Drinking water is a very important topic for people and society. Groundwater usually represents the most important source of drinking water in terms of quantity, safety and reliability. In mountain areas important sources of water supply are hosted in carbonate aquifers, in relation to groundwater quality and quantity. However, high effective infiltration coefficients and low groundwater residence time induce high flow variability and groundwater vulnerability, implying a high risk of contamination. This is stronger in mining areas.
The Baccatoio Torrent basin (NW Tuscany, Italy) includes all these features: different carbonate aquifer systems which locally interfere with abandoned mining areas, characterized also by acid mine drainages. This area includes several orebodies made of pyrite, baryte, and iron oxide. Pyrite ores contain high levels of PTE, including exceptional Tl concentration, 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 Tl and it is not routinely monitored).
This study analyses some springs (named Molini S. Anna-VSMSA, Moresco Alta-VSMOA, Moresco Sondaggio-VSMOS and Moresco Galleria-VSMOG) fed by carbonate aquifer systems. A multidisciplinary integrated approach, based on geological surveys, flow rate and level measurements, sampling, chemical and isotopic analyses, allows to characterize the springs and to understand preliminarily the groundwater flow and the origin of the contamination.
The first one (VSMSA), located at higher altitude and fed by a metamorphic dolostone-marble aquifer, resulted contaminated in Tl (5-30 µg/l) and removed from the local drinking water network. Tl-contamination results from the binary mixing of two sources: an uncontaminated component constituted by Ca-HCO3-type water that mixes with waters from a contaminated body formed by the interaction of acid drainages generated by the weathering of Tl-hosting mineral phases with dolostone rocks.
The second group of springs are located at lower altitude and are close to each other, but only VSMOG feeds the aqueduct, because the others occasionally contain PTE including Tl (up to 2.5 µg/l). Also the Moresco springs show a dominant Ca-HCO3 composition, confirming the presence of a carbonate aquifer system. Flow rates, hydraulic levels and EC hydrographs highlight two different groundwater flow systems: the first one, shallower, feeding VSMOA and VSMOS; the second one, deeper, feeding VSMOG and showing higher SO4 concentration, temperature and EC, as well as approximately constant isotopic composition, confirming the presence of a deeper circuit. Data point out a likely connection between Baccatoio Torrent and VSMOA and VSMOS springs, inducing contamination.
These examples highlight the importance of the knowledge of the aquifer systems and of the processes involved in groundwater circulation in order to use natural water resources as drinkable water.