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| Titel |
Hyporheic nitrogen dynamics in gravel bed rivers |
| VerfasserIn |
A. Marzadri, D. Tonina, A. Bellin |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023454
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| Zusammenfassung |
| Streams often suffer of excessive nitrogen inputs from agricultural and urban areas. These
inputs are the major responsible of streamś eutrophication and may be a source of
nitrous oxide an important greenhouse gas formed during same hyporheic processes.
Consequently, hyporheic exchange, which mixes surface and pore waters, affects both fluvial
and terrestrial ecosystems and its inclusion in nutrients and contaminant transport
model is necessary. In general, in-stream water continuously exchange between
stream and sediment through the ¨p umping¨m echanism, which stems primarily
from near-bed pressure gradients. Alternate zones of high (downwelling) and low
(upwelling) pressure induce a complex flow pattern within the hyporheic zone with
in-stream and pore waters entering the stream and the sediment, respectively. In the
present work, we focus on the export of ammonium (NH4+), nitrate (NO3-) and
their fate within the streambed of gravel bed rivers with alternate bars. We model
hyporheic exchange with analytical solutions of the intra-gravel flows induced by
streambed morphology and the fate of the inorganic compounds of nitrogen with a set of
transport equations coupled with first order kinetics. Transport is solved by particle
tracking, assuming negligible local dispersion and temperature dependant reaction rate
coefficients. Through a Lagrangian approach we present the transport equation
in term of hyporheic residence time, which is the controlling parameter of both
retention and nitrification-denitrificaton processes. We investigate the important
factors controlling the export of ammonium, nitrate, and production of nitrogen
gases by the hyporheic zone. Our results show that the hyporheic zone acts as a
sink of ammonium to an extent that depends on the nitrification rate but it may
act as a source or a sink of nitrate. Additionally, it can influence the emission of
nitrogen gases (N2 and N2O), depending on the ratio between ammonium and
nitrate concentrations in the stream and on the biomass uptake. Moreover, stream
morphology discriminates whether the hyporheic zone is source rather than a sink of
these inorganic nitrogen species . Nitrification processes dominates in small steep
streams because of the short residence time, while denitrification plays a major role in
low-gradient large streams. Additionally, emissions of nitrogen gases increase with water
temperature in small steep streams, but not with alluvium depth, because the hyporheic
flow occurs primarily near the surface. On the other hand, the emission of nitrogen
gases increases with both temperature and alluvium depth in low-gradient streams. |
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