Hydrochemical and Isotopic Characteristics of the Groundwater System of Alleydege Plain
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Date
2011-06
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Addis Ababa Universty
Abstract
On the bases of geomorphology which is controlled by en echelon normal faults, the area is
classified into two parts; the Alleydege plain, and Awash River valley. The groundwater
dynamics of the area with emphasis given to surface water and groundwater interaction,
origin of recharge; groundwater flow direction, groundwater evolutionary trend and direction,
causes and origin of salinity and general groundwater quality issues relative to temporal
variation are studied in this research using hydrochemical and isotopic data. Ultimately
conceptual hydrochemical groundwater model of the area is constructed.
XLSTAT _Pro version 7.5.2; for agglomerative cluster analysis, Aquachem 4.0 software
package; for graphical plots and representation of hydrochemical data, PHREEQC built in
aquachem 4.0 version; for inverse geochemical modeling, ArcGIS _9.3 software version; for
Analysis of areal distribution and variation of hydrochemical and isotopic data and for
production of different maps and Autocad-2007 version; for production of longitudinal
geologic and hydrogeologic profiles, are employed in this study.
Hydrochemical and isotopic studies reveal that there is interaction of Awash River with the
groundwater of the alluvial aquifer of the Awash River valley and the Alleydege plain that lie
within few kilometer distances from the Awash River during rainy season; during dry season
such interaction is not observed. Recent meteoric water contribution to the groundwater
reserve of the Alleydege plain is very limited. As a result subsurface inflow or percolation
along the slope from southern, southeastern, and eastern highlands are found to be the major
recharge mechanism for the Alleydege plain. Towards the rift, from the recharge areas of
southern, southeastern, and eastern highlands; the ionic concentration, salinity and isotopic
enrichment (δ2H and δ18O) increase following the groundwater flow paths. In general the
groundwater flow and evolutionary trend direction follows the surface topography flowing
from the south, southeastern and eastern elevated areas to the Alleydege plain and finally to
the rift floor converging to the Awash River in the northern direction. The recharge area is
characterized by Ca-HCO3, Ca-Mg-HCO3 or Ca-Na-HCO3 water type, where as the
Alleydege plain and the marginal areas are characterized by a Na-Ca-HCO3-Cl water types.
Na-HCO3-Cl and Na-HCO3 are the dominant water types in the Awash River valley and rift
floor. K-mica, olivine, pyroxene, plagioclase and illite are the primary minerals of the
Alleydege plain groundwater system.
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Keywords
Alleydege Plain