Compound Specific Isotope Analysis (CSIA)

Compound specific isotope analysis (CSIA) is an analytical method that measures the ratio of stable isotopes (e.g. 13C/12C, 2H/1H, or 37Cl/35Cl) of a contaminant.

Is Contaminant Degradation Occurring?

For some compounds, isotopic ratios change in predictable ways (e.g. isotopic fractionation) as the compound is degraded.  Conversely, physical processes like volatilization and dilution generally do not appreciably shift the isotopic ratios.

  • Therefore, CSIA can potentially provide direct evidence of ongoing contaminant degradation including reductive dechlorination of PCE, TCE, and daughter products.
  • The results can also be used to estimate the extent of contaminant degradation (fraction remaining or fraction degraded).

 

Why Does CSIA Work?

Degradation of some compounds can cause a shift in the isotopic ratios of the parent and daughter products in a process referred to as kinetic isotope fractionation.

Using the ratio of 13C /12C as an example…

  • Chemical bonds formed by the heavier isotope (13C) are slightly stronger than bonds formed by the lighter isotope (12C).
  • Therefore, molecules of the contaminant with the lighter isotope (12C) tend to be degraded more quickly than molecules containing the heavier (13C) isotope.
  • Faster or preferential degradation of molecules with the lighter isotope (12C) means that during degradation, the remaining parent compound becomes enrichened in 13C (increased δ13C) while the daughter compound is initially 13C depleted (decreased δ13C).
  • Ultimately when degradation is complete, the isotope ratio of the final product (e.g. ethene) will equal the initial isotope ratio of the parent compound.

 

Applicable Contaminants

Isotopes Analyzed to Assess Contaminant Degradation

Carbon Isotope Ratio (13C/12C):  Analysis of carbon isotopes is the most frequently used approach to assess degradation of a number of common groundwater contaminants including chlorinated ethenes, ethanes, and methanes.

Hydrogen Isotope Ratio (2H/H) and Chlorine Isotope Ratio (37Cl/35Cl):  Two dimensional compound specific isotope analysis (2D-CSIA) is simply the analysis of the isotope ratios of multiple elements (e.g. 13C/12C, 2H/1H, or 37Cl/35Cl).  2D-CSIA is more sensitive than single-element CSIA and should be considered for certain applications.

Reporting Stable Isotope Ratios

The ratio of stable isotopes is reported as a delta value (δ). For carbon isotopes, it is written as δ13C, and for chlorine isotopes it is reported as δ37Cl. The heavy isotope of hydrogen (2H) is also called deuterium (D), so the stable isotope ratio of hydrogen may be written as either δ2H or δD.

Range of Values of δ13C in Manufactured Compounds

If the δ13C value of a contaminant increases by 2‰ over time or downgradient, it is seen as proof that the compound is degrading. However, often the original δ13C value is unknown and must be approximated. There are two ways to approach this approximation, based on the EPA’s CSIA Guidance Document:

  1. The most negative δ13C value of the parent compound at or near the source area can be treated as the δ13Csource. As a parent compound degrades, the δ13C can only become more positive, so the most negative value found at the source area is most similar to the original value.
  2. A published literature value of the manufactured, undegraded parent compound can be used as an approximation.

This table contains the published δ13C values for some common, undegraded, chlorinated contaminants. Displayed, for each compound, are the most negative δ13C values recorded (δ13C Min), the most positive values recorded (δ13C Max), the average of all values recorded (δ13C Mean), the range of δ13C values for the compound, and the number of samples analyzed for that compound (n). Because the δ13C increases throughout degradation, selecting the most positive δ13C value (δ13C Max) of the contaminant is the most conservative approach. This table will be continually updated to reflect the current literature.

 

 


Enrichment Factors

As a compound is degraded, the delta value (i.e. δ13C, δ37Cl) for that compound will increase. An isotope enrichment factor (ɛ) relates the change in delta value (Δ δ13C) of an isotope within a compound to the fraction of that compound that remains after degradation. The enrichment factor is dictated by the isotope, the compound, the degradation pathway, and the site conditions.

A very negative enrichment factor for a compound results in a large change in delta value as a compound is degraded. A less negative enrichment factor (one that is closer to zero) means that there is little change in the delta value throughout the degradation process.

An enrichment factor can be used to infer information about the extent of biodegradation. For example, the equation

can be used to estimate the first order rate constant (kx) for degradation with distance along a flow path based on the change of δ13C (with distance (x) along a flow path.

There are several different approaches for choosing an isotope enrichment factor based on the EPA’s CSIA Guidance Document:

  1. Select a published enrichment factor that has been reported under similar conditions to your site. For the most conservative estimate of the extent of biodegradation, select the most negative enrichment factor that has been published for the contaminant of concern. When assuming a very negative ɛ, a large shift in delta value translates to less degradation than when assuming a more positive ɛ, making the most negative enrichment factor the most conservative option.
  2. Calculate the range of possible degradation rates using the most negative and least negative enrichment factors that have been published for the contaminant of concern.
  3. Base the degradation rate on the mean and standard deviation of all published enrichment factors for the contaminant of concern.

This table contains published ɛ for the carbon isotopes of some common chlorinated contaminants, along with information on the conditions of each site. The table will be continually updated to reflect the current literature.

Coming soon!

When submitting CSIA samples to Microbial Insights, the submitter will be provided a passcode. This passcode will allow access to comprehensive tables that document published enrichment factors and the original delta values of manufactured compounds. Using their uploaded concentration and CSIA data, the passcode will also enable the user to generate plots that can be incorporated into site characterization, degradation determination, and source distinction. These plots can be directly copied and pasted for easy reporting and record-keeping.