Abstract:
Off-target deposition is a potential route of pesticide entry into the environment. To
determine off-target deposition of pesticides, various deposition samplers are used. A study
was carried out to compare a number of deposition samplers (chromatography paper, glass
microscope slides, glass petri dishes, polyacrylic rods) used in spray drift determination of
atrazine (and terbuthylazine when present in the formulation). The first sampling campaign
was conducted to compare three horizontally oriented deposition samplers (glass microscope
slides, chromatography paper, and glass petri dishes) and two vertically oriented deposition
samplers. The second sampling campaign consisted of horizontal and vertical
chromatography paper only, as this sampler provided the best results in the first sampling
campaign. In each of the spraying events of atrazine three sampling lines were used. In the
first application, the sampling distances were 0, 10, 25, 50, 100, 200, and 400 m. In the
second application, the sampling distances were 0, 10, 30, 50, and 100 m. In addition,
corresponding upwind control deposition samplers were also included in each pesticide
sampling campaign. Analysis of pesticide deposition samples for both sampling campaigns
was done by GC-MSD. The limit of detection (LOD) for atrazine was 0.006 μg.ml-1 (1st
sampling campaign) and 0.01 μg.ml-1 (2nd sampling campaign), and 0.02 μg.ml-1, for
terbuthylazine (2nd sampling campaign) respectively. Excellent recoveries of >70% were
obtained by sonication extraction of chromatography paper, glass petri dishes, glass
microscope slides, and polyacrylic rods. The precision in terms of %RSD was less than 20%.
In the first sampling campaign, the chromatography paper was the best deposition sampler,
and atrazine levels deposited on the samplers were up to 0.012 μg.cm-2. In the second
sampling campaign, both atrazine and terbuthylazine were detected, and deposition levels
were up to 11 μg.cm-2, and 12 μg.cm-2 respectively. Comparison of field deposition data and
AgDISP modelled data showed that the model under-predicted the amount of atrazine in the first sampling campaign. In the second sampling campaign, the model under-predicted only at
0 m, but as the distance increased, there was agreement between the AgDISP and the field
deposition results. Thus, the findings of this study make it apparent that field deposition
studies are still important to arrive at buffer distances that accurately reflect environmental
concentrations of off-target deposition during pesticide application. For the first time, this
project has shown the comparison of various samplers with different materials of construction
in the off-target deposition of a pesticide active ingredient (atrazine), under local South
African conditions.
