The Impact of Pesticides - Academy Publish

The greatest number of pesticides are recorded in the months of May, July,September and November (see Figure 3). These figures derive from measurementsat around 400 monitoring sites (www.bestrijdingsmiddelenatlas.nl). In the wintermonitoring takes place at considerably fewer sites. This varied monitoring intensityis geared to the months in which most crops have their growing season and are thusstill in the field. And this has a direct relation with the months in which pesticide useis highest.In the months of February to August the percentage of compounds exceeding thestandards is often around 10%. In October this percentage peaks further, witharound 15% of the pesticides monitored exceeding the MPC. In this particularmonth about 300 compounds are included in the monitoring programme. The lowestpercentage of exceedances occurs in the winter months of November and December.These colder and often wetter months are obviously outside the growing season formany crops, implying less pesticide use, with the possible exception of soilfumigants used to prepare the soil for the next early growing season. During thisperiod monitoring is often focused more on the larger watercourses into whichmultiple streams flow. It should be noted, though, that there is little variation in thepercentage of compounds exceeding the standards in the course of the year, varyingessentially between 5 and 10% of the compounds. All in all, then, year-roundmonitoring of water quality, even outside the growing season of many crops, is byno means superfluous.Which compounds cause most breaches of MPC standards?Table 1 shows the compounds that exceeded the MPC most frequently in 1998,2004 and 2009, ranked according to degree of exceedance using the followingprocedure. a) Compounds are ranked on the basis of the weighted number ofmonitoring sites at which the MPC for the compound was exceeded, i.e. correctingfor the number of monitoring sites by taking the percentage exceedance of thestandards. b) Compounds monitored at fewer than ten sites have been ignored. c)Allowance has been made for the degree of standards exceedance, weighting theresults according to the following classes: 0 ( MPC and 2x MPC and 5x MPC exceedance). The compounds areAcademyPublish.org - The Impact of Pesticides196