Abstract
Spray drift is considered a major pesticide transport pathway to surface waters. Current research and legislation usually only considers direct spray drift. However, also spray drift on roads and subsequent wash-off to surface waters was identified as a possible transport pathway. Hydraulic shortcuts (storm drainage inlets, channel drains, ditches) have been shown to connect roads to surface waters, thus increasing the risk of drift wash-off to surface waters. However, the importance of this pathway has never been assessed on larger scales. To address this knowledge gap, we studied 26 agricultural catchments with a predominance of arable cropping (n = 17) and vineyards (n = 9). In these study sites, we assessed the occurrence of shortcuts by field mapping. Afterwards, we modelled the areas of roads drained to surface waters using a high-resolution digital elevation model (0.5 m resolution) and a multiple flow algorithm. Finally, we modelled drift deposition to drained roads and surface waters using a spatially explicit, georeferenced spray drift model. Our results show that for most sites, the drift to drained roads is much larger than the direct drift to surface waters. In arable land sites, drift to roads exceeds the direct drift by a factor of 4.5 to 18, and in vineyard sites by 35 to 140. In arable land sites, drift to drained roads is rather small (0.0015% to 0.0049% of applied amount) compared to typical total pesticide losses to surface waters. However, substantial drift to drained roads in vineyard sites was found (0.063% to 0.20% of applied amount). Current literature suggests that major fractions of the drift deposited on roads can be washed off during rain events, especially for pesticides with low soil adsorption coefficients. For such pesticides and particularly in vineyards, spray drift wash-off from drained roads is therefore expected to be a major transport pathway to surface waters.
Highlights
After pesticide application on agricultural crops, a certain fraction of the applied amount is not attained to the target crop, but is lost to nontarget ecosystems such as surface waters
We evaluated the potential of spray drift wash-off from roads to surface waters
We combined a field mapping approach with a spatially explicit, georeferenced spray drift model for a large set of agricultural catchments representing arable land and vineyards in Switzerland. We focused on these two crop types since they are two of the most important crop types in Switzerland with respect to coverage and average pesticide use
Summary
After pesticide application on agricultural crops, a certain fraction of the applied amount is not attained to the target crop, but is lost to nontarget ecosystems such as surface waters. In a Swiss arable land catchment, Schönenberger et al (in review) found that either spray drift on roads or spills from leaking spraying equipment led to increased pesticide concentrations in inlets of the road storm water drainage system In another Swiss arable land catchment, Ammann et al (2020) found – based on the field study described in Doppler et al (2012) – that the consideration of spray drift wash-off from roads could strongly reduce the uncertainty of exposure models. These studies show that spray drift wash-off from roads is a relevant transport pathway to surface waters in certain catchments. Given the paucity of empirical data on wash-off from these surfaces the results will be only tentative at this stage
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