Résumé
•Hydrochemical profiles, including fluoride, were run in a crystalline aquifer.•Conductivity profiles enabled to deduce F concentration from I.S.E. measurements.•Variations of F are restricted to the weathered/fractured layer of the aquifer.•Fluoride gradient depends on the fracture/fissure connectivity.•Profiles reveal an F-accumulation by water–rock interactions and fertilizers inputs.
Hydrochemical borehole-loggings with a submersible Idronaut Ocean Seven 302 multiparameter probe equipped of F- and NO3-ion-selective electrodes in combination with EC, pH and dissolved oxygen, were applied for characterizing fluoride (F) contamination in a crystalline (hard-rock) aquifer of a small Indian agricultural watershed where groundwater is intensively abstracted for rice irrigation. A high accuracy of F concentrations determined in-situ—shown by comparing with laboratory analyses—was obtained through using conductivity logs for ionic strength consideration. Large variations in chemical composition and particularly of F-concentration were observed inside boreholes, though restricted to the weathered/fractured layer down to 30–35m depth. This conforms to the hydrogeological model of a crystalline aquifer where most groundwater flow occurs in the shallow part of the fractured zone. The general trend of increasing F content with depth results from F accumulation through water–rock interaction, but the shape of the F profile depends on the connectivity of the fracture network close to the borehole. The concentrations seen within the water-table fluctuation zone locally suggest F input from fertilizers in groundwater, in addition to the earlier-demonstrated role of evaporation from irrigation return flow. It is also likely that, locally, the deepening of boreholes has contributed to increasing the population’s vulnerability by tapping F-enriched groundwater at depth.