Elucidating groundwater responses of shallow basaltic aquifers in the Ethiopian volcanic highlands

Abstract

Globally, basaltic aquifers are crucial for sustaining large populations. In Ethiopia, where volcanic formations predominate, shallow aquifers in basaltic regolith are the most accessible water sources for communities and urban populations, thereby supporting livelihoods. However, these aquifers demonstrate complex hydrogeological characteristics, including significant heterogeneities and compartmentalized flow systems. The lack of comprehensive studies of recharge–storage–discharge processes in basaltic shallow aquifers poses challenges, often leading to overextraction and water scarcity. Here, we examined spatiotemporal groundwater dynamics in basaltic shallow aquifers of the volcanic highlands of Ethiopia by combining recharge–storage–discharge linkages using observations from eight monitoring wells distributed across contrasting litho-structural settings. Lag-time analysis reveals the existence of multiple shallow groundwater recharge mechanisms: rapid recharge within 1–3 days through fractures and lithologic contacts, and delayed recharge persisting up to 44 days driven by slow infiltration into thick basaltic regolith aquifers. Aquifer memory ranged from one day to three months, with longer memory effects observed in weathered/fractured basaltic units constrained by massive volcanic domes. Consistent with these findings, aquifer drainage properties highlight variable recession characteristics: steep recessions characterize fractured aquifers underlain by impermeable basalts, indicating limited storage capacity, whereas gradual recessions occur in weathered/fractured aquifers with extensive regolith and alluvio-lacustrine deposits, reflecting sluggish drainage. These findings highlight the need for integrated, site-specific hydrogeological analyses to elucidate recharge–storage–discharge relationships in basaltic shallow aquifers. Such analyses are essential for addressing aquifer heterogeneity and safeguarding this resource from seasonal climatic variations and anthropogenic influences.