From point source to diffuse source of contaminants: The example of mercury dispersion in the Paglia River (Central Italy)

Abstract

Fluvial sediments of the Paglia River, a tributary of Tiber River (Central Italy), are contaminated by mercury (Hg) as a consequence of past mining activity (1846-1981, with main production 1900-1970) in the Monte Amiata district (the 3rd largest Hg producer worldwide). In this study, we combine a geomorphological analysis with geochemical data to try and understand the influence of fluvial dynamics on the spatial distribution of Hg in fluvial sediments over a 43 km segment of the river. By considering the evolution of the river course between 1883 and 2013, eight different geomorphic units (GUs) were recognised, including the active channel bed (baseflow channel and bar), the floodplain, and five orders of terraces. The distribution of Hg in sediments of these GUs reflects the timing of their formation with respect to evolution of the mining activity. In GUs formed before the main peak of mining activity, or after mine closure, sediments show mean Hg contents comparable to, or slightly higher than, the local background, estimated at 2-6 mg/kg; in GUs formed during the peak production, Hg mean contents are definitely higher (up to 26 mg/kg). The current floodplain also shows high contents (mean 19 mg/kg), because of continuous reworking and transport of older contaminated sediments during major flood events. Therefore, the point contaminant sources represented by mining centres evolved into a diffuse source spread over several tens of kilometres. By combining geochemical data with calculated sediment volumes, we estimate that not less than 63 tonnes of Hg are currently contained in the sediments of the investigated river stretch. Such amount of Hg will probably limit for the near future a full land use along the Paglia-Tiber course.

Modeling the effects of land cover change on sediment concentrations in a gold-mined Amazonian basin

“…In this sense, runoff and soil erosion have an important role in controlling nutrient concentrations in Amazonian rivers, since cations and bicarbonates are mainly provided by the weathering of surface soils by rainfall (Markwitz et al 2001). Moreover, it is worth mentioning the release of toxic chemicals such as mercury and cyanide into the rivers, as a result of the techniques and processes applied in the artisanal gold mining, as well as from contaminated soils of abandoned gold mining areas (Rhoades et al 2009;Colica et al 2019). In this context, knowing the proportions of SC derived by GMA and that derived by laminar soil erosion is useful in studies on the estimation of mercury and mercury-compound release, transport, deposition, remobilization, and fate in the ecosystems, distinguishing between anthropogenic, natural emissions, and remobilization of mercury from historic deposition, as mentioned in item (e) of the Article 19 of the Minamata Convention on Mercury (UNEP 2013).…”

“…Mercury pollution is a global concern due to its high toxicity, persistence, and capacity for long-range transport towards marine ecosystems (Amos et al 2014). Contaminated soil of abandoned gold mining sites become a source of mercury pollution potentialized by soil erosion related to deforestation (Amos et al 2014;Colica et al 2019). Thus, estimating sediment budgets and distinguishing the parcels of sediment derived by GMA from that generated by sheet erosion will give insights for the understanding of mercury and other gold mining-related chemical contamination in rivers and sediments (Rhoades et al 2009;Colica et al 2019).…”

“…Contaminated soil of abandoned gold mining sites become a source of mercury pollution potentialized by soil erosion related to deforestation (Amos et al 2014;Colica et al 2019). Thus, estimating sediment budgets and distinguishing the parcels of sediment derived by GMA from that generated by sheet erosion will give insights for the understanding of mercury and other gold mining-related chemical contamination in rivers and sediments (Rhoades et al 2009;Colica et al 2019). Such analysis contributes for improving knowledge on mercury release and transport, in compliance with the Article 19 of the Minamata Convention on Mercury, signed by about 140 countries, including Brazil (UNEP 2013).…”

Total Mercury Mass Load from the Paglia–Tiber River System: The Contribution to Mediterranean Sea Hg Budget

“…The mercury load from Tiber River estimated here must take into account that the Paglia River system represents a permanent diffuse source of Hg-rich sediments, with at least 60 × 10 3 kg of Hg stored in the overbank sediments and in the floodplain along the first tract of the Paglia River alluvial valley [ 22 , 24 , 25 ] and along its main tributaries [ 23 ]. Moreover, to the best of our knowledge, the amount of Hg stored in the Alviano reservoir and along the Tiber River stretch connecting this dam to the Mediterranean Sea is not known.…”

“…Tiber River, the main river of Central Italy, flowing through the city of Rome, receives waters from Paglia River, which drains the Monte Amiata mining district (MAMD; southern Tuscany, Italy), the third-largest Hg-producing district worldwide. Today, after more than forty years from the closure of mining and smelting activities in the area and the partial reclamation of the main mining areas (Siele and Abbadia San Salvatore) [ 19 ], Hg is still persistent in the environment as a result of the widespread presence of polluted stream sediments along rivers draining the MAMD [ 7 , 20 , 21 , 22 , 23 , 24 , 25 ].…”