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Pesticides in air and in precipitation and effects on plant communities SummaryPesticides in precipitation The concentration of pesticides in precipitation was investigated during the period May 1996 - December 1998 at three sites on Zealand in Denmark (Gadevang, Gisselfeld and Lorup). These sites had not been directly affected by local emissions. The samples were analysed for isoproturon (including 2 metabolites), the phenoxyalkanoic acid herbicides MCPA, mecoprop, and dichlorprop plus bentazone (including 2 metabolites), and DNOC. Effects of pesticides This project was part of a larger project, which studied the effects of pesticides in precipitation on plants and plant ecosystems. The highest measured concentrations were 0.9µg/L for isoproturon and 0.6µg/L for phenoxyalkanoic acid herbicides. Pesticides in precipitation when applicated In most cases the concentrations in precipitation were found at times when the pesticides were known to be applied to crops. Combined samples for these three sites for the period September 1996 - November 1997 were analysed for 44 compounds. Concentrations over the detection limit were only found for isoproturon, metamitron, DNOC (2-methyl-4,6-dinitrophenol), mecoprop, methabenzthiazuron, 2-hydroxyterbuthylazine, terbuthylazine and 2,4-D. DNOC in precipitation caused by photochemical reactions Unexpectedly high concentrations of DNOC (0.38 - 4.5µg/L) were found during the whole sampling period. Although DNOC has not been applied in Denmark since 1986, it has been detected in other investigations in the top layers of the ground water and in streams. Current literature indicates that DNOC is likely to be formed by photochemical reactions of toluene and nitrogen oxides in the atmosphere probably from traffic pollution. The atmospheric deposition of DNOC, mecoprop and isoproturon is respectively 7.5-, 0.3- and 0.3g/ha/year. Generally, pesticides will be transported over distances of more than several hundred kilometers before they are deposited, unless they are (photochemically) degraded in the atmosphere at a high rate.
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