Monitored Natural Attenuation Of Groundwater Nitrate


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MNA of Groundwater Nitrate, Presented by Brad Esser, PhD at AFCEE Conf. San Antonio, TX April 2010

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Monitored Natural Attenuation Of Groundwater Nitrate

  1. 1. MONITORED NATURAL ATTENUATION OF GROUNDWATER NITRATE Characterization using nitrate isotopic composition and excess nitrogen Dr. Bradley K. Esser Lawrence Livermore National Laboratory Robert A. Ferry Brown and Caldwell Victor Madrid Lawrence Livermore National Laboratory Mike Singleton Lawrence Livermore National Laboratory 08 April 2010 San Antonio, TX
  2. 2. Groundwater Nitrate Outline of talk <ul><li>LLNL investigations and capabilities </li></ul><ul><li>Denitrification </li></ul><ul><li>Nitrate isotopic composition </li></ul><ul><li>Excess nitrogen </li></ul><ul><li>An MNA case study </li></ul><ul><li>Regulatory framework </li></ul><ul><li>Upcoming work </li></ul>
  3. 3. LLNL analytical capabilities <ul><li>LLNL site investigations </li></ul><ul><ul><li>Successful approval of monitored natural attenuation as a CERCLA remedy for groundwater nitrate contamination on a high explosives test range </li></ul></ul><ul><li>LLNL Work for Others Program investigations for water agencies </li></ul><ul><ul><li>California Water Board: Groundwater Ambient Monitoring & Assessment (GAMA) program </li></ul></ul><ul><ul><li>California water districts: Orange County, Metropolitan, Santa Clara </li></ul></ul><ul><li>Strong collaborations with academic researchers </li></ul><ul><ul><li>University of California, several campuses </li></ul></ul><ul><ul><li>California State University, East Bay </li></ul></ul><ul><ul><li>University of Texas, Austin </li></ul></ul>
  4. 4. LLNL analytical capabilities <ul><li>Stable isotope mass spectrometry </li></ul><ul><ul><li>Nitrate-  15 N,  -  18 O ; H 2 O-  D, -  18 O; DIC/DOC-  13 C; SO 4 -  34 S </li></ul></ul><ul><ul><li>Bacterial denitrifier method for analysis of nitrate </li></ul></ul><ul><li>Membrane-inlet mass spectrometry </li></ul><ul><ul><li>Excess air and excess nitrogen: N 2 , O 2 , Ar, CO 2 , CH 4 </li></ul></ul><ul><li>Noble gas mass spectrometry </li></ul><ul><ul><li>Tritium- 3 He and 4 He groundwater age dating </li></ul></ul><ul><ul><li>Groundwater recharge temperature (xenon) </li></ul></ul><ul><ul><li>Excess air (neon) </li></ul></ul><ul><li>Trace constituent analysis: ICPMS, LC/GC-MS </li></ul><ul><li>Groundwater flow and contaminant transport modeling </li></ul>
  5. 5. Denitrification is a microbial redox reaction that converts nitrate to molecular nitrogen <ul><li>Heterotrophic denitrification </li></ul><ul><li> 4 NO 3 - + 5 CH 2 O ( organic C ) + 4 H +  2 N 2 + 5 CO 2 + 7 H 2 O </li></ul><ul><li>Autotrophic denitrification </li></ul><ul><li>14 NO 3 - + 5 FeS 2 ( pyrite ) + 4 H +  7 N 2 + 10 SO 4 2- + 5 Fe 2+ + 2 H 2 O </li></ul><ul><li>14 NO 3 - + 5 Fe +2 ( reduced iron ) + 7 H 2 O  0.5 N 2 + 5 FeOOH (goethite) + 9 H + </li></ul><ul><li>Denitrification requires </li></ul><ul><ul><li>Denitrifying bacteria </li></ul></ul><ul><ul><li>An electron donor </li></ul></ul><ul><ul><li>Low oxygen conditions (< 0.6 mg/L) </li></ul></ul>NO 3 -  NO 2 -  NO  N 2 0  N 2 Nitrate (+5) Nitrite (+3) Nitric Oxide (+2) Nitrous Oxide (+1) Nitrogen (0)
  6. 6. Monitored natural attenuation <ul><li>Site Characterization Objectives </li></ul><ul><li>Demonstrate active removal. </li></ul><ul><li>Determine the mechanism and rate of attenuation. </li></ul><ul><li>Determine the attenuation capacity of the aquifer. </li></ul>Ford, R. G., Wilkin, R. T., and Puls, R. W., 2007. U.S. Environmental Protection Agency.
  7. 7. Tools for characterization of denitrification <ul><li>“ Excess” nitrogen: end-product of denitrification </li></ul><ul><li>Nitrate isotopic composition: dual isotope approach </li></ul><ul><li>PCR surveys of denitrifying bacterial populations </li></ul><ul><li>Sulfur and carbon isotopic composition Stable isotopic composition of electron donor oxidation products </li></ul><ul><li>Geochemistry and geochemical modeling </li></ul><ul><li>Groundwater age dating and groundwater transport modeling </li></ul>
  8. 8. Nitrate isotopic composition Source attribution and process identification The “ dual isotope ” approach refers to the use of both nitrogen (nitrate-  15 N) and oxygen (nitrate-  18 O) isotopic composition to attribute nitrate source and to constrain nitrate cycling Ranges based on data compiled in Kendall (1998).
  9. 9. Identifying denitrification isotopically Nitrate isotopic composition dual isotope plot Denitrification enriches both 15 N and 18 O (the “heavy” isotopes) in residual nitrate. In most natural terrestrial systems, relative  18 O/  15 N enrichment is ~1/2, and distinguishes denitrification from ammonia volatilization+nitrification (which only enrich 15 N). Characteristic slope on dual isotope plot California dairies (Singleton, 2007, EST) Note : While excess nitrogen is only useful in the saturated zone, the dual isotope method can be used for both groundwater and sediment samples. California dairy sediment samples
  10. 10. Identifying denitrification isotopically Correlation between isotopics and concentration Inverse correlation between 15 N enrichment and residual nitrate concentration A strong case can be made for denitrification if changes in nitrate isotopic composition correlate to changes in nitrate concentration along a groundwater flow path; i.e., downgradient waters are low in nitrate and enriched in  15 N. California dairy sediment samples Note : The largest isotopic effects are often observed in samples with low concentrations of residual nitrate.
  11. 11. <ul><li>Uses Pseudomonas chlororaphi to generate N 2 O for isotopic analysis of both  15 N and  18 O from nitrate in water samples </li></ul><ul><li>Allows simultaneous  18 O and  15 N analyses on samples with low nitrate (down to 0.5 mg/L NO 3 - ) and small volume (< 4 mL) </li></ul><ul><li>Samples can be processed rapidly using an automated headspace sampler </li></ul>Measuring nitrate isotopic composition with the denitrifying bacteria method Inject samples into vials with denitrifying bacteria (NO 3 - -> NO 2 overnight) Separate out CO 2 and focus N 2 O Measure  18 O and  15 N of N 2 O with mass spec Collect gas from vials with automated headspace sampler
  12. 12. Nitrate isotopic composition analysis Advantages of the bacterial denitrifier method <ul><li>Advantages include </li></ul><ul><li>Significantly greater sensitivity: smaller samples and analysis of low-nitrate samples </li></ul><ul><li>Freedom from interferences , such as sulfate </li></ul><ul><li>Disadvantages include </li></ul><ul><li>Slightly less precise and requires corrections for fractionation </li></ul><ul><li>Highly contaminated samples can poison the cultures </li></ul>Method Combustion/ Ion Exchange Bacterial Denitrifier Facility Environmental Isotope Lab (University of Waterloo) Lawrence Livermore National Laboratory Nitrate-  15 N precision 0.2 permil 0.5 permil Nitrate-  18 O precision 0.5 permil 1.0 permil Mass nitrate required 5 mg 0.002 mg Volume required (at MCL) 500 mL 4 mL (typical sample size is 20-40 mL)
  13. 13. Dissolved gases in groundwater Groundwater contains atmospheric nitrogen <ul><li>The presence of dissolved nitrogen in groundwater does not by itself indicate denitrification </li></ul><ul><li>Groundwater contains dissolved gas from incorporation of atmospheric and soil gases during recharge </li></ul><ul><ul><li>Concentrations are above equilibrium solubility and are extremely variable </li></ul></ul><ul><ul><li>A significant fraction of this atmospheric gas component will be nitrogen, the most abundant gas (78%) in the atmosphere </li></ul></ul>Denitrification: NO 3 -  N 2
  14. 14. Excess nitrogen in groundwater Dissolved N 2 in excess of air-derived N 2 <ul><li>Excess N 2 = Total N 2 – Atmospheric N 2 </li></ul><ul><li>We assume that excess nitrogen is derived from denitrification </li></ul><ul><li>Denitrification: NO 3 -  N 2 </li></ul><ul><li>Excess nitrogen is determined by: </li></ul><ul><li>Measuring total dissolved nitrogen </li></ul><ul><li>Measuring an inert, non-biogenic atmospheric gas (Ar, Ne) </li></ul><ul><li>Estimating the atmospheric N 2 component from the inert gas concentration by using either an excess air model or an observed trend in non-denitrified groundwater </li></ul><ul><li>Subtracting out the atmospheric nitrogen component </li></ul>
  15. 15. Extent of denitrification Excess nitrogen allows estimation of initial nitrate Excess N 2 allows determination of the amount and extent of denitrification, and can be used with groundwater age or velocity to constrain rate. Extent of denitrification = 1 – f, Where f = fraction initial nitrate remaining Stratified aquifer underlying a California dairy operation Caveat: With extensive denitrification or methanogenesis, nitrogen gas can be lost through gas ebullition.
  16. 16. Quadrupole mass analyser Vacuum pump Gas-permeable membrane inlet Water sample Water trap <ul><li>Membrane inlet mass spectrometry (MIMS) </li></ul><ul><ul><li>Measures nitrogen, argon, oxygen, carbon dioxide, and methane </li></ul></ul><ul><ul><li>Fast, field-portable, and inexpensive </li></ul></ul><ul><ul><li>Uses standard VOC sampling method: three 40-mL VOA vials with no headspace </li></ul></ul>Peristaltic pump Determination of excess nitrogen LLNL built a small gas analyzer
  17. 17. Field determination of excess nitrogen Case study at a California dairy operation <ul><ul><li>Direct Push (DP) survey </li></ul></ul><ul><ul><ul><li>Synoptic water and soil sampling </li></ul></ul></ul><ul><ul><ul><li>Water : ions, excess N 2 , isotopics </li></ul></ul></ul><ul><ul><ul><li>Soil: preserved for microbial analysis </li></ul></ul></ul><ul><ul><li>Multi-level 2-inch diameter monitoring wells </li></ul></ul><ul><ul><ul><li>3-4 levels in perched aquifer </li></ul></ul></ul><ul><ul><ul><li>Continuous core recovered </li></ul></ul></ul>Nitrate and excess N 2 were determined in DP samples within 20 minutes, and used to screen nested monitor wells across a sharp vertical redox gradient G Bryant Hudson & field-portable MIMS
  18. 18. Excess nitrogen indicates that denitrification is occurring in the lower anoxic aquifer Excess nitrogen and denitrification Case study at a California dairy operation
  19. 19. Bacterial population profiles show that denitrification occurs at the oxic-anoxic interface PCR and denitrification Case study at a California dairy operation
  20. 20. Groundwater age and denitrification Case study at a California dairy operation Tritium-helium groundwater age dating provides constraints on the timing of nitrate contamination and the rate of denitrification
  21. 21. <ul><li>UPPER LOCAL AQUIFER </li></ul><ul><li>Chemical mitigation: degradation </li></ul><ul><li>Active denitrification mitigates impact of high-nitrate recharge </li></ul><ul><li>LOWER REGIONAL AQUIFER </li></ul><ul><li>Physical mitigation: transport </li></ul><ul><li>Confining layer prevents recharge of high-nitrate irrigation from overlying dairy </li></ul>Distinguishing different mechanisms for the occurrence of low-nitrate groundwater Case study at a California dairy operation
  22. 22. LLNL Site 300 A DOE HE testing facility in the California Coast Range
  23. 23. Site 300 case study Nitrate contamination threat to drinking water wells Off-site water supply wells
  24. 24. Site 300 hydrogeology Complex marine sedimentary sequence
  25. 25. Preliminary assessment Nitrate, dissolved oxygen, and groundwater flow <ul><li>Pattern consistent with denitrification: </li></ul><ul><li>Decrease in nitrate along flow path from oxic unconfined aquifer to anoxic confined aquifer, but … </li></ul><ul><li>Dissolved organic carbon is low, cannot support observed denitrification </li></ul><ul><li>Characterization goals: </li></ul><ul><li>Confirm denitrification </li></ul><ul><li>Identify electron donor </li></ul>Nitrate concentration Oxic Anoxic Ground water flow direction
  26. 26. Identification of denitrification Nitrate isotopic composition
  27. 27. Confirmation of denitrification Excess nitrogen and dissolved oxygen Dissolved N 2 detected Dissolved N 2 detected > 15 mg/L equivalent NO 3 Dissolved N 2 not detected
  28. 28. Confirmation of denitrification Excess nitrogen and nitrate-  15 N
  29. 29. Assimilative capacity Identification of electron donor Heterotrophic denitrification 4NO 3 - + 5CH 2 O( organic C ) + 4 H +  2N 2 + 5CO 2 + 7H 2 O Groundwater contains insufficient Dissolved Organic Carbon! <ul><li>Autotrophic denitrification </li></ul><ul><li>14 NO 3 - + 5 FeS 2 ( pyrite ) + 4 H +  7 N 2 + 10 SO 4 2- + 5 Fe 2+ + 2 H 2 O + </li></ul><ul><li>Observables for pyrite oxidation </li></ul><ul><li>Pyrite in sediment </li></ul><ul><li>Changes in downgradient water chemistry consistent with pyrite oxidation </li></ul><ul><li>Downgradient sulfate should have a lighter sulfate isotopic composition (  34 S) than upgradient sulfate </li></ul>
  30. 30. Identification of electron donor Thermodynamically constrained mass balance <ul><li>PHREEQC Simulation </li></ul><ul><li>Processes: </li></ul><ul><li>Pyrite dissolution </li></ul><ul><li>Fe(OH) 3 precipitation </li></ul><ul><li>Sulfide oxidation to sulfate </li></ul><ul><li>Acidification </li></ul><ul><li>CaMg(CO 3 ) 2 dissolution </li></ul><ul><li>Cation exchange: Ca ++ and Mg ++ for Na + </li></ul><ul><li>Denitrification </li></ul><ul><li>Dilution </li></ul>Observed changes in downgradient groundwater : Higher sulfate Lower Ca, Mg, K, and nitrate Observed changes in groundwater chemistry along flow path are consistent with autotrophic denitrification
  31. 31. Identification of electron donor Changes in sulfate isotopic composition 5.3 4.2 3.4 2.5 0.3 Observed changes in sulfate-S isotopic composition along flow path are consistent with autotrophic denitrification Oxidation of sulfide to sulfate favors the lighter isotope, and produces sulfate-S isotopically lighter than pyrite-S. The observed trend is consistent with addition of isotopically light sulfate through pyrite oxidation x.x  34 S data
  32. 32. Denitrification at Site 300 CERCLA Monitored Natural Attenuation remedy approved Consistent set of geochemical indicators demonstrating autotrophic denitrification driven by oxidation of naturally occurring pyrite
  33. 33. Monitored Natural Attenuation of Nitrate – Regulatory Aspects <ul><li>A tiered approach for evaluating MNA of nitrate is presented in the U.S. EPA guidance document: </li></ul><ul><li>Monitored Natural Attenuation of Inorganic Contaminants in Ground Water (EPA-600-R-07-140), October 2007 </li></ul>
  34. 34. U.S. EPA Tiered MNA Approach <ul><li>Tier I: Demonstration that the groundwater plume is not expanding, and that sorption onto aquifer solids is the predominant attenuation process. </li></ul><ul><li>Tier II: Determination of the mechanism and rate of attenuation. </li></ul><ul><li>Tier III: Determination of the attenuation capacity of the aquifer and the stability of the immobilized contaminants. </li></ul><ul><li>Tier IV: Design performance monitoring program and establish a contingency plan . </li></ul>
  35. 35. Role of Stable Isotopes in Supporting Nitrate MNA Remedies <ul><li>Enrichment of heavier isotopes with increasing distance from release point demonstrates the presence of a biological nitrate denitrification mechanism. </li></ul>
  36. 36. Role of Dissolved Gas Analyses in Supporting Nitrate MNA Remedies <ul><li>A progressive increase in excess dissolved nitrogen gas along the groundwater flow path demonstrates irreversible destruction of nitrate to a non-toxic degradation product. </li></ul>
  37. 37. Groundwater nitrate characterization at Edwards AFB (work in progress) <ul><li>AECOM, Brown and Caldwell, LLNL Team </li></ul><ul><li>Nitrate Source Identification: </li></ul><ul><li>Measure end-member isotopic signatures in soil samples collected beneath the release points of known nitrate-bearing materials: </li></ul><ul><ul><li>Naturally occurring (baseline) </li></ul></ul><ul><ul><li>Septic/sanitary sewer </li></ul></ul><ul><ul><li>Hydrazine/nitric acid </li></ul></ul><ul><ul><li>Ammonia </li></ul></ul><ul><ul><li>Photographic chemicals </li></ul></ul><ul><ul><li>Explosives </li></ul></ul><ul><li>Compare isotopic signature of source materials to that of groundwater within the nitrate plumes. </li></ul>
  38. 38. Use of Stable Nitrate Isotopes at Edwards AFB (cont.) <ul><li>Nitrate Microbial Attenuation Evaluation: </li></ul><ul><li>Evaluate microbial denitrification within nine groundwater plumes by collecting samples from monitoring wells located along the groundwater flow paths, from the release areas to the leading edges of the plumes. </li></ul><ul><li>Measure isotope ratios and excess dissolved nitrogen gas in the samples. </li></ul><ul><li>Use results to support Monitored Natural Attenuation remedies. </li></ul>
  39. 39. Acknowledgements <ul><li>The LLNL Nitrate Team </li></ul><ul><ul><li>Mike Singleton </li></ul></ul><ul><ul><li>Vic Madrid </li></ul></ul><ul><ul><li>Jean Moran (CSU-EB) </li></ul></ul><ul><ul><li>Steve Carle </li></ul></ul><ul><ul><li>G. Bryant Hudson (retired) </li></ul></ul><ul><ul><li>Walt McNab </li></ul></ul><ul><ul><li>Harry Beller (LBNL) </li></ul></ul><ul><ul><li>Staci Kane </li></ul></ul><ul><ul><li>Tracy LeTain </li></ul></ul><ul><li>University Collaborators </li></ul><ul><ul><li>University of California - Davis (T. Harter) </li></ul></ul><ul><ul><li>University of Arizona (B. Ekwurzel, K. Moore) </li></ul></ul><ul><ul><li>University of Texas – Austin (B. Cey, B. Scanlon) </li></ul></ul><ul><li>Sponsors </li></ul><ul><ul><li>DOE/NNSA </li></ul></ul><ul><ul><li>LLNL research funding </li></ul></ul><ul><ul><li>California State Water Quality Control Board </li></ul></ul>
  40. 40. Contact information <ul><li>Questions? </li></ul>Dr. Bradley K. Esser Lawrence Livermore National Laboratory, L-231 Livermore, CA 94551-0808 Email: [email_address] Voice: 925-422-5247 Robert A. Ferry Brown and Caldwell Email: [email_address] Voice: 925-872-7264