Recharge Source and Groundwater Dynamics in the Southern Sections of the Western Afar Rift Margin and Associated Rift Floor
No Thumbnail Available
Date
2025-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
This study employs a multidisciplinary approach integrating geological, hydrogeological, hydro-meteorological, hydrochemical, and isotopic analyses, supported by GIS and remote sensing tools, along with steady-state groundwater flow modeling to assess groundwater dynamics in the study area. Lithostratigraphic correlations indicate that the columnar-jointed lower basalt exhibits a continuous extension from the plateau to the rift margin; however, it is not present on the rift floor. Transverse structures trending NW-SE, NE-SW, and W-E connect the plateau, Western Afar rift margin, and rift floor. Hydrochemical analyses reveal that most of the groundwater samples from the plateau and northern rift escarpment are predominantly Ca-(Mg)-HCO3-type waters with low total dissolved solids (TDS). The stable isotope compositions plot near and along the Local Meteoric Water Line (LMWL). Integrating the hydrochemical and isotope results, these water groups represent early-stage geochemical evolution with limited water-rock interaction and have been recharged primarily from modern precipitation and local groundwater flow systems. Highland springs along escarpments have comparable water types, relatively depleted isotope signatures, and a high yield with the continuous flow from small catchments demonstrating that the influx of groundwater from the adjacent plateau through transverse structures supplemented by local precipitation is the main source of recharge. The deep groundwater from the north western plateau and western Afar rift margin have Na-Ca-HCO3 and Na-HCO3 water type and depleted isotope content but with a moderate increase of TDS towards the groundwater flow directions. In the rift floor, the groundwater from the shallow aquifers exhibits Ca-HCO3 water type, isotopically enriched, and low radon values, indicating evaporation-affected local recharge with minimal water-rock interaction. In contrast, the deep groundwater source is Na-HCO₃/SO₄2-/Cl- water type, isotopically depleted, with high radon and mineralization distinct from shallow systems, indicating separate recharge sources and mechanisms. Integrating the geological, lithostratigraphic correlations, hydrochemical and isotope analysis; deep circulating groundwater flow from the plateau area is the main source of recharge for the deep aquifer systems of the Western Afar rift margin and associated rift floor. The main controlling mechanisms are the columnar jointed lower basalt and transverse structures that connect the plateau to the rift margin, and finally to the rift floor which can be taken as preferential pathways. Hence, the plateau area serves as the primary source of groundwater recharge for the deep aquifers for both the rift margin and rift floor. Hydro meteorological study quantifies groundwater recharge by employing Water balance, WetSpass modeling, Chloride Mass Balance, and Baseflow separation controlled by radon measurement. Accordingly, the estimation of groundwater recharge using the WetSpass and Base-flow separation methods highlights that the rift margin has a significantly higher recharge rate than the rift floor. Similarly, the groundwater potential map delineated through GIS and RS techniques shows that
the rift floor has a very low to low groundwater potential than the rift margin. However, from the geological, hydrochemical, and isotope analysis, groundwater flows from the rift margin to the rift floor due to the presence of preferential pathways. Hence, the rift floor can have moderate groundwater recharge, leads moderate groundwater potential. In addition, the GIS and RS tools have shown that structures and geology are the determinants of thematic layers for the occurrence of groundwater both in the rift floor and rift margin. Steady-state groundwater flow modeling in the Jewuha–Robit catchment confirms substantial plateau-derived groundwater influx. Sensitivity analysis identifies hydraulic conductivity as the most influential parameter, while abstraction has minimal impact on flow dynamics.
Description
Keywords
Afar Depression, Transverse Structures, Columnar Jointed Basalt, Groundwater Dynamics