Hydrogeochemical mechanisms governing the mineralization and elevated fluoride $\left( {\text{F}^{ - } } \right)$ contents in Precambrian crystalline aquifer groundwater in central Benin, Western Africa

Abstract

In the central part of Benin (Western Africa), high fluoride $\left( {{\text{F}}^{ - } } \right)$ contents have been reported in groundwater from Precambrian crystalline bedrock aquifer which is the main source of drinking water. The hydrogeochemical mechanisms leading to such elevated fluoride concentrations are usually not fully understood. In this context, the objective is to identify the hydrogeochemical processes governing groundwater mineralization and the origin of the high fluoride concentrations. A dataset of 162 groundwater samples was collected from the aquifer consisting of a thin altered bedrock layer (shallow aquifer) and a deep fractured crystalline bedrock (deep aquifer). Geochemical approaches and multivariate statistics have been used to explore the data. Fluoride concentrations vary between 0.00 and 7.19 mg/L in groundwater. Samples collected in the southern part of the investigated area, close to Dassa-Zoumé, show the highest concentrations, with more than 75% greater than the guideline value of 1.5 mg/L. The deep fractured aquifer shows higher concentrations of fluoride than the shallow regolith reservoir. Results reveal that groundwater mineralization is derived mainly from the hydrolysis of silicate minerals, but it is also influenced by anthropogenic effects, particularly in the shallow reservoir. However, fluoride has a geogenic origin, primarily from the weathering of silicate minerals, primarily biotite. ${\text{Ca}}^{2 + }/{\text{Na}}^{ + }$ cation exchanges and ${\text{F}}^{ - }/{\text{OH}}^{ - }$ anion exchanges between groundwater and the rock matrix also occur as minor processes. Saturation of the water with respect to calcite and the precipitation of this mineral, which reduces calcium activity, also favor the release of fluoride from rocks.