Editing Hydrogeology of Djibouti

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{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||General Description
+
|Aquifer||General Description
 
|-
 
|-
 
|Wadi (valley) alluvium aquifers
 
|Wadi (valley) alluvium aquifers
||Generally High to Very High (but Variable)
 
 
||Alluvial sediments deposited along the main wadis, forming narrow, elongate aquifers that vary from tens to several hundreds of metres in width and from a few meters to a few tens of meters in thickness  (Jalludin and Razack 2004). Usually unconfined. Jalludin and Razack (2004) quote summary statistics of transmissivity values from 38 constant rate pumping tests from unconsolidated sedimentary aquifers, although it is not known whether these are wadi or alluvial plain aquifers:  the transmissivity values ranged from 38 to 14,150 m<sup>2</sup>/day, with an average of 1490 m<sup>2</sup>/day.
 
||Alluvial sediments deposited along the main wadis, forming narrow, elongate aquifers that vary from tens to several hundreds of metres in width and from a few meters to a few tens of meters in thickness  (Jalludin and Razack 2004). Usually unconfined. Jalludin and Razack (2004) quote summary statistics of transmissivity values from 38 constant rate pumping tests from unconsolidated sedimentary aquifers, although it is not known whether these are wadi or alluvial plain aquifers:  the transmissivity values ranged from 38 to 14,150 m<sup>2</sup>/day, with an average of 1490 m<sup>2</sup>/day.
 
|-
 
|-
 
|Alluvial plain aquifers
 
|Alluvial plain aquifers
||Generally High to Very High (but Variable)
+
||Alluvial plain aquifers, including coastal and inland basin aquifers, cover some 22 % of the country. The extent of individual aquifers varies between 40 km<sup>2</sup> and 1,500 km<sup>2</sup>, and the thickness of the alluvial sediments varies between 40 m and more than 300 m. Jalludin and Razack (2004) quote summary statistics of transmissivity values from 38 constant rate pumping tests from unconsolidated sedimentary aquifers, although it is not known whether these are wadi or alluvial plain aquifers: the transmissivity values ranged from 38 to 14,150 m<sup>2</sup>/day, with an average of 1490 m<sup>2</sup>/day.
||Alluvial plain aquifers, including coastal and inland basin aquifers, cover some 22 % of the country. The extent of individual aquifers varies between 40 km<sup>2</sup> and 1,500 km<sup>2</sup>, and the thickness of the alluvial sediments varies between 40 m and more than 300 m. Jalludin and Razack (2004) quote summary statistics of transmissivity values from 38 constant rate pumping tests from unconsolidated sedimentary aquifers, although it is not known whether these are wadi or alluvial plain aquifers: the transmissivity values ranged from 38 to 14,150 m<sup>2</sup>/day, with an average of 1490 m<sup>2</sup>/day.
 
  
 
Groundwater quality is generally poor, with 1 g/l to more than 2 g/l of total mineralisation (Jalludin and Razack 2004).
 
Groundwater quality is generally poor, with 1 g/l to more than 2 g/l of total mineralisation (Jalludin and Razack 2004).
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{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer type||Named formations||Aquifer Productivity||General Description
+
|Aquifer type||Named formations||General Description
 
|-
 
|-
 
|Regional scale stratiform volcanic aquifers - Quaternary  
 
|Regional scale stratiform volcanic aquifers - Quaternary  
 
||Gulf and stratiform basalts; stratiform rhyolites  
 
||Gulf and stratiform basalts; stratiform rhyolites  
||Generally High to Very High (but Variable)
 
 
||The stratiform basalt series covers an area of about 9000 km<sup>2</sup> in the southwestern and northwestern regions of the country, extending into Ethiopia, and can reach 1300 m thick. Groundwater flow is dominantly through fractures. Transmissivity values from 26 constant rate pumping tests ranged from 13 to 27,100 m<sup>2</sup>/day, with an average of 5350 m<sup>2</sup>/day; the average storage coefficient was 2 × 10<sup>−5</sup> (Jalludin and Razack 2004). Based on only 3 constant rate pumping tests, stratiform rhyolites show higher transmissivity values, from 4660 to 46,600 m<sup>2</sup>/day (Jalludin and Razack 2004).  
 
||The stratiform basalt series covers an area of about 9000 km<sup>2</sup> in the southwestern and northwestern regions of the country, extending into Ethiopia, and can reach 1300 m thick. Groundwater flow is dominantly through fractures. Transmissivity values from 26 constant rate pumping tests ranged from 13 to 27,100 m<sup>2</sup>/day, with an average of 5350 m<sup>2</sup>/day; the average storage coefficient was 2 × 10<sup>−5</sup> (Jalludin and Razack 2004). Based on only 3 constant rate pumping tests, stratiform rhyolites show higher transmissivity values, from 4660 to 46,600 m<sup>2</sup>/day (Jalludin and Razack 2004).  
  
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|-
 
|-
 
|Local scale volcanic aquifers, with minor interbedded volcaniclastic, alluvial sands, silts and clays, sometimes calcareous - Tertiary  
 
|Local scale volcanic aquifers, with minor interbedded volcaniclastic, alluvial sands, silts and clays, sometimes calcareous - Tertiary  
||Dalha and Somali basalts; Mabla rhyolites; Adolei basalts
+
||Dalha and Somali basalts; Mabla rhyolites; Adolei basalts.
||Generally Moderate to Very High (but Variable)
 
 
||Volcanic aquifers of limited extent (area <2000 km<sup>2</sup>), including basalt traps (series of individual lava flows several metres thick, which in total can reach >200 m thick) (Jalludin and Razack 2004). Groundwater flow is dominantly through fractures.  
 
||Volcanic aquifers of limited extent (area <2000 km<sup>2</sup>), including basalt traps (series of individual lava flows several metres thick, which in total can reach >200 m thick) (Jalludin and Razack 2004). Groundwater flow is dominantly through fractures.  
  
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|}
 
|}
  
====Consolidated Sedimentary - Intergranular and Fracture Flow====
+
====Consolidated sedimentary====
  
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||General Description
+
|Aquifer||General Description
 
|-
 
|-
 
|Jurassic-Cretaceous sandstones
 
|Jurassic-Cretaceous sandstones
||Probably Moderate to High
 
 
||These rocks have only a small outcrop in the southeast of the country, and little is known of their aquifer properties. Jalludin and Razack (2004) quote a single transmissivity value from a constant rate pumping test of 1800 m<sup>2</sup>/day.
 
||These rocks have only a small outcrop in the southeast of the country, and little is known of their aquifer properties. Jalludin and Razack (2004) quote a single transmissivity value from a constant rate pumping test of 1800 m<sup>2</sup>/day.
 
|}
 
|}

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