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