Editing Hydrogeology of Ethiopia

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|Alluvio-lacustrine sediments
 
|Alluvio-lacustrine sediments
||Variable productivity, but can be highly productive in places
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||Can be highly productive
 
||These sediments have highly variable permeability.  Fine sand deposits have the highest permeability, with some boreholes providing more than 10 l/s with minimal drawdown. Transmissivities range up to 700 m²/day and specific yields are of the order of 3.2 l/s/m. In several places higher transmissivities have been noted. For example, a 150 m deep borehole in alluvio-lacustrine deposits at the foot of the southern plateau has a transmissivity of 3012 m²/day. These aquifers can be both unconfined and confined; they vary in thickness from 0 to 400 metres; water table depth is typically in the range 1 to 60 metres; and the typical borehole depth is 100 m.
 
||These sediments have highly variable permeability.  Fine sand deposits have the highest permeability, with some boreholes providing more than 10 l/s with minimal drawdown. Transmissivities range up to 700 m²/day and specific yields are of the order of 3.2 l/s/m. In several places higher transmissivities have been noted. For example, a 150 m deep borehole in alluvio-lacustrine deposits at the foot of the southern plateau has a transmissivity of 3012 m²/day. These aquifers can be both unconfined and confined; they vary in thickness from 0 to 400 metres; water table depth is typically in the range 1 to 60 metres; and the typical borehole depth is 100 m.
  
 
Fine-grained sands interbedded with massive volcanic tuffs and fine ash are known to have low productivity in many places (e.g. in the central Ethiopian Rift). In the eastern part of the country the total thickness of these sediments can reach about 300 m. In most of the outcrops, they consist of conglomerates, sandstone and mudstone, which are gypsiferous and locally bear saline groundwater.
 
Fine-grained sands interbedded with massive volcanic tuffs and fine ash are known to have low productivity in many places (e.g. in the central Ethiopian Rift). In the eastern part of the country the total thickness of these sediments can reach about 300 m. In most of the outcrops, they consist of conglomerates, sandstone and mudstone, which are gypsiferous and locally bear saline groundwater.
 
||Variable salinity
 
||Variable salinity
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|Quaternary Alluvial Aquifers within Lake Tana basin
 
|Quaternary Alluvial Aquifers within Lake Tana basin
||Moderate to High Productivity
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||Moderate to high productivity
 
||These occur dominantly in the eastern part of the basin following the lower Rib and lower margin of Gumera and Fogera plain (East of Lake Tana). They also cover a significant area of the north part of the basin at the lower part of the Megech and Western shore of Lake Tana. However, the distribution of alluvial sediments is limited compared to the volcanic aquifers. The deposits vary in thickness from 1 to 400m. The aquifers can be unconfined or confined. Water table depth is typically in the range 1 to 60 metres, and typical borehole depth is 100 m.
 
||These occur dominantly in the eastern part of the basin following the lower Rib and lower margin of Gumera and Fogera plain (East of Lake Tana). They also cover a significant area of the north part of the basin at the lower part of the Megech and Western shore of Lake Tana. However, the distribution of alluvial sediments is limited compared to the volcanic aquifers. The deposits vary in thickness from 1 to 400m. The aquifers can be unconfined or confined. Water table depth is typically in the range 1 to 60 metres, and typical borehole depth is 100 m.
  
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|Wadi bed aquifers
 
|Wadi bed aquifers
||Moderate to High Productivity
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||Although localised, these intergranular aquifers have significant groundwater potential in water scarce arid and desert settings. Wadi bed length exceeds 30 000 km across Ethiopia, and total groundwater storage in these aquifers could be as much as 3 billion cubic metres. The most important wadi aquifers, which support the livelihoods of millions of people living a pastoral lifestyle, include those in Borena, Lower Omo, Ogaden, the Western Lowlands bordering Sudan, and in the Afar depression. The most productive wadi bed aquifers are those dominated by sandy and gravelly sediments with a low proportion of clay.  
 
||Although localised, these intergranular aquifers have significant groundwater potential in water scarce arid and desert settings. Wadi bed length exceeds 30 000 km across Ethiopia, and total groundwater storage in these aquifers could be as much as 3 billion cubic metres. The most important wadi aquifers, which support the livelihoods of millions of people living a pastoral lifestyle, include those in Borena, Lower Omo, Ogaden, the Western Lowlands bordering Sudan, and in the Afar depression. The most productive wadi bed aquifers are those dominated by sandy and gravelly sediments with a low proportion of clay.  
  
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|Talus slope, landslide bodies, alluvial terraces
 
|Talus slope, landslide bodies, alluvial terraces
||Moderate Productivity
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||These deposits form small outcrops 0.5 to 2 km² in area. Permeability is enhanced in areas where the material is loosely packed. Groundwater from mountain areas flows towards the talus slope and landslide deposits, from where springs commonly emerge. Typical spring yields are 1 to 2 l/s. These aquifers with readily available groundwater discharges support several villages in areas of relatively gentle slopes and good soil development. Recharged from groundwater flowing from higher elevation.
 
||These deposits form small outcrops 0.5 to 2 km² in area. Permeability is enhanced in areas where the material is loosely packed. Groundwater from mountain areas flows towards the talus slope and landslide deposits, from where springs commonly emerge. Typical spring yields are 1 to 2 l/s. These aquifers with readily available groundwater discharges support several villages in areas of relatively gentle slopes and good soil development. Recharged from groundwater flowing from higher elevation.
 
||Variable salinity
 
||Variable salinity
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|Rift volcanics
 
|Rift volcanics
||Moderate Productivity  
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||Generally Low Productivity  
||Borehole yields generally from 1 to 5 l/s.  In some conditions the aquifer is confined, leading to artesian conditions. Both direct rainfall and indirect (eg from river beds) recharge is common.
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||Generally low borehole yields, from 1 to 5 l/s.  In some conditions the aquifer is confined, leading to artesian conditions. Both direct rainfall and indirect (eg from river beds) recharge is common.
 
||High fluoride and salinity, often exceeding WHO limits.
 
||High fluoride and salinity, often exceeding WHO limits.
 
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|Upper basalt aquifer (Aiba, Alaji and Termaber formations)
 
|Upper basalt aquifer (Aiba, Alaji and Termaber formations)
||High Productivity
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||High to Very High Productivity
 
||This aquifer forms the most productive of the volcanic aquifers in Ethiopia. It is typically unconfined to semi-confined and often artesian.  Groundwater discharge occurs to wetlands and to springs on cliffs. The aquifer thickness ranges from 50 to 1000 m. The depth to water table varies from 0 to 250 metres. Borehole depths are typically from 100 to 150 m. Borehole yields are generally up to 20 l/s.  
 
||This aquifer forms the most productive of the volcanic aquifers in Ethiopia. It is typically unconfined to semi-confined and often artesian.  Groundwater discharge occurs to wetlands and to springs on cliffs. The aquifer thickness ranges from 50 to 1000 m. The depth to water table varies from 0 to 250 metres. Borehole depths are typically from 100 to 150 m. Borehole yields are generally up to 20 l/s.  
  
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|Lower basalt aquifer (Ashangie Formation)
 
|Lower basalt aquifer (Ashangie Formation)
||Low to High Productivity
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||Low to Very High Productivity
 
||These rocks are characterised by rugged topography with dissected and irregular morphology. The rocks are deformed, and in their northern section dip at up to 40°. They are thinly bedded, and in several areas are brecciated. Field evidence shows that the brecciated parts are characterized by lower permeability. The rocks are typically deeply weathered, when they are reddish in colour, but generally have low permeability. Both primary (vesicle) porosity and secondary (fracture) porosity have been modified and reduced by secondary mineralisation (e.g. by calcite, zeolite and silica).  
 
||These rocks are characterised by rugged topography with dissected and irregular morphology. The rocks are deformed, and in their northern section dip at up to 40°. They are thinly bedded, and in several areas are brecciated. Field evidence shows that the brecciated parts are characterized by lower permeability. The rocks are typically deeply weathered, when they are reddish in colour, but generally have low permeability. Both primary (vesicle) porosity and secondary (fracture) porosity have been modified and reduced by secondary mineralisation (e.g. by calcite, zeolite and silica).  
  
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|Hamanile, Gabredarre and Antalo formations (Jurassic limestones)
 
|Hamanile, Gabredarre and Antalo formations (Jurassic limestones)
||High Productivity
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|| The '''Gabredarre Formation''' is characterised by karst features, including caves. The limestones of the Sofomar caves region have the highest degree of karstification of Ethiopia's carbonate rocks. The aquifer has moderate permeability and productivity.   
 
|| The '''Gabredarre Formation''' is characterised by karst features, including caves. The limestones of the Sofomar caves region have the highest degree of karstification of Ethiopia's carbonate rocks. The aquifer has moderate permeability and productivity.   
  
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|Adigrat Formation (Jurassic sandstone)
 
|Adigrat Formation (Jurassic sandstone)
||Moderate Productivity
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||The highly cemented '''Adigrat Formation''' has low primary porosity, and the top part has been altered by heating by Cenozoic volcanism.  Fracturing has created secondary porosity and permeability.  The emergence of springs at the contact of the Adigrat sandstone and the overlying volcanic rocks is indicative of the low permeability of the Adigrat Formation.
 
||The highly cemented '''Adigrat Formation''' has low primary porosity, and the top part has been altered by heating by Cenozoic volcanism.  Fracturing has created secondary porosity and permeability.  The emergence of springs at the contact of the Adigrat sandstone and the overlying volcanic rocks is indicative of the low permeability of the Adigrat Formation.
  
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Recharge varies from 10 to 250 mm/yr depending on rainfall regime  
 
Recharge varies from 10 to 250 mm/yr depending on rainfall regime  
 
||Good quality  
 
||Good quality  
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