Gis-Based Groundwater Vulnerabilitiy Mapping of Holeta River Catchment, Oromia Regional State, West Shewa Zone

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Addis Ababa Universty


The studied area, Holeta River Catchment, lies in the central part of the country, 28 km west of Addis Ababa in Oromia Regional National State. The area is bounded between Latitude 8o53’23” to 9o13’28’’ North and Longitude 38o20’45” to 38o36’48”East. It is a sub-catchment of upper Awash drainage basin with a total surface area of 508 km2 and perimeter 125.54km.It has an elevation range of less than 2060 t0 3380m.a.s.l. The basaltic lava flows (Alaji and Tarmaber-Megezez formation), Trachytic and Rhyolitic Volcanic rocks, pyroclastic deposits and quaternary Alluvial sediments are major geologic units in the area. On annual basis, the area has 1094.5mm, 981.3mm, 595.8mm, and 304.2mm of mean total rainfall, Potential Evapotranspiration (PET), Actual Evapotranspiration (AET), and groundwater recharge respectively. The mean annual temperature of the area is 16.2oC. Population density of Holeta River catchment varies from 15.83 persons per hectare to 0.52 persons per hectare. The existing land use/land cover patterns in the area are broadly divided into six groups as: Agriculture/grass land area (63.95%), forest land (6.46%), bare rock (1.09%), urban area (0.69%), bushes/shrubs (26.13%), and wetland cover (1.69%) of the total land use cover. The dominant soil types in study area are; cambisols (67%), vertisols (0.78%), and Alluvilal soil (0.78%) of the total soil cover. The soil thickness varies from less than 2.1m to greater than 9.1m and the major soil texture classified as; silty clay, clay, and sandy loam respectively. Studies shows that, the major aquifer type of the area are: Alluvial deposits, buried valley deposits, and weathered and fractured basalts The major possible pollutant sources of groundwater are; Agricultural activities (both point and non-point sources), municipal/domestic waste disposal, sewage and also health centers, cemeteries, quarry mining, and market places. The groundwater in the area is characterized by mainly Ca-Mg-HCO3, Ca-Na-HCO3, and Ca-HCO3. In analyzed samples the dominant cation is Calcium ion and bicarbonate is the dominant anion. TDS, EC, and pH values range 50mg/l-648mg/l, 84μs/cm-607.8μs/cm, 6- 7.99 respectively. The general objective of the study is to identify and map the groundwater vulnerability to pollution in Holeta River Catchment by using a DRASTIC model with the assistance of GIS (Arc-map) software. In general, in intrinsic DRASTIC map, the vulnerability of groundwater increases from north to south of the catchment. This map reveled that major part (northern) of the catchment lies on medium risk area while the southern tip and central parts of the area are highly/extremely vulnerable to pollution due to high recharge rate, alluvial and highly fractured aquifer media, and gentle slope. Low/very low vulnerable area is quite small in western and eastern parts. From the specific vulnerability impact map due to population density of the catchment, it is possible to conclude that the both the urban sites (Holeta and Adis Alem) show high vulnerability index to groundwater due to their population size. Where as, major parts of the study area are very low vulnerable to pollution with respect to population density. Finally, as this work is conducted under serious constraints of resources, I recommend further detail research work to fill the knowledge gap and provide detailed data for future researchers.



Vulnerabilitiy Mapping