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| Titel |
Fate of nitrate and origin of ammonium during infiltration of treated wastewater investigated through stable isotopes |
| VerfasserIn |
Matthew Silver, Johanna Schlögl, Kay Knöller, Christoph Schüth |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144071
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| Publikation (Nr.) |
 EGU/EGU2017-7852.pdf |
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| Zusammenfassung |
| The EU FP7 project MARSOL addresses water scarcity challenges in arid regions, where
managed aquifer recharge (MAR) is an upcoming technology to recharge depleted aquifers
using alternative water sources. However, a potential impact to water quality is increasing
ammonium concentrations, which are known to be a problem resulting from bank filtration.
In the context of MAR, increasing ammonium concentrations have received little attention so
far.
A soil column experiment was conducted to investigate transformations of nitrogen
species when secondary treated wastewater (TWW) is infiltrated through a natural soil
(organic matter content 5.6%) being considered for MAR. The TWW contains nitrate and
dissolved organic nitrogen (DON), but typically very low (<0.2 mg/L) concentrations of
nitrite and ammonium. In addition to the nitrate and DON in the inflow water, nitrogen in
the soil organic matter is a third possible source for ammonium produced during
infiltration.
The experiment simulated MAR using a series of wetting-drying cycles. At the end of the
wetting phases, pore water samples were collected from six depths. Results show that
the largest decreases in nitrate concentration occur in the upper part of the soil,
with on average 77% attenuated by 15 cm depth and 94% by 30 cm depth. Starting
at 30 cm and continuing downward, ammonium concentrations increased, with
concentrations reaching as high as 4 mg-N/L (the EU drinking water limit is 0.41
mg-N/L).
Selected samples were also measured for stable nitrogen and oxygen isotopes. Nitrate
became isotopically heavier (both N and O) with increasing depth (samples collected at 5 and
15 cm below the soil surface), with most results forming a linear trend for δ18O vs. δ15N.
This pattern is consistent with denitrification, which is also supported by the fact that the
ammonium concentration first increases at a depth below where most of the nitrate is
consumed. However, the relationship between δ15N-NO3− and nitrate concentration is not
clearly logarithmic, so processes other than denitrification are not ruled out for explaining the
fate of nitrate.
The δ15N of ammonium in the water samples and of nitrogen in the soil were also
measured. With increasing depth and time, the δ15N-NH4+ (mean 4.3‰) decreases and
approaches the δ15N of the pre-experimental soil of 2.4‰. This suggests that ammonium is
formed at least in part from the soil organic matter, likely through a combination of leaching
and microbial processes.
Although most nitrate attenuates by 15 cm depth and very little ammonium is observed
here, some nitrate (usually <0.5 mg-N/L) was observed at depths of 30 cm and below,
especially early in the experiments. Starting at 30 cm depth, organic carbon concentrations
and thereby also C:NO3−ratios become high (>10), which are conditions sometimes
found to be favorable to dissimilatory nitrate reduction to ammonium. Rayleigh
enrichment factors also suggest that nitrate may be the source of some of the ammonium.
Measurements of additional samples and organic nitrogen isotopes are planned, in
order to further evaluate the fate of nitrate and the source(s) of the ammonium. |
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