Suspended load and mercury pollution: towards a simple method to measure Hg flux from the Monte Amiata Mining District (Southern Tuscany, Italy)

Mining activities produce huge amounts of waste material heavily polluted by toxic elements. Over time, these wastes can pollute fluvial ecosystems due to runoff. Moreover, mining can significantly alter river morphology by modifying sediments supply, erosion, transport, and (re-) deposition. The interplay between geomorphic processes (e.g., flooding events) and anthropic activities plays an important role in the contaminants redistribution across the environment, even at long range. Mercury (Hg) is listed as a critical contaminant due to its high toxicity, mobility, and persistence in the environment. Its use is progressively banned, and Hg mining is limited to a few countries. However, legacy mine wastes are still releasing Hg into the environment, particularly to fluvial ecosystems. The Monte Amiata Mining District (MAMD, Southern Tuscany) was the 3rd largest Hg producer worldwide. The Paglia River (PR) drains the SE sector of the MAMD and its catchment covers an area of 1320 km2. The widespread Hg pollution of this river basin and its low resilience to contamination was demonstrated in previous studies. Disastrous flooding events remobilized and redistributed massive amounts of polluted sediments across the catchment (Colica et al., 2019). The Hg flux discharged by PR to the Tiber River, and ultimately to the Mediterranean Sea, was estimated around 11kg/y (Rimondi et al., 2019; Fornasaro et al., 2022a; Fornasaro et al., 2022b). However, this estimate is based on spot samplings throughout the year. In this study, the relationship between Hg transported by particulate (Hgp) and total suspended solids (TSS) for PR was investigated to set up a method for the calculation of Hg fluxes from TSS monitoring. Water samples were collected during low and high river discharge. Samples were taken along the PR, upstream and downstream of the Elvella creek confluence (ECC), a tributary that is not polluted by Hg, in order to evaluate its effects on the Hg budget. The samples were filtered and the TSS collected on the filters were analyzed for Hg. TSS ranged between 1.3 and 621.4 mg/L, whereas Hg varied between 0.8 and 321.8 ng/L. The highest Hg and TSS values were measured during the recession phase of flooding events, whereas the lowest ones were found during low flow conditions. A linear relationship was found between Hg and TSS. Hg was higher in the upstream samples than in those collected downstream the ECC, confirming that Hg source is the heavily polluted PR basin. The relationship between the two parameters could be applied to the indirect, continuous measurement of Hg fluxes discharged by PR with an automated TSS/turbidity sensor. Such monitoring would allow assessing the variability of Hg pollution across the PR basin in real time especially in case of flooding, that are expected to become more frequent due to climate change, leading to an increase of Hg delivery to the Tiber River and ultimately to the Mediterranean Sea. Colica A., Benvenuti M., Chiarantini L., Costagliola P., Lattanzi P., Rimondi V. & Rinaldi M. (2019) - From point source to diffuse source of contaminants: The example of mercury dispersion in the Paglia River (Central Italy). Catena, 172, 488-500. Fornasaro S., Morelli G., Costagliola P., Rimondi V., Lattanzi P. & Fagotti C. (2022a) - Total Mercury Mass Load from the Paglia–Tiber River System: The Contribution to Mediterranean Sea Hg Budget. Toxics, 10(7), 395. Fornasaro S., Morelli G., Rimondi V., Fagotti C., Friani R., Lattanzi P. & Costagliola P. (2022b) - The extensive mercury contamination in soil and legacy sediments of the Paglia River basin (Tuscany, Italy): interplay between Hg-mining waste discharge along rivers, 1960s economic boom, and ongoing climate change. J. Soils Sediments, 22(2), 656-671. Rimondi V., Costagliola P., Lattanzi P., Morelli G., Cara G., Cencetti C., Fagotti C., Fredduzzi A., Marchetti G., Sconocchia A. & Torricelli S. (2019) - A 200 km-long mercury contamination of the Paglia and Tiber floodplain: Monitoring results and implications for environmental management. Environ. Pollut., 255, 113191.