Productivity, Water Wells Functionality and Hydraulic Structures of Selected Volcanic Aquifers in Ethiopia
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Date
2024-03
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Addis Ababa University
Abstract
The volcanic aquifers of Ethiopia play a vital role in providing groundwater used for
domestic, industrial and agricultural water supply purposes in both urban and rural areas.
However, in such volcanic aquifers, research indicated that declining water levels due to
over exploitation decreased natural recharge and climatic variability and failures of water
wells within a short life span after construction. Therefore, the objective of this dissertation
is to investigate volcanic aquifers’ variation in hydraulic structures, classification and
productivity based on a large data set of hydraulic parameters systematically examined for
the age (Eocene to Quaternary) and depth (18 - 882m). This work also addresses the use of
geospatial technique to estimate large-scale groundwater flow velocity (Vgw) and 222Rn
isotope in estimating Vgw, 222Rn recovery, water residence time and testing the applicability
of 222Rn technique in post-construction hand pump wells (HPWs) functionality monitoring.
The results revealed that there is a good correlation between the ages of the emplacement
of rock with hydraulic properties and there is no depth-wise variation in aquifer
productivity. The oldest (Eocene) basalts show lower productivity as compared to the
youngest (Quaternary) basalts. The insights gained from the analysis show that increasing
the depth of drilling does not necessarily increase aquifer yields and can inform the globalscale
groundwater flow modelling efforts. The groundwater flow velocity estimation (Vgw)
estimated from shallow volcanic aquifers reveals the characteristics of volcanic aquifers
productivity and flow dynamics. The values of Vgw estimated by the geospatial and 222Rn
methods show similar trends in all study sites where the high values were observed in
highly fractured and weathered basaltic aquifers, while the lowest values were in the
slightly weathered, less fractured basalt, trachyte and pyroclastic deposits. But, the Vgw
estimated by the geospatial technique (0 and 195 m/day) is higher than Vgw obtained from
222Rn isotope (0.37 and 69.1 cm/day) indicating the lower value in the 222Rn technique is
due to the effects of aquifer conditions and borehole efficiency effects. The HPWs have been
classified into four functionality classes: fully functional (FF), Unreliable (UR), low yield
(LY) and unreliable and low yield (UR&LY). FF HPWs show a high 222Rn recovery revealing
a quick through flow of groundwater into the wells, healthy functioning of boreholes
without screen clogging effects and a high permeability of the aquifer material in the
vicinity of the well. Whereas, UR&LY HPWs show the lowest 222Rn recovery revealing a
slow flow of water in the well owing to low permeability, declining water level, screen
clogging, poor initial design and high water stagnation in the boreholes. The substantial
difference in 222Rn recovery between the four categories reveals the applicability of a
naturally occurring 222Rn isotope as a promising independent tool to monitor postconstruction
changes in the performance of HPWs without the need for dismantling the
HPWs for inspection.
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Keywords
222rn Technique, Geospatial Technique, Productivity, Water Wells Functionality, Age and Depth Wise, Aquifer Properties, Aquifer Classes, Volcanic Aquifers, Ethiopian Highlands