Stable Isotopes and Environmental Tracers for

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Stable Isotopes and Environmental Tracers for
Detecting and Quantifying Denitrification in
Aquatic Systems JK Böhlke, U.S. Geological
Survey, Reston, VA Mark Altabet, U.
Massachusetts, New Bedford, MA
measurements without deliberate
manipulations Stable isotopes indicate
distribution and controls Hydrochronology and
stoichiometry give rates Sediments record spatial
and temporal variations
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Simplified N Cycles Apparent isotope
fractionations vary with reaction rates,
substrate concentrations, heterogeneity and
openness of the system Denitrification
stoichiometries, isotope mass balances, and
fluxes typically dominated by NO3- and N2
Intermediate species may exhibit complex
variations if formed and consumed at
different rates
a (15N/14N)p/(15N/14N)r K(eq) for
equilibrium isotope substitutions k15/k14
for irreversible transformations Typical kinetic
effects in N cycle k14 gt k15 a lt
1 note these terms can vary in the literature
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Denitrification N2 gas production Isotope
fractionation (15N/14N)r (15N/14N)r x
(Cr/Cr)(a-1) a (15N/14N)p/(15N/14N)r d15N
(15N/14N)i/(15N/14N)air -1 x1000 d18O
(18O/16O)i/(18O/16O)vsmow -1 x1000
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Princeton, Minnesota (Böhlke et al., 2002)
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Multiple isotope systems
Michalski and others (2004) GCA, in press.
Kendall and Aravena (2000) Casciotti et al.
(2002) Granger et al. (2004) Böhlke (unpub.)
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Denitrification 4NO3- 5C 2H2O ? 2N2 CO2
4HCO3- 6NO3- 2FeS2 2H2O ? 3N2 4SO42-
2FeOOH 2H 2NO3- 10FeO(SiO2) 2H 4H2O
? N2 10FeOOH (SiO2) NO2--NO-N2O Isotope
effects of denitrification (major species) NO3-
correlated fractionations of N and O N2 addition
of non-atmospheric component HCO3-, SO42-
addition of electron donor source components
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Atmospheric environmental tracers used for dating
young ground water
Data are for uncontaminated, mid-latitude,
northern hemisphere sites nuclear bomb
tests 36Cl, 14C, 3H(3He) Industrial production
and release CFCs, SF6, 85Kr, (3H) Cosmic ray
interactions 14C, (3H, 36Cl)
(modified from Cook and Böhlke, 1999)
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(modified from Böhlke, 2002)
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Unconfined aquifer, Minnesota ?8 m vertical
gradient (Böhlke et al., 2002)
Confined aquifer, SW Africa ?80 km horizontal
gradient (Vogel et al., 1981)
Reaction rates from ground-water dating and
reaction progress
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Multiple sources of N varying in space and time
Mississippi River basin (Goolsby and others,
1999) (Burkart and James, 2001) ? Different
sources trends ? Decadal time scale
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Same NO3- gradient Different reasons Future
gradients depend on which is right
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Sulfate-d34S varies inversely with nitrate-d15N
(pyrite is an electron donor) DIC-d13C does
not change with dentrification pyrite
roll-front, Holocene vs Anthropocene
Princeton, Minnesota (Böhlke et al., 2002)
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NO3- in ?discharge f(time, inputs, storage,
reactions, etc.) Past trends may not indicate
future trends
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SUMMARY (stable isotopes and environmental
tracers, ground water) Stable isotope signatures
of denitrification (distribution and
substrates) Environmental tracers of travel time
plus reaction progress give rates Combination
permits resolution of transient inputs, dilution,
denitrification, flow path configurations,
and residence times (predictions, optimization of
remediation) Temporal and spatial scales Stable
isotope effects all scales In situ rates 100 to
104 years (to 106?) 0.1 meter to 100s of
kilometers Denitrification rates 10-7 to
10-1 µmol/L/d (10-9 to 10-3/d) Strengths
Non-invasive Large temporal and spatial scales
Limitations Complex in open or heterogeneous
systems (soils, air/water interfaces, gw/sw
interfaces) Small scales and rapid rates not
accessible to some environmental tracers Mixing
zones versus boundaries (discontinuities in
isotope effects and rates)
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Present and Future (technical) Improved
analytical techniques (happening fast)
(small samples, low concentrations, high
precision, multiple species, automation,
in situ measurements, monitoring)
(e.g., NO3- bacterial method, noble gases with
N2, NBNaO isotopomers, D17O) (solve
calibration problems) Improved understanding of
isotope features (e.g., biodiversity
versus fractionation, signatures of transient
intermediates, transport vs.
reaction limitation) Incorporation of realistic
biogeochemical and geochronologic information in
models (integrated gw, sw, and watershed
flow systems) (extend site-specific
results by geology?)