干旱区生态环境知识资源中心

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 Potosi (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 Tothian 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 m(3)/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.