Hydrogeochemistry and strontium isotopes in the Arno River Basin (Tuscany, Italy): Constraints on natural controls by statistical modeling

Summary In this paper the chemistry of major and trace elements and, for the first time, strontium isotopic ratios measured in running waters from the Arno River Basin (Tuscany, central-northern Italy) and thermal springs discharging in the same hydrographic system are presented and discussed. Classical graphical methods (e.g. mixing diagrams) have here been improved to identify, in a correct statistical sample space, extreme chemical compositions attributable to the action of geochemical processes and/or inherited from specific lithologies (namely contributions or components 1, 2 and 3) to be used in inverse modeling procedures, due to the absence of clear end-members. A linear least squares problem, with non-negativity constraints and distances, as required for compositional data (convex linear mixing) was solved by considering the contribution of the most discriminant variables given by the 87 Sr/ 86 Sr isotopic ratios and the concentration of Ca 2+ , Mg 2+ , Sr and Rb. Following such approach, it can be assumed ( p 2+ (Mg 2+ )– HCO 3 - radiogenic-rich ( 87 Sr/ 86 Sr = 0.71274; Rb/Sr = 0.039), represents a dominant feature at basin scale, with a weight ranging from 69% to 100%. Much lower percentages are related to component 2, represented by a Ca 2+ (Mg 2+ )– SO 4 2 - facies with intermediate 87 Sr/ 86 Sr (0.70874) and low Rb/Sr (2.8 × 10 −4 ) ratios and component 3, identified by Ca 2+ – HCO 3 - facies with less radiogenic 87 Sr/ 86 Sr (0.70827) and low Mg 2+ /Ca 2+ (0.011) ratios. These components are mainly dominated by the dissolution of evaporitic rocks and/or mixing with thermal waters in the southern part of the Arno River Basin and by dissolution of the carbonatic fraction, kinetically favored with respect to that of the silicatic minerals, in the upper reaches of the main course and its tributaries, respectively.