Strontium and lead isotopes as environmental tracers in a water supply watershed in Southern Brazil

Urbanization and agriculture increase the risk of water contamination in watersheds. A special concern arises when this water is used for public supply. Conventional hydrochemical methods have been applied to identify contaminants in bulk water; however, there is lack of investigation of their origins. Environmental isotopes can be used to address such knowledge gap by tracking the harmful substance back to its origins. This study evaluates the interference of urbanization as well as inorganic fertilizers in the suspended sediments of the tributaries of two impoundments, whose water supplies 72% of the municipality of Caxias do Sul, Southern Brazil. To achieve this goal, the isotopic ratios 87 Sr/ 86 Sr, 208 Pb/ 204 Pb, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 206 Pb and 207 Pb/ 206 Pb were selected as geochemical tracers, showing promising results to evaluate sources of contamination in source water. The results of strontium isotopes showed two distinct groups, one marked by the geogenic origin of the sediment and the other by inorganic fertilizers. Lead isotopes indicated that suspended sediments have signatures compatible with the rocks that occur in the region and are in a range of isotopic ratios different from fertilizers or copper sulfate.