PCDD/F contamination over time in Japanese paddy soils.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs)were analyzed in preserved paddy soils periodically collected from 7 sites around Japan since 1960 to trace the changes in concentrations, to elucidate their sources, and to estimate their mass balance in Japanese paddy fields. Concentrations of sigma PCDD/Fs in paddy soils from all sites increased during the 1960s and the 1970s, then decreased. The results of principal component analysis and chemical mass balance based on functional relationship analysis indicate that the increase in sigma PCDD/F concentrations in paddy soils was due to the increased use of pentachlorophenol (PCP) and chlornitrofen (CNP); more than 95% of PCDD/Fs in all paddy soils were derived from impurities in these herbicides. The half-lives of PCDD/F for disappearance from the paddy soils were estimated to be 10 to 20 years (mean: 17.3 years). The estimated PCDD/F mass balance using the above half-life during the past 40 years in paddy fields indicates that about 80% of PCDD/Fs have disappeared. The soil puddling (mechanically mixing of paddy soil with pooled irrigation water) is implied as one reason for the PCDD/F disappearance from paddy fields. However, as the amounts of PCDD/Fs added through the use of PCP and CNP were extremely large, PCDD/F concentrations in Japanese paddy soils will decrease gradually, and PCDD/F runoff from paddy fields to surrounding catchments will continue.