using isotopic analysis to determine the source and fate of groundwater contamination
TRANSCRIPT
Using isotopic analysis to determine the source and fate of
groundwater contamination
identification of stable isotopes and chemical elements within chemical compounds.
Ability to demonstrate source and degradation of contaminants Environment, bioremediation Drug testing Quality control
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: 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
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
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
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
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
Anaerobic, unconfined aquifer
Within 6.5m deep glacial deposits
Underlying shale bedrock Watertable 0.3-1m below
ground Two sources of TCE
contamination
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
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
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
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
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