Identifying the structure of a volcanic aquifer and estimating its hydraulic properties: A case of Wama catchment in the Upper Blue Nile Basin, Western Oromia, Ethiopia

This study is aimed to delineate and determine its hydraulic properties in Wama catchment with drainage area of 3,390 km², in the Upper Blue Nile basin, Western Oromia, Ethiopia. The tertiary volcanic rocks are the predominant geologic units, which make up 83.36% of the region. The main tools utilized for data processing and interpretation include ArcGIS 10.4, Global mapper 23.0, Surfer 20, Strater 5, AQTEVSOLV Pro.4.5, and Microsoft Excell. Aquifers’ structures were manually delineated with Surfer via 19 boreholes and 13 shallow wells using lithologic units whereas hydraulic properties were estimated from constant pumping test data of 1 shallow well and 17 boreholes using AQTEVSOLV Pro based on a single well test approach by clustering into western, northern, north eastern, central and southern regions. Due to variations in the deposition of geologic units, the study area's aquifer structure's thickness varies both vertically and laterally. Materials like sand, gravel, scoria, and fractured and weathered volcanic rocks were considered as good aquifer whereas clay, pyroclastics, and massive basalts are aquitards. Semi-confined aquifer type dominates majority of the catchment except the central region which is confined aquifer based on available lithologic units, position of water level and pumping test data using AQTEVSOLV Pro. Transmissivity (T) of western, north eastern and central region varies from 0.94-64 m²/d, 22.4-60 m²/d and 23.4-30 m²/d respectively, indicating intermediate aquifer potential for extraction of local water supply (dominant). In northern region, transmissivity (T) varies from 12.3-827 m²/d, implying high aquifer potential for withdrawal of regional importance. Specific capacity (SC) of western, northern, north eastern and central regions ranges from 1.15-157 m²/d, 16-632 m²/d, 41-78 m²/d, and 4 - 43 m²/d respectively. Transmissivity of southern part is 10.4 m²/d (potential for local water supply). In this study, the correlation of T and SC is about 97% indicates direct relationship. Therefore, the higher transmissivity value shows the aquifer is supplying adequate water towards the well across aquifer thickness and the higher specific capacity shows the well has good efficiency for water extraction. 

Key words: Aquifer structure; hydraulic properties; Wama catchment; volcanic rocks.