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Establishment of a basis for administrative use of PestSurf

3 Pesticides and parameters

• 3.1 Parameterisation for PestSurf
• 3.2 Parameterisation for FOCUS SW

The test cases defined by Danish Environmental Protection Agency (DEPA) are listed in Table 3.1. The pesticides were used as input to:

• four different scenarios in  PestSurf, that is for a stream and a pond in a sandy and a sandy loam catchment respectively,
• FOCUS SW D3 ditch and D4 pond and stream,
• the standard waterhole scenario currently used in the Danish risk assessment.

3.1 Parameterisation for PestSurf

Information concerning the different pesticides were collected from EU monographies, the Pate database used by the DEPA and other available sources. As the aim of the exercise is to compare different methods rather than ensuring a perfect parameterisation of each pesticide, assumptions had to be made when the required data were not at hand.

Data requirement for Pestsurf and the status of the data are listed below.

Cas no.
The information was available for all compounds, see Table 3.2.

Molecular weight
The information was available for all compounds, see Table 3.2.

Information concerning the form: acid, basic, neutral
The molecular formula and pKa-value (if relevant) was available for all compounds, see Table 3.2.

pKa
The pKa-value was available for all compounds, where it was relevant. However, for pendimethalin, a pKa-value was given, while all other parameters indicated no dissociation. It was therefore assumed that the compound is neutral. The values used are listed in Table 3.2.

LogKow
If the compound is neutral, the listed Kow-value is used directly. If not, a Kow-value has to be calculated for the neutral and the dissociated species separately.

The assumed formula is:

Kow = Kow_AH* [AH] + Kow_A-*[A-]

If Kow is determined at more than one pH-value, it is possible to calculate Kow_AH and Kow_A- For two values, the equations can be solved. For three values, an optimised solution can be found. For a few compounds, only one value was reported and the missing value was set to ”1”. Values are shown in Table 3.3.

Water solubility
The information was available for all compounds. No attempt was made to change if the solubility was given at 25 °C instead of 20 °C. The values used are shown in Table 3.2.

Vapor pressure
A value was available for all compounds, but not always at 20 °C. If the vapor pressure was given at 25 °C, a recalculation was attempted. It was assumed that

Ln(P) = A – B/(t+273),

where P is the vapor pressure, t is temperature in °C and A and B are constants.

If vapour pressure was estimated at two temperatures, these were used for recalculation to 20 °C. If the boiling point was given, this was used together with a vapour pressure equal to 1 atmosphere. If the boiling point was not given because the compound disintegrates, the disintegration temperature was used. This is not correct, and must be considered a pragmatic solution. The values are shown in Table 3.4.

Henry’s law constant
The parameter was given for most compounds but for terbutylazin and fluazinam, the value is calculated from molar weight, vapor pressure and water solubility. For the rest of the values, the unit was changed, generally from Pa m³mol^-1 to dimensionless. This was done by division with the gas constant and the absolute temperature (assumed to be 20 °C if no temperature was listed with the value). The values are given in Table 3.5.

Sorption parameters
The Freundlich exponent and the Koc-value were derived from the reported sorption studies. Values were selected as recommended by FOCUS (2001). If adequate data were not readily available, a choice was made. The values applied are shown in Table 3.6.

DT50
The DT50 was derived from studies and values from different studies were recalculated to 20 degrees and pF2 as recommended by FOCUS (2001). Finally values were selected as recommended by the above mentioned FOCUS report (Aritmetric mean, geometric mean or median value, depending on the number of observations). If adequate data were not readily available, a subjective choice was made.The values are shown in Table 3.6.

Water/Sediment studies
The water/sediment studies were generally poorly documented in the available material. A choice had to be made to what was actually used as input to PestSurf. In most cases the water/sediment study value is used with the reported sediment concentration or the default value from PestSurf. For strongly sorbing compounds, where breakdown only seems to take place in the water phase, the DT50-value for pesticide in water is used with a sediment concentration of 0. The selected values are shown in Table 3.7.

Hydrolysis
For all compounds information on hydrolysis was available at three different pH-values. Three equations with three unknown were solved.

First the half live value (T1/2, days) were converted to rates:  ln(2)/(24*T1/2)

Then, the equations were made:

Measured ratε = K_acid*[H⁺] + K_neutral + K_{basic *} [H^-]

The equations were solved, and the three rates (K_acid, K_neutral, and K_basic were found. If one of the rates became 0, the rate was kept to 0 and an optimal solution found for the two remaining parameters.

Thirdly, the rates were converted back to T1/2-values: ln(2)/(24*K_x). The original and recalculated values are shown in Table 3.8.

Photolysis
Only two compounds had sufficient data for full parameterisation: bentazon and metamitron. Five compounds are not degraded by photolysis, and a 6^th is assumed not to degrade due to lack of information. The last six compounds were assigned a quantum yield and a spectrum that is poorly described. The selected values are shown in Table 3.9.

Plant uptake
Plant uptake was set to 0.5 for all pesticides.

3.2 Parameterisation for FOCUS SW

Information concerning the different pesticides to be used in FOCUS SW is generally similar to waht is required for PestSurf c.f. paragraph 3.1.

Molar mass and water solubility is parameterised according to estimates in Table 3.2, saturated vapour pressure according to estimates in Table 3.4, soil sorption coefficients, Freundlich exponents and half life in soil according to estimates in Table 3.6, and half life in water and sediment according to estimates in Table 3.7.

FOCUS SW require input of “molar enthalpy of vaporisation”. It was for all simulations set to 95000 J/mol. The “molar enthalpy of dissolution” was set to 27000 J/mol. These values were provided by the FOCUS SW software as default values. Similarly, FOCUS SW estimates for diffusion coefficients in water and air were used, these estimates obtained values of 4.3 × 10^-5 m²/d and 0.43 m²/d, respectively.

For all FOCUS SW simulations the factor for the uptake by plant roots in soil were set to a 0.50 and the wash-off factor from crop was set to 0.05 mm^{-1 in MACRO (both default values).}

Table 3.1. Pesticides and spraying conditions specified by DEPA for the study.
Tabel 3.1. Pesticider og udbringingsforhold specificeret af miljøstyrelsen for studiet.

Vejl. Planteværn =Planteværn Landbrug 2005, Dansk Landbrugs Grovvareselskab, Axelborg, 1503 København V, www.dlg.dk.

Table 3.2. Cas.no, molecular weight and charge, pKa-value and water solubility of the selected pesticides.
Tabel 3.2. Cas. nr., molekylevægt og ladning, pKa-værdi og vandopløselighed af de udvalgte pesticider.

Table 3.3. Listed and re-calculated values for Kow.
Tabel 3.3. Opgivne og beregnede Kow-værdier.

Table 3.4. Listed and re-calculated values for vapor pressure.
Tabel 3.4. Opgivne og beregnede værdier for damptryk.

Table 3.5. Listed and re-calculated values of Henry’s law constant.
Tabel 3.5. Opgivne og beregnede værdier for Henry’s konstant.

Table 3.6. Sorption and degradation parameters in soil.
Tabel 3.6. Sorptions- og nedbrydningsparametre i jorden.

Table 3.7. Degradation in water/sediment.
Tabel 3.7. Nedbrydning i vand/sediment.

Table 3.8. Listed and re-calculated values for hydrolysis.
Tabel 3.8. Opgivne og beregnede hydrolyse-parametre.

Table 3.9. Photolysis.
Tabel 3.9. Fotolyse.

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Version 1.0 December 2006, © Danish Environmental Protection Agency