Microbial community evolution in a marine shore porphyry tailings deposit throughout wetland remediation

Background Two porphyry copper mines located in southern Peru initiated the discharge of mine wastes (tailings) for a final shore deposition in the Bay of Ite (Pacific Ocean). 780 millions metric tons of tailings were deposited, which formed a delta with a surface of ca. 16 km2. A low-pH oxidation zone characterized by the presence of iron hydroxides developed on the surface, along with a strong build-up of efflorescent salts rich in heavy metals. Objectives A 3 years investigation was carried out in order to understand the biogeochemical processes resulting from the remediation of the oxidizing tailings by the implementation of a wetland cover1. Methods Analysis of the bacterial communities was carried out by molecular techniques (T-RFLP and cloning-sequencing). A combination of physical, chemical and mineralogical methods was applied for the characterization of solid tailings and water samples. Results The non-remediated tailings was characterized by a low-pH oxidation zone (pH 3 – 4), in which the oxidative dissolution of sulphide was identified as the dominant process, carried out mainly by Leptospirillum spp, Acidithiobacillus spp and Sulfobacillus spp. This dissolution led to an increase in metal concentrations in the pore water. Below the oxidation zone, the neutrophilic primary zone was characterized by SO42- and Fe reductions induced by sulphate- and iron-reducing bacteria (SRB and FeRB). Drastic changes were observed along the remediation process, as the heavy metal concentrations decreased progressively close or below the detection limits together with the gradual installation of a near neutral pH and more reducing conditions. SO42- and Fe reductions were still observed in the deep section of the remediated tailings. Close to the surface, the presence of lithoautotrophic neutrophilic iron oxidizing bacteria such as Mariprofundus ferroxidans, Galionella spp, Leptotrix mobilis suggested that iron oxidation played an important role in the iron cycle. Other ubiquitous bacteria related to fresh water and marine environment were also detected in the wetland. Conclusions The implementation of a wetland promoted neutralization of the acid mine drainage and induced favourable geochemical conditions for the development of both SRB and FeRB, which lead to the metal removal from solution. References Diaby, N. 2009. Biogeochemical evolution of a marine shore tailings deposit during bioremediation. Ph. D. Thesis, University of Lausanne, Switzerland. 217 pp.