Severe and contrasted polymetallic contamination patterns (1900–2009) in the Loire River sediments (France)

The Loire River basin (117,800 km2, France) has been exposed to multiple sources of metals during the last 150 years, originating from major mining districts (coal and non-ferrous metals) and their associated industrial activities. Geochemical archives are established here from the analysis of a 4 m sediment core in the downstream floodplain and then compared to stream bed sediments from pristine monolithological sub-basins and from bed and bank sediments in impacted tributaries. The contamination is assessed for 55 major and trace elements through their enrichment factors to Al (EF), normalized to the pre-anthropogenic background. Archives from 1900 to 2009 show enrichment (EF < 1.3) not only for Ba, Be, Cs, Ga, Rb, REE, Sr, V, and Zr but also for U and Th, despite U mining activities until the 1990s. From 1900 to 1950, the level of contamination is severe for Hg, Au, Ag (10 < EF < 30), important for Sb and Sn (3 < EF < 7) and moderate for Cu, Pb and Zn (1.5 < EF < 3). This state was mostly attributed to coal uses and metal mining. During the period 1950–1980, severe polymetallic contamination is noted for Hg (EF up to 53), Cd (23), Ag (18), Zn (6.2), Cu (6.0), Sn (5.6), Pb(4.8), Sb(4.4) and for new impacted elements as Bi (23.8), As (3.7), Cr (3.4), W (3.1), Mo (2.6), Ni (2.8), Co (1.65) due to mines, smelters, industries and from urban sewers, collected mostly after 1950 (total population of 8.4 million people). The limited dilution by detrital material (Loire sediment load about 1.5 Mt/year) is an additional cause of such severe contamination. After 1950, river eutrophication is well marked by the general increase of endogenic calcite (EF (Ca) = 4), diluting all other elements by 20%. From 1980 to 2009, all contaminants, except Au (EF = 100), decrease steadily.

► The environmental history of the Loire basin was browsed for the first time.
► The oldest source of Hg, Sb and Sn contamination was coal-mining and associated use.
► A severe polymetallic contamination (1945–1980) was related to mining activities.
► A generalized and gradual decontamination of the Loire sediments started in the 1980s.
► Endogenic calcite formation had a dilution effect on metal content by 20% in the 1990s.