Accumulation of arsenic from acidic mine waters by ferruginous bacterial accretions (stromatolites)

In the acidic stream (pH 2.2–4) of the Carnoulès Pb-(Zn) mine, Gard, France, very high As contents (from 9 to 20%) can be accumulated as ferric arsenate and arsenate-sulphate precipitates in rapidly growing bacteria-made structures. The main bacterial forms are rod-shaped and sheathed, their sheath is made of Fe-As-rich material and is coated with ferric arsenate colloidal particles or may be partially included in authigenic crystals. Living forms ofThiobacillus-type bacteria have been recognized in the precipitates. The cyclic development of bacterial colonies alternating with sand deposition and erosive episodes results in the formation of As-rich ferruginous accretions. These laminated and dome-shaped bacterial constructions are similar to those of stromatolites. The extremely high contents of solute As in upstream flow (250 mg/1) are lowered by 2–3 order of magnitude downstream. Lead is also precipitated and concentrated in this Fesingle bondAs-rich bacterial stromatolite (2500 ppm Pb). This accumulation and concentration of As and heavy metals via direct or induced microbial action limits pollution downflow. But seasonal storms could erode these Fesingle bondAssingle bondPb-rich deposits and drastically increase pollution.