Variation in hydraulic structure with respect to depth and age of a large igneous province in Ethiopia

Abstract

Depth-wise variation in hydraulic structures of volcanic aquifers is rarely investigated when compared to basement and other rocks. A comprehensive dataset is presented here on the hydraulic properties of volcanic aquifers from a large igneous province in Africa. Age- and depth-wise variation in transmissivity (T), yield (Q), hydraulic conductivity (K) and specific capacity (S_c) of volcanic aquifers and water wells were systematically examined for stratigraphic units of various ages (Eocene to Quaternary Period) and well depth (18–882 m). The T ranges from 0.02 to 9,830 m²/day. There is a good correlation between the age of the emplacement of the rocks and their hydraulic properties. The oldest (Eocene) basalts show lower productivity (T, K and Q) when compared to the youngest (Quaternary) basalts. There is no statistically significant depth-wise variation of T, K and Q when one single formation is investigated. The insights gained from the analysis show that increasing the depth of drilling does not necessarily increase aquifer yields and can inform global-scale groundwater modelling efforts. The data challenge the widely held assumption that K and Q decrease with depth. Unlike basement rocks, volcanic rocks show no statistically significant change in hydraulic properties along its depth profile to the depth of 900 m.