Identification of natural and anthropogenic sources of heavy metals in river ecosystems and their impact on biodiversity

Living organisms need varying quantities of specific Potentially Toxic Elements (PTEs) (such as Fe and Zn) for their development, but an excessive accumulation of these elements in river ecosystems can disrupt their equilibrium. PTEs in riverine systems may originate from both natural processes (e.g. rock weathering) and human activities (e.g. industrial), thus requiring geochemical tools that are able to discriminate their sources to gain insights into the ecological and geological interplay. Within this framework, this Ph.D. focuses on the investigation of both natural and anthropogenic sources of PTEs in the Ombrone Grossetano River Basin (OGRB), located in Southern Tuscany (Italy), and on the evaluation of their distribution among selected geological matrices, with the aim to gain insights on their mobility and potential impacts on the biotic component of the riverine ecosystem. The OGRB represents a particularly suitable case study, as it combines a marked lithological variability, characterized by the presence of sedimentary, igneous, metamorphic and ophiolitic units, coupled with a long history of ore exploitation as well as a current widespread agricultural land use. To address these objectives, a multi-matrix approach was adopted, focusing on stream sediments, suspended solid loads (SSLs) and stream waters. Major, minor and trace element concentrations were determined in all matrices, whereas the isotopic analysis of Sr and Pb was performed on a selection of samples, to better constrain the water-rock interaction processes and anthropogenic inputs in these samples. To infer the potential ecological risk of PTEs, the speciation of dissolved metals was assessed using both abiotic (WATEQ4F in PHREEQC) and biotic (Biotic Ligand Models, BLMs) models. The chemical composition of stream sediments mainly reflects the mineralogical and chemical features of the bedrock, and the anthropogenic input is limited to a restricted group of PTEs (i.e., Cu, Zn, Pb, As and Sb). The highest enrichment factors are located in historical mining districts and known mine-waste areas within the Farma-Merse and Gretano sub-basins. These enrichments are mostly confined to the tributaries and do not exhibit a strong influence on the composition of the main river. Compared to bottom stream sediments, suspended solid loads display distinct geochemical features, due to the dominant role of phyllosilicates, Fe-Mn oxyhydroxides and organic matter, which enhance the sorption capacity of these solids for specific PTEs. Accordingly, Cu and Zn are preferentially partitioned into the fine suspended fraction, which allows for a re-distribution of these elements along the river network; other elements, such as Pb, As and Sb, show higher enrichment factors in the stream sediments than in SSLs. The chemistry of stream waters mainly reflects the contribution of shallow subsurface flow and groundwater inputs, whose proportions are linked to the sampling season; carbonate and evaporitic lithologies play the major role in determining the surface waters characteristics. The concentrations of dissolved trace elements are generally low compared to those observed in solid matrices, indicating that most PTEs remain primarily within sediments and suspended particles. Nevertheless, Cu and Zn are the PTEs displaying the highest mobility into surface waters, particularly in samples collected in mining-impacted sub-basins and during first flush events. Antimony also displays some enrichments linked to localized anthropogenic inputs (i.e., mining and sewage-related sources). In contrast, Pb remains consistently low or below detection limits in stream waters, confirming its immobile behaviour under the hydro-geochemical conditions of the OGRB. Isotopic tracers provide additional and independent constraints on element sources: Sr isotopes primarily reflect the lithological control and confirm the dominant role of water-rock interaction in the geochemistry of the investigated matrices, whereas Pb isotopes allow for the discrimination between natural contribution and anthropogenic inputs linked to historical mining and, locally, urban aerosols. Therefore, the combined use of elemental and isotopic approaches provides a particularly effective tool for disentangling natural and anthropogenic contributions to the OGRB environmental matrices. The impact exerted by PTEs on the biotic component of the OGRB ecosystem has been investigated by evaluating the bioavailability of these elements using both abiotic (WATEQ4F) and biotic (BLM) models. The two models yield different results, but they both indicate that many metals occur predominantly in non-bioavailable forms (either complexed or particle-bound), hence limiting their biological uptake. In particular, the BLM model suggests a rather low ecological risk linked to PTEs across most of the basin, with only localized exceedances for Zn and, occasionally, Cu under low-discharge conditions. Mercury exhibits a peculiar behaviour: its presence is strongly associated to fine particles and organic matter and displays values exceeding the available environmental quality standards. However, it is noteworthy that the whole area is characterized by a Hg enrichment compared to European or worldwide references, which might overestimate the impact that Hg has on the OGRB. Overall, this research shows that, although past mining and other human activities have significantly influenced the geochemistry of the OGRB, the current hydrogeochemical conditions of the basin do not promote widespread transfer of most potentially toxic elements into bioavailable forms. Moreover, it highlights the importance of a holistic multidisciplinary approach that combines geochemistry, isotopic tracing, and bioavailability assessments on all the representative geological matrices characterizing a riverine system. By integrating these perspectives within a process-oriented framework, the work provides a robust method for distinguishing natural and anthropogenic sources of contamination and evaluating their environmental impacts. Such an approach can be applied to other geologically complex and human-impacted basins, supporting more targeted monitoring strategies and informing measures to minimize the effects of industrial and agricultural activities, ultimately contributing to sustainable freshwater management.