Geochemical Distributions of Heavy Metals and Cr Behavior in Natural and Cultivated Soils of Volcanic Jeju Island, Korea

Volcanic ash soils, representative soil series (Andisols) on Jeju Island, Korea were collected to investigate the geochemical and mineralogical characteristics. Distributions of heavy metals and Cr behavior, in particular, were further assessed according to land uses (natural or cultivated). The natural soil samples had ferromagnesian minerals mostly in mineralogy, high levels of organic matter, silty textures, and acidic pH. Low amounts of SiO2 and high contents of Al2O3 and Fe2O3, were found in the Jeju soils, which are typical characteristics of Andisols compared to other soil orders. The mean total concentrations of Zn (127 mg/kg), Ni (183 mg/kg), Co (84 mg/kg), and Cr (677 mg/kg) were higher than worldwide values in most of the natural soils. Especially, Cr was a main soil contaminant, exceeding 1000 mg/kg in some soil series. The relationships between heavy metals and major elements indicated a ferromagnesian mineral origin for heavy metals and secondary associations with Fe and Al oxides during weathering processes. Cultivated soils also had similar volcanic ash soil properties, but showed more clayey textures, greater SiO2, and lower Al2O3 and Fe2O3 contents compared to natural soils. The average heavy metal content was not affected by cultivation, and characteristic behavior was maintained, except for slight increases in Cu, Pb, and Zn contents in surface soils because of fertilizer inputs. The only distinct decrease was in Cr content in the surface and subsurface of cultivated soils. This decrease might reflect the formation of a soluble organic complex of Cr(III), based on the significant increase in the sodium pyrophosphate-extracted fraction in sequential extractions. However, the high reducing capacities of volcanic ash soils compared to inland non-volcanic soils suggest that the potential for contamination by Cr(VI) can be significantly very low.