The Impact of Pesticides - Academy Publish

atios of the pesticide amounts in the paddy soils increase according to the decreaseof paddy water volumes. The half-lives of herbicides mefenacet, dymron, andbromobutide in the water were estimated as 1.8–7.7 days, 2.1–9.6 days, and 2.4–3.1days (Ishii et al. 2004, Morohashi et al. 2012). The decrease of residue pesticides inthe paddy soils can be interpreted using first-order reaction kinetics. In addition, therespective half-lives of phthalide, mefenacet, dymron and bromobutide in the soilwere estimated as 20-31 days, 12-21 days, 30 days and 5.2-10 days, respectively(Iwashita et al. 2008, Ishii et al. 2004, Morohashi et al. 2012).Fig. 1. Diagram of an applied pesticide moving from a paddy field to a river.RUNOFF OF PESTICIDES FROM PADDY FIELDS TO RIVERSSome pesticides applied to paddy fields flowed into rivers via drainage channels.The runoff events of the aerially applied pesticides occurred in the nearest drainagechannels immediately after the application. The runoff events were also caused byrainfall (Hasukawa et al. 2009; Maeda et al. 2008). It is reported that transport ofpesticides by surface runoff during rainfall events is a major process contributing topesticide contamination of rivers (Fox et al. 2007; Guo et al. 2004). However, therunoff immediately after the application was the major process for the aerial appliedpesticides to paddy fields. In cases of ground applications, the runoff of pesticidesfrom paddy fields was caused mainly by the drainage of paddy waters at severaldays after application, and by rainfall (Morohashi et al. 2012).The runoff ratios of pesticides from paddy fields to drainage channels or rivers arepresented in Table 1. The reported values depended on the paddy fields themselvesand water control as well as the application methods and weather conditions asdescribed above. Among the pesticides, the runoff ratios of fungicides andAcademyPublish.org - The Impact of Pesticides204