| Front page | | Contents | | Previous | | Next |

Dry deposition and spray drift of pesticides to nearby water bodies

Appendix E. Example model runs

Example model runs were made for volatilisation and transport of pesticides to a river. The runs was made for all pesticides for which the physico-chemical properties were given by Smit et al. (1997) with a very few additions (316 pesticides at all).

The model runs were made for widths of non-spray zones and water bodies as defined by the FOCUS group for winter cereals (Table E-1).

Table E-1.
Dimensions of the non-spray zone and the water bodies used in the model runs.
Tabel E-1.
Størrelse af bufferzonen og vandløb/søer i modelkørslerne.

Water body	width non-spray zone (m)	width water body (m)
Stream	1.5	1.5
Pond	3.5	30.0
Ditch	1.0	1.0

In the model runs it has been assumed that the upwind area onto which pesticide was applied is 100 m. This assumption has some influence on the results, as the upwind length of the field has an effect on the shape of the vertical concentration profile.

The temperature of the air, the crop, the soil and the water body are assumed to be 15° C. It is also assumed that the water body does not contain pesticide, i.e. that the flux to the water body is not reduced by the presence of pesticide in the water body.

The model runs for rivers (streams) are made for the situations:

• A situation with a small surface resistance (rc). A flow velocity of 0.52 m s^-1, a depth of 1.37 m and a slope of 7.4× 10^-3 m m^-1 were adopted. This gives areaeration coefficient K_2d of about 44 day^-1 with the Thyssen and Erlandsen (1987) parameterisation. This is a rather large value and this leads to a rather small surface resistance, leading to an upper estimate of the dry deposition to the river.
• A situation with a relatively large surface resistance (rc). A flow velocity of 0.06 m s^-1, a depth of 0.12 m and a slope of 3× 10^-4 m m^-1 were adopted. Thisgives a reaeration coefficient K_2d of about 1 day^-1 with the Thyssen and Erlandsen (1987) parameterisation. This is a rather low value and this leads to a rather high surface resistance, leading to an lower estimate of the dry deposition to the river.
• A situation for a pond at a wind speed of 5 m s^-1.
• A situation for a ditch at a wind speed of 5 m s^-1.

It is assumed that the mass accommodation coefficient of the pesticide is so large that it will not have any influence on the uptake rate by water (see Appendix C). It is also assumed that the are no reaction of the pesticide in water. If there is a fast reaction in the water the surface resistance rc will become lower and the flux larger.

The vapour pressure needed to calculate the accumulated emission from crops is found from the actual temperature using equation (A-16) in appendix A and assuming a heat of evaporation of 95000 J mol^-1.

The Henry’s law coefficient needed to calculate the accumulated emission from normal to moist fallow soil is found from the actual temperature and equation (A-20) in appendix A, assuming a heat of dissolution at constant temperature and pressure of –68000 J mol^-1.

For the situation of emission from crops a surface roughness length z_0m of 0.1 m and a friction velocity u_* of 0.386 m s^-1 is taken. For the situation from emission from fallow soil a z_0m of 0.01 m is taken and a u_* of 0.284 m s^-1. These values will give the average wind speed in Denmark at 60 m height (Appendix G).

Some pesticides are applied to crops, others are applied to (almost) fallow soil. This difference will lead to differences in the volatilisation, due to different parameterisations of volatilisation from crops or from fallow soil. In the examples presented here emission is modelled either from crops or from fallow soil, although usually only one type of application is used. This is done because otherwise information had to be collected on the use of the pesticide in the real world.

In the calculations of the volatilisation from the soil Kd is calculated from Kow for all pesticides assuming that the pesticide is adsorbed at the organic matter in the soil (see Appendix A for a description of the method). This is done because otherwise information had to be collected on the adsorption of each pesticide, which was not possible within this project. Not all pesticides are, however, adsorbed to the organic matter in the soil and this may lead to errors.

So the user of these tables has to check which assumption on the volatilisation (from crop or from fallow soil) is most realistic and has to use the most likely results. Moreover, the user should not use the results presented here if the pesticide is not adsorbed to the organic matter in the soil but e.g. to minerals.

For some pesticides the parameterisation of the accumulated emission from crops will lead to a volatilisation of more than 100% of the dose. This is of course not correct. In that case the volatilisation is set to 100%. This is not necessarily correct either, but should be used as a first guess and an indication that the accumulated emission is rather large. The parameterisation of the accumulated emission from normal to moist soil has a maximum of 95.1%.

Table E-2 gives an overview of the model runs made and the associated tables.

Table E-2.
Overview of model runs made.
Tabel E-2.
Oversigt over modelkørslerne.

The results of the model runs are presented in Tables E-3 to E-7 give the following information:
4. Name of the compound.
5. Accumulated emission of the pesticide (in % of the dose).
6. Accumulated flux of the pesticide (kg m^-2) to the water expressed as a percentage of the accumulated emission flux (kg m^-2). This is the average flux over the whole width of the water body.
7. Accumulated flux of the pesticide (kg m^-2) to the water expressed as a percentage of the dose flux (kg m^-2). This is the average flux over the whole width of the water body. This number can be compared directly to the spray drift flux.

The results for abamectine 1a in Table E-3 show e.g. that if 1 kg m^-2 abamectine 1a is applied, 0.03% of that amount has been dry deposited per m^-2 of water surface, i.e. 3× 10^-4 kg m^-2.

Comparing the results for the deposition flux of pesticides emitted from crops to rivers with a small and a large surface resistance rc (Tables E-3 and E-4) shows that the results are often the same. This is because for most compounds the dry deposition flux is limited by the laminar boundary layer resistance rb and different surface resistances caused by different stream parameters have no influence on the dry deposition flux. Only for compounds with Henry’s law coefficient k_H (cg/cw) of the order of 2× 10^-3 and larger dry deposition flux is limited by the surface resistance, which is a function of the stream parameters. In that case a difference will be found between the results of the two runs.

The deposition flux of pesticides to ponds caused by pesticides emitted from crops is lower than to streams with a large surface resistance rc. This is mainly caused by the fact that the width of the non-spray zone and the width of the water body is larger for the pond than for the stream. Due to the average longer distance to the emission area in the case of ponds, the pesticide is more mixed up in the vertical and the average concentration will be lower than in case of the streams. Moreover, there is an effect due to the difference in r_c values for streams and ponds.

Comparing the results for deposition flux of pesticides emitted from crops to ponds and ditches, shows that the average deposition flux to ditches is almost a factor two higher than to ponds. This difference is caused by differences in the width of the non-spray zone and the water body.

The accumulated emission from crops and the accumulated emission from soil are quite different, which results in difference deposition fluxes (compare Table E-3 and E-7)

 
Click on the picture to see the html-version of: ‘‘Table E-3‘‘

Click on the picture to see the html-version of: ‘‘Table E-4‘‘

Click on the picture to see the html-version of: ‘‘Table E-6‘‘

Click on the picture to see the html-version of: ‘‘Table E-7‘‘

References

Smit, A.A.M.F.R., van den Berg, F., Leistra, M. (1997) Estimation method for the volatilization of pesticides from fallow soil. Report Environmental Planning Bureau Series 2, DLO Winand Staring Centre, Wageningen, the Netherlands.

Thyssen, N., Erlandsen, M. (1987) Reaeration of oxygen in shallow, macrophyte rich streams: II. Relationship between the reaeration rate coefficient and hydraulic properties. Int. Revue. ges. Hydrobiol. 72, 575-597.

| Front page | | Contents | | Previous | | Next | | Top |