Kouadio Koffi

Microbial Diversity in a Pyrite-Rich Tailings Impoundment (Carnoulès, France)

The microbial communities have been investigated in the subsurface waters of the Carnoulès pyrite-rich tailings impoundment (France) for two hydrological situations characterized by the presence of oxygenated waters during winter and suboxic conditions in early autumn. In these acidic waters (2–5) characterized by elevated concentrations of Fe (1608–3354 mg · l−1), As (130–434 mg · l−1) and sulfates (5796–14318 mg · l−1) and variable dissolved oxygen content, the cultivable bacteria found in these system are Thiomonas and Acidithiobacillus ferrooxidans. Molecular methods, Terminal-Restriction Fragment Length Polymorphism (T-RFLP), and 16S rRNA encoding gene library analysis indicate low diversity. The environment is dominated by only a few types of microorganisms, with 70–80% of the whole bacterial population assigned to two or three Terminal-Restriction Fragments (T-RFs). Most of these organisms are uncultured, newly described, or recently associated with acid mine drainage. Modifications of the community structure are observed as a function of the sampling period and seem to be related to the aqueous chemistry of the tailings water. At low Dissolved Oxygen (DO = 1 mg · l−1) concentrations and moderately acidic conditions (pH = 5.7), the dominant organisms are related to the uncultured clone BA31 affiliated with Desulfosarcina variabilis, a sulfate-reducing bacteria (SRB), Acidithiobacillus ferrooxidans and the uncultured clone BVB20, closely related to Thiobacillus. At high (12 mg · l−1) DO concentrations and low (< 2) pH values, the microbial diversity is less important and 65% of the population is assigned to the uncultured bacterium clone AS6 related to Desulfosarcina variabilis.

Geochemical Processes Controlling the Formation of As-Rich Waters Within a Tailings Impoundment (Carnoulès, France)

The distribution of arsenic (As(III), As(V)) and iron (Fe(II), Fe(III)) species was monitored during 1 year in a borehole drilled in the Carnoulès tailings impoundment which contains As-rich pyrite. The concentrations of total As and Fe in subsurface waters exhibited strong variations over one year, which were controlled by dissolved oxygen concentrations. At high oxygen levels, extremely high As (up to 162 mM) and Fe (up to 364 mM) concentrations were reached in the borehole, with the oxidised species predominant. As and Fe concentrations decreased 10-fold under oxygen-deficient conditions, as a result of pH increase and subsequent precipitation of As(V) and Fe(III). From drill core sections, it appeared that at low dissolved oxygen levels, As(III) was primarily released into water by the oxidation of As-rich pyrite in the unsaturated zone. Subsequent As and Fe precipitation was promoted during transport to the saturated zone; this reaction resulted in As enrichments in the sediment below the water table compared to the original content in pyrite, together with the formation of As-rich (up to 35 wt% As) ferruginous material in the unsaturated zone. High amounts of As(V) were released from these secondary phases during leaching experiments with oxygenated acid sulfate-rich waters; this process is believed to contribute to As(V) enrichment in the subsurface waters of the Carnoulès tailings during periods of high dissolved oxygen level.