Spraying drift data

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. 

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. 

Another factor that will increase the drift is the target size or number of spray swaths. If the spray block is more than about 200 yards wide, then the expected deposit amount could increase by a factor of 2- to 10-fold. There is a need for a great deal more drift data and for it to be summarized into generalized drift curves with some limitations placed on them as to the upper and lower limits of expected spray drift amounts under a variety of conditions so that they can be applied to various spray situations. There is not a great deal of drift data specific to forest spraying. 

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. 

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. 

A major route of contamination of surface waters by naled is spray drift and direct application for mosquito abatement. There are no data on the fate and transport of degradates containing only the organophosphate group, which form by cleavage of the P-O bond in naled and/or DDVP. 

Because there are no direct applications of PBO to water, the exposure of aquatic organisms to PBO is limited to spray drift at the time of application to adjacent lands and runoff associated with rainfall. Regarding drift the EPA uses a default assumption of 5 a- drift, which is defined as 5% of the actual application rate applied to the entire surface of the water body (pond) on a per acre basis. The compound applied is assumed to be instantaneously at equilibrium within the water column (Urban and Cook, 1%6). In modelling aquatic exposure, the amount of chemical in runoff is also added to the water body (pond) based on historical rainfall data, the properties of the chemical, and other factors. 

Vertical surface collectors can readily provide information on relative drift (e.g., the amount of drift from one field trial compared to another). However, it is difficult to obtain absolute data unless the precise collection characteristics are known for the droplet size spectrum at the point of spray collection, wind speed and air turbulence intensity. " The SDTF conducted studies in wind tunnels to compare the collection efficiency of different types of drift collector used in its field studies. These studies showed that collection efficiency on strings was several orders of magnitude higher for 0.8-mm diameter cotton string than for 2-mm diameter polyethylene line and vertical o -cellulose strips or squares. The higher collection efficiency for the cotton... 

Control field matrices are usually placed at the field site upwind and at a significant distance from the spray or re-entry area so as to avoid all obvious routes of contamination at the test site that may destroy the integrity of the control samples. However, the control matrices should not be placed so far away from the test site as to avoid any suspected contamination that might occur from drift or other sources of contamination. One may want to define better the conditions at the test site in order to interpret better the exposure data collected from the volunteers matrices. 

Consideration should be given to the settling velocity of the droplets which is a combination of terminal velocity and transport velocity due to atmospheric turbulence. There are data to suggest that droplets lQu settle before the applicator comes into contact with any drift from the previous row sprayed (15). 

The deposit of active chemical, the drift losses and drop size range can be found and would be functions of the spray formulations and application equipment which are under test In a given weather and application terrain. In order to compare different test run data, the results may be plotted as a series of 2nd degree polynomial regression curves (6). Actual chemical analysis of the released spray caught on the samplers provides the most accurate measure of deposit and airborne losses, but calculation of these functions from the drop sizes found can also be done. A total deposit recovery as a % of the amount released can be determined. 

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. 

The assumption that spray which is unaccounted for on the forest floor is lost by drift processes is not supported by data presented in this paper. 

For the purpose of attempting to predict the amount of drift one might expect in general from a spray operation, it is more useful to composite the data from a number of drift experiments into a generalized curve and extrapolate from that to the operation being considered. Table VII shows such data developed by Dr. Norman Akesson from the University of California, Davis. (2,3,4). This data is for a coarse spray... 

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