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Compound Specific Stable Isotope Analyses (CSIA)
Identification of multiple sources of chlorinated solvents of a groundwater plume
Patrick Jacobs, Tauw, GermanySiegmund Ertl, Hydroisotop, Germany
Ralf Engelhardt, Tauw, GermanyFrank Volkering, Tauw, The Netherlands
Teresa Roza, Grupo EPA, SP, BrasilSérgio Rameh, Grupo EPA, SP, Brasil
Aline Nagata, Grupo EPA, SP, Brasil
Isotopes - Fundamentals
• Isotopes = species of the same element (same number of protonsand electrons) but with a different number of neutrons.
• Chemical properties are virtually the same for isotopes of the same element but the mass differs according to the number of nucleons.
• Most chemical elements have two or more isotopes which may be stable or radioactive, e.g.:
• Carbon: 12C (stable, 12 u), 13C (stable, 13 u), 14C (radioa., 14 u)• Hydrogen: 1H 2H 3H
Stable Isotopes - Background
• Specialized isotope laboratory separate different chemical species (e.g. PCE, TCE, DCE, VC) from a groundwater sample and subsequently determine the isotope ratios as 13C/12C and/or 37Cl/35Cl
• Isotope ratios are expressed in ‰ relative to areference standard
standard
standardsamplesample R
RR −⋅= 1000δ
Stable Isotopes - Background
• 13C-PCE of -23.5 ‰ means: 13C/12C in the sample is2,35 % lower than in the reference standard.
13C standard: VPDB (Vienna Pee Dee Belemnite); the ratio R=13C/12C=0,0112372 is defined by IAEA (International Atomic Energy Agency)
37Cl standard: SMOC (Standard mean ocean chlorine) the ratio R=37C/35C=0,2422 is defined by IUPAC (International Union of Pure and Applied Chemistry; Coplen 2002)
Isotopic ratios in environmental samples
• Isotopic ratios are a function of the starting material and its manufacturing process.
• Stable isotope ratios change in systematic ways during the course of biodegradation or other processes.
conc
entr
atio
n
source
plume
PCE
TCE
DCEVC reductive dechlorination
δ13
C-P
CE
13C-PCE enrichment
0
c0
0
δ0
Compound Specific Stable Isotope Analysis
• CSIA measures these small changes in isotopic ratios very precisely.
• Those changes can be exploited to gain robust information about the source, transport, and fate of a compound.
Analytical methods used
•δδδδ13C of chlorinated hydrocarbons and ethene:
Purge-and-Trap and GC Combustion Isotope Ratio Mass Spectroscopy (P&TGC-C-IRMS)
•δδδδ37Cl of PCE:
Purge-and-Trap and GC Mass Spectroscopy (P&T-GC-MS).
Analytical methods used
13C12C
δδδδ13C-VC
13C12C
δδδδ13C-cDCE
13C12C
δδδδ13C-TCE
Isotope-RatioMass spectrometer
13CO2+
12CO2+
Inte
nsity
time
13CO213CO2
13CO2
Inte
nsiti
ty 12CO212CO2
12CO2
GC
time
VCc-DCE
TCE
Injection
column
combustionfurnance
Watertrap
Ref.gas
time
CO2
CO2
CO2
time
VC c-DCE
TCE
62 64
M+/Z
96 98
M+/Z
130132
M+/Z
Mass spectrometer chlorine isotopes
Calculation of δδδδ37Clby the
fracmentationratios
VOClsample
Analytical methods used
Gas chromatograph
MassspectroMeterδδδδ37ClIsotope ratio
massSpectrometerδδδδ13C/δδδδ2H
Purge&Trap
Liquid Nitrogen
Referencegases
combustion furnace ( 13C)organic compounds to CO 2
pyrolysis furnace ( 2H)organic compound to H 2
CSIA - application
• CSIA can give evidence which type of substance has been spilled.
• CSIA can reveal whether or not microbial or abioticdegradation of organics are taking place.
• CSIA can help identify multiple sources of the same substance and the respective plumes.
Forensic approach
Source 1δ13C-PCE -26 ‰
Source 2δ13C-PCE -32 ‰
Plume 1δ13C-PCE -25 ‰
Plume 1δ13C-PCE -24 ‰
Plume 2δ13C-PCE -31 ‰
Plume 2δ13C-PCE -30 ‰
Plume ?δδδδ13C-PCE -28 ‰
Example study
• The CSIA approach as shown was applied successfully on a site in Brazil.
• For the sake of confidentiality, however, the example given here does not present the real site. The example details the technical approach and data interpretation.
Example site settings
Example site settings
Example site settings
Cross section
Hotspot 1 Hotspot 2
Deepwells
Interpretation
• First thesis :• Hotspot 1 = source• Hotspot 2 = plume emanating from Hotspot 1• Contamination of downstream deep wells = plume emanating from Hotspot 1
• Extensive transport modeling (FEFLOW) does not provide satisfactory explanation for contaminant distribution
• Contaminant transport not directed to Hotspot 2 nor to deep well!• Comparably negligible contamination in intermediate aquifer!
� First thesis not plausible!
� Second thesis : multiple sources with uncertain contribution to plumes
How to falsify first thesis?How to verify the second thesis?
���� CSIA
Investigation Concept
area 1
area 2area 3,deep well
1
2
3
41
2
1
��area 3 - 1
�area 2 - 2
��area 2 - 1
��area 1 - 4
�area 1 - 3
�area 1 - 2
��area 1 - 1
δ37Cl (PCE)δ13C (PCE-VC + Eth)
neighbor site
study site
VOCl concentrations: totals and percentage
~ no degradation weak d.
0
10000
20000
30000
40000
50000
60000 Ethene(Eth) / µg/L
Vinylchloride(VC) / µg/L
trans-Dichloroethene(tDCE) / µg/L
cis-Dichloroethene(cDCE) / µg/L
Trichloroethene(TCE) / µg/L
Tetrachloroethene(PCE) / µg/L
0%
20%
40%
60%
80%
100%
Area 1 - 1 Area 1 - 2 Area 1 - 3 Area 1 - 4 Area 2 - 1 Area 2 - 2 Areea 3 - 1
moderate d.
-30
-20
-10
0
10
20
30
Are
a 1
- 1
Are
a 1
- 2
Are
a 1
- 3
Are
a 1
- 4
Are
a 2
- 1
Are
a 2
- 2
Are
a 3
- 1
delta
13C
(PC
E)
-1,5
-1
-0,5
0
0,5
1
1,5
delta
37C
l (P
CE
)
δ13C-PCE
δ37Cl-PCE
CSIA results
area 1
area 2
area 3,deep well
1234
12
1
neighbor site
study site
CSIA results
area 1
area 2
area 3,deep well
1234
12
1
neighbor site
study site
-30
-20
-10
0
10
20
30
Are
a 1
- 1
Are
a 1
- 2
Are
a 1
- 3
Are
a 1
- 4
Are
a 2
- 1
Are
a 2
- 2
Are
a 3
- 1
delta
13C
(PC
E)
-1,5
-1
-0,5
0
0,5
1
1,5
delta
37C
l (P
CE
)
δ13C-PCE
δ37Cl-PCE
Observation:• Isotopic fingerprints within
area 1 consistent
• Isotopic fingerprints of area 2
and area 3 are significantly
different from area 1
Conclusion:• Contamination in area 1 most likely
has only one common source
• PCE in area 2 and in area 3 are heavier (less negative δ13C) but:
• this is not necessarily due to different sources!
• Also fractionation during bio-degradation depletes 12C over 13C!
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
00,20,40,60,81
Multiple sources vs. bio -fractionation
Area 1:f(PCE) ≈ 1,0
Area 1:f(PCE) ≈ 1,0
Area 2:f(PCE) ≈ 0,8
Area 2:f(PCE) ≈ 0,8 Area 3:
f(PCE) ≈ 0,4
Area 3:f(PCE) ≈ 0,4
bio-fractionation, ϵ = -5,7 … -2,4 … -1,36 ‰
f (PCE)
δ13
C(P
CE
) *
1000
Rayleigh function
Fitting of area 3 „fingerprint“
-35
-30
-25
-20
-15
-10
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
PCE degradation
delta
13C
(pr
omill
e)
PCETCEcDCEVC
• Fractionation model (PHREEQC) including all chlorinated ethenes according to analyzed species distribution area 3.
• The resulting ϵ-factor falls into the lower/medium range
ϵ
PCE -1,8 ‰TCE -3,5 ‰DCE -12 ‰
Optimized fit
parameters
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
00,20,40,60,81
0
1
2
3
4
5
6
7
8
-25 -23 -21 -19 -17 -15
Multiple sources vs. bio -fractionation
δ13C(PCE) * 1000
δ37
Cl(P
CE
) *
1000
Area 1Area 1
Area 2Area 2
Area 3Area 3
bio-fractionation, ϵ13C = -5,7 … -2,4 … -1,36 ‰
ϵ37Cl = -2,2
Conclusion (1)
• The CSIA campaign clearly proved:• Area 1 is caused by a single PCE source as both 13C-PCE and 37-Cl
signatures do not show relevant variation within the area
• Area 2 is not caused by the same source as area1. The isotopic
composition of PCE in area 2 cannot result from biotic degradation of
the source material when assuming plausible enrichment factors.
– Postulating a sufficient enrichment of 13C-PCE (ϵ ≈ -22!) would
go along with an extremely light daughter product composition, which is not found on site.
Conclusion (2)
• The CSIA campaign clearly proved:• Area 3 is not caused by the same source as area1. Its isotopic
composition of PCE could only result from biotic degradation at extremely strong enrichment factors.
– Fitting the isotopic fingerprint using a fractionation model shows that rather “normal” enrichment factors must be assumed.
– At these enrichment factors, the isotopic ratios at area 3cannot derived from the same source material as present at area1
END