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 (Hg⁰) 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^II). Elemental mercury (Hg⁰) was undetectable in soils, yet we observed substantial Hg⁰ emission (20-1392 ng m^-2 h^-1) from 27 soil plots contaminated with Hg (0.2-390 mg Hg kg^-1). The emissions of Hg⁰ were calculated for 1274 parcels covering an area of 8.6 km² of which 12% exceeded the Swiss soil remediation threshold of 2 mg Hg kg^-1. The annual Hg⁰ emission from this area was approximately 6 kg a^-1, 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^II is likely an important pathway to form volatile Hg⁰ 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.