United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Athens, GA 30605-2700
Research and Development
EPA/600/SR-95/173 January 1997
oEPA Project Summary
Assessment of Nitrogen
Loads to Aquatic Systems
Avinash S. Patwardhan and Anthony S. Donigian, Jr.
Estimates of the contributions of atmo-
spheric nitrogen deposition to Chesapeake
Bay, Galveston Bay, and Tampa Bay are
reported. The estimates are based on a
refinement of a methodology developed
by the Environmental Defense Fund (EOF)
in a 1988 study of Chesapeake Bay. The
three-embayment study employed a
spreadsheet methodology that produced
estimates of nitrogen loadings from the
various nitrogen sources found in each of
the study areas. Sensitivity analysis
showed that atmospheric deposition has
a significant effect on the nitrogen load-
ings delivered to the aquatic systems. More
than 40% of the total nitrogen loads to the
three systems resulted from atmospheric
deposition.
This Project Summary was developed
by EPA's National Exposure Research
Laboratory's Ecosystems Research Di-
vision, Athens, GA, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Nitrogen, released by many mobile and
stationary sources, is ubiquitous in the en-
vironment where it can have both benefi-
cial and deleterious effects. Nitrogen im-
proves soil fertility and plant nutrition, but
also produces water and air pollution
through acidification and eutrophication of
surface waters, contamination of ground
water contribution to greenhouse gas emis-
sions, and effects on ozone distribution.
As part of a study of sources of nitrogen
to the Chesapeake Bay in 1988, the EOF
developed modeling systems that included
mass balance modeling of nitrogen on vari-
ous land categories to explore and quantify
both the sources of nitrogen and potential
reduction alternatives. Subsequently, the
U.S. Environmental Protection Agency
(EPA) through its Ecosystems Research
Division in Athens, GA, directed a research
project to refine the EOF methodology that
resulted in the development of a spread-
sheet format that incorporates commonly
available data sources, e.g., EPA/state
waste discharges, emission inventories,
soils and meteorologic data bases. The
refined procedures were then applied to
the Chesapeake Bay, Galveston Bay, and
Tampa Bay watersheds. Results of the EOF
and EPA studies were compared.
The spreadsheet procedure estimates
the contribution of atmospheric nitrogen
deposition to the three study sites with re-
spect to nitrogen contributions from nonpoint
and point sources. The land use categories
from which nonpoint source loadings were
calculated consist of forest, cropland, pas-
ture, and urban area. The point source
loadings consist of contributions of nitrogen
from industrial and sewage treatment plant
discharges. The annual wet atmospheric
deposition data were obtained from the Na-
tional Atmospheric Deposition program.
Percentage contributions from various
nitrogen sources in the three watersheds
are presented in Table 1. The Chesa-
peake Bay was divided into 14 subbasins
for the spreadsheet analysis; the EOF pro-
cedure considered the watershed to be a
single basin. The two procedures produced
results that were in close agreement with
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respect to the contribution of atmospheric
deposition to the total load to the Chesa-
peake Bay. The total nitrogen loads of
140 million kg/yr from the EOF study were
similar to, but slightly higher than, the 115
million kg/yr from the spreadsheet analy-
sis.
The spreadsheet methodology estimated
slightly higher contributions of nitrogen from
point sources as compared to the EOF
analysis. The higher percentage could re-
flect the data used to calculate point source
loads. The spreadsheet procedure used
average point source loadings as estimated
by EPA's Chesapeake Bay Program Office
for 1984-1987, whereas the EOF proce-
dure used point source data for 1985 only.
The spreadsheet estimate for the percent-
age contribution from manure was 3.2%
higher than the EOF estimate. In the EOF
analysis, manure was applied only to pas-
tures and a retention factor of 97.5% was
used, which resulted in very low nitrogen
loadings to the Bay as a result of manure
application. In the spreadsheet analysis,
manure was applied to both cropland and
pastures, and as cropland has lower reten-
tion of nitrogen than pastures, a slightly
higher percentage nitrogen load was esti-
mated.
The percentage nitrogen loadings to
Chesapeake Bay from fertilizer applica-
tions as estimated by the two procedures
did not agree. In the EOF analysis, county
fertilizer sales data were used to estimate
the total amount of fertilizer applied to the
entire watershed, and a single constant
retention factor was used for all the agri-
cultural cropland in the Chesapeake wa-
tershed. For the spreadsheet analysis, fer-
tilizer application data estimated by state
and county extension officials were used.
The total fertilizer application was 13 mil-
lion kg greater for the spreadsheet analy-
sis than for the EOF study. As variable
retention parameters (computed from unit
area loads) for all 14 subbasins were used
in the spreadsheet method, however, fer-
tilizer loads were lower than for the EOF
method.
For Galveston Bay, the nonpoint nitro-
gen loadings information used in the
spreadsheet study indicated that more than
50% of the total nonpoint load is due to
the land use area occupied by urban dwell-
ings. The total nitrogen load to Galveston
Bay was estimated to be 36 million kg. Of
the total 49% atmospheric deposition load
to Galveston Bay (Table 1), direct atmo-
spheric deposition to the Bay's surface
water accounted for 39%, with the water-
shed contributing the remaining 10%. Fer-
tilizer loadings to the Bay were quite low
as only 22% of the watershed was under
crop production.
For Tampa Bay, a total nitrogen load of 4.5
million kg was estimated. Direct atmospheric
deposition onto Tampa Bay surface water
accounted for 40% of the total 67% (Table 1)
atmospheric deposition load, with the water-
shed contributing 27%. The relatively large
contribution of atmospheric deposition may
reflect the considerably lower total nitrogen
load for Tampa Bay as compared to the loads
in the other two embayments. The relatively
small watershed area and large water surface
area of Tampa Bay also magnified the
relative contributions from atmospheric
sources.
In summary, the study results indicate
that atmospheric deposition of nitrogen
over portions of the U.S. may be a signifi-
cant contributor of nitrogen to aquatic sys-
tems. Application of the spreadsheet meth-
odology developed in this project to Chesa-
peake Bay, Galveston Bay, and Tampa
Bay supports this conclusion; more than
40% of the total nitrogen load that is deliv-
ered to these water bodies is estimated to
be the result of atmospheric nitrogen depo-
sition. Although not confirmed in this study,
increased emissions of nitrous oxides to
the atmosphere may be a source of the
increased nitrogen inputs to aquatic sys-
tems through the deposition pathway.
Users of the estimates reported here
should be aware of the assumptions on
which the spreadsheet methodology was
based. These assumptions are that (1)
dry atmospheric deposition is equal to wet
atmospheric deposition, (2) nitrogen in-
puts to the land use categories of pas-
tures, urban areas, and wetlands are only
from atmospheric deposition, (3) the river-
ine nitrogen loss parameter is constant for
all steams in all three study sites, (4)
biological fixation is ignored for crops and
forests, and (5) the retention parameter is
assumed to be the same for fertilizer ap-
plication and atmospheric deposition on
cropland. Further investigations may be
needed to support these assumptions.
Table 1. Nitrogen Contributions (Percent) from Various Sources to Chesapeake Bay,
Galveston Bay, and Tampa Bay
Nitrogen Source
EOF
Analysis
Chesapeake Bay
Spreadsheet Analysis
Chesapeake Bay Galveston Bay Tampa Bay
Point Source
Atmospheric
Deposition
Fertilizer
Manure
23
39
34
30
43
20
7
48
49
19
67
14
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Avinash Patwardhan and Anthony S. Donigian, Jr. are with AQUA TERRA Consultants,
Mountain View, CA 94043.
Thomas O. Barnwell, Jr. is the EPA Project Officer (see below).
The complete report, entitled "Assessment of Nitrogen Loads to Aquatic Systems," (Order
No. PB97-125 421; Cost: $25.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Ecosystems Research Division
National Exposure Research Laboratory
U.S. Environmental Protection Agency
Athens, GA 30605-2700
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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EPA
PERMIT No. G-35
EPA/600/SR-95/173
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