Using isotopic analysis to determine
the source and fate of groundwater
contamination
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identification of stable isotopes and chemical
elements within chemical compounds.
Ability to demonstrate source and degradation
of contaminants
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Environment, bioremediation
Drug testing
Quality control
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Stable isotopes: naturally occurring forms of
the same element with different numbers of
neutrons
C13/C12
 H1/H2
 Cl35/Cl37
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Stable isotope analysis based on determination
of the ratios of two stable isotopes in a sample
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Fractionation: partitioning of isotopes due to
chemical, biological, geological and
hydrological processes
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Differing molecular weights cause isotopes
react at different rates
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source material left "heavier” (heavier isotope more
abundant)
Kinetic Isotope Effect
Degradative vs non degradative processes
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Gas Chromatography – Isotope Ratio Mass
Spectrometry
Separates individual chemicals from a complex
mixture based on molecular mass
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Different isotopes have different masses!
High sensitivity, high precison, rapid
Provides an isotopic fingerprint that can be
compared among samples
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Isotopic composition conserved during
environmental processes
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Contaminant fractionated by environmental
processes
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Sources with distinct isotopic compositions
What processes have affected the contaminant and to
what extent
Bioremediation & degradation
Determination independent of non degradative
processes
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BTEX: benzene, toluene, ethylbenzene and xylenes
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Petroleum derivatives
 Ex. Gasoline
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CNS Toxin, carcinogen
TCE: Trichloroethene
Chlorinated ethene
 degreaser, industrial solvent, dry cleaning chemical
 CNS depressant, carcinogen, ALS, PD
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MTBE: methyl tert-butyl ether
Gasoline additive
 Potential carcinogen
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http://www.lenntech.com/images/gw-contamination.gif
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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
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Clear difference in isotope pools to be
compared
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Isotopic distinction greater than present
variations/imprecisions
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Source differentiation: various source pools
Degradation: degraded and undegraded
contaminant
Environmental processes
Instrumental precision
Predictable isotopic behaviour
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Anaerobic, unconfined
aquifer
Within 6.5m deep glacial
deposits
Underlying shale bedrock
Watertable 0.3-1m below
ground
Two sources of TCE
contamination
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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
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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
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Source can only be identified in a sample well
with no degradation/fractionation
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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
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Isotope ratio converted to an internationally
accepted standard value (δ)
Trends and samples analyzed by application of
statistical models
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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
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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