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Summary of monitoring results

Chapter 10, PLAP - Monitoring results May 1999 - June 2002

The monitoring data identified three different leaching patterns for the applied pesticides - no leaching, slight leaching and unacceptable leaching (see Table 28). It should be noted, though, that the present evaluation of the leaching risk of many of these pesticides is still preliminary as their potential leaching period extends beyond the current monitoring period. This does not apply to those pesticides marked with a single asterisk in Table 28. The monitoring results indicate an unacceptable degree of leaching by two of the applied pesticides or their degradation products.

  • Two degradation products of metribuzin - metribuzin-diketo and metribuzin-desamino diketo - leached from the root zone (1 m b.g.s.) at average concentrations exceeding 0.1 µg/l. Both degradation products appear to be relatively stable and leached throughout the entire monitoring period. Average concentrations reaching 0.1 µg/l were thus seen as much as three years after application. There was also evidence that their degradation products may be present in the groundwater several years after application. At both sandy sites (Tylstrup and Jyndevad), previous application of metribuzin has caused marked groundwater contamination with its degradation products.
  • The findings indicate that glyphosate, when applied in late autumn, can leach through
    the root zone at unacceptable concentrations in loamy soils. At the loamy sites Estrup and Silstrup, glyphosate leached from the root zone into the drainage water at average concentrations exceeding 0.1 µg/l. At Estrup its degradation product AMPA leached at an average concentration exceeding 0.1 µg/l. This appeared to be attributable to a combination of pronounced macropore flow occurring shortly after application and limited sorption and degradation capacity. Long-term leaching was especially pronounced with AMPA, which was frequently detected more than one and a half years after application. So far the leaching of AMPA and glyphosate has been confined to the depth of the drainage system and they have rarely been detected in monitoring screens located below the depth of the drainage system. Evidence of glyphosate leaching was only seen in the loamy soil, and the leaching risk was negligible at the coarse, sandy soil site at Jyndevad. Infiltrating water passed through a matrix rich in aluminium and iron, thereby providing good conditions for sorption and degradation.
The monitoring data also indicate leaching of a further fourteen pesticides, but not in unacceptable levels, however. Although the concentration exceeded 0.1 µg/l in several samples, the average concentration did not. This is summarized in Table 29, which shows the number of samples in which the various pesticides were detected at each site and the maximum concentration. Apart from the sandy soil site at Jyndevad, where incipient leaching of desethylterbuthylazine (degradation product of terbuthylazine) was observed, leaching within this group of pesticides was only observed at the loamy soil sites, where leaching was associated with pronounced macropore transport that resulted in very rapid movement of pesticides through the unsaturated zone.

Table 28. Pesticide leaching at the 6 PLAP sites. The number of asterisks indicates the number of monitoring periods the pesticide was included in the PLAP. The colours indicate the degree of leaching. Pesticides applied in spring 2002 are not included in the table.
Table 28

On several occasions single precipitation events caused leaching to the drainage water in high concentrations. In most cases the concentration decreased to a low level after a short period of time, and leached mass and average concentration in the drainage water were generally low. The observed leaching was typically confined to a 6-9 month period following pesticide application, exceptions being metamitron-desamino and bentazone. With these two compounds there was evidence of slight leaching one year after application.

Eleven of the 27 pesticides applied - about 40% - did not leach during the 3-year monitoring period. This group includes the three different sulfonylureas - metsulfuronmethyl, triasulfuron and tribenuronmethyl - which were tested on several field applications.

Table 29. Number of samples in which the various pesticides were detected at each site with the maximum concentration (µg/l) in parentheses. The table only encompasses those pesticides/degradation products detected in either several consecutive samples or in a single sample in concentrations exceeding 0.1 µg/l. Pesticides applied in spring 2002 are not included.
Table 29

Tribenuronmethyl was hence applied on 4 different sites under different hydrological conditions with percolation (1 m b.g.s.) during the first month after application ranging from 0 to 114 mm. The monitoring results provide no evidence of leaching of any of the applied sulfonylureas or their degradation products, including triazinamin and triazinamin-methyl.

Fenpropimorph and propiconazole were tested at all 6 sites. They were always applied during spring, and percolation during the first month after application ranged from 0 to 13 mm. With fenpropimorph the leaching risk was found to be negligible at all sites. With propiconazole, slight leaching was seen at just one of the 6 sites. These findings complement those of the sorption/degradation studies, which indicate that the leaching risk is low due to strong sorption to the topsoil. Why leaching of propiconazole was particularly prevalent at the Estrup site is unclear and no degradation/sorption data are available to help clarify the matter. The hydrological conditions could play a role since percolation started much earlier in the autumn and was much more intense at Estrup than at any of the other sites.
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