GROUNDWATER POTENTIAL AND WELL YIELD DISCREPANCIES: HYDROGEOLOGICAL CONTROLS, DRILLING CHALLENGES, AND AQUIFER PARAMETER–RESISTIVITY RELATIONSHIPS IN UPPER BILATE RIVER BASIN, MAIN ETHIOPIAN RIFT VALLEY, SOUTHERN ETHIOPIA

Abstract

Discrepancies in well yields within the Upper Bilate River Basin (UBRB) of the Ethiopian Rift Valley Lake Basin highlight the intricate hydrogeology of its volcanic aquifers and Quaternary deposits. This study examines the influence of drilling-challenges such as partial penetration, well loss coefficient, wellbore storage on yield variations, which may surpass the effects of natural hydrogeological variability. By integrating data from meteorological, hydrological, remote sensing, vertical electrical sounding, and pumping tests with historical well records, key aquifer parameters like transmissivity and hydraulic conductivity are quantified, while empirical relationships between aquifer productivity and resistivity are established. This study's water balance analysis for the Upper Bilate River Basin reveals a semi-humid system with a 254.6 mm annual surplus. A significant wet-season surplus facilitates groundwater recharge, estimated at 58.9 mm/year (6% of rainfall), indicating moderate infiltration and strong surface water-groundwater interaction A hydrogeological framework and groundwater potential zone map, generated through weighted overlay analysis of ten thematic layers, categorized the basin into excellent (1.39%), very good (17.9%), good (79.17%), and low (1.54%) potential zones. Most wells align with high-potential zones, confirming predictive accuracy: 74% of Wells are in very good zones and 26% in good zones, with none in low-potential areas. Transmissivity (T) in the study area varies from 0.05 to 841.10 m²/day, indicating a heterogeneous aquifer system. Moderate to moderately high transmissivity zones (59.77–89.93 m²/day) dominate, covering nearly 60% of the area, mainly in the central and northern parts, suggesting good aquifer productivity. Geophysical investigations identify Layer 6 (highly weathered and fractured pyroclastic rocks) as the most promising aquifer, followed by Layers 5 and 4, while upper shallow layers function as aquitards. A strong correlation between transmissivity and transverse resistance (r = 0.83, p = 1.32×10⁻¹³) supports the integration of geophysical and pumping test data for aquifer assessment. Well yield discrepancies in the UBRB are influenced more by drilling challenges than by aquifer natural heterogeneity. An analysis of 220 Wells indicates a partial penetration ratio (L/b) of 0.13 to 0.96, with a mean of 0.56, suggesting moderately penetrating wells. Wellbore storage shows that 49% of wells have high storage (Cw ≥ 0.9), while 27% have low storage (Cw < 0.1), reflecting variable aquifer connectivity. In a study of 25 wells, loss coefficients (C) range from 4.0×10⁻⁷ to 5.0×10⁻⁵ day²/m⁵, with 72% classified as severely clogged and none being properly developed. Well efficiency varies between 11.2% to 100% (mean 70.3%), with 18% rated Poor, 22% Fair, 30% Good, and 30% Excellent. This highlights the need for better well design, development, and maintenance practices. The correlation between transmissivity and well efficiency demonstrates that aquifer transmissivity primarily governs well performance, with high-transmissivity zones hosting the most efficient wells. In contrast, low efficiency in moderately transmissive areas mainly stems from technical issues—such as improper well design, partial penetration, or excessive wellbore storage rather than aquifer limitations. Enhancing well construction and maintenance practices is therefore crucial to fully realize groundwater potential in these zones.