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Spray drift surface water exposure assessments for pesticide registration in Great Britain and Northern Ireland

When applying for pesticide registration in Great Britain and Northern Ireland you need to assess the risk of exposure to surface water as a result of spray drift. You need to submit predicted environmental concentrations in water and sediment (PECsw and PECsed). This page explains how to calculate exposure in surface water and sediment from spray drift.

GB and NI PECsw spray drift calculations are based on an 'edge of field' scenario. The surface water body is next to a treated field and assumed to be static, 100 m long by 1 m wide, with 30 cm water overlying a 5 cm deep sediment layer with a density of 1.3 g/cm3. Substances are assumed to be evenly distributed through the water and sediment layers.

For product evaluations, the use of a risk envelope approach may also be possible. Details on the suitability of risk envelopes can be found at Risk envelope suitability for pesticide registration in Great Britain and Northern Ireland.

Standard spray drift

The drift values for a standard assessment, assuming application via standard hydraulic nozzles, are those provided by Rautmann et al, 2001. This quantifies a 90th percentile drift value for single applications and an overall 90th percentile drift for multiple applications.

For more information on the underlying principles behind the drift assessment refer to the Aquatic Guidance Document (SANCO/3268/2001, rev. 4 (final), 17 October 2002) and Chapter 6 of the Guidance on tiered risk assessment for plant protection products for aquatic organisms in edge-of-field surface waters (EFSA Journal 2013; 11(7):3290).

The drift values for various crop groups are shown within the 'PECsw spray drift (multiple buffer zones)' calculator and 'PECsw-sed spray drift' calculator available from the Environmental fate models: Excel calculator tools webpage.

The table below explains which values should be used for which crops, including details of drift values for early and late growth stages.

Crop groupings for standard spray drift assessments

Field crops

Most arable crops such as cereals and root crops such as potatoes.
Commonly treated by tractor mounted horizontal boom sprayer.

Fruit crops

Broadcast air assisted sprayers that blast upwards and outwards. Typically used in fruit crops such as apples, pears and cherries and associated with higher drift.

For orchards the canopy changes over time, presenting different levels of crop interception and spray drift. Fruit crops are divided into 'early' and 'late' stages, associated with higher and lower spray drift, respectively:

  • 'early' drift values for fruit crops up to and including BBCH 70
  • 'late' drift values for fruit crops at BBCH 71 and onwards

If the proposed use pattern spans early and late growth stages, provide an assessment using both sets of drift values.

Vines

As the canopy and agronomic practice changes through the season, vines are divided into early and late stages, associated with lower and higher spray drift values, respectively.

Growth stage 15 coincides with transition from first leaves to leaf development for vines. Therefore:

  • 'early' drift values for grapevines up to and including BBCH 15
  • 'late' drift values for grapevines at BBCH 16 and onwards

If the proposed use pattern spans early and late growth stages, provide an assessment using both sets of drift values.

Hops This crop is associated with air-assisted spraying and therefore relatively high drift values. Drift values specific to hops are given in the Rautmann data set.
Vegetables, small fruit and ornamentals

These crops do not have their own tab on the PECsw calculator tools, but are divided into two drift categories, according to 'height <50 cm' or '>50 cm'. It is important to note that these relate to the equipment used, rather than crop height.

For 'height <50 cm': the drift values match those for 'field crops', from use of a ground crop sprayer with a horizontal boom. If the product is applied by horizontal boom equipment, use the 'height <50 cm' data and assess exposure using the 'field crops' tab on the calculator, irrespective of crop height.

For 'height >50 cm: the drift values match those for late vines, from application by other types of machinery (for example broadcast air-assisted sprayers, variable geometry boom sprayers etc). Use the results from 'vines late' from the vines tab of the calculator

Note about soft fruit (not vines or orchards):
Soft fruit growers are also likely to use various application techniques, including non-standard sprayers (not covered by the drift data). As well as standard conventional boom sprayers and broadcast air assisted sprayers, growers may use air assisted conventional sprayers, where the nozzles can be angled in different directions.

Therefore, provide an exposure and risk assessment, assuming both:

  • conventional boom sprayer (using the 'field crops' tab of the PECsw spray drift (multiple buffer zone) calculator)
  • and broadcast air assisted sprayer (using the results for the 'late vines' from the 'vines' tab of the PECsw spray drift (multiple buffer zone ) calculator

If a LERAP phrase is needed for the broadcast air assisted sprayer, then include an additional phrase (other specific restriction) to inform any user of non-standard machinery to abide by the buffer zone distance required for the broadcast air assisted sprayer. For example, 'If using application machinery other than a standard horizontal boom sprayer please adhere to the LERAP mitigation specified for broadcast air assisted sprayers'

Some soft fruit such as blackcurrants may be treated by tunnel sprayer or curved booms that direct spray downwards and inwards but are not air assisted. For 'non-horizontal boom modes without air assistance', it is assumed that drift would be no worse than for a horizontal boom going over the same type of crop, so the standard 'field crop' drift data should be used.

Drift from multiple applications

For products applied multiple times, it is unrealistic to use 90th percentile drift values for each application. Instead, a reduced percentile is used for each individual application, which combined represents an overall 90th percentile. The correct spray drift percentile value is automatically selected when you use the PECsw spray drift (multiple buffer zones) calculator available from the Environmental fate models: Excel calculator tools webpage.

Where a compound dissipates with a short DT50 between applications, reducing carry over of residues between applications, the lower drift assumed for each spray event for multiple applications can lead to a lower PEC than from a single application. Therefore, you must always calculate the PEC for both single and multiple applications. Use whichever gives the highest PEC in the risk assessment.

Standard spray drift mitigation - Buffer zones

If the PEC results in an unacceptable aquatic risk assessment, risk mitigation measures may be needed. No spray buffer zones mitigate the risk from drift.

The GB and NI maximum buffer zone allowed to mitigate drift from horizontal boom sprayers for field crops is 20 m using standard nozzles. For fruit crops such as apples treated using air assisted broadcast sprayers, the maximum buffer zone is 50 m.

Buffer zones are applied on a crop basis for each product. Different buffer zone distances may be set for different crops for the same product.

PECsw spray drift values for a range of standard buffer zones can be determined using the PECsw spray drift (multiple buffer zone) calculator. It is useful to provide PECsw and PECsed values for a range of buffer distances in your application as it makes it easier to determine the appropriate buffer zone distance.

After product authorisation in Great Britain and Northern Ireland, users have the option to reduce a 5 m buffer zone set under the standard nozzle scheme using the LERAP scheme. However, the LERAP scheme cannot be used by growers to reduce buffer zones of between 5 m and 20 m set under the standard nozzle scheme.

Drift reduction technology

For field crops there is also the option to assess risks from products applied using drift reduction technology (DRT). This considers use of 3-star rated drift reducing nozzles using Van de Zande drift values. PEC concentrations are calculated using DRT drift values for up to 6, 12 or 18 m buffer zones. For more information on gaining a low drift star rating for equipment, refer to guidance on LERAP equipment accreditation.

Applicants must choose to present assessments based on either standard nozzle assumptions or DRT assumptions, not both.

Specification of DRT is on a product basis. It is not possible for a product label to specify use of DRT for one crop and use of standard nozzles for another crop. Different buffer zone distances may be set for different crops for the same product, for example 6 m 'crop A', 12 m 'crop B'.

Use of DRT to determine PECsw spray drift must result in the stipulation of DRT measures on the product label. The product label will need to specify that 3-star nozzles are required. For more information on labelling requirements please refer to the labelling handbook.

It should be noted that it is not possible to reduce a buffer zone determined by DRT methods using the LERAP scheme.

Standard PECsw for active substances

PECsw spray drift values are calculated for all active substances. The inputs necessary for use in the calculations are summarised in the table below.

Inputs to PECsw spreadsheets
Number of applications - Maximum proposed, up to 10 applications.
DT50 water days The longest non-normalised dissipation DT50 from the water phase of the water sediment study (OECD 308) is used. If the dissipation DT50 value is not available or cannot be calculated, as a worst-case, the longest total-system degradation DT50 value may be used.
Rate g/ha Highest application rate proposed.
Interval days Shortest proposed application interval.
Spray drift % Drift values from Rautmann et al, 2001 for crop, buffer zone distance and number of applications intended.
Number of days after application   Can be edited, if needed.
Specific inputs for the SEDIMENT TAB:
Max % sed % Maximum % of compound partitioned to sediment phase, normally from a laboratory water sediment study.
DT50 sediment days

Longest dissipation DT50 sediment from the laboratory water-sediment study (OECD 308).

If DT50 values were not calculated separately for sediment, the worst case DT50 for whole system may be used.

Use the 'PECsw spray drift (multiple buffer zones)' calculator available from Environmental fate models: Excel calculator tools). Appropriate drift values are automatically selected by the calculator.

If the GAP (good agicultural practice) is for multiple applications, determine the PECsw spray drift based on both single and multiple applications and use the highest PECsw in risk assessment.

The multiple buffer zone spreadsheet linked above only gives the initial PECsw from spray drift. If you need to derive PEC concentrations at later time points or time weighted average (TWA) values for use in the risk assessment use the PECsw-sed spray drift calculator available from the Environmental fate models: Excel calculator tools webpage. In this calculator you have to manually insert the appropriate drift value for the crop group and stage (for example for early or late applications). Determine the PECsw spray drift based on both single and multiple applications and use the highest PECsw in risk assessment.

Standard PECsw for metabolites

PECsw spray drift values should be determined for all ecotoxicologically relevant metabolites.

For GB and NI assessments PECsw concentrations for metabolites are calculated based on the maximum total dose of active substance, adjusted for maximum formation observed in the water compartment of the water sediment studies and corrected for molecular weight differences. This is a simple and conservative approach to reduce uncertainty associated with the fact that for multiple applications, formation and degradation of a metabolite will be simultaneous making it difficult to determine when the true peak occurs. However, the drift value used should still be that appropriate for the number of intended applications.

For example, 3 applications of 100 g a.s/ha to cereals would be entered as a single total dose of 300 g a.s./ha (corrected for molecular weight and metabolite formation), with drift for 3 applications of 2.01% at 1 m which represents the 77th percentile appropriate for 3 applications.

To perform the calculations use either the 'PECsw spray drift (multiple buffer zone)' calculator or the 'PECsw-sed spray drift' calculator available to download from the Environmental fate models: Excel calculator tools webpage). However, as spray drift is applied automatically in the PECsw spray drift (multiple buffer zone) calculator, you should determine the relative metabolite dose for each application (that is corrected for molecular weight and metabolite formation). This should then be entered into the calculator for each application, with the interval set to 0 days, not allowing any time for degradation between applications.

The calculator requires a value to be entered for DT50, but this does not influence the initial PECsw for a single application, so may be set to any number. Ensure interval is set to 0.

For a multiple application GAP, it is unlikely that the PECsw for a metabolite from a single application with 90th percentile drift would be higher than from maximum total dose, but this should be checked.

Endpoints:

Example:

Single application:
Rate of 250 g a.s./ha.
Parent molecular weight 378. Metabolite molecular weight 190.
Molecular weight correction factor 190/378 = 0.50.
Corrected rate 250 * 0.50 = 125
Maximum % observed of metabolite in water phase of water-sediment study 21%
Adjusted for max. formation of metabolite 125 * 0.21 = 26.25 g met/ha
Use 90th percentile drift for 1 application

* Occasionally, actual concentration of metabolite is reported instead of %AR or parent equivalent, in which case there is no need to adjust for molecular weight difference

For example:

Peak concentration of parent in field water sediment study was 3 µg/l.
Peak concentration of metabolite in same study was 1.5 µg/l.
Application rate of parent is 1000 g/ha,
'Application rate' of metabolite = 1000 x (1.5/3) = 500 g/ha.

Standard PECsw for the formulation

GB and NI specific formulation PECsw spray drift is determined for a single application of the product. It is not necessary to calculate exposure from multiple applications because it is considered unlikely that the formulation will remain intact between applications.

For liquid formulations the maximum dose should be converted to an application rate (g/ha) using the following formula:

Application rate g/ha = maximum dose rate (ml/ha) x density (g/cm3).

This gives mass of applied formulation in g/ha which can then be entered into the 'PECsw spray drift (multiple buffer zones)' calculator available to download from the Environmental fate models: Excel calculator tools webpage. The DT50 box can be left empty because only the initial PECsw spray drift is required.

DRT PECsw for active substances

For field crops there is also the option to apply for approval under the Drift Reduction Technology (DRT) scheme. Enter the number of applications, application rate, water DT50 and interval into the 'PECsw spray drift (drift reduction)' calculator available from Environmental fate models: Excel calculator tools. The spreadsheet automatically selects the appropriate drift value and generates a PECsw spray drift initial value for the buffer distances of 6, 12 and 18 meters.

Calculations under the DRT scheme do not differentiate between the level of drift from single and multiple applications. Therefore, for multiple applications you can just provide PECsw values from the multiple application pattern and do not need to check exposure from a single application.

DRT PECsw for metabolites

PECsw values for metabolites assuming DRT can also be calculated using the 'PECsw spray drift (drift reduction)' calculator available from Environmental fate models: Excel calculator tools. The application rate for the metabolite is calculated in the same way as described above for the standard PECsw for metabolites. The application interval should be set to 0 to allow calculation of PECsw following the principles of the total dose approach.

DRT PECsw spray drift for the formulation

The formulation PECsw spray drift value can be calculated using the 'PECsw spray drift (drift reduction)' calculator available from Environmental fate models: Excel calculator tools. The method is exactly the same as for a single application for the standard PECsw spray drift formulation.

Sediment

When assessing the risk to sediment dwellers you need to assess the risk of exposure as a result of spray drift. You need to submit predicted environmental concentrations in sediment. This section explains how to calculate exposure in sediment from spray drift.

The ecotoxicity of substances to sediment dwellers can either be expressed as a concentration in bulk sediment (usually in µg/kg) or as a concentration in the water phase (usually in µg/l). The calculation of sediment exposure must match the ecotoxicity endpoint for use in the risk assessment. For calculations of PECsed in bulk sediment refer to the sections on standard PECsed for active substances, standard PECsed for metabolites or DRT PECsed. If the DT90 in sediment is longer than 1 year you must also assess the risk from accumulation of residues in sediment according to the guidance on PECsed accumulation.

For calculating sediment exposure from multiple application patterns, the total dose approach must be used. This is a conservative approach, used to simplify the calculation and reduce uncertainty caused by the simultaneous partitioning and degradation of the substance in sediment between spray drift events. Further guidance is provided in the sections below.

Standard PECsed for active substances

For calculations of PECsed in bulk sediment (that is in µg/kg) as a result of standard spray drift use the 'PECsw-sed spray drift' calculator. For calculation of PECsed as a concentration in the water phase refer to the guidance on Pseudo PECsw for sediment dwellers.

To perform the calculations, select the 'Sediment' tab in the 'PECsw-sed spray drift' calculator available from Environmental fate models: Excel calculator tools.

In the yellow boxes enter:

Number of applications One.
Partitioning of active substance from water to sediment and its degradation in sediment occurs simultaneously. To reduce uncertainty and simplify the calculation, use the maximum total dose for multiple application GAPs.
DT50 sediment Longest dissipation DT50 sediment from laboratory water-sediment study.
If DT50 values were not calculated separately for sediment, the worst case DT50 for whole system may be used.
Rate Enter the maximum total dose (g/ha)
Interval Not relevant
Drift % drift values from Rautmann et al, 2001 for the proposed crop, buffer zone distance and number of applications intended (that is the drift value relevant for the number of applications in the GAP should be used). Drift values are provided in the 'Rautmann Drift Values' tab of the PECsw-sed spray drift calculator.
For a multiple application GAP, you should check that a single application with 90th percentile drift will not give a higher PECsed value than the maximum total dose approach. However, this is very unlikely as using maximum total dose is a conservative approach, assuming a higher rate overall and no degradation between applications. For a single application GAP, just use the 90th percentile drift value appropriate for the crop of interest.
% in sediment Maximum % of substance observed in sediment compartment of the laboratory water-sediment study.

Use the PECsed initial (µg/kg) for the risk assessment.

To determine the PECsed spray drift for different buffer zones, amend the spray drift % figure by using the value for the same number of applications, but different buffer zones. For example, for field crops with three applications the value at 1 m is 2.01%, at 3 m it is 0.68% and at 5 m is 0.41%.

Standard PECsed for metabolites

PECsed values should be determined for all metabolites identified as being ecotoxicologically relevant in sediment.

Follow the same approach as for active substances detailed above, except the relative maximum total dose for the metabolite must be determined. It is important to ensure the % formation in sediment is not 'double counted'. To determine the relative total dose, correct the total parent application rate for molecular weight difference only, then enter % metabolite observed in sediment into the spreadsheet within box '% in sediment'.

DRT PECsed

For calculations of the PECsed in bulk sediment (that is in in µg/kg) for a product applied to field crops using DRT, you must first calculate a total dose PECsw value from DRT using the 'PECsw spray drift (drift reduction)' calculator. Select a single application in the calculator and enter the total dose as rate (in g/ha).
You then need to convert the total dose DRT PECsw value to a bulk sediment value using the equation below. For metabolites, use the parent total dose PECsw but use the appropriate fraction in sediment for the metabolite and parent and metabolite molecular weights.

PECsed equals PECsw times fraction in sediment times water height (times metabolite molecular weight), divided by sediment bulk density times sediment depth (times parent molecular weight)

Where:

PECsed accumulation

If the DT90 in sediment is longer than 1 year you must assess the risk from accumulation of residues in sediment. The accumulated load in sediment can be calculated by determining a maximum accumulation factor using the following equation:

maximum accumulation factor (MAF) equals 1 divided by 1 minus e(minus k times i)

Where:

If, due to the lack of decline, it was not possible to calculate a sediment DT50, use a default DT50 of 1000 days. The maximum PECsed concentration for a substance from applications in one year can then be multiplied by the MAF to account for accumulation. Use the accumulated PECsed value in the risk assessment.

Pseudo PECsw for sediment dwellers

If the ecotoxicity of the active substance to sediment dwellers is expressed as a concentration in the water phase (usually in µg/l), for multiple application GAPs it will be necessary to calculate another PECsw value, known as a pseudo PECsw to assess risks to sediment dwellers. The pseudo PECsw for sediment dwellers is based on the water concentration resulting from application of a single maximum total dose and the appropriate multiple application spray drift value.

For single application GAPs you can use the existing PECsw value and compare it to the ecotoxicity effect endpoint expressed as a water phase concentration in the risk assessment for sediment dwellers. For single or multiple application patterns, if the DT90 in sediment is longer than 1 year you must assess the risk from accumulation of residues using the maximum accumulation factor approach outlined in the guidance on PECsed accumulation.

For metabolites formed within the duration of the sediment-dwellers study, it is likely that the risk assessment on the active substance will address the risks posed by the metabolite. For metabolites formed outside of this timescale, the PECsw already calculated for the metabolite is sufficient because this value for multiple application GAPs is already conservatively based on the total dose approach.

Pseudo PECsw for sediment-only metabolites

If a metabolite is only formed in sediment and not detected in water, but you need to calculate a PECsw value for use in the risk assessment, the PECsed concentration (calculated according to standard PECsed for metabolites or DRT PECsed) can be converted to an equivalent PECsw concentration by multiplying by 0.2167. The 0.2166 conversion factor is derived from the dimensions and properties of a standard edge of field water body (100 m long x 1 m wide, depth of 30 cm = 30,000 l; 5 cm sediment with bulk density of 1.3 g/cm3 = 6,500 kg of sediment; 6,500 / 30,000 = 0.2167).

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Updated 2021-01-19