Pesticides in air and in precipitation and effects on plant communities
Final conclusion
The report describes how volatility of pesticides can be determined by a laboratory model system; further models for pesticide diffusion and transport in the atmosphere as well as deposition are described and evaluated. Precipitation was collected and analysed for selected pesticides over a couple of years and further the effect of the herbicide mecoprop was determined on a number of sensitive test plants to see whether the found pesticide concentrations in rainwater possess a threat to plant communities.
The laboratory experiments have elucidated a great need for standardised laboratory systems to determine the volatility of pesticides. Especially the test conditions need to be described carefully.
Atmospheric transport and deposition models for pesticides have been developed, but information is lacking on some of the basis processes as emission, dry deposition, atmospheric reactions and conversion from the gaseous to the particulate phase. Information on transport, diffusion and wet deposition processes for pesticides is sufficiently known. The description of emission and dry deposition in such models can be improved if the processes that determine the concentration in the surface (soil, vegetation, water) are modelled at the same time.
The concentrations of pesticides found in rainwater seemed generally to be of the same order of magnitude as in other European countries. The pesticides appeared in the highest concentrations in the spraying season, and when the use of phenoxy-herbicides was limited in Denmark, they were only rarely found in the precipitation. This is known not to be true for the chlorinated hydrocarbon insecticides, which may be transported over long distances. The herbicide DNOC was found in the highest concentrations, but this compound seems to be formed in the atmosphere.
When the effects of pesticide concentrations found in rain water were determined on susceptible plant species, the experiments showed, that the NOEL (No Observable Effect Level) for mecoprop was more than three times higher than the maximum yearly deposition of mecoprop in Denmark. Tests have only been performed with single pesticides and need to be performed with the many combinations of pesticides that are normally found in rainwater.
Determination of volatilisation
Volatilisation of pesticides are influenced by many different factors under field conditions. It is difficult to study the influence of individual parameters (e.g. temperature, wind speed) on the emission with field experiments. Therefore standardised laboratory experiments can be a helpful tools. Since the German BBA-guidelines appeared in 1990 different methods have been developed.
In a test of the different methods with three pesticides, it was found, that chamber properties and size and experimental area and air exchange rates are the most important for the determination of volatilisation. Great variations between the different methods were found, which showed that it is very difficult to compare results from different test methods.
In this project is developed a laboratory system which makes it possible to determine evaporation from different surfaces and to compare different pesticides. Based on the findings from the experiments it is concluded, that:
 | The BBA guideline should be improved and the description of the chamber and the conditions should be more specific |
| Volatilisation chamber must be specified (a small chamber is recommended) |
| Sampling after 1, 3, 6 and 24 hours seems reasonable |
| It is important to measure the height over the surface and whether turbulent or laminar flow is used when air velocity is determined. A velocity of less than 1 m per sec. 1 to 5 mm above the surface might be sufficient. |
| Approximately 50% relative humidity should be used and the surface to which the pesticide is applied should have the same humidity during the whole experiment |
| Filter paper may be used to mimic leaf surface |
| Standard soil with specified pH should be kept at 60% of maximum water holding capacity during the whole experiment |
| It should be considered not to perform experiments with soil but only from artificial surface since volatility is always highest from the artificial surfaces. In this way a maximum emission rate can be determined. |
| Several concentrations of a given pesticide should be tested and experiment must be carried out with both formulated product and active ingredient |
| Preferably the test should be carried out at 10 and 30°C |
Modelling atmospheric transport and deposition
The behaviour of pesticides in the atmosphere is very much dependent upon their solubility in water and their vapour pressure. Modelling of the fate of pesticides in the atmosphere is difficult, because information about important properties of pesticides is not available or is uncertain. Further it is very difficult to generalise their behaviour, because the properties of pesticides are so different, there is a large variety of surfaces with different properties. For the approval of pesticides general information is provided by the manufactures, but this is not sufficient to predict their behaviour in the atmosphere.
Emission and dry deposition depend on atmospheric turbulence, temperature and properties of the surface of soil and plants and the processes that take place in the surface (degradation and sorption). Temperature is especially important because the evaporation of water and the vapour pressure and water solubility of the pesticide depend on it.
It has been shown that for highly soluble gaseous pesticides a maximum of less than 25% of the emission can be deposited within 2 km from the source. For moderately soluble gaseous pesticides it in the order of 7% and for slightly soluble gases about 1% only. The dry deposition relatively close to the source has not been measured although models have shown that in extreme cases about 20% of the emission could be dry deposited within a few hundred metres from the source.
If the concentrations in air and in precipitation are measured simultaneously, the rate at which pesticides are removed from the atmosphere by wet deposition can be calculated, if the mixing height is known. For gaseous pesticides the removal rate by dry and wet deposition increases with their solubility in water. For pesticides in particulate form the removal rate by dry and wet deposition depends on the size distribution of the particles. In general particles are removed rather efficiently by wet removal processes where clouds are involved.
Photochemical atmospheric reactions of pesticides are not thought to be important within 2 km from a source but can play an important role during transport over longer distances. They will limit the distance over which gaseous pesticides can be transported if dry or wet deposition is not sufficient.
Conversion of pesticides from the gaseous to the particulate phase is thought not to be important within 2 km from the source, but is important during transport over longer distances. The reason is that dry and wet removal rates for gaseous and particulate pesticides are different. A phase change will therefore influence the removal rate.
Models can never replace measurements, but they can provide a best estimate, and especially in designing experiments models are useful, since experiments for pesticides should be planned with great care because chemical analysis are expensive. Laboratory experiments are useful, because they make it possible to study processes under controlled conditions, with only one factor varied at a time. A good research strategy should include both field and laboratory experiments and model development.
Compared to the air pollutants sulphur dioxide and nitrogen oxides, modelling of pesticides in the atmosphere is far from easy, they occur in much lower concentrations, and there are hundreds of different pesticides around in the atmosphere. On the other hand, it needs less knowledge to come to sound policy decisions than to get a quantitative scientific description of the whole system.
Pesticides in air and in precipitation
Rainwater has been collected on three locations on the island of Zealand. The analysis of phenoxyalkanoic acids (mecoprop, dichlorprop and MCPA) and of isoproturon showed that there was an obvious connection between the findings of pesticides in rainwater and the time of spraying with the herbicides. Concentrations and deposition were of the same order of magnitude as found in other European countries.
Phenoxyalkanoic herbicides were detected during the first years of the experiment, but in 1998 no phenoxyalkanoic acid were detected on the three locations. This indicates that the limitations in the use of these herbicides have greatly reduced their appearance in precipitation.
The herbicide DNOC was found in high concentrations both during and outside the spraying season, even though this compound has not been allowed in Denmark for the last 10 years. The concentrations detected in the rain in Denmark are of the same order of magnitude as found in England, Germany and Switzerland. The finding of DNOC indicates that either the compound can be transported in the atmosphere over long distances or other sources of DNOC than pesticide use are important. Most likely, the finding of DNOC is caused by formation of the compound in the atmosphere, probably from reaction between toluene and nitrous oxides under influence of sunlight.
The pesticide concentrations found are regarded as minimum concentrations since the samples are not adequately stable especially in the summer season if the rain falls in the beginning of the collection period.
Effects on plants and plant communities
The herbicide mecoprop-P was used as model pesticide. The influence of low doses of this herbicide was determined on the following testplants, which are all very susceptible to phenoxyalkanoic acid herbicides (Capsella bursa-pastoris, Thlaspi arvense, Sinapis alba, Cirsium arvense, Lapsana communis, Chenopodium album and Stellaria media. Further the effects were determined on the competition between Capsella bursa-pastoris and Geranium dissectum.
The No observable Effect Level (NOEL) was defined as the ED10 value (the calculated dose that caused a 10% reduction in the growth of the test plant). The results show that the NOEL of mecoprop on the most susceptible plant species in the study was more than 3 times higher than the maximum yearly deposition of mecoprop in Denmark.
Plant biomass was found to be as susceptible as seed production to assess the pesticide influence. Further competitive ability between plant species was not found to be a more susceptible test method. Effects of the combinations of pesticides that have been found in rainwater still needs to be determined.