Dry deposition and spray drift of pesticides to nearby water bodies

Dry deposition and spray drift of pesticides to nearby water bodies

Summary and conclusions

Decision tool

A decision tool is being developed for the Danish Environmental Protection Agency to evaluate the risk of transport of pesticides to water bodies such as streams and lakes (PestSurf). The development of PestSurf is co-ordinated by DHI Water & Environment. In this report the tools to estimate the atmospheric contribution from the application of pesticides to fields close to water bodies are presented that will become part of the decision tool. The following processes are taken into account:

Dry deposition of gaseous pesticides that are volatilised from the field mainly after application. Dry deposition is transport to the surface by whirls in the air (turbulence). In order to get dry deposition to the water body the pesticide should volatilise first then be transported by the wind to the water body. Although volatilisation is usually highest just after application, it will continue for many days (~10-20 days).
Spray drift, i.e. transport of pesticides in drops generated during the spraying operation. Also in this case the droplets have to be transported to the water body by the wind. The deposition of these droplets to the water body is in this case mainly caused by gravitation and not by turbulence. Turbulence, however, plays a role in keeping a fraction of the drops somewhat longer in the atmosphere.

A model has been developed to describe the volatilisation of pesticides, the atmospheric transport/mixing and the dry deposition to water bodies.

Volatilisation

The volatilisation of pesticides from crops and fallow soil in this model is described by empirical relationships between the measured volatilisation of pesticides and their physico-chemical properties for a limited number of pesticides. The relationship for volatilisation from fallow soil also includes soil properties. These relationships were developed by Alterra, Wageningen, The Netherlands (Smit et al., 1997; Smit et al., 1998). They state that the volatilisation from crops is a function of the vapour pressure of the pesticide and that the volatilisation from fallow soil is a function of the fraction of the pesticide that is in the gas phase in the soil. These relationships are in the model used for all pesticides.

Atmospheric transport and mixing

The atmospheric transport and mixing in the model takes into account that the gaseous pesticide released at low heights is mixed up by whirls in the air (turbulence) and that the wind speed increases with height.

Dry deposition

The dry deposition in the model takes into account that the pesticide is transported downward towards the water surface by whirls (turbulence) and that the gas phase diffusivity plays a role in a thin layer close to the surface. The uptake of the pesticide by the water surface depends on the mixing in the upper part of the water body (aqueous phase mass transfer coefficient) and the Henry’s law coefficient. The Henry’s law coefficient is a measure of the solubility of the gas that describes the relation between the concentration of the pesticide in the gas phase and the concentration in the aqueous phase at equilibrium. The mixing in the upper part of the water body is different for different types of water bodies.

In rapidly running shallow waters the mixing in the upper part of the water body is created by friction at the river bottom. The mixing in the upper part of this type of water body in the model is described using empirically determined mass transfer coefficients for oxygen. A correction is applied to these coefficients to take into account the difference in diffusivity of the pesticide and oxygen in the aqueous phase. The empirically determined mass transfer coefficients are also function of the average velocity of the stream, its average depth and the slope (metre change in height per metre horizontal distance).

In lakes, slowly running or deep waters and the sea, mixing in the upper part of the water body is caused by the wind. The mixing in the upper part of these water bodies is described with empirical relationships between measured mass transfer coefficients and measured wind speed. These relationships are established for different gases, but the results are normalised to the exchange of CO2 at 20° C. In the model these normalised relationship is then used to calculate the aqueous mass transfer coefficient of the pesticide. Corrections are then made to take into account the difference in diffusivity of the pesticide and CO2 in water. These experimentally determined mass transfer coefficients increase with wind speed.

For most pesticides the resistance to transport in the atmosphere limits the dry deposition and not the resistance to transport in the water body.

Spray drift

Spray drift during field spraying is influenced by a number of factors. One can divide these into technical/agronomic factors, which can be influenced by the farmer and climatic conditions at the time of application. The following factors are influencing the spray drift potential:

Droplet size (nozzle choice)
Boom height
Driving speed
Air-assistance, shielding
Dose rate
Crop development, neighbour crop, shelter belt
Wind speed
Temperature and humidity

The first 5 points relate to the technique used and the droplet size is the most influential factor concerning spray drift from traditional field sprayers. The droplet size is influenced through the choice of nozzle and spray pressure. When fine atomising nozzles with a high drift potential are preferred for some applications then it is because a high biological efficacy is dependent on the use of fine or medium atomising nozzles. If a coarse drift reducing droplet size is used a reduced efficacy can be the result. This means that for some applications an increased dose rate might be needed in order to retain biological efficacy if drift-reducing nozzles are used for the application.

Raising the boom height above the recommended level increases the drift potential considerably because the travelling time of the small droplets increases significantly. The drift potential is correspondingly increased when the driving speed is increased due to the increased wind speed experienced by the spray swath. Different types of drift reducing equipment have been developed for traditional field sprayers. Probably the most widespread system is air-assistance. The system creates an air-stream parallel to the spray swath, which helps keeping the droplets in the spray cloud until they reach the target. One of the air-assistance systems, the Twin system, has documented a drift reduction of approximately 2/3 compared to the use of the same droplet size without air-assistance. Concerning biological efficacy, a neutral or positive influence of air-assistance is seen dependent on the type of application. Different types of shielding devices for traditional field sprayers has been developed but shielding devices are at the moment primarily used for orchard sprayers (tunnel sprayers) and for band sprayers.

The wind speed at the time of spraying is one of the most important factors influencing spray drift. Temperature and humidity influences spray drift through their effect on evaporation from the droplets during their travel to the target. In this way droplet size is reduced and spray drift potential increases. The effect of temperature and humidity on spray drift is not quantified under field conditions. At wind speeds above 1 m s^-1 spray drift increases more or less linearly with wind speed. The best estimate to normalise spray drift values to the same wind speed is by dividing the actual found drift values with the measured wind speed – 1 m s^-1. This means as an example that spray drift is doubled when the wind speed is raised from two to three m s^-1. The coastal climate in Denmark is characterised with more windy conditions than found in a continental climate. From this reason it seems that the 95 percentile Ganzelmeier values (Ganzelmeier.et al., 1995) are more representative for spray drift under average Danish conditions than the mean values found under German wind conditions.

Description of spray drift processes

The report gives also information on the processes necessary to model spray drift, but no spray drift model has been developed. The 95% percentile values for spray drift described in Ganzelmeier et al. (1995) is regarded as being representative of mean values for spray drift under Danish wind conditions. These values are being used to describe spray drift in PestSurf.

Conclusions: comparison of dry deposition and spray drift

In the last part of the report the contribution from dry deposition and from spray drift are compared. The comparison shows that the contribution from dry deposition potentially can be larger than from spray drift for those gaseous pesticides that are highly soluble (defined by having a small Henry’s law coefficient, which is here defined as: concentration in air (kg m^-3)/concentration in water (kg m^-3)) for waters with a high mixing rate in the upper layer. With potentially it is meant in the case that almost 100% of the applied pesticides volatilises. For pesticides that are rather volatile such as fenpropimorph, pendimethalin, bentazone the dry deposition is often more important than spray drift. If only a small fraction of the pesticide (a few %) volatilises spray drift becomes relatively more important. Spray drift decreases rather fast as a function of the downwind distance from the edge of the field, whereas dry deposition decreases much more slowly.