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| Titel |
Simultaneous measurement at multiple depths of in situ rates of denitrification in the bed of a groundwater-fed river |
| VerfasserIn |
Katrina Lansdown, Mark Trimmer, Kate Heppell |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040479
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| Zusammenfassung |
| Typically characterised by steep chemical gradients and variable redox conditions, the
hyporheic zone is considered a ‘hotpot’ or site of enhanced biogeochemical activity in the
aquatic environment. As such the importance of the hyporheic zone for the attenuation of
nutrients such as nitrate in a fluvial network has long been recognised. Controls on nitrogen
transformations, however, especially at depths greater than 10cm below the sediment-water
interface, remain comparatively less understood. Most work aimed at quantifying
denitrification in the hyporheic zone has involved laboratory incubation of recovered
sediments which is likely to affect the estimate of the true in situ rate. Results of such studies
are usually cited as ‘potential’ rates of denitrification and have undoubtedly improved the
understanding of nitrogen cycling in the aquatic environment. There is, however, a need for
in situ measurement to improve our knowledge of nitrogen cycling in the river
bed.
Here, rates of denitrification in the hyporheic zone have been measured at multiple
depths, simultaneously using “push-pull” methodology (e.g. Snodgrass and Kitanidis 1998).
The “push-pull” technique involves injection of a solution containing reactant(s) (e.g. nitrate)
and a conservative tracer (e.g. chloride) into the sediment and extraction of pore water
samples over time. Recovered samples are screened for the removal of reactant(s) and/or the
accumulation of products(s). Temporal changes in the conservative tracer are used to correct
the concentration of the reactant(s) and product(s) for dispersion and advection. The
disadvantage of the ‘traditional’ “push-pull” methodology is that rates of nitrate removal
are measured rather than rates of denitrification. In this research, comparison of
measured and ‘corrected’ nitrate concentrations allowed the rate of nitrate removal (or
production) to be quantified. In order to determine in situ rates of denitrification we used
15N-enriched nitrate as the reactant in the “push-pull” experiments and measured the
accumulation of 29N2 and 30N2 over time. Rates of denitrification were then calculated
using the isotope pairing technique (as per Nielsen 1992; Sanders and Trimmer
2006).
Measurements have been made at up to 40cm depth in the river bed using a miniprobe
system developed from the design of Sanders and Trimmer (2006) in order to validate the
experimental approach. Previous work conducted at the study site (R. Leith, Cumbria, UK)
suggested than ambient conditions would not be ideal for in situ measurement of
denitrification rates using “push-pull” methodology. Slow rates of denitrification were
measured during laboratory incubations of the sandy sediment (M. Trimmer, unpublished
data), and the velocity of flow in the hyporheic zone would limit the duration of the in situ
measurement (Käser et al. 2009). Despite difficulties posed by the site, we have measured in
situ rates of denitrification at multiple depths in the hyporheic zone simultaneously. We also
report measurement of in situ rates of denitrification below the zone of surface water –
ground water mixing. In future we will use the experimental technique described
in this paper to quantify (i) variation in N transformation rates with depth; and
(ii) spatial variation in rates of nitrate consumption/production within the river
bed.
Käser, DH, Binley, A, Heathwaite, AL and Krause, S (2009) Spatio-temporal variations
of hyporheic flow in a riffle-pool sequence. Hydrological Processes 23: 2138 -
2149.
Nielsen, LP (1992) Denitrification in sediment determined from nitrogen isotope pairing.
FEMS Microbiology Ecology 86: 357 - 362.
Sanders, IA and Trimmer, M (2006) In situ application of the 15NO3- isotope pairing
technique to measure denitrification in sediments at the surface water–groundwater interface.
Limnology and oceanography: Methods 4: 142 - 152.
Snodgrass, MF and Kitanidis, PK (1998) A method to infer in situ reaction rates from
push-pull experiments. Ground Water 36(4): 645 - 650. |
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