Abstract In the Carnoulès Pb-Zn mining site (Gard, France), abandoned 40 years ago, acidic waters (pH 3) with an extremely high As content (80–350 mg l −1 ) emerge from the base of a tailings stock containing As-rich pyrite (2–4% As). From the acidic spring, the oxidation — reduction potential (Eh) and O 2 parameters strongly increase within a few metres and a Fe-As-rich (up to 22% As) material precipitates and covers the bed of the acidic creek. Consequently, there is a sharp decrease in arsenic concentration of the acidic waters downflow (<10 mg l −1 As) and of the Fe-rich precipitates (down to 2% As). Seasonal variations in dissolved arsenic concentrations of the spring waters are important. Furthermore, the Fe-As-rich stream sediments that were stored, during drought periods, are reworked and transported downflow during rainy periods. The annual fluxes of total arsenic comprise between 2 and 6 t; the lifetime of the As-releasing system is that of several centuries. Fe and As speciation measurements have been carried out. Fe(II) and As(III) dominate all along the acidic stream. Fe(III) is rapidly precipitated. The removal of total dissolved arsenic mostly corresponds to a decrease in dissolved As(III) and results in the formation of As-rich ferric precipitates. A further decrease in arsenic concentration in water can be attributed to adsorption mechanisms on ferric hydroxides. In the Fe-As-rich products, the most common bacteria are long rod-shaped phenotypes of the genus Thiobacillus ferooxidans . Laboratory experiments have been carried out to investigate the potential catalytic role of these acidophilic oxidizing bacteria on the removal of arsenic: in the biotic systems, 60–80% As were removed in a few days against 5% for abiotic systems.