Groundwater and potentially toxic elements in a dismissed mining area: Thallium contamination of drinking spring water in the Apuan Alps (Tuscany, Italy)

Thallium is among the most toxic elements to humans. Environmental regulations of the European Community adopted by Italy pose a maximum concentration level for Tl in groundwater of 2 μg/L. However, at present drinkable water must not abide any concentration threshold for thallium and this element is not routinely monitored. In the past-mining area nearby the Valdicastello Carducci village (northern Tuscany, Italy), characterized by Tl-bearing ores, water from a spring that has been tapped for human consumption contains thallium in the range 4.3–27.8 μg/L. The challenge of this study is to highlight the mechanisms leading to Tl contamination in groundwater, by crossing hydrochemical and O-H isotopic data with hydrogeological information. We propose that the Tl-contamination in the spring results from a binary mixing that occurs within a karst system involving an uncontaminated Ca-HCO3 water (mainly interacting with limestone) and a component originated by the interaction of acid drainages (generated by the weathering of Tl-bearing mineral phases in the area) with dolomitic host rocks. In this process, most of the potentially toxic elements released during the weathering are scavenged by the carbonate neutralization of the acid plume due to adsorption onto the iron and manganese oxyhydroxides that formed during pH changes. Nevertheless, the aquifer has little or no attenuation capacity for thallium. Moreover, O-H isotopic data and hydrogeological constraints are compatible with a spring feeding mechanism related to a piston displacement effect.