Assessment On Hydraulic Properties Of The Ethiopian Tarmaber Formations

The Ethiopian Taber formation represents, Oligocene – Miocene basaltic shield volcanic on thernnorthwestern and southeastern plateaus covering an area of about 47,194Km2 (which is 8% of the totalrnflood basalt aerial coverage in the country). Two types of Taber formations have been mapped. ThernTaber Megahertz formation (Ntb) with an absolute age of (16 – 13 Ma, Kamikaze, 1979) is transitionalrnto alkali basalt which covers an area of about 22,058Km2 (which is 3.4% of the flood basalts (or traps)rnaerial coverage in the country). This formation commonly outcrops in the central highland plateau, thernescarpments and the rift floor and at some localities in the south western highland plateau part of therncountry (sees Fig.-2.1 & 2.3). The Taber Guess formation (PNtb), with an absolute age of (26 –rn16Ma, Kamikaze, 1979) is alkaline to transitional basalt, often ridge and cliff forming shield volcanoesrnwith minor trachea and phonologist flows. It covers a total area of about 25,136Km2 (which is 4.5% of thernflood basalt aerial coverage in the country). This formation dominantly outcrops in northwesternrnhighland plateau part of the country (see fig.-2.1). The Taber shield volcanoes become progressivelyrnyounger to southeastern and south western part of the northwestern Ethiopian plateaus. Well-log datarnand pump test database analyses show that, the Taber formations aquifer system can be categorized asrnconsolidated fractured aquifer category where the dominant aquifer types are, confined, doublernporosity fractured aquifer system and single plane vertical aquifer systems. The double porosity aquifersrnare related to deeply drilled wells reflecting presence of large and narrow fracture systems with highrnpermeability but lower storage capacity. It also shows that, Taber Megahertz formation (Ntb) hasrnbetter aquifer productivity than the Taber Guess formation (PNtb) and yet, both formations showrndecrease aquifer productivity with respect to increased drilled boreholes depth and increased age of thernformation. Besides, boreholes drilled within the Taber formations shows that, the wells have highrnwell loss coefficient values, indicating improper well site location, improper well design and constructionrnfactors and well yield deterioration with time due to clogging, corrosion and incrustation activities ofrnthe well screens. Spatially, the Ethiopian Tarmaber formations show an increasing aquifer productivityrntrend from the highland plateau areas towards the escarpment and the rift floor areas, and yet, fromrnthe north, south, northwest, southwest, northeast, southeast and east- west directions of the highlandrnplateau areas toward the Lake Tana basin areas. Generally, the Taber basalt aquifer productivity isrnhighly controlled by the location and morphological setup of the formation outcrop, nature andrndegree of weathering, hydro thermal processes and nature, extent, frequency and orientation of thernassociated structural features and yet, weathering, hydro thermal processes and other volcanic activitiesrntend to decrease aquifer permeability while, fracturing, faulting and other tectonic activities tend tornincrease aquifer productivity of the Taber formations.

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