Integration of geochemical and isotopic tracers for elucidating water sources and salinization of shallow aquifers in the sub-Saharan Drâa Basin,Morocco |
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| Institution: | 1. Division of Earth & Ocean Sciences, Duke University, Durham, NC, USA;2. Applied Geology and Geo-Environment Laboratory, Ibn Zohr University, Agadir, Morocco;1. Division of Earth and Ocean Sciences, Nicholas School of Environment, Duke University, Durham, NC 27708, USA;2. Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, 56124, Pisa, Italy;3. Dipartimento di Scienze della Terra e dell''Ambiente, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy;4. School of Geography and the Environment, Oxford University, Oxford OX11 9QA, UK;1. Laboratory of Applied Geosciences, Faculty of Sciences, University Mohammed 1, Oujda, Morocco;2. University Cadi Ayyad, Faculty of Sciences Semlalia, Laboratory of GEOHYD, Marrakech, Morocco;3. Ministry of Energy, Mines, Water and Environment, Rabat, Morocco;4. Director of Mohamed VI Museum for the Civilization of Water in Morocco, Ministry of Habous and Islamic Affairs, Marrakech, Morocco;1. Antek S.A., Geosciences Technology Center, Calle 25B No 85B-54, Bogotá, Colombia;2. Hocol S.A., Fluid Management Team, Floor 16th, Carrera 7 No 114-07, Bogotá, Colombia;1. Università di Genova, DISTAV, Genoa, Italy;2. Laboratory of Hydrogeology-Environment, Faculty of Sciences, University Mohamed 1st, Oujda, Morocco;3. Institut Scientifique, Université Mohammed V, Rabat, 10106, Morocco;1. Marine Geoscience and Soil Science Laboratory (URAC-45), Earth Sciences Department, University of Chouaïb Doukkali, Faculty of Sciences, El Jadida, Morocco;2. I2M UMR 5295 Laboratory, Environmental Civil Engineering Department, Bordeaux 1 University, Bordeaux France;3. Water and environment laboratory, University of Chouaïb Doukkali, Faculty of Sciences, El Jadida, Morocco;1. College of Construction Engineering, Jilin University, Changchun 130021, China;2. Department of Municipal and Environmental Engineering, Taiyuan College, Taiyuan 030000, China;3. School of Natural Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China;4. Geological Survey of Jiangsu Province, Nanjing 210018, China;5. Shandong Geological Engineering Investigation Institute, Jinan 250014, China |
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| Abstract: | In the arid sub-Saharan of southern Morocco, groundwater salinization poses a direct threat for agricultural production in six oases’ basins that are irrigated by water imported from the High Atlas Mountains. Here the geospatial distribution of salinity is evaluated in shallow groundwater, springs and surface waters in the Drâa Basin, integrating major and trace element geochemistry and isotopic tracers (O, H, Sr and B). The data show that water discharge from the High Atlas Mountains to the Upper section of the Drâa Basin is characterized by both low and high salinity, a distinctive low δ18O and δ2H composition (as low as ?9‰ and ?66‰, respectively), typical for meteoric water from high elevation, a 87Sr/86Sr range of 0.7078–0.7094, and δ11B of 12–17‰. The Ca–Mg–HCO3, Na–Cl–SO4, and Ca–SO4 compositions as well as the Br/Cl, 87Sr/86Sr, and δ11B values of the saline water suggest dissolution of Lower Jurassic carbonates and evaporite rocks in the High Atlas Mountain catchment. Storage and evaporation of the imported water in a man-made open reservoir causes an enrichment of the stable isotope ratios with a δ18O/δ2H slope of <8 but no change in the Sr and B isotope fingerprints. Downstream from the reservoir, large salinity variations were documented in the shallow groundwater from the six Drâa oases, with systematically higher salinity in the three southern oases, up to 12,000 mg/L. The increase of the salinity is systematically associated with a decrease of the Br/Cl ratio, indicating that the main mechanism of groundwater salinization in the shallow aquifers in the Drâa oases is via salt dissolution (gypsum, halite) in the unsaturated zone. Investigation of shallow groundwater that flows to the northern Drâa oases revealed lower salinity (TDS of 500–4225) water that is characterized by depleted 18O and 2H (as low as ?9‰ and ?66‰, respectively) and higher 87Sr/86Sr ratios (～0.7107–0.7115) relative to irrigation water and groundwater flow from the Upper Drâa Basin. This newly-discovered low-saline groundwater with a different isotopic imprint flows from the northeastern Anti-Atlas Jabel Saghro Mountains to the northern oases of the Lower Drâa Basin. This adjacent subsurface flow results in a wide range of Sr isotope ratios in the shallow oases groundwater (0.7084–0.7131) and appears to mitigate salinization in the three northern Drâa oases. In contrast, in the southern oases, the higher salinity suggests that this mitigation is not as affective and increasing salinization through cycles of water irrigation and salt dissolution appears inevitable. |
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