STABLE WATER ISOTOPES ANALYSIS IN INVESTIGATING GROUNDWATER RECHARGE AND FLOW PROCESSES OF BILATE WATERSHED, ETHIOPIA

Abstract

Understanding groundwater recharge and flow processes are very important for well site investigation and development. Environmental isotope tracers have been used as a useful tool in providing new insights into hydrologic processes. The main objective of this study is to understand the groundwater recharge variability, sources, and flow processes in Bilate River Watershed. For this purpose, integrating hydrological modeling and isotopic approach were used. Spatial and temporal recharge variability were studied by using the SWAT model. The long term recharge estimated by SWAT model shows a strong spatial variability and decrement from the highland towards the rift floor due to climate conditions, soil & land use land cover characteristics and topography & geology. On the other hand, water level and stable isotope results (δ18O and δD) from collected samples of rain, surface water and groundwater of the watershed were used to characterize groundwater recharge. The water samples used for the stable isotope analysis (δ18O & δ2H) were measured using Liquid Water Stable Isotopes Analyzer at Isotopes Laboratory of AAU. The local meteoric water line of the watershed is δ2H = 8.79+ 5.65*δ18O and the isotopic compositions of groundwater are mostly distributed close to the line. The δ18O of precipitation ranged between -3.28‰ and -0.97‰, with a mean of -2.26‰ ± 0.2%, the δ18O of shallow well groundwater ranged between -3.52‰ and -2.01‰, with a mean of -2.70‰ ± 0.2% and the δ18O of spring are in the range between -5.66‰ to -1.05‰ with the mean of -3.30‰ ± 0.2%. The δ18O of deep wells ranged between -6.08‰ and 4.43‰, with a mean of -1.11‰ ± 0.2‰. The plot of isotopic compositions of δ18O versus δD shows four recharge clustering zones: recharge from local rainfalls, recharge by higher altitudes, recharge by local rainfall & surface flow and regional deep aquifers recharged. Additionally, the plot shows that rainfall is dominant recharge processes for groundwater over the watershed and a significant sourc