Multi-proxy approach ( 2H/H, 18O/ 16O, 13C/ 12C and 87Sr/ 86Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia

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Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l −1 and in concentrations of Ca 2+, Mg 2+, Na + and Si(OH) 4 compared to the shallow type. The δ 2H and δ 18O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ 13C DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO 2 from local soil horizons, whereas δ 13C DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO 2, δ 13C DIC = +1‰ of the deep groundwater with highest PCO 2 = 10 −0.9 atm yields δ 13C CO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO 2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO 2 from soil and magmatic origin. The uptake of magmatic CO 2 results in elevated cations and Si(OH) 4 concentrations. Weathering of local basalts is documented by 87Sr/ 86Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO 2 from the solution leading to secondary calcite formation.