Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia |
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| Authors: | Tewodros Alemayehu Albrecht Leis Anton Eisenhauer Martin Dietzel |
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| Institution: | aInstitute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, A-8010 Graz, Austria;bDepartment of Earth Sciences, Mekelle University, P.O. Box 231, Mekelle, Ethiopia;cInstitute for Water, Energy and Sustainability, Joanneum Research, Elisabethstrasse 16 II, A-8010 Graz, Austria;dLeibniz Institute of Marine Sciences, IFM-GEOMAR, D-24148 Kiel, Germany |
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| Abstract: | 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 Ca2+, Mg2+, Na+ and Si(OH)4 compared to the shallow type.The δ2H and δ18O 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 δ13CDIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO2 from local soil horizons, whereas δ13CDIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO2, δ13CDIC = +1‰ of the deep groundwater with highest PCO2 = 10−0.9 atm yields δ13CCO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO2 from soil and magmatic origin. The uptake of magmatic CO2 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 CO2 from the solution leading to secondary calcite formation. |
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| Keywords: | Axum Carbon dioxide Groundwater Silicate weathering Stable carbon isotopes Strontium isotopes |
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