O. Atteia On the Role of chlorinated solvents degradation sequences for the simulation of natural attenuation at real sites

Transcription

1 On the Role of chlorinated solvents degradation sequences for the simulation of natural attenuation at real sites, C. Guillot de Suduiraut, Institut EGID, 1 Allée Daguin, Pessac Cedex

2 Natural attenuation of chlorinated solvents Their degradation has been shown at different sites Most of the degradation occurs under anaerobic conditions Several similar protocols are applied but long term prediction remains uncertain

3 Typical degradation sequence 1 O 2 CO 2 2 SO 4 CO 2 i H 2 O CH i S -II CH H + H 2 CO 2 k CH TCE DCE H + H 2 O 2 H 2 O k CO 2 VC DCE VC k H + H k 2 O 2 H 2 O CO 2 DCE VC Eth k H + H 2 k

4 Uncertainties Concurrence between microbes to use H 2 (methanogenesis, sulphate reduction, dechlorination) Kinetics of H 2 production during fermentation

5 Laboratory Kinetics 20 field days years Half-lives TCE

6 Is natural attenuation sufficient? Conditions to reach C=Co/1000 at 1 km from the source after a long time ( years) C=20 mg/l C=20 µg/l 1 km

7 For a tracer 1E+6 distance to reach C/Co = 1/1000 (km) 1E+5 1E+4 1E+3 1E+2 α T = 1 m α T = 10 m α T = 100 m 1E source width

8 Developed approach Rflow a semi-analytical reactive code developed at EGID Similar to Biochlor for Chlorinated solvents sequence Able to simulate any other reaction, e.g. production of H 2 due to fermentation Also work on irregular flow pattern

9 Test on basic solutions Mol/L 2.00E E E E E E E E E E E+00 TCE_PHAST DCE_PHAST VCE_PHAST TCE_Rflow DCE_Rflow VC_Rflow Tracer PHAST : coupled geochemistrytransport model (Phreeqc-RT3D) Rflow : Splitted transportgeochemistry, sequential chemistry 1000 times faster

10 Lateral profile 1.20E E-04 C (mol/l) 8.00E E E-05 TCE_phast DCE VCE O/5 TCE_Rflow DCE_Rflow VC O2/5 2.00E E transverse distance (m)

11 Limits of TCE T degradation C TCE TCE is degraded H 2 TCE simply dispersed x

12 Exploring the domain 250 Source width = 20 m 200 TCE half-life (days) at=0,5 m at=1 at=2 at=5 at= FOC (mg/l)

13 Concurrence for H 2 H 2 production Fast H 2 consumption by natural electron acceptors (O 2, NO 3, SO 4 ) H 2 remaining for dechlorination

14 smaller sources longer TCEplume! 250 ys = 10 m 20 m 30 m 200 TCE half-life (days) at=0,5 m at=1 at=2 at=5 at= FOC (mg/l)

15 TCE 1-10 mg/l mg/l > 100 mg/l O 2 ~10000 µg/l Offutt AFB: unsufficient attenuation 1-5 mg/l < 1 mg/l TCE biodegradation rate: t 1/2 2.7 to 4.7 years or! t 1/2 47 to 63 years Methane to 0.18 mg/l

16 Limits for DCE and VC C TCE potential H 2 DCE C max = C o /1000 VC x

17 Restricting the domain TCE half-life (days) DCE or VC at=0,5 m at=1 at=2 at=5 at=10 50 t 1/2 DCE ~ t 1/2 TCE t 1/2 DCE ~ 1.1 t 1/2 TCE FOC (mg/l)

18 Does DCE/VC oxidati tion helps? C no TCE potential H 2 O 2 DCE VC x

19 Extension of TCE and VC plumes St-Joseph TCE VC TCE 1000 VC Dover Plattsburg TCE VC

20 Field case : plattsburgh (1) data (Wiedmeier et al. 1997) model (α L =50 m, α T = 4 m, t =25 yr)

21 Field case : Plattsburgh (2) data model

22 constants and zones field estimated (TMB, Cl) A-B B-C C-E TCE DCE VC model A-C C-E TCE 1 0 DCE (O 2 ) VC (O 2 )

23 What happens in the field! Thermo-Chem sites, contaminated with DNAPL and LNAPL Source BTEX, PCE, TCE geoprobe profiles & multi-level wells 200 m

24 A D PCE B 100 C 35 BTEX TCE H 2 0 c-dce SO 4 VC 0

25 How to deal with real world calculated from field : α y <0.1 m, α z ~1 mm C TCE D B : no degradation PCE VC DCE A local velocity variations are higher than one order of magnitude what is the meaning of an average sample?

26 Sources need to be mixed LightNAPL DenseNAPL argile

27 Conclusion In a first approach, a complete dechlorination within acceptable distance requires drastic conditions A key issue is the potential entrance of electron acceptors in the plume (reduces H 2 for dechlorination) This depends on lateral dispersivity

28 Conclusion (2) Good news real lateral dispersivity is small we are in good conditions for DNAPL to degrade We cannot always use geoprobes & multilevel wells which type of models Take care that degradable organic matter and DNAPL are together