Risk Assessment of Heavy Metal-Contaminated Soils with Reference to the Aging Effect(Symposium 3.5.2 Risk Assessment and Risk Based Remediation,<Special Issue>International Symposium: Soil Degradation Control, Remediation, and Reclamation, Tokyo Metropoli

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To estimate the effect of aging on the availability of heavy metals in soil, the relationship between availability of metals and time after contamination was investigated with a range of soils and metals. Five metals, nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb), were added as nitrate solutions (2.5mmol kg^<-1> for single metals; 2.5mmol kg^<-1> for mixed metals, or 3 times their background levels as single metals) to 4 kinds of soil samples with variable chemical properties: dune soil (Psamments), alluvial soil (Aquents), red soil (Udults) and volcanic soil (Udands). The soils were incubated at field capacity for 1 day, 1 week, 2 weeks, 4 weeks or 6 months, and the exchangeable (readily available) fraction was extracted with 1M ammonium acetate by ion-exchange reaction. The available fraction of the metals was extracted with 0.005M DPTA solution (pH 6) by chelating reaction. The concentrations of the extracted metals were then determined by inductively coupled plasma atomic emission spectroscopy. The quantity of metals in the exchangeable and available fractions under single metal contamination conditions at the same rate (2.5mmol kg^<-1>) were generally in the order of volcanic soil<red soil<alluvial soil<dune soil for soils and in the order of Cu/Pb<Ni/Zn<Cd for metals. Mixed metal contamination conditions considerably increased the quantity of metals in both the exchangeable and available fractions, especially for Pb and Cu, compared with single metal contamination. For realistic metal contamination at the rate equivalent to 3 times the background level of the metals, a temporal decrease of metal availability, i.e. aging, was evident, especially for red soil with a high amount of oxides and for Ni and Zn. Temporal changes, however, diminished to <3% per week after 4 weeks for all soils and metals. These results enabled us to understand the natural attenuation of metal contamination and assess the risk of soil contamination by determining exchangeable or available fractions of metals in 4-week incubated soils after artificial single or mixed metal contamination.

Risk Assessment of Heavy Metal-Contaminated Soils with Reference to the Aging Effect(Symposium 3.5.2 Risk Assessment and Risk Based Remediation,<Special Issue>International Symposium: Soil Degradation Control, Remediation, and Reclamation, Tokyo Metropoli

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