Aerobic Denitrification: Implications for Nitrogen
Fate Modeling in the Missouri-Ohio-Mississippi

(MOM) River Basin

Robert Thomas1, John Washington1, Lidia Samarkina2

1US EPA ORD/NERL/ERD, Athens, GA; 2Senior Service America, Inc.

Introduction

Methods

¦	N-compunds are the cause in -50% of impaired water bodies.

•	1.6 million metric tons of N compounds discharged annually from the MOM Basin.

¦	NO3- is dominant N species; understanding its behavior is essential for N-fate modeling.

¦	NO3- has risen 3 -10 fold since the early 1900s in major US rivers.

¦	High NO3- renders groundwater impotable, rivers degraded & coastal productivity
impacted.

•	Denitrification is an important process for removing N03- from ecosystem because inert N2
is the final product; however denitrification is a step-wise biogeochemical reduction of N03_:

NO,

MO,

NO

n2o

N,

Moreover, convention holds that denitrification is a strictly anaerobic process.

pfiesteria?

•	Four microcosm reactors were constructed.

•	800 ml_ of filtered (0.2|im) river water added to each jar and
mixed with a magnetic stir bar.

•	Added Ca-nitrate to 10 mg/L as N final concentration

•	1 jar as control; 3 jars inoculated with 3 ml_ of agricultural
wetland sediment slurry (0.01 mg solids dry wt).

•	Reactors were sealed with atmospheric-gas headspace, which
was recirculated through the solution.

•	Headspace gas and the solution were sampled twice daily for
N20 and N03-, respectively

» Dissolved 02 measured by Fiber Optic Oxygen (FOXY)
Electrodes

l Eutrophication may stimulate harmful
i species

• The propensity for reduction reactions to proceed is often predicted
by the oxidation state of a system.

>	TEAP - widely accepted model for characterizing oxidation state.

- Microbes reduce oxidants sequentially from high to low energy.

» Thus, under the TEAP model denitrification does not proceed until all
of the oxygen is reduced or removed from the system.

>	However, with thermodynamic analyses of field data, Washington et
al. (2005) found dissolved oxygen and Corg draw other couples
toward potentials imposed by them, generating a bimodal clustering.

Results

Dissolved NO3 & N2O - Control

0.025 -

N03sv

2 0.02 -

y = 0.1181x -3680.6
R2 = 0.0105

B.*>K

-« Low

13-Apr 23-Apr 3-May 13-May

Control jar only modest increase in N20 to slightly above atmospheric values and no
significant decrease in N03", while the inoculated jar produced N20 almost 3 times greater
than atmosphere accompanied by significant reduction in N03- at p = 0.99. Dissolved oxygen
was -6.4 mg/L in both jars for the duration of the experiment.

* High

Dissolved NO3 & N^O - Inoculated

0.025 -|
i[ 0.02 -
° 0.015 -
£ 0.01 -

**.H W

S

930 \
910 o

13-Apr 23-Apr

3-May 13-May

Date (Mori'YR)	Log [Reactant] (M)

• While the redox clustering indicates thermodynamic control, the two potential clusters,

instead of one, suggests kinetic constraints as well.

> Moreover, the rate of reaction is determined more by concentration than relative potential
with reactants < 10_6M being slow to react & divergent from the clusters, whereas reactants
>10-6M are near equilibrium & in a cluster

These results have been corroborated with aerobic experiments on synthetic solutions we
have inoculated and several environmental samples, including: spring, wetland, and river
waters; GA wetland sediment, TN soil, SC pond sediment, and SC estuarine sediment
suspensions. In every environmental medium we have tested, without exception, we have
observed significant N20 ingrowth under aerobic conditions, supporting that aerobic
denitrification is a common environmental process.

Hypothesis

Conclusions

• These observations suggest that reactions of higher-potential oxidants with lower-potential
reductants commonly proceed simultaneously, regardless of the presence of other
potential reactants

>	Thus, redox reactions can proceed in parallel as opposed to sequentially as often
conceived in the TEAP conceptual model.

>	Conventional thought long has held that nitrate reduction, or denitrification, does not take
place when oxygen is present.

>	However, based on the thermodynamic modeling, dissolved 02 and N03- were reduced to
nearly equal redox potentials in every sample we analyzed, suggesting simultaneous
reduction of 02 and N03_.

We propose that denitrification is NOT a strictly anaerobic process

Disclaimer: Although this work was reviewed by EPA and approved for presentation, it may not necessarily reflect official Agency policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

•	Contrary to TEAP theory, N03" can be reduced in oxic settings (e.g., the Mississippi River)

•	Aerobic denitrification is expected to have a high impact on N-fate modeling as most
existing models call for denitrification to take place in anoxic settings, yet most impacted
surface waters are aerobic.

- For example, within the denitrification module of the EPA's Water Quality Analysis
Simulation Program (WASP), the kinetic expression for denitrification contains a first
order rate constant, a temperature correction term, and a DO correction term, which
calculates the decline in denitrification rate as DO levels rise above 0 using a user
specified half-saturation constant KN0 (KN03 = DO level where denitrification rate is
reduced by half). Currently, the default value is effectively zero (10-20), preventing this
reaction at all non-zero DO levels.

•	While our experiments have qualified aerobic denitrification, our future work will be towards
quantifying denitrification rates at varying DO concentration and temperature. Plans are to
collaborate with WASP developers to incorporate aerobic denitrification into the model.

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