Antimony and arsenic distribution in a catchment affected by past mining activities: influence of extreme weather events

A devastating rainfall in Tuscany in November 2012 provided the opportunity to study the diffusion of As and Sb along the Tafone mining area, under extreme conditions, by comparing their concentrations in water, soil, and river sediments in drought (equilibrium) periods against those observed immediately after a flooding event and after a period of 6 weeks rebalancing of the watercourse. The data obtained at equilibrium show a significant “natural” As and Sb contamination with the highest levels along the valley in the mine surroundings. Very high Sb and As concentrations in the sediment of the Tafone Lake cause a potential continuous supply of toxic elements to the river downstream. In surface waters (river water and lake), As concentrations exceed slightly, and only occasionally, the Italian regulatory limits, while, those for Sb are always widely exceeded. In addition, the soils collected along the Tafone valley show high Sb and As total content, and were studied by a novel nine-step sequential extraction protocol, proposed for assessing the bioavailability of As and Sb in sulphide ore mining-impacted soils. Experimental data show that As and Sb mobility is strongly affected by even small variations of the pH and redox conditions, thus controlling their release into the environment. The results obtained after the flood demonstrated that extreme weather events can induce two main problems: (1) the temporary increase of As and Sb amount transported by the river water to the sea, principally as particulate and colloidal material, and (2) the sediment transport out the mining area, reaching soils eventually used for agriculture, and the potential diffusion through the food chain.