Description of the study siteThe Carnoulès Pb-Zn mine, located in the south of France, produced 1.2 Mt of solid waste stored behind a dam and containing 0.7% Pb, 10% Fe and 0.2% As. The aquifer is not fed by the vertical percolation of rainwater through the tailings, but rather comes from natural sources buried under the tailings (Koffi et al., 2003). The water table is between 1 and 10 m below the surface of the tailings stockpile, depending on the season and location. With the exception of temperature which is almost constant with average values ​​around 15°C, the physicochemical parameters of groundwater vary depending on hydrological conditions (Casiot et al., 2003b). In 2001, groundwater beneath the tailings contained extremely high levels of arsenic: up to 10,000 mg·L-1 (Casiot et al., 2003b). The water emerges at the bottom of the dam, forming the source of Reigous Creek. This AMD is acidic (pH≤3), with high concentrations of sulfate (2000 to 7700 mg L-1), iron (500 to 1000 mg L-1) and arsenic (50 to 350 mg L -1). Iron and arsenic are mainly present in their reduced forms Fe(II) and As(III) (Casiot et al., 2003a). Natural attenuation of As is the result of microbiologically mediated As-Fe coprecipitation (Morin et al., 2003, Bruneel et al., 2006). 10 to 47% of Fe and 20 to 60% of As are eliminated from the aqueous phase in the first 30 meters of the stream. Beyond this point (COWG sampling site, located 30 m downstream from the source, fig. 1), the Reigous receives water from quarries and mining galleries, particularly after rainy events, which strongly influence its acidity and its metal content (Egal et al., 2010).Sampling procedure and measurement of physicochemical propertiesSix sampling campaigns were carried out in November 2007, February 2008, October 2008, March 2009, November 2009 and January 2010 out of five sampling sites, resulting in a set of 30 samples. Groundwater was collected from a borehole (S5, between 10 and 12 m deep) located inside the tailings. Water samples were also collected at four sites along Reigous Creek (collecting downstream seepage from the surrounding area) at the spring (S1), 30 m downstream of the spring (COWG), 150 m downstream (GAL) and 1,500 m downstream (CONF). , just before the confluence between the Reigous stream and the Amous river (Fig. 1). Water samples (300 ml) were immediately filtered through sterile 0.22 µm Nucleopore filters, which were transferred to a collection tube (Nunc), frozen in liquid nitrogen and stored at -80 °C until DNA extraction..