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| Titel |
Groundwater mixing pattern and origin of salinization in the Azraq Oasis, Jordan, revealed by noble gases |
| VerfasserIn |
Tillmann Kaudse, Werner Aeschbach-Hertig, Randa Tuffaha, Refaat Bani-Khalaf |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093285
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| Publikation (Nr.) |
 EGU/EGU2014-7877.pdf |
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| Zusammenfassung |
| Azraq Oasis, located in the eastern Jordanian desert, is an important freshwater resource of
the country. Shallow groundwater reserves are heavily exploited since the 1980s and in
consequence the groundwater table dropped by about 25 m and important wetlands dried out.
Furthermore, some wells of the major well field show an increasing mineralization over the
past 20 years. The fact that only a few wells show this behavior is surprising since the
wells are situated quite close together and are mostly drilled to the same depth. A
previous study using conventional tracers did not yield a satisfactory explanation
[1].
Application of dissolved noble gases reveals the complex mixing pattern leading to the
very localized salinization within the well field. It is found that the wells affected by
salinization 1) contain distinctly more radiogenic 4He than the other wells, indicating higher
groundwater age, and 2) exhibit a significantly enhanced 3He-4He ratio, implying
an influence of deep mantle fluids. Since the hydrogeologic system in the Azraq
Oasis comprises of three aquifer systems, separated by poorly permeable layers and
traversed by several deep fault systems, mantle influence is expected to be found in
the deeper aquifers. The data, therefore, indicate upward leakage into the shallow
aquifer.
However, the saline middle aquifer is virtually free of mantle helium. To our
knowledge, this is the first time a groundwater system is described where mantle helium
is found in an aquifer lying on top of one which is free of mantle impact. This
behavior can be explained by an upstream from an even deeper (and saline) source
through a nearby conductive fault, while the groundwater flow direction in the middle
aquifer is towards the fault and reversed in the shallow aquifer, towards the well
field.
This scheme explains how the mantle fluids (and also most probably the increased
salinity) infiltrates into the shallow aquifer, but not why only few wells are affected. The
shallow aquifer consists of chalky limestone and a far more permeable basalt shield on top.
Because the boreholes of the well field have no casing, water is potentially abstracted from all
depths. Initially, however, by far most water was abstracted from the basalt aquifer due to
the different permeabilities. As the groundwater table dropped, the basalt layer
fell progressively dry and subsequently more water from the deeper part of the
shallow aquifer was incorporated into the well’s discharge – which according to the
presented scheme is affected by salt and mantle fluids. The local depletion depends
strongly on the individual cone of depression around a borehole and, therefore,
can explain the local occurrence of the salinization phenomenon. The admixing of
deep groundwater is further supported by warmer discharge temperatures and other
parameters.
[1] Al-Momani et al. (2006), IAEA TecDoc 1507, 177-211 |
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