Water resources are a major concern in dry or semi-dry countries. They are crucial to the development of human, economic and social activities [Ragab et al., 2002b;Williams., 1999;Misra., 2014;Wang et al. 2017]. Algeria has adopted a new policy to protect and safeguard water resources [Negm et al., 2020;Qadir et al., 2007], as population and urbanization increases, industrial units and farm land have resulted in the degradation of groundwater and surface water quality, combined with a very significant decline in the groundwater resources, the only water reserves for the supply of populations.It is critical to understand the groundwater quality because it is a key mark of its suitability for human consumption, agriculture, and/or industry. In recent years, there has been a worldwide increasing concern for water quality. Several studies [Khan et al., 2020; Rao et al., 2012;Babiker et al., 2007] were conducted to assess the quality of groundwater.Groundwater chemistry has been used as an instrument for evaluating the quality of water for human use and irrigation. [Srinivasamoorthy
Problem statement: The underground waters in the oriental regions of the Algerian Sahara, present real chemical quality problems. Their content in fluorides always exceeds the limit of the recommended levels. That is 0.8 mg L-1, according to the maximum temperature in the region. Combined to a high salinity, it affects the health of the population living around the region. The present work, deals with the presence of fluoride and the geochemical origin in the Complex Terminal aquifer of Ouargla, rarely examined in the Algerian Sahara. Approach: Is based on the following aspects: Sampling and physico-chemicals analysis of water, statistical treatment of the physico-chemical water parameters and simulation to natural and isotherm 25°C evaporation of water parameters. Results: The results show the presence of fluoride in the studied water. The rates vary between 1 and 2 mg L-1. The calculation of water saturation index in relation with the preponderant minerals, using the thermodynamic model phreeqci reveals a sensitivity of carbonate minerals towards precipitation and dissolution of evaporitic minerals and clayey fluorides as well. Conclusions/Recommendations: The increasing alkalinity of water in contact with the aquifer during long periods of stay decreases the chemical activity of calcium and helps with alteration of clayey minerals and fluoride as a necessary condition for a possible fluorite mineralization. Knowing the origin of that fluoridation leads to possible solutions, through the optimization of a water treatment meets the standards
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