U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3077

2010

Sustainability of Natural Attenuation of Nitrate in 

Agricultural Aquifers

I ncreased concentrations of 

nitrate in groundwater in agricultural areas, coinciding with 

increased use of chemical and 

organic fertilizers, have raised 

concern because of risks to environmental and human health. At 

some sites, these problems are 

mitigated by natural attenuation 

of nitrate as a result of microbially 

mediated reactions. Results from 

U.S. Geological Survey (USGS) 

research under the National WaterQuality Assessment (NAWQA) 

program show that reactions of 

dissolved nitrate with solid aquifer 

minerals and organic carbon help 

lower nitrate concentrations in 

groundwater beneath agricultural 

fields. However, increased fluxes 

of nitrate cause ongoing depletion 

of the finite pool of solid reactants. 

Consumption of the solid reactants 

diminishes the capacity of the 

aquifer to remove nitrate, calling 

into question the long-term sustainability of these natural attenuation 

processes.

Increased use of chemical and 

organic fertilizers in agricultural areas (fig. 

1) over the past 50 years has coincided 

with increased nutrient concentrations in 

groundwater (see, for example, Böhlke, 

2002). Today nitrate (NO3

−

) is one of the 

most abundant groundwater contaminants 

in the United States (Nolan and Stoner, 

2000). This is cause for concern because 

elevated NO3

−

 concentrations have been 

associated with human health risks (World 

Health Organization, 2004), and when 

NO3

−

-laden groundwater discharges to 

surface waters, it contributes to eutrophication (Howarth and Marino, 2006).

One important factor determining the 

extent and duration of NO3

−

 contamination 

is denitrification, a type of reduction-oxidation or “redox” reaction. These reactions 

involve the chemical transfer of electrons 

from reduced compounds (“reductants”) 

to electron-accepting compounds (“oxidants”). A commonly observed example in 

everyday life is the corrosion of exposed 

iron. In the presence of water, electrons 

pass from the reductant (iron) to the oxidant (oxygen gas) to form rust consisting 

of iron oxides, hydroxides, and oxyhydroxides. Microorganisms facilitate many redox 

reactions such as denitrification. In groundwater, electron transfer from organic or 

inorganic reductants to NO3

−

 generates 

energy for denitrifying microorganisms. 

The denitrification reaction creates N2, a 

harmless gas, and consumes NO3

−

, helping 

to prevent or reverse groundwater contamination by this compound.

Figure 2 shows a schematic of redox 

processes important to the fate of groundwater NO3

−

. Some reactants required for 

denitrification arrive in groundwater as 

dissolved solutes originating near the land 

surface and recharging through the unsaturated zone to the groundwater table. Others 

are released from the solid phase, meaning 

they are derived from aquifer rocks or sediments. Dissolved O2 and NO3

−

 infiltrating 

from the surface are important oxidants, 

while solid-phase organic carbon (OC), 

solid minerals with iron (Fe) and sulfur (S), 

and dissolved OC (DOC) are important 

reductants. The subsurface microorganisms 

that mediate redox reactions can be categorized as aerobic (requiring the presence of 

O2) or anaerobic (thriving in the absence 

of O2). Aerobic microorganisms use dissolved O2 to oxidize available reductants. 

When dissolved O2 is exhausted, anaerobic 

microbes use NO3

−

 to oxidize available 

reductants (denitrification). Understanding 

the source and availability of reductants 

used by microbes for denitrification is 

central to predicting the extent to which this 

natural attenuation process can help control 

NO3

−

 contamination in groundwater.

The U.S. Geological Survey (USGS) 

has made groundwater NO3

−

 contamination 

Figure 1. Agricultural uses of water and fertilizers affect the chemistry of underlying aquifers. In this photo, irrigation water percolates into a wheat field in Yuba, Arizona. Photo by Jeff Vanuga , U.S. Department of Agriculture 

Natural Resources Conservation Service.
in agricultural areas a focus of its National 

Water-Quality Assessment (NAWQA) 

Program because survey data show that 

this problem is widespread (Dubrovsky and 

others, 2010). The factors controlling O2

broader NAWQA study initiated in 2004 on 

the fate and transport of agricultural chemicals. Sites selected for intensive study were 

in important agricultural systems in California, Washington, Nebraska, and Maryland 

of this Agricultural Chemicals Team (ACT) 

affect natural attenuation of agricultural 

contaminants in groundwater. To character-

-

ticated analyses, including various stable 

isotopes, dissolved gases in groundwater, 

solid-phase mineralogy and chemistry, 

groundwater age-tracers, and major ion 

chemistry (Green and others, 2008a).

Denitrification at ACT Sites

Combined results from investigations 

completed at the ACT study sites show 

that O2

controlled by solid phases of OC as well 

as Fe- and S-minerals in the aquifers. At 

these sites, reduced Fe and S are absent 

in recharging water, and DOC concentrations are typically too low to account 

for the mass of O2 and NO3

−

 that has 

Therefore, the observed O2 reduction and 

reactions with solid-phase OC, Fe, and(or) 

S, which are relatively plentiful, but in 

Although some DOC is present in 

recharging water, patterns in DOC concentrations indicate that it has limited reactivity at most ACT sites. As O2 and NO3

−

 are 

consumed, concentrations of DOC remain 

relatively constant at Maryland, Washinga decrease in DOC concentration does 

occur, suggesting some involvement of 

DOC in the redox reactions; however, the 

total amount of DOC supplied in recharging water is too small to account for the 

amount of O2

-

(SUVA) of DOC gives an indication of its 

reactivity or labile fraction (Weishaar and 

others, 2003). The SUVA analyses at these 

sites suggest that there are no substantial 

differences in DOC reactivity in aerobic 

DOC in recharging water at these sites 

appears not to play a major role in O2

solid-phase reductants in the aquifer has 

implications for the long-term sustainability of NO3

−

 natural attenuation. In many 

agricultural areas, the concentration of 

NO3

−

 in deep soils and shallow groundwater has greatly increased since the mid 

1900’s as a result of increased fertilizer use. 

The vertical velocities of water, NO3

−

, and 

O2 have also increased because of groundwater pumping and irrigation, as well as 

At the ACT sites and others with moderate 

2 reduction, 

2 and NO3

−

 result 

in deeper penetration of NO3

−

, especially 

around high-production wells pumped for 

irrigation or for domestic water use. The 

2 and NO3

−

 result in 

from the solid phase. As the most reactive 

solid phase reductants become depleted in 

sediments will probably decrease, resulting 

in further NO3

−

 transport to deeper groundwater and surface water bodies.

Implications for Future Nitrate 

Fluxes and Water Quality

These results show that with current agricultural practice, the depletion of 

aquifer reductants will result in decreased 

Aerobic

Anaerobic

O2, NO3

−, DOC

Not to scale

NO3

−, DOC

DOC

Groundwater recharge area Groundwater 

discharge

area Groundwater table

Solid aquifer 

material

Water and 

solutes

NO3

−

N2 Pyrite

Organic

matter

S, Fe

N2

OC

NO3

−

Figure 2. Schematic diagrams showing the fluxes of oxidants and reductants in an aquifer (left) and in 

water-filled pore-spaces between aquifer solids (right). The oxidants O2 (dissolved oxygen gas) and NO3

−

(nitrate) percolate through the unsaturated zone to the groundwater table and into groundwater. In shallow 

aerobic (containing O2) groundwater, microorganisms consume O2 until it is depleted. In the anaerobic zone 

(without O2) microorganisms reduce NO3

−. At four studied sites, dissolved organic carbon (DOC) passes 

through these aerobic and anaerobic zones without substantial changes (see fig. 5), indicating that some 

or all DOC originating from the land surface may be nonreactive. As shown on the right, microorganisms 

reduce NO3

− using reductants such as sulfur (S), iron (Fe), and organic forms of carbon (OC). Reactive OC 

can originate from solid organic matter, while S and Fe can originate from minerals such as pyrite. Because 

of limited availability of reductants, some NO3

− does not react and passes farther into the anaerobic zone.

Less than 1

1–4

5–9

10–24

25–39

40 or more

Percent of farm

land irrigated 

ACT

study site 

25,000 

fertilized acres

WA

CA

NE MD

0 100 Miles

Figure 3. Irrigation and agricultural fertilization in the conterminous United States, with the locations 

of the four study sites in California (CA), Maryland (MD), Nebraska (NE), and Washington (WA). Each 

small blue dot represents 25,000 acres treated with commercial fertilizer, lime, and(or) soil conditioners. 

The yellow to brown color shades show the percent of irrigated land relative to total land in farms for 

each county (adapted from U.S. Department of Agriculture, National Agricultural Statistical Service 2007 

Census of Agriculture, accessed September 27, 2010, at http://www.agcensus.usda.gov/Publications/2007/

Online_Highlights/Ag_Atlas_Maps/Farms/).
response, however, is uncertain. Of the total 

quantity of reductants identified in the ACT 

studies, a portion will not react because of 

physical sequestration or chemical recalcitrance. Previous studies have shown that 

a large fraction of the total solid-phase 

reductants are nonreactive on the time 

scales over which contaminant transport 

occurs (van Helvoort and others, 2007). 

Furthermore, although solid OC and Fe 

are plentiful relative to S, previous studies 

have often found preferential reactivity of 

S over OC (for example, Postma and others, 1991; van Helvoort and others, 2007). 

At the ACT sites, preferential reactivity 

of the Fe and S mineral phases is supported by higher ratios of Fe/OC and S/

OC in reduced zones of the aquifer relative 

to aerobic zones, where these reductants 

have been depleted. This suggests that the 

sustainability of denitrification depends on 

a relatively small reactive fraction of solidphase reductants, especially the minerals 

containing Fe and S. Existing studies show 

that the relative abundance and reactivity of 

the various solid-phase reductants can vary 

widely, depending on depth, grain size, 

and duration of exposure of sediments to 

the atmosphere during and after deposition 

(Hartog and others, 2002, 2005). Improved 

understanding of the sustainability of denitrification will require additional information about the reactivity of these heterogeneously distributed aquifer reductants.

The importance of the rates and 

sustainability of denitrification varies 

Figure 4. Data on redox reactions from the four study sites in Maryland (MD), Washington (WA), California (CA), and Nebraska (NE). A, Pie charts show the fraction 

of O2 and NO3

− that have reacted with reductants, as well as the fraction remaining in groundwater. The amount of reduced O2 and NO3− corresponds to a “demand,” 

defined as the amount of a reductant needed to cause the observed extent of reactions. Possible reductants include (B) organic carbon, (C) reduced iron, and (D) 

reduced sulfur; mmol L−1, millimoles per liter. On the bar charts, the dark red bars show the median demand for each reductant at each site. Light blue bars show the 

median amount of each reductant present in solid aquifer materials. Dark blue bars show the median amount of a reductant in recharging water at the water table 

(greater than 0.01 mmol L−1 for organic carbon only). The lines above and below the tops of the bars show the minimum and maximum values. Observed quantities of 

solid iron and organic carbon were adequate at every site to explain the measured reduction of O2 and NO3

−, whereas sulfur was sufficient at some sites. Dissolved 

reductants were not sufficient to account for the measured reduction of O2 and NO3

− at any site, implying that reactions with solid-phase organic carbon, iron, and(or) 

sulfur are responsible for O2 reduction and denitrification at these sites.

depending on local site conditions. Rates 

of denitrification are similar among the 

ACT study sites and are typically low 

(Green and others, 2008a; Green and others, 2009). At sites with relatively high 

rates of recharge, such as Maryland, Washington, and California (Green and others, 2008b), denitrification is too slow to 

substantially affect oxidant and reductant 

concentrations during transport through 

shallow groundwater. Declines in denitrification will likely produce minor changes 

relative to the large total flux through 

shallow groundwater. At sites such as 

Nebraska, where recharge rates are variable, or Mississippi (another ACT site currently being studied), where recharge rates 

are low (Green and others, 2009), declines 

in denitrification rates or increased NO3

−

flux will likely cause dramatic increases 

of NO3

−

 fluxes to deeper groundwater and 

adjacent water bodies. Because denitrification rates are low at all of the ACT sites, 

sustaining potable groundwater quality 

will require long-term efforts to optimize 

the efficient use of nitrogen fertilizers and 

irrigation.

Although this study focuses on NO3

−

, 

the results apply to other groundwater 

contaminants affected by redox geochemistry. For example, the increased fluxes of 

O2 and NO3

−

 from agriculture can result in 

release of metals such as arsenic and uranium bound in mineral reductants (Böhlke, 

2002; Izbicki and others, 2008) and may 

facilitate transport of these redox-sensitive 

solutes by oxidizing portions of the aquifer 

that would otherwise reduce the solutes to 

immobile forms.

MD

WA

CA

NE

Oxygen Nitrate

reduced

remaining

Organic carbon (mmol L−1)

Iron (mmol L−1)

Sulfur (mmol L−1)

0.01

0.1

1

10

100

1,000

10,000

MD WA CA NE demand in solids

in water

B

0.01

0.1

1

10

100

1,000

10,000

A C MD WA CA NE

0.01

0.1

1

10

D MD WA CA NE

Figure 5. Comparisons of dissolved organic carbon (DOC) concentrations and specific ultraviolet absorbance 

(SUVA) in aerobic zones (O2>1 mmol L−1) versus anaerobic zones (O2<1 mmol L−1). Units: mmol L−1, millimoles 

per liter; L mg-C−1m−1, liters per milligram carbon per meter; SUVA is defined as the UV absorbance at 254 

nanometers measured in inverse meters (m−1) divided by the DOC concentration measured in milligrams 

of carbon per liter (mg-C L−1). Vertical lines connect the minimum and maximum values, boxes show the 

interquartile range (containing the 50 percent of samples closest to the median), and horizontal lines inside 

the boxes show the median. The concentration of DOC does not change substantially (as shown by overlapping interquartiles) between aerobic and anaerobic zones at the Maryland, Washington, and California 

sites. At the Nebraska site, DOC in anerobic waters is less concentrated, which suggests O2 reduction has 

consumed some DOC. However, reactivity of DOC as indicated by SUVA does not change substantially at 

any of the sites. Also, as shown in figure 4, total quantities of DOC in recharge are insufficient to account 

for the measured reduction of O2 and NO3

−. Overall, these results indicate that involvement of DOC as a 

reductant of O2 is limited.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

1

2

3

4

SUVA (L mg-C−1 m−1)

Maryland Washington California Nebraska Maryland Washington California Nebraska Nonreactive Reactive aerobic anaerobic

DOC (mmol L−1)
Christopher T. Green and Barbara A. Bekins

Edited by Peter H. Stauffer

Graphic design by Darlene A. Ryan

For more information please contact:

Christopher T. Green

(650) 329-4728

ctgreen@usgs.gov

Paul Capel, Agricultural Chemicals Team Leader

(612) 625-3082

capel@usgs.gov

See also USGS fact sheets about individual 

agricultural chemical studies:

Maryland: http://pubs.usgs.gov/fs/fs08003

California: http://pubs.usgs.gov/fs/fs08103

Washington: http://pubs.usgs.gov/fs/fs08203

Nebraska: http://pubs.usgs.gov/fs/fs08303

This Fact Sheet and any updates to it are available 

online at:

http://pubs.usgs.gov/fs/2010/3077/

Conclusions

Results of this research to date justify the 

following conclusions:

• NO3

−

 contamination is prevalent in 

groundwater at the studied agricultural 

sites.

• Denitrification in these aquifers relies 

on a finite reservoir of solid-phase 

reductants.

• Because of increased fluxes of O2 and 

NO3

−

 beneath agricultural lands, this 

finite reservoir of solid-phase reductants 

is being depleted. 

• Further work is needed to quantify 

the existing reduction capacity and to 

determine the long-term sustainability 

of NO3

−

 natural attenuation in agricultural aquifers.

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NO3

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Groundwater table

Aerobic

Anaerobic

Not to scale

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(NO3

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of arrows indicates the concentration of oxidants (NO3

− and O2), and their length indicates the velocity of 

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and NO3

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