Geochemistry and radiogenic isotopes of total suspended solids (TSS) from the Nievole River Valley (Tuscany, Central Italy)

The Nievole River Valley (NRV) is a sub-basin of the Arno River Basin located in Tuscany (Central Italy). Nievole, Pescia di Collodi and Pescia di Pescia rivers, and many other artificial channels, crosscut the northern part of NRV and collect effluents from many small cities and industrial districts, before inflowing into the Padule di Fucecchio, a swampy zone whose waters are regulated by the Usciana River. The NRV is characterized by a complex ecosystem including many relevant anthropic activities and small-medium enterprises, such as paper mill industries, flora-nursery farms, thermal spas, and one of the most productive tanning districts in Italy. For these reasons, the NRV was selected for this study, being aimed at identifying natural and anthropogenic inputs that affect the geochemical composition of liquid and suspended matrices and stream sediment. In this work we determined the chemical and isotopic composition of the total suspended solids (TSS) collected from the NRV river system during high and low river discharge periods in 2021 along the main courses and tributaries, distributed in an area of about 320 km2. Major, minor, and trace element concentrations were measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) after total acidic digestion while Sr, Pb, and Nd isotopic compositions were performed by Thermal Ionization Mass Spectrometry (TIMS). According to the Italian Legislative Decree 152/2006, heavy metal concentrations in TSS approach, or even exceed, those for the Italian soils intended for green public areas for almost all the regulated elements, suggesting that a strong anthropogenic pressure is acting on the NRV. Vanadium, Cr, Co, Cu, Zn, As, Sn, and Pb showed values up to 149, 428, 31, 182, 476, 37, 22, and 109 mg/kg, respectively. Cesium concentrations were up to 46 mg/kg and decreased along the river course, highlighting a dilution trend along a tributary of the Padule di Fucecchio. Lead isotopes in TSS displayed a signature typically associated with the presence of anthropogenic contributions, especially for those samples collected from the most urbanized and industrialized areas. Enrichment Factors for Nickel, Mo, and Pb showed a positive correlation with Pb isotopic ratio with the higher values reflecting a clear anthropogenic contribution.