The groundwaters of the Cornia River alluvial aquifer (southern Tuscany) were studied to determine the origin of their characteristic high-boron content. The interpretation is based on concentrations of major ions and selected minor and trace elements (B, Br, F, and Li). Mass balance calculations confirm that the main processes in the entire system are mixing of three water types and cation exchange between waters and sediments. The observed geochemical patterns indicate that the CaHCO3 waters recharging the aquifer interact with the alluvial sediments, increasing the TDS contents of the groundwaters without changing their chemical facies. High SO4 (UP to 1840 mg/l) and F (up to 2.6 mg/l) thermal (hot) waters are discharged at several points in the Cornia basin. These Ca-SO4 waters mix with the Ca-HCO3 waters in the upstream part of the basin. Near the coastline, seawater intrusion occurs as a result of over-pumping, giving rise to Na-Cl-dominated waters near the shore. Further inland, within the transition zone between fresh and salty waters, cation exchange on the clay fraction of the aquifer leads to a loss of Na and a gain of Ca, shifting the composition towards Ca-Cl and mixed-anion waters. Interaction with clay minerals explains the higher contents of B and Li in the Cornia groundwaters than in the seawater. The geochemical data, presented in the form of GIS-based geochemical maps, provide a baseline that can be used as a useful diagnostic tool to monitor the hydrochemical evolution of the Cornia groundwaters in view of possible future degradation of their quality in response to natural causes and human activities, including geothermal developments in the general Larderello area. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.