The impact of periphyton layers on the performance of shallow-water covers to prevent oxidation of reactive tailings was studied at Louvicourt Mine (Aur Resources). A previous study reported the growth of a periphyton layer (biofilm) at the surface of the tailings, submerged under a 30 cm water cover in field experimental cells. The present study was initiated nine years after the tailings were submerged and integrated geochemical, mineralogical and biological data to provide an overall assessment of the long-term performance of the shallow water cover. Geochemical changes were monitored by in situ measurements of porewater chemistry. Samples of the biofilm, the oxic surface layer of tailings and the deeper tailings were also analyzed for total metals content, solid-phase speciation using sequential chemical extractions and detailed mineralogical analysis by SEM-based image analysis. Finally, molecular biological techniques (16S rRNA) were also used to characterize the microbial community associated with the biofilm. The mineralogical and chemical data confirmed that mobilisation of trace metals occurred in surface tailings but also indicated that the overlying biofilm effectively trapped the released metals. Results also indicate metal specificity in the nature of the processes involved and resulting fluxes. The sequential extractions suggested that Cd and Zn were captured by sorptive processes within the biofilm, while Cu was scavenged by the organic matter present in the biofilm. There was a 10 fold decrease in Cd and Zn fluxes to the overlying water over the last seven years whereas tailings switched from a sink to a source of Cu for the water cover. The Cu, Cd and Zn net releases to the overlying water calculated in this study were several orders of magnitude lower than the fluxes reported at other sites. Furthermore, the estimated water cover concentrations resulting from these fluxes would be well below discharge limits and equal to or below water quality guidelines. The results also highlighted that the disposal system had not reached equilibrium after nine years of operation. The tailings were still acting as a net source of dissolved solids to the overlying water and chemical changes were still observed in the deeper part of the porewater profiles. Finally, the pH and oxygen microprofiles and the molecular biological analyses of the biofilm community structure indicated that the biofilm included a variety of different metabolic pathways, and acted both as a source and a sink of H+ and oxygen depending on diurnal light conditions. This study concludes that biofilm development was beneficial since it likely protected tailings from resuspension and contributed to the maintenance of anoxic conditions in the underlying sediments. In addition, the biofilm trapped mobilized dissolved metals and decreased fluxes out of the tailings into the overlying water column.