The impact of ancient mining activity on the critical zone: the example of the Peisey-Nancroix Pb-Ag mine (19th-20th century)

International audienceA better understanding of long-term trace metals dispersal and their bioavailability in the critical zone is required toanticipate future impacts of present-day mining activity. The Pb-Ag district of Peisey-Nancroix (northern French Alps) isan excellent target to study the long-term environmental impacts (> 150 years) of former mines and trace metal dispersalin the critical zone, considering that every steps of the mining cycle, from ore extraction to ore processing, were locatedon a few hectares site that has been unmodified since mine closure in 1866 (no reclamation). A cross-disciplinary studyis conducted using several proxies (ore, waste dumps, soil, water, stream sediments and related microbial communities,vegetation) in order to characterize present-day contamination on the mining site, to determine trace metals transfer inthe various compartment of the critical zone, and to evaluate the effect of trace metals on microbial communitiesfunctioning.Topsoils are significantly enriched in Pb, and locally in As, Sb and Cu compared to the local geological background.Soils developed on top of waste deposits contains up to 3.4% Pb, 1695 ppm Zn, 381 ppm Sb and 168 ppm Cu. Tracemetal transfer from contaminated soils to the vegetation is evidenced by XRF profile across larch tree trunk sections.Contamination of streambed sediments is restricted to the vicinity of the mine and to the mine drainage outlets. Theconcentration of Pb2+ in pore waters, which ranges from 18 to 56 µg/L (EU potability limit = 10 µg/L), is well correlatedto the sediment total Pb content and represents the main source of bioavailable metal. Microbial investigations will beperformed to evaluate the ecotoxicological effects of Pb and Cu exposure on the genetic structure and the ecologicalfunctions (respiration, denitrification, organic matter decomposition...) of the bacterial communities naturally present inthe contaminated sediments and soil