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These multiple metrics provide several ways of looking at the nitrogen cascade and its impact on human health
and the environment. However, there are many impacts that remain unaccounted for in any of these metrics. Some
impacts might be quantified, but the necessary data have yet to be collected. Economic losses due to damage to
commercial fisheries in the Bay are an example that is likely to be significant but has not yet been quantified.
Similarly, economic losses due to climate change and ozone depletion from N2O emissions have not been fully
evaluated. Impacts such as loss of biodiversity cannot be readily quantified at all, so it is desirable to consider a set of
qualitative and non-quantified metrics in addition to the quantitative ones.
Other parts of the country such as the Mississippi valley or the Central Valley of California are expected to show
very different patterns of cost damages, with terrestrial and freshwater emissions causing proportionally higher
damage costs, and emissions to the atmosphere causing a lower percentage of damages. But those very differences
would assist EPA and the generators of those emissions in setting priorities for mitigation.
It is important to recognize that Nr is not the only stressor that can affect both human and environmental health.
Researchers are challenged to comprehensively understand cause-and-effect relationships in a complex environment
and to balance management actions and costs to ensure that risk-minimizing management strategies are effectively
implemented.
As these multiple metrics indicate, decisions about which fluxes of Nr to mitigate depend upon which metric is
utilized. The cascading economic costs of damage highlight the importance of regulating air emissions because of
their impacts on human heath as well as their large contribution to the degradation of Chesapeake Bay water quality.
Hence, if one is interested in reducing water impacts of Nr, the total reduction of damage may rely nearly as much
on stricter enforcement of the Clean Air Act as the Clean Water Act. This challenges our traditional approach to
regulation, but that is a consequence of comprehensively examining Nr guided by the nitrogen cascade.
Table 12: Marginal abatement cost per tonne ofNrby source
*ion in the N case'
where emitted
Air
lource/poiiuian
Electric utilities/NOx14
lndustrial/NOx 15
Mobile sources/NOy 16
Non-agricultural/NHs
aiemeni cost per tonne o
$4,800
$22,000
$14,000
No estimate
Agriculture/nitrate 17
$10,000
Land
Urban and mixed open land
uses/nitrate 17
$96,000
Fresh water
Point sources/nitrates 17
$18,000
impacts have been documented. Based extensively on
European work, CLs for aquatic ecosystems are Nr inputs
on the order of 2-15 kg N/ha/yr (Bobbink et al., 2010).
There are numerous locations within the U.S. where
deposition to surface waters falls within this range.
Water quality standards
Section 303 of the CWA requires states to adopt
water quality standards and criteria that meet the state-
identified designated uses (e.g., uses related to "fishable"
and "swimmable") for each waterbody. Specifically, "a
water quality standard defines the water quality goals of
a water body, or portion thereof, by designating the use
or uses to be made of the water and by setting criteria
necessary to protect the uses" (40 CFR § 131.2). Further,
"such standards serve the dual purposes of establishing
the water quality goals for a specific water body and serve
14 See U.S. EPA, 2005c
15 See U.S. EPA, 1998
16 See Krupnick et al., 1998
17 See Chesapeake Bay Program, 2003a,b
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as the regulatory basis for the establishment of water
quality-based treatment controls and strategies beyond the
technology-based levels of treatment required by sections
301(b) and 306 of the Act" (40 CFR § 131.2).
The EPA sets minimum requirements for approvable
standards and criteria including: use designations;
water quality criteria sufficient to protect the designated
uses; and an antidigradation policy (40 CFR § 131.6).
Traditionally, Nr and other land, air, and water pollutants
are measured in terms of quantity (mass) released per unit
time (e.g., kg/day) or as a concentration (e.g., milligrams
per liter, ml/L). Therefore, regulations often specify mass
loading limits or maximum concentrations in permits.
In the mid-to-late 1990s, EPA began to emphasize the
development of numeric nutrient criteria for both P and
N through the state standards-setting process because,
according to the 1996 Water Quality Report to Congress
(U.S.EPA, 1997), 40% of the rivers, 51% of the lakes and
ponds, and 57% of the estuaries assessed for the report
were exhibiting a nutrient-related impairment. Few states
had adopted numeric nutrient criteria for all affected
waterbodies, especially for N, often relying on narrative
criteria or secondary effects such as chlorophyll-a
concentration, dissolved O2, or water clarity. EPA's
strategy, driven by President Clinton's Clean Water
Action Plan (U.S. EPA and USD A, 1998) mandated
numeric nutrient criteria to begin to address the problem
(U.S. EPA, 1999). To move the objectives of the Clean
Water Action Plan forward, EPA published national
nutrient criteria guidance for lakes and reservoirs (U.S.
EPA, 2000d), rivers and streams (U.S. EPA, 2000b),
estuaries and coastal waters (U.S. EPA, 200Ib), and
wetlands (U.S. EPA, 2007c), based upon ecoregional
guidance for lakes and reservoirs and rivers and streams.
To date, relatively few states have adopted new numeric
criteria into their water quality standards. While some
successes are evident in promulgating P criteria for
freshwater systems, which has a richer history of numeric
criteria incorporation into state water quality standards,
development of numeric nitrogen criteria has been elusive
for a variety of reasons.
Nr management in multiple media and across
jurisdictions can be complicated because the CWA has
little authority over atmospheric sources, and individual
states explicitly lack authority to control upstream
sources. For example, extensive monitoring and analysis
of the sources of reactive nitrogen in the Raccoon
River of western Iowa have shown that point sources
from municipal treatment plants and residential septic
tanks account for less than 8% of the total nitrogen
load to the system, with agricultural runoff being the
overwhelming source (Jha et al., 2010). This disparity is
similar statewide (Libra et al., 2004). As a result, nutrient
management strategies that are focused on the control
of point sources can often result in inefficient allocation
of resources if non-point sources are not also addressed.
In addition it is often the case for estuaries such as the
Gulf of Mexico or Chesapeake Bay, that management
goals that meet water quality standards cannot be attained
without interstate compacts or a strong federal role. This
may be resisted by upstream states that may have to bear
the cost but do not necessarily reap the benefits of the
water quality improvement. Such a dilemma underscores
the need for an integrated approach to Nr management.
The Committee notes that a State-EPA Nutrient
Innovations Task Group has considered some options for
improving control of nutrient pollution sources (State-
EPA Nutrient Innovations Task Group, 2009).
Populated (urban/suburban/developed) land areas
provide significant loads of Nrto the environment, both
by generation (e.g., deposition of NOX emissions) and
by transfer (e.g., domestic sewage from food imported
into the watershed). Categorical sources include sewage
treatment plants (STPs), industries, subsurface (septic)
systems, atmospheric deposition, domestic animal and
wildlife waste, and fertilizers used on lawns, gardens
and landscapes. Infrastructure (e.g., storm sewers) and
landscape conditions (e.g., increased impervious cover)
more efficiently move Nr associated with surface runoff
to receiving waters and may also inject or infiltrate Nr into
ground water. Landscape changes, primarily increases in
impervious cover, soil disturbance and compaction, and
wetland/hydric soil losses, have also reduced the capacity
for natural systems to treat Nr inputs by recycling or
denitrification. Other disruptions in chemical condition
(e.g., acidification), biology (e.g., vegetative cover), and
physical character (e.g., temperature increase) alter the
nitrogen cascade, which may have both negative and
positive consequences for Nr amelioration on the populated
landscape and in air and water. Populated lands are
estimated to export as much as 10 times the total nitrogen
that was exported under pre-development conditions.
Finding 15: Intervention to control Nr under most
water management programs generally occurs in three
ways:
Prevention or source controls.
Physical, chemical, or biological "dead ending" or
storage within landscape compartments where it is
rendered less harmful (e.g., long-term storage in soils
or vegetation; denitrification, primarily in wetlands;
reuse).
• Treatment using engineered systems such as wastewater
treatment plants or BMPs for stormwater and nonpoint
source runoff.
While most management programs focus on the
third (treatment) approach, there are opportunities for
combining the three that can be more effective and cost
less. Furthermore, it is important to recognize that in
some cases total reduction of water impacts of Nr may
rely nearly as much on stricter enforcement of the Clean
Air Act as the Clean Water Act.
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Recommendation 15: To better address Nr runoff
and discharges from the peopled landscape the Committee
recommends that EPA:
15a.
• Evaluate the suite of regulatory and non-regulatory
tools used to manage Nr in populated areas from
nonpoint sources, stormwater and domestic sewage,
and industrial wastewater treatment facilities,
including goal-setting through water quality standards
and criteria.
• Determine the most effective regulatory and voluntary
mechanisms to apply to each source type (recognizing
that in some cases total reduction of the impacts ofNr
may rely nearly as much on stricter enforcement of
the Clean Air Act as the Clean Water Act) with special
attention to the need to regulate nonpoint source and
related land use practices.
15b.
• Review current regulatory practices for point sources,
including both wastewater treatment plants and
stormwater, to determine adequacy and capacity
towards meeting national Nr management goals.
• Consider technology limitations, multiple pollutant
benefits, and funding mechanisms as well as potential
impacts on climate change from energy use and
greenhouse gas emissions, including nitrous oxide.
15c.
• SetNr management goals on a regional/local basis,
as appropriate, to ensure most effective use of limited
management dollars.
• Fully consider "green " management practices such as
low- impact development and conservation measures
that preserve or re-establish Nr-removing features to the
landscape as part of an integrated management strategy,
along with traditional engineered best management
practices.
15d.
• Research best management practices that are effective in
controlling Nr, especially for nonpoint and stormwater
sources, including land and landscape feature preservation
and set Nr management targets that realistically reflect
these management and preservation capacities.
• Construct a decision framework to assess and determine
implementation actions consistent with management goals.
15e.
• In cooperation with the Departments of Agriculture and
Army, the Fish and Wildlife Service and the Federal
Emergency Management Agency, the EPA should
develop programs to encourage wetland restoration
and creation with strategic placement of these wetlands
where reactive nitrogen is highest in ditches, streams,
and rivers. The Agency should also address the means
of financing, governance, monitoring, and verification.
Such programs might be modeled on the Conservation
Reserve Program or extant water quality and
environmental trading programs, but need not be limited
to current practices (as discussed in section 5.3.4).
4.4. Water Quality Monitoring
and Assessment
Under Section 106 of the CWA, the EPA provides
funds to assist state and interstate agencies and tribes
to conduct monitoring of the nation's waters to ensure
adopted water quality criteria and designated uses are
met. Further, primarily under Section 305(b) of the CWA,
those entities are required to report, on a biennial basis, on
the health and status of their jurisdictional waters. These
assessments are presented by the states to the EPA to
categorize attainment of designated uses. EPA published
these reports up until 1998 (U.S. EPA, 2000a), after
which it transitioned into a Water Quality Report in 2000
(U.S. EPA, 2002) and a National Assessment Database
in 2002 (U.S. EPA, 2010c). States also prepare a list of
"impaired" waters under Section 303(d) of the CWA and
EPA develops a synthesis of the CWA Section 305(b) and
303(d) reporting under a Consolidated Assessment and
Listing Methodology (CALM) approach.
As discussed above, the EPA compiles the approved
state 303(d) lists into a national listing (U.S. EPA,
2010e). The list provides information by state as well
as by impairment cause, and identifies the TMDLs
completed to date. The most current data available
on the EPA Web site includes reporting from most
entities through 2008. The report identifies 6,816
impairments related to "nutrients" (almost 9% of all
identified impairments), although other impairments
may ultimately have a nutrient enrichment cause. For
example, organic enrichment/oxygen depletion (6,410),
turbidity (3,046), noxious aquatic plants (981), algal
growth (539), and ammonia (general toxicity 356), can
all have a common cause such as N or P enrichment. It
should also be clear that impairments may have multiple
causes so, for example, waters identified as impaired by
C>2 depletion may also be impaired by nutrients.
There are other initiatives promoted by EPA to
monitor and assess the nation's waters, generally
implemented in collaboration with, or by, the state and
interstate agencies and tribes having jurisdiction over the
waters. These include the Wadeable Stream Assessment
(U.S. EPA, 2006c), the National Coastal Assessment
and its National Coastal Condition Reports (U.S. EPA,
200 la, 2004a, 2006b), the Survey of the Nation's Lakes
and Survey of the Nation's Rivers and Streams, and
more recently, probabilistic monitoring efforts in lakes,
streams, and estuaries (U.S. EPA, 2010d). Many of
these are aimed at including a biological assessment
component that is often lacking in water pollutant and
chemistry efforts described above.
The USGS collects data on surface and underground
waters and disseminates these data to the public, state and
local governments, public and private utilities, and other
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federal agencies involved with managing water resources.
The Committee encourages EPA to work closely with
USGS on monitoring and assessment activities.
The National Oceanic and Atmospheric Administration
has periodically produced estuarine assessments under
the National Estuarine Eutrophication Assessment
(NEEA) program. The most recent report was released
in 2007 (Bricker et al, 2007). The report has a focus on
nutrient enrichment and its manifestations in the estuarine
environment and relies on participation and interviews of
local experts to provide data for the assessment. Among
the key findings were:
Eutrophication is a widespread problem, with the
majority of assessed estuaries showing signs of
eutrophication - 65% of the assessed systems,
representing 78% of assessed estuarine area, had
moderate to high overall eutrophic conditions.
The most common symptoms of eutrophication were
high spatial coverage and frequency of elevated
chlorophyll-a (phytoplankton) - 50% of the assessed
estuaries, representing 72% of assessed area, had
excessive chlorophyll-a ratings.
4.5. Clean Air Act and Air Quality
Regulation and Management
The modern history of American air pollution control
legislation begins with the 1963 Clean Air Act (CAA)
which, along with its amendments, requires the EPA
to establish and revise National Ambient Air Quality
Standards (NAAQS) and to prepare state of the science
reviews such as the Criteria Documents and more recently
the Integrated Science Assessments (ISA) (U.S. EPA,
2005a, 2006a, 2007a). There are six criteria pollutants:
carbon monoxide, lead, NC>2, ozone, 862, and PM.
These have been determined to endanger public health
or welfare. The CAA as currently written requires a
review of the scientific criteria for these standards at five-
year intervals. Although NC>2 is the only Nr compound
specified as a criteria pollutant, NHX and NOy play a
major role in formation of the secondary pollutants ozone
and paniculate matter.
The CAA has been amended several times since its
inception. In 1970, the CAA was amended "to provide
for a more effective program to improve the quality of
the nation's air." The CAA was amended again in 1977,
primarily to mandate reductions of emissions from
automobiles. Despite evidence that NOX is the central
pollutant in photochemical smog formation (Chameides
and Walker, 1973; Crutzen, 1973, 1974; Fishmanand
Crutzen, 1978; Fishman, et al., 1979), federal regulations
did not require automobiles to control NOX emissions to
below 1 g/mi (0.14 gNperkm) until 1981. Few locales
violate the standards for NC>2,18 but the secondary effects
of several of these gases also pose health and welfare
concerns. If a city had an annual average NO2 level
Table 13: Federal primary ambient air quality standards that involve Nr, effective February 2010.
Pollutant
Ozone (Os)
1-hr average
8-hr average
Nitrogen Dioxide (NO2)
1-hr average
Annual average
Particulate Matter, coarse (PM-io)
Diameter < 10 |jm, 24-hr average
Annual average
Particulate Matter, fine (PM2.s)
Diameter < 2.5 urn, 24-hr average
Annual average
Federal Primary Standard (NAAQS)
0.12 ppmv
0.08 ppmv
100 ppb
0.053 ppmv (100 ug/m3)
1 50 ug/m3
50 ug/nr
35 ug/m3
1 5 ug/m3
Note: Secondary standards are currently identical to the primary standards. Source: www.epa.gov/air/criteria.html
18 In 2010, EPA promulgated a new 1-hour standard of 100 ppb for NC>2 [Primary National Ambient Air Quality Standards for Nitrogen Dioxide; Final
Rule, Federal Register 75 (26): 6474-6537]. Monitoring for compliance with this new standard is required, but it will not be known for several years
which if any locales violate this standard.
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anywhere near the NAAQS for NO2, it would risk severe
photochemical smog - the summertime efficiency for
ozone production ranges from 4 to 10 ppb 63 per ppb NOX.
As previously discussed, the focus on compliance
monitoring for NO2 ignores the other, equally important
members of the NOy family such as HNOs that deposits
quickly onto the earth's surface. It is clear that a causal
relationship exists between current levels of N and S
deposition and numerous biologically adverse effects
on ecosystems across the U.S. (U.S. EPA, 2008d).
Conversion of the existing network of NOX monitors to
NOy monitors with a detection limit of 0.1 ppb would
still demonstrate compliance with the NO2 standard but
greatly increase the utility of the measurements for model
evaluation as well as for understanding nitrate deposition
and formation of photochemical smog, and haze.
Air pollution, especially ozone and PM, continued to
be a problem in many American cities and the CAA was
again amended in 1990. The Nr-relevant aspects were
aimed at controlling urban smog and acid deposition.
States were required to develop emissions inventories for
reactive organic compounds, carbon monoxide, and NOX,
but not NH3 or N2O. Over the U.S., sulfate and nitrate are
responsible for about two-thirds and one-third, respectively,
of the direct deposition of acids. The CAA Amendment of
1990 required emissions decreases of 10 million tons of SO2
and 2 million tons of NOX relative to 1980 levels. Ammonia
and ammonium, although they contribute to acidity after
entering terrestrial ecosystems (Galloway et al., 2003; NRC,
2003) and are expected to play an increasing role (Pinder et
al., 2008), were not regulated by this legislation.
The 1997 revision of the CAA and related regulations
changed the standards for ozone and PM (see Table 13). A
sizable fraction of the mass of PM less than 2.5 microns,
PM2 5, is condensed Nr. As stated above, these particles
have adverse health consequences. PM is also controlled
by the Regional Haze Regulations (40 CFR 51). These
regulations require that by the year 2064, states must
restore Class I areas defined in the regulations to their
natural levels of atmospheric clarity.
Ozone and PM, the two most recalcitrant of the criteria
pollutants, cover large spatial scales. All of the ozone and
much of PM are secondary pollutants in that they are not
released at the tailpipe but form in the atmosphere. Ample
evidence shows that much or most of the PM in American
cities is secondary (e.g., Donahue et al., 2009). Violations
are declared on urban scales, responsibility for their control
was assigned to states, but the physics and chemistry of
smog and haze are regional. In the eastern U.S., ozone
episodes often cover several states and involve pollutants
emitted in upwind states that do not themselves experience
violations (Husar et al., 1977; Logan 1989; Moy et al.,
1994; Ryan et al. 1998). The 1990 amendments to the Clean
Air Act authorized, in part as a response to this scaling
problem, the Ozone Transport Assessment Group (OTAG)
and the Ozone Transport Commission (OTC). These have
jurisdiction extending from Washington, D.C. to Maine.
Progress has been made on regional control of emissions;
the NOX State Implementation Plan (SIP) call, implemented
in 2003 and 2004, has led to measurable improvements in
ambient ozone and nitrate levels (Gego et al., 2007; Sickles
and Shadwick, 2007a). Experiences with ozone and PM
provide a useful demonstration of why it is necessary to
develop an integrated approach to management of Nr.
Atmospheric thresholds for Nr
As shown in Table 13 the metric used for safe, upper
limits in the atmospheric environment is concentration
(in mass per unit volume of air or volume mixing ratios)
averaged for a given time period, usually 1 hour, 8 hours,
24 hours, or annually. The thresholds for excess Nr in the
atmosphere remain an area of active research. The only
Nr compound for which there is currently a NAAQS is
NO2, which may not exceed 0.053 ppm (100 ug/m3) for
the annual arithmetic mean and 100 ppb for the one-hour
average. This standard, based on the direct health effects,
is certainly inadequate because NO2 concentrations well
below 0.053 ppm lead to concentrations of secondary
pollutants well above acceptable levels (i.e., PM2 5 and 03).
The NO2 concentration required to achieve the current 75
ppb ozone standard has not been rigorously established,
but it must be well below 0.053 ppm, because information
provided by EPA indicates that areas currently in violation
of the ozone standard typically have NO2 concentrations
below 0.020 ppm (U.S.EPA, 2010a). The NO2 concentration
required to achieve the current 15 ug/m3 PM2 5 standard
is probably also below the 100 ug/m3 standard for NO2
because of the role of NO2 in secondary paniculate
formation. States in the eastern U.S. are considering
substantial additional NOX emissions reductions in order to
comply with the new 8-hour 75 ppb ozone standard. One
scenario being tested (G. Aburn, Maryland Department
of Environment, personal communication) involves the
following reductions: (1) reducing NOX emissions for point
sources by 65%, (2) reducing NOX emissions for on-road
sources by 75 percent, (3) reducing NOX emissions for
nonroad sources by 35%, and (4) reducing VOC emissions
by 30% for all source groups.
As further discussed in Section 6.2, it is the opinion of
the Committee that a decrease in NOX emissions of 2 Tg N/
yr relative to the 2002 baseline level can be achieved in the
near term. Emissions decreases implemented since 2002
have already substantially improved ozone concentrations
(Gego et al., 2007). The absolute amount of decrease and
the positive impact it would have on human health is region
dependent, but further decreases will result in further
beneficial decreases in PM2 5 and 03 concentrations.
The threshold for total Nr in the atmosphere is yet to be
fixed, but depends on its rate of deposition to the surface
and the sensitivity of the receptor(s). The immediate
need for determining thresholds for atmospheric Nr is
monitoring of NOy and NHX.
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4.6. Thresholds for Excess Nr Effects on
Terrestrial Ecosystems
In parallel with the original concept of critical loads
developed by Nilsson and Grennfelt in 1988 and now
widely used for air quality management in Europe
(Appendix D), thresholds in general and critical loads
specifically for Nr effects on terrestrial ecosystems in the
U.S. should be understood to be "quantitative estimates
of exposure to air concentrations of Nr compounds below
which harmful effects on specified sensitive elements
within ecosystem of concern do not occur according
to present knowledge" (Nilsson and Grennfelt, 1988;
Heittelinghetal, 2001).
In developing these quantitative estimates of thresholds
and/or critical loads for terrestrial ecosystems in the U.S.
(e.g., Fenn et al., 2003), it is imperative to understand
the extraordinarily wide diversity and Nr-sensitivity
of various components of terrestrial ecosystems in
different parts of the U.S., as well as the huge differences
in purposes and intensity of management and public
perceptions of the value of these ecosystem components
to various sectors of American society. Thus, the critical
loads appropriate for maintaining species diversity in a
natural grassland in northern Minnesota or a wilderness
area in the Mediterranean climate of southern California
are likely to be very different from those associated
with direct effects on similar systems in other regions
of the U. S. - or even for beneficial and/or adverse
effects on other components of the same terrestrial
ecosystem. For example, the threshold or critical load for
adverse effects of excess Nr on understory vegetation,
beneficial mycorrhizae, or lichen communities in a
forest ecosystem is likely to be very different from the
threshold for adverse effects on the dominant forest trees
in that same ecosystem. Thus, public perceptions of
"specified sensitive elements within the ecosystem" may
be important in determining what specific thresholds or
critical loads should be considered in order to minimize or
avoid specific adverse effects of concern.
At present, the sum total of directly measured wet
plus dry-deposited chemically oxidized (NOy) and
chemically reduced (NHX) inorganic Nr loads in various
states within the contiguous U. S. are on the order of 3
to 15 kg N/ha/year (NADP, 2010; CASTNET, 2010). As
shown in Appendix A, a three-year run of the Community
Multiscale Air Quality (CMAQ) model also provided
estimates of the average annual total Nr loads (including
organic forms as well as inorganic NOy and NHX forms of
Nr) in the contiguous U.S. These model estimates varied
from minimal deposition values of about 3 kg N/ha/year to
maximum estimated values of about 17 kg N/ha/year. This
range agrees well with the range of the measurements.
These directly measured and modeled estimates of total
(wet plus dry) deposition of organic and inorganic forms
of Nr indicate that there are several areas, especially in
the eastern U.S., and a few areas of the western U.S.,
where current total Nr loads are already very close to, or
will very likely soon exceed, the recommended threshold
and critical load estimates provided by Bobbink et al.
(2010) in their review of scientific evidence regarding
the impacts of atmospheric nitrogen deposition on plant
diversity in terrestrial ecosystems.
4.7. Comments on Nr Critical Loads
In recent years, the Acid Rain Action Plan developed
by New England governors and eastern Canadian
Premiers has led to evaluations of critical loads to surface
waters and forests in that region. Those studies identified
many waters and forest lands that met or exceeded
critical load capacity for combined sulfur and nitrogen
deposition both in the New England States and in the
eastern Canadian provinces. The plan set target decreases
of 20 to 30% for nitrogen oxide emissions by 2007 and a
50% decrease in sulfur dioxide emissions by 2010. These
targets are intended to decrease long-range transport of
air pollutants, acid deposition, and nutrient enrichment of
marine waters in this region.
In May 2006, a Multi-Agency Critical Loads Workshop
was held, which led to the formation of a Critical
Loads Ad-Hoc Committee (CLAD) within the National
Atmospheric Deposition Program (NADP). A goal of
the program is to "provide consistency in development
and use of critical loads in the U.S." One outcome is a
project undertaken by the Northeast States for Coordinated
Air Use Management (NESCAUM) to: "estimate critical
loads of sulfur and nitrogen in atmospheric deposition
for areas where sufficient knowledge, data, and methods
exist" and "to demonstrate the use of critical loads as a tool
for assessing environmental policies and programs and
managing natural resources."
A February 2007 Workshop sponsored by EPA on
"The Assessment of Health Science for the Review of the
National Ambient Air Quality Standards (NAAQS) for
Nitrogen (NOX) and Sulfur Oxides (SOX)" expansively
reviewed both ecosystem and human health effects toward
revision of the NAAQS. Policy discussions at this workshop
raised the questions of whether critical loads assessments
were an effective means of improving ecosystem
management, and whether the science was understood well
enough to use critical loads as a management tool. The
conclusion was that, although there was a substantial body
of accumulated scientific evidence, there was only limited
use of critical loads approaches for management of air
quality in the U.S. The Multi-Agency Workshop on Critical
Loads (mentioned above) was cited at EPA's 2007 workshop
as an agenda-setting effort to resolve some of the science
and policy issues that could help advance critical loads
approaches in the U.S. The Integrated Nitrogen Committee
believes that the primary reason critical loads are not now
used in the U.S. is that policy makers in this country have
so far not been willing to adopt unfamiliar air and water
quality management approaches or approaches that have not
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been evaluated directly in this country. Thus, the Committee
recommends that EPA consider implementation of the
critical loads concept for management of deleterious Nr
effects in various parts of the U.S.
Finding 16: The Committee finds that there have been
persistent increases in the amounts of Nr that have been
emitted into and retained within various ecosystems,
affecting their functioning. Unless this trend is reversed,
it will become increasingly difficult for many of these
ecosystems to provide the services upon which human
well-being is dependent. The Committee believes that
there is a need to regulate certain forms of Nr to address
specific problems related to excess Nr, and we believe that
the best approach for an overall management strategy is
the concept of defining acceptable total Nr critical loads
for a given environmental system.
Recommendation 16: The Committee recommends
that the Agency work toward adopting the critical loads
approach concept in determining thresholds for effects
of excess Nr on terrestrial and aquatic ecosystems.
In carrying out this recommendation the Committee
recognizes that it will in many cases be necessary for
the Agency to enter into new types of research, policy,
and regulatory agreements with other federal, state, and
tribal units based on cooperative, adaptive, and systemic
approaches that derive from a common understanding of
the nitrogen cascade.
4.8. Tradeoffs of Nr Impacts in Risk
Reduction Strategies
Because nitrogen is such an abundant and widespread
element, and Nr is such a critical component of the earth's
biosphere, associated impacts are many and pervasive.
In many cases, strategies to manage the impacts of Nr
involve tradeoffs, i.e., mitigating one type of impact may
exacerbate others. Given the interactions among oxidized
and reduced N species, it is important to recognize the
potential for unintended consequences to occur as a result
of strategies that are aimed at limiting one form of Nr in air
or water but lead to the increased production of other forms
of Nr, or the formation and release of other contaminants
of concern. For example, stringent control of point sources
of Nr can be energy-intensive, requiring significant energy
investments for chemicals, electricity, and other support,
and this may in turn lead to the production of more reactive
nitrogen and increased CO2 emissions. Furthermore,
there may be environmental impacts of these treatment
processes, particularly in the production of solid wastes
that can be significant environmental hazards. This is
the main reason that a life cycle approach is necessary
in evaluating any remediation or treatment scheme. In
addition, as discussed in Section 3.1.2, numerous lakes,
reservoirs, rivers, and fjords worldwide exhibit N and P co-
limitation, either simultaneously or in seasonally-shifting
patterns. Therefore, strategies are needed to reduce both
P and N inputs. Not all control practices will be effective
for dual nutrient reduction and this must be taken into
consideration. Four categories of tradeoffs examined below
are: ammonia release from concentrated feed lot operations
(CAFOs), concerns about human nutrition, nitrification and
denitrification, and nitrogen-carbon related impacts.
Ammonia release from CAFOs
As a result of effluent guidelines for NH3 in aquatic
systems, state and federal regulations and programs
under the CWA were developed to address water
quality protection from CAFOs. The resulting manure
management systems utilized NH3 volatilization as a
means to remove N and decrease the N in the manure
when land applied. Only recently has the resulting
increase in NH3 emission into the air been viewed as a
potential problem with respect to air quality concerns and
N deposition.
Finding 17: Current EPA policy (40 CFR Part 51,
Clean Air Fine Particle Implementation Rule) discourages
states from controlling ammonia emissions as part of
their plan for reducing PM2 5 concentrations. In this
rulemaking, EPA states that "ammonia reductions
may be effective and appropriate for reducing PM2 5
concentrations in selected locations, but in other locations
such reductions may lead to minimal reductions in PM2 5
concentrations and increased atmospheric acidity."
Ammonia is a substantial component of PM2 5 in most
polluted areas of the United States at most times. While
it is true that reducing NH3 emissions might increase the
acidity of aerosols and precipitation, the net effect of NH3
on aquatic and terrestrial ecosystems is to increase acidity.
After being deposited onto the earth's surface, NH4+ is
under most circumstances quickly nitrified, increasing the
acidity of soils and waters. The Committee is unaware of
any evidence that NH3 reduces the toxicity of atmospheric
aerosols or that high concentrations of NH3 occur
naturally over any substantive area of the United States.
It has not yet been established which components of PM
have substantive impacts on human health, but the total
concentration of PM2 5 correlates with morbidity and
mortality, and NH3 contributes to PM2 5. The visibility
degradation and other adverse effects associated with
PM2 5 are related to aerosol surface area or mass where
NH4+ certainly plays a role.
Recommendation 17: The Committee recommends that
the EPA presumption that NHj is not a PM2.5 precursor
should be reversed and states should be encouraged to
address NH$ as a harmful PM^ 5 precursor.
Swapping N between environmental systems
Nitrous oxide is produced in "natural" and agricultural
soils, and all aquatic systems, almost exclusively as
a result of the microbial processes of nitrification and
denitrification. As NH4+ ion is the initial mineral N
product formed during organic matter mineralization
and most of the fertilizer used worldwide is NH4+ based
(e.g., urea, ammonium sulfate) (FAO, 2007), the suite
-------
of microbiological reactions that result in the release of
gaseous N products need to be considered.
Nitrification is the oxidation of NH4+ ion to
NO3~ (Figure 17). Most commonly, nitrification is a
chemolithotropic process consisting of the conversion of
ammonium to nitrite, which is then converted to NCV by
a second group of bacteria. The ammonium oxidizing
bacteria (AOB) are obligate aerobes with some species
that are tolerant of low-oxygen environments. The most
common genera of autotrophic NH4+ oxidizers are
Nitrosospira and Nitrosomonas. AOB are found in most
aerobic environments where ammonium is available
through the mineralization of organic matter or where N
compounds are added.
Biological denitrification is the dissimilatory reduction
of NO3~ and nitrite to produce NO, N2O, and N2 by a
taxonomically diverse group of bacteria. These bacteria
synthesize a series of reductases that enable them to
utilize successively more reduced N oxides as electron
acceptors in the absence of oxygen. The general reductive
sequence is shown in Figure 17. In addition to the
free-living denitrifiers, symbolically -living Rhizobia in
root nodules of legumes are able to denitrify nitrate and
produce nitrous oxide (Mosier and Parkin, 2007).
The abundant denitrifiers are heterotrophs, which
require sources of electron-reducing equivalents contained
in available organic matter. Factors that most strongly
influence denitrification are oxygen, nitrate concentration,
pH, temperature, and organic carbon. The reductive
enzymes are repressed by oxygen but not by NH4+.
Nitrous oxide reductase appears to be more sensitive to
oxygen than either NOs" or nitrite reductase. Therefore N2
production predominates in more anoxic sites and N2O
production may be greater in more aerobic conditions.
However, the ratio of N2 to N2O emitted may also be
affected by high NOs" concentrations and associated higher
levels of electrical conductivity and osmotic stress and soil
pH (low pH favors N2O production).
Given these interactions among oxidized and
reduced N species (discussed above), it is important to
recognize the potential for unintended consequences
to occur as a result of strategies that may be aimed at
limiting one form of Nr in air or water but lead to the
increased production of other forms of Nr. One such
instance is the potential offsetting of the benefits of
NO3~ remediation at the expense of increasing input of
N2O to the atmosphere. An example of such a situation
involves NOs" leached from agricultural fields, much of
which could be removed from drainage water in natural
or reconstructed wetlands. This process is ideal if the
denitrification process goes to completion, i.e., only
N2 is produced. If, however, the process is incomplete,
and NO and N2O gases are emitted, then the end result
may create a compensating risk that could be greater
than that posed by the nitrate that is removed. This is
Nlrrosomonas
Nttrosospira
NitmsocGccus
Main Controls
Substrate. Oz, H3O, T
-a
>^
r •
PI*
>^ Ntirvbacte t
NjO
DcnitrifU-Htion
• NO
NO,
F.iculLitive Anaerobic Bacterij
Main Controls
Substrate, available C.Oj, HjO, T
Figure 17: Diagram of the nitrification and
denitrification processes
Source: Mosier and Parkin, 2007. Reprinted with
permission; copyright 2007, Taylor & Francis Group LLC
- Books.
because NO continues to be reactive in the atmosphere
and is eventually redeposited in aquatic or terrestrial
systems, and N2O is a GHG that has an atmospheric
life time of approximately 100 years and a radiative
forcing of approximately 300 times that of CO2 on a
hundred-year time frame (IPCC, 2001). N2O is also a
major source of NO in the stratosphere and depletes
stratospheric ozone (Crutzen, 1981). If more of the NOs"
denitrified is converted to N2O in wetlands than upstream
or downstream, the environmental cost may be high.
Hernandez and Mitsch (2007) found that permanently
flooded wetlands had lower N2O/N2 ratios of emissions
than did intermittently flooded wetlands. They also found
that the ratio was higher in the cold months even though
the flux rates are much lower then. A full risk assessment
needs to be made to determine how much of such
"pollutant swapping" is advisable.
A similar potential exists for Nr mediation in sewage
treatment. The current practice is to convert ammonia/
ammonium that mineralizes from excreted organic matter
to nitrate through the nitrification process. As nitrate-
containing effluent from sewage treatment flows into
aquatic systems the nitrate may be denitrified, resulting
in N2O production if denitrification is not complete. The
protein consumption by some 301 million humans in the
U.S. results in the processing of ~ 2 Tg of N annually
(-18.4 g N/ person/day), much of which flows through
sewage treatment facilities and ultimately leads to the
production of between 0.06 and 0.1 Tg of N2O-N /yr in
aquatic systems or soils to which sewage sludge is applied.
Tradeoffs among C and N-driven impacts
Reactive N also contributes to many impacts on the
environment that are also impacted by other chemical
species, notably carbon. As depicted in Figure 18, there
are several points of tangency between the global C and
N cycles. These are: combustion, agricultural production,
industrial production, soil and sediment processes, and
-------
end-of-life disposition of products. The implication of these
interactions is that, in many instances, the perturbation
of one cycle cannot be fully assessed without including
effects on the other. For example, proposals to develop
bio-based products (biofuels, but also other products) as the
preferable alternative to fossil-based resources are not free
from impacts. Such trade-offs may involve a single impact
(e.g., global climate change to which both carbonaceous
gases and N2O contribute) but may also involve trade-offs
between impacts that are not easily compared. Figure 19
shows the latter case in the form of climate change impacts
(to which C is a principal contributor) versus eutrophication
impacts (to which nitrogen is a principal contributor) for
several different biofeedstock-product combinations which
are evaluated relative to the substituted commercial product
made from fossil C. A value of 100% on the y-axis would
mean that the bio-based alternative is no better than the
fossil-based counter-product, while the negative region
of the y-axis in Figure 19 represents net C sequestration.
It is difficult to make direct comparisons across disparate
impact categories, however Figure 19 suggests that, in
choosing among alternatives, policies that aim to minimize
both sets of impacts would be preferred.
Finding 18: The Committee notes that the effective
management of Nr in the environment must recognize the
existence of tradeoffs across impact categories involving
Nr transformations and the cycling of other elements.
Recommendation 18: The Committee recommends
that the integrated strategies for Nr management outlined
in this report be developed in cognizance of the tradeoffs
associated with reactive nitrogen in the environment
(consistent with the systems approach of overarching
recommendations 2 and 3 discussed in Section 6.2 of this
report). Specific actions should include:
Establishing a framework for the integrated
management of carbon and reactive nitrogen;
Implementing a research program that addresses the
impacts of tradeoffs associated with management
strategies for carbon, reactive nitrogen, phosphorus,
and other contaminants of concern;
Implementing a research and monitoring program
aimed at developing an understanding of the combined
impacts of different nitrogen management strategies
on the interchange of reactive nitrogen across
environmental media.
C flows N flows
Bioproduct, material,
and energy flows
ATMOSPHERE
FOSSIL FUELS AND
CHEMICAL
PRODUCTION
Energy and chemicals
Figure 18: Combined carbon and nitrogen global cycles
Source: Miller et al., 2007 (Figure 1, p. 5178). Reprinted with permission; copyright 2007, American Chemical Society.
-------
4.9. Interactions of the N Cascade
and Climate
Weather and climate vary substantially on many time
scales including the interannual. Long-term (decadal or
more) changes in climate as have been predicted by IPCC
(2007a,b) may have profound effects on the N cycle;
conversely, changes in the biogeochemical cycle of Nr can
induce climate forcing. While it is beyond the scope of
Carbon Nitrogen Tradeoffs for Various Bloproducts
CSET
0.5 *SL 1.0 1.5 2.0 2.5 3.0
Eutrophlcatlon : •. n i m..i -, KIO,-'MJ)
Figure 19: Comparisons between Global
Warming Potential (GWP) and eutrophication
impact categories for various bioproducts
Abbreviations: BD=Biodiesel; CET=Corn Ethanol;
CSET=Corn & Stover Ethanol; PLA=Polylactic Acid
(Corn); RL=Rapeseed Lubricant; SL=Soybean Lubricant;
STET=Stover ethanol; SWEL=Switchgrass Electricity;
SWET=Switchgrass Ethanol.
Source: Adapted from Miller et al., 2007 (Figure 2,
p. 5180). Adapted with permission; copyright 2007,
American Chemical Society.
this report to fully address how cycles of C and N interact
(see Figure 18 for a general treatment of the intersection
points of C and N cycles), there are several ways in which
climate impacts the biogeochemical cycle of Nr and vice
versa (e.g., Yienger and Levy, 1995; Holland etal., 1997;
Hungate et al., 2003; Hungate et al., 2004; Sutton et al.,
2007; Thornton et al., 2007; Levy et al., 2008; Sokolov
et al., 2008). These are highly interactive and nonlinear
systems. The following important interactions are noted:
Increased deposition of Nr into terrestrial and aquatic
ecosystems can alter the sequestration of carbon, while
increased ambient CO2 can change the deposition and
uptake of Nr.
Nitrate flux from fields to surface waters increases
with increasing rainfall (see Box 5:The Impact of
Climate Change on Agricultural Discharge of Reactive
Nitrogen).
Increasing temperature can both increase and decrease
atmospheric loading of paniculate matter.
Aerosols (PM) have direct and indirect (through cloud
microphysics) effects on radiative forcing of climate
and on the hydrological cycle.
N2O and 63 are greenhouse gases.
• Soil Nr chemistry and emissions of N2O, NH3, and
NO depend on environmental conditions such as
temperature and soil moisture.
• The amount of Nr deposited and exported from the U.S.
depends on meteorological variables including wind
speeds and convection.
Numerical models, when verified against past climates.
can provide insight into possible future climates and their
impacts on the nitrogen cycle. For example, increasing
temperatures increase the amount of NOX control
necessary to achieve the same amount of photochemical
smog control (Bloomer et al., 2009; Jacob and Winner.
2009). The EPA program for studying the impact of
climate change on photochemical smog (air pollution
ozone) production offers a useful model; see Jacob and
Winner (2009) for an overview.
Finding 19: The biogeochemical cycle of Nr is
linked to climate in profound, but nonlinear ways that
are, at present, difficult to predict. Nevertheless, the
potential for significant amplification of Nr-related
impacts is substantial, and should be examined in more
complete detail.
Recommendation 19: The EPA should support cross-
disciplinary and multiagency research on the interactions
of climate and Nr. To determine the interactions of global
biogeochemical Nr cycles and climate, the Committee
suggests that EPA follow a series of steps such as:
1. Select several likely scenarios for global climate from
the IPCC report for the year 2050.
2. Down-scale statistics or nest regional climate models
within each of these global scenarios to generate
meteorological and chemical fields (e.g., temperature,
relative humidity, winds, precipitation, CO 2) for a few
years around 2050.
3. Run several independent biogeochemical Nr models
(earth system models that include air/water/land) for
North America for these years with current Nr and
emissions and application rates.
4. Rerun models with decreased Nr emissions/application
to evaluate strategies for controlling impacts such as
those described in this report.
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Box 3: The Impact of Climate Change on Agricultural Discharge of Reactive Nitrogen
The discharge of reactive nitrogen from intensively managed agroecosystems is characterized by a number of
attributes that often exhibit a high degree of variability: fluctuating material flows associated with the degree of
nitrogen fixation and the extent of denitrification, the interdependence of crops in rotation, and dependence on
geography, weather patterns (particularly rainfall intensity, duration, and frequency), soil type, and agricultural
practices.
One way to gauge the impact of climate change on such systems is to examine the ranges exhibited by historical
data that collectively encompass the range of impacts that are anticipated. The assumption is that a changing climate
will systematically alter governing attributes in plausibly predictable ways, for example increased annual rainfall
and temperature over a large geographic region. The IPCC has provided general climate-induced impacts for world
regions (IPCC, 2007a,b).
Lognormal Distribution
Total NO3 Load from E low
(Historical Data
Discharge, Tg/yr
Figure 20: Probability of given discharge level for nitrate in the watersheds of eastern Iowa
Based on the simulation model of Miller et al. (2006). Red markers are historical data of discharges according to
year as reported by Powers (2007). Green bars represent a log-normal distribution.
The general impact of climate change on the discharge of reactive nitrogen from agroecosystems can be
discerned from the information in Figure 20. This figure shows a probability distribution for nitrate discharged
from the watersheds of eastern Iowa (approximately 50,000 square km), which are dominated by corn-soybean
agroecosystems (a general description of the region can be found in Kalkhoff et al., 2000). It is derived from
information on the input of synthetic fertilizers in the region during the period 1989-1999, and includes factors that
describe the transformation and transfer of Nr once applied. The distribution shown was generated using a Monte
Carlo technique, details of which can be found in Miller et al. (2006). Also included in Figure 20 (in green) is a
standard log-normal distribution, which the simulation most closely fits, and independently measured annual nitrate
runoff data (an output of the system) over the same time period, as reported by Powers (2007). The simulation is
not perfect, but it does capture the extremes of reactive nitrogen discharge, as represented by data for the years 1993
and 1998.
Figure 20 shows that the interannual variation in nitrate discharged is nearly 30-fold during the 11-year
observation period. While the impact of climate change on such a system cannot be predicted for a given year,
Figure 20 provides a basis for visualizing shifts in nitrate discharge due to changes in those factors that affect Nr
transformation and transfer. For example a climate change scenario that predicts a general increase in precipitation
amount and frequency, other factors being constant, will tend to shift the distribution of Figure 20 to the right,
resulting in generally higher discharges of nitrate (see for example Vanni et al., 2001; the data point for 1993 in
Figure 20 corresponds to precipitation in the region that was approximately 1.8 times the long-term annual average).
Other factors, of course, may amplify or retard such impacts. Understanding whether or not implementation of
best management practices and advanced technological methods can counteract climate change trends that favor
increases in discharge would require a series of significant research studies and advances in modeling capabilities.
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Integrated Risk Reduction Strategies
for Nr
5.1. Importance of Integrated Risk
Reduction Strategies
Chapters 3 and 4 of this report presented the
environmental impacts and metrics associated with the
emission of the various forms of Nr and reviewed ways
of organizing these into impact "categories." As noted,
Nr has many impacts on the environment, impacts
that are interrelated through the nitrogen cascade. As
previously stated, the nature of reactive nitrogen demands
an integrated approach within EPA and across other
relevant federal agencies, as reactive nitrogen cycles
through the environment in different forms. A number
of risk reduction approaches and the importance of
considering Nr control points in the nitrogen cascade are
discussed below.
5.2. Control Strategies for Nr
There are several ways in which the release and control
of Nr in the environment can be approached. In general
these can be classified as follows:
1. Improved practices and conservation - in which the
flux of Nr that creates an impact is lowered through
better management practices, including those that
preserve or enhance Nr controlling ecosystem
services (e.g., on-field agricultural practices,
controlled combustion conditions, ecosystem function
preservation and management)
2. Product substitution - in which a product is developed
or promoted which has a lower dependency on or
releases less Nr (e.g., N-bearing wastes instead of
corn grain as a feedstock for biofuels, development of
alternative power sources such as wind and solar)
3. Transformation - in which one form of nitrogen
is converted to another form (e.g., nitrification of
wastewater, denitrification in engineered or natural
systems)
4. Source limitation - in which the amount of Nr
introduced into the environment is lowered through
preventive measures (e.g., controls onNOx
generation)
5. Removal - in which Nr is sequestered from impacting
a particular resource (e.g., ion exchange)
6. Improved use or reuse efficiency - in which the
efficiency of production that is dependent on Nr is
improved (e.g., increased grain yields for lower Nr
applied), or Nr wasted from one source is reused in
another (e.g., algal farming)
Effective management of Nr requires combinations
of these approaches; none is a perfect alternative for
controlling Nr in the environment. Table 14 provides
a summary of the pros and cons of each of these
approaches.
5.3. Management Strategies for Nr in the
Environment
Four types of management strategies for the control of
Nr, and other pollutants, in the environment have evolved
over the past 40 years:
1. Command-and-control - in which an entity's
discharge of pollutants is regulated through a series
of permitted limitations on emissions, violations of
which may result in penalties being assessed
2. Government-based programs for effecting a policy,
such as directed taxes, price supports for a given
commodity, subsidies to bring about a particular end,
and grants for capital expansion or improvement
3. Market-based instruments for pollution control in
which market trading schemes are used to bring about
a desired policy end, often at reduced overall cost.
4. Voluntary programs in which desired ends are
achieved using private or government-initiated
agreements or through outreach and education.
5.3.7. Command-and-control19
Policy makers have traditionally used command-and-
control strategies requiring individuals and dischargers
to meet mandatory guidelines. Such an approach evolved
as the country was gearing up to meet the requirements
first established nationally through the CWA and CAA
enabling legislation in the 1970s. Because U.S. capabilities
to monitor contaminant concentrations and predict
environmental impacts were, generally, rudimentary, early
emphasis was placed on "technology-based" approaches
for managing emissions. This resulted in the promulgation
of "best practicable technology" controls, and eventually
"best available technology" controls, the idea being that
mandating some level of control, even with uncertain
improvements on impacts, would be better, and less
arbitrary, than other approaches of the time.
19 Based on Models in Environmental Regulatory Decision-Making, National Research Council, 2007.
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Table 14 : Advantages and limitations of various approaches to Nr control in forestry and agriculture
^^^^bMjlJjMrajfjUjIikW
Improved practices,
conservation
Product substitution
Transformation
Source limitation
Removal
Improved efficiency
^J
Lessens one or more impacts;
utilization of existing ecosystem
services
Lessens the need for Nr, allows for
more targeted uses of Nr
Reduces one or more impacts
to which Nr contributes, for
denitrification closes the
nitrogen cycle; utilizes natural
biogeochemical processes that may
be available ecosystem services
Reduces one or more impacts to
which Nr contributes
Reduces one or more impacts to
which Nr contributes; natural land
features/processes and ecosystem
services may be used
Reduces the need for Nr
Education cost; availability and cost
of preserved lands
Questions of acceptability,
technological issues
May contribute to other impacts; hu-
man presence has modified and di-
minished ecosystem service values
Decreased crop yields, in some
cases few viable alternatives yet
developed
Residuals containing Nr must still
be managed effectively; availability,
location and cost of land for natural
or enhanced Nr removal
Research and education costs
Nevertheless, both the CWA and the CAA had
more specific goals that were aimed at protecting
human health, public welfare, and ecosystem health.
For example, the CAA required states to develop
implementation plans (SIPs), the approval of which
depended on their ability, once implemented, to meet
ambient clean air standards. Likewise, the CWA
required greater controls to be implemented for certain
water bodies for which technology-based limits alone
were insufficient to meet standards (this became the
TMDL program).
Over time, and as our abilities to monitor, predict, and
understand impacts improved, it became possible, or at
least plausible, to tailor emission levels on a source-by-
source basis, allowing the firm in question to decide its
own technological approach. Thus permits, which place
strict limits on the amount of pollution a firm is allowed
to discharge over a specified period of time, have become
the main method for managing the majority of point
source contaminants, including the various forms of Nr,
in the environment.
While the CWA has had considerable success in
controlling point source discharges, it has been largely
unsuccessful in limiting nonpoint discharges, and it
is these sources that are particularly important for
managing nutrient flows into receiving waters. The
National Research Council has addressed this deficiency
and pointed to the need to fully implement TMDL plans
and establish numerical nutrient standards for nutrients
(National Research Council, 2008b, 2009).
5.3.2. Government Taxes and Subsidies to
Achieve Policy Ends
Government taxes and subsidies have created a
variety of results, some in conflict with and some to
further the ends of Nr management. Examples include
U.S. agricultural and land-use policies, energy and
transportation policies, and both point and nonpoint
source mandated controls on N-bearing aquatic resources,
including domestic and industrial wastewaters and
agricultural runoff.
Current and future energy policy with respect to
vehicle efficiency and biofuels will help determine
the amount of Nr released into the environment from
these sources. Some states have chosen to place modest
taxes on fertilizer containing Nr, though the demand
impact is slight at best. However, revenues may be
dedicated to improved Nr utilization efficiency. Crop
subsidies and crop insurance may at times expand land
use and even encourage increased use of fertilizers,
effectively increasing Nr in the environment. There
are various agricultural conservation programs in
the U.S. administered by the USD A. These include
the Conservation Reserve Program and the Wetland
Reserve Program (CRP and WRP). The former takes less
suitable land out of cultivation and the latter encourages
wetland protection and restoration. Both can contribute
to better Nr management. The Environmental Quality
Incentives Program (EQIP) directly subsidizes nutrient
management efforts by crop and livestock producers.
Of concern to the Committee is the need for more
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effective approaches aimed at encouraging farmers
and land managers to adopt proven conservation and
Nr management practices in fields and feedlots. The
extent of proven practices, such as variable rate fertilizer
application and installation of stream buffers, fall far
below today's technological frontier.
5.3.3. Market-Based Instruments for Pollution
Control20
A fundamental shift in environmental management
philosophy was initiated with the 1990 Clean Air Act
Amendments, which combined regulatory requirements
with market flexibility allowing lower compliance costs
through tradable credits. Most market-based policy
instruments operate on the principle that if the regulatory
framework or some other factor sufficiently alters
the relative value of available decision choices for an
individual or firm, subsequent decisions they make will be
in alignment with the policy makers' objective.
As an example, if a government wants to limit
pollution in a river where a number of polluters
discharge, it need not adopt a uniform command and
control limit on each discharger. Instead, a regulatory
cap on the total pollutant loadings can be established and
individual permit limits can be issued to all dischargers,
with provisions that allow the dischargers to trade
between their individual limits as long as the overall cap
is not exceeded. Those dischargers having low pollution
control costs will have incentive to control more
pollution than their permit limit and thus generate water
quality credits that can be sold to dischargers with high
costs of pollution control. Because the overall cap on the
pollutant is fixed, the regulatory goal is achieved. Water
quality trading thus brings about the desired reduction in
pollution level at lower cost than if all dischargers were
required to use traditional onsite treatment technology.
Water quality trading also encourages cost-effective
pollution control investment by giving each firm a clear
economic signal to invest in new technology to reduce
pollution at a level that corresponds to the market value
of the permit.
As with control strategies for Nr, there is no one
universal market-based strategy that is applicable to
every policy maker's objective. For example, the nature
of incentives available to and effective with producers
involved in over-fishing is different from landowners
providing environmental amenities. In the former case,
the objective is to restrict the intensity of fishing. In
the latter case the objective is to encourage private
landowners to provide environmental goods and services
at the lowest cost possible.
Evolution of new market-based strategies is a
continuous process. Most strategies have been customized
over time to meet local needs. One can group such market
based approaches under the following conceptual headings:
1. Water Quality Tradable Credits - Every polluting
entity is allowed to discharge pollutants up to a certain
pre-determined limit, defined in concordance with
the terms of the CWA. The entities discharging less
than their allocated limit generate credits. Under this
strategy, credits can be traded with other polluting
entities that have exceeded their allocated limit
provided that water quality standards are not exceeded.
2. Auction-Based Contracting - Environmental or
conservation contracts are auctioned, where individual
landowners place their bids to provide such goods or
services from their land. Two factors jointly determine
the selection of the bids: the amount of the bid and the
expected value of the environmental or conservation
benefit resulting from accepting the bid.
3. Individual Transferable Quotas - An individual
transferable quota (ITQ) is an allocation privilege
to extract a specified quantity of a resource among
a selected number of quota holders. The distinctive
feature of the ITQ is that the privilege is transferable
or leasable. An ITQ may be a right to produce under
favorable circumstances, such as a tobacco quota
when tobacco production would normally be limited.
4. Risk Indemnification for Specified Behavior - An
example of this is crop insurance designed to protect
farmers from uncertainty in the adoption of best
management practices that provide a public good but
are inherently riskier.
5. Easements - Conservation easements or conservation
servitudes refer to the case in which a landowner
enters into a legally binding agreement to surrender
certain property rights for a specified period of
time, either voluntarily or for compensation. Such
arrangements usually provide public goods relative to
the environment or conservation.
Policy maker objective, local conditions, and several
other factors determine the suitability of a particular
market based strategy. For example, water quality
trading is well suited where there are a variety of
dischargers at different levels of contribution and with
varying control costs. A policy framework that facilitates
the emergence of multiple options for dischargers to
meet their permit limits, such as buying from more
efficient controllers of discharge or investing in new
equipment to achieve further reductions, is likely to
accomplish the desired level of water quality at the least
possible cost to the economy. Table 15 illustrates the
potential effective application of a number of market-
based approaches in specific situations. Accompanying
this chapter are two examples of the application of
' Based on Canchi, D., P. Bala and O. Doering, Market Based Policy Instruments in Natural Resource Conservation, Report for the Resource
Economics and Social Sciences Division, NRCS, USDA, Washington D.C., March 3, 2006, pp. 4-9.
-------
market-based approaches for the design of water
quality trading schemes for Nr in watersheds (Box 4:
Water Quality Trading to Meet the Long Island Sound
Wasteload Allocation in Connecticut, and Appendix C:
Water Quality Trading in the Illinois River Basin).
Table 15 shows pair-wise comparison between
different market-based strategies. The objective and
the incentive structure of the participants determine the
suitability of one market based strategy over another.
Each pair of cells briefly lists the most relevant set of
conditions for which the respective strategy may be
optimal (left cell points to strategy at the top of the
column and right cell points to the strategy at the end
of the row). Consider the two strategies (illustrated
below): Auction-Based Contracting and Tradable
Credit. If the participation of every private entity is
essential, then Auction-Based Contracting works best.
For example, if the objective is to preserve a large tract
of privately owned contiguous land, Auction-Based
Contracting is the appropriate strategy. This requires
the participation of every private land owner to set
aside a portion of their land. An auction designed to
reveal the individual's land owner's reserve price for
participation leads to the most efficient solution. A
classic example of this is the Australian Government
of Victoria's auction based Bush Tender program. Here
it was essential to enlist blocks of land with particular
hydrological and other characteristics to maximize the
reduction in salinity and provide other environmental
benefits. (Department of Sustainability and
Environment, 2008). The goal was to bring about the
reintroduction of native vegetation, its protection, and
management where it would slow the development of
salinity in soils. Offsets would not accomplish this, and
as one looks at Table 15's characteristics of alternative
approaches, neither would quotas, or insurance for
BMPs. Easements might be used, but auctions were
much more cost effective and more suited to the long
term management commitment needed, as indicated
in the second row of Table 15 under "Easements."
Compared to this, if the objective is an overall
Auction Based
xx
V
When no offsets exist;
The participation of
every private entity is
critical;
reduction of a pollutant regardless of the individual
private entity's contribution to the abatement, the
Tradable Credit strategy with a cap is more appropriate.
As another example, if aggregate depletion is of
concern (as with fisheries) then individual transferable
quotas are appropriate. However, auction-based
contracting is preferable to individual quotas when no
offsets exist.
Although there are significant differences between
water and air quality trading, there are also several
potential barriers to effective trading systems for
both media. These are related to: accountability and
monitoring; establishing standards and management
goals; complexities of cross media and multiple source
trading, including parity of sources; insurance that
outcomes would reduce risk (environmental benefit);
economics and marketability of traded credits; and
transparency of the program, including public outreach
and stakeholder involvement.
5.3.4. Biophysical and Technical Controls (con-
trol points) on Transfer and Transformations of
Nr in and between Environmental Systems
Within the nitrogen cascade there are a number of
places where the flow of Nr is constrained or regulated,
either by nature or by human intervention, or a
combination of the two. This report refers to these places
in the cascade as "control" points. The control points
may restrict the flow of Nr species within environmental
systems (atmospheric, terrestrial, aquatic) or between
them. The control points vary from primary controls
where Nr is minimized through conservation measures or
through after-the-fact measures that attempt to convert Nr
that is emitted or not fully used to nonpolluting products
(such as conversion to N2 by denitrification or through
long-term storage). The discussion of control points in
this section is primarily focused on biophysical controls in
terrestrial and aquatic environmental systems. However,
the section concludes with a discussion of possibilities for
decreasing NOX emissions from combustion.
Offsets are
possible;
Aggregate effect
is of concern, not r
each individual L
entity's
contribution;
Tradable
Credit
-------
Table 15: Summary of market-based instruments for pollution control with conceptual examples
Auction Based
Contracting
When there
exist no
offsets; The
participation
of every
private entity
is critical;
Offsets are
possible;
Aggregate
effect is of
concern,
not each
individual
entity's
contribution;
Individual
Transferable
Quotas
When the
depletion is of
concern;
Aggregate
depletion is of
concern;
When the
discharge is of
concern;
When there
exist no
offsets; The
participation of
every private
entity is
critical;
Insurance for the
Adoption of BMPs
Homogenous
polluters;
Offsets not
feasible;
Excessive
pollution is
primarily
to mitigate
uncertain
profits;
Modest
short-term
objective;
Tied to a
production
process;
When risk
averseness
of the entity
can be used
to motivate
participation;
Discharge of
effluents is of
concern;
Not
homogenous
polluters;
Offsets are
possible;
Pollution is
an absolute
consequence
of the
production
process;
Not tied to any
production
process;
Suited for
motivating
participants
to engage in
secondary
activities;
Depletion of a
resource is of
concern;
Easements
Unidirectional;
When offsets
are not
possible; One
entity retiring
more property
rights cannot
trade with the
other retiring
less property
rights.
Auction based
contracting
can bs sssn
as a rsfinsd
and improved
cost~sfficisnt
altornativo to
sassmsnts'
Retirement
of rights is of
concern;
No uncertainty;
No action
required on
the part of the
participant;
Bidirectional;
Offsets are
possible;
Requires
specific action
on the part of
the participant
to accomplish
the objective;
Designing of
auction based
contracting
requires
considerable
professional
expertise;
Acquisition
of rights is of
concern;
Tied to a
production
process:
Trad able
Credit
Auction
Based
Contracting
Individual
Transferable
Quotas
Insurance
for the
Adoption of
BMPs
Each pair of cells briefly lists the most relevant set of conditions for which the respective strategy may be optimal (left
cell points to strategy at the top of the column and right cell points to the strategy at the end of the row).
Box 4: Water Quality Trading to Meet the Long Island Sound Wasteload Allocation in Connecticut
Pollutant trading is increasingly being promoted as a cost-effective means for attaining water quality standards.
Connecticut and New York have been working with the EPA Long Island Sound Study (LISS) for more than
20 years to address low oxygen conditions (hypoxia) in Long Island Sound that have been linked to excessive
loadings of nitrogen. A Total Maximum Daily Load (TMDL) for nitrogen, drafted by the two states and approved
by the EPA in 2001, set a 58.5% nitrogen reduction target in 2014 from point and nonpoint source/stormwater
sources. Connecticut has initiated a point source trading program for 79 municipal sewage treatment plants (STPs)
to facilitate implementation of the TMDL wasteload allocation (WLA) and is investigating the potential for
incorporating nonpoint source/stormwater into the existing Nitrogen Credit Exchange (NCE).
Several prerequisite conditions essential to the success of the current point source trading program have been
met. Briefly, (1) all the STPs contribute to the same water quality problem; (2) the technology to remove N and meet
the targets exists; (3) there are compelling member benefits to participate, especially cost savings; (4) sources can
easily be monitored and tracked by end-of-pipe monitoring; (5) credit cost calculations are based on established and
agreed upon protocols founded in state legislation; (6) sources of N are diverse and create viable supply and demand
conditions while reducing overall cost, with close control by a Nitrogen Credit Advisory Board (NCAB); and (7)
transaction costs are low relative to credit prices. In operation since 2002, the NCE has proven to be a viable and
effective mechanism for meeting the nitrogen WLA.
The economic record of the NCE demonstrates the vigor of trading over the first five years of completed trades
from 2002 to 2006 (Table 16). In sum, more than 10 million credits have been traded on the NCE, representing more
than $22 million in economic activity.
-------
The use of geographically-based trading ratios is instrumental to the relative cost of meeting N reduction limits at
the 79 treatment plants, which are scattered throughout the state (Figure 21). Because N is reactive as it travels down
rivers into the Sound, and the Sound's currents further affect relative impacts as they transport N and the resulting
algal blooms to the hypoxic areas at varying efficiencies, location of each treatment plant makes a difference in
relative impact on dissolved oxygen per pound of N discharged at end of pipe. Generally, the closer a POTW is to
the edge of the Sound, and the closer to the hypoxic zone, the higher the trading ratio (Figure 22). For plants with
high trading ratios, economics often favor treatment, while those with lower ratios may find the purchase of credits
economically advantageous over treatment.
Table 16: Performance of the Nitrogen Credit Exchange
2002
2003
2004
2005
2006
Total
$1.65
$2.14
$1.90
$2.11
$3.40
$1,317,223
$2,116,875
$1,786,736
$2,467,757
$3,828,114
$2,357,323
$2,428,636
$2,659,804
$1,315,392
$2,394,956
$11,516,705 $11,156,111
798
989
940
1,170
1,126
5,023
1,429
1,135
1,400
623
704
5,291
Source: Connecticut Department of Environmental Protection, 2007
STP Nitrogen Loads
N Load llbs/dl
• 0-200
• 200 - 600
• 600 - 1300
• 1300 - 2900
• 2900 - 6500
Figure 21: Relative nitrogen discharge (Ibs/day) from 79 POTWs.
Source: Connecticut Department of Environmental Protection, 2007
The point source NCE does not reflect a free market approach to trading. Demand is set by the annual general
permit limit and supply of credits is constrained by the availability of Clean Water Fund dollars and the timing and
location of N removal projects. Credits are bought and sold from the state, thus the number of credits purchased does
not need to match the number of credits sold (as would typically be true in a tradable permit system). Nevertheless,
there is a tendency towards implementing cost effective projects as sewage treatment plant authorities decide
whether it is less expensive to treat or buy credits, and try to predict when that break-even point might occur that
would warrant application for project funding.
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Nitrogen Trading Zones
Figure 22: Trading ratios for municipalities in Connecticut.
Source: Connecticut Department of Environmental Protection, 2007
Incorporating a nonpoint source/stormwater (NPS/SW) component into the existing point source trading program
presents some difficult challenges. Among the seven prerequisite conditions listed above that are well met by the
current point source program, NPS/SW trading does not provide compelling economic benefits for members; NFS/
SW N is difficult to quantify and track; credit cost estimation does not have a strong foundation in any existing
programs; NPS/SW credit costs, though geographically diverse, may not result in significant implementation
savings; and transaction costs (or time spent negotiating the ground rules for NPS/SW trading) may be considerably
higher than for point source credits. Many of these obstacles can be overcome by deferring to models and textbook
costs and efficiencies for NPS/SW BMPs. Tracking will still be a challenge because of the sheer number and
distribution of BMPs that can be applied throughout the state.
Basic economic principles suggest that a free-market arrangement will not produce many NPS/SW credits for
market. Costs are much higher than for point source credits and a regulatory approach must therefore be instituted to
formalize the load allocation for nitrogen and to structure participation by municipalities.
If a NPS/SW trading component were to be added in the future, it would most likely also be an incentive-based
program rather than a free-market approach. Nitrogen is difficult and costly to control in Connecticut's urban/
suburban setting, and reductions are unlikely to be cost- competitive with POTW credits in a free market system.
However, because municipalities are required to implement the Phase II stormwater permit, and various federal,
state, and local programs require or emphasize NPS/SW management, there may be benefits for an incentive-based
approach to offset some of those costs. For example, payment for NPS/SW reductions at the same credit prices paid
to POTWs under the NCE would help defray costs and encourage additional nitrogen reductions from stormwater/
NPS sources. Connecticut and the NCAB will continue to evaluate and explore the viability of these options.
Market approaches and trading can lower costs and increase economic efficiency of Nr control. The approaches
may well have to be situation specific and depend on a structured regulatory framework to create the market or
trading opportunity. As with the 1990 Amendments to the Clean Air Act, the design of market based instruments
is a product of technical capability, regulatory design, and public preference. Implementation can be tedious but
the benefits in efficiency substantial, even after being balanced against equity concerns. However, there can be
something of a geographical and supply/demand mismatch between nonpoint sources and point sources that might
be trading partners. Ribaudo and Nickerson identify only 142 of 710 eight-digit Hydrologic Watershed Units
containing waters impaired by nitrogen where trading would be most likely (Ribaudo and Nickerson, 2009).
Further, the cost for management will be enormous. EPA's Clean Water Needs Survey (EPA, 2008b) has identified
more than $200 billion in wastewater management infrastructure needs, and those needs do not fully address nutrient
control from both traditional point and nonpoint/stormwater sources or consider alternative technologies.
-------
Biophysical controls in terrestrial environmental
systems
As indicated in Figure 2, approximately 36 Tg of
new Nr is introduced into the U.S. each year. This new
Nr is derived from sources that include consumption of
-11 Tg of synthetic N fertilizer, ~8 Tg of N that is fixed
biologically by crops, and ~ 5 Tg that is emitted from
fossil fuel combustion annually. This N is used to produce
food and fiber (~15 Tg) or is formed during electrical
generation, industrial production, or transportation.
Efforts to decrease the creation of new Nr should first
look to conservation.
Reduction in use of fossil fuel and/or decreased Nr
emission can come through a variety of mechanisms such
as more energy-efficient industrial processes, homes, and
vehicles. Further gains are possible through conservation
practices and alternatives to wasteful approaches, such
as improving public transportation to minimize use of
personal automobiles, and use of local products that do
not require long-distance shipping.
Improvements in food and fiber production and
changes in diet can also play an important role in limiting
Nr. Because agriculture is the largest consumer and
producer of Nr, consumption of fertilizer N could be
decreased by changes in diet and increasing fertilizer N
use efficiency in crop and fiber production systems. The
control points discussed in this section include: protein
consumption in the human diet; removing croplands that
are highly susceptible to Nr loss from crop production;
decreasing fertilizer N demand by increasing fertilizer
use efficiency in crop and fiber production, as well as
on residential and recreational turf grass; and better
management of Nr in manure from livestock production
in CAFOS.
Decreasing the amount of fertilizer N needed
through changes in human diet
Along with increasing fertilizer N use, continued high
intake of protein in developed countries and changes in the
diet of people in developing countries will likely lead to
greater N losses from global food production in the future.
The first aspect of changes in food production concerns
the increasing protein consumption that is occurring as
global population increases and gets wealthier. This is
likely to require increased N input into food production
(Naylor et al., 2005; Galloway et al., 2007).
The average protein supply per person in developed
countries is presently -100 g per day, while in the
developing countries it is only ~65 g per day (Food and
Agricultural Organization Statistical Database (FAO
FAOSTAT, 2010a). There is a direct proportionality
between protein and nitrogen composition of food (ca
0.16 g N per 1 g protein). On average in 1995, developed
countries consumed -55% of total protein from animal
sources while developing countries derived -25% of total
protein from animals. Protein consumption was highest
in the U.S. and Western Europe, -70 and -60 g animal
protein per person per day, respectively. In 2003, total
protein consumption in the U.S. was 115 g per person
per day (74 derived from animals and 41 from vegetable)
(FAO FAOSTAT, 2010a). In developing countries,
the greatest change in animal protein consumption
has occurred in China where the consumption of meat
products has increased 3.2 fold (from -10 to -32 g per
person per day) since 1980. In Sub-Saharan Africa there
has been no increase in either total (-50 g per person
per day) or animal protein (-10 g per person per day)
consumption during the past 30+ years (Mosier et al.,
2002).
The reason for focusing on the consumption of
animal protein is that more N is needed to produce a
unit of animal protein than an equal amount of grain
protein. Bleken et al. (2005) note that the N cost of
animal production in Norway and the Netherlands was
approximately five units of N in feeds for each unit of
N produced. Approximately 2.5 units of N are required
to produce a unit of wheat protein-N. Bequette et al.
(2003) report that dairy cattle consume four units of N
in feeds (including forage and grains) for every unit of
N that appears in milk. Using a range of efficiencies
for animal production practices, Kohn et al. (1997)
estimated that 4 to 11 units of fertilizer N would be
used in a whole farm system to produce a unit of milk
protein. This ratio would be lower when using legume
N to feed cattle, as is commonly done. Based upon the
extra N required to produce animal protein compared
to grains, continued high protein consumption in
developed countries and changes to higher protein diets
in developing countries will likely increase N input and
losses in food production.
Moderating this increase by decreasing the average
amount of total protein consumed in developed countries
is one mechanism of limiting part of the expected
increased N requirement in food production. One
example of a country with a healthy diet and moderate
consumption of animal protein is Italy in 1963. At that
time, food supply was adequate to ensure sufficient
nutrition to all groups of society (Bleken, 1997). Total
protein consumption was 85 g per person per day, and
consumption of animal protein was 32 g, roughly half
of the current U.S. diet, and yet much higher than the
average of developing countries. Another example is
Japan, where animal protein consumption has traditionally
been low, although it has increased from 25 g in 1963 to
54 g animal protein per person per day in 1995. In the
same period the total protein consumption has increased
from 73 g to 96 per person per day.
Bleken (1997) analyzed the relation between human
diet and global N need for food production. Her analysis
indicates that the total N needed for diets with high
animal protein intake (comparable to many industrialized
countries today) is almost twice as high as the N needed
-------
for the average diet in Italy 1963, or for Turkey in 1993.
Based on her analysis, the Committee assumes that in
the high-N input regions, per capita N need for food
production may be reduced by 45%, which would reduce
present-day N inputs by 15% worldwide.
Switching to a lower protein diet may not, however,
reduce N losses if the new diet includes increased
quantities of fruits, vegetables, and nuts, in addition to
staple grains, beans and pulses. Vegetables, fruit, and
nuts are high value crops that typically require large
inputs of fertilizers and pesticides when produced at a
large, commercial scale, and N fertilizer losses can be
considerably larger than for grain crops. Having a very
diverse diet that includes a wide range of high-value
fruits and vegetables available year round (whether
they are in-season locally or not) also has consequences
for N inputs/outputs from agriculture - both within the
U.S. and globally. EPA and USDA are encouraged to
develop programs that stress how both human health and
environmental health will improve with a greater focus on
the human diet. It has been estimated that 30% - 40% of
the food prepared for consumption in the U.S. is wasted
(Kantor et al., 1997; Hall et al., 2009). Thus, additional Nr
may be conserved by decreasing the amount of food that
is wasted.
Removing croplands that are susceptible to Nr
loss from crop production
An analysis of NO3~ loading in the Mississippi
River Basin (Booth and Campbell, 2007) provides
estimates of N input from agricultural lands. Similar
estimates were provided by Del Grosso et al. (2006).
Recommendations in this analysis are essentially the
same as those arrived at in the original national hypoxia
assessment, which suggested that the most leaky lands
be taken out of production (Doering et al. 1999). Booth
and Campbell state:
Nitrogen derived from fertilizer runoff in the
Mississippi River Basin (MRB) is acknowledged as
a primary cause of hypoxia in the Gulf of Mexico.
To identify the location and magnitude of nitrate
runoff hotspots, and thus determine where increased
conservation efforts may best improve water quality,
we modeled the relationship between nitrogen inputs
and spring nitrate loading in watersheds of the MRB.
Fertilizer runoff was found to account for 59% of
loading, atmospheric nitrate deposition for 17%,
animal waste for 13%, and municipal waste for
11%. A nonlinear relationship between nitrate flux
and fertilizer N inputs leads the model to identify a
small but intensively cropped portion of the MRB
as responsible for most agricultural nitrate runoff.
Watersheds of the MRB with the highest rates of
fertilizer runoff had the lowest amount of land enrolled
in federal conservation programs. Our analysis
suggests that scaling conservation effort in proportion
to fertilizer use intensity could reduce agricultural
nitrogen inputs to the Gulf of Mexico, and that the
cost of doing so would be well within historic levels
of federal funding for agriculture. Under this simple
scenario, land enrolled in conservation programs
would be increased by about 2.71 million hectares, a
29% increase over 2003 enrollments, while land taken
out of traditional fertilized agriculture and enrolled in
conservation programs would constitute about 3% of
2003 fertilized hectares.
The Booth and Campbell approach places the leakiest
intensively cropped lands into government programs
like the Conservation Reserve Program - where they
would be put into grass or cover crops. Doering et al.
(1999) had a somewhat different approach. Under their
analysis, nitrogen use or nitrogen loss reductions were
imposed on agriculture, and the U.S. Agricultural Sector
Mathematical Programming (USMP) model adjusted
crop rotations, tillage practices and fertilizer inputs
within the Mississippi Basin - meeting the given Nr
constraint while maximizing producer and consumer
welfare. The model favored those crops and cropping
systems at different points in the landscape having low
nitrogen leakage. Where the model could not find a crop
production system having positive returns while meeting
the Nr restrictions, the land was retired from production.
This analysis suggests opportunities for maintaining land
in agricultural production while still reducing Nr losses
through better matching of land characteristics with crops
and cropping systems.
This 1999 analysis of the Mississippi Basin was
carried out in the context of cost effective approaches -
starting with the most cost-effective (in terms of producer
and consumer welfare) and moving to less cost-effective
approaches as more and more nutrients were controlled.
This included both restriction of fertilizer inputs, buffers,
and wetland remediation as well as the land use changes
and crop rotations referred to above. The suggestions
presented by the Committee for Nr reductions that could
be achieved from agriculture with existing technology
are consistent with the cost effective approaches in the
1999 Hypoxia Assessment's economic analysis. Cost
effectiveness and alternative cropping systems were
considered in the SAB report, Hypoxia in the Northern
Gulf of Mexico: An Update by the EPA Science Advisory
Board (U.S. EPA SAB, 2007) but unfortunately as
pieces from individual study examples rather than as an
integrated approach like the 1999 Hypoxia Assessment
(Doering etal., 1999).
Decreasing fertilizer N demand by increasing
fertilizer use efficiency in crop and fiber production
The largest input of Nr in North America is N
fertilizer used for crop production. The mean annual
N fertilizer input to North America between 1999 and
2003 was 12.5 Tg. Of this fertilizer N, 66% was used to
fertilize cereal crops, mainly corn and wheat (Dobermann
and Cassman, 2005).
-------
As previously discussed, corn yield in the U.S. has
increased (from an average of 100 bushels per acre in
1985 to 136 bushels per acre in 2005) as a result of
improved nutrient and pest management, expansion
of irrigated area, conservation tillage, soil testing, and
improved crop genetics (yield and pest resistance)
(Council for Agricultural Science and Technology
[CAST], 2006). From 1980 to 2000, N-fertilizer use
efficiency (NFUE, kg grain produced per kg applied N,
or kg grain / kg N) increased from 42 to 57 kg grain / kg
N, a 35% efficiency gain during a period when average
U.S. corn yields increased by 40% (Fixen and West,
2002). Despite this steady increase in NFUE, the average
N fertilizer uptake efficiency for corn in the north-central
U.S. was 37% of applied N in 2000 based on direct
field measurements (Cassman et al. 2002). These results
indicate that greater than 50% of applied N fertilizer
is vulnerable to loss pathways such as volatilization,
denitrification, runoff, and leaching. The results also
suggest there is substantial room for improvement in
N efficiency currently achieved by farmers. Although
progress has been made to increase both cereal yield
and NFUE, a concerted effort to further increase NFUE
remains a logical control point to reduce production
costs, because N fertilizer represents a significant input
cost, and to limit Nr leakage (e.g., NHs, NOX, N2O,
NCV) from agroecosystems.
The goal of reducing Nr while sustaining adequate
rates of gain in cereal production to meet expected
food demand will require increases in NFUE, which in
turn will require innovative crop and soil management
practices. This need is exacerbated by the recent increase
in demand for corn to produce ethanol biofuel. The
concept of improved N synchrony (practices that better
match the amount, timing, and geospatial location of
applied N to crop-N demand and the N supply from
indigenous soil resources) is generally viewed as the
most appropriate approach for improving NFUE (e.g.,
Appel, 1994; Cassman et al., 2002). The challenge is to
attain greater synchrony between crop N demand and the
N supply from all sources (e.g., soil, fertilizer, organic
inputs such as manure, compost, or green manures)
throughout the growing season. Losses from all N-loss
mechanisms increase in proportion to the amount of
available N present in the soil profile at any given time.
Several promising technologies and combinations of
technologies have emerged in recent years. Significant
increases in NFUE are often achieved through reducing
N fertilizer use by 10 to 30%, while still maintaining
or even slightly increasing yields (Giller et al. 2004).
Figure 23 indicates where the greatest gains in NFUE
are expected to be realized from investments in different
technology options. Improvements in crop and soil
management practices will contribute to higher NFUE
by achieving greater congruence in timing of the supply
of applied N with crop-N demand and the N supply from
- "" Genetic engineering
Plant and strain
selection and breeding
Improved agronomic management
Existing Knowledge
Titncscale
Source: Giller et al., 2004 (Figure 3.2, p. 48). Reprinted
with permission from Island Press; copyright 2004,
SCOPE.
indigenous soil resources. While there is relatively small
scope for specific biotechnology traits to improve NFUE,
overall improvement in crop genetics from commercial
breeding efforts that focus on increasing yield and
yield stability will continue to play a significant role in
improving overall NFUE. However, large investments
in research, extension education, and technology
transfer will be required, and significant incentives must
be implemented, to achieve the degree of improved
synchrony needed to make substantial improvements
in NFUE. The need to accelerate the rate of gain in
crop yields to meet increasing demand for human food,
livestock feed, and biofuels represents an additional
new challenge. Crop prices are expected to rise as they
more closely track the price of petroleum (Council for
Agricultural Science and Technology, 2006). Higher crop
prices will motivate farmers to achieve higher yields, and
higher crop yields require a greater amount of N uptake
to support increased biomass production (Greenwood et
al., 1990). Therefore, an explicit emphasis on developing
technologies that contribute to both increasing yields
and NFUE will be needed to ensure that the goals of
food security, biofuel production, and protection of
environmental quality are met.
Alternatives to current urban landscaping practices
Section 2.2.4 discussed the use of turf grasses as a
prominent feature in U.S. urban landscapes with over
1 TgN used to fertilize lawns each year (Table 9).
New developments are most amenable to landscaping
practices that may minimize the need to use supplemental
fertilizer. These practices include preservation of the
natural soil profile, use of turf types that require little
or no fertilizer, minimizing turf areas, using organic
-------
maintenance techniques, and choosing alternatives
to lawns and exotic plant species such as naturalistic
landscaping. Many of these practices are part of a low
impact development philosophy, which can also combine
other best management practices to mitigate the effects
of impervious cover and landscape changes. Existing
development is also amenable to many of these practices,
especially conversion of typical residential
and commercial lawns to natural landscapes and
retrofitting other BMPs that promote infiltration, such as
rain gardens.
Structural and non-structural Best Management
Practices (BMP) to treat runoff
There are probably hundreds, if not thousands, of
BMPs that have been designed and manufactured to treat
runoff from both urban and agricultural lands. Whether
applied to new development or existing agricultural or
urban land use, most follow basic principles that simulate
natural land features and processes that remove pollutants
from runoff. They promote infiltration to take advantage
of the cleansing value of passage through soils and to
reduce runoff volumes, and provide for biological or
chemical conditions that help remove pollutants (NRC,
2008b, 2009).
The most notable of the processes for managing Nr is
providing conditions that are adequate to denitrify Nr in
the waste-stream in a process called biological nitrogen
removal (BNR). BNR simply creates conditions that
convert initial forms of nitrogen to nitrate via oxidation,
and convert nitrate to dinitrogen gas by providing
conditions (especially high carbon and low oxygen)
where the denitrification process can occur. These
simulate natural conditions such as nitrification that
occurs in oxic soils as water-borne nitrogen infiltrates into
the soils and groundwater, and denitrification that occurs
in highly-organic, saturated soils such as in wetlands
where oxygen is low.
Most BMPs are considered structural, and may
be highly engineered "package" plants that can treat
sewage or runoff, depending on scale and structure,
or simple detention basins that allow sediments and
adhered pollutants to settle out. "Artificial" wetlands are
a good example of a more sophisticated BMP that takes
advantage of natural processes, and may be created at the
end of the stormwater pipe, or at edge of field. Structural
BMPs are an important part of any strategy to limit
reactive nitrogen loss to the environment. For example,
The State of Iowa contains some of the most productive
agricultural land in the world. Of the 36 million acres of
land, 23 million acres are planted in corn or soybeans.
Approximately 39 percent of the corn/soybean acres
are drained with an estimated 800,000 miles of tile
(Cutler, 2000). Each year, thousands of miles of new or
replacement tile are installed. This drainage network is
responsible for the conveyance of 90 percent (Crampton
et al., 2006) of the nitrate that appears in Iowa's surface
waters. Control of nitrogen discharge from drainage
tile will be needed to limit reactive nitrogen loss to the
environment from agriculture.
Various approaches have been proposed with varying
degrees of success. Constructed wetland, bio-reactors,
and drainage tile encapsulated with biomass have
proven to reduce 90 percent of the reactive nitrogen
loss to the environment (Blowes et al., 1994). It would
seem reasonable to require any new or replacement
drainage tile to implement a control strategy at the time
of construction and a retrofit program for the remaining
drainage systems. With an average of two years to pay
back the cost of drainage systems due to increased crop
yield, the added cost for nitrogen control does not seem to
be unreasonable.
Non-structural BMPs are often preservation actions,
as discussed earlier, or activities that prevent pollutants
from entering the waste stream such as street sweeping or
fertilizer limitation.
Engineered and restored wetlands to decrease
NO3~loading of aquatic systems
The construction and/or restoration of wetlands have
received considerable attention in the past two decades
as a conservation method. Such an approach has several
positive attributes including promoting denitrification in
watersheds containing or receiving Nr, flood protection,
habitat preservation, and recreational potential (Hey and
Philippi, 1995). In the upper Mississippi basin optimum
siting of wetlands could result in as much as 0.4Tg of NOs"
converted to N2 (Mitsch et al., 1999, 2001; Hey, 2002).
Much of the nitrate leached from agricultural fields
could be removed from drainage water in natural,
created, or restored wetlands. Nitrate removal from the
water column in wetlands is performed by plant uptake,
sequestration in the soils, and microbial transformation
that includes immobilization and denitrification. Plant
uptake and microbiological immobilization result in
temporary storages in the system since most nitrogen
will eventually return to the wetland via plant death and
decomposition. In contrast, denitrification can constitute
a real nitrogen sink because NCV is converted mainly
to N2 that is emitted to the atmosphere (Clement et al.,
2002). As discussed in Section 4.7, the potential for the
formation of N2O is of concern if such systems are not
operated properly.
In addition to preserving existing wetlands, there are
two other basic approaches that utilize wetlands to reduce
the Nr and other nutrients reaching rivers, streams, and
vulnerable downstream coastal systems. These approaches
are: 1) creation and restoration of ecosystems, principally
wetlands and riparian forests, between farms and adjacent
ditches, streams and rivers; and 2) diversion of rivers into
adjacent constructed and restored wetlands all along the
river courses (Mitsch and Jorgensen, 2004; Mitsch and
Gosselink, 2007).
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The Committee notes that if wetlands can be
economically and effectively restored where croplands
now exist on hydric soils within the 100-year floodplain,
this may be an important NO3~ control mechanism.
Cropland on hydric soil in the floodplain occupy about
6.9 million acres (2.8 million hectares). If this area and its
wetlands were given back to the Mississippi River, over
a million tons of NO3" -N would be annually removed or
prevented from reaching the Gulf of Mexico (Mitsch et
al., 1999; Hey, 2002; Hey et al., 2004). To give scale to the
solution needed, restoration of over 4.9 million acres (2
million hectares) of wetlands is needed in the Mississippi-
Ohio-Missouri (MOM) river basin to reduce the nitrogen
load to the Gulf of Mexico sufficiently to ensure a
reduction in the size of the hypoxic zone in the Gulf of
Mexico (Mitsch et al., 2001).
At a series of workshops on restoration of the MOM
river basin in 2003-2004 (Day et al., 2005; Mitsch and
Day, 2006), scientists and managers were asked to
focus on needed research and chokepoint opportunities
for managing N in that basin. They concluded that a
major, interdisciplinary research program (as a lead-
in to the actual restoration of wetlands and rivers) was
needed, with sufficient funding, study sites, and time
to reduce remaining uncertainties about the efficacy of
wetlands to solve pollution problems related to N. It
was recommended that, to implement this program, 20
to 30 full-scale, existing and new agricultural/wetland
demonstration projects should be located throughout the
country and instrumented to study agricultural runoff into
wetlands in a variety of soil conditions. Pilot and full-scale
studies of diversions into riparian systems along river
channels were recommended in order to determine the
effectiveness of these systems for nutrient removal. The
Committee notes that these research and demonstration
projects have not been undertaken, and that there is a
continuing need for this work.
Further illustration of the use of wetlands as a tool for
Nr management tool is presented in Appendix C, Water
Quality Trading in the Illinois River Basin (D. Hey,
personal communication).
Technical controls (control points) on transfer
and transformation of atmospheric emissions of
Nr in and between environmental systems: NH3
Newly fixed Nr is produced biologically or added as
fertilizer to meet the demand for food and fiber production.
Much of the N is used in cereal crop production and cereal
crops are then used to feed livestock. The new Nr is then
recycled through the livestock production system where
it becomes again susceptible to losses to the atmosphere
as ammonia and NOX, and is available for additional N2O
production and movement into aquatic systems as NH4+
and NO3-.
The bulk of the N fed to livestock ends up in manure,
and where this manure (approximately one-half in urine
and one-half in feces) is produced, there is often a much
greater supply than can be efficiently or economically
used as fertilizer on crops. For large concentrated animal
feeding operations there is considerable expense associated
with disposal of the manure. Various storage systems have
been developed to deal with this excess manure, the most
interesting of which, from the standpoint of integrated
policy on N, convert the urea to N2. The fraction of
manure N that can be and is converted to N2 remains a
major unanswered scientific or technical question.
The NRC (2003) noted the paucity of credible data on
the effects of mitigation technology on rates and fates of
air emissions from CAFOs. The report did, however, call
for the immediate implementation of existing atmospheric
emission technology. The NRC (2003) also called for a
mass balance approach in which the losses of N species
such as NH3, NO, N2, and N2O are expressed as a fraction
of the total N loss. Quoting from the NRC report:
Storage covers for slurry storage tanks, anaerobic
lagoons, and earthen slurry pits are being studied
as a method to decrease emissions from those
containments. Both permeable and impermeable
covers are being studied. Tested covers range from
inexpensive material such as chopped straw (on slurry
containments only) to more expensive materials such
as high density polyethylene. Covers can decrease
emissions from storage but their net effect on
emissions from the system is conditional on how the
effluent is used on the farm.
Anaerobic digestion in closed containment has been
studied for many types of applications. Anaerobic
digestion is the process that occurs in an anaerobic
lagoon. When conducted in closed vessels, gaseous
emissions including methane, carbon dioxide and
small amounts of other gasses (possibly ammonia,
hydrogen sulfide, and VOCs) are captured and can be
burned for electricity generation, water heating, or
simply flared. The in-ground digester being tested on
a swine farm in North Carolina is an example of the
ambient temperature version of this technology (there
are also mesophilic and thermophilic designs). The
concentration of ammonia remaining in effluent from
that digester is higher than the concentration in lagoon
effluent and can be volatilized once exposed to air.
Recent research (e.g., Bicudo et al., 2004; Funk et al.,
2004a,b; Shores etal., 2005) demonstrates reduction in
NH3 emissions after a permeable cover was installed.
Miner et al. (2003) reported that a polyethylene cover
can reduce NH3 emissions by -80%, but it is not clear
what fraction of that N was converted to N2. Harper et
al. (2000) reported that in a well-managed swine lagoon
denitrification N2 losses can be equivalent to N lost as
NH3, in other words about 50% efficiency. Kermarrec
et al. (1998) reported that sawdust litter helped reduce
NH3 emissions from pig manure with 44-74% of manure
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N converted to N2, but greater than 10% of the manure
N was released as N2O. Sommer (1997) found that NH3
was emitted from cattle and pig slurry tanks at the rate
of 3.3 kg N m^r1 until covered with straw. After straw
application NH3 emissions were below detection limit.
Mahimairaja et al. (1994) reported that NH3 volatilization
was reduced by 90-95% under anaerobic conditions.
Section 2.2.4 contains a discussion of best management
practices to minimize NH3 emissions from livestock
waste, and presents finding and recommendation 6 on the
need for a framework for manure management.
Technical controls (control points) on transfer
and transformations of atmospheric emissions of
Nr in and between environmental systems: NOX
As previously discussed, a major contributor to Nr
in the atmosphere is fossil fuel combustion. During the
combustion process NOX (NOX = NO + NO2) are released
to the atmosphere. Globally the production of NOX has
accelerated in the last few decades, primarily through the
increase in fossil fuel combustion (Galloway et al., 1995;
2008). With this increase in emissions from ~5 Tg N in
1940 to ~ 25 Tg N in 2005, combustion of fossil fuels
accounted for about 50% of the total global NOX emissions
for 1990. Of the anthropogenic sources, fossil fuel,
aircraft, biomass burning, and part of the soil emission are
most important (Holland et al., 1997). Although global
NOX emissions continue to increase, these emissions are
declining in the U.S. (see Section 2.2.1).
Nitrogen oxide is formed during combustion by three
mechanisms:
Thermal NOX where N2 and O2 combine at high
temperatures (thermal pathway dominates at
temperatures greater than approximately 1500 C) to
form NO through the Zeldovich mechanism
Fuel NOX where nitrogen from a fuel (e.g., coal and
biofuels) is released as some intermediate and then
combines with O2 to form NO
Prompt NOX where N2 reacts with hydrocarbon radicals
in flames, forming various compounds including
hydrogen cyanide and other cyano radicals. These
in turn form NOX. Contributions of prompt NOX are
usually low as compared to fuel NOX.
There are several ways to control emissions of NOX.
The most common controls are on coal-fired electric utility
generators and those are discussed below. Following the
discussion of electric utility generator controls, or external
combustion systems, there is a discussion on internal
combustion controls.
Reduction of the temperature limits the kinetics of
the N/O2 reaction. Temperature can be controlled by
using a fuel-rich mixture versus fuel lean. In this case
the reactions take place at lower temperatures. Fuel-
rich mixtures also reduce the amount of O2 available
for reaction and there are changes in the chemical
mechanisms which limit the oxidation of N2. If fuel-
lean mixtures are used for temperature control, while the
temperature is lower there is a significant amount of O2
present. Typically, in external combustion systems controls
are implemented by using less excess air and using staged
combustion. In addition, flue-gas recirculation (FOR) is
used to lower the temperature. Low-NOx burners operate
under the principle of internally staging the combustion.
To reduce fuel NOX, air and fuel staging are used to reduce
the peak temperature where air and fuel are admitted in
separate locations.
Chemical reduction of NOX is also possible. These
methods include: selective non-catalytic reduction
(SNCR), selective catalytic reduction (SCR), and fuel
reburning. SNCR is an add-on technology where urea or
NH3 is injected in a controlled temperature zone to allow
the reduction of NOX. SCR is also an add-on technology
where the flue gas must pass through a catalyst bed to
allow reaction between ammonia and NOX. Care must
be taken with both technologies to avoid NH3 slip. Fuel
reburning requires the injection of a fuel to create a zone
where NOX is reduced to N2. Low NOX burners may also
use an internal fuel reburning to reduce the NOX.
For internal combustion engines, the same mechanisms
discussed above are used in a variety of different
ways, since these systems are using high pressure and
predominately have thermal NOX versus fuel NOX
formation. Most technologies involve the need to reduce
the peak temperature and duration of high temperatures
of the combustion zone. For example, gas turbines utilize
low NOX burners, while spark ignition engines utilize a
three-way catalyst which requires less than 0.5% O2. In
this case, additional NOX is reduced by utilizing unburned
fuel as a reagent over the catalyst for chemical reduction
of NOX. It should be noted, however, that a side reaction
for the three-way catalyst system produces ammonia. NOX
emissions can be reduced in diesel engines by delaying the
injection of the fuel, and by retarding the timing in spark-
ignited engines. Engines also use exhaust gas recirculation
(EGR) to reduce the peak temperatures. Recent road side
studies have indicated high efficiency (-90%) for NOX
removal from the American light-duty fleet (Bishop and
Stedman, 2008).
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This concluding chapter contains the Integrated
Nitrogen Committee's findings and its recommendations to
EPA. Section 6.1 discusses the need for a comprehensive
program to monitor reactive nitrogen. Section 6.2 provides
the Committee's overarching recommendations to EPA.
Section 6.3 contains suggestions for near-term actions
that might be taken by EPA and other management
agencies to decrease Nr entering the environment from
various sources. Section 6.4 contains specific findings
and recommendations corresponding to each of the
Committee's four study objectives.
The first objective of the study was to identify and
analyze, from a scientific perspective, the problems Nr
presents in the environment and the links among them.
To accomplish this objective, the Committee examined
the flows of Nr within the food, fiber, feed and bioenergy
production systems and developed lands in the U.S.,
paying special attention to the locations within each
of these systems where Nr is lost to the environment.
The same process was employed for fossil fuel energy
production but, since all the Nr formed and released
during energy production is lost to the environment, the
Committee identified the important energy producing
sectors that contribute to Nr emissions. The Committee
found that agriculture and domestic use of fertilizers
to produce food, feed, and fiber (including bioenergy
and BNF) and combustion of fossil fuels are the largest
sources of Nr released into the environment in the U.S.
The Committee also examined the fate of the Nr
lost to the environment, estimated the amount stored in
different systems (e.g., forest soils) and tracked Nr as
it is transferred from one environmental system (e.g.,
the atmosphere) to another (e.g., terrestrial and aquatic
ecosystems). Source and fate analyses set the stage for
identifying the environmental and human health problems
Nr presents, and the links among them. Using the nitrogen
cascade, the Committee identified the impacts Nr has
on people and ecosystem functions as it moves through
each system and contributes to adverse public health and
environmental effects, including photochemical smog,
nitrogen-containing trace gases and aerosols, decreased
atmospheric visibility, acidification of terrestrial and
aquatic ecosystems, eutrophication of coastal waters (i.e.,
harmful algal blooms, hypoxia), drinking water concerns,
freshwater Nr imbalances, GHG emissions and subsequent
climate change, and stratospheric ozone depletion.
The second objective of the study was to evaluate
the contribution an integrated N management strategy
could make to environmental protection. To accomplish
this objective the Committee identified actions that
could be taken to better manage Nr. These actions take
into account the contributions of all Nr sources and
chemical species that adversely impact human health
and environmental systems, and the need to ensure that
solving one problem related to Nr does not exacerbate
another problem or diminish ecosystem services that
support societal demands.
The third objective of the study was to identify
additional risk management options for EPA's
considerationJn addressing this objective, the Committee
identified four major goals for management actions that
collectively have the potential to decrease Nr losses to
the environment by about 25%. Decreasing Nr emissions
by these actions will result in further decreases in
Nr-related impacts throughout the nitrogen cascade.
The Committee has suggested a number of ways to
attain these management goals, including conservation
measures, additional regulatory steps, voluntary actions,
application of modern technologies, and end-of-pipe
approaches. The Committee notes that these are initial but
significant actions; however, others should be taken once
the recommended actions are completed and assessed,
and further opportunities are explored in an adaptive
management approach.
The fourth objective of the study was to make
recommendations to EPA concerning improvements in
Nr research to support risk reduction. The Committee has
provided numerous recommendations for additional Nr
research to support risk reduction activities.
6.1. Need for Comprehensive Monitoring
ofNr
In previous sections of this report the Committee
has discussed the importance of monitoring reactive
nitrogen in the environment. The Committee recommends
establishing a program for comprehensive monitoring
of the multiple forms of reactive nitrogen as both
stocks and flows as they pass through different media
and ecosystems. There are two major reasons for this
overarching recommendation. The first purpose of
monitoring is to provide the observational data on trends
that will inform research into the complexity of the
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nitrogen cascade to better identify the most effective
intervention points to reduce damage to human health and
the environment by reactive nitrogen. The second need
for monitoring is to be able to assess the effectiveness
of policy interventions overtime, and to apply the
principles of adaptive management. As it becomes clear
which strategies and policy instruments are effectively
reducing the amounts of Nr entering the environment,
and which are ineffective, it will be necessary to modify
those interventions in response to the monitoring data. As
conditions change (e.g., shifts in the technology of electric
power production, new fuels for transportation, changing
land use patterns and climate change), the nitrogen
cascade will be modified, and the relative importance
of sector specific policies will change. Only through
comprehensive monitoring will it be possible to manage
Nr effectively.
Finding 20: The Committee has determined that an
integrated approach to monitoring that includes multiple
media (air, land, and water) components and considers
a suite of environmental and human concerns related to
reactive nitrogen in the environment (e.g., Nr effects,
climate change, human health) is needed. Some of the
phenomena presented in this report need more definition
and verification but, more importantly, as controls are
brought to bear on Nr, improvements need to be measured
to verify and validate successful management strategies.
If the desired improvements are not realized as shown by
the collected data, corrective measures will be required.
Such an adaptive approach acknowledges the likelihood
that management programs will be altered as scientific
and management understanding improve.
Recommendation 20: The Committee recommends
that EPA initiate discussions and take action to develop a
national, multi-media monitoring program that monitors
sources, transport and transition, effects using indicators
where possible, and sinks ofNr in keeping with the
nitrogen cascade concept. This comprehensive program
should build upon existing EPA and state initiatives as
well as monitoring networks already underway in other
federal agencies such as the U.S. Geological Survey
programs and the NADP effort.
6.2. Overarching Recommendations
Human activities have significantly increased the
introduction of Nr into the U.S. environment and, through
radical alterations of land use, have eliminated many
of the natural features that once may have provided
pollutant treatment. While there have been significant
benefits resulting from food production, there have also
been, and continue to be, major risks to the health of both
ecosystems and people due to the introduction of Nr into
the nitrogen cascade.
In its 1990 report, Reducing Risk, the Science Advisory
Board recommended that the EPA increase its efforts
to integrate environmental considerations into broader
aspects of public policy in as fundamental a manner as
are economic concerns. Other federal agencies often
affect the quality of the environment, e.g., through
the implementation of tax, energy, agricultural, and
international policy, and EPA should work to ensure
that environmental considerations are integrated, where
appropriate, into the policy deliberations of such agencies.
In the current era of increasing responsibilities without
commensurate budgets, intergovernmental cooperation,
partnerships and voluntary programs have become vital
tools for agencies needing to stretch their resources to
fulfill their missions.
Optimizing the benefits of Nr, and minimizing its
impacts, will require an integrated nitrogen management
strategy that not only involves EPA, but also
coordination with other federal agencies, the States, the
private sector, universities, and a strong public outreach
program. The Committee understands that there are real
economic costs to the recommendations contained in this
report. For each recommendation there will of necessity
be tradeoffs derived from the varying cost-effectiveness
of different strategies.
The Committee makes four overarching
recommendations:
Overarching Recommendation 1
The Committee recommends an integrated approach
to the management ofNr. This approach will likely use
a combination of implementation mechanisms. Each
mechanism must be appropriate to the nature of the
problem at hand, be supported by critical research on
decreasing the risks of excess Nr, and reflect an integrated
policy that recognizes the complexities and tradeoffs
associated with the nitrogen cascade. Management efforts
at one point in the cascade may be more efficient and cost
effective than control or intervention at another point.
This is why understanding the nature and dynamics of the
N cascade is critically important.
Overarching Recommendation 2
The framing of the reactive nitrogen cascade provides
a means for tracking nitrogen as it changes form and
passes through multiple ecosystems and media. This
complexity requires the use of innovative management
systems and regulatory structures to address the
environmental and human health implications of the
massive amounts of damaging forms ofNr. It is difficult
to create fully effective regulations de novofor such a
complex system so we recommend utilizing adaptive
management to continuously improve the effectiveness
and lower the cost of implementation policies. This in
turn will require a monitoring system that will provide
feedback on the effectiveness of specific actions taken to
lower fluxes and concentrations ofNr.
Overarching Recommendation 3
EPA should form an intra-Agency Nr management
task force that will build on existing Nr research and
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management capabilities within the Agency. This
intra-Agency task force should be aimed at increasing
scientific understanding of: (1) Nr impacts on terrestrial
and aquatic ecosystems, human health, and climate, (2)
Nr-relevant monitoring requirements, and (3) the most
efficient and cost-effective means by which to decrease
various adverse impacts ofNr loads as they cascade
through the environment.
Overarching Recommendation 4
Successful Nr management will require changes in
the way EPA interacts with other agencies. To coordinate
federal programs that address Nr concerns and help
ensure clear responsibilities for monitoring, modeling,
researching and managing Nr in the environment, the
Committee recommends that EPA convene an Inter-
agency Nr management task force. It is recommended
that the members of this inter-agency task force include at
least the following federal agencies: U.S. Department of
Agriculture, U.S. Department of Energy, U.S. Department
of Housing and Urban Development, U.S. Department
of Transportation, National Oceanic and Atmospheric
Administration, U.S. Geological Survey, U.S. Forest
Service, and Federal Emergency Management Agency.
This task force should coordinate federal programs that
address Nr concerns and help ensure clear responsibilities
for monitoring, modeling, researching, and managing Nr
in the environment. The EPA Office of International and
Tribal Affairs should work closely with the Department of
State to ensure that EPA is aware of international efforts
to control Nr and is developing national strategies that
are compatible with international initiatives.
Similar recommendations for coordination and joint
action among and between agencies at both state and
federal levels have been made in the National Research
Council's recent reports on the Mississippi Basin
(NRC, 2008b, 2009). These intra and inter-agency Nr-
management task forces should take a systems approach
to research, monitoring, and evaluation to inform public
policy related to Nr management. The Committee
proposes that the intra and inter-agency task forces
coordinate the following activities to implement a systems
approach to Nr management.
Development of methods
Implementation of a systems approach will require
development of methods to facilitate various aspects of Nr
management. These include methods for: (1) establishing
and evaluating proposed Nr budgets; (2) using life cycle
accounting approaches for Nr management; (3) gathering
and using data on N fertilizer use and other Nr sources
and fluxes as the basis for informed policies, regulations
and incentive frameworks for addressing excess Nr loads;
(4) evaluating the critical loads approach to air and water
quality management; (5) identifying and using indicators
of excess Nr's economic damage and effects on human
health and the environment; and (6) using systems-based
approaches for controlling Nr releases to the environment.
Implementing best management practices
(BMPs)
It will be necessary to improve the scientific
understanding of BMPs that can be used for specific
applications to manage Nr. In particular, this includes
better scientific understanding of: (1) Nr requirements
in agriculture to ensure adequate food, feed, fiber, and
bioenergy feedstock supply while also avoiding negative
impacts on the environment and human health; (2) Nr
requirements for urban landscapes (e.g., residential
and commercial) and their maintenance while avoiding
negative impacts on the environment and human health;
(3) planning and pollution prevention, including low
impact development and natural ecosystem service
preservation; (4) use of natural land features and
attributes, such as wetland preservation and enhancement,
natural soil profiles, and buffer strips; and (5) improved
removal of Nr from sewage waste steams at both large-
scale wastewater treatment facilities and individual
subsurface (septic) systems. In addition, proactive
extension and technology transfer approaches will need to
be established to facilitate adoption of BMPs.
Developing appropriate tools and metrics for
assessing impact from adoption of best manage-
ment practices
Assessment activities will also be an important element
of the systems approach to managing Nr. These activities
should include: (1) quantifying the effectiveness and
impact of policies and regulations focused on reduction
of negative environmental impacts from Nr; (2) assessing
combined carbon (C) and Nr effects on terrestrial and
aquatic ecosystems; (3) assessing indicators/endpoints,
costs, benefits, and risks associated with impairment of
human health and decline and restoration of ecosystem
services; (4) reviewing existing and proposed legislation
for purposes of better integrating or designing regulatory
activities that recognize the nitrogen cascade or
streamlining procedures for enacting Nr risk reduction
strategies; and (5) evaluating economic incentives,
particularly those that integrate air, aquatic, and land
sources of excess Nr.
Education, outreach, and communication
It will be necessary to develop new education,
outreach, and communication initiatives. As discussed in
this report, this includes a range of targeted outreach and
education programs to manage Nr and achieve desired
environmental outcomes.
6.3. Near-term Management Goals
The Committee puts forward four goals for actions
that could be taken by EPA or other management agencies
to decrease Nr entering the environment from various
sources. We believe these goals can be attained over the
near term (approximately 10-20 years) using existing and
emerging technologies and practices. These suggestions, if
implemented, have the potential to reduce total Nr loadings
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to the environment in the U.S. by approximately 25%
below current levels. The Committee believes that these
represent realistic and attainable near-term goals, however
further reductions are undoubtedly needed for many
N-sensitive ecosystems and to ensure that health-related
standards are maintained. The Committee understands that
actual policy decisions on the implementation of programs
to limit Nr releases to the environment may differ from
those listed below for a variety of reasons, but believes that
an aggressive level of action, as represented by these goals,
is critical given the growing demand for food and fiber
production and energy use from population pressures and
economic growth. The rationale for these goals is set forth
below, along with recommended management options for
achieving the goals.
Management Goal 1. Controls on NOX emissions
from mobile and stationary sources
The Clean Air Act (1970) and its Amendments (1990)
have resulted in NOX emissions that are less than 50
percent of what they would have been without existing
controls. While this is an admirable accomplishment,
there is still a need to seek improvements. NOX emissions
are an order of magnitude greater than at the beginning
of the twentieth century. As a consequence, there
remain significant negative impacts on both humans
and ecosystems. In 2002, coal-fired utilities generated
approximately 1.3 Tg N annually (see Figure 3). If all
coal-fired plants used state-of-the-art NOX controls, this
number could be reduced by 0.6 Tg N/yr (calculations
performed by Cohen, 2008); in fact, 2008 emissions have
been reduced by 0.3 Tg N/yr from 2002 levels (see Figure
3), so in essence, half the reduction has already been
accomplished. The EPA should continue to reduce NOX
emissions from major point sources, including electric
generating stations and industrial sources, expanding
the use of market mechanisms such as cap and trade.
Under this scenario, it is likely that high-efficiency, low-
emission power plants will be built for energy needs.
Some controls on NOX emissions are implemented only
in the ozone season (May to September) (U.S. EPA
2009c). To protect welfare and avoid adverse effects
on ecosystems, NOX emissions controls should be
implemented year round.
For mobile sources, emissions for highway and
off-highway sources are approximately 2.2 Tg N/yr
and 1.2 Tg N/yr, respectively. EPA is in the process of
implementing a number of regulations that will reduce
NOX from mobile sources (see Appendix F). The
implications of these recent regulations are not reflected
in the quantitative analysis presented in this report. EPA
projects future year decreases in emissions (see Figure
5 in Section 2.2.1). However, better controls are needed
for on-road heavy duty diesel vehicles and off-road
vehicles, which include locomotives, construction, farm,
landscaping equipment, and marine vehicles. For these
off-road vehicles, 80-90% NOX removal is technically
achievable (deNevers, 1995; Koebel et al., 2004).
Assuming a 40% reduction for these sources, there is a
potential reduction of 1.4 Tg. The total reduction for both
mobile and stationary sources is then approximately 2
Tg N/yr. Part of achieving such levels of compliance will
require the implementation of inspection and maintenance
programs or road-side monitoring.
The Committee cautions, however, that achieving
such a goal may be inadequate for many areas to meet
the new 60 to 70 ppb ozone standard recommended
by the EPA Clean Air Scientific Advisory Committee
(CASAC) (U.S. EPA CASAC, 2008) or even the 75 ppb
standard currently promulgated. Additional measures
such as increasing the role of solar- and wind-generated
electricity, wider use of hybrid and electric cars, and
public transit conducive to energy conservation and
reduced emissions should be promoted.
Management Goal 1. The Committee estimates
that if EPA were to expand its NOX control efforts
for emissions of mobile sources and power plants
and include implementation of year round controls
on stationary sources to protect welfare and avoid
adverse effects on ecosystems, a 2.0 Tg N/yr decrease
in the generation of reactive nitrogen could be
achieved. It is believed that coal-fired utilities could
experience a reduction of 0.6 Tg N/yr. Since 2002,
emissions have already been reduced by at least 0.3
Tg N/yr; hence, this represents an additional 0.3 Tg
N/yr. Approximately 3.4 Tg N/yr can be attributed to
mobile sources (highway, off-highway). Assuming a
conservative 40% reduction, an additional 1.4 Tg N/yr
could be reduced.
Management Goal 2. Nr discharges and emis-
sions from agricultural lands and landscapes
Section 5.3.4 of this report reviews the various
methods that can be used to improve Nr management in
agricultural systems. The Committee estimates that crop
N-uptake efficiencies can be increased by up to 25% over
current levels through a combination of knowledge-based
practices and advances in fertilizer technology (such as
controlled release and inhibition of nitrification). Crop
output can be increased while reducing total Nr by up
to 20% of applied synthetic fertilizers, approximately
2.4 Tg N/yr below current levels of Nr additions to the
environment. These are appropriate management goals
with today's available technologies. Further progress is
possible through expanded research programs.
The Committee is concerned about current policies
and practices governing biofuel development. Acreage
devoted to corn production has increased substantially
for corn based ethanol production during the past several
years (with nearly one-third of the crop currently devoted
to bioethanol production), with fertilizer nitrogen use on
corn increasing by at least 10% (an additional 0.5 Tg N/
yr), largely to meet biofuel feedstock crop demand. In the
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absence of Nr controls and a failure to implement best
practices, current biofuels policies will make it extremely
difficult to reduce Nr transfers to soils, water and air
(Simpson et al., 2008). Integrated management strategies
will be required.
The Committee also notes with concern the increase
of N2O in the atmosphere. The Committee believes that
GHG emissions trading will provide both opportunities
and challenges for mitigating Nr environmental and health
impacts. Policies and regulations should consider how to
reward reductions of N-related GHG. Biofuel subsidies that
accurately account for Nr contributions to GHG emissions,
certification of individual biofuel plants for GHG impact,
and rewards for farmers who reduce N2O emissions
are examples of how an integrated strategy can reduce
agricultural GHG impacts. For additional production of
liquid biofuels beyond the grandfathered amount in the
Energy Independence and Security Act (EISA), EPA has
the power to exercise some controls on N2O emissions
through the life cycle GHG accounting requirements. It is
the opinion of the Committee that Section 204 of the EISA
calling on the Agency to adopt a life cycle approach to the
assessment of future renewable fuel standards is a positive
step toward a comprehensive analysis.
Section 5.3.4 of this report reviews methods of
controlling Nr from landscape runoff through the use of
natural or restored wetlands, urban areas, and through
the use of best management practices. The Committee
finds that flows of Nr into streams, rivers, and coastal
systems can be reduced by approximately 20% (~1 Tg N/
yr) through improved methods of landscape management
and without undue disruption to human commercial and
aesthetic activities.
Management Goal 2. The Committee estimates that
crop N-uptake efficiencies can be increased by up to
25% over current practices through a combination of
knowledge-based practices and advances in fertilizer
technology amounting to -2.4 Tg N/yr below current
amounts ofNr additions to the environment. The
Committee further estimates that excess flows of
Nr into streams, rivers, and coastal systems can be
decreased by approximately 20% (~1 Tg N/yr) through
improved landscape management and without undue
disruption to agricultural production.
Management Goal 3. Ammonia emissions from
livestock management and manure handling
In spite of gains made over the last several decades in
decreasing the amount of NOX emitted from stationary
and mobile combustion sources, the total amount of Nr
released into the atmosphere has remained relatively
constant. This is related to the essentially unregulated
release of ammonia from livestock operations.
As discussed in Section 2.2.3, at the present time,
fewer livestock are required to produce more animal
products than in the past. For example, since 1975 milk
production has increased linearly at the rate of ~ 180
kg milk per cow /yr while milk cow herd population
decreased at the rate of-69,000 head peryr (i.e., the
60% greater amount of milk produced in 2006 compared
to 1970 required 25% fewer cows). Animal inventories
declined by 10% for beef brood cows from 36 million
head in 1970 to 33 million head in 2006, and the
inventory of breeder pigs and market hogs declined
8% from 673 million head to 625 million head in the
same period, even with similar or greater annual meat
production. These trends resulted from greater growth
rates of animals producing more meat in a shorter
amount of time. In 1970, broilers were slaughtered after
80 days on feed at 1.7 kg live weight, but by 2006 the
average weight was 2.5 kg after only 44 days on feed.
These trends in requiring fewer animals to produce
more animal food products through improved diet and
increased production efficiency will continue.
Implementation of improved methods of livestock
management and manure handling and treatment to
decrease NH3 emissions that have been developed
since 1990 will further decrease ammonia and other
gases and odor emissions. For example, sawdust litter
helps decrease NH3 emissions from pig manure with
44-74% of manure N converted to N2. Storage covers
for slurry storage tanks, anaerobic lagoons, and earthen
slurry pits decrease emissions from those containments.
Anaerobic digestion in closed containment has been
studied for many types of applications. Recent research
demonstrates reduction in NH3 emissions after a
permeable cover was installed (e.g., a polyethylene cover
decreased NH3 emissions by -80%). A well managed
swine lagoon can denitrify approximately 50% of the
excreted N to N2. Recently engineered developments
utilizing closed loop systems (Aneja et al., 2008a)
substantially reduce atmospheric emissions of ammonia
(> 95%) and odor at hog facilities. Based upon recently
demonstrated reduction of NH3 emissions from swine
and poultry production, a moderate reduction of 50%
from 1990 NH3 emission estimates for swine and poultry
production should be attainable (Table 17). Because
of the larger land area involved in dairy and beef
production and the lack of effort that has been exerted in
mitigating NH3 emissions, a more modest and reachable
goal of decreasing NH3 emissions by 10% through
improvements in animal diet and manure management is
proposed (Table 17).
Management Goal 3. The Committee estimates that
livestock-derived NH^ emissions can be decreased by
30% (a decrease of 0.5 Tg N/yr) by a combination of
BMPs and engineered solutions. This is expected to
decrease PM2.s by -0.3 jj.g/m3 (2.5%), and improve
health of ecosystems by achieving progress towards
critical load recommendations. Additionally we
estimate that NH^ emissions derived from fertilizer
applications can be decreased by 20% (a decrease
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Table 17: Estimates for potential decreases in NH3 emissions from livestock manure in the
United States
NH3 Source
Dairy
Beef
Poultry
Swine
Goat/sheep
Horse
Total
23.1
27.1
27.5
17.5
1.6
2.9
100
^^^
0.37
0.44
0.44
0.28
0.03
0.05
1.61
Estimated D
10
10
50
50
10
10
ecrease of NHa
Tg N/yr
0.040
0.040
0.220
0.140
0.003
0.005
0.45
Estimate is based on livestock emissions of 1.6 Tg from Table 1.
of-0.2 Tg N/yr), through BMPs that focus on
improvements related to application rate, timing,
and placement.
Management Goal 4. Discharge of Nr from
developed lands and point sources
National loadings of Nr to the environment from
public and private wastewater point sources are relatively
modest in comparison with global Nr releases; however,
they can be important local sources with associated
impacts, especially in highly-populated watersheds. The
Committee has estimated that sewage containing Nr from
human waste contributes 1.3 Tg N/yr to the terrestrial
inputs of nitrogen (Table 1).
The Committee has also estimated that turf fertilizer
usage contributes 1.1 Tg N/yr to terrestrial inputs, a load
that could potentially be cut by about one third (Section
2.2.4). The Committee did not provide estimates for
general stormwater and nonpoint source runoff nitrogen
load reductions specific to developed or urban areas
- runoff concentrations and loads are highly variable
reflecting geographic and climatic conditions throughout
the U.S. and equally variable removal efficiencies from
standard treatment BMPs. This is shown in a summary
of the International Stormwater Best Management
Practices Database (Geosyntec Consultants, Wright
Water Engineers, Inc., 2008). However, most BMPs are
effective because they provide the beneficial biochemical
conditions of wetlands, the biophysical controls described
in Section 5.3.4 and Appendix C. These benefits, and the
application of BMPs, are recommended in overarching
recommendation 4, as well as in the preceding
Management Goal 2 as applied to agricultural lands.
Similar stormwater and nonpoint source management
benefits specific to developed lands should be anticipated
with BMP application in those areas.
Denitrification processes as applied to human
waste at sewage treatment plants are well-studied and
growing in application. Performance of these engineered
solutions, collectively referenced as biological nitrogen
removal (BNR), can be more rigorously controlled
than for stormwater and nonpoint source BMPs. Recent
publications by the U.S EPA (2007f, 2008e,f) have
summarized the state of and the capability for nitrogen
removal, and have reported that technologies to achieve
effluent concentrations of 3 mg total nitrogen per liter
(TN/L) or less are readily available. However, plant
capacity and design, wastewater characteristics, and
climate conditions can all affect the ability of a facility
to remove nitrogen. EPA's review of 2003-2006 data
for 16 facilities that remove nitrogen to varying degrees
found a range of final effluent TN concentrations of 1.0
to 9.7 mg/L, with an average of 5.6 mg/L. In general,
very small facilities (<0.1 MOD) do not perform as
well, with a final TN concentration ranging from 6-12
mg/L. Treatment performance varied and exceeded 5 mg
TN/L at some of the facilities. Given these conditions,
and performance uncertainties, it seems reasonable to
conclude that removal efficiencies in the range of 40-
60% below standard effluent nitrogen loads could be
readily attained. Based on the human waste load of 1.3
Tg N/yr, this would yield a decrease in total nitrogen
load of between 0.5 and 0.8 Tg/yr. Using data provided
by Maryland Department of the Environment (2006) and
the Connecticut Department of Environmental Protection
(2007), two states that have promoted nitrogen removal
technologies as solutions to coastal eutrophication, EPA
(2007f) has constructed cost estimates of upgrading the
performance of sewage treatment plants ranging from
$990,000 to $1.74 million per MOD treated.
There are two funding sources of significance
authorized in the CWA that are used to fund projects
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relevant to the control of Nr. Section 319 establishes
state nonpoint source management programs to plan for
and implement management measures that abate sources
of nonpoint pollution from eight source categories,
including both urban and agricultural sources; however,
the CWA disallows use of 319 funds for NPDES
permit requirements, so urban areas with stormwater
permits do not qualify for Section 319 funding. Over
the years, section 319 has made available, through 60%
matching funds, over $1.6 billion in assistance. The
much larger source of funding comes under Title VI of
the CWA, which has provided over $24 billion (federal)
for the construction of treatment facilities for point
sources of wastewater over the past 20 years, although
only a fraction of this amount has been dedicated to
denitrification processes. Title VI "state revolving" loan
funds can be used for stormwater management, as well as
other water pollution management activities, but not all
states have chosen to use funds beyond traditional sewage
treatment plant infrastructure needs because of the large
backlog of demand for those purposes.
In 2009, under the American Reinvestment and
Recovery Act (ARRA), the CWA Clean Water State
Revolving Fund (CWSRF) received a $4 billion boost
for clean water infrastructure and the CWSRF for fiscal
year 2010 was tripled over the prior years to two billion
dollars. These additional funds not only provide for jobs
creation, as intended by Congress, but provide states with
resources to reduce the backlog of clean water projects,
which also often include nutrient management needs.
The ARRA funds also emphasized the use of CWSRF
dollars for stormwater and nonpoint source management
and energy savings under a "green infrastructure"
requirement. A 20% set-aside for green infrastructure
was a requirement of AARA CWSRF funding and was
used widely for projects that included reductions in GHG
emissions, land-based low impact development BMPs
to reduce runoff and improve runoff quality, and other
innovative practices to treat wastewater and runoff. A
green infrastructure requirement is being continued in the
fiscal year 2010 CWSRF allocation.
Management Goal 4. The Committee recommends
that a high priority be assigned to increasing funding
for nutrient management. We estimate that a decrease
in Nr emissions from human sewage of between 0.5
and 0.8 TgN/yr can be achieved, with additional
decreases likely with increased stormwater and
nonpoint source BMP application support.
6.4. Summary of Specific Findings and
Recommendations Corresponding to the
Four Study Objectives
In this report the Committee has provided specific
findings and recommendations to assist EPA in its
understanding and management of nitrogen-related air,
water, and soil pollution issues. The specific findings and
recommendations corresponding to each of the four study
objectives are summarized below.
Study Objective #1: Identify and analyze, from a
scientific perspective, the problems Nr presents
in the environment and the links among them.
In general, the Committee finds that uncertainty
associated with rapid expansion of biofuels, losses of Nr
from grasslands, forests, and urban areas, and the rate and
extent of denitrification have created the need to measure,
model, and report all forms of Nr consistently and
accurately. Addressing this need will decrease uncertainty
in the understanding of the fate of Nr that is introduced
into the environment and lead to a better understanding
of the impacts of excess Nr on the health of people and
ecosystems. This should be accomplished through a
coordinated effort among cognizant federal and state
agencies and universities.
The Committee recommends that EPA routinely
and consistently account for the presence of Nr in the
environment in forms appropriate to the medium in which
they occur (air, land, and water) and that accounting
documents be produced and published periodically.
Specific Findings:
Rapid expansion of corn-ethanol production has the
potential to increase N fertilizer use through expanding
corn production and its associated N fertilizer inputs.
Development of cellulosic ethanol industry will require
cultivation for cellulosic crops, which will also require
N fertilizer. Distillers grains are changing animal
diets and affecting N recycling in livestock. Both
have important consequences for the effective future
management of Nr. (Finding #4 - also pertains to study
objectives 2 and 4)
Although total N budgets within all terrestrial systems
are highly uncertain, Nr losses from grasslands and
forests (vegetated) and urban (populated) portions
of the N cascade appear to be higher, on a percent of
input basis, than from agricultural lands. The relative
amount of these losses ascribed to leaching, runoff,
and denitrification are as uncertain as the N budgets
themselves. (Finding #9)
Denitrification of Nr in terrestrial and aquatic systems
is one of the most uncertain parts of the nitrogen cycle.
Denitrification is generally considered to be a dominant
N loss pathway in both terrestrial and aquatic systems,
but it is poorly quantified. (Finding #10 - also pertains
to study objective 4)
The Committee finds that there is a need to measure,
compute, and report the total amount of Nr present in
impacted systems in appropriate units. Because what is
measured influences what we are able to perceive and
respond to, in the case of Nr, it is especially critical to
measure total amounts and different chemical forms at
regular intervals over time. (Finding #13 - also pertains
to study objective 4)
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Specific Recommendations:
The Committee recommends that EPA routinely and
consistently account for the presence of Nr in the
environment in forms appropriate to the medium
in which they occur (air, land, and water) and that
accounting documents be produced and published
periodically (for example, in a fashion similar to
National Atmospheric Deposition Program summary
reports). The Committee understands that such an
undertaking will require substantial resources, and
encourages the Agency to develop and strengthen
partnerships with appropriate federal and state agencies
and private sector organizations with parallel interests
in advancing the necessary underlying science of Nr
creation, transport and transformation, impacts, and
management. (Recommendation #13 - also pertains to
study objective 4)
EPA should work with USDA and universities
to improve understanding and prediction of how
expansion of biofuel production, as mandated by the
2007 EISA, will affect Nr inputs and outputs from
agriculture and livestock systems. Rapid expansion
of biofuel production has the potential to increase N
fertilizer use through expansion of corn production area
and associated N fertilizer inputs, and from extending
cultivation of cellulosic materials that will also need
N inputs. Current models and understanding are not
adequate to guide policy on how to minimize impact of
biofuel expansion on environmental concerns related to
Nr. (Recommendation #4)
EPA should join with USDA, DOE, and universities
in efforts to ensure that the N budgets of terrestrial
systems are properly quantified and that the magnitudes
of at least the major loss vectors are known.
(Recommendation #9 - also pertains to study objectives
2 and 4)
EPA, USDA, DOE, and universities should work
together to ensure that denitrification in soils and
aquatic systems is properly quantified, by funding
appropriate research. (Recommendation #10 - also
pertains to study objective 4)
Study Objective #2: Evaluate the contribution an
integrated N management strategy could make
to environmental protection.
In general, the Committee finds that effective
management of Nr in the environment must recognize
the existence of tradeoffs across a number of impact
categories involving the cycling of nitrogen and other
elements. In addition, an integrated multi-media approach
to monitoring Nr is needed.
The Committee recommends that EPA:
1. Develop a uniform assessment and management
framework that considers the effects of Nr loading
over a range of scales reflecting ecosystem, watershed,
and regional levels. The framework should include
all inputs related to atmospheric and riverine delivery
of Nr to estuaries, their comprehensive effects on
marine eutrophication dynamics and their potential for
management
2. Examine the full range of traditional and ecosystem
response categories, including economic and
ecosystem services, as a basis for expressing Nr
impacts in the environment, and for building better
understanding and support for integrated management
efforts.
Specific Findings:
There has been a growing recognition of eutrophication
as a serious problem in aquatic systems (NRC, 2000).
The last comprehensive National Coastal Condition
Report was published in 2004 (EPA, 2004) and
included an overall rating of "fair" for estuaries,
including the Great Lakes, based on evaluation of
over 2000 sites. The water quality index, which
incorporates nutrient effects primarily as chlorophyll-a
and dissolved oxygen impacts, was also rated "fair"
nationally. Forty percent of the sites were rated "good"
for overall water quality, while 11% were "poor" and
49% "fair." (Finding #11)
The Committee finds that reliance on only one
approach for categorizing the measurement of Nr is
unlikely to result in the desired outcome of translating
N-induced degradation into the level of understanding
needed to develop support for implementing effective
Nr management strategies. (Finding #14)
Effective management of Nr in the environment must
recognize the existence of tradeoffs across impact
categories involving the cycling of other elements,
particularly carbon and phosphorus. (Finding #18)
The Committee has determined that an integrated
approach to monitoring that includes multiple media
(air, land, and water) components and considers a
suite of environmental and human concerns related
to reactive nitrogen in the environment (e.g., Nr
effects, climate change, human health) is needed.
Some of the phenomena that presented in this
report need more definition and verification. More
importantly, however, as controls are brought to bear
on Nr, improvements need to be measured to verify
and validate successful management strategies. If
the desired improvements are not realized as shown
by the collected data, corrective measures will be
required. Such an adaptive approach acknowledges
the likelihood that management programs will be
altered as scientific and management understanding
improve. (Finding #20 - also pertains to study
objective 3)
Specific Recommendations:
The Committee recommends that EPA develop a
uniform assessment and management framework that
considers the effects of Nr loading over a range of
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scales reflecting ecosystem, watershed, and regional
levels. The framework should include all inputs related
to atmospheric and riverine delivery of Nr to estuaries,
their comprehensive effects on marine eutrophication
dynamics, and their potential for management.
(Recommendation #11)
It is recommended that the EPA examine the full range
of traditional and ecosystem response categories,
including economic and ecosystem services, as a basis
for expressing Nr impacts in the environment, and for
building better understanding and support for integrated
management efforts. (Recommendation #14)
The Committee recommends that the integrated
strategies for Nr management outlined in this report be
developed in cognizance of the tradeoffs associated with
reactive nitrogen in the environment, consistent with the
systems approach of overarching recommendations 2
and 3. Specific actions should include:
• Establishing a framework for the integrated
management of carbon and reactive nitrogen
• Implementing a research program that addresses the
impacts of tradeoffs associated with management
strategies for carbon, reactive nitrogen, and other
contaminants of concern
• Implementing a research and monitoring program
aimed at developing an understanding of the
combined impacts of different nitrogen management
strategies on the interchange of reactive nitrogen
across environmental media. (Recommendation #18)
In cooperation with the Department of Agriculture,
U.S. Army Corps of Engineers, U.S. Fish and
Wildlife Service, and the and the Federal Emergency
Management Agency, the EPA should develop
programs to encourage wetland restoration and
creation, with strategic placement of these wetlands
where reactive nitrogen is highest in ditches, streams,
and rivers. The Agency should also address the
means of financing, governance, monitoring, and
verification. Such programs might be modeled on the
Conservation Reserve Program or extant water quality
and environmental trading programs, but need not be
limited to current practices. (Recommendation #15e -
also pertains to study objective 3)
The Committee recommends that EPA initiate
discussions and take action to develop a national,
multi-media monitoring program that monitors sources,
transport and transition, effects using indicators where
possible, and sinks of Nr in keeping with the nitrogen
cascade concept. This comprehensive program should
build upon existing EPA and state initiatives, as well as
monitoring networks already underway in other federal
agencies such as the U.S. Geological Survey programs
and the NADP effort. (Recommendation #20 - also
pertains to study objective 3)
Study Objective #3: Identify additional risk
management options for EPA's consideration.
In general, the Committee finds that a number of risk
management actions should be considered to reduce Nr
loading and transfer to the environment. The Committee
recommends risk management actions that include farm-
level improvements in manure management, actions to
reduce atmospheric emissions of Nr, and interventions to
control Nr in water management programs.
Specific Findings:
Farm-level improvements in manure management can
substantially reduce Nr load and transfer. There are
currently very few incentives or regulations to decrease
these transfer and loads, despite the existence of
management options to mitigate. (Finding #6)
Scientific uncertainty about the origins, transport,
chemistry, sinks, and export of Nr remains high, but
evidence is strong that atmospheric deposition of Nr
to the earth's surface, as well as emissions from the
surface to the atmosphere, contribute substantially to
environmental and health problems. Nitrogen dioxide,
NC>2, is often a small component of NOy, the total
of oxidized nitrogen in the atmosphere. The current
NAAQS for NO2, as an indicator of the criteria pollutant
"oxides of nitrogen," is inadequate to protect health
and welfare. NOy should be considered seriously as a
supplement or replacement for the NO2 standard and in
monitoring. Atmospheric emissions and concentrations
of Nr from agricultural practices (primarily in the form
of NH3) have not been well monitored, but NH4+ ion
concentration and wet deposition (as determined by
NADP and NTN) appear to be increasing, suggesting
that NH3 emissions are increasing. Both wet and dry
deposition contribute substantially to NHX removal,
but only wet deposition is known with much scientific
certainty. Thus, consideration should be given to adding
these chemically reduced and organic forms of Nr to the
list of criteria pollutants. (Finding #8)
Meeting Nr management goals for estuaries, when a
balance should be struck between economic, societal,
and environmental needs, seems unlikely under current
federal law. Enforceable authorities over nonpoint
source, stormwater, air (in terms of critical loads),
and land use are not adequate to support necessary Nr
controls. Funding programs are presently inadequate
to meet existing pollution control needs. Furthermore,
new technologies and management approaches are
required to meet ambitious Nr control needs aimed at
restoring national water quality. (Finding #12 - also
pertains to study objective 4)
Intervention to control Nr under most water
management programs generally occurs in three ways:
• Prevention or source controls
• Physical, chemical or biological "dead ending" or
storage within landscape compartments where it
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is rendered less harmful (e.g., long-term storage
in soils or vegetation; denitrification, primarily in
wetlands; reuse)
• Treatment using engineered systems such as
wastewater treatment plants or BMPs for stormwater
and nonpoint source runoff
While most management programs focus on the
third (treatment) approach, there are opportunities for
combining the three that can be more effective and cost
less. (Finding #15)
The Committee finds that there have been persistent
increases in the amounts of Nr that have been emitted
into and retained within various ecosystems, affecting
their functioning. Unless this trend is reversed, it
will become increasingly difficult for many of these
ecosystems to provide the services upon which human
well-being depends. The Committee believes that there
is a need to regulate certain forms of Nr to address
specific problems related to excess Nr, and we believe
that the best approach for an overall management
strategy is the concept of defining acceptable total
Nr critical loads for a given environmental system.
(Finding #16 - also pertains to study objective 4)
Current EPA policy (40 CFR Part 51, Clean Air Fine
Particle Implementation Rule) discourages states from
controlling ammonia emissions as part of their plan
for reducing PM2.5 concentrations. In this rulemaking
(Federal Register 72(79): 20586-20667), EPA has
stated that "ammonia reductions may be effective
and appropriate for reducing PM2 5 concentrations in
selected locations, but in other locations such reductions
may lead to minimal reductions in PM2 5 concentrations
and increased atmospheric acidity." Ammonia is a
substantial component of PM2 5 in most polluted areas
of the U.S. at most times. While it is true that reducing
NH3 emissions might increase the acidity of aerosols
and precipitation, the net effect of NH3 on aquatic and
terrestrial ecosystems is to increase acidity. After being
deposited onto the earth's surface, NH4+ is, under most
circumstances, quickly nitrified, increasing the acidity
of soils and waters. The Committee is unaware of any
evidence that NH3 reduces the toxicity of atmospheric
aerosols or that high concentrations of NH3 occur
naturally over any substantial area of the U.S. It has
not yet been established which components of PM
have substantive effects on human health but the total
concentration of PM2 5 correlates with morbidity and
mortality, and NH3 contributes to PM2 5. The visibility,
degradation, and other adverse effects associated with
PM2 5 are related to aerosol surface area or mass where
NH4+ certainly plays a role. (Finding #17)
Specific Recommendations:
A policy, regulatory, and incentive framework is
needed and should be developed to improve manure
management to reduce Nr load and ammonia
transfer, taking into account phosphorus load issues.
(Recommendation #6)
EPA should re-examine the criteria pollutant "oxides of
nitrogen" and the indicator species, NO2, and consider
adding chemically reactive nitrogen as a criteria
pollutant and NHX and NOy as indicators to supplement
the NO2 National Ambient Air Quality Standards.
(Recommendation #8a)
The Committee recommends that monitoring of NHX
and NOy begin as soon as possible to supplement
the existing network of NO2 compliance monitors.
(Recommendation #8b)
The Committee recommends that EPA reevaluate
water quality management approaches, tools, and
authorities to ensure Nr management goals are
attainable, enforceable, and the most cost-effective
available. Monitoring and research programs should
be adapted as necessary to ensure they are responsive
to problem definition and resolution, particularly
in the development and enhancement of nitrogen
removal technologies and best management practices,
and continue to build our level of understanding
and increase our ability to meet management goals.
(Recommendation #12 - also pertains to study
objective 4)
To better address Nr runoff and discharges from the
peopled landscape, the Committee recommends that
EPA:
• Evaluate the suite of regulatory and non-regulatory
tools used to manage Nr in populated areas from
nonpoint sources, stormwater and domestic sewage,
and industrial wastewater treatment facilities,
including goal-setting through water quality standards
and criteria; and
• Determine the most effective regulatory and
voluntary mechanisms to apply to each source
type with special attention to the need to regulate
nonpoint source and related land use practices.
(Recommendation #15a)
• Review current regulatory practices for point sources,
including both wastewater treatment plants and
stormwater, to determine adequacy and capacity
towards meeting national Nr management goals; and
• Consider technology limitations, multiple pollutant
benefits, and funding mechanisms as well as potential
impacts on climate change from energy use and
greenhouse gas emissions, including nitrous oxide.
(Recommendation #15b)
• Set Nr management goals on a regional/local basis,
as appropriate, to ensure most effective use of limited
management dollars; and
• Fully consider "green" management practices such as
low-impact development and conservation measures
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that preserve or re-establish Nr removing features to
the landscape as part of an integrated management
strategy along with traditional engineered best
management practices. (Recommendation #15c)
The Committee recommends that the Agency work
toward adopting the critical-loads approach concept
in determining thresholds for effects of excess Nr on
terrestrial and aquatic ecosystems. In carrying out
this recommendation, the Committee recognizes that
in many cases it will be necessary for the Agency to
enter into new types of research, policy, and regulatory
agreements with other federal, state, and tribal
units based on cooperative, adaptive, and systemic
approaches that derive from a common understanding
of the nitrogen cascade. (Recommendation #16 - also
pertains to study objective 4)
The Committee recommends that the EPA presumption
that NH3 is not a PM2 5 precursor should be reversed
and states should be encouraged to address NH3 as a
harmful PM2 5 precursor. (Recommendation #17)
Study Objective #4: Make recommendations to
EPA concerning improvements in Nr research to
support risk reduction.
The Committee has recommended research in
key areas to support risk reduction. The Committee's
recommendations include research to advance the
understanding of: the quantity and fate of Nr applied
to major crops; how to accelerate crop yields while
increasing N fertilizer uptake efficiency; agricultural
emissions of forms of Nr; atmospheric deposition of
Nr; and the potential for amplification of Nr-related
climate impacts.
Specific Findings:
Crop agriculture receives 60% of U.S. annual new
Nr inputs from anthropogenic sources (9.8 Tg from
N fertilizer, 7.7 Tg from crop BNF versus 29 Tg of N
anthropogenically introduced into the U.S. environment
peryear) and accounts for 58% (7.6 Tg) of total U.S.
Nr losses from terrestrial systems to air and aquatic
ecosystems, yet current monitoring of fertilizer use
statistics by federal agencies is inadequate to accurately
track trends in quantities and fate of N applied to major
crops and the geospatial pattern by major watersheds.
(Finding #1)
Nr inputs to crop systems are critical to sustain crop
productivity and soil quality. Moreover, given limited
land and water resources, global population growth,
and rapid economic development in the world's most
populous countries, the challenge is to accelerate
increases in crop yields on existing farm land while
also achieving a substantial increase in N fertilizer
uptake efficiency. This process is called "ecological
intensification" because it recognizes the need to
meet future food, feed, fiber, and energy demand of
a growing human population while also protecting
environmental quality and ecosystem services for future
generations (Cassman, 1999). More diverse cropping
systems with decreased Nr fertilizer input may also
provide an option on a large scale if the decrease in
Nr losses per unit of crop production in these diverse
systems can be achieved without a decrease in total
food production, which would trigger indirect land
use change to replace the lost production and negate
the benefits. However, crop cultivars and agronomic
practices are changing rapidly, which changes N
requirements, but current efforts in research, extension,
and conservation programs on N management within
these rapidly evolving systems are not adequate to
meet the challenge of providing better information to
increase NFUE. (Finding #2)
Nitrous oxide emissions from the Nr inputs to
cropland from fertilizer, manure, and legume
fixation represent a large proportion of agriculture's
contribution to greenhouse gas emissions, and
the importance of this source of anthropogenic
greenhouse gas will likely increase unless NFUE
is markedly improved in crop production systems.
Despite its importance, there is considerable
uncertainty in the estimates of nitrous oxide emissions
from fertilizer, and research should focus on reducing
this uncertainty. (Finding #3)
There are no nationwide monitoring networks in the
U.S. to quantify agricultural emissions of greenhouse
gases, NO, N2O, reduced sulfur compounds, VOCs,
and NH3. In contrast, there is a large network in place
to assess the changes in the chemical climate of the
U.S. associated with fossil fuel energy production, i.e.,
the National Atmospheric Deposition Program/National
Trends Network (NADP/NTN) which has been
monitoring the wet deposition of sulfate (SO42"), NO3",
and NH4+ since 1978. (Finding #5)
Synthetic N fertilizer application to urban gardens
and lawns amounts to approximately 10% of the total
annual synthetic N fertilizer used in the U.S. Even
though this N represents a substantial portion of total
N fertilizer use, the efficiency with which it is used
receives relatively little attention. (Finding #7)
The biogeochemical cycle of Nr is linked to climate
in profound but nonlinear ways that are, at present,
difficult to predict. Nevertheless, the potential for
significant amplification of Nr-related impacts is
substantial, and should be examined in more complete
detail. (Finding #19)
Specific Recommendations:
The Committee recommends increasing the specificity
and regularity of data acquisition for fertilizer
application to major agricultural crops in terms of
timing and sufficiently small application scale (as
well as for urban residential and recreational turf) by
county (or watershed) to better inform decision-making
-------
about policies and mitigation options for reducing Nr
load in these systems, and to facilitate monitoring and
evaluation of impact from implemented policies and
mitigation efforts. (Recommendation #1)
To obtain information on Nr inputs and crop
productivity the Committee recommends that:
• Data on NFUE and N mass balance, based on
direct measurements from production-scale fields,
be generated for the major crops to identify which
cropping systems and regions are of greatest concern
with regard to mitigation of Nr load, and to better
focus research investments, policy development,
and prioritization of risk mitigation strategies.
(Recommendation #2a)
• Efforts at USD A and universities be promoted to:
(1) investigate means to increase the rate of gain in
crop yields on existing farm land while increasing
N fertilizer uptake efficiency, and (2) explore the
potential for more diverse cropping systems with
lower N fertilizer input requirements to the extent
that large-scale adoption of such systems would not
cause indirect land use change. (Recommendation
#2b)
• EPA work closely with the U.S. Department of
Agriculture (USD A), Department of Energy (DOE),
the National Science Foundation (NSF), and
universities to help identify research and education
priorities to support more efficient use and better
mitigation of Nr applied to agricultural systems.
(Recommendation #2c)
The Committee recommends that EPA ensure that the
uncertainty in estimates of nitrous oxide emissions
from crop agriculture be greatly reduced through the
conduct of EPA research and through coordination of
research efforts more generally with other agencies
such as USD A, DOE, NSF, and with research
conducted at universities. (Recommendation #3)
The status and trends of gases and paniculate matter
emitted from agricultural emissions (e.g., NOs" and
NH4+) should be monitored and assessed utilizing a
nationwide network of monitoring stations. EPA should
coordinate and inform its regulatory monitoring and
management of reactive nitrogen with the multiple
efforts of all agencies including those of the U.S.
Department of Agriculture and NSF-supported efforts
such as the National Ecological Observatory Network
(NEON) and the Long Term Ecological Research
Network (LTER). (Recommendation #5)
To ensure that urban fertilizer is used as efficiently as
possible, the Committee recommends that EPA work
with other agencies such as USDA as well as state and
local extension organizations to coordinate research
and promote awareness of the issue. (Recommendation
#7a)
Through outreach and education, supported by
research, improved turf management practices should
be promoted, including improved fertilizer application
and formulation technologies and maintenance
techniques that minimize supplemental Nr needs and
losses, use of alternative turf varieties that require less
fertilization, alternative ground covers in place of turf,
and use of naturalistic landscaping that focuses on
native species. (Recommendation #7b)
EPA should pursue the longer term goal of monitoring
individual components of Nr, such as NO2 (with
specificity), NO, PAN, and HNOs, and other inorganic
and reduced forms, as well as support the development
of new measurement and monitoring methods.
(Recommendation #8c)
The scope and spatial coverage of the Nr concentration
and flux monitoring networks (such as the National
Atmospheric Deposition Program and the Clean Air
Status and Trends Network) should be increased, and
an oversight review panel should be appointed for these
two networks. (Recommendation # 8d)
EPA in, coordination with other federal agencies,
should pursue research goals including:
• Measurements of deposition directly both at the
CASTNET sites and in nearby locations with non-
uniform surfaces such as forest edges
• Improved measurements and models of convective
venting of the planetary boundary layer (the lowest
layer of the atmosphere) and of long range transport
• Improved analytical techniques and observations
of atmospheric organic N compounds in vapor,
paniculate, and aqueous phases
• Increased quality and spatial coverage of
measurements of the NH3 flux to the atmosphere
from major sources especially agricultural practices
• Improved measurement techniques for, and numerical
models of, NOy and NHX species (especially
with regard to chemical transformations, surface
deposition and off-shore export and linked ocean-land-
atmosphere models of Nr). (Recommendation #8e)
Research should be conducted on: best management
practices that are effective in controlling Nr, especially
for nonpoint and stormwater sources (including
land and landscape feature preservation); setting
Nr management targets that realistically reflect
these management and preservation capacities; and
constructing a decision framework to assess and
determine implementation actions consistent with
management goals. (Recommendation 15d)
The EPA should support cross-disciplinary and
multiagency research on the interactions of
climate and Nr. To determine the interactions of
global biogeochemical Nr cycles and climate, the
-------
Committee suggests that EPA follow a series of steps
such as:
1. Select several likely scenarios for global climate from
the IPCC report for the year 2050
2. Down-scale statistics or nest regional climate models
within each of these global scenarios to generate
meteorological and chemical fields (e.g., temperature,
relative humidity, winds, precipitation, CC^) for a few
years around 2050
3. Run several independent biogeochemical Nr models
(earth system models that include air/water/land) for
North America for these years with current Nr and
emissions and application rates
4. Rerun models with decreased Nr emissions/application
to evaluate strategies for controlling impacts such as
those described in this report. (Recommendation #19)
6.5 Conclusions and Observations
Nitrogen is not only an essential resource for humans,
for example its use in food production, but also a
byproduct of essential processes such as combustion.
The increasing amounts of nitrogen that humans capture
through the Haber-Bosch process for useful purposes
and the increasing amounts of nitrogen released from
combustion also add increasing amounts of excess
reactive nitrogen (Nr) to the environment. In the United
States, the production of crops receives 60% of the
new Nr inputs from anthropogenic sources, but also
accounts for almost the same proportion of total Nr losses
from terrestrial systems to air and aquatic ecosystems.
Nitrogen fertilizer use efficiency by crops seldom reaches
50% and may be 33% or lower. Fossil fuel combustion is
another major contributor of excess Nr in the production
of essential energy. However, the excess Nr that flows
from these activities is not benign. There are serious
negative impacts from excess Nr on both human health
and the environment. Table 1 gives a range of examples
of these.
The nature of the problem
Dealing with excessive reactive nitrogen is an
extraordinarily complex issue. Part of this relates to the
nature of nitrogen and its ability to change its form and
flow through different media, as evidenced conceptually
by the nitrogen cascade. Nitrogen's transformative nature
only increases the difficulty of dealing with its negative
aspects. Further, unlike the linear problems that society
has been more accustomed to dealing with in the past,
excessive Nr in the environment is in a class of problems
sometimes characterized as "wicked" (Batie 2008,
Kreuter 2004). For a "wicked" problem;
• There is not universal agreement on what the problem
is - different stakeholders define it differently
• There is no defined end solution, the end will be
assessed as "better" or worse"
• The problem changes over time
• There is no clear stopping rule - stakeholders,
political forces and resource availability will make
that determination on the basis of "judgments"
• The problem is associated with high uncertainty of
both components and outcomes
• Values and societal goals are not necessarily shared
by those defining the problem or those attempting to
make the problem better
This is not to say that society does not try to deal with
such problems (healthcare, environmental degradation,
water resource management, food safety, etc.). What it
does say is that different approaches are needed to deal
with such problems as compared with better defined
problems that are amenable to disciplinary linear science
where experts can define the problem and its endpoint.
The Integrated Nitrogen Committee has provided a
number of findings and recommendations with respect to
the problems caused by excess Nr. Some of these respond
to specific defined science based concerns. Others relate
to broader concerns the Committee raised in dealing with
the overall Nr dilemma. The following points synthesize
some of the important lessons learned from the more
than four years of study on this issue by the Integrated
Nitrogen Committee.
Recognizing the problem and building
consensus
It is critically important that the problems caused
by integrated nitrogen be widely recognized. Many
recognize that we add nitrogen to the environment for
specific useful purposes. Fewer recognize that there are
both direct and indirect impacts from this that damage
both the environment and human health. Until there is
general recognition that there is a problem and we need
to deal with the negative externalities of excess reactive
nitrogen in our environment, there will be no willingness
to tackle this issue. Education, communication and
outreach are critically important to engender in the
public sufficient will to tackle this widespread problem.
Education, communication and outreach will be critical
to the formation of a common definition of the problem,
an essential step once it is recognized. Following that,
there will have to be some degree of public consensus
for actions that will effectively reduce excess reactive
nitrogen in the environment. These steps will not be
possible unless there is a process of public consensus
building with respect to Nr. The first essential step in
trying to deal with a "wicked" problem is getting some
measure of agreement across different participants and
stakeholders about the problem itself.
The importance of the nitrogen cascade
Understanding the problem will require recognition
of the nature of Nr. In this case, the Integrated Nitrogen
Committee found the concept of the nitrogen cascade
-------
(pictured in the Executive Summary and Chapter 1)
to be an essential guide to approaching the problem.
This conceptual framework traces the flows of new
nitrogen through atmospheric, terrestrial, and aquatic
environmental systems where Nr is received, stored
and passed on in one form or another. The Committee
initially spent much of its effort on understanding
these flows and then determining as best as possible
the magnitudes of the flows and sinks through these
systems. This is the critical baseline information
needed to understand the problem, and many of the
recommendations in this report relate to gaining a better
understanding of this phenomenon. Unless it is well
understand how Nr flows through these systems, what
the Nr sinks are in these system components, and what
the magnitudes are of both the sinks and flows, any
attempts to better control Nr may well be ineffective.
Understanding and being able to quantify the nitrogen
cascade will allow the identification of the major excess
Nr contributors, an understanding of the different forms
of Nr at various points along the cascade, and enable
the determination of more effective interdiction points.
Without this basic understanding of the flows of Nr, their
nature, and their magnitude, attempts to deal with excess
Nr will have high uncertainty. Chapter 2 summarizes
much of this critical information that the Committee was
able to determine and assemble from other sources.
Integrated approaches are essential
Given what we know about the way Nr behaves,
efforts to deal with excess Nr must be organized in a way
that reflects the nature of the problem. Unfortunately,
many of our approaches are narrowly disciplinary
focused, and our policy and regulatory institutions are
often focused on one or another media where excess Nr
may temporarily reside or on a sector that contributes
excess Nr. The regulatory structure that has evolved for
problems affecting human health and the environment
has been specifically narrow, following policymakers
focus on such things as clean water or clean air. These
silos have to be broken down if excessive Nr is to be
dealt with effectively. Current efforts by EPA and other
agencies to encourage more integration across these
silos, and include other institutions and stakeholders,
are absolutely essential. Research efforts to better
understand excess Nr and better mitigate its negative
impacts must be trans-disciplinary. What will also be
critical is greatly increased collaboration and cooperation
among and between agencies and interested stakeholders.
Several of the Committee's recommendations address
this issue, calling for formal mechanisms to encourage
this to take place. The Nr problem has boundaries much
broader than the boundaries of the institutions that we
have relied on in the past to protect human health and
the environment. We are not going to be effective if we
do not both expand these boundaries and adopt a broad
multi-institutional reach.
Essential monitoring and research
In the effort to understand and quantify the nitrogen
cascade the Committee became aware of areas of needed
research and monitoring. Much of this is essential for
improving our knowledge of what goes on in the cascade.
It is also essential for benchmarking current Nr flows and
sinks and for targeting where actions are best taken to
reduce excess Nr. There is not sufficient information and
understanding of these flows and sinks to allow maximum
benefit from utilizing the full power of the cascade
concept. The Committee has recommended improved
monitoring and research to enhance our understanding
in air, land and water environments. In some cases
our knowledge has such wide margins of error that we
cannot identify or quantify important concentrations or
flows sufficiently for necessary decisions. In other cases
we need much better understanding of the efficacy of
actions that might be taken to control Nr. In some cases
we need to know more about the indirect impacts of Nr
as well as the indirect impacts of measures to control
Nr. Monitoring is both an essential part of the research
needs as well as being a critical guide to what we face
and whether our efforts to better control Nr are being
effective. Such environmental monitoring is often not
considered to be critically important. It is critically
important for effective approaches to reducing excess Nr.
Where to begin
The approach taken in this report and its
recommendations should result in helping enable the
control of excess Nr to proceed on the basis of starting
where it is most technically and economically effective.
Some of this may be low hanging fruit, or particular
niches where institutions and stakeholders already have
a common purpose. Part of the value of the nitrogen
cascade is that its characterization of Nr sinks and
flows allows the comparison of alternative points of
entry and interdiction. Further monitoring and research
should also allow the comparison of different modes of
interdiction. There will be choices to be made between
preventing increased Nr at the source, stabilizing it, or
treating the medium involved to remove it. Additional
research and actual experience in such efforts, coupled
with monitoring, will be essential for making these
choices. One product of the cascade exercise is a better
understanding of what and where the major contributors
of Nr are. The big sources of Nr have to be addressed
if there is to be meaningful impact on reducing excess
Nr. Part of the decision about where to begin will relate
to the efficacy of measures to reduce excess Nr, and our
knowledge is by no means complete on this. Beyond
the technical considerations of effectiveness, there will
have to be policy decisions about the trade-offs and
interdependencies between approaches such as market
mechanisms, regulation, incentives, and voluntary actions.
Market mechanisms generally require regulations that
are the basis for the creation of the market situation that
-------
makes such things as emission trading viable. Voluntary
actions are more likely if there is some prospect of future
required action.
Metrics matter
What is measured is critical to determining the
dimension of the problem, what to tackle, and whether
progress is being made. Kilograms removed (or prevented
at the source), the percentage of Nr removed from a
particular cascade or sink, or dollars of damage avoided
by the removal of Nr are all reasonable measures to use.
Measuring the improvement in physical units of Nr can
lead to very different source targets, control measures and
estimations of success as compared to measuring dollars
of damage. This relates to the dilemma presented by
"wicked" problems in both the original problem definition
and the determination of whether actions have made
things better or worse. An example from the Chesapeake
Bay is cited in the report that illustrates this. If the goal is
less Nr in the bay waters, then all sources are important.
If one is concerned about reducing the economic damage
of Nr and uses dollars of damage, then atmospheric
deposition becomes the primary focus because of the
high value of health damages from atmospheric pollution
related to Nr.
Setting goals for action
Finally, nothing is going to be accomplished if goals
are not set and efforts do not get underway. There are
sufficient findings and recommendations presented by
the Committee in addition to what we already know to
enable multiple agencies to begin to reduce the excess
Nr entering the environment. Some of the trade-offs
between alternative approaches are also well known.
The Committee suggests actions that might be taken by
EPA or other management authorities to reduce nitrogen
in the environment and a 25% reduction of excess Nr is
suggested as attainable with current technology over the
near term. Actions being suggested need testing, refining
and require monitoring. A start, even on a pilot scale, in
one portion of the cascade or sector will yield valuable
information about the efficacy of the approaches used,
further demonstrate the necessity for a multi-agency joint
stakeholder approach, and help further define the problem
and where it can initially be best addressed. This necessity
of getting underway is one of the main recommendations
of the National Research Council's series of reports on
improving water quality in the Mississippi (National
Research Council, 2008b and 2009). These reports
also emphasize the institutional arrangements that are
necessary. These are also echoed in this report. Following
on the National Research Council's reports, the USDA
Natural Resources Conservation Service has recently
begun an effort that targets nutrient reductions in the
Upper Mississippi. It is time to get more efforts underway
with effective collaboration between public and private
institutions and stakeholders.
Because this report addresses the needs of EPA's
research mission, there is substantial emphasis on gaps
in knowledge about Nr and the research that needs to
be done to fill these gaps. However, the report contains
an extensive knowledge base about Nr which was a
necessary precursor to addressing the objectives of the
report. The report then identifies the problems posed
by Nr, assesses the necessity of an integrated strategy
to deal with these problems, identifies some risk
management options for EPA's consideration, and makes
recommendations for improved research and monitoring
to support risk reduction. This information is sufficient
to allow initial determinations of where and how the Nr
problem can be addressed effectively with positive results.
As efforts progress, more will be learned and improved
methods, targeting, and analysis can be applied to this
truly wicked problem.
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Appendix A:
Nitrogen Deposition from the Atmosphere to
the Earth's Surface
Review ofNr wet deposition
Substantial progress has been made in monitoring Nr
wet deposition as is summarized in information provided
by the National Atmospheric Deposition Program/
National Trends Network (NADP) established in 1979.
This network monitors precipitation composition at over
250 sites in the U.S. and its territories (http://nadp.sws.
uiuc.edu). Precipitation at each station is collected weekly
according to well established and uniform procedures and
sent to the Central Analytical Laboratory for analysis of
acidity, NO3', NH4+, chloride, as well as the base cations
calcium, magnesium, potassium and sodium. For greater
temporal resolution, the Atmospheric Integrated Research
Monitoring Network AIRMON, composed of seven
sites, was formed in 1992 as part of the NADP program
to study wet deposition composition and trends using
samples collected daily. The same species are measured
as in NADP. By interpolating among sites, NADP is able
to estimate the wet deposition of NH4+ (reduced N), and
NO3~ (oxidized N) for the 48 contiguous states (Table A-l
and Figure A-l).
Table A-1: Annual wet deposition of reduced
(NH4+), oxidized (NO3-), and total N to the 48
contiguous states
NADP/NTN deposition estimates
reduced N in oxidized N in total wet N
precipitation, precipitation, deposition,
kg/ha/yr kg/ha/yr kg/ha/yr
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
1.49
1.63
1.66
1.49
1.72
1.46
1.48
1.50
1.59
1.72
1.70
1.65
1.65
1.68
1.67
1.80
1.74
1.78
1.58
1.62
1.57
1.55
1.55
1.52
1.41
1.40
3.17
3.30
3.45
3.24
3.49
3.04
3.10
3.07
3.14
3.27
3.22
3.06
3.05
g 55
a 54
I 53
I 52
§ 51
50
49
48
'£ 47
I45
oxidized N in precipitation, %
• reduced N in precipitation, %
Source: NADP/National Trends Network (NTN) http://
nadp.sws.uiuc.edu
Figure A-1: Percent contribution of oxidized
(NO3~) and reduced (NHf) nitrogen wet deposi-
tion from 1994 to 2006.
As emissions of NOX have decreased, the relative
importance of NHX has increased (data from National
Atmospheric Deposition Program, 2010).
Although individual regions vary, the NADP data for
the entire 48 states indicate an apparent decrease in NCV
wet deposition, but not in NH4+ deposition (Table A-l and
Figure A-2). Ammonium wet deposition shows a weak
increase, although the correlation coefficient is small. As
NOX controls have become more effective, the role of
reduced N appears to have grown in relative importance.
The nitrate data appear to show a statistically significant
trend and quantifying the response of deposition to a
change in emissions would be useful to both the scientific
and policy communities. A notable reduction in power
plant NOX emissions occurred as the result of the NOX
State Implementation Plan (SIP) call (McClenny et al.,
2002; Gilliland et al., 2008; Bloomer et al., 2009). EPA
should pursue a rigorous analysis of the emissions and
deposition data, including identifying monitors and
methods that are consistent from the beginning to the end
of the record, as indicated in Recommendation 8.
How is Nr deposition related to emissions?
The relationship between emissions of Nr and
observed deposition is critical for understanding the
efficacy of abatement strategies as well as for partitioning
local and large-scale effects of emissions. Only a few
studies covering several individual sites have sufficient
monitoring consistency and duration to determine
rigorously long-term trends in NCV and NH4+ and their
relationship to emissions, and here we consider several
-------
i.
1"
!M
SI
J
1
ft
•e
g
o
g
g
2.00
1.90
130
1.70
1.60
1.50
1.40
1.30
1.20
1.10
100
* * * *
*~ * * * *
y = 0.009 3x- 17.0
R! = 0.13
1993 1995 1997 1999 2001 2003 2005 2007
2.00
1.90
1.80
1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
* *
Figure A-2: Trend in reported wet deposition of
NH4+ and NO3~ for the 48 contiguous states
Note the sampling methods and locations have not been
tested for temporal or spatial bias (data from National
Atmospheric Deposition Program, 2010).
examples (Kelly et al, 2002; Butler et al., 2005; Likens et
al, 2005). These sites tend to be in the eastern U.S. where
monitoring is more concentrated and has a longer history
and where upwind sources and downwind receptors are
relatively well known. Examination of these studies
reveals that concentrations of gaseous and paniculate
N species in the atmosphere, as well as the Nr content
of precipitation over the eastern U.S., shows significant
decreases. Correlation with regional emissions is stronger
than with local emissions, in keeping with the secondary
nature of the major compounds - NCV and NH4+.
Decreases in NH4+ concentration and wet deposition are
attributed to decreases in SC>42~ concentrations, meaning
that more of the reduced Nr remains in the gas phase. For
the period 1965 to 2000, NCV levels in bulk deposition
correlate well with reported NOX emissions. For shorter
and earlier time periods the correlation is weaker, and the
authors attribute this to changes in the EPA's methods of
measuring and reporting emissions; they find evidence of
continued errors in emissions from vehicles. Decreases in
deposition will probably not be linearly proportional to
decreases in emissions; for example a 50% reduction in
NOX emissions is likely to produce a reduction of about
35% in concentration and deposition of nitrate.
The relationship between chemically reduced N
emissions and deposition is more complex. The maps of
ammonium deposition (Figure A-3) show that maxima
occur near or downwind of major agricultural centers
where emissions should be high. The full extent of the
deposition record (see http://nadp.sws.uiuc.edu) shows the
large intensification of NH4+ wet deposition in selected
areas. The southeastern U.S., particularly North Carolina,
has seen a long-term rise (Aneja et al., 2001; Aneja
et al., 2003; Stephen and Aneja, 2008). The increase
in deposition coincides with the increase in livestock
production, but a swine population moratorium appears to
have helped abate emissions (Stephen and Aneja, 2008).
Ammonium ion wet deposition, 2007
Nitrate ion wet deposition, 2007
Inorganic nitrogen wet deposition from nitrate
and ammonium, 2007
*" i*
i» „• • *- -
&*
• :• • o
-
U-4.D
U.I.D
tt-ta
ifl-ID
Figure A-3: Annual NH4+, NO3-, and total
inorganic N deposition for the year 2007 showing
spatial patterns of deposition
Source: National Atmospheric Deposition Program, 2010.
-------
Concentrations of aerosol NH4+ have decreased in many
parts of the country, and this may appear to contradict
the trend in wet deposition, but a decrease in condensed
phase NH4+ will be accompanied by an increase in
vapor phase NH3 if SO42~ and NO3" concentrations
decrease; see http://vista.cira.colostate.edu/improve/. This
potentially misleading information highlights the need for
measurements of speciated NHX (Sutton et al., 2003).
Review of dry deposition observations for the
eastern United States
Monitoring dry deposition presents a greater
challenge than monitoring wet deposition. The Clean
Air Standards and Trends Network (CASTNET) and
Atmospheric and Integrated Research Monitoring
Network (AIRMON) were established to monitor
chemical and meteorological variables to infer dry
deposition in order to study the processes leading from
emissions to atmospheric concentrations and through
deposition to ecosystem effects. AIRMON dry deposition
monitoring was discontinued in 2003. See www.epa.gov/
castnet/, www.arl.noaa.gov/research/programs/airmon.
html, and http://nadp.sws.uiuc.edu.
Recent reviews (Sickles and Shadwick, 2007a,b)
analyze the seasonal and regional behavior of
concentration and deposition of a variety of primary and
secondary pollutants including Nr and investigated trends
from 1990 to 2004 for the U.S. east of the Mississippi
River. The investigators evaluated observations from
more than 50 sites in the eastern states and concluded
that for 2000-2004, the mean annual total measured N
deposition for this area was 7.75 kg N per hectare per
Table A-2: Deposition of N to the eastern United
States in units of kg N/ha/yr
Dry NH4+
Wet NH4+
Dry HNO3 + NO3-
Wet NO3-
Total measured N Dep.
Est. dry other NOy
Est. dry NH3
Est. total NO,,
Est. total NH3 + NH4+
Est. Grand Total
ial deposition
IM/h;
0.41
2.54
1.E
2.92
7.75
0.94
1.90
5.74
4.85
10.59
Data are from the U.S. CASTNET program for the period
of 2000-2004. Monitored species for 34 sites east of the
Mississippi include vapor-phase HMOs, participate NOs',
and NH4+; unmonitored are other oxidized species such
as NOX and PAN and gas-phase reduced N species
most notably NH3 (Sickles and Shadwick, 2007a). For
an explanation of how deposition of unmeasured species
was estimated see text.
year (expressed as kg N/ha/yr); see Table A-2. This value
includes vapor phase HNO3, paniculate NO3", and NH4+;
it does not include deposition of other oxidized species
such as NOX and PAN, nor gas-phase reduced N species,
most notably NH3. The measured deposition rates peak
in spring and summer, but unaccounted for ammonia
deposition is probably a substantial fraction of the total,
and the true annual cycle remains uncertain.
Estimated total N deposition to the eastern
United States
CASTNET monitors HNO3 and NCV, but not other
members of the NOy family - notably NOX. Dennis
(U.S. EPA, 2007d) estimated that the unmeasured NOy
species account for about 50% of the dry deposition of
nitrates. Half of 1.88 (see Table A-2) is 0.94 kg N /ha/yr.
Ammonia is also unmeasured by CASTNET, and model
estimates (Mathur and Dennis, 2003) of NH3 indicate
that dry deposition should account for 75% of wet NH4+
deposition; 75% of 2.54 is 1.9 kg N /ha/yr. Adding these
two values to the total from Table A-2 yields a reasonable
estimate, within about ±50% absolute accuracy, of total
deposition of about 10.6 kg N /ha/yr for the eastern U.S.
Characteristics of N deposition to the eastern
United States
Analysis of production of N2 and N2O via gas phase
reaction is provided in Appendix E. Warmer temperatures
are conducive to release of NH3 from soils and manure
as well as from atmospheric particles, thus ammonia
concentrations are typically highest in summer. Diffusion
of gases is faster than diffusion of particles, and dry
deposition of vapor-phase Nr is faster as well; for
example the mean CASTNET reported HNO3 deposition
velocity is 1.24 cm/s while that for paniculate NO3~ is
0.10 cm/s. In 2003 and 2004 substantial reductions in
emissions from electricity-generating units (power plants)
were implemented under the NOX State Implementation
Plan (SIP) call. Many of these power plants are
located along the Ohio River generally upwind of the
measurement area. Significant reductions (p = 0.05) were
found between the 1990-1994 and 2000-2004 periods
(Sickles and Shadwick, 2007a).
Uncertainty in measured deposition
Analysis of uncertainties in the deposition of Nr is
challenging. The coefficient of variation for total regional
N deposition for 2000-2004 is 23%, representing a
minimal value of uncertainty. Concentrations of some of
the NOy species are monitored, as is the wet deposition of
major oxidized and reduced N species, but concentrations
of ammonia and other Nr species are not monitored. The
network for monitoring dry deposition is sparse and has
not been evaluated for spatial bias. The monitors are
located in flat areas with uniform surfaces - advective
deposition into, for example, the edges of forests are
estimated to contribute substantially to the uncertainty
(Hicks, 2006). Other sources of error include the model
-------
used to convert weekly average concentrations and
micrometeorological measurements into depositions.
Precision can be determined from collocated sites and
is estimated at 5% for nitrate and 15% for ammonium
in precipitation (Nilles et al, 1994). The uncertainty in
estimated dry deposition arises primarily from uncertainty
in deposition velocities (Hicks et al., 1991; Brook et
al., 1997) and can be as high as 40% for HNO3. Total
uncertainty for deposition of Nr based on measurements is
at least 25% and may be as high as 50%.
Deposition estimates from numerical models
The EPA Community Multiscale Air Quality model
(CMAQ) was run for North America at 36 km resolution
(R. Dennis et al., personal communication, January 2008).
Simulation of Nr deposition is hampered by the lack of
emissions information (especially for NH3), by the need
to parameterize planetary boundary layer (PEL) dynamics
and deep convection, as well as by simplified multiphase
chemistry. This run of CMAQ did not account for NOX
emissions from marine vessels, and these amount to
about 4% of the total NOX emissions in 2000. Calculated
nitrogen deposition for the 48 contiguous states (Table
A-3) was broadly consistent with direct measurements
(Table A-2). CMAQ NOX emissions were 5.84 Tg N for
the year 2002; of that, 2.74 Tg N were deposited. This
suggests that -50% was exported - a number somewhat
higher than has been reported in the literature; this
discrepancy is discussed below.
Table A-3: Results from CMAQ* for total
deposition in 2002 to the 48 contiguous states of
oxidized and reduced N
kg N/ha/yr Tg N/yr
Oxidized N
Reduced N
Total N Deposition
3.51
2.66
6.17
2.74
2.07
4.81
The CMAQ results were adapted from Schwede et
al., (2009) The Watershed Deposition Tool: Atool for
incorporating atmospheric deposition in water-quality
analyses, htttp://www.epa.gov/amad/EcoExposure/
depositionMapping.html.
Ammonia emissions and ambient concentrations can
be measured, but are not routinely monitored. For Nr, the
CMAQ numerical simulation employed inverse modeling
techniques - that is, NH3 emissions were derived from
observed NH4+ wet deposition (Gilliland et al., 2003;
Mathur and Dennis, 2003; Gilliland et al., 2006). Model
determinations of NH3 therefore do not provide an
independent source of information on NH4+ deposition.
The three-year CMAQ run gives an indication of the
spatial pattern of deposition (Figures A-4). For NHX, wet
and dry are equally important, but for NOy, dry deposition
accounts for about two-thirds of the total deposition,
while wet deposition accounts for about one-third. While
this does not hold for the eastern U.S. it is true for the
U.S. as a whole; in arid southern California, for example,
dry deposition of Nr dominates. Based on CMAQ, total
NOy deposition is 2.79 times the wet deposition and total
NHX deposition is 1.98 times the wet deposition. Using
the data from Table A-l for the average wet deposition for
the period 2000- 2004, total deposition of oxidized N is
4.36 kg N /ha/yr (2.79 x 1.56 = 4.36). The total deposition
for reduced N is 3.17 kg N /ha /yr (1.98 x 1.60). The
grand total (wet and dry oxidized and reduced) is then
about 7.5 kg N /ha /yr.
The model has highly simplified organic N deposition.
Note that these values reflect emissions before the NOX
SIP-call, which resulted in substantial reductions in NOX
emissions from point sources over the eastern U.S.
For comparison purposes, a collection of chemical
transport models (CTMs) (Dentener et al., 2006) yielded
total (wet plus dry) deposition to the whole U.S. of about
3.9 Tg N /yr oxidized Nr and 3.0 Tg N /yr ammoniacal
N for current emissions. The fate of NOX is assumed to
be primarily HNO3 or aerosol NO3~; organic N species
are generally not modeled in detail. Because this analysis
includes Alaska, a better estimate for NOX for the 48
contiguous states is 4.6 Tg N /yr. The variance among
models was about 30% (one standard deviation) for
deposition fluxes in regions dominated by anthropogenic
emissions. Globally, the calculations from the ensemble
of 23 CTMs estimated that 36-51% of all NOy and NHX
emissions are deposited over the ocean. This load could be
important to estuarine N loading estimates, as offshore N is
carried inshore by currents or through advective processes.
Deposition estimates from mass balance
From estimated total emissions of Nr compounds
and observed or simulated export, a reasonable estimate
of rate of deposition can be obtained by mass balance
- deposition equals emissions minus export. Although
substantial uncertainty (about a factor of two) exists for
the emissions of NH3, NOX release is reasonably well
known. In general, advection in the boundary layer and
lofting through convection followed by export at higher
altitudes are the two main mechanisms that prevent
removal of NOy and NHX by deposition to the surface of
North America (Luke et al., 1992; Li et al., 2004).
Experimental observations have been conducted over
the eastern U.S. for more than two decades (Galloway
et al., 1984; Galloway and Whelpdale, 1987; Luke and
Dickerson, 1987; Galloway et al., 1988). Most recent
estimates (Dickerson et al., 1995; Li et al., 2004; Parrish
et al., 2004b; Hudman et al., 2007), agree that annually
7-15% of the emitted NOX is exported in the lower to
mid-troposphere.
CTMs derived small export values - on the order of
30% of the total NOX emitted into the lower atmosphere
-------
TOTAL NITROGEN DEPOSITION (KG-N/HA)
Hour: 00
Min 0.18 at (40,8). Max- 36.41 at (93,29)
Figure A-4: CMAQ annual average (wet plus dry and oxidized plus reduced) nitrogen deposition (in
kg-N/ha/yr) across the United States.
This is based on three years of differing meteorology - one dry, one wet, and one average precipitation year - across
the Eastern United States
Source: U.S. Environmental Protection Agency, 2007f
(Penner et al., 1991; Kasibhatla et al, 1993; Holland
et al., 1997; Horowitz et al., 1998; Liang et al., 1998;
Galloway et al., 2004; Park et al., 2004; Holland et al.,
2005; Doney et al., 2007). Reviewed publications using
the mass balance approach have substantial uncertainty
but indicate with some consistency that 25-35% of the
NOy emitted over the U.S. is exported.
Comparison of models and measurements of
oxidized N deposition
Both ambient measurements and numerical models
of NOy have reached a level of development to allow
reasonable estimates of deposition. For reduced
nitrogen, neither ambient concentrations nor emissions
are known well enough to constrain models. Recent
model estimates of the U.S. N budget are reasonably
uniform in finding that about 25-35% of total NOX
emissions are exported.
Results from CMAQ runs described above indicate
that, of the NOX emitted over the continental U.S., 50%
is deposited and 50% is exported. This is within the
combined error bars of other studies, but well under the
best estimate of 70% deposition. One possible source
of this discrepancy is underestimation of deposition of
organo-nitrogen compounds. The chemical mechanism
used in CMAQ was highly simplified - only about 2-3%
of the total Nr deposition can be attributed to organo-
nitrogen compounds (R. Dennis personal communication,
2008). Ammonia from fossil fuel combustion, while
important locally, is probably a small component of
national Nr deposition.
Major sources of uncertainty in modeled and observed
values include missing deposition terms and poorly
constrained convective mass flux. As indicated above,
convective mass flux (rapid vertical transport) is uncertain
because most convective clouds are smaller than a grid
box in a global model. There is evidence for nonlinearities
in NO2 deposition velocities with greater transfer from the
atmosphere to the surface at higher concentrations (Horii
et al., 2004; 2006).
-------
Emissions from Canada and Mexico can have a emissions from Canada and Mexico are each 10-15% of
substantive impact on atmospheric Nr over the U.S. near those of the U.S. and the bulk of the Mexican population
major sources, such as downwind of industrial Ontario is distant from the U.S. We expect the overall impact of
and major cities of San Diego, CA, and Tijuana, Mexico neighboring countries to add about 10% uncertainty to the
(Wang et al., 2009). While Nr is imported into the U.S. estimated Nr budget for the 48 contiguous states.
from these border countries, there is also export. The
-------
Appendix B:
Sources and Cycling of Nr Input into
Terrestrial Systems in the United States
Most of the new Nr introduced into terrestrial
systems in the U.S. was used to produce food for human
consumption and forage and feed for livestock and poultry
(-17.7 Tg total with 9.7 Tg from synthetic fertilizer and
~8 Tg from biological N fixation; Table B-l). In addition
to new Nr and Nr that was recycled from livestock and
human excreta, crop production releases Nr that was
stored in soil organic matter (see Section 2.3.2). The N
in cereal crops is typically derived from added fertilizer
(synthetic or manures) and from mineralization of soil
organic matter (conversion of complex organic molecules
to ammonium) in about equal amounts. As discussed
in Section 2.2 and Section 5.3.4, crop production is not
efficient in using Nr so only 30-70% (a global average
of 40%) of all the N mobilized for crop production is
harvested in the crop. The remainder is in crop residue
(roots and above ground stover) stored in the soil, leached
to aquatic systems as NCy, volatilized to the atmosphere
as NH3 or NOX or denitrified (see Section 4.8, Figure 18)
to produce NOX, N2O and N2. An additional ~1.1 Tg of
synthetic fertilizer N is used to maintain turfgrass in the
urban environment (see Section 2.2.5) and another 0.1-0.2
Tg N is used to enhance forest production.
Within the nitrogen cascade (Figure 1), the interactions
between the agricultural and populated portions of the
terrestrial system dictate the production and flow of Nr.
Although occupying the largest area, forest and grassland
portions of terrestrial ecosystems serve mainly to absorb
atmospheric deposition and provide a source of forest
products and forage for livestock production. Reactive
nitrogen input into these systems is from biological N
fixation in unmanaged lands, atmospheric deposition, and
Nr from livestock manure that is deposited. The livestock
that is grazing within grasslands (Table B-l) may lead
to the N saturation of unmanaged forest and grassland
ecosystems (Galloway et al., 2004; Bobbink et al., 2010).
This report uses the Nr input numbers from Table B-l
and food production numbers to estimate the flow of Nr
through agricultural and populated parts of the terrestrial
system (Table B-2). The FAO (200lOb); www.fao.org/
es/ess/top/country.html) lists the 20 largest agricultural
commodities produced, imported, and exported in the
U.S. in 2002. Of these commodities, corn (229 Tg),
soybeans (75 Tg), wheat (44 Tg), and cow's milk (77 Tg)
were produced in the greatest amount. Using commodity
N content data derived from data used to calculate crop
Table B-1: Sources of reactive N into terrestrial systems in the United States in 2002 (from Table 1
data sources; in Tg N/yr).
Atmospheric
N fixation
Synthetic N
Animal manure##
Human sewage##
1.3
7.7
9.7
1.2
0.1
1.4
0.1
1.9
6.4
3.8#
0.4
1.1
1.2
6.9*
14.1
10.9
6.0#
1.3
The amount of atmospheric Nr deposition is based on area of each environmental system within the continental U.S.
The total area does not sum to 100% because non-arable lands are not included in this table.
"""Synthetic fertilizer N used for managed pasture fertilization is included in the agricultural land classification.
#Unrecoverable livestock manure deposited on grasslands, the unaccounted for ~1 Tg of Nr assumed to be lost
through ammonia volatilization, leaching, or denitrification (U.S. EPA, 2007e).
##Note that livestock manure and human sewage used as fertilizer are recycled N components of the nitrogen cascade
and not new Nr inputs.
-------
residue N in the EPA inventory of U.S. greenhouse gas
emissions and sinks (U.S. EPA, 2007e), an estimated 9
Tg of N was marketed in three crops, soybeans (4.4 Tg
N; from U.S. EPA, 2007g), corn (3.2 Tg N), and wheat
(0.9 Tg N). Whole milk contained ~ 0.5 Tg of N while
other meat and egg produce contained ~1.4 Tg of N,
totaling ~ 1.9 Tg N. Grain, fruits, nuts and vegetables
contained ~9.3 Tg of N. If the total N input use efficiency
is 40%, then ~23 Tg of N from all sources is required to
produce 9.3 Tg of vegetative commodities. Table B-2
lists the estimated Nr input into agricultural systems (~
20 Tg) and additional N input from crop residue that was
returned to the field the previous year (4.4 Tg) and from
mineralization of soil organic matter (4.7 Tg). All of this
N input totals ~29 Tg of N that is actually involved in
the production of the 9.3 Tg of crop commodity N. If one
assumes that return of crop residue to the field is directly
proportional to crop production, then 24.3 Tg of N was
required to produce the 9.3 Tg of crop commodity N.
These estimates indicate that -38% of the total annual
input of N that went into the agricultural crop production
system was contained in the main crop commodities
produced in the U.S. in 2002.
Of the 24.3 Tg N required to produce crop commodity
N in 2002, approximately 2.5 Tg was used to grow feed
used for milk, egg, and meat production. This estimate is
made assuming that 4 units of N are required to produce
a unit of milk, eggs, or meat (see Section 5.3.4). This
estimate also assumes that one-third of N required for
livestock production comes from commodities in the
FAO top-20 list and the remaining two-thirds comes
from alfalfa, silage, and grass over the course of a year
(Oitjen and Beckett, 1996). Approximately 4.3 Tg of N in
agricultural commodities (2.8 Tg in soybeans, corn, and
wheat) were exported, while ~0.15 Tg N was imported
in various food and drink commodities. The U.S. human
populace consumed —1.96 Tg of N in 2002 (292 million
people, consuming 114.7 g protein/person/day, 0.16 gN/g
protein, 365 days) (approximately 1.2 Tg from animal
protein-N and 0.7 from vegetative protein).
These three consumption areas - internal consumption
of vegetable N for livestock production, human
consumption, and export - account for 77% of the
commodities produced. The unaccounted for commodity
N is likely partly in annual storage. Some smaller fraction
of annual production is used for pet food and a small
fraction is returned to the terrestrial environment because
of spoilage and handling losses.
In forests and grasslands (vegetated system) N input
in 2002 was -3.5 Tg of anthropogenically introduced
N, with the remaining —10.1 Tg derived from BNF and
livestock manure deposition. Of this anthropogenic N,
-21% was retained in soil and tree biomass, while the
remainder was removed in tree harvest (-0.2 Tg, see
Section 2.3.2.) or lost to other parts of the environment
through NH3 volatilization and NO3 leaching and runoff
(Table B-2). Total N input into agricultural systems
was -20 Tg, with - 11 Tg being removed as products
including the transfer of-2 Tg N as food to the human
population. Almost 40% of the N input into agricultural
systems is lost through NH3 volatilization, nitrification/
denitrification, and NO3" runoff. The 4.2 Tg of Nr of
Haber-Bosch N that is used for industrial feedstock is
not included in this assessment. Of the input of-3.3 Tg
of N into the populated system -80% is lost through
human excreta processed in sewage treatment plants,
denitrification in soils, and leaching and runoff of NO3~
(Table B-2).
Table B-2 summarizes the input and flow of Nr in
the main terrestrial systems within the continental U.S.
Anthropogenic input of Nr into forests and grasslands
totaled -3.5 Tg in 2002, with an estimated 6.4 Tg of Nr
being introduced through natural biological N fixation.
Of this Nr - 0.7 Tg was stored in vegetation and soils
(see Section 2.3.2) and -2 Tg removed as livestock
forage, while the remainder was lost to the atmosphere
and aquatic systems, or removed as forest products and
livestock forage. The largest anthropogenic Nr input (-20
Tg) was into agricultural production where -11.2 Tg was
removed as agricultural product, - 2 Tg transferred as
Table B-2: Nr input and flows (Tg N/yr) in the terrestrial portion of the Nitrogen Cascade (Figure 1)
within the continental United States in 2002
Vegetated
13.6
0.7
Icultural & Transfers to
Aquatic or
. .xmospheric
2.2
10.7
79
Agricultural
19.6
0.8
11.2
7.6
39
Populated
3.3
0.1
0
3.2
97
The Environmental Systems are those noted in the terrestrial portion of the N cascade shown in Figure 1. Data from
Table B-1, derived from regrouping information from Table 1 data sources, are shown in Table B-2.
"""Estimates are from section 2.3.2 of this report.
-------
edible product to the "populated" portion of the terrestrial
system, -0.8 Tg was stored in agricultural lands, and -7.6
Tg N was lost to the atmosphere and aquatic systems.
New N input into the "populated" portion totaled ~3.3 Tg,
which came from N transfer in food and use of fertilizer
N in lawns, gardens and recreational areas. Within these
areas an estimated 0.12 Tg was stored in urban forests.
-------
-------
Appendix C:
Water Quality Trading in the Illinois
River Basin
For various reasons, wetland restoration has been
proposed and the magnitude of needed restoration
estimated. For the Wetland Reserve Program (WRP), the
Farm Bill of 1990 set a goal of restoring approximately
1 million acres. A few years later, the NRC (1992)
proposed a national goal of restoring 10 million acres of
inland and coastal wetlands by 2010. The NRC went on
to recommend that 400,000 miles of streams and rivers
be restored by 2012 and that 1 million acres of lakes
be restored by 2000, both of which would further the
control of reactive nitrogen. While none of these goals
has been or is likely to be met by the recommended
date, they articulated a need for wetland restoration
addressing the important relationship between wetlands
and water quality.
Taking into account the economics of using wetlands
to manage Nr adds yet another dimension to site
selection. Based on the results of the Water Environment
Research Foundation's study (Hey et al., 2005a,b), the
Kinship Foundation sponsored a study (Scott et al., in
preparation) to define the market for producing and
selling Nr (as NO3~) credits. For this analysis, a real
potential market area was selected: the Illinois River
watershed in Illinois - the tributaries draining Wisconsin,
Indiana, and Michigan were excluded. The producers
of nitrogen credits were identified as "nutrient farmers"
and they became the "sellers" of N credits. The "buyers"
of nitrogen credits were restricted to municipal and
industrial wastewater treatment facilities, those facilities
that hold an NPDES permit. This restriction, of course,
resulted in a considerable understatement of the market
size since the identified buyers emit less than 11% of the
total aquatic N load (David and Gentry, 2000) that finds
its way to the Mississippi River - air emission/deposition
and agriculture account for the remaining 89%.
The watershed was divided into 19 sub-watersheds,
spatially locating credit supply and demand. A linear
programming model was developed and used to: (1)
examine the potential extent and distribution of nitrogen
credit demand and supply; (2) compare the average
seasonal demand levels to the supply capacity of nutrient
farms; and (3) evaluate the relative effects of seasonality.
Market efficiency was imposed through the objective
function: the least costly distribution of credit production
to meet the given monthly demand. Thereby, sellers
and buyers were identified and linked and the spatial
characteristics of the market mapped by sub-watershed.
At the same time, the equilibrium price of a credit, or the
prevailing price at which buyers and sellers are willing
to trade, was determined. The market, as represented by
the model, determined where the most intensive wetland
investment (i.e., wetland restoration) would be, the
revenues returned to these investments, and the costs and
savings to the buyers.
All 290 permitted buyers are geographically
distributed as shown in Figure C-l. The mass loading
of the buyers (2,423 tons/month) is reflected in Figure
C-2. Eighty-nine percent of the demand comes from the
northeastern corner of the Basin (Upper Fox, Des Plaines,
and Chicago/Calumet sub-watersheds), the Chicago
metropolitan area. As illustrated by Figure C-3, 41% of
the wetland restoration area (using the criteria discussed
above) was identified in the southwestern corner of the
watershed (Lower Illinois, La Moine, Macoupin, Lower
Sangamon, and Middle Illinois sub-watersheds), where
the floodplain is almost entirely leveed. For the market
study, the available load of Nr (NCV) by season and
sub-watershed was mapped as illustrated in Figure C-4.
The N load was computed using water quality and flow
data collected by the U.S. Geological Survey from 1987
to 1997. The wetland and wastewater cost functions are
described in Hey et al., 2005; however, the wetland cost
functions were modified for the market study to reflect the
variability of land costs across the watershed (i.e., higher
land values in urban Chicago and lower land values in
rural Illinois). This variability is reflected in the spatial
distribution marginal costs shown for the spring marginal
costs depicted in Figure C-5. Wetland treatment costs vary
by time of year because the level of microbial activity,
which drives the denitrification process, varies with water
temperature. Therefore, treating an equivalent load of Nr
requires more wetland area in winter than in summer.
Three regulatory scenarios
Regulatory agencies may require that dischargers and
nutrient farms be located in proximity to each other and
could impose "penalties" when the two are not. Thus,
for the sake of analysis, the Committee created three
regulatory scenarios:
1. Unrestricted - buyers can purchase nitrogen credits
from nutrient farmers anywhere in the watershed
without regard to location. The result of this scenario is
given in Figure C-6.
2. Restricted intra-watershed - the buyer must purchase
all available credits within its own sub-watershed
before buying in other sub-watersheds
-------
3. Accrued 10% penalty - buyers pay an increasing
"tax" on credits purchased in consecutive downstream
watersheds
The three regulatory scenarios were analyzed for each
of the four seasons. (D. Hey, Wetlands Research, Inc.,
Personal Communication.)
The "unrestricted" scenario is the least expensive
because nutrient farms in this scenario are located
downstate, where land is least expensive. In the other two
scenarios, credits were purchased a little more evenly
throughout the watershed. Still, most of the credits in the
southern corner of the watershed were purchased. The
Industrial Dischargers
Major Municipal Dischargers
Minor Municipal Dischargers
River
County Boundary
Q Watershed Boundary
Figure C-1: Distribution of municipal discharge
and industrial dischargers in the Illinois River
Watershed
Municipal dischargers shown are those exceeding one
million gallons per day. Symbols may represent more
than one discharger at that location.
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
A/ River
Watershed
Acres Available
(thousands)
7-15
15-25
40-65
65-120
Figure C-3: Potential land availability in the
100-year flood zone for nutrient farming in each
sub-watershed in the Illinois River watershed
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
A/ River
Watershed
Tons of Nitrogen
1-5
6-20
21-35
• 36-100
B101 -350
351 -1,750
Figure C-2: Distribution of total nitrogen
emissions by sub-watershed
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
River
I—I Watershed
Tons of Nitrogen
0-50
51 -125
• 126-275
• 276 - 400
••401-750
• 751-1,500
Figure C-4: Spring available total nitrogen load
by sub-watershed
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
-------
iver
l~~l Watershed
Cost
dollars par ton
1,000-1,950
1,951 -2,100
2,101 -2,250
2,251 - 2,500
• 2,501 - 2,750
•• 2,751 - 3,000
Figure C-5: Spring marginal cost (price) by
watershed
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
"restricted intra-watershed" and "accrued 10% penalty"
scenarios resulted in more credits being purchased. This
resulted in the sale of N credits exceeding the mass
of Nr emitted by wastewater treatment, which would
benefit the overall control of reactive nitrogen. It also
would increase the value of the market and the profits
of the nutrient farmer. The downside of such regulatory
controls is that they would drive up the effective price of
nitrogen credits. If a buyer had to buy a 1.5 tons for every
ton discharged because credits are not available in the
tributary watershed, the effective price of a credit would
be 1.5 times the price of the tributary sub-watershed. If
prices rise too much, "concrete and steel" technologies
may become competitive.
Considering all of the point source dischargers
in the Illinois River watershed, between 29,000 and
36,000 tons TN/year could be removed through nutrient
farming under the studied trading schemes (Table C-l).
The range of removal is a function of the penalties
imposed on the market by the regulatory agencies.
V River
CH Watershed
Tons of Nitrogen
0
1 -200
201 -400
401 -1,000
• 1,001 -1,500
§•1501-2000
Figure C-6: Unrestricted spring credit sales
(tons/month) by sub-watershed
Source: D. Hey, Wetlands Research, Inc., Personal
Communication
Accordingly, the market revenue would range from
$70 million to $121 million/year. This is a sizeable
market that could generate substantial profits, from $6
million to $38 million with the return on investment
varying from 5 to 25%. If the savings are shared evenly
between the seller and buyer, the nutrient farmer could
earn between $200 and $300/acre/year net profit, which
in many cases is greater than the profits from corn
or soybean production. Further, these profits do not
include any earnings from flood control or recreation, as
suggested in a McKnight study report (Hey et al., 2004).
With such profits, sufficient land should be available for
nutrient farming.
This analysis indicates that appropriate lands
are available and that wetlands can be effectively
restored and efficiently used to control Nr. The market,
structured as discussed above, could generate the capital
to accomplish the needed large-scale wetland restoration
while saving taxpayers the cost of upgrading their
municipal wastewater treatment plant (TWI, 2007).
Table C-1: Nutrient farm market parameters under three trading scenarios
Parameter
Total Credits Sold (tons)
nrestnctei
29,078
Restricted
ra-watershe
29,078
:crued 10%
Penalty
35,781
Total Revenue 21
$69,925,497
$99,571,889
$121,457,652
Total Cost to Produce Credits
$63,258,006
$66,193,924
$83,288,747
Profit
$6,667,491
$33,377,968
$38,168,905
Source: D. Hey, Wetlands Research, Inc., Personal Communication
21 Assumes all credits were sold at the cheapest cost within the Illinois River watershed.
-------
-------
Appendix D:
Management of Nr Measures Based on the
Concept of Critical Loads
The European Union has undertaken broad measures,
based on the critical-loads concept, to manage Nr.
Tables D-l, D-2, and D-3 summarize several different
environmental impacts currently used as indicators
and identify whether there are current limit values
set by the United Nations Economic Commission for
Europe (UNECE) or European Union (EU). These
tables identify the main links to the cascade of reactive
nitrogen in the environment, the relevance and link to
Nr of the effect/pollutant, and existing agreements in
which the effect is currently addressed. In addition,
some impacts are more relevant than others in relation
to societal importance and the connection to the nitrogen
cascade. The categorization on a scale of 1 (highest
relevance) to 5 (unimportant) provides a first-level
prioritization for future mitigation activity. The last
column summarizes existing links to international
regulations and conventions.
Where there is a limit and the relevance for the
nitrogen cascade is high, then this might be the limiting
factor for Nr production and its associated losses to the
environment. Some limits might be more relevant in
specific areas and less relevant in others. For example
NO2 concentrations relevant for human health are
limited to 40 ppb in urban areas, limiting industry and
traffic, but would probably not be an issue of concern
in remote areas with low population densities. In
these areas, however, loss of biodiversity might limit
nitrogen deposition and therewith the sources in the
region. The only way to determine the extent that
critical thresholds are limiting is by overlaying them
on different regions and determining, through the use
of monitoring data or by modeling exercises, where
and which sources contribute to exceeding the critical
threshold. Then the best methods for putting caps on
relevant sources can be identified. A pre-classification
of regions might be useful, e.g., urban regions, remote
regions, marine areas, etc. One aspect of this global
view of nitrogen impacts and metrics that is evident is
the mix of "classical-" and "service"-based categories,
consistent with the need for an integrated approach to
the management of nitrogen.
-------
Table D-1: Summary of the effects of excess Nr on human health in relation to metrics, current
international regulations and conventions, and the link to the nitrogen cascade
Respiratory disease in people
caused by exposure to high
concentrations of:
Ozone
Other photochemical
oxidants
Fine particulate
aerosol
Direct toxicity of nitrite
NO2-
Nitrate contamination of
drinking water
Depletion of stratospheric ozone
Increase allergenic pollen
production, and several
parasitic and infectious human
Blooms of toxic algae and
decreased swimability of
in-shore water bodies
^^^^T-^TVrV^^I
Sum of ozone
over 35 ppb
Org. N03, PAN
concentration
(atm)
PM-io, PM2 5
concentration
(atm)
NO2-
concentration
N03-
concentration
(aq.)
NOX, N20
concentration/
flux (atm)
Chlorophyll a
NO3- (&P)
concentration
(aq)
YES
NO
YES
YES
YES
NO
NO
NO
Link to Nr
NOX
emissions
NOX
emissions
NOX, NH3em
NOX
N03-
leaching
NOX, N20
Runoff, Nr
deposition
3
5
1
2
2
3
5
1
Regulatory or
political
JBK
Convention on Long-
range Transboundary Air
Pollution
Clean Air for Europe
Convention on
Long-range
Transboundary Air
Pollution et al.
Convention on Long-
range Transboundary Air
Pollution
Clean Air for Europe
World Health
Organization
Convention on Long-
range Transboundary Air
Pollution
Clean Air for Europe
EU Essential
Facilities Doctrine
Montreal Protocol
None
Convention for the
Protection of the Marine
Environment of the
North-East Atlantic
Helsinki Commission
Barcelona Convention
atm - atmospheric; aq - aqueous
*Relevance and link to nitrogen incorporates societal priority and N contribution: 1. highest relevance, 2. high
relevance, 3. significant relevance, 4. some relevance, 5. unimportant.
-------
Table D-2: Summary of the effects of excess Nr on ecosystems related to currently used metrics, the
existence of European regulatory values, and the link to the nitrogen cascade.
Ozone damage to crops,
forests, and natural
ecosystems
Acidification effects on
terrestrial ecosystems, ground
waters, and aquatic
ecosystems
Eutrophication of
freshwaters, lakes
(incl. biodiversity)
Eutrophication of coastal
ecosystems inducing
hypoxia (incl. biodiversity)
Nitrogen saturation of soils
(incl. effects on GHG
balance)
Biodiversity impacts on
terrestrial ecosystems
(incl. pests and diseases)
Me,rics
AFstY(O3flux),
AOT40"
Critical loads
Biological
Oxygen De-
mand,
NO3~ cone (aq)
Critical loads
BOD, NO3- cone
(aq)
Critical loads
Critical loads
Critical loads,
critical levels
(NH3, NOX)
^g
YES
YES
YES
NO
BOD, N03-
conc (aq)
Critical load
YES
YES
Link to Nr
cascade
NOX
Nr deposition
Runoff, Nr
deposition
Runoff, Nr
deposition
Nr deposition
Nr deposition
2
2
3
1
1
1
Regulatory or
political
convention
Convention on Long-
range Transboundary
Air Pollution
Clean Air for Europe
Convention on Long-
range Transboundary
Air Pollution
Clean Air for Europe
WFD
Water Framework
Directive
Convention for the
Protection of the
Marine Environment of
the North-East Atlantic
Helsinki
Commission
Barcelona
Convention
Convention on Long-
range Transboundary
Air Pollution
Clean Air for Europe
Convention on Long-
range Transboundary
Air Pollution
Clean Air for Europe
Convention on
Biological Diversity
atm - atmospheric; aq - aqueous
*Relevance and link to nitrogen incorporates societal priority and N contribution: 1. highest relevance, 2. high
relevance, 3. significant relevance, 4. some relevance, 5. unimportant.
"""Accumulated ozone exposure over a threshold of 40 parts per billion
-------
Table D-3: Summary of the effects of excess N on other societal values in relation to metrics
and regulatory values in current international regulations and conventions and the link to the
nitrogen cascade
Metrics
\\atory or
>litical
ventio
Odor problems as-
sociated with animal
agriculture
Acidity in precipitation.,
03,
PM
YES
NOX, NH3
Convention on
Long-range
TransboundaryAir
Pollution
Effects on
monuments and
engineering
materials
PM2 5 cone (atm)
NO
NOX, NH3
Global climate
warming induced by
excess nitrogen
N2O, cone/flux (atm)
NO
NOX, NH3
United Nations
Framework
Convention on Climate
Change
Regional climate
cooling induced by
aerosol)
PM2.5 cone (atm)
NO
NOX, NH3
United Nations
Framework
Convention on Climate
Change
atm - atmospheric; aq - aqueous
*Relevance and link to nitrogen incorporates societal priority and N contribution: 1. highest relevance, 2. high
relevance, 3. significant relevance, 4. some relevance, 5. unimportant.
-------
Production of A/2 and A/2O via gas-phase
reactions
Atmospheric conversion of NOX and NHX to less
reactive N2 or N2O appears to play a minor role in the
global N budget, but currently is not well quantified. The
gas-phase reactions in the troposphere that convert NH3
and NOX to N2 and N2O, start with attack of NH3 by OH:
NH3 + OH-^NH2-+H2O (1)
Several potentially interesting fates await the NH2
radical:
NH2-+O3^NH,NHO,NO (2)
NH2 • + NO2 -> N2O + H2O (3)
NH2-+NO^N2 + H2O (4)
kO3 = 1.9xlO-13 cm3 s'1
kNO2=1.8xlO-11cm3s-1
kNO=1.8xlO-11cm3s-1
The first step, attack by OH, is slow. The rate constant
for the Reaction 1 is 1.6 x 10"13 cm3 s"1 and the lifetime
of NH3 for a typical concentration of 106 OH cm'3
is about 70 days. In most areas of the world where
concentrations of NH3 are high, concentrations of sulfates
are also high, and NH3 is removed by conversion to
condensed phase ammonium sulfate or bisulfate on time
scales much faster than 70 d. The mean lifetime of these
aerosols with respect to wet deposition is about 10 d.
There are some areas of the world, notably California
and South Asia, where NH3 and NOX are emitted in large
quantities, but SO2 is not, and there gas-phase conversion
can take place. NH3 is usually removed by wet or dry
deposition. Reaction 2 is relatively slow and oxidation of
ammonia is in general an unimportant source of NOX. But
Reactions 3 and 4 may be atmospherically noteworthy. As
an upper limit to current N2O production, we can assume
that each of these regions covers an area of 106 km2 and
that they contain ammonia at a concentration of 10 g N nr3
in a layer 1,000 m deep. The annual production of N2 and/
or N2O would then be on the order of 0.1 Tg N, a minor
but nontrivial contribution to denitrification and about 1%
of the anthropogenic N2O production. If NH3 rich air is
lofted out of the boundary layer into the upper troposphere
where deposition is impeded, it will have an atmospheric
residence time on the order of months, and the probability
of reaction to form N2O or N2 becomes greater. This
possibility has not been investigated extensively. It is also
possible than Europe and North America will continue to
reduce S emissions without reducing NH3 emissions and
the atmospheric source of N2O will grow in importance.
In the stratosphere, N2O photolysis leads to loss of
Nrvia
N2O
N2 + O (5)
While reaction with an electronically excited oxygen
atom O(1D) leads to production of NO via
N2O + O(!D) -> 2NO (6)
Photolysis (Reaction 5) dominates, but a large
enough fraction of the N2O reacts with O^D) that this
is the main source of NOX in the stratosphere. The fate
of this oxidized nitrogen (NOy) is transport back into
the troposphere where it is removed by wet deposition.
Downward transport of the odd N from the oxidation of
N2O is a minor (~1%) source of NOy in the troposphere.
Most of the N2O released into the atmosphere is
eventually converted to N2 - the problem is that it
destroys stratospheric ozone in the process.
In summary, our current understanding of the
chemistry of atmospheric ammonia suggests that in situ
conversion to N2 and N2O plays a minor (~1%) role
in global N budgets, but if assumptions about kinetics
or concentrations are in error, these mechanisms could
become important.
SPARROW model for estimating watershed Nr
Estimates of Nr transfers in aquatic ecosystems are
difficult to quantify at the national scale, given the need
to extrapolate information from sparse monitoring data
in specific watersheds to the geographic boundaries of
the nation. One excellent tool for estimating Nr loads at
regional scales is the spatially referenced regression on
watershed attributes (SPARROW) modeling technique.
The SPARROW model has been employed to quantify
nutrient delivery from point and diffuse sources to
streams, lakes, and watershed outlets at the national scale
(Smith etal., 1997). The model infrastructure operates
in a geographic framework, making use of spatial data
to describe sources of pollutants (e.g., atmospheric
deposition, croplands, fertilizers) and characteristics of
the landscape that affect pollutant transport (e.g., climate,
topography, vegetation, soils, geology, and water routing).
Though empirical in nature, the SPARROW modeling
approach uses mechanistic formulations (e.g., surface-
water flow paths, first-order loss functions), imposes mass
balance constraints, and provides a formal parameter
estimation structure to statistically estimate sources and
fate of nutrients in terrestrial and aquatic ecosystems. The
spatial referencing of stream monitoring stations, nutrient
sources, and the climatic and hydrogeologic properties
of watersheds to stream networks explicitly separates
-------
landscape and surface-water features in the model. This
allows nutrient supply and attenuation to be tracked
during water transport through streams and reservoirs,
and accounts for nonlinear interactions between nutrient
sources and watershed properties during transport. The
model structure and supporting equations are described
in detail elsewhere (Smith et al, 1997, Alexander et al.,
2000, Alexander et al., 2008). Figure E-l provides an
estimate of contemporary Nr loading in surface waters
of the U.S., representing long-term average hydrological
conditions (over the past three decades). There are hot
spots of high Nr yields to rivers associated with land use
and watershed characteristics, and SPARROW allows
considerations of the fate of these Nr inputs to streams
and rivers as they flow downstream to coastal receiving
waters (Alexander et al., 2008).
0-5
6-10
11-17
18-26
27-50
Figure E-1: Total Nr yields (kg/ha/yr) in large rivers of the U.S.
Data Source: Alexander et al., 2008
-------
Appendix F:
Recent Major EPA Mobile Source Rules to
Control NOX
EPA informed the Committee that it is in the process
of implementing a number of regulations to reduce NOX
from a variety of mobile sources22. These include clean
diesel regulations for trucks and buses and nonroad
engines, as well as locomotives and smaller marine
vessels. EPA first regulated NOX emissions from motor
vehicles for the 1973 model year and since then has
tightened these standards. EPA's efforts to control NOX
emissions from nonroad vehicles, locomotives, and
commercial marine vessels started in the 1990s. NOX
reductions for each rule were calculated by EPA based
on inventories available at the times of the rules.
1. Light Duty Tier 2 Rule - EPA's Tier 2 Vehicle and
Gasoline Sulfur Program (65 FR 6698, February 10,
2000). This program requires new cars, sport utility
vehicles (SUVs), pickup trucks, and vans to be 77
to 97% cleaner than 2003 models, while reducing
sulfur levels in gasoline by 90%. EPA estimates that
as newer, cleaner cars enter the national fleet, the new
tailpipe standards will reduce emissions of nitrogen
oxides from vehicles by 3 million tons, or about
74% in 2030. Prior to that, the EPA Tier 1 vehicle
regulations, effective with the 1995 model year, also
resulted in significant NOX reductions.
2. EPA's Clean Heavy Duty Truck and Bus Rule. When
the Agency finalized the Heavy Duty Truck and Bus
Diesel Rule (66 FR 5002, January 18, 2001) in 2001,
trucks and buses accounted for about one-third of
NOX emissions from mobile sources. In some urban
areas, the contribution was even greater. With model
year 2010, all new heavy duty trucks and buses will
result in NOX emission levels that are 95% below
the pre-rule levels. EPA projects a 2.6 million ton
reduction of NOX emissions in 2030 when the current
heavy-duty vehicle fleet is completely replaced with
newer heavy-duty vehicles that comply with these
emission standards.
3. Clean Air Nonroad Diesel - Tier 4 Rule (69 FR
38957, June 29, 2004). In 2004, EPA adopted a
comprehensive national program to reduce emissions
from future nonroad diesel engines by integrating
engine and fuel controls as a system to gain the
greatest emission reductions. EPA estimates that in
2030, this program will reduce annual emissions of
NOX by about 740,000 tons.
4. Marine-Related NOX Reductions from 1999 to 2003.
EPA completed three rulemakings with respect to the
diesel marine sector that will reduce NOX emissions.
These rules are now in effect and being phased-in.
In 1999 (64 FR 73299, December 29, 1999), EPA
promulgated NOX requirements for diesel engines
used in commercial boats (large inland and near-
shore boats) and commercial vessels (ocean-going
vessels). EPA estimates that these reduced emissions
from these vessels by about 30%. In 2002 (67 FR
68241, November 8, 2002), EPA promulgated rules
reducing NOX emissions from diesel engines used in
recreational marine vessels by 25%. In 2003 (68 FR
9746, February 28, 2003), EPA promulgated another
rule further reducing NOX from diesel engines used in
commercial vessels by about 20%. EPA projects that
on a nationwide basis, these four programs will reduce
marine-related NOX by more than 1 million tons in
2030.
5. Locomotive and Marine Diesel Rule (73 FR 25098,
May 6, 2008). In March 2008, EPA adopted standards
that will reduce NOX emissions from locomotives
and marine diesel engines. The near-term emission
standards for newly-built engines phased in starting
in 2009. The long-term standards begin to take effect
in 2015 for locomotives and in 2014 for marine diesel
engines. EPA estimates NOX emissions reductions
of 80% from engines meeting these standards. EPA
projects that in 2030, about 420,000 tons of NOX will
be reduced from the locomotive engines, and 375,000
tons of NOX will be reduced from commercial and
recreational marine engines.
6. Non-road Spark-Ignition Engines (73 FR 59034,
October 8, 2008). In 2002, EPA promulgated emissions
standards for large spark-ignition engines. These took
effect in 2004 for Tier 1 standards and in 2007 for Tier
2 standards. EPA promulgated emissions standards
for small spark-ignition engines in 2008. EPA projects
that, when fully implemented, the new standards will
result in a 35% reduction in HC+NOX emissions from
new engines' exhaust, reduce evaporative emissions by
45%, and that together these programs will reduce NOX
by more than 585,000 tons in 2030.
7. EPA's Coordinated Strategy for Control of Emissions
from Ocean-Going Vessels (www.epa.gov/otaq/
oceanvessels.htm). EPA's coordinated strategy to
22 The information in Appendix F was provided to the Integrated Nitrogen Committee by Mazrgaret Zawacki of the U.S. EPA Office of Transportation
and Air Quality.
-------
control emissions from ocean-going vessels consists
of actions at the national and international levels.
On December 22, 2009, EPA finalized emissions
standards for ocean-going vessels which take effect
in 2011. In addition to this rule the U.S. Government
has also amended MARPOL Annex VI to designate
U.S. coasts as an Emission Control Area (EGA) in
which all vessels, regardless of flag, will be required
to meet the most stringent engine and marine fuel
sulfur requirements in Annex VI. New engine emission
and fuel sulfur limits contained in the amendments to
Annex VI are also applicable to all vessels regardless
of flag and are implemented in the U.S. through the Act
to Prevent Pollution from Ships (APPS). EPA projects
that when fully implemented, the coordinated strategy
will reduce NOX emissions from ocean-going vessels
by 80% and that in 2030, the coordinated strategy
is expected to yield a reduction in NOX of about 1.2
million tons.
. EPA's Voluntary Clean Diesel Programs. EPA has
created a number of programs designed to reduce
emissions (including both PM and NOX) from the diesel
fleet. In conjunction with state and local governments,
public interest groups, and industry partners, EPA has
established a goal of reducing emissions from the over
11 million diesel engines in the existing fleet by 2014.
Looking at these engines, EPA determined there were
general sectors that provided the best opportunity to
obtain significant reductions and created programs for
Clean Agriculture, Clean Construction, Clean Ports,
Clean School Bus, and SmartWay Transport.
9. Section 177 of the Clean Air Act allows states outside
of California to adopt California emissions standards,
once EPA has granted such a waiver. As a result,
several northeastern states have adopted California
standards. Maryland adopted its California LEV II
NOX standards as part of its Low Emission Vehicle
Program (COMAR 26.11.34, effective December 17,
2007). These standards take effect with the 2011 model
year. Maryland submitted that program to EPA as a
SIP revision. Pennsylvania adopted California LEV II
NOX standards as part of its Clean Vehicles Program
(codified at Pa. Code Chapters 121 and 126, effective
December 9, 2006). Pennsylvania's program began
with model year 2008 vehicles. Pennsylvania submitted
this program as a SIP revision.
-------
Nitrogen contamination ofgroundwater
In addition to environmental concerns about
N-nutrient loading to freshwaters from a groundwater
pathway, there are also potential human health impacts
from elevated levels of N in groundwater, especially
from NO3. It has been long established that excess NO3
in drinking water supplies can cause blue baby syndrome
(methemoglobinemia) (Knobeloch and Proctor 2001;
Ward et al. 2005, 2006), the indicator of which is MetHb.
To protect public health from effects of NO3 in drinking
water, EPA has established a maximum contaminant level
goal (MCLG) that considers a lifetime exposure plus a
margin of safety. For NO3 in drinking water, the MCLG
is 10 mg/L. Nitrite-N also has an established MCLG of
1 mg/L, and the combined NO3 and NO2 MCLG set by
EPA is 10 mg/L. These same values are used to regulate
NO3 and NO2 as maximum contaminant levels (MCL),
which are the highest levels of contaminants allowed in
drinking water (40 CFR § 141.62).
The drinking water standard is commonly exceeded
in streams and rivers of the U.S., particularly in the
agricultural Midwestern U.S. For example, there were 13
episodes over a 25- year period of formal warnings by
authorities to local citizens in Columbus, Ohio, about not
drinking tap water because nitrate-nitrogen was higher
than 10 mg-N/L (Mitsch et al., 2008). These episodes
lasted from one to several weeks each. The pattern is
generally for high concentrations of nitrate-nitrogen
in Midwestern rivers from February through June or
July. In one pattern, averaged over 7 years with weekly
river sampling, nitrate-nitrogen in a central Ohio river
peaked with an average of 7 mg-N/L in June after which
concentrations decrease to 1-2 mg-N/L for the rest of the
summer and fall. The year-to-year variability was high
for that month as well (Mitsch et al. 2005) as these spring
"high-nitrate" floods do not occur every year. The nitrate
"pulses" generally are part of flood events after fertilizer
has been applied to fields in the watershed.
Public policy by water supply agencies is to treat
high concentrations of nitrate-nitrogen in drinking water
supplies as a real public health threat. Recent studies
have brought the concern of high nitrate-nitrogen in
drinking water into dispute (Ward et al., 2005, 2006).
While there is a definite link between excessive nitrate
in drinking water and methemoglobinemia, there is also
a need to better understand the interaction of the range
of environmental factors (e.g., cofactors such as diarrhea
and respiratory diseases reportedly increase MetHb
levels) that promote methemoglobinemia. This will
help identify the environmental conditions under which
exposure to nitrate in drinking water poses a risk of
methemoglobinemia.
According to the USGS (Barber, 2009) Summary of
Estimated Water Use in the U.S. in 2005, total water
withdrawals in the U.S., excluding thermoelectric power
usage, were 210 billion gallons per day, of which 44,200
million gallons per day (MOD) were for public water
supply. About two-thirds of that supply is provided by
surface water, the rest is from wells and about 58%
(25,600 MOD) of public water supply goes towards
domestic use, including drinking water. Private wells
(Figure G-l) that are not part of public water supply
systems are estimated to provide an additional 3,830
MOD, providing domestic water for 42.9 million people
(14% of the U.S. population in 2005).
Groundwater N in forested and low intensity (<10%)
agriculture or urban land use areas is estimated to be
fairly low, having 75th percentile concentrations of 0.5
and 1.1 mg/L in two USGS studies (Nolan and Hitt,
2002). They consequently concluded that a "reasonable"
background concentration, as NO3-N, would be 1.1 mg/L,
which would include effects in more sensitive aquifers
with nominal loading for urban or agricultural sources.
Nitrate can enter groundwater from a variety of
sources, including all of those described in this report,
but fertilizer and animal waste in rural, agricultural areas
are especially prominent sources (Nolan and Ruddy,
1996). Other sources include septic systems, more
important in densely-developed and unsewered urban
areas, and atmospheric deposition. Vulnerability to
elevated NO3 levels is also variable, but an assessment
and model by Nolan and Hitt (2006) predicted that"...
areas with high N application, high water input, well-
drained soils, fractured rocks or those with high effective
porosity, and lack of attenuation processes..." are
especially vulnerable (r2 = 0.801).
As illustrated in Figure G-2, surveys confirm that
NO3 in groundwater is elevated in many areas of the
U.S., well above the 1.1 mg/L background upper bound
described above (Figure G-2). The background colors
on the map in Figure G-2 are indicative of different types
of aquifers. In a 1992-1995 survey (Nolan and Stoner,
2000), shallow groundwater underlying agricultural areas
was found to be most severely impacted by elevated
NO3-N levels (median concentration of 3.4 mg/L).
Urban shallow aquifers were less impacted (median
concentration of 1.6 mg NO3-N/L) and deeper, major
-------
aquifers, in general, had a median NO3-N concentration
of 0.48 mg/L. However, NO3-N concentrations did
exceed the 10 mg/L MCL threshold set by EPA for
drinking water in more than 15% of the groundwater
samples in the survey from drinking water aquifers.
In the most recent survey of domestic well water
quality (DeSimone et al., 2009), USGS found
concentrations of NO3-N greater than 10 mg/L in 4.4%
of the wells sampled. Concentrations exceeding the
nitrate MCL were most frequently encountered in certain
basins of the Southwest and California, west-central
glacial aquifers in the Upper Midwest, and coastal plain
aquifers and Piedmont crystalline rock aquifers in central
Appalachia. Lowest concentrations were found in the
coastal plain aquifers of the Southeast. In general, higher
NO3-N concentrations were found near agricultural
lands. In an additional analysis of shallow groundwater
wells in agricultural areas, separate from the national
survey, nearly 25% of the sampled wells exceeded the 10
mg/L MCI for NO3-N. DeSimone et al. (2009) suggested
that redox could be a defining factor in some cases, and
perhaps was the reason for low NO3-N concentrations in
the Southeast in soils that promote denitrification, as well
as higher NO3-N levels in other areas where aquifers
were better oxygenated.
Ammonia toxicity in freshwater systems
The EPA and states have long regulated ammonia
(NH3) in the environment, not because of its nutrient
contribution to cultural eutrophication, but because of
its toxicity to freshwater aquatic life (U.S. EPA, 1986).
The un-ionized ammonia molecule has been identified
as the primary toxic form, rather than the ammonium
ion (NH4+), and research has further demonstrated
the relationship between pH, temperature, and NH3
partitioning from the total ammonia pool in freshwaters
(U.S. EPA, 1999). Because of the relationship to
temperature in particular, and the variable sensitivity of
species and life stages of aquatic organisms, state water
quality standards in application generally consider cold
and warm water conditions, as well as acute and chronic
exposures to life stages of sensitive organisms, plus a
margin of safety to derive criteria.
While water quality criteria were initially set for
concentrations of NH3, as criteria development skills
and understanding improved, it made sense to develop
criteria for total ammonia concentration for specific
water quality conditions, e.g., cold or warm (salmonids
present or absent) with consideration of ambient pH
factors as appropriate, for protection of the most
sensitive species likely to be present (early life stages
present or absent), plus a margin of safety. This was
because of evidence that the NH4+ fraction may also
be contributing to toxicity. Criteria could be presented
as formulas in adopted state criteria to calculate total
ammonia thresholds based on prevailing pH and
temperature conditions and organisms present/absent,
as appropriate.
Based on this research and analysis, EPA currently
recommends adoption of ammonia criteria as criterion
continuous concentration (CCC) and criterion maximum
concentration (CMC) as described in Box G-l (U.S.
EPA, 1999).
EXPLANATION
PopuUrion (199 5) supplied by
domestic well!, in percent of
total county population
IB >40snd<60
S3 >20and<40
CD <20
In many parts of the United States, domestic wells supply drinking water for large percentages ol the population.
Nationwide, more than (3 million people rely on domestic wells. Data shown are from Solley and others (1998)
Figure G-1: U.S. Population (1995) supplied by domestic drinking water wells
Source: DeSimone et al., 2009.
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vj \)
EXPLANATION
• =-10
3 il
Concentrations of nitrate were greater than the U.S. Environmental Protection Agency Maximum
Contaminant Level of 10 milligrams per liter (mg/LI as N in 4.4 percent of the wells. Elevated concentrations of
nitrata occurred throughout the United Slates, but least commonly in the Southeast Concentrations greater than
1 mg'L as N. which is usually indicative o1 human activities in many areas (Nolan and Hitt, 2003), were found in
about 40 percent of wells located throughout the sampled areas
Figure G-2: Nitrate concentrations in U.S. domestic drinking water wells
(Background colors on the map are indicative of different aquifer types)
Source: DeSimone et al., 2009
Recent research on the sensitivity of freshwater
unionid mussels (Family, Unionidae), and the rare and
endangered status of several unionid species, has led
the EPA to issue a draft update to the ammonia criteria
guidance (U.S. EPA, 2009b) that would supersede
the current 1999 guidance. Mussels have been found
to be more sensitive to ammonia toxicity than the
most sensitive species used to derive the 1999 criteria
(Augspurger et al., 2003). Augspurger et al. (2003) found
that the CMC with unionids considered would range
from 1.75 to 2.50 mg total ammonia-N/L, 60% lower
than the current method calculation of 5.62 mg total
ammonia-N/L, for example. In the draft 2009 update,
EPA proposed a two-tiered process for ammonia CMC
and CCC development for waters with and without
sensitive unionid species.
With this new sensitivity identified, ecosystem
imbalances due to eutrophication and the presence and
die off of invasive species may result in toxic levels of
ammonia for mussels in freshwater systems, in addition
to the conventional sewage and agricultural sources. For
example, Cooper et al., (2005) suggested that die offs of
invasive Asian clam (Corbicula fluminea), common in
most southeastern U.S. waters, could produce enough
sediment pore water ammonia during decay to be lethal
to sensitive unionid mussels. Further, combination of
ammonia with other toxic substances may compound
toxic effects, as found by Wang et al. (2007a,b) for
ammonia and copper. Their research suggests that the
1999 criteria for total ammonia might not be protective
of sensitive mussel species.
Impacts ofNr on freshwater ecosystems
Reactive nitrogen (Nr), including reduced
(ammonium, organic N compounds) and oxidized
(nitrate, nitrite) forms, play central roles in modulating
and controlling (limiting) primary and secondary
production and species composition in freshwater
ecosystems. These include lakes, reservoirs, streams,
rivers, and wetlands (Goldman, 1981; Paerl, 1982; Elser
et al., 1990, 2007; Wetzel, 2001). While phosphorus
has been considered the primary limiting nutrient in
freshwater ecosystems (c.f. Schindler, 1971; Schindler
et al., 2008), there are numerous examples where Nr
plays either a primary or secondary (i.e., co-limiting)
role as a limiting nutrient (Paerl, 1982; North et al.,
2007; Wurtsbaugh et al., 1997; Lewis and Wurtsbaugh,
2008). In particular, oligotrophic, alpine, tropical and
subtropical, and other lakes having small watersheds
-------
Box G-1: The National Criterion for Ammonia in Fresh Water
The available data for ammonia, evaluated using the procedures described in the "Guidelines for Deriving
Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses," indicate
that, except possibly where an unusually sensitive species is important at a site, freshwater aquatic life should be
protected if both of the following conditions are satisfied for the temperature (T) and pH of the waterbody:
1. The one-hour average concentration of total ammonia nitrogen (in mgN/L does not exceed, more than once every
three years on the average, the CMC (acute criterion) calculated using the following equations. Where salmonid
fish are present:
CMC =
0.275
39.0
+ 10
7.204-/>//
1+](K//-7.204
Or where salmonid fish are not present:
0.411 58.4
CMC =
1 + 10
7.204-pH
-7.204
2A. The 30-day average concentration of total ammonia nitrogen (in mg N/L) does not exceed, more than once
every three years on the average, the CCC (chronic criterion) calculated using the following equations.
When fish early life stages are present:
0.0577 2.487
ccc =
1 + 10
7.688-p//
1 + 10
ptf-7.688
M//X2.85, 1.45-10
(0.28-25-7") •
When fish early life stages are absent:
0.0577 2.487
ccc =
•1.45-10
Q.Q2«-(25-MAX(T,l))
v 1 + 10*" «j
2B. In addition, the highest 4-day average within the 30-day period should not exceed 2.5 times the CCC.
Source: U.S. EPA, 1999.
relative to the lake surface/volume, and lakes
experiencing incipient stages of eutrophication, tend to be
N-limited (Wetzel, 2001; Lewis and Wurtsbaugh, 2008).
N limitation was illustrated for Lake Tahoe (in California
and Nevada) which was highly sensitive to N enrichment
during its early stages of eutrophication (Goldman.
1981; 1988). As the lake accumulated anthropogenic
N inputs from both land-based runoff and atmospheric
deposition within the Tahoe Basin, it began exhibiting
symptoms of accelerating eutrophication, including
noticeable "greening" of its formerly transparent near-
shore waters and excessive epiphytic growth and fouling
on its rocky bottom. Continued excessive N loading in
the 1960s through 1980s has led to accelerating rates
of algal primary production and a tendency to shift to
more P limited conditions due to excessive N, relative
to P, loading (Goldman, 1988). This greater than 30-year
progression to more eutrophic, and less desirable (from
ecological, trophic and economic perspectives—i.e..
tourism, water use) conditions has largely been spurred
on by excessive N loading. Recent measures taken to
reduce N inputs have been successful in reducing the
lake's rate of eutrophication (Goldman, 2002). Similarly.
Lake Erie, which has experienced P-driven nuisance algal
blooms starting in the 1950s, is now facing excessive N
loading. This is largely a result of P input restrictions.
which have been enacted since the 1970's, accompanied
by a lack of control on ever-increasing N loads. This
shift in nutrient loading (increasing N:P) has led to a
resurgence of toxic cyanobacterial blooms dominated
by the non-N2 fixing genus Microcystis, an indicator of
excessive N loading (North et al., 2007).
Numerous lakes, reservoirs, rivers, estuaries (e.g., the
Gulf of Mexico), and fjords worldwide exhibit N and
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P co-limitation, either simultaneously or in seasonally-
shifting patterns (Dodds et al, 1989; Elser et al, 1990,
2007; Elmgren and Larsson, 2001; Forbes et al., 2008;
Scott et al., 2008; Wetzel, 2001; North et al., 2007; Xu
et al., 2010). For example, many reservoirs in Texas
exhibit seasonal N limitation in the river-reservoir
transition zone, regardless of their trophic status (Scott
et al., 2009). Under these circumstances, N inputs tend
to determine the spatial and temporal extent of summer
nuisance algal blooms, a key symptom of degrading
water quality (Dodds et al., 1989; Paerl, 2009; Xu et al.,
2010). N inputs, including those from increasing levels
of atmospheric deposition, impact nutrient stoichiometry,
with cascading effects on nutrient limitation, productivity,
and lake nutrient cycling characteristics (Elser et al.,
2009). Therefore, the inputs of N play a critical role in the
overall trophic response, trophic state, and water quality
conditions of affected freshwater ecosystems.
In Florida lakes, algae are often limited by the
availability of Nr (Kratzer and Brezonik, 1981). The
most well-studied example is Lake Okeechobee, the
largest lake in the Southeastern U.S., and a system that
periodically displays large blooms of noxious blue-green
algae. This lake has high availability of reactive P, and
changes in the availability of Nr control the wax and
wane of algae. In the 1980s and 1990s, blooms of algae
were predominantly caused by cyanobacterial nitrogen
(N2) fixer Anabaena. However, the most widespread
recent bloom, which covered almost the entire lake
surface in summer 2006, was caused by Microcystis, a
non-N2- fixing cyanobacterium that depends on dissolved
inorganic N (DIN: ammonium, nitrate, nitrite) and
possibly organic N for its growth. This alga is the most
common producer of toxins in Florida lakes, and it has
the ability to "luxury consume" P from lake sediments
and then rise through the water column, increasing
its biomass to a level that largely is controlled by the
amount of DIN. Because Lake Okeechobee's sediments
contain massive quantities of reactive P (Havens et al.,
2007), successful control of Microcystis blooms will
require reduction in both P and N inputs to this lake.
In addition to the importance total N loads play
in determining water quality status and trends, the
supply rates and ratios of various Nr forms play an
important role in structuring microalgal and macrophyte
communities mediating freshwater primary production
(Paerl, 1988; McCarthy et al., 2007, 2009; Lin et al.,
2008). For example, the ratio of ammonium to oxidized
N was related to the proportion of cyanobacteria
composing the total phytoplankton community of
Lake Okeechobee (McCarthy et al., 2009). Non-N2-
fixing cyanobacteria, such as Microcystis, are superior
competitors for reduced N (Blomqvist et al., 1994),
but even N2-fixing cyanobacteria will preferentially
assimilate ammonium if it is available (Ferber et al.,
2004). Ammonium is the initial N form produced by
recycling processes (via invertebrate excretion and
bacterial mineralization), but standing concentrations
often remain very low because they are assimilated
rapidly. Ammonium and other reduced N forms, such
as dissolved free amino acids, are more available than
oxidized N forms (nitrate and nitrite) to bacteria (Vallino
et al., 1996) and cyanobacteria because less energy is
required to incorporate reduced N into biomass than for
oxidized forms (Syrett, 1981; Gardner et al., 2004; Flores
and Herrero, 2005).
Lastly, it should be pointed out that both freshwater
and marine systems do not respond to nutrient inputs
in isolation. These systems are hydrologically and
biogeochemically connected and coupled, functioning
as a freshwater to marine continuum (Paerl, 2009).
Nutrient limitation may shift along the continuum, and
eutrophication and other symptoms of N and P over-
enrichment, including harmful algal blooms, hypoxia,
loss of biodiversity, and food web alterations impact
water quality, habitat condition, use, and sustainability
of downstream waters. Therefore, excessive N loading
in upstream freshwater ecosystems, ranging from the
headwaters of pristine alpine streams to lowland lakes,
reservoirs, and rivers can adversely affect downstream
estuarine and coastal marine waters (Conley et al., 2009;
Paerl, 2009). Examples of such continuum-scale impacts
include such prominent systems as Chesapeake Bay,
Albemarle-Pamlico Sound, Florida Bay, Mississippi
River plume (Gulf of Mexico), Baltic Sea, and Coastal
North Sea (Elmgren and Larsson, 2001; Boesch et al.,
2001; Paerl, 2009).
Impact of nitrogen on wetlands
In this section, the possible impact of reactive nitrogen
on wetlands is discussed. In Chapter 5 considerable
attention is devoted to the subject of wetlands serving
as effective nitrogen sinks. There are about 110 million
ha of wetlands in the U.S., with more than half of
those in the state of Alaska (Mitsch et al., 2009). Of
those wetlands, roughly 97% are inland (and mostly
freshwater) and 3% are estuarine (and mostly saline). Of
the total wetlands, approximately half (166 million acres
or 55 million ha) are peatlands, which, by their nature as
low-nutrient systems, are most susceptible to nitrogen
loadings, either from the atmosphere or from rivers and
streams. More than any other ecosystem, wetlands are
central to the cycling of nitrogen because they have both
aerobic and anaerobic conditions that allow for a wide
variety of important nitrogen processes, not the least of
which is denitrification.
Wetlands are similar to lakes and streams and any
other ecosystem in that their productivity is limited by
nutrient availability. But with wetlands the hydrology
limits or enhances productivity as well (Mitsch and
Gosselink, 2007). The addition of excessive nutrients
to wetlands, while often done purposefully when the
wetlands are so-called treatment wetlands (Kadlec and
-------
Wallace, 2009), can cause vegetation shifts and decreases
in plant diversity. Verhoeven et al. (2006) suggested 4.5
g-N nr2 yr1 as critical loading rate of nitrogen, generally
from atmospheric sources, for peat-dominated wetlands.
Morris (1991) had suggested that bogs and fens generally
had loading rates of 1 to 6 g-N nr2 yr1 respectively.
Thus Verhoeven et al. (2006) were suggesting that
wetlands should not be loaded beyond what is currently
occurring in fen peatlands. These limitations do not
apply for most mineral soil wetlands, particularly those
connected to streams and rivers. Most freshwater and
tidal marshes have nitrogen loading rates closer to 60
g-N nr2 yr1 and they maintain a reasonably high and
sustainable productivity. Using a sustainable rate of
nitrogen retention as a measure, Mitsch and Jergensen
(2004) suggest a range of nitrogen retention rate of 10-20
g-N nr2 yr1 for wetlands to maintain their biodiversity
while being nitrogen sinks at the same time. Overall, this
is a fruitful direction for wetland research to determine
the assimilative capacity of wetlands for nutrients,
including nitrogen, while not surpassing a limit that will
dramatically change the wetland's structure and function.
Impacts of Nr on coastal systems
Mitsch et al. (2001) suggest that streams and rivers
themselves are not always as much affected by nutrient
loading as are lakes, wetlands, coastal areas, and other
lentic bodies of water. However, in most cases, these
nutrient-enriched waterways flow to the sea, with
eutrophication of coastal waters the unfortunate result.
This problem now occurs regularly throughout the world
(World Resources Institute, 2008), in locations such as
the Gulf of Mexico (Rabalais et al., 1996), the Baltic Sea
(Larson et al., 1985), and the Black Sea (Tolmazin, 1985).
During the past century, following large-scale use of
synthetic N fertilizers in agriculture, rapid expansion
of industrial and transportation-related fossil fuel
combustion and coastal urbanization, humans have
significantly altered the balance between "new" N inputs
and N losses in the marine environment (Codispoti et al.,
2001; Galloway and Cowling, 2002). During this time
frame, terrestrial discharge and atmospheric N emissions
have increased 10-fold (Howarth et al., 1996; Holland
et al., 1999). This number keeps growing as human
development continues to expand in coastal watersheds
(Vitousek et al., 1997a,b).
Researchers have long recognized this growing
imbalance, especially in estuarine and coastal waters
where anthropogenically-derived N over-enrichment
has fueled accelerated primary production, or "cultural"
eutrophication (Vollenweideretal., 1992; Nixon, 1995).
Eutrophication is a condition where nutrient-enhanced
primary production exceeds the ability of higher ranked
consumers and organic matter-degrading microbes to
consume and process it. D'Elia (1987) characterized
this condition as "too much of good thing" or over-
fertilization of N-limited marine ecosystems with "new"
N, the bulk of it being anthropogenic (Howarth et al.,
1996; Vitousek et al., 1997a,b; Galloway and Cowling,
2002). Symptoms of N-driven eutrophication vary: from
subtle increases in plant production to changes in primary
producer community composition; to rapidly accelerating
algal growth, visible discoloration or blooms, losses in
water clarity, increased consumption of oxygen, dissolved
oxygen depletion (hypoxia), which is stressful to resident
fauna and flora; to, in the case of total dissolved oxygen
depletion (anoxia), elimination of habitats (Paerl,
1988, 1997; Diaz and Rosenberg, 1995; Rabalais and
Turner, 2001). Other effects include submerged aquatic
vegetation (SAV) losses, possible impacts on tidal
wetland health, and disruption of estuarine food chain
dynamics that may favor an imbalance towards lower
trophic levels (e.g., jellyfish).
Anthropogenic or cultural eutrophication has
been closely linked to population densities in coastal
watersheds (Peierls et al., 1991; Nixon, 1995; Vitousek
et al., 1997a,b). Primary sources of N enrichment
include urban and agricultural land uses as well as
wastewater treatment plants, many of which have not
been designed to remove N. A significant, and in many
instances increasing, proportion of "new" N input can
also be attributed to remote sources residing in airsheds.
Delivery routes can also be complex, especially when
via subsurface aquifers outside the immediate watershed,
which can confound source definition and create long
delays in delivery and management response (Paerl, 1997;
Jaworski et al., 1997; Galloway and Cowling, 2002; Paerl
et al., 2002).
The availability of N controls primary production in
much of the world's estuarine, near-shore coastal, and
open-ocean waters (Dugdale, 1967; Ryther and Dunstan,
1971; Nixon, 1995; Paerl, 1997; Boesch et al., 2001).
As previously discussed, nitrogen can also play a role
as either a primary or secondary limiting nutrient in
freshwater environments, especially large lakes (e.g.,
Lake Tahoe, Lake Superior). As such, the fertility of
these waters is often closely controlled by N inputs,
which are provided either internally by regeneration of
pre-existing N and biologically-fixed atmospheric N2, or
supplied externally (i.e. "new" N) as combined N sources
delivered via surface runoff, sub-surface groundwater, or
atmospheric deposition.
The extent to which accelerated N loading promotes
eutrophication and its symptoms varies greatly among
marine ecosystems. Receiving waters exhibit variable
sensitivities to N and other nutrient [phosphorus (P), iron
(Fe), and silica (Si)] loads that are controlled by their size,
hydrologic properties (e.g., flushing rates and residence
times), morphologies (depth, volume), vertical mixing
characteristics, geographic and climatic regimes and
conditions. In addition, the magnitude and distribution of
N in relation to other nutrient loads can vary substantially.
In waters receiving very high N loads relative to
-------
requirements for sustaining primary and secondary
production, other nutrient limitations may develop. This
appears to be the case in coastal waters downstream of
rivers draining agricultural regions that are enriched in N.
On the ecosystem level, estuarine and coastal waters
exhibit individualistic responses to N loads over seasonal
and longer (multi-annual, decadal) time scales. The
degree to which these systems are exposed to freshwater
discharge, tidal exchange, and vertical mixing is critical
for determining how they respond to specific N loads
(Vollenweideretal., 1992; Nixon, 1995; Cloern, 1999,
2001; Valdes-Weaver et al., 2006; Paerl et al., 2007).
Another variable is the manner in which N loading takes
place, which may range from acute pulsed events such
as storms and associated flooding, to longer-term gradual
(chronic) increases in N loading associated with more
predictive seasonal, annual and inter-annual hydrologic
cycles. There are striking contrasts in ecosystem response
to N inputs that reflect a range in physical (hydrodynamic,
optical) and climatic conditions (Cloern, 1999, 2001).
Examples include contrasts between strong tidally-
driven estuarine systems, such as Delaware Bay and San
Francisco Bay, and non-tidal, lagoonal systems, such as
North Carolina's Pamlico Sound and Texas's Luguna
Madre, or semi enclosed coastal systems, such as Florida
Bay and the Long Island Sound (Bricker et al., 1999;
Valdes-Weaver et al., 2006; Paerl et al., 2007).
Externally-supplied N comes in various forms,
including organic N and inorganic reduced (NH3 and
NH4+ ion) and oxidized (NO3~) N, all of which are
potentially available to support new production and
eutrophication. Laboratory experiments on phytoplankton
isolates and bioassays with natural phytoplankton
communities have indicated that these contrasting
forms may be differentially and preferentially utilized,
indicating that, depending on composition of the affected
phytoplankton community, some forms are more reactive
than others (Collos, 1989; Stolte etal., 1994; Riegman,
1998). Phytoplankton community composition can also
be altered by varying proportions and supply rates of
different forms of N (Dortch, 1990; Stolte et al., 1994;
Harrington, 1999; Pinckney et al., 1999; Piehler et al.,
2002). Monitoring and research on dissolved organic N
inputs and their effects should be conducted in receiving
streams, rivers, lakes, estuarine, and coastal waters, since
there is evidence that these compounds can be utilized by
phytoplankton, including harmful bloom species (Paerl,
1988; Antia et al., 1991; Carlsson and Graneli, 1998;
Gilbert et al., 2006). In addition, specific N compounds
may interact with light availability, hydrodynamics and
other nutrients, most notably P, Si, Fe, and trace metals,
to influence phytoplankton community growth rates and
composition (Harrison and Turpin, 1982; Smith, 1990;
Dortch and Whitledge, 1992).
Over the past 25 years, there has been a growing
recognition of cultural eutrophication as a serious
problem in coastal estuaries (NRC, 2000). Globally,
Selman et al. (2008) have reported "Of the 415 areas
around the world identified as experiencing some form
of eutrophication, 169 are hypoxic and only 13 systems
are classified as 'systems in recovery."Comprehensive
surveys of U.S. estuaries have been conducted by the
National Oceanic and Atmospheric Administration
(NOAA) as part of the National Estuarine Eutrophication
Assessments (NEEA) in 1999 and 2004 (Bricker et al.,
1999, 2007). The most recent report, released in 2007
(Bricker et al., 2007) focused on nutrient enrichment
and its manifestations in the estuarine environment and
relies on participation and interviews of local experts
to provide data for the assessment. Among the key
findings for nearly 100 assessed U.S. estuaries were that
eutrophication is a widespread problem, with the majority
of assessed estuaries showing signs of eutrophication—
65% of the assessed systems, representing 78% of
assessed estuarine area, had moderate to high overall
eutrophic conditions. The most common symptoms of
eutrophication were high spatial coverage and frequency
of elevated chlorophyll a (phytoplankton)—50% of the
assessed estuaries, representing 72% of assessed area, had
a high chlorophyll a rating.
Further field evaluations by EPA and state and
university collaborators under the National Coastal
Assessment (NCA) used probabilistic monitoring
techniques. The NCA National Coastal Condition Reports
(NCCR) (U.S. EPA, 200la, 2004, 2006b) are more closely
related to nutrient enrichment assessments, especially for
manifestations of nutrient enrichment such as hypoxia,
nuisance algal blooms, and general habitat degradation.
The last comprehensive national NCCR was published
in 2004 (U.S. EPA, 2004) with a more recent assessment
focused on 28 National Estuary Program estuaries
published in 2007 (U.S. EPA, 2006). The 2004 NCCR
included an overall rating of "fair" for estuaries, including
the Great Lakes, based on evaluation of more than 2,000
sites. The water quality index, which incorporates nutrient
effects primarily as chlorophyll-a and dissolved oxygen
impacts, was also rated "fair" nationally. Forty percent
of the sites were rated "good" for overall water quality,
while 11% were "poor" and 49% "fair."
Attainment of water quality management goals
and standards for coastal systems
Estuarine systems, where bio-available Nr is
more likely to be the limiting nutrient, are most often
susceptible to Nr enrichment (Paerl, 1997; Boesch et al.,
2001). Defining single-number criteria for nutrients or
related indicators representative of undesirable levels of
productivity (e.g., chlorophyll-a) is difficult, even using
the ecoregional approach recommended by EPA. State
managers more often use the formal TMDL process or
collaborative estuarine management plans to set site- or
estuary-specific N management targets to meet existing,
related water quality criteria (e.g., dissolved O2 or
-------
chlorophyll a). Some of the more prominent efforts and
targets for nitrogen control are summarized in Table G-l.
These targets all exceed nitrogen load reduction goals
that the Committee has found to be readily achievable
using existing technology and management authority
(i.e., less than 25% from specific source categories).
Some sources of nitrogen loading to the estuaries in
Table G-l pose greater management challenges and
the expectation is that reductions in Nr loadings to
estuaries would cumulatively be less than 25%. This
suggests that efforts will be needed to enable even
greater nitrogen load reductions in the future. Many of
the management actions the Committee has proposed in
this report would require substantive changes in national
programs, regulatory authority, management technologies
and societal demands to be accomplished. This is a
nutrient management concern that state managers are
well aware of as they develop TMDLs and management
plans that range above attainment potential, not only
for Nr but more frequently for other pollutants that are
predominately nonpoint source and stormwater loaded
(including atmospheric source contributions).
The Chesapeake Bay Program, for example, is
a model for Nr and P management in many ways.
Considerable resources were committed, and many
BMPs were implemented. Yet despite regional efforts
and commitments from all watershed states, and more
funding than any other estuary program is likely
to see, management targets have not been met, and
recent data (2007) reveal the occurrence of a severe
hypoxic episode. Concerns over the slow progress in
restoring the Chesapeake Bay led to the issuance of an
Executive Order on May 15, 2009, establishing a Federal
Leadership Committee led by the EPA to develop and
implement a plan to restore the Bay in collaboration with
state agencies (Federal Register 74(93): 23097-23104).
Similarly, the adoption of the Long Island Sound TMDL,
which was driven by the presence of reactive nitrogen
(see Box G-2), sets an implementation plan that could
attain Connecticut and New York dissolved oxygen
criteria, but only if "alternative technologies" such as
mechanical aeration of the Sound or biological harvesting
of nutrients, are used.
Table G-1: Estuaries with nitrogen management plans or
TMDLs and percent nitrogen load reduction targets
Casco Bay, Maine
trogen Load
.duction Target
45%
TMDL or Plan
Plan
Chesapeake Bay
>40%
Plan
Northern Gulf of Mexico
Mississippi Plume Region
45%
Plan
Long Island Sound
60% for CT & NY sources
TMDL
Neuse River Estuary, NC
30%
TMDL
Tampa Bay, FL
Maintain TN (total nitrogen)
load at 1992-1994 levels
TMDL & Plan
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Box G-2: Long Island Sound Total Maximum Daily Load: Focus on Reactive Nitrogen
A TMDL sets a goal for reducing the load of a specific pollutant that is causing impairment to a waterbody. In
the case of Long Island Sound, the impairment constitutes low concentrations of dissolved O2 that violate both
Connecticut's and New York's water quality standards. Nitrogen has been identified as the pollutant that causes
substandard levels of dissolved oxygen in Long Island Sound and, accordingly, Connecticut's and New York's
environmental agencies have developed a TMDL that assigns nitrogen reductions from both point sources (the
wasteload allocation or WLA) and nonpoint sources (the load allocation or LA) in their respective states to meet the
established 58.5% reduction of anthropogenic sources.
The Long Island Sound TMDL is set at 23,966 tons of N/year, which represents a 23,834 ton/year reduction from
the total baseline (anthropogenic + natural sources considered) of 47,788 tons/year from Connecticut and New
York only. Most of that N load comes from point sources - POTWs (publicly owned treatment works) and CSOs
(combined sewer overflows) - accounting for 38,899 tons/yr of the total N load from the two states, or 81% of the
load. For that reason, the focus has been on managing point sources, although attainment of water quality standards
will require more widespread reductions from atmospheric deposition, stormwater, and nonpoint sources, and from
other watershed states north of Connecticut.
Connecticut and New York have some flexibility in the apportionment of those reductions between the WLA and the
LA, but must have completed 40% of the required reductions by 2004, 75% by 2009, and 100% by 2014 when the
final TMDL will be met. However, the TMDL is presently undergoing revision to incorporate findings from a new
model of Long Island Sound, and to reflect changes in dissolved O2 criteria in both states. The revised TMDL will
likely require more aggressive reductions of nitrogen to meet dissolved O2 criteria and may formalize targets for
upstream state contributions and atmospheric deposition.
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Appendix H:
Nr Saturation and Ecosystem Function
There are limits to how much plant growth can be
increased by N fertilization. At some point, when the
natural N deficiencies in an ecosystem are fully relieved,
plant growth becomes limited by availability of other
resources such as phosphorus, calcium, or water and the
vegetation can no longer respond to further additions of
Nr. In theory, when an ecosystem is fully Nr-saturated
and its soils, plants, and microbes cannot use or retain
any more, all new Nr deposits will be dispersed to
streams, groundwater, and the atmosphere. Nr saturation
has a number of damaging consequences for the health
and functioning of ecosystems. These impacts first
became apparent in Europe almost three decades ago
when scientists observed significant increases in nitrate
concentrations in some lakes and streams and also
extensive yellowing and loss of needles in spruce and
other conifer forests subjected to heavy Nr deposition. In
soils, most notably forest soils because of their natural
low pH, as NH4+ builds up it is converted to nitrate by
bacterial action, a process that releases hydrogen ions
and contributes to soil acidification. The buildup of NO3~
enhances emissions of nitrous oxides from the soil and
also encourages leaching of highly water-soluble NO3~
into streams or groundwater. As negatively charged NO3~
seeps away, positively charged alkaline minerals such as
calcium, magnesium, and potassium are carried along.
Thus, soil fertility is decreased by greatly accelerating
the loss of calcium and other nutrients that are vital
for plant growth. As calcium is depleted and the soil
acidified, aluminum ions are mobilized, eventually
reaching toxic concentrations that can damage tree roots
or kill fish if the aluminum washes into streams (Vitousek
etal., 1997a,b).
Forests, grasslands, and wetlands vary substantially
in their capacity to retain added nitrogen. Interacting
factors that are known to affect this capacity include
soil texture, degree of chemical weathering of soil,
fire history, rate at which plant material accumulates,
and past human land use. However, we still lack a
fundamental understanding of how and why N-retention
processes vary among ecosystems, much less how they
have changed and will change with time and climate
change (Clark and Tilman, 2008).
An overarching impact of excess Nr on unmanaged
terrestrial ecosystems is biodiversity loss. In North
America, dramatic reductions in biodiversity have been
created by fertilization of grasslands in Minnesota
and California. In England, N fertilizers applied to
experimental grasslands have led to similarly increased
dominance by a few N-responsive grasses and loss
of many other plant species. In formerly species-rich
heathlands across Western Europe, Nr deposition has
been blamed for great losses of biodiversity in recent
decades, with shallow soils containing few alkaline
minerals to buffer acidification (Vitousek et al., 1997a,b;
Bobbinketal., 2010).
Losses of biodiversity driven by Nr deposition can in
turn affect other ecological processes. Experiments in
Minnesota grasslands showed that in ecosystems made
species-poor by fertilization, plant productivity was much
less stable in the face of a major drought. Even in non-
drought years, the normal vagaries of climate produced
much more year-to-year variation in the productivity of
species-poor grassland plots than in more diverse plots
(Vitousek etal., 1997a,b).
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