Natural and anthropogenic contributions of heavy metals from suspended solids, sediments, and freshwaters from the Valdinievole sub-basin, Padule di Fucecchio and Usciana River (Tuscany, Italy)

A river system consists of three main geological matrices: waters, suspended solid loads (SSLs), and sediments. A combined geochemical and isotopic study of these matrices allows for a detailed assessment of the river’s environmental status. While natural factors like lithology, climate, and groundwater interactions influence the river's composition, anthropogenic activities often alter it significantly. Human activities release Potentially Toxic Elements (PTEs) into the environment, which can be dissolved, complexed, or adsorbed onto SSLs and sediments, posing ecological and health risks. The Valdinievole sub-basin (Central Italy) is an ideal case study due to diverse human pressures—agriculture, urbanization, thermal spas, paper mills, and tanneries. Water samples showed signs of human impact (e.g., high reduced nitrogen species, high N/P and Br⁻/Cl⁻ ratios), with thermal waters from Montecatini Terme characterized by low Br⁻/Cl⁻ ratios and elevated Li and Cs. Tanneries contributed Cr, Ni, Sb, and other metals. The Sr isotopic composition reflected both geological and anthropogenic sources. Solid matrices showed Zn, Cu, and V enrichment from various human sources, while Pb isotopes indicated agricultural contributions. Distribution coefficients revealed the mobility of elements like Li, Cs, and Zn from solids to water, raising environmental concerns. The study underscores the value of a multidisciplinary geochemical-isotopic approach to distinguish natural vs anthropogenic sources and guide mitigation efforts.