Hatfield J.L., Gandini A. (eds.) Nitrogen in the Environment

194 Nitrogen in the Environment

nitrate ranks. Unconsolidated aquifers were found to have the largest nitrate concen-

trations followed by alluvial aquifers and carbonate aquifers ( Figure 11 ). Carbonate

aquifers, when close to the land surface, can be directly connected to the surface

through karst features such as enlarged fracture systems and sink holes that provide

direct recharge paths not available in the other types of aquifers. However, although

generally thin, the soils, colluvium, and glacial deposits that overlie these aquifers

may provide a sufficient barrier to nitrate leaching to protect many carbonate aqui-

fers. Unconsolidated and alluvial aquifers are both composed of sand and gravel,

but may differ in the nature and thickness of overlying materials. The significantly

larger nitrate concentrations found in unconsolidated aquifers may result from the

overlying soils being developed directly in the sand and/or gravel. Alluvial aquifers,

on the other hand, can be buried under varying thicknesses of fine-grained flood-

plain deposits that are less permeable than sand, contain substantial organic matter

fractions, and have low dissolved oxygen, typical of groundwater discharge areas.

These differences in overlying materials or terrain and related flow systems are suf-

ficient to reflect significant differences in the nitrate concentrations.

Analysis of the four vulnerability classes ( Figure 8 ) shows the cumulative

effects of soil permeability and irrigation on the distribution of nitrate concentra-

tions in shallow aquifers ( Figure 12 ). It was hypothesized that vulnerability to

nitrate contamination increased when shallow aquifers were overlain by soils

with permeability exceeding 64 mm/h. It was further hypothesized that irrigation

provided an increase in the potential for nitrate contamination. Four vulnerability

0

30

20

10

Nitrate concentration (mg/L)

1389

596

446

A

B

C

Unconsolidated

Alluvial

Carbonate

Figure 11 . Nitrate concentrations grouped by shallow aquifer types.

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Nitrogen in Groundwater Associated with Agricultural Systems 195

classes were defined using permutations of irrigation intensity and presence or

absence of high-permeability soils. There were significant differences among sam-

ples from all four vulnerability classes at 0.05 level using Tukey ’ s multiple variable

comparison applied to ranks of nitrate concentration. Shallow aquifers in highly

irrigated areas yielded significantly larger nitrate concentrations than those in non-

irrigated areas regardless of overlying soil permeable (  64 mm/h). Since nitrate

concentrations in nonirrigated areas with low-permeability soils were significantly

larger than in areas with permeable soils is not intuitive. This apparent conflict with

the hypothesis that permeable soils are more susceptible to contamination may

result from influence of other factors, particularly the combination of agricultural

systems associated with irrigation and less permeable soils.

4 . CONCLUSIONS

This chapter focused on processes by which aquifers can be affected by nitro-

gen derived from agricultural systems. The primary form of nitrogen of concern for

drinking water, nitrate, is costly to remove in water treatment. Many major aqui-

fers used for urban drinking water are buried deep beneath large population centers.

These aquifers are geographically removed from recharge areas near agricultural sys-

tems. However, shallow aquifers in urban areas may be contaminated by atmospheric

and turf fertilizer sources of nitrogen. Groundwater sources for large municipal water

Figure 12 . Nitrate concentrations grouped by aquifer vulnerability classes.

0

30

20

10

Nitrate concentration (mg/L)

A

BC

D

729

143

702

143

Irrigated with

permeable soils

Irrigated without

permeable soils

Nonirrigated without

permeable soils

Nonirrigated with

permeable soils

CH07-P374347.indd 195CH07-P374347.indd 195 5/31/2008 7:30:49 PM5/31/2008 7:30:49 PM

196 Nitrogen in the Environment

supplies can be and often are blended with several sources allowing dilution of

any nitrate contamination. Most rural drinking-water supplies, however, are served

through individual or a limited number of wells that are usually completed in shal-

low aquifers. These shallow aquifers are commonly recharged beneath agricultural

activities. Limited research results have shown that once a shallow aquifer has been

contaminated by nitrate, it may take decades for the groundwater quality to improve

even after pollution controls have been implemented. Few programs exist that rou-

tinely monitor private groundwater systems for contamination from agricultural nitro-

gen. This makes it difficult for well owners to know trends in nitrate contamination

of their aquifer over time. Consequently, local and regional understanding about vul-

nerable aquifers beneath certain agricultural systems becomes critical information for

preventive and effective protection of water supplies, particularly in rural settings.

Two lines of evidence support several factors that contribute to groundwater

vulnerability to nitrate contamination in agricultural settings. Research from sev-

eral regions of the world provides a collection of spatially anecdotal information to

hypothesize globally applicable hydrologic and agricultural factors contributing to

groundwater vulnerability. Preliminary analysis of a United States dataset compiled

by the US Geological Survey NAWQA Program from a variety of sources confirms

these hypotheses for most agricultural systems.

Shallow unconfined aquifers have been most susceptible to nitrate contamina-

tion associated with agricultural systems. Unconsolidated aquifers and alluvial aqui-

fers are more vulnerable, although shallow carbonate aquifers provide a smaller but

substantial contamination risk. In areas dominated by irrigation, shallow aquifers

are more vulnerable to nitrate contamination than areas without irrigation. The pres-

ence of permeable soils over shallow aquifers compounds the risk of contamination

in irrigated areas.

Three agricultural systems (cattle and grains; corn, soybean, and hogs; and small

grains) produced significantly larger concentrations of groundwater nitrate than other

agricultural systems. However, significant differences of nitrate concentrations among

these three systems could not be confirmed. Irrigation, particularly in corn, soybean,

and hogs systems was found to have consistently larger groundwater nitrate concen-

trations in the United States data as well as in studies from outside this country.

Varying time lags exist in shallow groundwater responses to changes in agri-

cultural inputs at the surface. If trends in increased fertilizer use and groundwater

nitrate in the United States are repeated in other regions of the world, Asia may

experience increasing problems because of recent and substantial increases in ferti-

lizer use in that region. Both the American continents also continue to increase their

use of inorganic nitrogen fertilizer, albeit at rates less than those seen prior to the

1980s and those presently seen in Asia. Scientists and policymakers should be inter-

ested in learning if there will be a reduction in the trend of increasing concentra-

tions of nitrate in groundwater where fertilizer inputs have been reduced. The most

rapid responses may be seen in areas with extensive macropore flow where land-

use changes may produce the earliest changes in groundwater quality. It will be

CH07-P374347.indd 196CH07-P374347.indd 196 5/31/2008 7:30:49 PM5/31/2008 7:30:49 PM

Nitrogen in Groundwater Associated with Agricultural Systems 197

particularly interesting to monitor changes in groundwater nitrate in both Western

and Eastern Europe as well as in the former USSR where fertilizer use overall has

dropped since the early 1990s. Groundwater nitrate measurements in these regions

may provide tests of hypotheses that reduced nitrate contamination will follow

reduced inorganic fertilizer inputs. Fertilizer-use trends may be useful to estimate

long-term changes in nitrogen contamination of groundwater throughout the world.

Use of these trends to strategically locate long-term monitoring will help answer

questions about whether and when proportional changes in concentrations of nitrate

will follow these changes in fertilizer.

If the trend in concentrated livestock production seen in the United States is

global, there may be an accompanying trend of increasing nitrogen contamina-

tion locally in groundwater. Concentrated livestock operations provide both point

sources of nitrogen in the immediate area of the confinement as well as larger areas

of intense nonpoint sources as fields close to facilities become used for manure

disposal.

A major contributor to groundwater vulnerability is the distribution of irrigated

cropland. Regions were this practice expands, such as in Asia, may experience its

greatest impact. More data and research will be needed in Asia to determine if pat-

terns of water-quality degradation in irrigated areas is repeated in this region.

ACKNOWLEDGMENTS

The authors thank David James and Robert Jaquis of the USDA, National Soil

Tilth Laboratory for their statistical support and for generating the maps and graphs

that illustrate this chapter and David Lorenz, US Geological Survey, who prepared

estimates of fertilizer use in the United States. We also appreciate the thoughtful

and thorough manuscript reviews by US Geological Survey scientists Bernard

T. Nolan and Dana W. Kolpin. Their comments and suggestions were very helpful

to improve the early drafts of this chapter.

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203

Chapter 8. The Importance and Role of Watersheds in the

Transport of Nitrogen

T.J. Sauer

a

, R.B. Alexander

b

, J . V. Brahana

c

and R.A. Smith

b

a

USDA-ARS, National Soil Tilth Laboratory, Ames, IA, USA

b

US Geological Survey, Reston, VA, USA

c

Department of Geosciences, University of Arkansas, Fayetteville, AR, USA

A recent report to Congress concerning water quality in the United States indi-

cated that 35%, 45%, and 44% of the assessed rivers and streams, lakes, and estuaries,

respectively, were impaired by one or more pollutants ( US Environmental Protection

Agency, 1999 ). Nutrients, primarily nitrogen (N) and phosphorus (P), contributed to

the impairment of 30% or 135,000 km of the nation ’ s impaired rivers and streams,

44% of the impaired lakes, and 23% of the impaired estuaries. Excessive nutrient

loads are implicated in the eutrophication of lakes and reservoirs in the United States

and coastal ecosystems where N is most limiting to primary productivity ( Vitousek

et al., 1997 ; Carpenter et al., 1998 ). Efforts are currently underway to establish Total

Maximum Daily Load (TMDL) values for pollutants, including nutrients, of impaired

water bodies as described under Section 303(d) of the Clean Water Act of 1972.

The movement of N in the terrestrial environment is intimately related to the

movement of water. Water in the form of precipitation, flowing across the soil sur-

face as runoff, and percolating through soil layers to ground water can all be sig-

nificant carriers of organic and inorganic N constituents. The relative importance

of these transport mechanisms is a complex function of N sources and transforma-

tions, hydrologic processes, climate patterns, and land use. While some elements

of the N cycle can be studied in the laboratory under controlled experimental con-

ditions, many can be studied in a meaningful way only in the natural and cultur-

ally affected environments of watersheds. By considering N transport over a range

of spatial and temporal scales, it is possible to improve our understanding of the

factors affecting the fate of N in watersheds, including the effects of land use and

N sources (point, nonpoint, agricultural, urban, organic, and inorganic), N trans-

formations (mineralization, nitrification, denitrification, and immobilization), and

transport mechanisms (runoff, percolation to ground water, and ground water trans-

port). Knowledge of the variability in N transport in relation to these factors is criti-

cal to developing and implementing effective strategies for mitigating unacceptably

high N inputs to receiving waters.

Nitrogen in the Environment: Sources, Problems, and Management

J.L. Hatfield and R.F. Follett (Eds)

CH08-P374347.indd 203CH08-P374347.indd 203 5/31/2008 6:04:52 PM5/31/2008 6:04:52 PM

Hatfield J.L., Gandini A. (eds.) Nitrogen in the Environment

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