Growing lake with growing problems: integrated hydrogeological investigation on Lake Beseka, Ethiopia

The complex tectonic and volcanic processes in the Ethiopian rift valley have resulted in the formation of volcano-tectonic structural depressions that became sites for many rift valley lakes. Lake Beseka is one of the rift valley lakes in the northern section of the MER near to the Afar triangle. The lake plays an important role in the ecology of birds and wildlife, as it is located in the northern part of the Awash National Park. Lake Beseka has been expanding at an astounding rate since the late 1960s and early 1970s. This growth has had a detrimental effect on the surrounding physical, hydrological and infra-structural environment. This study is conducted with primary objective of understanding the hydraulic interaction of Lake Beseka with the surrounding groundwater system, and identifying and quantifying the role of groundwater in the hydrology of the lake. Integrated approaches of hydrochemistry, isotope hydrology, recharge estimation (water table fluctuation, EARTH modeling and chloride mass balance), and groundwater modeling (MODFLOW) are applied for hydrogeological characterization of the study area. The flow system in the watershed of Lake Beseka is analyzed by coupling a groundwater level map with the hydrochemical and isotopic composition of the water bodies. Hierarchical multi-element cluster analysis (HCA) is used to classify hydrochemical water samples of the study area into different groups. The lake is characterized by a Na-HCO3-Cl type of water, and has a hydrochemical signature similar to that of the groundwater system flowing from the western part of the watershed. The groundwater system in the western part of the watershed and hot springs that emerge at the western edge of the lake are characterized by a Na-HCO3 type of water with an average isotopic composition of -2.8 ‰ in δO and -10.7 ‰ in δH, which is comparable to the isotopic concentration of the input signal. The input signal has an isotopic value of -3 ‰ in δO and -9.3 ‰ in δH, and is derived from the intersection point of the local meteoric water line of Addis Ababa rainfall and the local evaporation line, which plots along evaporated waters of Lake Beseka. Hydrochemical and isotopic evidence indicates that groundwater flows from the western part of the watershed and discharges to the lake in the form of hot springs. This groundwater inflow constitutes the major water inflow to Lake Beseka and forms an integral part of its water budget. This fact is well supported by the groundwater modeling results, which estimate that 51% of the total water inflow to Lake Beseka comes from groundwater seepage to the lake. This seepage is computed by the model to be 33.8Mm annually. It is evident from the recharge estimation that recharge of the groundwater system comes from infiltration of local precipitation in the watershed. However, this recharge is estimated to be only 17.4 Mm annually. Thus, the groundwater recharge that seasonally replenishes the aquifer system within the lake watershed is not significant enough to explain the expansion of Lake Beseka. The model estimated that 30.45Mm of groundwater laterally flows to the aquifer system of the lake watershed annually across the surface boundaries. This lateral inflow might be related to recharge from the mountains. It is most likely that the expansion of Lake Beseka is related to changes in the amount of this groundwater, which first flows to the lake watershed and then to Lake Beseka. Owing to the geologic setting of the study area, tectonically induced modification of hydraulic gradient of the groundwater regime in the region might have resulted in an increase in the discharge of the hot springs that continuously flow to the lake.