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Synergies of Excess Sediment and Nitrogen |
Researchers: Geoffrey C. Poole, Krista L. Jones, Chris Bennett, and Ashley M. Helton.
In the face of the increasing size and intensity of agriculture and
urbanization in many landscapes, protecting streams and rivers from the
negative effects of pollution is an important management goal.
Although waterways are degraded by many different types of pollution,
this research project focuses on the effects of excess nitrogen (such
as that found in water draining from fertilized agricultural fields)
and sediment (which often enters streams when the earth is tilled or
disturbed) on the ecology of rivers and streams. Excess sediment
is known to degrade the physical habitat of rivers, while excess
nitrogen can cause problems such as promoting blooms of nuisance algae
and, ultimately, can create large, oxygen-starved “dead zones” where
nitrogen-laden river water mixes into the sea.
This project will investigate the fate, transport, and
interactions between sediment and nitrogen once in streams. The
project is based on the hypothesis that sediment and nitrogen may be
synergistic pollutants, meaning that their effects may be magnified
when delivered to rivers simultaneously. In essence, while river
ecosystems may exhibit some resilience to modest increases in nitrogen
delivery alone, we expect that sediment can reduce this inherent
resiliency. Specifically, fine sediment deposited in a river may
interfere with hyporheic exchange of water within a river – the natural
and continuous recirculation of water between a river channel, the
stream bed, and the flood-plain aquifer. Since hyporheic exchange
brings river water into contact with streambed and underground microbes
that can absorb nitrogen, any limitation of hyporheic exchange by fine
sediments could reduce a river ecosystem’s capacity to accommodate
excess nitrogen.
To investigate this question, we will extend
an existing state-of-the-art computer simulation model of river
ecosystem dynamics. Improvements to the model will allow us to
simulate: a) hyporheic exchange at multiple spatial scales
simultaneously (in the streambed, through meander bends and gravel
bars, and deeper in the floodplain aquifer); b) transport and
deposition of fine sediments in river channels; and c) associated
changes in expected rates of nitrogen removal within the hyporheic
zone. An extensive data library of field data from the Umatilla
River, Oregon, will be used as a reference to ensure that simulated
ecosystem dynamics are similar to those observed in rivers and
streams. Then, through a series of modeling experiments and
manipulations, we will determine whether the model shows strong
interactions between sediment and nitrogen in river, and if so, under
what circumstances.
This research will provide new insights into
the role of the hyporheic zone in imparting river resilience to excess
nitrogen loads. It will also help resolve the question of whether
excess nitrogen and sediment can degrade water quality
synergistically. Together, these outcomes will help natural
resource managers understand the potential for management and
restoration of the hyporheic zone.
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Research 
