A multi-isotopic approach to evaluate the origin of Fluoride in groundwater in the Main Ethiopian Rift Valley

n the Main Ethiopian Rift Valley (MER) high levels of fluoride (F-) in drinking water supply, commonly associated to the geologic substratum, are recognized as one of the major public health problems in most rural villages, where the majority of population exhibit various degrees of fluorosis symptoms. To better define the origin and dynamic of this contamination, a multisotopic investigation (δ18O and δ2H of H2O, δ15N and δ18O of NO3, 87Sr/86Sr, δ11B and 3H) has been implemented within the frame of the H2O20 FLOWERED Project (http://www.floweredproject.org) for a portion of the MER, complemented by a conventional hydrochemical study. Beside the geochemical-statistical estimation of the fluoride background concentration in groundwater based on literature data, two field surveys were carried out. The target area was divided in three physiographic zones: the rift, the transitional escarpments and the plateau. Specifically, the escarpment and the low-lying rift are mainly covered by Late Pliocene and Quaternary volcanic rocks, and both lacustrine and alluvial sediments. Aquifer systems are often confined or semiconfined in the fractured and/or weathered volcanic formations and phreatic in the sediments covering parts of the rift floor. The sampling was performed along both transversal (from the plateau to the Rift floor) and intra-rift (along the Rift floor) transects within the main aquifer formations with the purpose of highlighting the difference between groundwater from Rift-parallel flow and escarpment-to-Rift flow. Groundwater evolves from Ca–HCO3 to Na–HCO3 type in the plateau and in the Rift floor respectively. In general, most of the groundwater fall on the local meteoric water line, indicating a meteoric origin. Deep boreholes waters, both located in the Rift floor and in the escarpments, showed long residence time although, for the former zone, shorter residence times were detected especially for the water interacting with lake waters. Higher concentrations of F−, above the permissible limit for drinking water, were detected along the Rift floor zone. Notwithstanding, high values were detected also in correspondence of the plateau zone but just for the thermal waters. Fluoride origin was found to be not exclusively related to geologic sources, since a strong relationship with organic and inorganic pollution sources, attributable to both untreated sewage and synthetic fertilizer, was identified. The results inferred through the use of a hidrochemical and multi-isotopic approach represent an appropriate tool for differentiating geogenic and anthropogenic sources of fluoride contamination and correctly address protection and remediation strategies of water resources