Stable Isotope Analysis of Organic Groundwater...

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Luc ZwankLuc ZwankMichael BergMichael BergRené SchwarzenbachRené SchwarzenbachTorsten SchmidtTorsten SchmidtStefan HaderleinStefan Haderlein

Stable Isotope Analysis of OrganicStable Isotope Analysis of OrganicGroundwater Contaminants:Groundwater Contaminants:

Applications and Field StudiesApplications and Field Studies

12C 12C

13C 12C

Stable Isotope Analysis of Organic Groundwater Contaminants:Stable Isotope Analysis of Organic Groundwater Contaminants:Applications and Field StudiesApplications and Field Studies

Compound-specific isotope analysis (CSIA) is a new approach to identify both, in-situ transformation processes ofpollutants in contaminated aquifers, and, sources of groundwater contaminants. Various laboratory studies havedemonstrated the potential of this tool for contaminant hydrology (1–3). However, CSIA is not very sensitive and limited tovery high pollutant concentrations.

With the target to significantly improve the method detection limits of CSIA, we therefore developed solid-phasemicroextraction (SPME) and purge&trap (P&T) pre-concentration techniques for isotope ratio mass spectrometry (IRMS)analysis. For both extraction techniques reproducible procedures for isotopic measurements of organic contaminants wereestablished. The detection limits for δ13C-determinations of volatile organic compounds could drastically be lowered to <5µg/L with the developed P&T-GC/IRMS method.

The SPME-technique allowed the simultaneous determination of δ13C-values of methyl tert-butyl ether (MTBE) and itsdegradation product tert-butyl alcohol (TBA) in a highly contaminated aquifer. The results support earlier investigations,which found in-situ degradation of MTBE to TBA. Yet, this new technique yields additional information about the MTBE-plume. The carbon isotopic data strongly suggest that the contaminated aquifer has two different zones, one with isotopefractionation linked to MTBE-degradation and a second where the observed concentration decrease is mainly due todilution or other non-fractionating processes. Furthermore the stable isotope data allow the identification of different spillzones at the site.

P&T extraction was successfully applied to study the fate of halogenated solvents in a contaminant plume down-gradient ofa municipal landfill in Switzerland. Contrary to previous evaluations based on concentration data alone, the isotopicsignatures show the absence of in-situ degradation of trichloroethene (TCE), despite the presence of cis-dichloroethene, aknown metabolite of TCE.

References

(1) Ahad, J.M.E, Lollar B.S., Edwards E.A., Slater G.F. and Sleep B.E. (2000) Environ. Sci. Technol. 34, 892–896.

(2) Hunkeler D., Aravena R. and Butler B.J. (1999) Environ. Sci. Technol. 33, 2733–2738.

(3) Meckenstock R.U., Morasch B., Warthmann R., Schink B., Annweiler E., Michaelis W., and Richnow H.H. (1999) Environ. Microbiol. 1,409–414.

OutlineOutline

Definitions•

Applications of Compound-specific Isotope Analysis (CSIA)Source identificationAssessment of in-situ degradation

Analytical SetupGas Chromatography Isotope Ratio Massspectrometry combined with different enrichment techniques

Field Application: Methyl tert-butyl ether (MTBE)•

Field Application: Trichloroethene (TCE)•

Conclusions•

DefinitionsDefinitions

„Delta Notation“

13 10×

dardSUnkown

Standard material for carbon:

PeeDee Belemnite (PDB): 0‰

Applications of CSIAApplications of CSIA

Source identification of groundwater contaminants

[Warmerdam, E. M. v et al. (1995), Applied Geochemistry 10: 547-552.]

Source identification of groundwater contaminants

Applications of CSIAApplications of CSIABiological transformations and chemical transformations

cause important isotopic fractionation

[Meckenstock, R. U et al. (1999), Environmental Microbiology 1(5): 409-414.]

Biological transformations and chemical transformations cause important isotopic fractionation

Isotopic FractionationIsotopic Fractionation

Biological and abiotic transformations cause importantisotopic fractionation (Kinetic Isotope Effect)

Analytical SetupAnalytical Setup

12C 12C

12CO212CO2

Analytical SetupAnalytical Setup13CO2

13 C 12 C

ChromatogramChromatogram

Analytical ChallengesAnalytical ChallengesCSIA is a promising tool in contaminant hydrology

MS requirement: 0.8 MS requirement: 0.8 nmolnmol C on column C on column

lliquidiquid on-column injection: 74 mg/L PER (1.5 on-column injection: 74 mg/L PER (1.5 µµL)L)

Groundwater concentrations are often below these detection limitsGroundwater concentrations are often below these detection limits

=> => Enrichment techniquesEnrichment techniques neededneeded

but it its broad use is limited by high detection limits

Analytical ChallengeAnalytical Challenge

Biological and abiotic transformations cause importantisotopic fractionation (Kinetic Isotope Effect)

Physical Processes such as evaporation, sorption, condensationhave also an effect on the isotopic signature of compounds(Equilibrium Isotope Effect)

=> Do enrichment techniques cause isotopic fractionation ???

Extraction procedure:Fiber material: Carboxen-PDMSExtraction times: 5-60 minutesNaCl concentration 4M

Solid Phase MicroextractionSolid Phase Microextraction

Purge and TrapPurge and Trap

Detection LimitsDetection Limits

Compound On Column [�g/L]

SPME [�g/L]

P&T [�g/L]

trans-1,2-dichloroethene 75’000 130 1.5

cis-1,2-dichloroethene 71’000 92 1.1

trichloroethene 84’000 94 1.4

tetrachloroethene 74’000 66 2.2 methyl tert-butyl ether 24’000 16 0.6

benzene 19’000 22 0.3

toluene 9’000 9 0.2

δδδδ1313C-ReproducibilityC-Reproducibility

-27.27 + 0.20-27.37 + 0.36-27.56 + 0.07-27.88 + 0.20benzene

-27.75 + 0.09-29.67 + 0.40-27.91 + 0.06-28.13 + 0.15methyl, tert-butylether

-26.76 + 0.19-28.16 + 1.31-27.19 + 0.17-27.32 + 0.14tetrachloroethene

-26.11 + 0.20-26.83 + 0.13-27.37 + 0.12-26.59 + 0.08trichloroethene

P&T(n=10)

SPME(n=10)

On Column(n=10)

ElementalAnalyzer (n=3)

Compound

Methyl tert-butyl EtherMethyl tert-butyl Ether

Used as oxygenate in gasoline (up to 15 % Vol.)

High aqueous solubility (ca. 50 g/L) + limited degradability

=> High mobility in groundwater

High production chemical (ca 20 Mio tons/year)

Major degradation product: tert-butanol (TBA)

Field Application: MTBEField Application: MTBE

-19.8-12.7-25.7

-13.9-18.4

-18.2-22.1

-26.9‰

-26.8‰

-26.1‰

Dilution

Degradation

=> Shifts in isotopic signature of MTBE and TBA indicate transformation

t [‰

Plume1

Plume2

Plume3

MTBE TBA

fCC tSubstrateSubstrate ln0,

1313 ⋅+∂=∂ ε

Quantification of the isotope effect:

The simplified Rayleigh equation (for ε <20 permil)

Typical ε for biological transformation of MTBE: 1.5 to 2.5 permil

Gray, J. R. et al. (2002). Environmental Science and Technology 36(9): 1931-1938.

Hunkeler, D. et al. (2001). Environmental Science and Technology 35(4): 676-681.

R2 = 0.9999

R2 = 0.7054

-10-7-6-5-4-3-2-10

ln (remaining MTBE fraction)

Center of Plume Fringe Zone of PlumeLinear (Center of Plume) Linear (Fringe Zone of Plume)

ε = -4.8 permil ε = -1.8 permil

-10-7-6-5-4-3-2-10

ln (remaining MTBE fraction)

All DataData from Plume 1

Preliminary conclusions for this site:

• MTBE in-situ degradation takes place

it seems however that degradation is strongestin the highly contaminated part of the aquifer

Anaerobic MTBE-degradation ?

Cometabolic MTBE-degradation ?

• Multiple spill sites contribute to the contamination

Further investigation will include more hydrological data as well as D/H-isotopic data

TrichloroetheneTrichloroetheneH

TCE cis-DCE

Used as metal degreasing agent (widespread)

High production chemical (190‘000 t/year Germany, 2000)

cis-DCE has no industrial use

Degraded to cis-DCE under anoxic conditions

=> Detection of cis-DCE is evidence for degradationof higher chlorinated compounds

Field Application: TCEField Application: TCE

1 2 3 4

Monitoring Well

TCE [ug/L]DCE [ug/L]Chloride [mg/L]

TCETCEDCE

ConclusionConclusion

P&T allows detection limits in the low µg/L-rangecorresponding to a 20-50 fold reduction compared to SPME

Compound-specific isotope analysis is a valuable tool in contaminant hydrology

The reduction of detection limits allows its application atmany field sites

ConclusionConclusion

Scientific application: Combination of CSIA and hydrological modelling to understand in-situ reaction mechanisms and gain in-situ degradation rates

Technical application: Determination of contamination sourcesDetermination of occurence of in-situ degradationDetermination of type and localization of in-situ degradation

AcknowledgementAcknowledgement

EAWAGJakov BolotinSamuel LuziCarsten Schubert

Institut BachemaMichel Schurter

Amt für Umwelt Kanton ThurgauDaniel Müller Andreas Scholtis

δ13C of pure phase

δδδδ1313C-ReproducibilityC-Reproducibility

On Column Injection Solid Phase Microextraction Purge and Trap

Understanding of Reaction Mechanisms

Reductive Dechlorination of CCl4

Understanding of Reaction Mechanisms

Reductive Dechlorination of CCl4

Iron Porphyrine (1 e- transfer)

Sulfide (2 e- transfer)

Fe(II) sorbed to Goethite

ε [‰

Solid Phase MicroextractionSolid Phase Microextraction

δ13C of pure phase

Methyl tert-butyl ether (MTBE)

0 10 20 30 40 50 60

Extraction time [min]

δ13C [

Extraction Efficiency [%] d13C [permil]

Purge and TrapPurge and Trap

δ13C of pure phase

Tetrachloroethene (PCE)

01020304050607080

0 10 20 30 40 50 60

Extraction time [min]

δ13C [

Extraction Efficiency [%] d13C [‰]

Luc Zwank is originally from Luxembourg. He graduated at the Swiss Federal Institutefor Technology in Zurich (ETHZ) and got his masters degree in Environmental Sciences,with the focus on environmental organic chemistry, at the Ralph M. Parsons Laboratory(MIT, Cambridge, MA, USA), under the supervision of Prof. P. M. Gschwend. Since May2000 is a Ph.D. student at the Swiss Federal Institute for Environmental Science andTechnology, supervised by Prof. R. P. Schwarzenbach, and focusses on thecharacterization of transformation processes of groundwater contaminants.

E-mail: Luc.Zwank@eawag.ch

Stable Isotope Analysis of Organic Groundwater...

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