
Applying isotope methods the degradation of a substance or a substance group, can be determined through the shift of the stable isotope ratios in the not yet degraded substance. This way the degradation can be verified and conclusions about the real efficiency of the degradation can be drawn. Since the impact of other processes on the isotope ratios is insignificant the natural attenuation can be quantified specificly with respect to the microbial degredation of the pollutants. A potential reduction of the pollutants by dilution or adsorption processes will not affect the isotope ratios.
The following effects cause a reduction of the polluntants althrough natural attenuation does not occur:
- Dissolution and dilution
- Precipitation or dissolution processes of anorganic substances with the effect of degradation of pollutants (redox reactions, dechloration)
- Diffusion in groundwater due to differences in concentration
- Adsorption at organic or anorganic soil substances
- Degassing of volatile compounds
For a more detailed evaluation the 13C balance sum is derived from the mol fractions and the analysed δ13C values of the single substances of the chlorinated ethenes.
Σδ13C(PCE-VC) represents the molar weighted sum of the δ13C values of the compounds PCE, TCE, cDCE, tDCE, and VC.
Σδ13C(PCE-ethene) represents the molar weighted sum of the δ13C values of the compounds PCE, TCE, cDCE, tDCE, VC, and ethene.
In case there are very high biological activities in the area of interest (HCH degradation processes similar to lab conditions) a shift of the δ13C values (isotope fracitonation) might be observed due to the microbial degradation. This isotope effect is to be found both in the primary substance (isotope enrichment; shift to "heavier" values) as well as in the produced metabolites (isotope depletion with subsequent shift to mostly "heavier" values compared to the initial value). Analysing this data against calculated isotope contents allows to recognize the shift and to further interpretate it if applicable. The calculation is done by means of kinetics of first order which is used to successfully describe the isotope effects in many applications. The 13C primary signature of the technical initial substance acts as free parameter in the calculation respectively adjustment.
The enrichment factors (ε) known from lab studies of the metabilisation of PCE, TCE, cDCE, and VC are summerized in Tab. 1..
Tab.1: Fraction factors ε and their variation at different degradation types found in literature.
|Compound||ε [‰ ]||Degradation|
|PCE - TCE||-0.4 to -16.1||anaerobic|
|TCE - cDCE||-2.5 to -22.9||anaerobic|
|cDCE - VC||-12.0 to -21.1||anaerobic|
|VC - ethene||-21.5 to -31.1||anaerobic|
The so-called "sum signature" is used to evaluate the biological degradation. This requires a reduction of the complex multi-component system of degradation of PCE or TCE to non-chlorinated compounds to the one-component system (chlorinated compound reacts to non-chlorinated compound) for which the mass preservation is given. The sum of the enrichment factors εΣ for each single step provides a measure for the isotopic enrichment.Starting point for the description is the "lightest" plausible primary signature including the analytical uncertainty. Due to the enrichment of the sum signature the part of the natural attenuation which describes the degradation in groundwater can be determined.
If all components describing this system are captured ideally the 13C sum values Σδ13C(PCE-VC) will be on the degradation curve. The distance of the analytical value on the Σδ13C(PCE-VC) axis to the primary signature may be used as a measure of the residual HCH potential. However, should reactions occur indicating a further degradation (e.g. the metabolism of ethene via ethane to CO2 and CH4) the residual components will be further enriched and should be found above the calculated curve. This means the bigger the difference between Σδ13C(PCE-VC) and the primary signature the smaller the residual HCH potential.