Legacy heavy metal pollution from ancient silver-lead mines and floods: Chronicles from the Aulne estuary sedimentary banks (Brittany, France)

Metal mining can engender pollutions upon very large areas along the land-sea continuum, threatening both continental and marine ecosystems for decades to millennia after closure of the mines. This paper examines the pollution issued by silvereous lead mines of Huelgoat-Poullaouen (western Brittany, France) since the mid-18th century, from the point of view of downstream Aulne river estuary sediments. Sediment cores were retrieved from the estuary banks and analyzed using sedimentological, geochronological, geochemical and isotopic tools. Main results are: (i) An intense Pb, Zn and Cd pollution is stored within the sediments of the estuary, placing the Aulne amongst the most polluted river and estuary in France. (ii) Lead stable isotopes unambiguously attribute the pollution to the mines. (iii) The onset of the pollution dates back to 1770 CE, with levels reaching up to 1.300 mg.kg −1 for Pb, 1.600 mg.kg −1 for Zn and 12 mg.kg −1 for Cd at the apex of mine production (1770–1810 CE). Pollution levels then decrease to present-day levels, nonetheless showing peaks in line with subsequent phases of mine exploitation and floods which act as important conveyors of the pollution. (iv) Pollution stabilized around the 1950's at levels of around 300 mg.kg −1 for Pb and 500 mg.kg −1 for Zn. Such levels of pollution are still measured today on the surface sediments, showing that the pollution is still ongoing, due to the absence of any action being taken by the authorities at the level of the mining sites.