Mercury emission from industrially contaminated soils in relation to chemical, microbial, and meteorological factors.

The Minamata Convention entered into force in 2017 with the aim to phase-out the use of mercury (Hg) in manufacturing processes such as the chlor-alkali or vinyl chloride monomer production. However, past industrial use of Hg had already resulted in extensive soil pollution, which poses a potential environmental threat. We investigated the emission of gaseous elemental mercury (Hg) from Hg polluted soils in settlement areas in the canton of Valais, Switzerland, and its impact on local air Hg concentrations. Most soil Hg was found as soil matrix-bound divalent Hg (Hg). Elemental mercury (Hg) was undetectable in soils, yet we observed substantial Hg emission (20-1392 ng m h) from 27 soil plots contaminated with Hg (0.2-390 mg Hg kg). The emissions of Hg were calculated for 1274 parcels covering an area of 8.6 km of which 12% exceeded the Swiss soil remediation threshold of 2 mg Hg kg. The annual Hg emission from this area was approximately 6 kg a, which is almost 1% of the total atmospheric Hg emissions in Switzerland based on emission inventory estimates. Our results show a higher abundance of Hg resistance genes (merA) in soil microbial communities with increasing soil Hg concentrations, indicating that biotic reduction of Hg is likely an important pathway to form volatile Hg in these soils. The total soil Hg pool in the top 20 cm of the investigated area was 4288 kg; hence, if not remediated, these contaminated soils remain a long-term source of atmospheric Hg, which is prone to long-range atmospheric transport.