Mercury Partitioning and Behavior in Streams and Source Areas Affected by the Novo-Ursk Gold Sulfide Tailings (West Siberia, Russia)

We studied the behavior of mercury in acid mine drainage (AMD) and in portions of the Ur River affected and non-affected by AMD near the Ursk sulfide tailings (Siberia) before (2007–2009) and after (2011–2019) the beginning of tailings reprocessing operations. Mercury occurs in water as dissolved plus colloidal (Hg dc ) and colloidal (Hg C ) species or is adsorbed on suspended particles (Hg susp ). The mercury species were classified as either reactive (Hg R ) or non-reactive (Hg NR ), depending on their capacity to reduce to Hg 0 by reacting with SnCl 2 . The composition and pH of the AMD and river waters change downstream of the AMD input. Mercury concentrations ranged from 1.8 to 89 μg/L (for the entire monitoring period) and increased with AMD pH. High-pH conditions are unfavorable for the precipitation of jarosite, which can adsorb Hg from water, and thus mercury remains mobile. Hg NR is more abundant than Hg R in the river and in AMD particulates. Non-reactive mercury in the river water is associated with CH 3 Hg + , which correlates with total organic carbon (TOC), while Hg in the AMD samples is bound to HgS (s) , m-HgS (s) , HgSe (s) , and Hg-jarosite. Hg R species are associated with particulates in all water bodies as Hg 0 (liq) , Hg(OH) 2(aq) , HgCl 2(aq) , and Hg 2+ adsorbed by OH groups on the surfaces of mineral grains. Judging by the TOC concentration and the pH and Eh of river water, which local people use for fishing, Hg is prone to methylation upstream and downstream of the AMD input. The Hg enrichment of local surface waters is due to both a naturally elevated background in a Hg-rich province and to the mining and processing operations.