Heavy metals and Pb isotopes as tracers in environmental matrices: the Valdinievole sub-basin river system (Tuscany, Italy) case study

Heavy metals distribution and behaviour in natural systems have been received increasing attention due their impact on the environment and human health. These elements, indeed, can be essential for life, but also toxic when present in high concentrations causing critical diseases in the ecosystems. The geochemistry of trace and ultra-trace elements, coupled with Pb isotopic ratios, is a powerful tool for the discrimination of natural and anthropogenic sources and related geochemical processes. The geological matrices that constitute the riverine environment (water, suspended solid load, and sediment) are intimately linked and their simultaneous investigation allows to increase the knowledge of the trace elements mobility and distribution, as well as to distinguish between geogenic and human contributions to their heavy metal budget. The Valdinievole (Tuscany, Central Italy), is a secondary basin of the Arno River Basin and includes the Padule di Fucecchio, one of the most relevant Tuscan swampy zones. Besides, the Valdinievole sub-basin hosts a large number of important productive sectors such as paper mills, flora nursery farms, thermal spas, and tanning industries, thus representing an ideal study case to evaluate the impact on water, sediment and suspend solid load (SSL) of industrial and urban sewages. The water geochemistry highlighted a wide compositional variability, which ranged from Ca2+(Mg2+)-HCO3- to Na+-Cl-(SO42-) and Total Dissolved Solids up to 6,390 mg/L. The relatively high contents of the reduced N-species, which were up to 6.2 (NO2 -) and 23 (NH4+) mg/L, suggested the presence of a fresh and diffuse pollution. A few samples of water, SSL and sediment showed Cs contents up to 170 μg/L, 57 mg/L, and 46 mg/L, respectively. The SSLs and sediments were characterized by high heavy metal concentrations, e.g., Pb, Cu, Zn and Cr up to 174, 766, 1,899 and 7,953 mg/kg, respectively, likely including a variable contribution from anthropogenic sources. This hypothesis is further supported by i) the Pb isotopic ratios, which allowed to identify a clear tendency from a geogenic to an anthropogenic signature and ii) the heavy metals Enrichment Factors, whose values are indicative of the significant impact exerted by local human activities on riverine environmental matrices. EFs of Pb, Cu, Zn and Cr, calculated using the world suspended solid load concentration averages, reached values up to 12, 163, 56 and 134, respectively, confirming the influence of the enterprises situated in the Valdinievole sub-basin on the geochemical abundances of heavy metals.