Fluoride-bearing groundwater in the complex terminal aquifer (a case study in Hassi Messaoud area, southern Algeria): hydrochemical characterization and spatial distribution assessed by indicator kriging

According to the World Health Organisation (WHO), the human consumption of water containing a high concentration of fluoride (> 1.5 mg/l) can increase significantly many health problems such as dental and skeletal fluorosis. This study investigates fluoride abundance, origin, and its spatial distribution in groundwater from Complex Terminal (CT) aquifer in Hassi Messaoud area (Southern Algeria) where the CT water constitutes the main source of drinking water with a high daily intake. Available water wells were sampled and analysed on their major physico-chemical parameters including fluoride content. Hydrochemical characterization was constrained using the Durov diagram together with a PCA statistical treatment. The saturation indices were computed and used to track fluoride origin. The spatial distribution of fluoride in the studied aquifer was mapped by indicator kriging (IK). The results show that fluoride content ranges from 1.6 to 2.9 mg/l (average = 2.1 ± 0.4 mg/l) exceeding WHO drinking water standards. The Durov diagram, PCA and SI indicate that water acquires mineralization principally by leaching of evaporite minerals. Furthermore, SI evidences that fluoride concentration in water increases with CaCO₃ precipitation leading to CaF₂ dissolution. IK spatial distribution allows estimating the probability of not exceeding (2.1 mg/l) critical threshold. The cross-validation displays good performance of IK estimation (mean error = 0.05; mean standard error = 0.09). The obtained map shows a low to moderate probability of not exceeding the selected threshold in the whole aquifer. Therefore, the use of CT water for human consumption poses a risk to public health. These results can be used in water management framework and for selecting an ideal position to drill new boreholes for drinking water.