Marc Ribaudo and Robert Johansson
Abstract—Agricultural production
releases residuals, like sediment and pesticides, that may degrade
the quality of water resources and impose costs on water users.
Agriculture is the leading source of impairments in the Nation's
rivers and lakes and a major source of impairments to estuaries.
However, the extent and magnitude of this degradation is difficult
to assess because of its nonpoint nature.
Introduction
The production practices and inputs used by agriculture can result
in a number of pollutants entering water resources, including sediment,
nutrients, pathogens, pesticides, and salts. Farmers, when
making production decisions, often do not consider offsite impacts
associated with runoff or leaching. Documenting the links between
agriculture and water quality can help policymakers provide appropriate
incentives to farmers for controlling pollution that originates
on farms.
Agriculture is widely believed to have significant impacts on
water quality. While no comprehensive national study of agriculture
and water quality has been conducted, the magnitude of the impacts
can be inferred from several water quality assessments. A general
assessment of water quality is provided by EPA's
2000 Water Quality Inventory. Based on State assessments of
19 percent of river and stream miles, 43 percent of lake acres,
and 36 percent of estuarine square miles, EPA concluded that agriculture
is the leading source of pollution in 48 percent of river miles,
41 percent of lake acres (excluding the Great Lakes), and 18 percent
of estuarine waters found to be water-quality impaired, in that
they do not support designated uses. This makes agriculture the
leading source of impairment in the Nation's rivers and lakes,
and a major source of impairment in estuaries. Agriculture's
contribution has remained relatively unchanged over the past decade.
The significance of water pollutants commonly produced by agriculture
is suggested by information on impaired waters provided by States,
tribes, and territories to EPA in accordance with Section 303(d)
of the Clean Water Act. These are waters that do not meet water
quality standards, and cannot meet those standards through point-source
controls alone. The most recent information (2005) indicates that
25,823 bodies of water (stream reaches or lakes) are impaired nationwide.
Pathogens, sediment, and nutrients are among the top sources of impairment,
and agriculture is a major source of these pollutants in many areas.
A U.S. Geological Survey (USGS) study of agricultural land in watersheds
with poor water quality estimated that 71 percent of U.S. cropland
(nearly 300 million acres) is located in watersheds where the concentration
of at least one of four common surface-water contaminants (nitrate,
phosphorus, fecal coliform bacteria, and suspended sediment) exceeded
generally accepted instream criteria for supporting water-based
recreation activities (Smith, Schwarz, and Alexander, 1994). Another
USGS study found that structural changes in animal agriculture between
1982 and 1997 put upward pressure on stream concentrations of fecal
coliform bacteria in many areas of the Great Plains, Ozarks, and
Carolinas (Smith et al., 2005).
Major Agricultural Pollutants
Sediment is the largest contaminant of surface
water by weight and volume (Koltun et al., 1997) and is identified
by States as the second leading pollution problem in rivers and
streams and the third leading problem in lakes (USEPA, 2002). Sediment
in surface water is largely a result of soil erosion (see AREI
Chapter 4.2), which is influenced by soil properties and the
production practices farmers choose. Sediment buildup reduces the
useful life of reservoirs. Sediment can clog roadside ditches and
irrigation canals, block navigation channels, and increase dredging
costs. By raising streambeds and burying streamside wetlands, sediment
increases the probability and severity of floods. Suspended sediment
can increase the cost of water treatment for municipal and industrial
water uses. Sediment can also destroy or degrade aquatic wildlife
habitat, reducing diversity and damaging commercial and recreational
fisheries.
Regions with the greatest potential to discharge sediment from
cropland to surface waters include parts of the Heartland, Mississippi
Portal, and Prairie Gateway (see ERS
Resource Regions for a description) (fig. 2.2.1).
Figure 2.2.1—Potential delivery of sediment to surface
waters
Nitrogen and phosphorus are important
crop nutrients, and farmers apply large amounts to cropland each
year. They can enter water resources through runoff and leaching.
The major concern for surface-water quality is the promotion of
algae growth (known as eutrophication),
which can result in decreased oxygen levels, fish kills, clogged
pipelines, and reduced recreational opportunities. USGS has found
that high concentrations of nitrogen in agricultural streams are
correlated with nitrogen inputs from fertilizers and manure used
on crops and from livestock
waste (see AREI Chapters 4.4,
4.5). Nine percent of
domestic wells sampled by USGS's National Water Quality Assessment
Program (NAWQA) during 1993-2000 had nitrate concentrations exceeding
EPA's drinking water standard (maximum contaminant level or MCL)
of 10 milligrams per liter, and agriculture was identified as the
major source. EPA reported in its Water Quality Inventory that nutrient pollution is the leading cause
of water quality impairment in lakes and a major cause of oxygen
depletion in estuaries.
Watersheds with a high potential to deliver nitrogen to surface
water are primarily in the Heartland and Southern Seaboard regions
(fig. 2.2.2). Watersheds with a high potential to discharge nitrogen
to ground water are primarily in the Southern Seaboard, Fruitful
Rim, Heartland, and Prairie Gateway regions (fig. 2.2.3). Watersheds
with a high potential to discharge phosphorus to surface water are
located primarily in the Heartland, Southern Seaboard, and Northern
Crescent regions (fig. 2.2.4).
Figure 2.2.2—Potential delivery of nitrogen to surface
waters
Figure 2.2.3—Potential nitrogen leaching to ground
water
Figure 2.2.4—Potential delivery of phosphorus to
surface
water
Eutrophication and hypoxia (low oxygen levels) in the northern
Gulf
of Mexico have been linked to nitrogen loadings from the Mississippi
River (NOS, NOAA, 1999). Agricultural sources (fertilizer, soil
inorganic nitrogen, and manure) are estimated to contribute about
65 percent of the nitrogen loads entering the Gulf from the Mississippi
Basin (Goolsby et al., 1999). As much as 15 percent of the nitrogen
fertilizer applied to cropland in the Mississippi River Basin makes
its way to the Gulf of Mexico.
The Gulf of Mexico is not the only coastal area affected by nutrients.
Recent research by the National
Oceanographic and Atmospheric Administration has found that
44 estuaries (40 percent of major U.S. estuaries) exhibit highly
eutrophic conditions, caused primarily by nitrogen enrichment (Bricker
et al., 1999). These conditions occur in estuaries along all coasts,
but are most prevalent in estuaries along the Gulf of Mexico and
Mid-Atlantic coasts. Watersheds with a high potential to discharge
nitrogen from agriculture to estuaries are located primarily in
the Heartland, Mississippi Portal, and Southern Seaboard regions.
Farmers apply a wide variety of pesticides to
control insects (insecticides), weeds (herbicides), fungus (fungicides),
and other problems (see AREI
Chapter 4.3). Well over 500 million pounds (active ingredient)
of pesticides have been applied annually on farmland since the 1980s,
and certain chemicals can travel far from where they are applied.
Pesticide residues reaching surface-water systems may harm freshwater
and marine organisms, damaging recreational and commercial fisheries.
Pesticides in drinking water supplies may also pose risks to human
health. At least one of seven prevalent herbicides was found in
37 percent of the groundwater sites examined by USGS as part of
the National Water Quality
Assessment Program, but all at low concentrations.
Watersheds with a high propensity to discharge pesticides to surface
water are located primarily in the Heartland and Mississippi Portal
regions (fig. 2.2.5). Watersheds with a high propensity to discharge
pesticides to ground water are primarily in the Heartland, Prairie
Gateway, and Southern Seaboard regions (fig. 2.2.6).
Figure 2.2.5—Potential pesticide runoff from cropland
Figure 2.2.6—Potential pesticide leaching from cropland
Some irrigation water applied to cropland may run off the field
into ditches and to receiving waters (see AREI Chapters 2.1
and 4.6). These irrigation
return flows often carry dissolved salts as well
as nutrients and pesticides into surface or ground water. Increased
salinity levels in irrigation water can reduce crop yields or damage
soils such that some crops can no longer be grown. Increased concentrations
of naturally occurring toxic minerals—such as selenium, molybdenum,
and boron—can harm aquatic wildlife and impair water-based
recreation. Increased levels of dissolved solids in public drinking
water supplies can increase water treatment costs, force the development
of alternative water supplies, and reduce the lifespans of water-using
household appliances.
Dissolved salts and other minerals are a significant cause of
pollution in the Prairie Gateway and arid portions of the Fruitful
Rim and Basin and Range. Selenium is of particular concern because
of its adverse biological effects. Selenium in irrigation return
flows was identified as the cause of mortality, congenital deformities,
and reproductive failures in aquatic birds in Kesterson Reservoir
in western San Joaquin Valley, California (Seiler et al., 1999).
A Department of Interior study of the Western United States found that 4,100
square miles of land irrigated for agriculture is susceptible to
selenium contamination, along with adjacent land that may receive
return flows (Seiler et al., 1999). Affected areas are primarily
in California, western Kansas, eastern Colorado, and western South
Dakota.
The possibility of pathogens contaminating water
supplies and recreation waters is a continuing concern. Bacteria
are the largest source of impairment in rivers and streams, according
to EPA's water quality inventory. Potential sources include
inadequately treated human waste, wildlife, and animal feeding operations
(see AREI Chapter 4.5).
Diseases from micro-organisms in livestock waste can be contracted
through direct contact with contaminated water, consumption of contaminated
drinking water, or consumption of contaminated shellfish. Bacterial,
rickettsial, viral, fungal, and parasitic diseases are potentially
transmissible from livestock to humans (CAST, 1996). Fortunately,
proper animal management practices and water treatment minimize
this risk. However, protozoan parasites, especially Cryptosporidium
and Giardia,
are important sources of waterborne disease outbreaks. Cryptosporidium and Giardia may cause gastrointestinal illness, and Cryptosporidium may lead to death in persons with compromised immune systems. These
parasites have been commonly found in beef herds and Cryptosporidium is widespread on dairy operations (USDA, APHIS, 1994; Juranek, 1995).
Government Response to Agricultural Pollution
While agriculture's impacts on water resources are widespread
and considered to be significant, the control of agricultural pollution
is a challenge. The primary reason for this is that pollution from
agriculture is generally "nonpoint" in nature. Nonpoint-source
pollution has four characteristics that have an important bearing
on the design of policies for reducing it.
- Nonpoint emissions are generated diffusely over a broad land
area. These emissions leave from fields in so many places that
it is generally not cost effective to accurately monitor emissions
using current technology.
- Nonpoint emissions (and their transport to water or other resources)
are subject to significant natural variability due to weather-related
events and other environmental characteristics.
- Nonpoint emissions and the associated water quality impacts
depend on many site-specific characteristics, such as soil type,
topography, proximity to the water resource, climate, etc.
- Nonpoint pollution problems are often characterized by a very
large number of nonpoint polluters.
The difficulties in measurement, variability of discharges, and
the site-specific nature make regulations used for point sources
(factories and sewage treatment plants) largely inappropriate for
nonpoint sources. As a consequence, water quality laws such as the
Clean Water Act (see AREI
Chapter 5.7) generally do not regulate agricultural pollution,
but, instead, pass most of the responsibility on to the States. This
has resulted in quite varied responses, reflecting the States'
particular resource concerns and organizational capacity. Thirty-three
States have laws with provisions that regulate agriculture under
certain conditions, such as when voluntary approaches fail to achieve
water quality goals. States commonly use technology standards that
require farmers to implement conservation plans that contain recommended
management practices (Ribaudo and Caswell, 1999), such as conservation
tillage, nutrient management, pesticide management, and irrigation
water management. These plans can be required statewide, or
in areas particularly vulnerable to agricultural pollution.
By contrast, the Federal Government relies primarily on voluntary
approaches, such as education and financial assistance (policy
instruments), to encourage farmers to protect water quality.
Major USDA programs such as the Environmental Quality Incentive
Program and Conservation Security Program are important sources
of information and assistance for farmers concerned with water quality
(see AREI Chapter 5.4).
Between 1997 and 2004, 37 percent of EQIP funds were devoted to
water quality and conservation.
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