Speciation, mobility and bioaccessibility of Hg in the polluted urban soil of Idrija (Slovenia)

- The study uses a comprehensive dataset of geochemical data.
- Hg distribution and mobility was tested in a critically Hg polluted urban soil.
- Water-soluble and bioaccessible Hg concentrations are elevated in Idrija's soil.
- About 40% of Hg is bound to potentially transformable Hg matrix components.

The presented study deals with the distribution of mercury (Hg) binding forms, water-soluble Hg fraction and bioaccessible Hg fraction in the urban soil of Idrija, where the world's second largest Hg mine is situated. Mercury had been extracted for 500 years, leaving behind a legacy of highly contaminated soils and aqueous ecosystems.The soil was sampled at two different depths (0-10 cm and 10-20 cm) in 45 locations situated in urban green spaces. Additionally, four garden soil samples were collected in order to compare the results with respect to land use. Solid-phase Hg binding forms were determined with the Hg thermal desorption technique, aqueous Hg mobility with the use of water leaching test and gastric bioaccesibility with the use of simulated stomach acid extraction.The results of the Hg thermal desorption measurements detected the presence of matrix-bound Hg and cinnabar, which are very similarly distributed at both soil depths. The median relative amount of matrix-bound Hg, which is potentially transformable, is ~ 40%, whereas that of cinnabar is mostly immobile, at ~ 60%. Water-soluble Hg concentrations range between 3.9 and 106.5 μg/kg corresponding to 0.002%-1.2% of aqua regia soluble Hg (ARS Hg) in topsoils (n = 45), and between 2.7 and 178.7 μg/kg (0.001%-0.29% of ARS Hg) in subsoils (n = 45). In garden soils (n = 4), water-soluble Hg ranges between 20.3 and 48.8 μg/kg (0.01%-0.09% of ARS Hg), and the concentrations remain within the concentrations in adjacent urban top- and subsoil. Gastric bioaccessible concentrations range between 0.9 and 166.1 μg/kg (0.002%-3.3% of ARS Hg) in topsoils (n = 44). Gastric bioaccessible Hg in garden soils (n = 4) ranges between 74.7 and 442.2 μg/kg (0.02%-2.04% of ARS Hg), showing similar or higher concentrations than in adjacent urban topsoils, which indicates a possible positive influence of gardening practices on gastric Hg bioaccessibility.Both water-soluble and gastric bioaccessible concentrations in Idrija's urban soil are increased and, as such, important for the local population due to their long-term daily-based exposure. While water-soluble Hg may lead to elevated Hg concentrations in edible plants or groundwater, gastric bioaccessible Hg may lead to elevated concentrations of Hg in the target vulnerable groups of population (children and gardeners), who often undergo hand-to-mouth exposure.