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Stable Isotopes Forensics – When Isotopes Work. Using isotopic analysis to determine the source and fate of groundwater contamination. What is stable isotope analysis?. identification of stable isotopes and chemical elements within chemical compounds.

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Stable Isotopes Forensics – When Isotopes Work

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Stable Isotopes Forensics – When Isotopes Work

Using isotopic analysis to determine the source and fate of groundwater contamination


What is stable isotope analysis?

  • identification of stable isotopes and chemical elements within chemical compounds.

  • Ability to demonstrate source and degradation of contaminants

    • Environment, bioremediation

    • Drug testing

    • Quality control


Isotope Ratios

  • Stable isotopes: naturally occurring forms of the same element with different numbers of neutrons

    • C13/C12

    • H1/H2

    • Cl35/Cl37

  • Stable isotope analysis based on determination of the ratios of two stable isotopes in a sample


Fractionation

  • Fractionation: partitioning of isotopes due to chemical, biological, geological and hydrological processes

    • source material left "heavier” (heavier isotope more abundant)

  • Differing molecular weights cause isotopes react at different rates

    • Kinetic Isotope Effect

  • Degradative vs non degradative processes


GC - IRMS

  • Gas Chromatography – Isotope Ratio Mass Spectrometry

  • Separates individual chemicals from a complex mixture based on molecular mass

    • Different isotopes have different masses!

  • High sensitivity, high precison, rapid

  • Provides an isotopic fingerprint that can be compared among samples


Source vs Fate

  • Isotopic composition conserved during environmental processes

    • Sources with distinct isotopic compositions

  • Contaminant fractionated by environmental processes

    • What processes have affected the contaminant and to what extent

    • Bioremediation & degradation

  • Determination independent of non degradative processes


Where are these compounds found?

  • BTEX: benzene, toluene, ethylbenzene and xylenes

    • Petroleum derivatives

      • Ex. Gasoline

    • CNS Toxin, carcinogen

  • TCE: Trichloroethene

    • Chlorinated ethene

    • degreaser, industrial solvent, dry cleaning chemical

    • CNS depressant, carcinogen, ALS, PD

  • MTBE: methyl tert-butyl ether

    • Gasoline additive

    • Potential carcinogen

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“Isotopic analysis is a powerful tool for investigating and monitoring of the sources and fates of environmental contaminants, provided it is applied in situations where the basis of knowledge is sufficient for isotopic forensics to work.” pg. 22


Criteria to identify degradation or source

  • Clear difference in isotope pools to be compared

    • Source differentiation: various source pools

    • Degradation: degraded and undegraded contaminant

  • Isotopic distinction greater than present variations/imprecisions

    • Environmental processes

    • Instrumental precision

  • Predictable isotopic behaviour


Case Study: TS-1, a TCE contaminated site

  • Anaerobic, unconfined aquifer

  • Within 6.5m deep glacial deposits

  • Underlying shale bedrock

  • Watertable 0.3-1m below ground

  • Two sources of TCE contamination


Sampling

  • Repeated samples must be taken both upstream and downstream (groundwater flow) of the contamination site

  • Multiple transects

  • Sufficient data for identification of trends

  • Decreased effect of variation


Source Zone

  • Characterized by very high depletion, saturation of contaminant

    • Depletion (low relative levels heavy isotope) indicates a lack of degradation

    • Enrichment (high relative levels heavy isotope) indicates degradation

  • Source can only be identified in a sample well with no degradation/fractionation


Temporal Variability

  • Repeated contamination events over time from a single or multiple sources

  • Extent of the effect on source determination determined by quantity added

  • Worst Case: enrichment must proceed beyond potential range source variation

  • Given a linear trend, the assumption can be made that temporal variability does not have an effect


Analysis

  • Isotope ratio converted to an internationally accepted standard value (δ)

  • Trends and samples analyzed by application of statistical models

    • Linear relationship between concentration and δ

  • Sampling and variability taken into account

  • Requires only prior knowledge of the enrichment factor (ε)

    • Available in literature

    • Based on known rates of degradation of isotopes of interest

    • Can be used to determine fractionation, degradation or source


Summary Slide

  • Site specific determination of presence of contaminants, bioremediation and sources of contamination

  • Highly sensitive

  • Highly precise

  • Growing field with potential to expand to include many other isotopes & unique applications


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