Natural controls validation for handling elevated fluoride concentrations in extraction activated Tóthian groundwater flow systems: San Luis Potosí, Mexico

Abstract

Fluoride concentration in groundwater supply above the guideline value of 1.5 mg/L is a health hazard for the population living in two thirds of the Mexican territory. Enhanced groundwater extraction in the city of San Luis Potosí (SLP), Mexico, led to a substantial territorial increase in water with high fluoride (F−) which originates from thermal water–rock interaction with regional rhyolites. Previous knowledge of the Tóthian groundwater flow systems around SLP City and their F− concentrations from 1987 data provided an insight into natural F− controls for the construction and operation of boreholes. During the period 1987–2007, the number of new boreholes increased as well as the relocation of boreholes whose production diminished. Overall estimated extraction augmented from 2.6 to 4.1 m3/s. Results obtained for 2007 suggest that F− controls defined for 1987 data (e.g., variable portions of F−-rich deep thermal water in borehole yields) are also valid in newly constructed boreholes. Water authority actions related to groundwater extraction lack consideration of proposed F− controls, so constructed boreholes progressively tapping the high F− groundwater flow system resulted in a 85% increase in the F− affected territory (> 2 mg/L) between 1987 and 2007. Reduction in F− extraction following the proposed natural control mechanisms (e.g., fluorite precipitation) was also confirmed. Applying geochemical and mineralogical analysis, rhyolites surrounding the SLP graben basin and contributing to its volcano-clastic sedimentary filling were identified as the primary F− source for elevated concentrations in groundwater of the area under investigation.