Hydrogeochemical and isotopic signatures of groundwater in the Andasa watershed, Upper Blue Nile basin, Northwestern Ethiopia

Abstract

This study aims to evaluate the hydrogeochemical and isotopic signatures of water (surface and groundwater) of the Andasa watershed in the upper Blue Nile Basin, northwest Ethiopia using integrated hydrogeochemical and isotopic methods with the help of Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) techniques. To achieve this, major cation and anion (n = 61) and stable isotope (δ2H and δ18O; n = 36) analyses have been done. The result revealed that geogenic (rock-water interaction) and anthropogenic (waste disposal landfill site, agricultural practices, diverted river via open canal) effects are the major water quality controlling factors. Rock-water interaction is more dominant factor that produced alkaline earths bicarbonate hydrogeochemical facies. Moreover, the Cl and δ18O relationship shows a shift towards the right from vertical axis (δ18O) which signifies anthropogenic effects. Using HCA and PCA techniques, EC, TDS, Ca, Na, Mg, HCO3, SO4, Cl and NO3 were identified as the leading hydrogeochemical parameters in determining the area. Accordingly, Mg-Ca-HCO3, Ca-Mg-HCO3 and Ca-Na-HCO3 found to be the main water types. Silicate minerals are responsible for controlling the hydrogeochemical property of the waters in the area through hydrolysis and cation exchange processes. Most of the samples signify evaporation prior to recharge in the wet seasons and irrigation return flow in the dry seasons. Most river samples show depleted δ18O and δ2H reflecting that their source is highland area of the watershed. From hydrogeochemical and isotopic signatures, the north and northeast parts of the area are the discharging zones and southern and central parts are the recharging zones.