From the western central part of the municipality of São Paulo, 47 deep wells (150-200 meters) were sampled in an area covering 150 km² and encompassing two distinct urban environments: residential and industrial. Each groundwater sample was analyzed for 8 major and 13 minor elements. With respect to total dissolved solids, there was a very notable difference between water extracted from the upper sedimentary aquifer (3-45 mg/l) and that extracted from the deeper crystalline rock aquifer (80-500 mg/l). The principal ions responsible for the higher total dissolved solids in the crystalline aquifer are: sodium, calcium, magnesium and bicarbonates. The low mineralization level of water from the sedimentary aquifer is attributed to the well-leached nature of São Paulo basin sediments which are essentially composed of quartz, kaolinite and oxides and hydroxides of iron. The distinctly higher mineralization levels in the crystalline rock aquifer, on the other hand, are attributed to the weathering of minerals in the rocks, principally by the hydrolysis of feldspars. With regard to samples taken in the industrial section of the study area, relatively high concentrations of the major ions were encountered as well as the minor elements of iron, manganese, barium, chromium and lead. These results indicate that industries also contribute to the mineralization of these groundwaters.
