Hydrogeological Controls in Sandstone of Ambo Area
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Date
2001-06
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Addis Ababa University
Abstract
The study area is a sub catchment of Abay dra inage basin with a total surface area
and perimeter of 838km2 and 171 km respectively. Two other regional drainage basins
border the catchment: the Awash drainage basin to the east and southeast, and the
Ghibe- Orno drainage basin to the south.
The main rock outcrops in the area are the basaJlic lava now of Tertiary and
Quaternary age, trachytic lava flow and pyroclastic deposits of Quaternary age, and the
Mesozoic sandstone. Application of di ITerent modeling techniques shows that the
sandstone is part of the Upper sandstone sequence.
On annual basis, the area has 1143.3mm, 809mm and 162.8mm of mean total
rainfall , Actu al Evapotranspiration (AET) , and grou ndwater recharge respectively.
There exist various water bodies in the area: two crater lakes (Lake Dandi and Lake
Wanchi with a total surface area of 7.4 and 4km2 respect ively), river, streams, and
spri ngs (both thermal and cold springs).
Hydrogeological descriptions of the lithological units in the area is mainly based on
qualitative interpretation employing various approaches such as degree of weath ering
and fracturing, existence/ or absence of springs, vegetation cover, soil cover, e tc.
Despite the presence of many wells there is only a limited well data. Howeve r, there are
some wells with well data from which some hydraulic parameters have been
determined. The existing well data as well as the qualitative interpretation s how that
the main aquifer in the area are the fractu red and / or weathered basaltic lava flow
VII
followed by alluvial sediments along some streams, and s81ldstone. The first two are
aquifers for cold groundwa ter whereas the sandstone is mainly 811 aquifer of the
thermal/ or mineral water. Some hydraulic parameters have been determined for these
aqu ifers; accordingly, the basaltic lava now has a hydrau lic conductivity and
transmissivity of 3.7 x 10 sm/s to 3.9 x 10 sm/s and 97· 100 m2/day respectively. The
al luvial sediments have a transmissivity of 23.9m2/day on average. Where as, the
sand stone has a t ransmissivity of 76.5m2 / day.
The existence of highly permeable units (in tens ively fractured trachytic lava now and
unwelded tuff) at the elevated lands of Dandi and Wanchi at the southern part of the
study area favors them to be a recharge area. If comparison is made with in the limit of
the study area, good ground wa ter potential with low Total Dissolved Solids (TDS) is
fou nd to the eastern side of Ambo town .
Thermal and/or mineral water in the a rea is stru ctu ral ly controlled ; the al ignment of
the the rmal springs along the nearly east· west oriented fault line in Ambo area C81l
justify the situation. Currently, the thermal springs are limited to a given area, mainly
in Ambo town; but their past extent can be inferred from the travertine deposit in the
area whose thermal origin can be verified from the existence of silica sinter with it.
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Keywords
Sandstone of Ambo Area