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UK surface water exposure from spray drift

General Information

Further information on the method for the calculation of PECsw spray drift and PECsed spray drift for the UK registration of pesticide products is presented here. FOCUS SW modelling is not yet adopted for UK national assessments. For information on the presentation of PECsw assessments in core evaluations see EU PECsw calculations and associated pages.

In the UK PECsw/sed concentrations are calculated separately for spray drift and drainflow. The following information  relates to the drift aspect. Please also refer to first tier drainflow PECsw and higher tier drainflow from Webfram. for information on assessment of surface water exposures from drainflow.

UK standard assessment of drift is based on the use of German "Rautmann" spray drift data for PECsw assessments where application by standard hydraulic nozzles are required.  Use of Drift Reduction Technology (DRT) is now accepted in the UK as a regulatory risk mitigation measure. The DRT scheme is an additional option running alongside existing schemes.  DRT assessment uses Dutch "Van de Zande" drift data and PECsw assessments assume the use of 3 star nozzles under the DRT scheme (assumes 75% drift reduction).  The submission to the HSE's Chemicals Regulation Division (CRD) must state whether the product assessment is based on standard spray drift or DRT.  However, product assessments cannot use a combination of both approaches.  Further details of each scheme are given below.

For product evaluation the use of a "Risk Envelope" approach may also be possible.  Care must be taken especially if the risk envelope has any form of mitigation, or assumes crop is grown every other year or third year due to crop rotation considerations.  Details on the suitability of risk envelopes may be found at Risk envelope suitability.

Standard Drift Values

UK PECsw spray drift calculations are based on the assumption that the surface water is static water with a depth of 30cm, with 5cm deep sediment and effective 100:1 ratio for field to water body.  The drift values used are those provided by Rautmann et al 2001 described in a previous Aquatic Guidance document (SANCO/3268/2001 dated 1 October 2001). The drift values are shown within the spreadsheets: PEC sw Multiple Buffer Zones and PEC sw-sed (spraydrift) .

The drift values are crop specific and alter depending upon the no spray buffer zone (distance between the edge of spraying and the surface water) and the number of applications.

The drift values are crop specific and alter depending upon the no spray buffer zone (distance between the edge of spraying and the surface water) and the number of applications.

For a set distance, the percentile spray drift values decrease with each subsequent application; eg one application has a 90th percentile, two applications each have 82nd percentile;  this ensures that the overall assessment (considering for example two applications each with 82nd percentile) gives the 90th percentile.

For field crops, drift values (and associated percentiles) are provided up to 8 applications, above which the drift value is the same as for 8 applications (67th percentile).

Vegetables <50cm, >50cm:

Where drift values refer to 'height <50 cm or >50cm' this relates to application machinery used rather than crop height.  All field crops sprayed using a field crop sprayer with a horizontal boom should use the 'Field crops' value in the table, (which is the same as the 'Vegetables <50 cm' value).  When application is by other types of machinery (eg broadcast air-assisted sprayers, variable geometry boom sprayers etc), higher drift values for 'Vegetables >50 cm' (identical to the 'Late grapevine' values) should be used.

Early and Late spray drift:

Orchard Fruit crops

Spray drift data used in FOCUS surface water and UK surface water assessment provide different spray drift values for early (higher spray drift) and late (lower spray drift) orchard crops due to changes in the canopy throughout the season. CRD consider that BBCH 71and later correlates with the 'late' spray drift values as this is in agreement with crop interception values given in the EFSA guidance on DegT50 in which maximum crop interception in apples occurs from BBCH 71.

The decision is therefore made that:

Grape vines

Spray drift data used in FOCUS surface water and UK surface water assessment provide different spray drift values for early (lower spray drift) and late (higher spray drift) grapevine crops due to changes in the canopy and agronomic practice throughout the season. In original trials conducted to determine the drift values the early applications were made mid-May at growth stage 15 and the late trial applications at GS31 in mid-July.  Growth stage 15 also coincides with the transition from first leaves to leaf development for the vines.

When assessing vines in UK assessments the decision is therefore made that:

Standard spray drift mitigation - Buffer Zones

If the PEC concentrations result in an unacceptable aquatic risk assessment (ie the toxicity: exposure ratio is less than the appropriate assessment factor as specified in Commission Regulation  (EU) 546/2011), risk mitigation measures may be needed.  No spray buffer zones mitigate the risk from drift.

The UK maximum buffer zone allowed to mitigate drift from horizontal boom sprayers for arable crops is 20 m using standard nozzles. For orchard crops treated using air assisted broadcast sprayers, the maximum buffer zone is 50 m.

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

Using thePEC sw Multiple Buffer Zones spreadsheet the PECsw spray drift value for a range of standard buffer zones may be determined. It is advisable to provide PECsw and PECsed values for a range of buffer distances in the registration report as it will facilitate ecotoxicological risk assessment and determination of the relevant buffer zone distance.

Product authorisation may be given for either standard nozzles or DRT: 3* rated nozzles, not both.

Note: After product authorization in the UK, 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 5m and 20m set under the standard nozzle scheme.

Drift Reduction technology

For arable crops only, there is also the option for the Applicant to apply for authorisation under the Drift Reduction Technology Scheme (DRT).  This was introduced by CRD in March 2014 .

This takes into account use of 3* rated drift reducing nozzles and PEC concentrations are calculated using DRT drift values  for up to 6, 12 or 18 m buffer zones. (Van de Zande drift reduction PECsw calculator.xls).

DRT operating conditions must be maintained until 30m from the top of the bank / surface of the water body. Beyond 30m the standard operating conditions apply.

If a standard spray drift assessment fails at 20m (maximum buffer for standard spray drift) then DRT method can be considered.  Applicants may also use DRT assumptions to reduce buffer zones of between 6m and 20m calculated using standard nozzle assumptions identified at the first tier. However in this latter case, Applicants must choose to present buffer zones based either on standard nozzle assumptions or DRT assumptions.

Use of the DRT method 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.

Specification of DRT is on a product basis, ie 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 eg 6 m 'crop A', 12 m 'crop B'.

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

There are a number of methods for calculating surface water exposures from spray drift, summarised below

1. Standard PECsw, spray drift for active substance

PECsw spray drift values are calculated for all active substances in a PPP.

DT50 water: For a UK spray drift assessment the longest non-normalised dissipation DT50 from the water phase of the water sediment study is used. Dissipation DT50 are not always reported in EFSA conclusions, as degradation rates are needed for FOCUS surface water calculations. However, the dissipation rate may be reported within the main body of the DAR/RAR.

If the dissipation DT50 value is not available, as a simple worst case the longest degradation DT50 value may be used. However as dissipation can be a significant process, if the use of the DegT50 value leads to a requirement for buffer zones for multiple applications, it is recommended that a dissipation value be calculated and used in the calculation.

DT50 values may also sometimes be derived from a representative mesocosm/ microcosm study.

Dose:Where multiple applications of a product are intended, an issue arises from use of lower spray drift values for individual applications in a multiple application scheme, compared to spray drift values for a single application.  Dependent on the water phase DT50 and application interval, the use of the 90thpercentile drift value for a single application could give a higher PECsw, spraydrift, than the overall 90thpercentile for multiple applications, (for example if the DT50 was much shorter than the application interval). Therefore, PECsw should be calculated for both a single application and the multiple application scheme to determine the greatest concentration, for use in the risk assessment.

Interval: Use the shortest interval where a range is specified.

Method 1A. for determining Standard PECsw spray drift for active:

Use the PEC sw Multiple Buffer Zones spread sheet and see Instructions tab.  Appropriate Rautmann drift values are calculated by the spreadsheet automatically.

If GAP is for multiple applications, determine the PECsw spray drift based on both one application and multiple applications to determine the highest PECsw spray drift.

Method 1B. for determining Standard PECsw spray drift for active:

The spreadsheet above only gives the initial PECsw, spraydrift.  To derive PEC concentrations at later time points or time weighted average (TWA) another spread sheet can be used: PEC sw-sed (spraydrift).

2. DRT PECsw, spray drift for actives

For arable crops only, there is also the option for the Applicant to apply for approval under the Drift Reduction Technology Scheme (DRT). This takes into account use of 3* rated drift reducing nozzles and PEC concentrations are calculated using Van de Zande drift values.

Use the PEC sw Drift reduction spreadsheet .  Enter the number of applications, application rate, Water DT50 (see notes above) and interval. The spreadsheet automatically enters the Van de Zande drift value and generates a PECsw, spraydrift initial value for the buffer distances of 6, 12 and 18 meters.

3. Standard PECsw, spray drift for metabolites

PECsw spray drift values should be determined for all metabolites observed in the water compartment of water sediment studies at 10% AR or more, or at 5% AR or more on two consecutive occasions, or >5% AR and still increasing at study end,in line with data requirements (Commission Regulation 283/2013).  (Metabolites observed at the same levels in aqueous photolysis studies also require assessment).

For UK national 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 conservative approach to reduce uncertainty associated with the fact that for multiple applications, formation and degradation of a metabolite will be simultaneous making it unclear when the true peak occurs.

However, the drift value used should still be that appropriate for the number of intended applications. eg 3 applications of 100 g a.s/ha to cereals would be entered as a single dose of 300 g a.s./ha (corrected for molecular weight & formation), with drift for 3 applications of 2.01% at 1m (not 2.77% for a single application).

You can use PEC sw Multiple Buffer Zones or PEC sw-sed (spraydrift).

Note: for PEC sw Multiple Buffer Zones spray drift is applied automatically, so you should determine the relative metabolite dose (see below) for each application and then enter the details into the spreadsheet for each application, with the interval is then set to 0 days - thus not allowing any time for degradation between applications.

The spreadsheet 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 it may be worth checking.

Endpoints:

Water DT50: Set to any value >0 (no influence on initial PECsw for single application).
Dose: Total dose of active substance, adjusted for the molecular weight difference* of the metabolite and parent and for maximum occurrence.  (Highest observed %AR in the water compartment of water/sediment studies).

Where metabolite concentrations are reported as % AR or parent equivalents adjust parent dose for the molecular weight of the metabolite. eg
Parent molecular weight = 350.            Metabolite molecular weight = 300.
Molecular weight correction factor = 300/350 = 0.857
Maximum formation in laboratory study was 60% AR (ie 0.6)
Application rate of parent is 1000 g/ha,
'Application rate' of metabolite = 1000 x 0.6 x 0.857 = 514.2 g/ha.

*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.

4. DRT PECsw, spray drift for metabolites

The PEC sw Drift reduction can be used to determine DRT PECsw value. The application rate for the metabolite is calculated in the same way as in the standard PECsw metabolites (section 3 above).

Note: If values for surface water exposure values for active substances are presented using DRT methods, then appropriate DRT results for metabolites will also be necessary.

5. PECsw standard spray drift formulation

UK specific formulation PECsw spray drift is determined for one application of the product. It is appropriate to calculate an initial formulation PECsw spray drift for a single application, as it is over precautionary to assume product will persist, since all components will behave independently and dissipate at different rates.

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

This gives weight of applied formulation in kg/ha which can then be entered into PEC sw Multiple Buffer Zones for the appropriate crop. The DT50 box can be left empty where only Initial PECsw, spraydrift is required. The spray drift value for a single application should be used.

6. DRT PECsw spray drift formulation

If one of the actives in the formulation has resulted in the need to use DRT, then the formulation PECsw spray drift value must also be determined in the PECsw Drift reduction. The method is exactly that same as for the standard PECsw spray drift formulation (method 5) given above.

7. Pseudo PECsw for sediment dwellers

If the sediment-dwelling organism ecotoxicity study is dosed in the overlying water instead of sediment layer, the toxicity end point (NOEC) will be expressed as µg/l not µg/kg, and will need to be compared to a concentration in µg/l instead.

Thus it may be necessary to calculate another PECsw value, known as a pseudo PECsw for sediment dwellers; this is only pertinent for multiple applications.
The PEC for the active substance is based on the water concentration resulting from the maximum total dose approach and the appropriate multiple application spray drift value.

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 metabolite. For metabolites formed outside this time-scale, the PECsw already calculated is sufficient.

Method:

Use the maximum, total dose for the GAP, and the drift value appropriate to the number of applications. Proceed as per PECsw method 1B using the Use the maximum, total dose for the GAP, and the drift value appropriate to the number of applications. Proceed as per PECsw method 1B using the PEC sw-sed (spraydrift).

PECsed spray drift - standard drift: Introduction

Applicants should decide whether PECsed calculations are required for risk assessment, this typically being dependent on the data available for sediment-dwelling aquatic organisms and how the toxicity endpoints for such species are determined.  If it is decided that PECsed calculations are not required, a suitable case should be presented for CRD to consider.  In the absence of such a consideration, PECsed calculations must be provided.

If required, PECsed spray drift values should be determined for metabolites observed in the sediment compartment of the water sediment studies at 10% AR or more, or at 5% AR, or more on two consecutive occasions, or >5% AR and still increasing at study end, in line with data requirements (Commission Regulation 283/2013).

Where the GAP consists of multiple applications, the maximum total dose should be used in calculating PECsed concentrations (µg/kg) for active substance and metabolites. This reduces uncertainty related to simultaneous degradation and partitioning of the substance to sediment.

For metabolites the dose should be adjusted for molecular weight differences and maximum % observed in sediment (as described earlier). 

As for PECswit is also necessary to ascertain whether PECsed from a single application using higher % drift for one application would give a higher concentration in sediment over multiple applications.  This applies to both a.s. and metabolites.

PECsed methods

PECsed standard spray drift for actives

Use spread sheet PEC sw-sed (spraydrift).
Select the Sediment tab.
Enter data into the yellow boxes.
Set number of applications to 1 (maximum total dose), but use the Rautmann spray drift value for the intended number of applications.
RATE: g/ha (expressed as maximum total dose for multiple applications)
Spray drift %: Value from Rautmann tables (see PEC sw-sed (spraydrift)) appropriate to intended number of applications and buffer zone.
% in sediment: max formation in sediment within the water sediment study.

To determine the PECsed spray drift for different buffer zones alter 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 1m is 2.01%, at 3m is 0.68% and at 5m is 0.41%.
Record the PECsed ini µg/kg.

For multiple applications, it is necessary to ascertain whether a single individual application (with a % drift value for one application) does not give a higher PECsed concentration.

PECsed standard spray drift for metabolites:

As for active substance (PECsed method 8 above), except that the relative maximum total dose for metabolite, must be determined.

(NOTE Ensure the % formation in sediment is not 'double counted'.)
Either:

Correct the application rate for molecular weight difference only, then enter % AR of metabolite observed in sediment into the spreadsheet within box '% in sediment'

Or:

Correct the application rate for molecular weight difference and % formed in sediment; and then enter 100% into the box titled '% in sediment'. )

8. PECsed DRT

To determine PECsed for a product which has DRT applications you must determine the PECsed based on maximum total dose. This is achieved by converting the total dose DRT PECsw value.

For parent take parent PECsw calculated for maximum total dose (based on method 2 above).  Use PEC sw-sed (spraydrift)' spreadsheet.

For metabolite either use PECsw for metabolite calculated for total dose (method 4 above), OR, use parent maximum total dose PECsw with actual molecular weights entered in parent & metabolite molecular weight. boxes

These total dose PECsw values are converted to PECsed total dose values using the PEC sw-sed (spraydrift).

Bulk density = 1.3 g/cm3 default
PECsw = total dose µg/L
Height of water = 30 cm default
Sediment layer = 5cm default
% in sediment/water study = the maximum % in sediment observed in the water sediment study

For the active substance (or metabolite if you have a PECsw for the metabolite) enter 100 in both boxes requiring molecular weight.

If you have a Parent PECsw only and need a metabolite PECsed enter the relevant molecular weights in the boxes.

9. PECsed and PECsw for metabolite in Sediment only:

The PECsed is determined as for PECsed method 9 above.

If a metabolite is formed only in sediment and not in water, and if a PECsw is required, PECsed can be calculated as for PECsed method 9, and converted to a PECsw concentration by multiplying PECsed x 0.2166.

†The figure 0.2166 is derived from:
Water body of 100m long x 1m wide, with depths of 30cm water and 5cm sediment of 1.3 g/cm3 bulk density.
Sediment concentration of a µg/kg will be equal to a total sediment load of a
µg/kg x 6,500 kg = b µg.
If total load from sediment transferred to water phase, the water phase concentration would be b µg/30,000l = c µg/l.
Multiplication by 6500, then division by 30,000 equates to multiplication by 0.2166.

Updated 2017-03-28