A Robust Model for Biotic and Abiotic Degradation of Chlorinated Aliphatic Hydrocarbons in an In Situ Bioreactor

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1 A Robust Model for Biotic and Abiotic Degradation of Chlorinated Aliphatic Hydrocarbons in an In Situ Bioreactor Robert Edwards- Noblis Gabriel Moreno-Ferguson Camp Stanley Storage Activity Chris Beal - Portage Environmental Susan Baez-Cazull, PhD - Noblis 1

2 Site Background Former disposal trenches Groundwater contaminated with PCE, TCE, and cis-1,2- DCE DNAPL in fractures of vadose zone Bioreactor created to degrade contaminants in extracted groundwater: PCE TCE DCE VC Ethene Debris removed and trenches backfilled with wood mulch carbon substrate/electron donor Operation began in early 2007 Bioaugmented with culture KB-1 in February 2008 Bioreactor sumps pink circles West Bay multi-level monitoring wells green circles Extraction wells dark red/magenta 2

3 Bioreactor Construction 3

4 Bioaugmentation of Bioreactor Trench 4

5 Bioreactor Anomalies During the first seven months of operation, dechlorination occurred in absence of DHC functional genes (i.e., tcea, vcra, and bvca) Reductive dechlorination does not seem to account for all degradation occurring in bioreactor Redox conditions cycle between manganese reducing and methanogenic conditions Can other degradation pathways account for these observations? 5

6 Results For DHC 16S and Functional Genes 6

7 Reductive Dechlorination Pathway in Bioreactor Post-Bioaugmentation 7

8 900 Redox Processes October 2009-August 2010 T Methanogenesis Sulfate Reduction and Methanogenesis Mn (μg/l) CH4 and SO4, μg/l Mn Reduction Mn[II] SO4 CH /25/ /14/ /3/2009 1/22/2010 3/13/2010 5/2/2010 6/21/2010 8/10/2010 9/29/2010 Month 10 8

9 Project Objectives and Execution Understand the biogeochemical processes that may contribute to contaminant degradation in bioreactor Biotic reductive dechlorination, anaerobic oxidation Abiotic dechlorination by metals Execution: Evaluate concentrations of metals and inorganics to identify potential involvement in contaminant degradation Deploy Stable Isotope Probes to investigate alternate degradation pathways 9

10 Project Objectives and Execution - concluded Identify key monitoring parameters to track remedy performance Selected microbial populations by qpcr Key indicators ( such as sulfate, manganese, iron) of biogeochemical processes involved in contaminant degradation Execution: Identify other microbes that may participate in degradation pathways other than reductive dechlorination Identify optimization opportunities to accelerate cleanup Develop conceptual model to facilitate implementation of full-scale remedy 10

11 Stoichiometric Relationship Between Mn[II] Production and CAH Disappearance in Bioreactor electron µeq Required for Mn[IV] Reduction Average Total CAH Bioreactor, µm electron µeq Produced by cis-dce Oxidation Ratio of electron µeq Produced to electron µeq Required µm Mn[II] Total CAH Δ of CAH, Sampling Period Produced Input, µm µm Aug08-Oct Nov08-Jan Feb09-Apr May09-Jul Aug09-Oct Average For All Periods Anaerobic Oxidation Pathway CH 2 CHCl + Mn[IV] + 4H 2 O 2CO 2 + Mn[II] + Cl + 11H + (equation 1) CHClCHCl + 2Mn[IV] + 4H 2 O 2CO 2 +2Mn[II] + 10H + + 2Cl (equation 2) Abiotic Reduction of Manganese H 2 S + MnO 2 S 0 + Mn[II] + 2 OH - (equation 3) 11

12 Deployment of 13 C-PCE Stable Isotope Probes (SIP) Biotrap contained 13 PCE and a sorbent Microbes utilize 13 PCE in degradation reactions; daughter products are sorbed in Bio-Trap for subsequent analysis 12

13 Results for Stable Isotope Probes Containing 13 C-PCE Deployed in Bioreactor Sample T1-1 Sample T1-2 Sample T1-3 % 13 C-Contaminant Loss % 13 C Incorporation % 13 C Mineralization DIC δ 13 C ( ) Average PLFA δ 13 C ( ) Data From July-August 2010; DIC = Dissolved Inorganic Carbon; PLFA = Phospholipid Fatty Acid Isotopic fractionation of 13 C in DIC Above background values (-5 to +5 ) Consistent with oxidation of 13 DCE/VC to CO 2 Isotopic fractionation of 13 C in PLFA Above typical background values (-20 to -30 ) Incorporation of some 13 C by an assimilatory pathway (i.e., oxidative acetogenesis) Acetate can be further oxidized to CO 2 using Mn[IV] and Fe[III] as electron acceptors PLFA analysis indicated presence of metal-reducing bacteria Results of SIP supports anaerobic oxidation pathway for DCE/VC 13

14 Reduction of SO 4-2 Concentrations in Bioreactor May be Tied to Reduction of Iron Sampling Period Weighted Average SO 4-2 Input, mm Average SO 4-2 Bioreactor, mm Δ in SO 4-2, mm % Reduction in SO 4-2 Aug08-Oct Nov08-Jan Feb09-Apr May09-Jul Average For All Periods SO e 2HS 2FeS 2Fe[III] + S 2 O e HS + SO 4-2 IRB + 2FeOOH Fe[II] C 2 Cl 4 C 2 Cl 3 (equation 4) Results for SO 4-2 are consistent with the hypothesis that FeS is formed by reduction of Fe[III] by HS, and subsequently oxidized back to Fe[III] during dechlorination, forming thiosulfate which disproportionates to HS and SO 4-2 (equation 4) 14

15 Iron Reducing Bacteria Are a Significant Component of Bioreactor Microbial Population Bioreactor Sample IRB/SRB, copies/l APS, copies/l T E E T E E T E E % IRB IRB Iron Reducing Bacteria SRB Sulfate Reducing Bacteria APS adenosine phosphosulfate reductase The presence of iron reducing bacteria supports the presence of a biotic reduction of Fe[III] coupled with an abiotic reductive dechlorination and oxidation of Fe[II] back to Fe[III] 15

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17 Bioavailable Metals in Minerals T1-2 T1-3 T2-2 Background Units Acid Volatile Sulfide (FeS) < mmoles/kg Bio-available Fe[III] mmoles/kg Bio-available Mn[IV] <0.131 <0.116 < mmoles/kg CES (as FeS 2 ) < <8.334 <8.334 mmoles/kg Strong Acid Soluble Mn[II] < < mmoles/kg Strong Acid Soluble Fe[III] < mmoles/kg Strong Acid Soluble Fe[II] < mmoles/kg Interpretation: - Fe[II] in minerals is electron donor (i.e., dechlorination of PCE) - Fe[III] is - not due to oxidation by DO as bioreactor conditions are anoxic - bioavailable as an electron acceptor in oxidation of organic substrates - by IRB, and - also cycles back to Fe[II] by IRB - Mn is apparently depleted in minerals 17

18 Summary Anaerobic oxidation of cis-dce can theoretically account for most of the Mn observed in bioreactor Stable Isotope Probe results: Indicate anaerobic oxidation of CAH to CO 2 occurs Identified presence of metal-reducing bacteria AMIBA Analysis Reduced Fe has been formed, possibly by reaction with HS -1 An Fe cycle between oxidized and reduced appears to be operating Reduced Fe may contribute to dechlorination However, Mn has been depleted in minerals in bioreactor 18

19 Conclusions Reductive dechlorination by Dehalococcoides is only one pathway operating to degrade contamination Geochemistry of a natural system is important! Anaerobic oxidation and dechlorination by metals may be important Key monitoring parameters may include Metals in soil (AMIBA analysis) Metal-reducing bacteria Carbon isotope ratios of DIC Optimization opportunity Addition of MnO2 19

20 Proposed Bioreactor Model HS -1 + SO 4-2 Disproportionation S 2 O 3-2 H 2 S Dissimilatory reduction SO 4-2 SRB Bioreactor carbon substrate, VFAs Mn[II] e -1 CO 2 (microbial) Fe[II] HS -1 PCE/TCE/DCE e -1 (abiotic) (biotic) e -1 CHO (microbial DHC, SRB?) Mn[IV] DCE/VC Fe[III] TCE/DCE/VC Ethene e -1 CO 2 IRB Bioreactor carbon substrate, VFAs