RIP Today, RC Tomorrow, Optimize Always RIP Today, RC Tomorrow, Optimize Always
MONITORED NATURAL ATTENUATION OF
GROUNDWATER NITRATECharacterization using nitrate isotopic
composition and excess nitrogen
Dr. Bradley K. EsserLawrence Livermore National Laboratory
Robert A. FerryBrown and Caldwell
Victor MadridLawrence Livermore National Laboratory
Mike SingletonLawrence Livermore National Laboratory
08 April 2010
San Antonio, TX
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Groundwater NitrateOutline of talk
LLNL investigations and capabilities Denitrification Nitrate isotopic composition Excess nitrogen An MNA case study Regulatory framework Upcoming work
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LLNL analytical capabilities
LLNL site investigations Successful approval of monitored natural attenuation
as a CERCLA remedy for groundwater nitrate contamination on a high explosives test range
LLNL Work for Others Program investigations for water agencies
California Water Board: Groundwater Ambient Monitoring & Assessment (GAMA) program
California water districts: Orange County, Metropolitan, Santa Clara Strong collaborations with academic researchers
University of California, several campuses California State University, East Bay University of Texas, Austin
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LLNL analytical capabilities
Stable isotope mass spectrometry Nitrate-15N,-18O; H2O-D, -18O; DIC/DOC-13C; SO4-34S Bacterial denitrifier method for analysis of nitrate
Membrane-inlet mass spectrometry Excess air and excess nitrogen: N2, O2, Ar, CO2, CH4
Noble gas mass spectrometry Tritium-3He and 4He groundwater age dating Groundwater recharge temperature (xenon) Excess air (neon)
Trace constituent analysis: ICPMS, LC/GC-MS Groundwater flow and contaminant transport modeling
Denitrification is a microbial redox reaction that converts nitrate to molecular nitrogen
Heterotrophic denitrification
4 NO3- + 5 CH2O (organic C) + 4 H+
2 N2 + 5 CO2 + 7 H2O
Autotrophic denitrification
14 NO3- + 5 FeS2 (pyrite)+ 4 H+
7 N2 + 10 SO42- + 5 Fe2+ + 2 H2O
14 NO3- + 5 Fe+2 (reduced iron)+ 7 H2O
0.5 N2 + 5 FeOOH (goethite) + 9 H+
Denitrification requires Denitrifying bacteria An electron donor Low oxygen
conditions(< 0.6 mg/L)
NO3- NO2
- NO N20 N2
Nitrate (+5)
Nitrite (+3)
Nitric Oxide (+2)
Nitrous Oxide (+1)
Nitrogen (0)
Monitored natural attenuation
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Site Characterization Objectives Demonstrate active removal. Determine the mechanism and
rate of attenuation. Determine the attenuation capacity
of the aquifer.
Ford, R. G., Wilkin, R. T., and Puls, R. W., 2007. U.S. Environmental Protection Agency.
Tools for characterization of denitrification
“Excess” nitrogen: end-product of denitrification Nitrate isotopic composition: dual isotope approach
PCR surveys of denitrifying bacterial populations Sulfur and carbon isotopic composition
Stable isotopic composition of electron donor oxidation products
Geochemistry and geochemical modeling Groundwater age dating and groundwater transport
modeling
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Nitrate isotopic compositionSource attribution and process identification
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Ranges based ondata compiled in Kendall (1998).
The “dual isotope” approach refers to the use of both nitrogen (nitrate-15N) and oxygen (nitrate-18O) isotopic composition to attribute nitrate source and to constrain nitrate cycling
Identifying denitrification isotopicallyNitrate isotopic composition dual isotope plot
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Denitrification enriches both 15N and 18O (the “heavy” isotopes) in residual nitrate. In most natural terrestrial systems, relative 18O/15N enrichment is ~1/2, and distinguishes denitrification from ammonia volatilization+nitrification (which only enrich 15N).
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
Identifying denitrification isotopicallyCorrelation between isotopics and concentration
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Inverse correlation between 15N 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 15N.
California dairy sediment samples
Note: The largest isotopic effects are often observed in samples with low concentrations of residual nitrate.
Uses Pseudomonas chlororaphi to generate N2O for isotopic analysis of both 15N and 18O from nitrate in water samples
Allows simultaneous 18O and 15N analyses on samples withlow nitrate (down to 0.5 mg/L NO3
-) and small volume (< 4 mL)
Samples can be processed rapidly using an automated headspace sampler
Measuring nitrate isotopic composition with the denitrifying bacteria method
Inject samples into vials with denitrifying
bacteria(NO3- → NO2
overnight)
Separate out CO2
and focus N2O
Measure 18O and
15N of N2O with
mass spec
Collect gas from vials
with automated headspace
sampler
Method Combustion/ Ion Exchange
Bacterial Denitrifier
Facility Environmental Isotope Lab (University of Waterloo)
Lawrence Livermore National Laboratory
Nitrate-15N precision 0.2 permil 0.5 permil
Nitrate-18O 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)
Nitrate isotopic composition analysisAdvantages of the bacterial denitrifier method
Advantages includeSignificantly greater sensitivity: smaller samples and analysis of low-nitrate samples Freedom from interferences, such as sulfate
Disadvantages includeSlightly less precise and requires corrections for fractionationHighly contaminated samples can poison the cultures
Dissolved gases in groundwaterGroundwater contains atmospheric nitrogen
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The presence of dissolved nitrogen in groundwater does not by itself
indicate denitrification
Groundwater contains dissolved gas from incorporation of atmospheric and soil gases during recharge
Concentrations are above equilibrium solubility and are extremely variable
A significant fraction of this atmospheric gas component will be nitrogen, the most abundant gas (78%) in the atmosphere
Denitrification: NO3- N2
Excess nitrogen in groundwaterDissolved N2 in excess of air-derived N2
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50%
100%
150%
200%
250%
90% 100% 110% 120% 130% 140% 150%
LLNL Site 300 groundwater
Dis
solv
ed n
itro
gen
(%
sa
tura
tion
)
Dissolved argon (% saturation)
Excess N2
Excess air
Equilibriumsolubility
Excess N2 = Total N2 – Atmospheric N2
We assume that excess nitrogen is derived from denitrification
Denitrification: NO3- N2
Excess nitrogen is determined by:
•Measuring total dissolved nitrogen
•Measuring an inert, non-biogenic atmospheric gas(Ar, Ne)
•Estimating the atmospheric N2 component from the
inert gas concentration by using either an excess air model or an observed trend in non-denitrified groundwater
•Subtracting out the atmospheric nitrogen component
Extent of denitrificationExcess nitrogen allows estimation of initial nitrate
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4
6
8
10
12
14
16
18
20
0%20%40%60%80%100%
KCD groundwaters
Dep
th (
m)
Initial Nitrate Remaining
Denitrification
Excess N2 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
Nitrate-N
Nitrate-N + Excess nitrogen-Nf
Caveat: With extensive denitrification or methanogenesis, nitrogen gas can be lost
through gas ebullition.
Quadrupole mass analyser
Vacuum pump
Gas-permeable membrane inlet
Water sample
Water trap
Membrane inlet mass spectrometry (MIMS)
• Measures nitrogen, argon, oxygen, carbon dioxide, and methane
• Fast, field-portable, and inexpensive
• Uses standard VOC sampling method: three 40-mL VOA vials with no headspace
Peristalticpump
Determination of excess nitrogenLLNL built a small gas analyzer
Field determination of excess nitrogenCase study at a California dairy operation
Direct Push (DP) survey Synoptic water and soil sampling Water: ions, excess N2, isotopics Soil: preserved for microbial analysis
Multi-level 2-inch diameter monitoring wells 3-4 levels in perched aquifer Continuous core recovered
Nitrate and excess N2 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
Excess nitrogen indicates that denitrification is occurring in the
lower anoxic aquifer
Excess nitrogen and denitrificationCase study at a California dairy operation
Bacterial population profiles show that denitrification occurs at the oxic-anoxic interface
PCR and denitrificationCase study at a California dairy operation
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Groundwater age and denitrificationCase study at a California dairy operation
Tritium-helium groundwater age dating provides constraints on the
timing of nitrate contamination and the rate of denitrification
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UPPER LOCAL AQUIFER Chemical mitigation: degradation Active denitrification mitigates
impact of high-nitrate recharge
LOWER REGIONAL AQUIFER
• Physical mitigation: transport
• Confining layer prevents recharge of high-nitrate irrigation from overlying dairy
Distinguishing different mechanisms for the occurrence of low-nitrate groundwater
Case study at a California dairy operation
LLNL Site 300 A DOE HE testing facility in the
California Coast Range
Site 300 case studyNitrate contamination threat to drinking water wells
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Off-site water supply wells
Site 300 hydrogeologyComplex marine sedimentary sequence
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Preliminary assessmentNitrate, dissolved oxygen, and groundwater flow
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Nitrate concentration
Oxic
Anoxic
Ground water flow direction
Pattern consistent with denitrification:Decrease in nitrate along flow path from oxic unconfined aquifer to anoxic confined aquifer, but…
Dissolved organic carbon is low, cannot support observed denitrification
Characterization goals:1.Confirm denitrification2.Identify electron donor
Identification of denitrification Nitrate isotopic composition
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Confirmation of denitrificationExcess nitrogen and dissolved oxygen
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Dissolved N2 detected
Dissolved N2 detected> 15 mg/L equivalent NO3
Dissolved N2 not detected
0
10
20
30
40
0 2 4 6 8 10
LLNL Site 300 groundwater
Exc
ess
nitr
oge
n (
mg
/L a
s n
itra
te)
Dissolved oxygen (mg/L)
Confirmation of denitrificationExcess nitrogen and nitrate-15N
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0
5
10
15
20
25
0% 20% 40% 60% 80% 100%
LLNL Site 300 denitrification study
Gro
un
dw
ate
r n
itra
te
15 N
Groundwater nitrate removed by denitrification (from excess nitrogen method)
Assimilative capacity Identification of electron donor
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Heterotrophic denitrification
4NO3- + 5CH2O(organic C) + 4 H+
2N2 + 5CO2 + 7H2O
Groundwater contains insufficient Dissolved Organic Carbon!
Autotrophic denitrification
14 NO3- + 5 FeS2 (pyrite)+ 4 H+
7 N2 + 10 SO42- + 5 Fe2+ + 2 H2O+
Observables for pyrite oxidation
Pyrite in sediment
Changes in downgradient water chemistry consistent with pyrite oxidation
Downgradient sulfate should have a lighter sulfate isotopic composition (34S) than upgradient sulfate
PHREEQC Simulation
Processes:• Pyrite dissolution
• Fe(OH)3 precipitation
• Sulfide oxidation to sulfate• Acidification
• CaMg(CO3)2 dissolution
• Cation exchange: Ca++ and Mg++ for Na+ • Denitrification• Dilution
Identification of electron donorThermodynamically constrained mass balance
-100%
-80%
-60%
-40%
-20%
0%
20%
40%
60%
80%
100%
Ca Mg Na K Cl SO4 HCO3 NO3
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
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5.3 4.2
3.42.5
0.3
x.x34S data
Identification of electron donorChanges in sulfate isotopic composition
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
Denitrification at Site 300CERCLA Monitored Natural Attenuation
remedy approved
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Consistent set of geochemical indicators demonstratingautotrophic denitrification driven by oxidation of naturally occurring pyrite
Monitored Natural Attenuation of Nitrate – Regulatory Aspects
A tiered approach for evaluating MNA of nitrate is presented in the U.S. EPA guidance document:
Monitored Natural Attenuation of Inorganic Contaminants in Ground Water (EPA-600-R-07-140), October 2007
U.S. EPA Tiered MNA Approach
Tier I: Demonstration that the groundwater plume is not expanding, and that sorption onto aquifer solids is the predominant attenuation process.
Tier II: Determination of the mechanism and rate of attenuation.
Tier III: Determination of the attenuation capacity of the aquifer and the stability of the immobilized contaminants.
Tier IV: Design performance monitoring program and establish a contingency plan.
Role of Stable Isotopes in Supporting Nitrate MNA Remedies
Enrichment of heavier isotopes with increasing distance from release point demonstrates the presence of a biological nitrate denitrification mechanism.
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Role of Dissolved Gas Analyses in Supporting Nitrate MNA Remedies
A progressive increase in excess dissolved nitrogen gas along the groundwater flow path demonstrates irreversible destruction of nitrate to a non-toxic degradation product.
Groundwater nitrate characterization at Edwards AFB (work in progress)
AECOM, Brown and Caldwell, LLNL Team
Nitrate Source Identification: Measure end-member isotopic signatures in soil samples collected
beneath the release points of known nitrate-bearing materials: Naturally occurring (baseline) Septic/sanitary sewer Hydrazine/nitric acid Ammonia Photographic chemicals Explosives
Compare isotopic signature of source materials to that of groundwater within the nitrate plumes.
Use of Stable Nitrate Isotopes at Edwards AFB (cont.)
Nitrate Microbial Attenuation Evaluation: 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.
Measure isotope ratios and excess dissolved nitrogen gas in the samples.
Use results to support Monitored Natural Attenuation remedies.
Acknowledgements
The LLNL Nitrate Team Mike Singleton Vic Madrid Jean Moran (CSU-EB) Steve Carle G. Bryant Hudson
(retired) Walt McNab Harry Beller (LBNL) Staci Kane Tracy LeTain
University Collaborators University of California - Davis (T. Harter) University of Arizona (B. Ekwurzel, K. Moore) University of Texas – Austin (B. Cey, B.
Scanlon)
Sponsors DOE/NNSA LLNL research funding California State Water Quality Control Board
Contact information
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Questions?
Dr. Bradley K. Esser
Lawrence Livermore National Laboratory, L-231
Livermore, CA 94551-0808
Email: [email protected]
Voice: 925-422-5247
Robert A. Ferry
Brown and Caldwell
Email: [email protected]
Voice: 925-872-7264
