Application of hydrochemical and isotopic techniques to understand groundwater recharge and flow systems in the Dawa River basin, southern Ethiopia

Abstract

The study of groundwater recharge and of flow systems is crucial to understand availability and sustainability of groundwater resources. This study uses hierarchical cluster analysis (HCA) and various graphical plots of hydrochemical and isotopic data to examine groundwater recharge and the dynamics of flow system in the Dawa River basin. HCA has classified the water samples into five distinctive clusters. Clusters I, II, and III represent the volcanic and most parts of the basement terrain. These clusters are distinguished by low EC, high percentages of HCO3− and Ca2+, Mg2+, or Na+, and dominantly (Ca, Mg, Na, K)–HCO3-type water. Cluster IV contains sulfate-type water with high percentage of Ca2+ + Mg2+ and represents sedimentary terrain. Cluster V, characterized by high EC and abundant Na+ and SO42− + Cl−, is sited at few locations along dry riverbeds. In the basement terrain, the chemical composition of groundwater varies greatly over short distances. In most parts of the basin, groundwater contains elevated levels of tritium at amount comparable to local rainfall. These chemical characteristics supported with tritium data indicate the dominance of groundwater of local flow systems, short residence time, and modern recharge in the basin. Stable isotope data indicate that in the semi-arid region, recharge occurs from high-intensity rainfall. Difference in δ18O and δ2H between the northern and the southern and southeastern groundwater supports distinct recharge sources and the absence of regional groundwater flow between the two regions. Converging evidences reveal that the traditional regional groundwater flow model which is common in most large river basins of Ethiopia does not hold true in the Dawa River basin.