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[en] Following the detection of a severe thallium contamination of the drinkable water from the public distribution system of Valdicastello Carducci-Pietrasanta (northern Tuscany, Italy), and the identification of the source of contamination in the Molini di Sant'Anna spring (average Tl content ≈ 15 μg L−1), the replacement of the contaminated water with a virtually Tl-free one (Tl < 0.10 μg L−1) caused an increase in Tl concentration in the drinkable water. This suggested that the pipeline interior had become a secondary source of Tl contamination, promoting its mineralogical and geochemical study. Rust scales samples taken from several pipeline segments, as well as leaching products obtained from these samples, were investigated through scanning electron microscopy, X-ray fluorescence chemical analyses, inductively coupled plasma – mass spectrometry, X-ray diffraction, and X-ray absorption spectroscopy. Thallium-rich rust scales (up to 5.3 wt% Tl) have been found only in pipeline samples taken downstream the water treatment plant, whereas the sample taken upstream contains much less Tl (~ 90 μg g−1). The Tl-rich nature of such scales is related to the occurrence of nano- and micro-spherules of Tl2O3 and less abundant nanocrystalline μm-sized encrustations of TlCl. Leaching experiments on Tl-rich rust scales indicate that a fraction of the available Tl is easily dissolved in tap water; X-ray absorption spectroscopy suggests that monovalent thallium occurs in water equilibrated with the rust scales, probably related to the dissolution of TlCl encrustations. Therefore, Tl dissolved as Tl+ only in the water from the Molini di Sant'Anna spring was partially removed through oxidative precipitation of Tl2O3 and precipitation of TlCl. This highlights the critical role played by the addition of chlorine-based oxidants in water treatment plants that could favour the deposition of Tl-rich coatings within the pipelines, giving rise to unexpected secondary sources of contamination. - Highlights: • Thallium-rich rust scales have been found within drinkable water pipelines. • Both Tl(I) and Tl(III) oxidation states have been detected by XAS measurements. • Solid Tl-phases within scales are represented by Tl2O3 and TlCl. • Tl-rich rust scales acted as a secondary source of Tl pollution of drinkable water. • The addition of Cl-based oxidants to water induced the precipitation of Tl phases.