Browsing by Author "Mekonta, Loemessa"

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    Hydrogeological Controls in Sandstone of Ambo Area
    (Addis Ababa University, 2001-06) Mekonta, Loemessa; Alemayehu, Tamiru (PhD)
    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.