Persistent imprint of historical metallurgy in an alpine watershed evidenced from lake sediments Pb isotopes

The Pb-Ag mine of Peisey-Nancroix was operated between 1734 and 1824. The associated smelters emitted Pb-rich fumes that were reported as threatening for local people all along their period of activity. Lake La Plagne is located at 2 100 m a.s.l., 7 km uphill the former mine. Considering that smelters fumes were transported uphill by prevailing winds, studying the metal contamination within Lake La Plagne sediments offers the rare opportunity to reconstruct the local atmospheric contamination, as well as the remnant catchment area contamination in a context where historical conditions of exploitation are well-constrained.

Sediments deposited before mining and smelting only contains 30 mg/kg of Pb, whereas the sediments deposited during smelting contains up to 148 mg/kg of Pb. Recent sediments deposited after mining activity period also present an enrichment in Pb (up to 58 mg/kg). Mineralogical observations (SEM-FEG) suggest that within contamination peaks, Pb is essentially associated with infra µm-scale Mn-Fe (hyrdr-)oxides.

Pb isotopes were measured on selected samples collected along the lake sediment core prior mining, during mining and smelting, and after mining. The Pb isotopic ratios of all lake sediments (n=12) indicate mixing between the isotopic ratios of the ore (n=39; galena : ²⁰⁸Pb/²⁰⁶Pb =2.092 ± 0.004 and ²⁰⁶Pb/²⁰⁷Pb= 1.173 ± 0.002), and those of the deepest lake sediments (n=2; ²⁰⁸Pb/²⁰⁶Pb =2.041 ± 0.002 and ²⁰⁶Pb/²⁰⁷Pb= 1.209 ± 0.0004) that are representative of the geochemical background. The sediments deposited long after mining still present a significant influence of local ore-derived Pb, suggesting remobilisation from the watershed of Pb inherited from the smelting period.