Assessment of Surface and Groundwater Interaction Using Environmental Isotope Tracers in Fiche, Central Ethiopia

Abstract:

Environmental stable isotopes of oxygen (δ ¹⁸O) and hydrogen (δ ²H) are natural conservative tracers and provide information on water source, flux and quantitative estimate of surface and groundwater interaction. Clear understanding of surface and groundwater interaction and establishing seasonal water pathway is vital in nutrient and contaminant tracing studies. To examine the link between river and groundwater system and their response to local precipitation events, we examined their respective isotopic compositions. We collected a total of 88 daily precipitation, weekly river and weekly groundwater samples during the main rainy season from Fiche, a small catchment in the Blue Nile Basin, central Ethiopian highland. Here we present the intra seasonal variation of δ ¹⁸O, δ ²H and d-excess in summer precipitation, showing a large range of daily δ ¹⁸O (-12.6‰ to 3.7‰), δ ²H (-83.3‰ to 45‰) and d-excess (9.8‰ to 23.7‰). The river and groundwater δ ¹⁸O, δ ²H and d-excess values show a relatively narrow range with average of -3.1‰, -9.4‰ and 15.0‰, respectively. The isotopic similarity between river and groundwater suggest high degree of surface and groundwater interactions. Comparison of the groundwater and summer precipitation isotope data from this study (δ²H = 8.4δ¹⁸O + 19.8) as well as the closest long-term precipitation isotope data from Global Network of Isotopes Precipitation (GNIP) station ( δ²H = 8.0δ¹⁸O + 15.0) alluded that summer precipitation is the main source of groundwater recharge. Furthermore, the high precipitation and groundwater d-excess values (>10‰) suggest the importance of continental recycled moisture as source of rain for the central Ethiopian highland during the main rainy season. Thus, as surface and groundwater are the two main important component of water resource in the region, clear understanding of their interconnection and link to local precipitation is critical in sustainable water resource management.