First tier drainflow calculation for pesticides registration in Great Britain and Northern Ireland

Surface water exposure via drainflow – first tier approach

When you are applying for registration of a pesticide in Great Britain or Northern Ireland, you need to submit an assessment of surface water exposure via drainflow. Drainflow may lead to pesticide exposure in surface water and sediment. This document explains how to calculate exposure at the first tier.

Contents

Are calculations for drainflow exposure necessary?

Calculate drainflow exposure if the use pattern meets both the following criteria:

A drainflow calculation is necessary only if both criteria apply.

Consider the need for a drainflow assessment on a case-by-case basis. If in doubt, conduct the assessment.

General considerations for first tier drainflow calculations

A large proportion of GB agricultural land contains subsurface artificial drainage systems. This allows cultivation of certain crops on soils which would otherwise be too heavy or wet to use. The land drainage systems speed up the passage of water through the soil profile and into surface waters. This increases the potential for contamination of surface water bodies.

Exposure of both the water and sediment compartments may occur following drainflow events. Calculate the exposure of both compartments for the active substance(s). Also calculate the exposure for metabolites observed in soil, surface water and sediment. Please note the following caveats:

Use the first tier tool PECsw-sed drainflow (tier 1) calculator – Version 1.0 (October 2015) to calculate possible exposure.

The tool assigns a mobility classification to a substance based on its Koc value. The tool then applies a corresponding proportion lost to drains.

Percentage assumed lost to drains based on Koc value:

Koc (mL/g) Mobility Classification Lost to Drains (%)
0-14 Very mobile 1.9
15-74 Mobile 1.9
75-499 Moderately mobile 0.7
500-999 Slightly mobile 0.5
1000-4000 Slightly mobile 0.02
>4000 Non-mobile 0.008

Two tabs are available. The tab you select will depend on the application timing, as described by the 2 scenarios below.

Applications between 1 October and 30 April

Application Between 1 May and 30 September

Application between 1 October and 30 April

Drains are most likely to be flowing between 1 October and 30 April. You must consider drainflow for any use pattern involving applications between these dates. The first-tier approach assumes that a drainflow event occurs soon after application with no degradation taking place. Check that the application dates correspond to the proposed BBCH growth stages. Consider earliest and latest possible application timings. In this case, use the tab 'Inside drainflow period'.

If any first tier drainflow calculations result in ecotoxicologically unacceptable PEC values, a higher tier drainflow (HTDF) assessment is required. For further information go to MACRO higher tier drainflow modelling for pesticide registration in Great Britain and Northern Ireland and WEBFRAM higher tier drainflow modelling for pesticide registration in Great Britain and Northern Ireland pages.

Active Substance assessment - Application between 1 October and 30 April

Step 1

Open the PECsw-sed drainflow (tier 1) calculator from Environmental fate models: Excel calculator tools and navigate to the 'Inside drainflow period' tab.

Screen shot of the 'Inside drainflow period' tab

Step 2

Enter the application rate.

Multiply by a correction factor to convert the PECsoil value in mg/kg to a pseudo application rate. The correction factor accounts for soil density (usually 1.5g/cm3) and soil depth. Most PECsoil calculations are derived over a depth of 5cm, and this can be converted to a pseudo application rate by multiplying the PECsoil value by 750. Use an alternative conversion factor to calculate the PECsoil over a different depth. The derivation of the conversion factor is:

Equation 1. To convert PECsoil (mg/kg) to application rate (g/ha):

Application rate = PECsoil × (100 × bulk soil depth × soil density)
Where 100 is a unit correction factor
Where usually soil depth (cm) = 5 and soil bulk density (g/cm3) = 1.5 so that:
Application rate = PECsoil  × 750
For soil density of 1.5g/cm3:
If 2.5cm soil depth, multiply by 375
If 10cm soil depth, multiply by 1500
If 20cm soil depth, multiply by 3000

Step 3

For single application use patterns, enter the crop interception. Take the crop interception values from the EFSA DegT50 Guidance document. Ensure that crop interception is set to 0% if already taken into account elsewhere, for example as part of PECsoil and pseudo application rate calculations for multiple applications in Step 2 above.

Step 4

Enter the mean Koc. This will normally be the agreed mean Koc value used for FOCUS modelling.

If only the Kom value is available, convert this to a Koc value by multiplying by 1.724.

If there is evidence of pH dependence, use the lowest Koc as a first-tier approach.

Approach clay dependence on a case by case basis. This will depend on the nature of the relationship, the relevance of the existing data and the properties of the soil types most vulnerable to drainflow in Great Britain and Northern Ireland (for example Denchworth type soils with 40-65% clay).

Step 5

Enter the maximum observed level in sediment as a fraction. (Note that this is not the same as the percentage level observed. Calculate the fraction by dividing the percentage by 100.  The fraction is ≤1.) Take the maximum level in sediment from the water-sediment studies section of the agreed List of Endpoints. This will be the mean of replicates at a timepoint and not the highest individual replicate.

Metabolites assessment - Application between 1 October and 30 April

Metabolites formed in soil

The timing of peak formation of a metabolite is uncertain as formation and decline processes occur concurrently. Consequently, use a conservative approach as a first tier. For multiple applications, assume the individual applications are applied together (maximum total dose approach).

Step 1

Open the PECsw-sed drainflow (tier 1) calculator from Environmental fate models: Excel calculator tools and navigate to the 'Inside drainflow period' tab.

Step 2

Enter the application rate. This is the maximum total dose of the parent, corrected for maximum occurrence and molecular weight differences. 

Examples:

Adjust for molecular weight if metabolite concentrations are parent equivalents or applied radioactivity (AR).  For example:

If a study reports the actual concentration of the metabolite instead of the % AR or parent equivalent there is no need to adjust for the molecular weight of the metabolite. For example:

Step 3

Enter the crop interception. Take the crop interception values from the EFSA DegT50 Guidance document. 

Step 4

Enter the mean Koc for the metabolite. This will normally be the agreed mean Koc value used for FOCUS modelling.

If only the Kom value is available, convert this to a Koc value by multiplying by 1.724.

If there is evidence of pH dependence, use the lowest Koc as a first tier approach. Approach clay dependence on a case by case basis. This will depend on the nature of the relationship, the relevance of the existing data and the properties of the soil types most vulnerable to drainflow in Great Britain and Northern Ireland (for example Denchworth type soils with 40-65% clay).

Step 5

Enter the fraction in sediment as a fraction. As a worst case 1st tier approach, use a fraction of 1 (i100% formation in sediment). If refinement is necessary, calculate the fraction using the following formula:

Equation 2

Application rate=PECsoil x (100 x bulk soil depth x soil density)

Metabolites formed in water

Parent substance present in soil may reach the water body via drainflow. It can then degrade to form metabolites in water. To calculate PECsw for water metabolites, correct the parent drainflow PECsw for maximum metabolite formation in water (as a fraction) and molecular weight differences. Use the highest levels of metabolites in water from the water-sediment study.

Equation 3

Metabolite PECsw drainflow =
Parent PECsw drainflow x mol weight correction x max fraction metabolite in water

Metabolites formed in sediment

Calculate PECsed for a metabolite formed in sediment in a similar way to metabolites formed in water but apply an extra correction factor of 4.615. This converts the concentration to a PECsed value. The correction factor uses standard GB and NI assumptions of 30cm water body, 5cm deep sediment layer and sediment bulk density of 1.3g/cm3.

Equation 4

Metabolite PECsw drainflow =
Parent PECsw drainflow x mol weight correction x max metabolite in sediment x 4.615*
*4.615 correction factor calculated as:
30 x 100
100 x 1.3 x 5
(where 30cm is the water depth, 5cm is sediment depth and 1.3g/cm3 is the sediment density)

Application between 1 May and 30 September

Calculations may also be necessary for products applied at other times of the year. Active substances or metabolites may have a long enough DT50 to remain in soil when drains begin to flow.  For example, a substance may be applied at the start of September but have a DT50 of 90 days. This DT50 suggests that significant residues will remain in soil on 1 October (when drainflow starts). Drainflow exposure must still be considered for this substance. Use the tab 'Outside drainflow period' for substances applied 1 May to 30 September. The tool allows for degradation of soil residues before 1 October.

Active Substance assessments - Application between 1 May and 30 September

Step 1

Open the PECsw-sed drainflow (tier 1) calculator from Environmental fate models: Excel calculator tools and navigate to the 'Outside drainflow period' tab.

Screen shot of the 'Outside drainflow period' tab

Step 2

Enter the use pattern information in the yellow boxes. 

  1. Enter the application rate (g/ha) and crop interception.
  2. For multiple application use patterns fill in the application interval. (Note that for single application use patterns leaving this blank or filling in with 0 or another number does not affect the result). The tool assumes the same application interval between all applications. For use patterns with several application intervals choose the shortest interval to be conservative.
  3. Fill in the date of the final application, based on the latest likely application date for the BBCH growth stages proposed.

Step 3

Enter the DT50 value.  This will be the longest non-normalised DT50. It is usually the same as the DT50 value used in the PECsoil calculation.

Step 4

Enter the mean Koc for the metabolite. This will normally be the agreed mean Koc value used for FOCUS modelling.

If only the Kom value is available, convert this to a Koc value by multiplying by 1.724.

If there is evidence of pH dependence, use the lowest Koc as a first-tier approach.

Approach clay dependence on a case by case basis. This will depend on the nature of the relationship, the relevance of the existing data and the properties of the soil types most vulnerable to drainflow in Great Britain and Northern Ireland (for example Denchworth type soils with 40-65% clay).

Step 5

Enter the maximum observed level in sediment as a fraction. (Note that this is not the same as the percentage level observed. Calculate the fraction by dividing the percentage by 100. The fraction is ≤1.) Take the maximum level in sediment from the water-sediment studies section of the agreed List of Endpoints. This will be the mean of replicates at a timepoint and not the highest individual replicate.

Results are given in the green boxes.

If the 1st tier assessment fails, it may be possible to adjust the latest application date. Change the application date to obtain an acceptable 1st tier PECsw value. Combine this if necessary with a label phrase restricting the application timing. A suitable label phrase might be 'To protect aquatic life, do not apply this product later than [end of month xxxx] in the year of harvest'. Take this approach as an alternative to providing a higher tier exposure assessment. Please bear in mind however that this will shorten the period of use for the product. Please note that the restriction should be to the end of a month and not a specific day in the month.

Metabolite assessment - Application between 1 May and 30 September

Metabolites formed in soil

You should not normally need to refine metabolite PECsw values to account for applications outside of the drainflow period. This is because the timing of peak levels of soil metabolites is uncertain. It can thus be difficult to show that metabolite peaks will occur before drains are flowing.

The exception is if the metabolite clearly peaks in soil outside of the drainflow period. For example, a parent substance may have a very short DT50 and metabolite peaks may be observed shortly after application. In this case total loading of the metabolite can be corrected to allow for dissipation prior to drainflow. To achieve this, correct the total parent application rate for molecular weight differences and maximum soil occurrence of the metabolite.

Metabolites formed in water or sediment

Base the metabolite PECsw and PECsed values on the parent PECsw value. Use the basic method outlined in 'Application between 1 October and 30 April'.

Crops grown on drained soils/not grown on drained land

Crops considered to be grown on drained land in Great Britain and Northern Ireland:

Crops unlikely to be widely grown on the worst case heavy drained soils:

(A simple reasoned case is usually acceptable to explain why these crops are not being considered for drainflow)

Natural surfaces not intended to bear vegetation and permeable surfaces overlying soil - typical uses are likely to be quite targeted such as around buildings, along fence lines and around amenity vegetation/trees. Soft surfaces such as these are considered unlikely to be drained in a similar manner or extent as the agricultural situations that the first tier drainflow assessment was designed to be used for and therefore these uses do not need to be included in the exposure assessment.

Combined drainflow for multiple actives

Consider the combined exposure for products containing more than one active substance. For further information please refer to Combined risk assessment for pesticide registration in Great Britain and Northern Ireland. Consider the need for combined risk assessment on a case-by-case basis. Combined exposure assessment is not usually needed if all actives pass the first tier drainflow assessment on an individual basis. This is because there is sufficient margin of safety relative to RAC values. Do not assess combined exposure if the risk assessment is driven by a single substance. For example, a higher tier assessment may be necessary for only one active substance. This indicates that this substance drives the risk. If other substances in the formulation are close to the RAC value at the first tier however, consider the potential contribution of these substances towards the risk. Further information is also available in the Ecotoxicology section of the website.

Crop specific considerations

The following points apply to specific crops:

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Updated 2021-08-10