IMPORTANCE OF THE VERTICAL COMPONENT OF GROUNDWATER-FLOW - A HYDROGEOCHEMICAL APPROACH IN THE VALLEY OF SAN-LUIS-POTOSI, MEXICO

Fractured volcanics exert a control on groundwater flow in the San Lui s Potosi (SLP) valley. The chemical composition and temperature of wat er pumped from boreholes partially penetrating the fractured volcanics indicate that the produced water originates from an upward vertical f low. Most of the thermal groundwater has been detected in areas relate d to regional faults and lineaments. Intensive and uncontrolled pumpin g from the upper 1/4 of the aquifer (total depth > 1500 m) causes the rise of water from a deep regional flow system that mixes with the sha llower waters. The deep waters contain high fluoride concentrations th at contaminate the mixture and cause substantial health related effect s. The recharge controls on the regional flow system require further r esearch; however, hydrogeochemical evidence supports the view that the origin of this recharge is limited to the western bounding Sierra Mad re Occidental. Higher levels of dissolved Na+, Li+, F- (and SO4-2) der ived from Tertiary volcanics have been introduced into the exploited r egion; the concentrations indicate lengthy and deep circulation flow. Li+ concentration was used as an indicator of groundwater residence ti me, and therefore of the length of the groundwater flow path. Hydrogeo chemical interpretation indicates the presence of three flow systems: a shallow local one controlled by a clay layer that subcrops most of t he valley floor, an intermediate system in which water infiltrates jus t beyond the boundary of the clay layer, and a deep regional system wh ich originates outside the surface catchment. The local and intermedia te systems circulate through materials with comparatively low hydrauli c conductivity. Low Cl- concentrations suggest rapid flow in the regio nal system. Concentrations of Li+ and F- can be used to calculate perc entages of waters in mixtures of regional and intermediate flows. Conc entrations of Na+, Ca2+ and SO4-2 appear to be controlled by water-roc k reactions. Uncontrolled pumping increases will tend to enhance fluor ide concentration and its undesirable effects. Lack of management of a quifers in basins similar to the SLP valley can result in groundwater contamination, not only from surface anthropogenic sources, but also f rom natural water-rock interactions.