Gis-based Groundwater Vulnerabilitiy Mapping Of Holeta River Catchment Oromia Regional State West Shewa Zone

The studied area, Holeta River Catchment, lies in the central part of therncountry, 28 km west of Addis Ababa in Oromia Regional National State.rnThe area is bounded between Latitude 8o53’23” to 9o13’28’’ North andrnLongitude 38o20’45” to 38o36’48”East. It is a sub-catchment of upperrnAwash drainage basin with a total surface area of 508 km2 and perimeterrn125.54km.It has an elevation range of less than 2060 t0 3380m.a.s.l.rnThe basaltic lava flows (Alaji and Tarmaber-Megezez formation), Trachyticrnand Rhyolitic Volcanic rocks, pyroclastic deposits and quaternaryrnAlluvial sediments are major geologic units in the area.rnOn annual basis, the area has 1094.5mm, 981.3mm, 595.8mm, andrn304.2mm of mean total rainfall, Potential Evapotranspiration (PET),rnActual Evapotranspiration (AET), and groundwater recharge respectively.rnThe mean annual temperature of the area is 16.2oC.rnPopulation density of Holeta River catchment varies from 15.83 personsrnper hectare to 0.52 persons per hectare. The existing land use/land coverrnpatterns in the area are broadly divided into six groups as:rnAgriculture/grass land area (63.95%), forest land (6.46%), bare rockrn(1.09%), urban area (0.69%), bushes/shrubs (26.13%), and wetlandrncover (1.69%) of the total land use cover. The dominant soil types inrnstudy area are; cambisols (67%), vertisols (0.78%), and Alluvilal soilrn(0.78%) of the total soil cover. The soil thickness varies from less thanrn2.1m to greater than 9.1m and the major soil texture classified as; siltyrnclay, clay, and sandy loam respectively.rnStudies shows that, the major aquifer type of the area are: Alluvialrndeposits, buried valley deposits, and weathered and fractured basalts The major possible pollutant sources of groundwater are; Agriculturalrnactivities (both point and non-point sources), municipal/domestic wasterndisposal, sewage and also health centers, cemeteries, quarry mining, andrnmarket places. The groundwater in the area is characterized by mainlyrnCa-Mg-HCO3, Ca-Na-HCO3, and Ca-HCO3. In analyzed samples therndominant cation is Calcium ion and bicarbonate is the dominant anion.rnTDS, EC, and pH values range 50mg/l-648mg/l, 84μs/cm-607.8μs/cm, 6-rn7.99 respectively.rnThe general objective of the study is to identify and map the groundwaterrnvulnerability to pollution in Holeta River Catchment by using a DRASTICrnmodel with the assistance of GIS (Arc-map) software. In general, inrnintrinsic DRASTIC map, the vulnerability of groundwater increases fromrnnorth to south of the catchment. This map reveled that major partrn(northern) of the catchment lies on medium risk area while the southernrntip and central parts of the area are highly/extremely vulnerable tornpollution due to high recharge rate, alluvial and highly fractured aquiferrnmedia, and gentle slope. Low/very low vulnerable area is quite small inrnwestern and eastern parts.rnFrom the specific vulnerability impact map due to population density ofrnthe catchment, it is possible to conclude that the both the urban sitesrn(Holeta and Adis Alem) show high vulnerability index to groundwater duernto their population size. Where as, major parts of the study area are veryrnlow vulnerable to pollution with respect to population density.rnFinally, as this work is conducted under serious constraints of resources,rnI recommend further detail research work to fill the knowledge gap andrnprovide detailed data for future researchers.

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