&EPA
www.epa.gov/research
EPA/600/F-13/223
science in ACTION
INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE
Development of a Tool to Identify Nitrogen Sources and Sinks within a
Watershed Framework
The management of reactive
nitrogen in watersheds requires
sound scientific tools.
Although nitrogen fertilization is vital to
maintain a productive and vigorous food
supply, excess reactive nitrogen (N)
released to the environment causes
deleterious effects on human and
ecosystem health. Excess N in aquatic
systems brings about harmful algae and
weeds, causes eutrophication induced
hypoxia in coastal systems that leads to
fish kills, and excess N contaminates
many potable drinking water supplies.
In order to manage the widespread N
pollution problem in watersheds it is
requisite for decision makers to have a
strong technical grasp of the factors that
control N sources, sinks (regions or areas
that can retain N), and pathways along
which nitrogen is moved and transformed
(U.S. EPA 2011). However, this
scientific rigor should not serve to
confuse the problem but communicate
alternatives in such a way that
management of land use is attainable for
decision makers (U.S. EPA 2012). A
fruitful approach to communicate the
relationships between N processing and
land use or land cover is through static
and interactive maps supported by models
of N processing (Kellogg et al. 2010).
The N-Sink nitrogen management tool
summarizes the complex biological,
chemical, and physical conveyance of N
to support watershed N management
decisions with sound scientific
information and a means by which to
visualize alternative decisions.
Since the sources, sinks, and conveyance
of N is highly landscape and hydrology
dependent, the N-Sink tool helps decision
makers summarize the influence of
sources and sinks within a watershed. In
terms of sources, N export from urban
and suburban watersheds is much higher
than from forested watersheds, although
lower than from agricultural watersheds
This screen capture shows calculation of a flow path from source to watershed
outlet (red line). A web-based beta version of N-Sink is available at
http://clear.uconn.edu/projects/nsink
(Groffman et al. 2004). High
concentrations of nitrate in shallow
groundwater and streams are correlated
with agricultural land use and unsewered
residential developments (Nowicki and
Gold, 2008). Sink areas include wetlands,
hydric soils, reservoirs, small-order
streams and impoundments (Groffman et
al., 2003). In particular, riparian wetlands
can be a significant sink for N due to the
combination of surface filtering of
sediments, plant and microbial uptake,
and subsurface denitrification (Gold et
al., 2001). Studies in both urbanizing
(Kaushal et al. 2008) and agricultural
(Clausen et al. 2000) watersheds have
demonstrated that riparian restoration can
reduce the delivery of nitrogen to
streams. Given the complexity of N
dynamics, there is a need to incorporate
our understanding of land use and
nitrogen cycling relationships into a tool
that can guide land use decisions that
support sustainable and healthy
communities. In most areas local land
use decision makers have little
knowledge about N sources and sinks,
and thus no way in which to factor N
pollution into their land use policies and
decisions. A relatively easy-to-use
decision support tool that identifies N
sources and sinks in a watershed, and
estimates delivery efficiencies from
particular locations in the watershed to
the outlet is a valuable asset for
communities to help reduce N pollution
to their waters. This is the objective
behind the development and testing of the
N-Sink tool.
The N-Sink Tool
N-Sink is a customized ArcMape
program that provides maps of N sources
and sinks within a watershed, and
estimates the delivery efficiency of N
movement from sources to the watershed
outlet. The primary objective of N-Sink is
to assist land use planners, watershed
U.S. Environmental Protection Agency
Office of Research and Development
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managers, and land conservation
organizations to evaluate proposed
development, prioritize restoration
projects, and focus resources in areas that
are most likely to yield reductions in N
delivery to coastal waters.
Land cover and N movement
relationships taken from the scientific
literature are used to identify N source
and sink areas, and to roughly quantify
their impact on the N budget. This
information, combined with modeling of
water flow at the catchment scale using
the ArcHydro extension to ArcGIS
(Maidment, 2002) is used to estimate
downstream delivery efficiencies. N-Sink
is developed with widely available
national databases in order to provide
broad applicability. These data include:
• Topography (National Elevation
Dataset, USGS)
• Hydrography (National Hydrography
Dataset, USGS)
• Soils (Soil Survey Geographic
Database, USDA/NRCS)
• Land cover (National Land Cover
Dataset, multi-agency consortium)
N-Sink incorporates many complex
biogeochemical and hydrologic
relationships inaccessible to decision
makers. However, the goals of broad
applicability and ease of use dictate a
trade-off in precision of the N delivery
estimates. N-Sink works as a
prioritization and visualization tool,
which enables users to understand how N
moves in a given watershed and
investigate the relative N-related impacts
of various land use scenarios. As such,
the numeric outputs of the tool focus on
percent removal of nitrogen from source
to receiving water, rather than specific
loading estimates.
The U.S. EPA's Nitrogen Management
Group as part of the Sustainable and
Healthy Communities program helps
support the ongoing development of N-
Sink. The early prototype base model for
N-Sink created by the University of
Rhode Island (URI), in partnership with
Arizona State University and the
University of Connecticut (UConn)
(Kellogg et al., 2010) was funded by the
USDA/NIFA Water Program. Together,
these groups work to bring N-Sink to
community decision makers.
Project Objectives
The objectives of the current project team
from UConn, URI, and EPA are to:
(1) Develop a web-based version (figure).
The original version, developed in 2010,
requires significant reprogramming to
take advantage of technological
improvements and modern ability to
bring the tool to the public. Geospatial
experts at URI and UConn are testing the
tool for the technical soundness of with
the modern upgrades and works to ensure
N-Sinks functionality in today's desktop
and internet environment.
(2) Develop visual (map) outputs of the
tool for their utility at the local level.
UConn's Center for Land Use Education
and Research, most notably the Nonpoint
Education for Municipal Officials
(NEMO) program, has a long and
successful history of taking geospatial
environmental information and folding it
into educational programs and products
that assist local land use decision makers
(Arnold et al., 2000).
(3) Characterize N sources and sinks in
two small (USGS Hydrologic Unit Code
12-digit) pilot watersheds in Connecticut
and/or Rhode Island. The Niantic River
watershed in Southeast Connecticut has
already been selected as one of these
pilots.
With a successful outcome to this project,
the N-Sink team hopes to incorporate the
N-Sink maps and information into
educational outreach programs for local
land use officials, watershed groups, and
nonprofit organizations beyond the pilot
watersheds.
References
Arnold, C.L., Jr., D.L. Civco, S. Prisloe J.D. Kurd,
and J. Stacker. 2000. Remote sensing-enhanced
outreach education as a decision support system
for local land use officials. Photogrammetric
Engineering & Remote Sensing 66(10):1251-
1260.
Clausen, J. C., K. Guillard, C. M. Sigmund and K.
M. Dors. 2000. Water Quality Changes from
Riparian Buffer Restoration in Connecticut.
Journal of Environmental Quality 29(6): 1751-
1761
Gold, Arthur J., Peter M. Groffman, Kelly Addy, D.
Q. Kellogg, Mark Stolt and Adam E. Rosenblatt.
2001. Landscape attributes as controls on ground
water nitrate removal capacity of riparian zones.
Journal of the American Water Resources
Association 37(6): 1457-1464.
Groffman, Peter M., D. J. Bain, L.E. Band, K.T.
Belt, G.S. Brush, J.M. Grove, R.V. Pouyat, I.C.
Yesilonis, and W.C. Zipperer. 2003. Down by the
riverside: urban riparian ecology. Frontiers of
Ecology andthe Environment 1:315-321.
Groffman, Peter M., Neely L. Law, Kenneth T.
Belt, Lawrence E. Band, and Gary T. Fisher.
2004. Nitrogen fluxes and retention in urban
watershed ecosystems. Ecosystems Volume 7:
393-403.
Kaushal, Sujay S., Peter M. Groffman, Paul M.
Mayer, Elise Striz, and Arthur J. Gold. 2008.
Effects of stream restoration on denitrification in
an urbanizing watersheds. Ecological
Applications 18(3): 789-804.
Kellogg, D. Q., Arthur J. Gold, Suzanne Cox, Kelly
Addy and Peter V. August. 2010. A geospatial
approach for assessing denitrification sinks within
lower-order catchments. Ecological Engineering
(36): 1596-1606.
Maidment, D.R. (Ed.), 2002. Arc Hydro: GIS for
Water Resources. Environmental Systems
Research Institute, Inc., Redlands, CA.Mass. DEP
website, accessed 01/03/13: http://www.mass.gov/
dep/water/resources/tmdls.htm
Nowicki, B. and A. J. Gold. 2008. Nutrient
transport in groundwater at the coastal margin.
In: Desbonnet, A. Costal-Fierce, B.A. (Eds),
Science foe Ecosystem-based Management:
Narragansett Bay in the 21st Century. Springer
Series in Env. Mngt, New York NY. Pages 67-
100. http://www.dec.nv.gov/docs/water pdf
tmdlnitrpecn.pdf
U.S. EPA Science Advisory Board. 2011. Reactive
Nitrogen in the United States: An Analysis of
Inputs, Flows, Consequences and Management
Options. A Report of the EPA Science Advisory
Board (EPA-SAB-11-013) http://www.epa.gov/sab
U.S. EPA Science Advisory Board. 2012. Science
Integration for Decision Making at the U.S.
Environmental Protection Agency (EPA-SAB-12-
008) http://www.epa.gov/sab
Contacts:
Dr. Ken J. Forshay
US EPA Office of Research and Development
(580)4368912
Forshav. Ken(g)EPA. gov
Dr. Q Kellogg
University of Rhode Island
(401)874-4866
qfgtedc.uri.edu
Chet Arnold
University of Connecticut
(860) 345-5230
Chester, arnoldfgjuconn.edu
Dr. David Burden
US EPA Office of Research and Development
(580) 436 8606
Burden.David(a).EPA.gov
U.S. Environmental Protection Agency
Office of Research and Development
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