 |
| Titel |
Disentangling dissolved oxygen sources in shallow riparian groundwater by
stable isotope analysis |
| VerfasserIn |
Michael Mader, David Porst, Christian Schmidt, Robert van Geldern, Johannes Barth |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152269
|
| Publikation (Nr.) |
 EGU/EGU2017-17087.pdf |
|
|
|
|
|
| Zusammenfassung |
| Dissolved oxygen (DO) is one of the strongest oxidation agents in aquatic environments.
Besides gas-water-exchange, mixing and mineral oxidation, it is a key player in
fundamental biogeochemical processes such as respiration and photosynthesis. These
processes also systematically influence stable isotope ratios of DO and of dissolved
inorganic carbon (DIC). Simultaneous measurements of DO and DIC concentrations
in conjunction with their stable isotope ratios (δ18ODO and δ13CDIC) can thus
provide useful tools to quantify oxygen and carbon sources and sinks in natural
waters.
This study focused on the Selke River in the Harz Mountains (Germany) with steep DO
gradients between the stream water and the shallow, adjacent groundwater and associated
stable isotope shifts. δ13CDIC values decreased from -13 ‰ to -18 ‰ versus the Vienna Pee
Dee Belemnite standard (VPDB) from May to November 2016 and indicated the dominant
influence of microbial respiration on the observed DO gradients. With such respiration
dominance, we have expected a simultaneous enrichment of δ18ODOto values higher than the
one of atmospheric O2 (+23.9 ‰ versus Vienna Standard Mean Ocean Water standard -
VSMOW). However, our measurements revealed anomalously low δ18ODO values between
+22 ‰ and +18 ‰ versus VSMOW for the same time period. These δ18ODO values
were lower than those found in the river. Latter were close to equilibrium with
the atmosphere (24.9 ‰ versus VSMOW). The observed δ18ODO ratios in the
shallow groundwater can be explained with DO from the river that is subject to
fractionation by microbial respiration with a typical fractionation factor (αr) of 0.995.
In addition, mass balances revealed that this oxygen pool receives contributions
of up to 25 % by diffused oxygen from the vadose zone. Consequently, isotope
shifts by respiration and admixture with surface water are masked by diffusion
effects that result in a decoupling of carbon and oxygen isotope systematics in
the near river subsurface environment. They also demonstrate that DO in shallow
groundwaters has additional sources than admixture from surface waters alone. |
|
|
|
|
|
|