The potential wildfire effects on mercury remobilization from soil and biomass in the Mt. Amiata mining district

Natural wildfires are responsible for ~10% of total mercury (Hg) emissions to the atmosphere due to Hg volatilization from the tree biomass and Hg thermal desorption from soils. Mercury geochemical anomalies, such as Hg mining areas, utmost contribute to these emissions due to the occurrence of high Hg concentrations in vegetation and soil. Climate changes, increasing the occurrence of wildfires, are expected to further exacerbate this process, providing spikes of Hg concentrations in the atmosphere, possibly near to human sensible sites. In this study, we calculate the release of Hg by a hypothetical wildfire occurring in the Mt. Amiata Mining District (MAMD) in southern Tuscany (Italy), where past Hg mining affects local atmospheric Hg concentration. Bark (n= 25), wood (n=18), and leaves (n=3) of black pines (Pinus nigra Arn.) and chestnuts (Castanea sativa Mill, leaves were not considered due to winter sampling), were collected in two areas of the MAMD and in a background site in the Appennino Pistoiese (AP – 150 km from MAMD). The two MAMD areas (Abbadia San Salvatore, ABS and Vivo d’Orcia, VO) are located at increasing distance from the metallurgical site (1.5 and 3 km, respectively). The soil (top and sub) was collected (n=22) below trees at each location. For all plant portions we quantified: i) the Hg content; ii) the involved biomass by means of allometric equations, extrapolating the result to woodland areas of 630 m2 and 1066 m2 for ABS and VO; iii) the mass of Hg stored in each area. Topsoil and subsoil were similarly quantified for Hg and for Hg storage. To estimate Hg emissions during the wildfire we assumed that: i) barks and leaves release 100% Hg (complete burning); ii) only 20% of wood is combusted; ii) the topsoil burning releases 100% of Hg; iii) subsoil does not contribute to Hg emissions. Mercury was measured by means of a direct Hg analyzer (DMA), based on EPA method 7473. For all plant substrata, higher Hg contents were observed in pines with respect to chestnuts. At all sampling sites, Hg concentrations vary in the order soil > bark > leaves > wood, and especially high at ABS site. The calculated Hg storages (plant + soil) are 1375, 321 and 58 g/ha for ABS, VO, and AP, respectively. At ABS, soils act as the major reservoir for Hg, trapping 90-95% of the total Hg/ha. Among biomass, wood is the major reservoir for Hg (56%) followed by barks (42%); leave contributions seem to be negligible (2%). These findings should be included in the development of fire risk prevention strategies and for the implementation of existing fire management activities