Disentangling natural vs. anthropogenic sources of heavy metals in river ecosystems: a geochemical perspective to support biodiversity in multiple sourced environments

River ecosystems are suffering a profound biodiversity crisis being threatened by multiple stressors, e.g., pollution, land use modifications, climate change. These ecosystems can be regarded as open thermodynamic systems, whose stability derives from a delicate balance between their biotic and abiotic components. The introduction of persistent and non-biodegradable heavy metals (HMs) that can be accumulated in living organisms, including humans, as they are exposed to contaminated food and water, can modify this equilibrium. As HMs can derive from both natural (rock weathering) and anthropic (e.g., urban and industrial activities, agricultural practices) sources, geochemical and isotopic tools able to discriminate the origin of these elements are of pivotal importance in the perspective of ecosystem management and biodiversity conservation. As part of the CN5-Spoke 3 Assessing and monitoring terrestrial and freshwater biodiversity and its evolution: from taxonomy to genomics and citizen science project, supported by the National Biodiversity Future Centre (NBFC) and funded by the Ministry of University within the National Recovery and Resilience Plan (PNRR), Mission 4-Component 2-Investment 1.4, the present study aims to identify the source(s) of HMs in the Ombrone Grossetano river basin (OGRB, Italy) by combining geochemical (major, minor and trace elements) and isotopic (Sr, Nd, Pb) analysis of water, sediments, and suspended solids. OGRB represents one of the major river ecosystems in Tuscany. It drains an area of 3,500 km2 and is an ideal site to identify and test innovative geochemical tracers able to discern the origin of HMs having potential impact on biodiversity considering i) its low population density (50 people/km2), and ii) the co-presence of protected areas, abandoned mining sites and thermal discharges contributing to the Ombrone river and its tributaries. Water and suspended solid samples will be collected during 6 sampling surveys in 2023-2025 during both high and low river discharges and diverse seasonal and weather conditions, whilst bottom sediments will be retrieved during one single campaign. Sediments and suspended solids will be analysed for total HMs content and by sequential extraction procedure to assess the potential bioavailability of metals. Major focus will be on the isotopic composition of Pb, Nd and Sr from different environmental matrices. A further step of the project is to combine the geochemical and isotopic data with river biodiversity assessments based on innovative monitoring techniques (eDNA and eRNA analysis) to explore the relationships between abiotic and biotic components of fluvial ecosystems and identify specific perturbations induced by HMs on the riverine biota.