Geochemistry of toxic trace elements (As, Sb, Pb, Cr, Mo, Bi, Cd, Tl) in the soils from the Kanto District, Japan: Preliminary study for the soil geochemical mapping (part 3)

摘要

We have studied the geochemical behavior and the origin of toxic trace elements in volcanic ash and alluvial soils. The volcanic ash and soil samples were collected from the southern to northeastern Kanto district, Japan. The concentrations of heavy metals containing toxic trace elements in the tephra layers are generally lower than those of loam layers in the Kanto Loam Formation. The tephras are proved to be poor in alkali and alkali-earth metals, and rich in Al_2O_3, TiO_2, H_2O, and toxic trace elements in most cases through its weathering process. The elements, such as As, Sb, Pb, Bi and Cd are generally enriched in the uppermost layer of the volcanic ash soils. This may be caused by biogenic concentration process, but not by recent environmental pollution. The vertical variation of the elemental concentrations in alluvial soils are generally smaller than that of volcanic ash soils Several sand layers derived from basement rocks are found in the core samples. Compared to the mud layers, sand layers are poor in heavy metals including the toxic trace elements. The alluvial soils existed under the effects of saline water in the past tense are significantly dominant in As and Sb, and depleted in Mo. This may results from the fixation and diffusion of these elements during diagenesis. Compared to the crustal abundance, both the volcanic ash and alluvial soils are rich in As, Sb, Pb and Bi, while there is no significant difference in the Cd and Tl contents. The enrichment of the four toxic elements may be caused by biogenic and diagenetic enrichment process, contribution of seawater and the supply of long-range Aeolian dust derived from the Asian continent.