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Higher tier drainflow from MACRO

Background

Higher tier drainflow exposure assessments using the MACRO model have become the standard calculation method submitted by Applicants in order to refine aquatic risk assessments in the UK. These assessments are needed when the aquatic risk assessment fails the UK first tier drainflow calculation method (see first tier drainflow PECsw. There is also the potential for higher tier drainflow assessments to be made using the Webfram model (see higher tier drainflow from webfram).  Applicants should note that either approach to refining first tier exposure assessments is acceptable and it is not necessary to pass both methods.

The MACRO simulation model is based on the approach of Brown et al (20041) in which a range of vulnerable drained soil scenarios coupled to dry, medium or wet weather data sets are simulated. Further guidance on the selection of appropriate parameters for use with the MACRO model can be found in the research report for defra project 'Development of guidance on parameter estimation for MACRO'.

The general approach is considered valid by CRD. However after a number of years of experience with submissions, CRD released the following guidance on the presentation of higher tier drainflow modelling results for UK submissions (particularly with the presentation of the results).

Typically the maximum daily concentrations on an annual basis from each of up to 30 years worth of a simulation have been generated. These values have been compared with a regulatory concentration derived from the Ecotoxicology database and the appropriate Annex VI triggers to estimate the numbers of years where the Toxicity Exposure Ratio (TER) would be unacceptable (ie where the maximum daily concentration exceeds the Regulatory Acceptable Concentration (RAC) on at least one day per year). This is normally presented for each soil and weather scenario separately. An overall level of exceedance can be estimated if the individual percentages failing at each scenario are weighted to reflect the extent to which each scenario represents drained soils for the relevant crop of interest in England and Wales. An example of this form of presentation is provided at the end of this guide in Appendix A.

An additional refinement that has been used for some substances has been to calculate the 90th percentile value of daily concentrations for each scenario. This value has been calculated by ranking all daily drainflow concentrations greater than 0.001µg/l (to weight the assessment in favour of the periods of peak drainflow events) before calculating the 90thpercentile. The 90th percentile value for each scenario has then been compared directly with the regulatory concentration. The rationale behind this approach has been that the 90th percentile level has often been used in environmental assessments as being an appropriate realistic worst case value (eg typically a 90th percentile spray drift value is derived from the larger database of experimental drift values for use in aquatic risk assessments; additionally the FOCUS groundwater scenarios aim at describing an overall vulnerability approximating to the 90th percentile).

However there were concerns about the appropriateness of the different methods of presenting the results, particularly the use of a 90th percentile drainflow value. Although there is a general understanding that the 90th percentile level may be an appropriate realistic worst case value in other areas of environmental risk assessment, CRD considers that the frequency of drainflow events predicted by models such as MACRO should also be considered when assessing the overall protectiveness of such an approach. For example, when a 90th percentile spray drift value is used for a product that may be applied once per year, the resulting Predicted Environmental Concentration (PEC) value implies that for 9 out of 10 spray events for the receiving water body (ie a static 30 cm deep ditch) the PEC would be lower than predicted. Conversely, 1 out of 10 would be greater.

When a 90th percentile drainflow concentration is selected, it would mean that for 9 out of 10 drainflow events for that water body, the PEC would also be lower than predicted. However drainflow events are generally predicted to occur much more frequently than spray drift events. Therefore, for this type of drainflow modelling, the 90th percentile concentration value is not considered comparable to a concentration derived from a 90th percentile spray drift value, nor is it comparable to the protection goals of the FOCUS groundwater scenarios, in either the temporal frequency of exceedances or the spatial level of exceedance. Therefore it is not possible to say that the 90th percentile drainflow concentration protects either 90 percent of soil scenarios, or that it protects 90 percent of the simulated years. For this reason the use of the 90th percentile drainflow concentration approach was no longer be considered a valid approach to refine such exposure assessments by CRD.

Therefore, since March 2009, all such risk assessments have been based initially on a consideration of the maximum daily concentrations versus the regulatory concentration on an individual scenario basis to determine the number of years where the risk assessment would be acceptable or not. Further details in the form of a worked example are included in Appendix A.

As regards a suitable precedent in terms of the numbers of combined scenario years failing the relevant TER trigger, CRD proposes that a pass rate of 90% should be set as a guide to an appropriate value (ie the total scenario years failing plus any land area that remains unquantified by the modelling should be no greater than 10%). Such a trigger is likely to be appropriate for use against effects endpoints derived from tests on species with the potential for rapid recovery (eg algae and aquatic plants). Such a trigger may not be appropriate when the critical risk is identified to be against other species eg fish. Currently our experience is mainly limited to situations where effects against either algae, aquatic plants or invertebrates are driving the risk assessment and if your risk assessment is being driven by either an acute or chronic effect endpoint against fish you should consult CRD.

In addition such a trigger alone may not be sufficient to demonstrate an acceptable risk where the number of individual failures in any one scenario is particularly high. A consideration of the significance of the individual levels of exceedance within each scenario should therefore also be provided. Such a consideration could take into account the ecology of the specific group of organisms of concern in the aquatic risk assessment, as well as a consideration of the relative distribution of each scenario within the agricultural landscape. It is therefore important that the detail of all scenarios be presented in the final regulatory submission in order that each assessment can be fully evaluated by CRD (see the worked example in Appendix A for further details).

It is recommended that any higher tier drainflow approaches to support a UK approval should be discussed with CRD in the first instance if they are likely to deviate significantly from the guidance outlined above. For example, if you intend to include additional refinements to the assessment such as calculating exposure concentrations that exceed the regulatory concentration in a time window appropriate to the critical effect endpoint this will need to be fully justified from both an exposure and effects point of view. In addition if you intend to take into account either the duration of exposure relative to the time scale of effects, recovery effects and/or the effects of repeated exposures these issues will again need to be fully supported from both the exposure and effects side.

Footnotes

1. Brown, Dubus, Fogg, Spirlet and Gustin (2004) Exposure to sulfosulfuron in agricultural drainage ditches: field monitoring and scenario based modelling. Pest . Manag. Sci. 60 pp765-776.

Appendix A

Appendix A:  Example of presentation of higher tier MACRO drainflow modelling results based on full reporting of scenario exceedances

Risk assessment based on a regulatory concentration of 5.0µg/l for 'Substance X' for a simulated use on wheat

Number of years with ditch concentrations of 'Substance X' >5.0 µg/l on at least one day for each of the modelling scenarios for actual daily concentrations (total years simulated per scenario = 30 ; values in parentheses are percentages of years exceeding 5.0µg/l on at least one day per year):-

Number of years of 30 year simulation with daily ditch concentration >5.0µg/l on at least one day
Soil Dry (<625 mm per annum) Medium (625-750 mm per annum) wet (750-850 mm per annum)
Denchworth

5/30 (16.7)

10/30 (33.3)

8/30 (26.7)

Hanslope

4/30 (13.3)

6/30 (20.0)

5/30 (16.7)

Brockhurst

1/30 (3.3)

2/30 (6.7)

1/30 (3.3)

Clifton

0/30 (-)

1/30 (3.3)

0/30 (-)

Quorndon

0/30 (-)

0/30 (-)

0/30 (-)

Proportion of total wheat growing land in England and Wales accounted for by each scenario (figures taken from Brown et al.,2004):-

Soil type Extent of soil within each climate scenario (%) Total extent
Dry Medium Wet >850mm p.a. (%)
Undrained

 -

 -

 -

 -

45.9

Peaty soils

 -

 -

 -

 -

3.5

Denchworth

2.7

3.0

0.7

0.5

7.0

Hanslope

9.0

5.6

0.5

0.4

15.5

Brockhurst

4.8

7.6

1.8

0.9

15.1

Clifton

1.5

5.2

1.6

0.9

9.2

Quorndon

2.5

0.9

0.3

0.2

3.8

Total

20.4

22.3

4.9

2.9

100

Results for each scenario can be weighted according to their prevalence across England and Wales.  For example the 16.7% exceedance predicted at the 'Denchworth dry' scenario above can be multiplied by the value of 2.7% which represents its relative distribution within the total wheat growing land of England and Wales: 16.7% * 2.7% = 0.45%.  The cumulative level of scenario year exceedances can be obtained by summing the relative contributions for all 15 scenario combinations:-

Soil type Extent of soil within each climate scenario (%) Total extent
Dry Medium Wet (%)
Denchworth

16.7 * 2.7%
= 0.45

33.3 * 3.0% = 1.00

26.7 * 0.7%
= 0.19


= 1.64

Hanslope

13.3 * 9.0%
= 1.20

20.0 * 5.6%
= 1.12

16.7 * 0.5%
= 0.08


= 2.4

Brockhurst

3.3 * 4.8%
= 0.16

6.7 * 7.6%
= 0.51

3.3 * 1.8%
= 0.06


= 0.73

Clifton

0 * 1.5%
= 0

3.3 * 5.2%
= 0.17

0 * 1.6%
= 0


= 0.17

Quorndon

0* 2.5%
= 0

0 * 0.9%
= 0

0 * 0.3%
= 0


= 0

Combining all years and scenarios and considering the number of scenario years with actual concentrations of 'Substance X' where concentrations were greater than 5.0µg/l on at least one day, this occurred in a combined total of 5.0% of scenario-years.  The concentration was <5.0µg/l in 92.1% of scenario years and the risk was not quantified in 2.9% of scenario years (the unquantified area represents the proportion of wheat growing land where annual rainfall was >850mm).

In addition, a consideration of the significance of the individual levels of exceedance within each scenario should be provided by Applicants.  For example in the worked example above the maximum number of exceedances is seen at the 'Denchworth medium' scenario and here an assessment of the acceptability of the level of exceedance of 10 out of 30 years should be provided in the regulatory submission.  As previously stated such a consideration could take into account the ecology of the specific group of organisms of concern in the aquatic risk assessment (eg algae, aquatic plants, invertebrates etc), as well as a consideration of the relative distribution of each scenario within the agricultural landscape.  Based on experience, CRD now differentiates between the following groups when making regulatory decisions:

Algae and aquatic plants

When the risk to aquatic plants and algae are being assessed via the use of MACRO, we use the criteria of no more than 10% overall failure rate and no more than 60% in any one scenario. Note that if an applicant has carried out Webfram, then the same criteria should be used.

Fish and aquatic invertebrates

There are no clear criteria (for either outputs from MACRO or Webfram) for the acceptance of exceedances for aquatic invertebrates and fish. This is due to our more limited experience of higher tier drainflow assessments for these groups.

In light of this, it is therefore necessary to consider the issue on a case-by-case basis considering such issues as the exposure profile in regards to the magnitude, frequency and the duration of the exceedances (only available from MACRO), the time to effect (requires raw data and suitable assessment intervals) and the recovery or potential species that may be affected. Please consider these exceedances further against the following metrics:-

Updated 2017-02-22