Identification of Groundwater Flow Systems and Recharge Sources and Estimation of Groundwater Recharge Using Stable Isotopes (δ18O and δ2H) and Chloride Mass Balance in Mekelle Area, Tigray, Northern Ethiopia

Identification of recharge sources in the Mekelle area is controversial, as some studies indicate that precipitation is the only source of recharge, while others indicate the existence of inter-catchment/inter-basin groundwater transfers with the possibility of intermediate-to-regional flow system recharge sources. The objectives of this study were, therefore, to identify the sources of recharge, quantify the groundwater recharge and develop a local meteoric waterline (LMWL) for the Mekelle area using the stable isotopic compositions (δ18O and δ2H) of rainwater. To that end, 13 precipitation, 34 groundwater, and two surface water samples were analyzed for δ18O and δ2H signatures. Accordingly, the local meteoric water line (LMWL: δ2H = 6.78 δ18O + 9.83) was developed from ten monthly accumulated precipitation samples and three rainfall event samples. Moreover, the spatial variations in the isotopic compositions of the groundwater with respect to the LMWL showed the existence of three zones/groups with different groundwater flow conditions, namely, shallow, intermediate, and deep flow systems. Two groundwater recharge sources were also identified from the dual-isotope slope of the groundwater isotope lines with respect to the LMWL. Furthermore, the groundwater recharge was estimated to be 49 mm/year using the chloride mass balance (CMB) method.