Lead isotopes in groundwater as an indicator of water–rock interaction (Masheshwaram catchment, Andhra Pradesh, India)

This study reports on the lead geochemistry and Pb-isotope ratios of ground waters along a small (53 km2) endoreic granitic catchment in India (Masheshwaram, Andhra Pradesh). The main focus is on granitic parent rock composition, water chemistry and isotope tracing, through the determination of lead isotopes in water by Multi-Collector ICP-MS using an improved new procedure. This method is efficient to give reliable Pb-isotope ratios with uncertainties (i.e. lower than 0.3% on each 206Pb, 207Pb, 208Pb ratios normalized to 204Pb) low enough to allow the different types of groundwater to be discriminated. Lead concentrations span over one or two orders of magnitude up to approximately 20 µg L− 1. Pb isotopes fluctuate largely as exemplified by the 206Pb/204Pb ratio, ranging from 17.01 up to 25.35. Most of the lead in the ground waters is of geogenic origin, and through the lead isotopic signature in groundwater, we have traced and fingerprinted the processes of water–rock interaction considering the granite matrix. Combining a weathering model and field observations, we were able to define a two step weathering process that includes a control on the Pb-isotope ratios by accessory phases and by the main mineral from the granite in a second step of weathering.