Spray drift potential

The low speed wind-tunnel method for evaluating spray drift potential from various liquid systems was found to estimate relative amounts of small, drift-susceptible droplets produced in the spray with sufficient... 

Field studies are required to provide a more reaUstic picture of the dissipation of the parent compound and those degradates determined to be significant. Under field conditions pesticides are exposed simultaneously to the individual dissipation processes that were examined separately in the laboratory studies. Thus, in field studies, some dissipation processes may be altered due to competition and interaction. Requirements for spray drift data were outlined in draft Subdivision R, but the EPA agreed that data generated on a generic basis by an industry consortium could represent the potential for drifting of individual pesticides. 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

Spray nozzle type plays an important role in the success of agrochemical application. For broadcast applications to soil, flat fan nozzles should be used. Newer spray tips such as the DG TeeJet, XR TeeJet, Turbo TeeJet and similar nozzles supplied by Lechler and Hardy have provided acceptable results in a number of studies. For a given nozzle type, the lower the application pressure, the larger is the spray droplet size and the less potential for spray drift. Similarly, the closer the boom is positioned to the soil surface, the less is the potential for spray drift." Most applications are made with spray tips having 80° or 110° spray angles and boom heights of about 50 cm above the soil surface. 

Although the downward transport of triazines by water is the most important route in evaluating the potential for presence in groundwater, other modes of transport away from the site of application should also be taken into consideration. These include plant uptake, upward transport to the soil surface by water, transport in surface runoff water and sediment, volatilization from the soil surface, spray drift during application, and movement on wind-eroded particles. This chapter will cover triazine transport across the soil surface and through the soil profile. 

The interpretation of the effects of such drift, particularly its potential for adverse effects on human health, is dependent on some of the parameters of environmental behavior shown on Table VIII. The dose is given at 2 lbs/acre and translated into a deposit level of 20 mg/square foot, which is more useful in the interpretation of exposure data. The figures given for the deposit amount from spray drift at 100 yards and 1/2 mile are the figures for drift from a coarse spray on flat land for small target areas and are average drift amounts. The figure of 20 mg/kg is the NOEL for 2,4-D. 

The inspections are a combination of information gathering and information verification. While the application form and annual return, which are filled in by the operator, contain a great deal of information, it is the inspectors job to verify this information. They also collect information not normally supplied in the annual returns and applications, such as potential spray drift, constituents of animal feeds and dates of fumigation and processing. 

Lightweight particles, such as dusts and wettable powders, are easily carried by moving air. Granules and pellets are much heavier and tend to settle out of air quickly. Small spray droplets also are easily carried in air currents. High-pressure and fine nozzles produce very small spray droplets that are very likely to drift. Lower pressure and coarse nozzles produce larger droplets with less drift potential. 

Southcombe ESE, PCH Miller, H Ganzelmeier, IC van de Zande, A Miralles and AI Hewitt, 1997. The international (BCPC) spray classification system including a drift potential factor. Proceedings of the Brighton Crop Protection Conference - Weeds, 1997. November 1997. Brighton, UK, 371-380. 

Compared to some ground equipment, aircraft release sprays from greater heights. This may increase the drift potential... 

So what is the future of agricultural aviation The author remains confident that aircraft provide a viable and efficient means of pesticide delivery in a wide range of environments and situations. How then should aerial application be managed to reduce the potential threat of off-target damage and spray drift ... 

Spray drift - this can result in losses of pesticide from the target site, but more concerning is the potential of the pesticide to cause harm to the neighbouring crop or environment. 

Possibly the first significant means by which a pesticide might be diverted from its target can occur immediately after spraying. This is the potential of the spray droplets to be diverted away from the crops or weeds by the action of climatic conditions. This process, commonly known as spray drift , is now becoming a greater concern, due to a number of trends in the pesticide user environment ... 

While the subject areas of spray drift and retention have received much attention in the past, this does not appear to be the case for all the subsequent pesticide loss mechanisms. This is partly a result of the fact that, whereas the former are relatively universal effects, not all pesticides suffer from all post-application loss mechanisms. The following list of potential loss mechanisms is not by any means exhaustive ... 

The data sets reviewed, document our knowledge on the deposition of aerial sprays released over coniferous forests. Conifers are relatively efficient collectors of spray drops as more drops are consistently observed on the ground in open areas than beneath trees. Spray which penetrates the upper canopy, and is unaccounted for on samplers in the lower canopy, probably was filtered out by foliage. More deposits are observed in the upper crown than in the lower crown. Data are lacking, however, on the fate of drops which do not penetrate the canopy. There is a potential for these drops to penetrate the canopy downwind or to drift off target. 

Potential routes of exposure to molinate include inhalation (for mixers, applicators, field workers, and residents of rice-growing regions), dermal (for mixers, applicators, field workers, and anyone exposed to drift of spray droplets or residues on plants), and dietary (from drinking water sources contaminated with molinate and from residues on rice and rice products). 

Spray nozzles designed for both aircraft and ground equipment can also be used to enlarge droplet size of the spray. Application equipment can also be modified to reduce drift. For example, shrouding the spray booms of ground equipment keeps droplets from swirling up into the air, thus reducing the potential for drift and applicator exposure. 

Large drops, with their greater kinetic energy, can cause problems as they impact on crops. Drops greater than 200 im diameter (Brunskill, 1956) have the potential to cause spray runoff and contamination of the soil. With the lowering of spray volume rates, and the use of more fine nozzles, this problem has reduced in importance in recent years, but the trend towards the use of coarse sprays for drift reduction may reintroduce the problem in the near future. 

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