Mercury content in volcanic soils across Europe and its relationship with soil properties

Abstract

Volcanoes are a natural source of Hg, whose deposition can occur in neighbouring soils. This study examines the role of soil compounds in the geochemical behaviour of total Hg (Hg T ) in volcanic soils. An estimation of Hg from lithological origin is also assessed to ascertain the relevance of other sources in Hg T accumulated in volcanic soils. Twenty soil profiles developed from volcanic materials and located across European volcanic regions were selected for this study. The general characterisation of soils included total C, N and S content and Al and Fe distribution determined using traditional methods. The total content of major and trace elements was determined using X-ray fluorescence spectrometry (XRF). The total Hg content of soil samples was measured with atomic absorption spectroscopy using a solid sample Hg analyser. Lithogenic Hg was calculated in the uppermost soil considering Al, Ti and Zr as conservative reference elements. Several statistical analyses (Pearson correlations, Mann–Whitney tests, stepwise multiple regressions and analysis of variance) were carried to ascertain the role of soil parameters and characteristics in the Hg accumulation in volcanic soils. The total Hg ranged from 3.0 to 640 ng g–1 and it tended to diminish with soil depth except in some soils where the lithological discontinuities resulted in high values of Hg T in the Bw horizons. More than 75% of the Hg T variance could be attributed to distinct contents of organic matter, Al– and Fe–humus complexes and inorganic non-crystalline Al and Fe compounds in “andic”, “vitric” and “non-andic” horizons. The degree of pedogenetic soil evolution notably influenced the Hg T soil content. Lithogenic Hg (1.6–320 ng g–1) was correlated with Al–humus complexes and clay content, suggesting the relevance of pedogenetic processes, whereas exogenic Hg (1.4–180 ng g–1) was correlated with total C and cation exchange capacity, indicating an origin predominantly from atmospheric deposition. The total Hg content of soils from volcanic areas demonstrated the role of volcanism as a source of this metal. The degree of evolution of the volcanic soils and their typical compounds (metal–humus complexes, organic matter and inorganic non-crystalline Al and Fe compounds) are involved in Hg accumulation. The mercury accumulated in the analysed soils is a mixture of the Hg that is present in volcanic soil parent material and the Hg that is deposited from the atmospheric pool, which combines natural and anthropogenic sources.