Change and behavior of residual mercury in paddy soils and rice of Japan.

Paddy soils collected near Agano River, Niigata Prefecture in Japan. Agano River is located on an attack area of Niigata-Minamata disease. Soil samples were collected from 122 sites in 1989 and 120 sites in 1997. As a result, the total mercury concentrations were ranged from 0.019 to 0.62ppm based on dry weight with a mean 0.155ppm in 1989, and from 0.015 to 0.34 with a mean 0.146ppm in 1997. The decrease of residual mercury concentrations in paddy soils was only 0.009ppm in the interval of eight years. However, the mercury concentrations in paddy soils were about 3 times as large as that of uncultivated soils in its surroundings. It is suggested that the soils in paddy fields still contain the mercury residues to be influenced by some agricultural fungicide. Mercury concentrations in rice were natural background level. A comprehensive evaluation shows that the concentrations in boil rice with an average content of under 0.001ppm total mercury pose no health risk to Japanese people consuming rice of 200g/day.