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| Titel |
Predicting the denitrification capacity of sandy aquifers from in situ measurements using push–pull 15N tracer tests |
| VerfasserIn |
W. Eschenbach, R. Well, W. Walther |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 8 ; Nr. 12, no. 8 (2015-04-17), S.2327-2346 |
| Datensatznummer |
250117904
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| Publikation (Nr.) |
 copernicus.org/bg-12-2327-2015.pdf |
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| Zusammenfassung |
Knowledge about the spatial variability of in situ denitrification rates
(Dr(in situ)) and their relation to the denitrification capacity in
nitrate-contaminated aquifers is crucial to predict the development of
groundwater quality. Therefore, 28 push–pull 15N tracer tests for the
measurement of in situ denitrification rates were conducted in two sandy
Pleistocene aquifers in northern Germany.
The 15N analysis of denitrification-derived 15N-labelled N2
and N2O dissolved in water samples collected during the push–pull
15N tracer tests was performed using isotope ratio mass spectrometry
(IRMS) in the lab and additionally for some tracer tests online in the field
with a quadrupole membrane inlet mass spectrometer (MIMS) in order to test
the feasibility of on-site real-time 15N analysis. Aquifer material
from the same locations and depths as the push–pull injection points was
incubated, and the initial and cumulative denitrification after 1 year of
incubation (Dcum(365)) as well as the stock of reduced compounds (SRC)
was compared with in situ measurements of denitrification. This was done to
derive transfer functions suitable to predict Dcum(365) and SRC from
Dr(in situ).
Dr(in situ) ranged from 0 to 51.5 μg N kg−1 d−1.
Denitrification rates derived from on-site isotope analysis using
MIMS satisfactorily coincided with laboratory
analysis by conventional IRMS, thus proving the
feasibility of in situ analysis. Dr(in situ) was significantly higher in
the sulfidic zone of both aquifers compared to the zone of non-sulfidic
aquifer material. Overall, regressions between the Dcum(365) and SRC of
the tested aquifer material with Dr(in situ) exhibited only a modest
linear correlation for the full data set. However, the predictability of
Dcum(365) and SRC from Dr(in situ) data clearly increased for
aquifer samples from the zone of NO3−-bearing groundwater.
In the NO3−-free aquifer zone, a lag phase of denitrification after
NO3− injections was observed, which confounded the relationship
between reactive compounds and in situ denitrification activity. This
finding was attributed to adaptation processes in the microbial community
after NO3− injections. It was also demonstrated that the microbial
community in the NO3−-free zone just below the
NO3−-bearing zone can be adapted to denitrification by
NO3− injections into wells for an extended period. In situ
denitrification rates were 30 to 65 times higher after pre-conditioning with
NO3−. Results from this study suggest that such pre-conditioning
is crucial for the measurement of Dr(in situ) in deeper aquifer material
from the NO3−-free groundwater zone and thus for the prediction of
Dcum(365) and SRC from Dr(in situ). |
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