Upward mercury transfer by anecic earthworms in a contaminated soil.

Abstract Mercury (Hg) is a contaminant of global importance but its fate and impact in soils is overlooked. Earthworms are responsible for soil bioturbation but their interaction with Hg is poorly described. This study was conducted on a Hg contaminated site in Switzerland. The objectives were to determine the Hg tolerance of Lumbricus terrestris and Aporrectodea nocturna , and their capability to redistribute Hg from deep contaminated soil layers to remediated topsoil layers. Earthworms were incubated for 30 days in 35 cm height soil columns with soil Hg contents ranging from 0.19 to 83 mg Hg kg−1. 100% survival was observed except for the highest soil concentration (67% survival). Corresponding bioaccumulation factors ranged from 1 to 17. In parallel, Hg upward transfer by earthworms from a deep contaminated soil layer to a non-contaminated 40 cm thick surface layer was studied in 80 cm repacked soil columns. After 70 days, total Hg content of surface casts was 10 times higher than the non-contaminated soil. Furthermore, the 25–30 and 35–40 cm layers had a Hg content of 0.537 and 8.54 mg Hg kg−1 respectively, both exceeding the local threshold intervention value of 0.5 mg Hg kg−1. We concluded that (i) earthworms not acclimated to polluted soils can survive Hg levels as high as 85 mg Hg kg−1 in mesocosms, and (ii) a 40 cm thick remediated layer would be recontaminated after 6 months of earthworm activity if the underneath layer remained contaminated. Highlights • L. terrestris and A. nocturna survive mercury levels as high as 85 ppm in soil mesocosms. • Bioaccumulation factor is proportional to soil Hg content and ranges from 1 to 17. • Casts Hg content is proportional to soil Hg content with a regression slope of 0.7 • Anecic worms lift up Hg from a 40–80 cm deep contaminated layer to a clean top layer. • Remediated topsoil is polluted again after 6 months of worm activity. [ABSTRACT FROM AUTHOR]