Downwind spray drift

An important feature of using these manual sprayers is that it is possible to do selective spot treatments, for example, confining the application of herbicides to patches of weeds. Also, where there is concern about downwind spray drift, placement of the nozzle nearer to the target and using a low pressure can enable treatments closer to a sensitive area. In conjunction with an attractant such as molasses or protein hydrolysate, insecticide bait sprays have been used in discrete spots for fruitfly control. Knapsack sprayers are often used in difficult terrain where access to vehicles is difficult or impossible. However, the lack of water in some situations, and the high requirement for labour, have created a demand for alternative application systems (see below). 

It is important for the public to recognize that there is no fixed distance or buffer zone for a safe distance to provide protection from spray drift. This will be a function of the pesticide being used, its toxicity and environmental behavior such as bioaccumulation, the nature and sensitivity of the downwind sites, and the nature of the application method, meteorological conditions, and so on. 

Another type of experiment has been used to assess the chemical reactivity of pesticides in the air. This principally employs downwind sampling from a treatment site during application (for measuring conversion in the spray drift) and for several days following application (for conversions involving volatilized residues) (24). The principal data are in the form of product(s)/parent ratios with increasing downwind distance, from which estimates of the rate of conversion can be made knowing the air residence time calculated from windspeed measurements. 

Water droplets were collected by the immiscible liquid method of Fraser and Eisenklam (8) at 2.4, 3.9, and 5.9 feet downwind from the spray nozzle. The observed impacting droplet sizes showed excellent agreement with the predicted sizes (Table I), thus lending confidence to the use of in-flight evaporation theory as a tool for interpreting wind tunnel spray drift data. 

Soil samples collected from waste lands downwind and downhill from the Hood River orchard were analyzed for DDT and its analogs. The results indicated the extent to which spray drift during application and surface runoff during rainfall or irrigation had contaminated these areas with pesticides. The samples were collected at distances up to 200 yards from the edge of the orchard. The average DDT content of ten 0—6-inch samples was 2.64 p.p.m. (total of DDT plus isomers), with the range 0.36-8.3 p.p.m. On a pound per acre basis, this amounts to approximately 5 pounds of total pesticide per acre. 

Figure 6 shows the resulting downwind drift-loss pattern when an oil and a water-base spray of the same drop size is used. Here the evaporation of the water reduces the deposit at points closer to the application while the non-evaporative oil shows a higher deposit out to about one mile, where the two curves cross. The characteristic of non-evaporative oil sprays is to deposit in greater amounts out to about one mile distance when compared with water base sprays, also the oil appears to have less material left to deposit beyond the one mile. Thus a low evaporative base spray produces a wider extended swath and requires wider buffers than a water-base application. 

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. 

Drift Budget. Four tests were conducted in atmospheric conditions which ranged from slightly stable through neutral to moderately unstable but with very similar mean wind speeds at 46 m above ground. The results of Crabbe et al. (7) for the airborne fraction of the applied spray are shown in Table II. At 400 m downwind of the swath 31 % of the material is still airborne while under neutral and unstable conditions the drifting fraction decreased to 12% and 9%, respectively. This trend is supported by measurements at 1200 m where under neutral atmospheric conditions 10% of the spray is still drifting while in the unstable case, no airborne droplets were detectable at this distance. 

The FSCBG aerial spray computer program is the result of more than a decade of refinement and verification of spray dispersion models used by the USDA Forest Service and the U. S. Army for predicting the drift, deposition and canopy penetration of particles and drops downwind from aircraft releases. This paper describes the mathematical framework of the models and selected applications of the models to military and Forest Service projects. 

Evaporation Module. Evaporation can significantly alter the aerosol size distribution as the spray cloud descends from the aircraft release height to deposit on the ground. The net effect of evaporation, because of reductions in the drop size and thus a decrease in gravitational settling velocity, is to decrease deposition near the source and increase the downwind drift of spray drops or vapor. The FSCBG model has two options that can be used to account for the evaporation of material. 

There are relatively few studies which relate to volatilization of pesticides from conifers. Yule et al.(16) have monitored the level of phosphorus in air (both vapours and particulate material) at five sites in New Brunswick during a spraying season in which 300 tons of fenitrothion were applied to over 10b ha. Average daily concentrations ranged up to 3 ug/tri and were generally between 0.5 and 1.5 ug/nrr (background 0.5 ug/m ). The atmosphere contamination was due partly to local application and partly to downwind drift of pesticide. 

Yoe (47) reports that sedimentation and impaction are usually the most important particle factors in drift hazard. Spray droplets, ranging from 10-50 microns in diameter, usually produce the greatest ground contamination several miles from the source of application, while droplets of 100 microns usually do not present a drift hazard unless winds are high. About 80% of the particles are deposited within short downwind distances when they are larger than 200 microns while droplets of less than 5 microns do not produce appreciable deposits and drift for many... 

Gerhardt, P. D., Witt, J. M., Summary of Downwind Drift Limits, Comparison of Dust vs. Spray, Pesticide Residue Study, Univ. of Arizona, September 1963. 

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