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Active substance PECsw calculations (for UK specific authorisation requests)

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At the current time, PECsw for national (UK) approvals must be calculated using the separate considerations of spray drift and drainflow:

a)Spray drift

The current UK accepted approach for calculation of PECsw by spray drift is described in a previous Aquatic Guidance document (SANCO/3268/2001 dated 1 October 2001). This makes use of German spray drift data to give 90th percentile drift for single applications and overall 90th percentile drift for multiple applications. Tables with specific spray drift values for individual applications are available at the back of this guidance document. The latest guidance document has been published on the Europa website and is dated 18 July 2013 and can be found as Guidance on tiered risk assessment for plant protection products for aquatic organisms in edge-of-field surface waters  (EFSA Journal 2013; 11(7):3290). This latest version does not include the spray drift data needed for UK assessments, but does include the underlying principles in Chapter 6.

For Plant Protection Products which may be used over the whole growing season, Applicants must consider different spray drift percentiles for fruit crops and vines which differ between early and late season use to determine which gives the greatest PECsw, similarly with vegetables which are <50cm tall and those which may be >50cm tall, a consideration of increased drift with the taller crops to determine which gives the greatest PECsw.

The following spray drift assumptions are used by HSE's Chemicals Regulation Division (CRD):

Fruit crops

Early spray drift values for fruit crops are used up to and including BBCH 70.
Late spray drift values for fruit crops should apply at BBCH 71 and onwards.


Early spray drift values for grapevines are used up to and including BBCH 15.
Late spray drift values for grapevines should apply at BBCH 16 and onwards.

In addition, HSE's Chemicals Regulation Division (CRD) have extended the buffer zones which can be used with Plant Protection Products for field crops with buffer zones of up to 20m permitted.  To assist Applicants with the calculation of PECsw, spraydrift, HSE's Chemicals Regulation Division (CRD) have produced a simple Excel spreadsheet PEC sw-sed (spraydrift) which can be downloaded "as is".  CRD have also produced a simple Excel spreadsheet PEC sw Multiple Buffer Zones which can be downloaded "as is" to calculate the PECsw,spraydrift to identify the buffer zones necessary for an acceptable risk assessment. Both spreadsheets are based on Rautmann (2001) drift data.

An important issue arises from the use of lower percentile 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 90th percentile drift value for a single application could give a higher PECsw than the overall 90th percentile for multiple applications. Thus it is always important with multiple application scenarios to initially conduct calculations for both a single application and multiple applications to determine which gives the greatest PECsw. HSE's Chemicals Regulation Division (CRD) has produced a worked example of a buffer zone assessment for spray drift for a horizontal boom sprayer. Note this is based on the older LERAP scheme but includes useful information nonetheless.

In the case of applications in which an acceptable use cannot be achieved with a buffer zone of up to 20 metres for field crops from the PEC sw Multiple Buffer Zones, as a higher tier, the applicant may submit a new risk/exposure assessment incorporating drift reduction technology (DRT), see PECsw via spraydrift.

Applicants may make applications using three star DRT for authorisation of new uses or for re-authorisation of existing uses which are currently based on standard equipment. Applications should specify that they are being made under the DRT scheme and be supported by the appropriate PEC calculations and aquatic risk assessments. The drift model to determine the PEC for this scheme and instructions for its use are available in an Excel spreadsheet PEC sw Drift reduction. The drift model contains the appropriate regression values (based on Van de Zande drift data, IMAG v1.2, May 2003) to calculate the initial surface water PEC due to spray drift (PECsw) for buffer zones at 6, 12 and 18 m intervals.

Applicants may apply to use DRT to reduce buffer zones of between six metres and twenty metres identified at the first tier. The use of DRT must be stated on the product label. In addition it should be noted that the method of application must be only by conventional nozzles OR 3 star DRT nozzles, not both.  The use or recommendation for both conventional and DRT applications on the label is NOT permitted.

At present, the use of DRT is only available for field crops (height <50 cm).

Further information on calculation of surface water exposure from spraydrift can be found at PECsw via spraydrift.

b) Drainflow

Up to 30% of the United Kingdom's cereal crops may be grown on heavy clay soils (Cannell et al, 1984), as may significant proportions of other crops (eg winter oilseed rape). Many of these soils require the use of sub-surface drainage systems to ensure that water logging does not occur and so to maximise yield. Such drainage systems, by a combination of both bypass and chromatographic flow, accelerate the passage of water through the soil profile and into surface waters. This, in turn, may increase the possibility of the contamination of surface waters with pesticides, which may be carried in the drainwater either in solution or sorbed to particulate matter. Exposure to aquatic organisms from dissolved residues may be significant.

Estimation of such exposure is therefore needed in addition to estimates of exposure by other routes (eg spray drift).

When should PECsw (drainflow) be estimated?

The following factors need to be considered before estimating exposure via this route.

Soil on which a crop is grown

It is known that a significant proportion of the UK cereal crop is grown on drained soils. However other crops may also be grown on such soils. Therefore, consideration needs to be given to whether the proposed crop is grown on drained soils. SEISMIC is a suggested source of such information, however applicants are free to use other sources as appropriate.

To assist Applicants further HSE's Chemicals Regulation Division (CRD) have proposed the following positive list of crops where drainflow assessments have been requested in the past because the crop has the potential to be grown on heavy drained soils. This positive list also reflects decisions taken by the Advisory Committee on Pesticides/Expert Committee on Pesticides with regards to the need for drainflow assessments for certain crops.

Main crops considered to be grown on drained soils requiring a formal quantitative assessment: all cereals, maize, oilseed rape, potatoes, cabbage and related brassicas, Brussels sprout, top fruit (eg apples, pears etc), all forms of beans and peas, strawberries, asparagus, all forms of grassland (eg permanent, rotational, amenity etc) and green cover on land temporarily removed from production.

Similarly HSE's Chemicals Regulation Division (CRD) have accepted that certain crops are highly unlikely to be widely grown on the worst case heavy drained soils, and in such cases, for foliar spray applications, no formal quantitative assessment of the drainflow route of exposure has been needed.  A simple reasoned case will normally be sufficient to address the risks posed by drainflow on these crops.  Crops falling into this category are typically root crops such as carrot, parsnip, turnip and all forms of beet crop, but also crops such as hops, vines, onions and leeks and soft fruit (eg blackberry and raspberry).  However, the above list is not absolute in that the above crops may be grown on soils which are artificially drained.  For seed treatments for example, drainflow may well be the dominant exposure route for surface water in which case an assessment of exposure via drainflow may be necessary.

Note that the above lists of crops and situations are not comprehensive or exhaustive, and for any crop not included above you should carefully consider the likely soil types that could support crop production when putting together your environmental fate submission. If you are in any doubt over whether the specific crop could be grown on drained soils, you are advised to consider submitting a simple first tier drainflow assessment in the first instance. The first tier is an empirical calculation based on application rate and the substance Koc, for which CRD have produced a simple Excel spreadsheet 'PECsw-sed(drainage) ' which can be downloaded "as is".

Timing of application

Loss of pesticides to surface water in drainflow is considered to be a potential problem at the times of the year when rainfall is greatest, net water movement is downwards and evaporation from the soil surface is low. There is therefore a greater potential of surface water contamination via drainflow occurring after autumn/winter applications, however, it may also occur in early spring. Drainflow is therefore considered most likely in the months October- April inclusive.

For the simplest assessments, if the crop is grown on heavy, drained soils and the pesticide is to be applied in the period October to April, then an estimation of the potential exposure of surface waters via drainflow must be performed.

Other factors needing consideration

Persistence of the pesticide

A pesticide may be applied to drained soils in the summer, however, it may be persistent and thus significant concentrations may remain in soil later in the year when drainflow may occur. In such instances an estimate should be made of the maximum concentration of the pesticide which could occur in soil in the period when drainflow could occur and an estimate of the concentration in drainflow be derived from this in order to input a suitable 'application rate' in the 'PECsw-sed(drainage) ' spreadsheet.

If the first tier PECsw drainflow exceeds the Environmentally Acceptable Concentration, then Higher Tier Drainflow calculations need to be carried out. Guidance on this methodology is published in a number of pages on the HSE's Chemicals Regulation Division (CRD) website. The UK approach has evolved from a number of Research Projects commissioned by Defra and HSE's Chemicals Regulation Division (CRD) which can be found on the R&D project page.

Additional guidance on exposure calculations for UK PPP from drainflow can be found at first tier drainflowPECsw and higher tier drainflow from webfram.


Where significant concentrations (>10% of the applied dose of active substance) of metabolites may be present in soil in the period when drainflow may occur then an estimate should also be made of the concentrations of these metabolites occurring in drainflow. This estimate should use the maximum concentration of the metabolite which may occur in soil in this period in order to input a suitable 'application rate' in the first tier 'PECsw-sed(drainage) ' spreadsheet.

As a general rule you should always consider drainflow when submitting a UK product authorisation application to HSE's Chemicals Regulation Division (CRD). This may take the format of a formal quantitative drainflow exposure assessment (see first tier drainflow PECsw and higher tier drainflow from webfram). Alternatively the submission can be a reasoned case to argue that for the specific combination of crop, substance and/or application timing that the drainflow route of surface water exposure is not relevant.

The above approaches cover surface water exposure through the use of Plant Protection Products on crops grown on porous surfaces. However, there are a number of other situations of use which can lead to environmental exposure and for which routes of exposure will differ.

Assessment of Surface Water Exposure from Use on Hard Surfaces

Where recommendations are made for use on hard surfaces, CRD guidance on assessment of surface water exposure from such uses should be followed. Current guidance on calculating the PECsw for the UK from hard surface use is included at HardSPEC.  A number of background documents describing the development and rational behind the HardSPEC model are included.

It is recommended that Applicants check the website periodically for updates to the HardSPEC model and full details of all reports can be found on the HardSPEC page.

Metabolite PECsw calculations (for UK specific authorisation requests)

The principles applying to the calculation of PECsw for metabolites are similar to those for PECsoil for metabolites, particularly in respect to uncertainty regarding peak formation under multiple application conditions. Thus always calculate PECsw for metabolites on the basis of the maximum total dose of the active substance adjusted for maximum formation observed in the studies and the molecular weight (appropriate for radiolabeled studies). The drift value should be the appropriate overall 90thpercentile value for the number of applications in the GAP, ie if the GAP specifies three applications on a cereal crop, the application rate should be based on the maximum total dose, but the % drift would be 2.01% at 1m, NOT 2.77% at 1m.

Because of the maximum total dose approach for metabolites under multiple application scenarios, it is unlikely that the PECsw for a metabolite from a single application, ie using a higher drift value, would be higher than from the maximum total dose. However, this may be worth checking.

Remember that soil metabolites may also reach surface water by drainflow. The approach for calculation of drainflow with soil metabolites is similar to that for an active substance, but using the peak formation observed in an appropriate soil study.

PEC sediment for active substance and metabolites calculations (for UK specific authorisation requests)

PECsed calculations for UK authorisation requests follow the same principles as the UK approaches for PECsw, but PECsed is calculated on the basis of the highest amount of substance in the sediment in the sediment/water study (important to specify the time in the study this occurs) and the amount in sediment at the end of study (usually 100 – 120 days)


Updated 2020-07-17