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

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EPA/600/SR-95/173

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