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| Titel |
Isotopes as tracers of groundwater circulation in a peat land |
| VerfasserIn |
Philippe Négrel, Catherine Guerrot, Romain Millot, Emmanuelle Pételet-Giraud |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109246
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| Publikation (Nr.) |
 EGU/EGU2015-9139.pdf |
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| Zusammenfassung |
| Like all wetlands ecosystems, the bio-diversity of peatlands is strongly fragile as it is
requiring very specific wet conditions. In the past 20 years, increasing efforts were engaged
to restore degraded wetlands, to create new wetlands and to manage wetlands sustainably in
order to assess their multiple benefits. Engaging specific actions to restore and preserve
wetlands require an important knowledge on the water cycle in such systems. In this study,
we use chemical and multi-isotopic approaches to trace the fluxes of water and
dissolved element in a peatland and for the drainage network. Aims are to evaluate the
origin of dissolved elements and possible anthropogenic impacts through Li, Sr, O
and H isotope systematics. This approach clearly demonstrated its effectiveness
for improving our understanding of the hydrological functioning of this wetland
ecosystem.
Water circulation and mixing were traced using strontium and lithium isotopes. The use
of Ca/Na ratios coupled with Sr isotopes constraint the end-members and mixing
calculations support that in the site, Sr-isotope signatures of water could be explained
solely by mixing Sr from water/rock interactions (the rainwater after run-off, basalts
and carbonate weathering) without involving any other process. As no carbonate
outcrops occur in the area, the carbonate end-member is allocated to the use of
Ca-amendment. Lithium contents in the water fluctuate significantly and δ7Li are extremely
variable reaching values up to +1226oṪhis extremely enriched 7Li signature of the
groundwaters was explained by an external input due to Ca-amendment, used in local
agriculture. The main results are that at least three strong groundwater fluxes with distinct
chemical and isotopic signatures supply water to the peatland and water volume
flowing out is almost negligible as low δ7Li are observed in the stream draining the
area.
We therefore applied B isotopes to complement the investigation of δ7Li. The δ11B values
increase between rivers draining basalts (around 0) and the springs bordering the peatland
(δ11B > 25). Peatland groundwaters have intermediate δ11B values in the range 7.8 to 19.4o
range accompanied by an increase in the Ca contents between the river draining basalts and
water in the peatland. Regarding the δ11B versus chloride as conservative element, the
isotope ratio decrease with no variation in the Cl content from rain water to river draining
basalts (as indicator of water rock interaction), then increase with slight increase in the Cl
content in the groundwater in the peatland and end with large increase in δ11B and Cl
contents in the springs (as indicators of fertilizer inputs). Cross interpretation of δ11B and
δ7Li confirms the role of water rock interaction and present day fertilizer inputs in the river
draining basalts and in the springs bordering the peatland. However, B isotopes
do not support the extremely enriched 7Li signature of the groundwaters in the
peatland as representative of an input due to present day Ca-amendment used in
local agriculture. Thus this study opened a new field for B-Li isotope investigations
in hydro-systems through their capability to trace present vs. day-past activities. |
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