**Compound Specific Isotope Analysis (CSIA)**

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

- Chemical bonds formed by the heavier isotope (
^{13}C) are slightly stronger than bonds formed by the lighter isotope (^{12}C). - Therefore, molecules of the contaminant with the lighter isotope (
^{12}C) tend to be degraded more quickly than molecules containing the heavier (^{13}C) isotope. - Faster or preferential degradation of molecules with the lighter isotope (
^{12}C) means that during degradation, the remaining parent compound becomes enrichened in ^{13}C (increased δ^{13}C) while the daughter compound is initially ^{13}C depleted (decreased δ^{13}C). - 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 (^{13}C/^{12}C): 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 (^{2}H/H) and Chlorine Isotope Ratio (^{37}Cl/^{35}Cl): Two dimensional compound specific isotope analysis (2D-CSIA) is simply the analysis of the isotope ratios of multiple elements (e.g. ^{13}C/^{12}C, ^{2}H/^{1}H, or ^{37}Cl/^{35}Cl). 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 δ^{13}C, and for chlorine isotopes it is reported as δ^{37}Cl. The heavy isotope of hydrogen (^{2}H) is also called deuterium (D), so the stable isotope ratio of hydrogen may be written as either δ^{2}H or δD.

**Range of Values of δ**^{13}C **in Manufactured Compounds **

If the δ^{13}C 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 δ^{13}C value is unknown and must be approximated. There are two ways to approach this approximation, based on the EPA’s CSIA Guidance Document:

- The most negative δ
^{13}C value of the parent compound at or near the source area can be treated as the δ^{13}C_{source}. As a parent compound degrades, the δ^{13}C can only become more positive, so the most negative value found at the source area is most similar to the original value. - A published literature value of the manufactured, undegraded parent compound can be used as an approximation.

This table contains the published δ^{13}C values for some common, undegraded, chlorinated contaminants. Displayed, for each compound, are the most negative δ^{13}C values recorded (δ^{13}C Min), the most positive values recorded (δ^{13}C Max), the average of all values recorded (δ^{13}C Mean), the range of δ^{13}C values for the compound, and the number of samples analyzed for that compound (n). Because the δ^{13}C increases throughout degradation, selecting the most positive δ^{13}C value (δ^{13}C 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. δ^{13}C, δ^{37}Cl) for that compound will increase. An isotope enrichment factor (ɛ) relates the change in delta value (Δ δ^{13}C) 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 (*k*_{x}) for degradation with distance along a flow path based on the change of δ^{13}C (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:

- 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.
- 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.
- 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.

**Get the most out of your data for better site management decisions.**

**The MI CSIA Database offers manufactured ranges, enrichment factors, and associated references compiled into ready to use tables. Access available with your CSIA report.**

The MI Workstation allows you to create and download custom CSIA plots that combine your site data with appropriate literature values to simplify interpretation.

Demonstrate

contaminant degradation.

Delineate

contaminant sources with Dual Isotope Plots.

Investigate

degradation mechanisms and extent of degradation with Modified Kuder Plots.

Plus

How-to guidance and examples