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 catch::lent of Abay drainage biCsin wi th a total surface area
and ?erimeter of 838km2 and 171km respectively. Two , :her regional drainage basins
border the catchment: the Awash drainage basin to th e east and southeast, and the
Ghib"- Omo drainage basin to (, e south.
The :nain rock outcrops in L0.e area are the basalt:: lava flow of Tertiary and
Quaternary age, trachytic lava [ ow and pyroclastic depose,s of Quaternary age . and the
Mesozoic sandstone. Application of different modeling techniques shows that the
sandstone is part of the Upper 5aIldstone sequence.
On annual basis, the area has 1143.3mm, 809mm 2..c'ld 162.8mm of mean total
rainfall, Actual Evapotranspiration (AET), and grounr:water recharge respectively.
There exist various water bodies in the area: two crate~ lakes (Lake Dandi and Lake
Wanchi with a total surface area of 7.4 and 4km2 re s~ectively), river , streams, and
springs (both thermal and cold springs) .
Hydrogeological descriptions of the lithological units in the area IS mainly based on
qualitative interpretation emplo,ing various approaches such as degree of weathering
and fracturing, existence/or absence of springs, vege:ation cover, soil co,'er, etc.
Despite the presence of many wells there is only a limiter: well data. However , there are
some wells with well data tom which some hydraulic parameters ha,'e been
determined. The existing well data as well as the qualiE tive interpretation show that
the main aquifer in the area are the fractured andj or weathered basaltic lava flowfoHowed by alluvial sediments along some streams, and sandstone. The first two are
aquifers for cold groundwater whereas the sandstone is mainly an aquifer of the
thermal/or mineral water. Some hydraulic parameters have been determined for these
aquifers; accordingly, the basaltic lava flow has a hydraulic conductivity and
transmissivity of 3.7 x lO·sm/s to 3.9 x 10sm/s and 97- 100 m2/day respectively. The
alluvial sediments have a transmissivity of 23.9m2/day on average. Where as, the
sa:1dstone has a transmissivity of 76.5m2/day.
The existence of highly permeable units (intensively fractured trachytic lava flow and
u nwelded tuf~ at the elevated lands of Dandi and Wan chi 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 water potential with low Total Dissolved Solids (TDS) is
found to the eastern side of Ambo town.
Thermal and/or mineral water in the area is structurally controlled; the alignment of
the thermal springs along the nearly east- west oriented fault line in Ambo area can
justify the situation. Currently, the thermal springs are limited to a given area, mainly
in Ambo tov-n; 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
Hydrogeological Controls