When forced under pressure through sprayer nozzles liquids emerge as thin elongated sheets with edge instabilities that break up into small aerosols or particles having nearly a thousand fold range in spherical diameters. Owing to gravitational forces and the viscosity of air, the rate of fall to ground can be predicted by Stokes Law and is proportional to the radius of the particles. The rate of fall before a particle hits the ground (or conversely how long a particle remains in air before it falls a given distance) is modified by entrainment in a mobile air mass. Rate of fall of a spray particle will also be influenced by the rate of evaporation of the liquid constituting the aerosol. The longer the aerosol remains in air before falling to ground (or alternatively striking an object above ground) the greater the opportunity to be carried away from its intended target (e.g., crop canopy). In general, all size classes of spray particles are capable of movement off-target, but the smallest particles would move the farthest before depositing on the ground. Drift has been historically considered to be the movement of pesticide residues via air masses during and after application. Post application movement of pesticide residues (i.e., after deposition on plants or soil) via volatilization can be classified as secondary or indirect drift. A comprehensive review of spray drift and its mitigation was published in 2005 (Felsot 2005)1 and is currently the subject of an IUPAC funded project2. A publication based on this project should be available in 2010.
Although drift has a negative connotation because of its usual association with off-target (or out of field) impacts, spray drift within, for example, the canopy during application can increase the potentially bioavailable material on foliage. On the other hand, off-target or out-of-field drift during application may produce a high concentration of residues that potentially have an immediate or acute effect on nontarget organisms.
Highly concentrated agrochemical residues generated during spray application can move (drift) beyond target foliage (or soil when a pre-emergent herbicide or fumigant is used) to nontarget receptors including water, plants and animals. Nontarget receptors may be acutely exposed and therefore face the greatest risk of adverse effects during and immediately after spray application. In addition to movement of agrochemical residues in turbulent air masses downwind of application, residues can also become concentrated in inversions or stable air masses and be transported long distances. Similarly, agrochemicals can volatilize from plant and soil surfaces in comparatively high concentrations for several days after application. These secondary drift residues also pose a hazard to nearby nontarget receptors. Factors affecting spray drift include droplet size, which can be modified by the nozzle type, nozzle spray angle and nozzle spacing, certain formulation adjuvants, wind direction, wind speed, air stability, relative humidity, temperature and height of released spray relative to the crop canopy3,4.
In the US the assessment of spray drift has been studied in detail by the Spray Drift Task Force which was set up in 19905 by 38 agricultural chemical companies to generate data to fulfil US Environmental Protection Agency (EPA) spray drift data requirements. This Task Force has developed an evaluation tool to estimate the environmental exposure from spray drift at time of application6. In Europe drift is incorporated into the FOCUS model which predicts the concentration of pesticides in surface water due to run-off, erosion and drift7.
The US Spray Drift Task Force has issued several publications dealing with aerial application12, ground hydraulic application13, airblast application in orchards14 and chemigation15 and their relationship to spray drift. In Germany the Ganzelmeier Tables have been developed using different types of sprayers and nozzles so that applicators can choose equipment that will minimise drift16,17. In the UK a government website is available which allows applicators to examine the drift reduction ratings of many types of equipment18. In the EU there is a move to introduce a “Standardised Procedure for the Inspection of Sprayers in Europe” –SPISE19
US Environmental Protection Agency, Pesticide Registration (PR) Notice 90-3: Announcing the Formation of an Industry-Wide Spray Drift Task Force, April 6, 1990
FOrum for Co-ordination of Pesticide Fate Models and their Use (FOCUS), Overview of FOCUS Surface Water
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