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| Titel |
Chemical and U–Sr isotopic variations in stream and source waters of the Strengbach watershed (Vosges mountains, France) |
| VerfasserIn |
M. C. Pierret, P. Stille, J. Prunier, D. Viville, F. Chabaux |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 10 ; Nr. 18, no. 10 (2014-10-07), S.3969-3985 |
| Datensatznummer |
250120491
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| Publikation (Nr.) |
 copernicus.org/hess-18-3969-2014.pdf |
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| Zusammenfassung |
This is the first comprehensive study dealing with major and trace element
data as well as 87Sr/86Sr isotope and (234U/238U)
activity ratios (AR) determined on the totality of springs and brooks of the
Strengbach catchment. It shows that the small and more or less monolithic
catchment drains different sources and streamlets with very different
isotopic and geochemical signatures. Different parameters control the
diversity of the source characteristics. Of importance is especially the
hydrothermal overprint of the granitic bedrock, which was stronger for the
granite from the northern slope; also significant are the different meteoric
alteration processes of the bedrock causing the formation of 0.5 to 9 m
thick saprolite and above the formation of an up to 1m thick soil system.
These processes mainly account for springs and brooks from the northern slope
having higher Ca / Na, Mg / Na, and Sr / Na ratios, but lower
87Sr/86Sr isotopic ratios than those from the southern slope. The
chemical compositions of the source waters in the Strengbach catchment are
only to a small extent the result of alteration of primary bedrock minerals,
and rather reflect dissolution/precipitation processes of secondary mineral
phases like clay minerals.
The (234U/238U) AR, however, are decoupled from the
87Sr/86Sr isotope system, and reflect to some extent the level of
altitude of the source and, thus, the degree of alteration of the bedrock.
The sources emerging at high altitudes have circulated through already
weathered materials (saprolite and fractured bedrock depleted in 234U),
implying (234U/238U) AR below 1, which is uncommon for surface
waters. Preferential flow paths along constant fractures in the bedrocks
might explain the – over time – homogeneous U AR of the different spring
waters. However, the geochemical and isotopic variations of stream waters at
the outlet of the catchment are controlled by variable contributions of
different springs, depending on the hydrological conditions.
It appears that the (234U/238U) AR are a very appropriate,
important tracer for studying and deciphering the contribution of the
different source fluxes at the catchment scale, because this unique
geochemical parameter is different for each individual spring and at the same
time remains unchanged for each of the springs with changing discharge and
fluctuating hydrological conditions. This study further highlights the
important impact of different and independent water pathways on fractured
granite controlling the different geochemical and isotopic signatures of the
waters. Despite the fact that soils and vegetation cover have a great
influence on the water cycle balance (evapotranspiration, drainage, runoff),
the chemical compositions of waters are strongly modified by processes
occurring in deep saprolite and bedrock rather than in soils along the
specific water pathways. |
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