Environ. Sci. Technol. 2007, 41, 2210-2215
PCDD/F Contamination over Time in Japanese Paddy Soils N O B U Y A S U S E I K E , * ,† NOBUHISA KASHIWAGI,‡ AND TAKASHI OTANI† National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan, and The Institute of Statistical Mathematics, 4-6-7 Minami-Azabu, Minato-Ku, Tokyo 106-8569, Japan
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 ΣPCDD/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 ΣPCDD/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.
Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) are persistent toxic compounds ubiquitously found in the environment. PCDD/Fs are released into the environment mainly through the incineration of wastes. In addition, a variety of chlorinated compounds such as polychlorinated biphenyls (PCBs) (1, 2), pentachlorophenol (PCP) (3-5), and 2,4,6-trichlorophenyl 4-nitrophenylether (chlornitrofen, CNP) (4-6) contain considerable amounts of PCDD/Fs as impurities. In Japan, PCP and CNP were commonly used as paddy herbicides in the past. PCP was used from 1958 to 1990, and CNP was used from 1965 to 1996. It is estimated that a total of 170 × 106 kg of PCP and 78 × 106 kg of CNP (active ingredient bases) were used (7). PCDD/Fs released from PCP and CNP were estimated to be 120-440 and 180-270 kgWHO TEQ, respectively (4, 5). The annual emissions of PCDD/ * Corresponding author phone: +81-29-838-8329; fax: +81-29838-8199; e-mail:
[email protected]. † National Institute for Agro-Environmental Sciences. ‡ The Institute of Statistical Mathematics. 2210
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 7, 2007
Fs from combustion in 1997 were estimated to be 6.9 to 7.1 kg-WHO TEQ (8). PCDD/Fs emissions from PCP and CNP were extremely large compared with those from combustion. According to a nationwide survey performed by the Ministry of the Environment and the Ministry of Agriculture, Forestry and Fisheries of Japan (9), WHO TEQ in paddy soils was significantly (P < 0.05) higher than that in uncultivated soils (10). Additionally, PCDD/F congeners such as OCDD and 1,3,6,8-/1,3,7,9-TeCDD are major PCDD/Fs in PCP and CNP (3-6) as impurities, respectively, and are accumulated in surface sediments (11-13) and fish (6, 11, 13, 14) in Japan. It can be indicated that the PCDD/Fs from these herbicides applied to paddy fields have flowed out into the surrounding catchments. Thus, paddy fields have played an important role as a secondary source of PCDD/Fs in the aquatic environment for half a century. It is therefore important to understand the temporal trends of PCDD/F levels in paddy soils for the prediction of future contamination status. Archived soils (15, 16), vegetation (17, 18), and sediment cores (19-25) are very useful materials for tracing the temporal change of contaminants in the environment. In previous studies of Japanese coastal and lake sediments core (22-25), PCDD/F concentrations were increased dramatically during the 1960s, but began decreasing gradually in the 1970s to early 1980s. Moreover, these reports indicated that the OCDD concentration increased intensely during the early 1960s due to the increased usage of PCP. On the other hand, 1,3,6,8-/1,3,7,9-TeCDD concentrations were increased dramatically during the late 1960s due to the increased usage of CNP. Thus, temporal variation of PCDD/F levels and sources in sediment cores from limited sites were evaluated. Nevertheless, in paddy soils, the temporal trends of PCDD/F levels and sources, and mass balance data are still meager. Paddy soils have been periodically collected all over Japan since 1960 and preserved at the National Institute for AgroEnvironmental Sciences. Results from analysis of these archived soils can derive the temporal change of PCDD/Fs contamination status exactly compared with sediment core analysis. In this study, we analyzed them to trace the changes in PCDD/F concentrations. Qualitative and quantitative identification of PCDD/F sources were evaluated in the paddy soils by principal component analysis (PCA) and chemical mass balance (CMB) (26), respectively. The half-lives of PCDD/Fs for disappearance from the paddy soils were calculated in order to estimate the PCDD/F mass balance over the past 40 years and to predict the future contamination status of Japanese paddy fields.
Experimental Section Samples. Air-dried and sieved (
