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| Titel |
Magnetic resonance soundings as a tool to assess past and present groundwater recharge pattern in the Lake Chad basin |
| VerfasserIn |
M. Boucher, G. Favreau, A. B. Moussa, M. Ibrahim, Marc Descloitres |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068151
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| Zusammenfassung |
| Determining past and present surface water - groundwater interactions is challenging in flat
sedimentary basins where small variations in the water balance may result in large changes in
river courses or lake surface area. In semiarid sub-Saharan Africa, Lake Chad is an endorheic
lake that remains fresh because of most of the lake water infiltrates to the aquifer. Large
changes in the Lake surface area occurred during the Holocene and had an impact
on aquifer recharge, yet little is known on past and present pattern of the aquifer
hydrodynamics. Magnetic Resonance Sounding (MRS) is a method that allows non invasive
determination of the free water content and hydraulic properties of aquifers. Two
complementary MRS field surveys were conducted in the semiarid northwestern
part of Lake Chad Basin in 2008 (11 soundings) and 2010 (12 soundings). High
MRS water contents (10-35 %) and aquifer permeability values (10-3 to 10-2 m/s)
were shown below present-day Lake Chad. At distance, below the former clayey
deposits of the Megalake Chad, lower MRS water content (8-13 %) and permeability
(10-4 to 10-3 m/s) values were estimated for the unconfined aquifer. These low
porosity and permeability values suggest limited aquifer fluxes, in accordance with
the occurrence of isotopically determined (O18/D) palaeo-groundwaters below
the clayey plain. Intermediate and homogenous hydraulic properties (MRS water
content and permeability values in the ranges of 16 to 25% and 1.10-3 to 3.10-3
m/s, respectively) were found in the Komadougou River valley, a seasonal river
known by piezometric surveys to represent the main source of recharge for the
aquifer. In the Lake Chad sedimentary basin, MRS revealed a distribution of aquifer
hydrodynamics properties that depends on the distance to surface waters, and proved to be a
decisive tool to quantify past and present surface water / groundwater exchanges. |
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