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PECsw / sed via drainflow- UK first tier


Within the UK a significant proportion of agricultural land is drained by means of artificial sub-surface land drains. This enables cultivation of certain crops on soils which would otherwise be too heavy/ wet to use.  The land drains, by a combination of both bypass and chromatographic flow, accelerate the passage of water through the soil profile and into surface waters.  As a result of this, pesticides and their degradation products, can move through the soil via a preferential flow pathway and this can increase the possibility of contaminating surface water.

For the drains to make a significant difference to the flow of soil water there must be sufficient water present. Therefore conventionally at the first tier the drainflow period is considered to occur from October to April (inclusive) to represent the most likely period in the UK when soils are at field capacity. The following describes the CRD preferred method for the calculation of PECsw drainflow and PECsed drainflow for the UK authorisation of pesticide products at the first tier.

Remember that for product evaluations the use of a "Risk Envelope" (see Risk envelope suitability to a previously evaluated product could provide an efficient alternative approach and should always be considered prior to assessing new exposure values.  However, caution is advised when 'risk enveloping' to old products because historical approaches to drainflow may differ from those currently in place.  Care must be taken especially if the risk envelope has any form of mitigation that would reduce drainflow exposure eg timing restrictions.  Care should also be taken when risk enveloping to a different crop if that different crop required a higher tier drainflow exposure assessment, since the higher tier assessments may be much more crop specific than tier 1.

For higher tier drainflow assessments see higher tier drainflow from webfram and Higher tier drainflow from MACRO.

Crops which can be grown on (and are not grown on) drained soil are listed, however, this list is not exhaustive.

If the first tier assessments described here result in PECsw drainflow values which exceeds the Regulatory Acceptable Concentration (RAC), then higher tier drainflow should be considered.

PECsed drainflow introduction

A calculation of PECsed is not always required as the Ecotoxicology data package may not have effects data against sediment dwellers, or the end point may be based on a water concentration due to the conduct of the study.

The calculation of a PECsed is based upon the maximum occurrence of a substance in the sediment in water/sediment studies.  The spreadsheet referred to in the guidance calculate the PECsed based on various assumptions - please read the assumptions tab within the spreadsheet.

Note that the calculation of PECsed from drainflow is likely to be quite conservative.  Ditches receiving substantial volumes of drainflow are likely to be highly dynamic, and the peak occurrence levels from the static water-sediment studies are therefore likely to be conservative.  However for consistency, where ecotoxicologically relevant, it is proposed that PECsed drainflow is always calculated at the first tier.  Where the first tier risk assessment fails, further consideration of the relevance of the PECsed drainflow calculation may be needed.  Due to the likely conservatism of the approach it is not necessary to calculate the PECsed drainflow for active substances using the total dose approach (as would be the case for PECsed, spray drift – see PECsw via spraydrift.


For pesticide product evaluations, since early 2014, it has been the accepted CRD approach to only calculate environmental exposure levels for metabolites which have been deemed as ecotoxicologically relevant in the appropriate EFSA conclusion.

If required PECsw drainflow calculations are determined for metabolites formed within surface water OR soil, though care should be taken, as the two routes are calculated differently. Where metabolites are observed in both water and soil, a PECsw value should be determined for each route using the appropriate method and maximum occurrence in the relevant environmental compartment. The highest PEC obtained should then be reported within the fate assessment to ensure ecotoxicology base the risk assessment on the highest value.  Due to the conservatism of the first tier, it is not necessary to sum the two routes when determining the relevant PEC value.

PECsed for metabolites (if required due to its ecotoxicological relevance in sediment) must also be determined for metabolites formed in surface water, soil or sediment.


There are several different calculations presented below, the standard calculations are:

  1. PECsw and PECsed drainflow for all products applied in the drainflow period (ie October to April)
    1. Active
    2. Metabolites formed in soil
    3. Metabolites formed in water or sediment
  2. PECsw and PECsed drainflow for products applied before the drainflow period
    1. Actives
    2. Metabolites formed in soil
    3.  Metabolites formed in water or sediment
  3. Combined drainflow - for multiple actives

1. PECsw and PECsed drainflow for products applied in the drainflow period

This method is used to determine the PECsw and PECsed for actives and metabolites when the product is applied to an appropriate crop between 1stOctober and 30thApril.
This is the first tier approach and as such is simplified so that it is based on a total dose approach for metabolites.

1a. Actives


Use the ''In drainflow period 'tab within the spreadsheet: Drainage PECs v1.1
Enter the endpoints into the yellow boxes.

Results of PECsw and PECsed are given in the white boxes.

Note the concentrations in the stream are the relevant values for the purposes of the aquatic risk assessment.

Assumptions used for this calculator are provided in the assumptions tab

Endpoints (Yellow boxes):

Application rate:

For single application GAPs enter the maximum application rate. For multiple application GAPs the simplest method is to convert the peak PECsoil value (obtained as described in Soil) to an effective application rate by multiplying the PECsoil (5cm) value by 750 to convert to g a.s./ha.  Note this is preferable over a total dose approach.

Crop interception: Use the Crop interception from 'Generic guidance for FOCUS ground water scenarios, or for applications submitted after the 1st May 2015 applicants will need to use the updated crop interception values from the EFSA degT50 guidance.  If the PECsoil value used to derive the application rate has already accounted for crop interception, be careful not to double count for this factor and set interception to 0.

Koc: This should usually be the mean Koc. It should come from the Annex I listing (EFSA conclusion). If only the Kom is available, the Koc is calculated by Kom x 1.724.  Where pH dependence occurs, you should use the lowest Koc as a first tier approach.  Where clay dependence occurs, a case by case decision will need to be made, depending on the relationship, the relevance of the existing data and the properties of the soils types most vulnerable to drainflow in the UK (for example Denchworth type soils with 40-65% clay).

Fraction in sediment: This must be entered as a fraction eg 26% is 0.26. This should come from the ESFA conclusion from the water sediment studies.

Screen shot of the 'In drainflow period' tab with in the spreadsheet: Drainage PECs:
Diagrams for use in wiki page

Note:  The Drainage PEC v1.1 spreadsheet assumes the worst case drainflow occurrence.  For applications within the drainflow period, it is set on the day of the last application (no degradation for the last application).

1b. Metabolites formed in soil

For a conservative first tier approach this is based on the total applied dose for multiple application GAPs.  This is appropriate where there is likely to be uncertainty associated with the peak occurrence of the metabolite and proximity to drainflow events.

Use the 'In drainflow period' tab with in the spreadsheet: Drainage PECs v1.1 as shown above.

Application rate: Calculation is based on the total application dose of the parent adjusted for the molecular weight difference of the metabolite and parent and maximum occurrence in soil:

If the metabolite concentrations are reported in terms or parent equivalents or % AR the fate evaluator must make an adjustment for the molecular weight of the metabolite.

For example:

Parent molecular weight = 350
Metabolite molecular weight = 300
Molecular weight correction factor = 300/350 = 0.857
Maximum formation seen in laboratory study (soil) was 60% AR (ie 0.6)
Application rate of parent is 1000 g/ha,
Therefore 'application rate' of metabolite = 1000 x 0.6 x 0.857 = 514.2 g/ha.

Very occasionally the actual concentration of the metabolite is reported in a study instead of the %AR or parent equivalent. In those cases there is no need to adjust for the molecular weight of the metabolite: For example:

Peak concentration of parent in field dissipation study was 3.0 mg/kg.
Peak concentration of metabolite in same study was 1.5 mg/kg.
Application rate of parent is 1000 g/ha,
Therefore 'application rate' of metabolite to be used is 1000 x (1.5/3.0) = 500 g/ha.
Crop interception: see section above re active.
KOC:  Use the mean Koc for the metabolite

Fraction in sediment: Use the value for the metabolite IF a PECsed for the metabolite is required.

In the relatively rare case that a metabolite is only formed in sediment this method can still be followed.  However in this case the peak occurrence in soil should be set to 100%.  In this case the parent application rate is only corrected for molecular weight differences.

1c. Metabolites formed in water:

PECsw for metabolites formed in water

Because the maximum formation % for water metabolites is taken from a water sediment study, it is not appropriate to adjust the application rate to the soil (as per metabolites formed in soil).  Instead the metabolite PECsw value is simply determined based on the Parent PECsw drainflow value. This allows for the parent to have been applied to soil and then reach the surface water via drainflow prior to degradation to the metabolite.  Therefore the calculation required is:

Metabolite PECsw drainflow (via water) = Parent PECsw drainflow x molecular weight correction factor x maximum occurrence of metabolite in water.

Note this approach means that the PEC for metabolites formed in water is based on a multiple dose, whereas the PEC for metabolites formed in soil is based on a total dose approach.  This is to account for the possibility that in soil the metabolite may not drain until all applications are made, ie the soil system is static, whereas in water the system is dynamic and it is assumed that there is reduced carry over of the parent between applications.

PECsed for metabolites formed in water

If required, the easiest way to calculate PECsed for metabolite formed in water is to use the same simple equation as above for PECsw.  An additional correction factor of 4.615 is applied.  This simply converts the PECsw value to PECsed, based on standard UK assumptions (30cm water body, 5cm sediment layer with bulk density of 1.3g/cm3):-

Metabolite PECsed drain flow (via water) = Parent PECsw drainflow x molecular weight correction factor x (maximum occurrence of metabolite in sediment (as a fraction)) x 4.615

2. PECsw and PECsed drainflow for products applied before the drainflow period

If a product is used on a relevant crop, but the applications are made before the drainflow period, it will still be necessary to consider drainflow if the compound soil DT50 is long.  In these situations, despite being applied before the drainflow period, there could still be substantial compound in the soil at the start of the drainflow period (1st October).

Note:  The Drainage PEC v1.1 spreadsheet assumes the worst case drainflow occurrence.  If the last application is before the drainflow period, the drainflow event is set at the 1st October (earliest possible)

2a. Actives

The first tier approach in this situation is to base the calculation on the likely minimum interval between the final application and the onset of drainflow (assumed to be the 1st October).  Use the 'Outside drainflow period' tab within the spreadsheet: Drainage PECs V1.1.

Fill in the GAP information in the yellow cells ie application rate in g/ha and crop interception.

For single application GAPs only fill in information for the 1st application.  For multiple application GAPs, fill in the application interval tab (note all applications are assumed to have the same interval – if the GAP allows for varying intervals, choose the shortest to be conservative).

Fill in the date of final application.  This should be the latest likely application date for the proposed GAP.  The format should be in dd/mm/2014 (no other year can be entered).

DT50: Use the longest un-normalised soil DT50 (SFO only) – usually this will be the same value as used in the simple tier 1 PECsoil calculation.  The simple worst case value is used here because the model does not account for temperature and soil moisture differences, as would be possible in a FOCUS model for example.

KOC:  Use the mean Koc (see above for comments on pH or clay dependence)

Fraction in sediment: When required, this must be entered as a fraction eg 26% is 0.26.

Results are in the white boxes.  Concentrations in the stream (and sediment if required) can be read off the worksheet.

In the absence of a higher tier exposure assessment, if the first tier assessment fails it may be possible to use this worksheet to adjust the latest application date to an earlier date until an acceptable first tier PECsw values is obtained.  This could be used as the basis for applying a label phrase eg  To protect aquatic life, do not apply this product later than 'the 1st of September' in the year of harvest.  However in most cases a failure at tier 1 will require refinement at a higher tier.

2b. Soil metabolites:

Due to the inherent uncertainties associated with establishing when soil metabolites may form at peak levels, it is generally not appropriate to refine the soil metabolite PECsw drainflow values to account for applications made outside of the drainflow period.

Instead the PECsw / PECsed values for soil metabolites are determined based on the standard total application dose of the parent adjusted for the molecular weight difference of the metabolite and parent, and the maximum occurrence as explained for the first tier calculation (1b) above.

Only where it can be clearly established that the metabolite will be formed at peak levels in soil outside of the drainflow period could the effective total loading of the metabolite be corrected to account for additional dissipation prior to drainflow.  This may be possible, for example, when the parent substance degrades very rapidly and peak levels of metabolite are formed almost instantaneously.

2c. Metabolites formed in water or sediment:

For metabolites formed in water or sediment, where these are formed from an a.s. applied outside the drainflow period, one can follow the basic methodology outlined in 1c since this is based on the parent PECsw (which in this specific case should itself be calculated following method 2a).

3. Combined drainflow - for multiple actives

If a product contains multiple actives and the route of entry to surface water is via drainflow then the potential additive affects should be considered.

As a first tier approach simply add the two first tier drainflow (concentration in stream µg/l) results.  This applies to the PECsw values irrespective of whether they are applied inside or outside the drainflow period.

However, if this simple approach fails then this will require a more sophisticated combined PECsw drainflow. See Combined Risk Assessments.

Before progressing to a higher tier assessment, all available options to refine the first tier assessment should be considered (eg to account for degradation between applications).

NOTE. Additional information is provided in the Assumptions tab of the spreadsheet.

Updated 2016-07-12