When you apply for registration of a pesticide in Great Britain and Northern Ireland, you need to submit an assessment of surface water exposure via drainflow. If you cannot show an acceptable risk using the first tier method, you will need to submit a higher tier drainflow exposure assessment.
Information on lower tier methods
MACRO tools
You can conduct higher tier calculations using the ‘MACRO 5 PECsw drainflow (higher tier MACRO)’ tool (which operates on Windows 7 and Windows 10 operating systems). A step-by-step user guide is included in the download folder.
The tool allows modelling of 4 soil types linked to 30-year datasets for dry, medium and wet weather climates. Each of the 4 soils is combined with the climate datasets to give 12 different soil-climate scenarios. You can model both an active substance and up to one metabolite.
If you want to use any methods that deviate from this guidance please contact HSE.
The MACRO 5 tool was developed in 2022 from the previously available MACRO 4 tool. Further information on the development of the MACRO 5 tool is available at: Background information regarding the development of the higher tier MACRO 5 drainflow tool.
Acceptability criteria of higher tier MACRO drainflow modelling in pesticide risk assessment
When you are preparing higher tier MACRO modelling for an application for pesticide registration in Great Britain and Northern Ireland, there are two criteria that must be met in order to demonstrate acceptable risk: Approach 1 (individual exceedance years) and Approach 2 (weighted level of exceedance).
Approach 1: Individual exceedance years
This criterion uses the largest modelled annual concentration each year and compares this to the Regulatory Acceptable Concentrations (RACs) for the relevant non-target groups. Different considerations are required depending on whether the RAC is derived from standard tier effects studies or higher tier effects studies and the species tested.
RACs for higher tier drainflow
Standard tier effects studies in this context means RACs based on toxicity studies where concentrations are either maintained at >80% nominal or a mean measured concentration is used i.e. Tier 1, Tier 2A (geometric mean) or Tier 2B (species sensitivity distribution) as defined by the EFSA (2013) aquatic guidance document.
Higher tier effects studies in this context means RACs based on studies with variable exposure, either Tier 2C (modified exposure studies) or Tier 3 (mesocosm studies where concentrations decline following initial application(s).
RAC derived from standard tier effects studies
The following steps should be undertaken. Please note, these steps are undertaken automatically when using the processing calculators described below in the ‘Processing of higher tier MACRO drainflow modelling’ section:
- compare the largest annual concentration each year to the RAC
- determine the number of years where the largest concentration exceeds the RAC. This is the number of exceedance years
- repeat for each scenario
- consider the significance of the number of exceedance years within each scenario. HSE differentiates between the following groups when considering the significance of the number of exceedance years:
- Algae and aquatic plants
For algae and aquatic plants, there must be no more than 60% of exceedance years in each scenario. This equates to no more than 18 years out of 30 exceeding the RAC. If these conditions are met, the risk is acceptable and you do not need to provide any further information regarding Approach 1. The 60% is an absolute upper limit threshold that cannot be exceeded in any scenario in order to conclude on an acceptable risk. It is not therefore possible to present a case justifying exceedances above 60%.
- Fish and aquatic invertebrates
For fish and aquatic invertebrates there is a lower limit threshold value. The risk is acceptable if there are no more than 10% of exceedance years in each scenario. This equates to no more than 3 years out of 30 exceeding the RAC. If these conditions are met, the risk is acceptable and you do not need to provide any further information regarding Approach 1 (unless the RAC is derived from a higher tier effects study). - For fish and aquatic invertebrates there is no defined upper limit threshold for Approach 1 in the same way that the 60% applies for algae and aquatic plants. If in the assessment for fish or aquatic invertebrates the exceedance years are above 10% for any scenario, it may still be possible to show an acceptable risk. This will need a more detailed case-by-case assessment. This should consider the size, frequency and the duration of exceedance events. Applicants must consider all scenarios where exceedances are above 10%. Use the following metrics:
- the size of the maximum exposure peak in relation to the RAC
- duration of exceedance events
- the number of exposure peaks above the RAC within each year
- the Higher tier drainflow processing (annual data) spreadsheet can be used to process the higher tier drainflow results for up to three compounds, whilst also undertaking the combined assessment if required (in line with Option 2 of the combined higher tier drainflow assessment HSE guidance). Due to file size, it is recommended this calculator be used in the first instance
- the ‘Higher tier drainflow processing (daily data)’ spreadsheet can be used to process the higher tier drainflow results for up to four compounds, whilst also undertaking the combined assessment if required (in line with Option 3 of the combined higher tier drainflow assessment HSE guidance). This calculator should be used if an unacceptable combined risk assessment is being predicted using the ‘Higher tier drainflow processing (annual data)’ calculator or if a combined assessment of 4 compounds is required
- instructions on how to use the processing spreadsheets are provided within the spreadsheets
- PL0501 Pesticide residues in water - ADAS-SWRC hydrological studies in Brimstone III.
- PL0502 Brimstone III - leaching of pesticides to drainage water from a structured clay soil.
- PL0503 Brimstone III - Pesticide residues in water.
- PL0509 Pesticide behaviour in soils and movement to drainage.
- PL0516 Evaluation of the use of preferential flow models to predict the movement of pesticides to water
- PL0538 Development of guidance on parameter estimation for MACRO
In addition, information from Approach 2 on the overall weighted level of exceedance may be useful to supplement the case-by-case assessment using the above metrics.
RAC derived from higher tier effects studies
Additional consideration is required when using a RAC from a higher tier effects study in higher tier drainflow modelling. In this case, in addition to consideration of the magnitude of the RAC compared to predicted ditch concentrations, additional parameters should be included. This includes area under the curve (AUC) and duration of the predicted ditch concentrations compared to the respective parameters in the mesocosm. This should be performed over the entire drainflow period and should include all peaks which exceed an appropriate baseline and are considered to be ecotoxicologically dependent (please refer to the EFSA 2013 aquatic guidance document). These metrics should be used in the same way as described for the peak exposure for Approach 1 above, giving the exceedance years in each scenario which are compared to the same triggers depending on the organism. Additional guidance to support the use of higher tier effects data in the higher tier drainflow assessment is available here.
The appropriate baseline is usually the Tier 1 RAC, however in some cases the higher tier data show that the assessments factors used at tier 1 were not, in that specific case, sufficient to account for the variability in sensitivity between species (the first tier RAC is higher than the higher tier RAC). This means that the first tier RAC cannot be used as the base for measuring area under the curve or duration of exposure. This means a different value should be used and justified (for practical reasons zero shouldn’t be used). One option is the LOQ in the higher tier study used to set the RAC as this can be used to calculate the area under the curve and the duration of exposure events.
Approach 2: Weighted level of exceedance
This criterion uses information on the extent of each soil-climate scenario within the crop. The percentage of exceedance years for each soil-climate scenario (from Approach 1) is multiplied by the extent of the crop grown under each soil-climate condition and summed to derive an overall weighted level of exceedance. The weighted level of exceedance must not exceed 10%. Approach 2 calculations are always required regardless of whether the RAC is derived from standard or higher tier effects studies, and the species tested.
Please note, the Approach 2 calculations are undertaken automatically in the processing calculators described below in the ‘Processing of higher tier MACRO drainflow modelling’ section.
Relevant cropping data is provided in the processing spreadsheets detailed in the ‘Processing of higher tier MACRO drainflow modelling’ section below. The cropping data is obtained from a series of different geospatial datasets. Further information is provided in the ‘SEISMIC Replacement Tool Report’.
Processing of higher tier MACRO drainflow modelling
Two higher tier drainflow processing calculators are provided in the ‘MACRO 5 PECsw drainflow (higher tier MACRO) tool’ download folder. Both calculators generate results based on Approach 1 (for RACs derived from standard tier effects studies only) and Approach 2:
Summary
When an unacceptable risk to surface water via drainflow is being predicted using the first tier method, a higher tier drainflow assessment is required using the ‘MACRO 5 PECsw drainflow (higher tier MACRO)’ tool. In order to conclude acceptable risk at the higher tier, two criteria must always be met: Approach 1 and Approach 2.
Approach 1 compares the maximum PECsw to the RAC. For standard tier algae or aquatic plant RACs, the total number of exceedances must not exceed 60%. For standard tier fish or aquatic invertebrate RACs, the total number of exceedances should not exceed 10%. However, if >10% exceedances are being predicted, it may still be possible to demonstrate acceptable risk by providing a detailed case-by-case assessment. For higher tier effect study RACs, additional consideration is always required, regardless of the number of exceedances being predicted (including zero exceedances).
Approach 2 weights the number of exceedances to the proportion of crop coverage under relevant soil and climatic conditions. The weighted level of exceedance must not exceed 10%. Approach 2 calculations are always required.
If an unacceptable risk is being predicted with either Approach 1 or Approach 2, a change to the use pattern may need to be considered to refine the risk assessment. For example, this may involve lowering the application rate or restricting the use to either less vulnerable application timings or restricting applications to later growth stages (increasing the level of crop interception). Generation of additional ecotoxicology effects data to refine the risk assessment may also be considered. If wishing to undertake a novel approach, this should be discussed with HSE beforehand.
Resources
MACRO 5 PECsw drainflow (higher tier MACRO) tool from Environmental fate models: Excel calculator tools.
Background information regarding the development of the higher tier MACRO 5 drainflow tool
EFSA PPR Panel (EFSA Panel on Plant Protection Products and their Residues), 2013. Guidance on tiered risk assessment for plant protection products for aquatic organisms in edge-of-field surface waters. EFSA Journal 2013;11(7):3290, 268 pp. doi:10.2903/j.efsa.2013.3290.
SEISMIC Replacement Tool Report
Additional ecotoxicology guidance to support the use of higher tier effects studies