Water Samples from Carnoules Mine in Southern France

Description of the study site

The Pb-Zn Carnoulès mine, located in southern France, produced 1.2 Mt of solid wastes that are stored behind a dam and contain 0.7% Pb, 10% Fe and 0.2% As. The aquifer is not fed by vertical percolation of rainwater through the tailings, but rather originates from natural springs that were buried under the tailings (Koffi et al., 2003). The water table is 1-10 m below the surface of the tailings stock, 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 the groundwater vary as a function of the hydrological conditions (Casiot et al., 2003b). In 2001, the groundwater below 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 the Reigous Creek. This AMD is acid (pH≤3), with high concentrations of sulfate (2000-7700 mg L-1), iron (500-1000 mg L-1) and arsenic (50-350 mg L-1). Iron and arsenic are mainly present in their reduced forms Fe(II) and As(III) (Casiot et al., 2003a). The 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 removed from the aqueous phase within the first 30 m of the creek. Beyond this point (COWG sampling site, located 30 m downstream from the spring, Fig. 1), the Reigous receives water from quarries and mine galleries, especially after rainfall events, which strongly influence its acidity and metal content (Egal et al., 2010).

Sampling procedure and measurement of physicochemical properties

Six sampling campaigns were carried out in November 2007, February 2008, October 2008, March 2009, November 2009 and January 2010 at five sampling sites, resulting in a set of 30 samples. Groundwater was collected from a borehole (S5, between 10 and 12 m deep) located within the tailings. Water samples were also taken at four sites along Reigous Creek (collecting downstream seepage water from the surroundings) at the spring (S1), 30 m downstream from the spring (COWG), 150 m downstream (GAL) and 1500 m downstream (CONF), just before the confluence between Reigous Creek 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.