Case study on the use of risk mitigation measures to reduce spray drift

Cases where the risk assessment concludes on the need to reduce exposure so that the safety factors between Predicted Exposure Concentrations (PECs) and thresholds for effects are frequent, as standard calculations usually consider water bodies located 1 metre away from the spray boom and no interception nor risk reduction measure in the first place.  Hence there can be a need to integrate risk mitigation options in the risk assessment or at the risk management level, to reflect different agricultural conditions, greater distances to the water body or non-target environment to be protected, and/or landscape features that may intercept the drift and/or special equipment in use by farmers that reduces spray drift at the source.

In the SETAC MAgPIE workshop it was noted that a flexible toolbox approach seems a most appropriate means of supporting an effective and targeted means of customising the risk mitigation strategy to local conditions (agricultural, landscape and environmental) and/or availability of equipment that may reduce drift at the point of application.  Within the conceptual framework of the SETAC MAgPIE proposals the level of drift mitigation is first considered, as calculated for a product and representative use, e.g. at zonal level, based upon risk assessment procedures.  There is then the possibility to implement risk mitigation measures from the toolbox either at the management level or after their inclusion into a risk assessment – adapted to the needs of the individual assessments:

  • At the risk management level: Based on the mitigation need stated on the label (for example, in terms of total % mitigation needed), farmers may then choose single or multiple measures with defined effectiveness from an official list to achieve the required effectiveness

  • After inclusion into the risk assessment process: Based on the mitigation need, modelling evaluates different measures and combinations thereof to achieve the required overall effectiveness (%). All eligible measures and combinations are then listed for this product on the label.

The following example illustrates how practically the expected level of spray drift reduction may be achieved using one or several risk mitigation measures, in the case of a season treatment (spring-summer) on vegetables in order to achieve an overall mitigation objective (e.g. at least 90% mitigation).

The most widespread risk mitigation option among Member States is the consideration of a Buffer Zone, or distance between the top of the water body bank and the last nozzle of the sprayer (sometimes also referred to as a “No Spray Zone”).  As the distance increases, the quantity and density of droplets that will deposit onto the water body decreases.  Where a Buffer Zone is recommended on the product labelling the first checkpoint of the farmer should be the distance separating the field to be treated to the nearest water body or non-target environment to be protected.  If this distance is greater than the recommended Buffer Zone then no further action should be needed.  In the cases where the field is closer to the non-target environment to be protected, the following aspects (amongst others from the toolbox) could be taken in consideration:

  • Is there high vegetation between the plot to be treated and the water body?  If so, this may contribute to reduce spray drift and can be taken into account if the height of the vegetation is higher than the field to be treated and if the vegetation is present over the whole length of the field along the water body.

  • Is the sprayer equipped with Spray Drift Reducing Nozzles? If so, the level or class of drift reduction that may be achieved with the nozzles is reported on their label, after measurement according to ISO, and this may be taken into account.

  • Is the sprayer equipped with any device aiming at providing directed spray or shield, together with a drift reduction class or range?

  • Is the last row to be sprayed vertically or towards the inside of the field?

The tables below illustrate how the level of spray drift reduction may be estimated if one or several of these measures are being implemented while spraying, taking the example of the risk mitigation measures recommended in Italy alongside the proposed classes and categories, for a treatment on vegetables (vertical spray over the field):

If for example, 10 metres only separate the field from the water body, but the label indicates that a minimum of 12-14 metres (or based on the above a reduction of 90% is needed) then additional measures need to be considered.

Presence of vegetation between the field and the water body

The vegetation can be taken into account if higher than the field canopy to be treated, and if present all along the water body to be protected from spray drift, as illustrated below:

In Italy, drift reduction rates are recommended for hedges, if these are 1 metre higher than the field canopy.  These rates are 25% for a bare hedgerow and 75% for a fully developed hedgerow.

A way to estimate the cumulated drift reduction of multiple risk reduction tools is provided in the method used in Italy.  This method is presented in the table below.

In our example, the 10 m distance to the water body, or buffer zone, provides a drift reduction of 50%.  This is O1, or 50% output.

If a hedge is present alongside the water body and is fully developed, then the drift reduction assigned to the hedge (M2) is 75% reduction.  The drift achieved after the two measures altogether would be:

O2 = 50-(50 x 75/100) = 50 – 37.5 = 12.5%

The cumulated drift reduction achieved with 10 m and the presence of vegetation (here a hedge) is therefore 87.5%.

Options to further reduce the drift is to spray the last row towards the inside of the field, and or to use drift reduction nozzles.

Taking as a source the numbers recommended in Italy again, this correspond to:

  • 35% reduction for an inwards spray of the last row.

 

In our example, the overall drift achieved would be:

O3 = 12.5 – (12.5 x 37.5/100) = 12.5 – 4.69 = 7.81%

The overall reduction is then 92,19%.

If vegetation is present but does not qualify for drift reduction

In this case, an inwards application onto the last row can still be taken into account if implemented, which can lead to an output of:

 

O2 = 50-(50 x 35/100) = 50 – 17.5 = 32.5%

 

The cumulated risk reduction would be 67.5%.

 

The use of risk reducing nozzles might provide addition drift reduction.  To reach the expected 90%,

Class D Drift reducing nozzle would provide a reduction of 75-94%, hence leading the cumulated drift to:

 

O3 = 32.5-(32.5 x 75/100) = 32.5 – 24.38 = 8.125%

 

The cumulated risk reduction would be 91.88%.

 
 
Drift Mitigation Calculator

A Drift Mitigation Calculator has been developed to simulate the achievable drift mitigation that can be achieved when one or more mitigation methods are used alone or in combination. These various methods range from technical measures to landscape features such as hedges or other. Individual drift mitigation percentages have been taken over from the Italian model developed by Azimonti et al. However, a second simulation tool integrating other features (such as methods from the Swedish Helper and values from the Ganzelmeier studies) will be set up, representing a more "experimental" approach. The Drift Mitigation Calculator based on the Italian model is to be considered a "worst case": the calculation is based on default of drift of 100%, which, in reality, is very rarely encountered.

Additional mitigation options may be possible and can be represented here. It is noted that robust representation requires supportive research into efficacy to satisfactorily characterise the potential for supplementary mitigation.

Remark: This Drift Mitigation Calculator tool works with most modern browsers such as Chrome, Safari, Edge, Internet Explorer and Firefox. There might be problems with older version of Internet Explorer, as it can not load some of the graphical elements used in this simulator.

En English translation of the Italian guidance document that is the basis for the underpinning assumptions and informations on this case study page has been provided as a courtesy by Dow AgroSciences. You can access the document here.

Link to the Italian page on risk mitigation measures with regard to drift and runoff

Azimonti, G., Balsari, P., Fanelli, R., Ferrero, A., Gigliotti, G., Marchini, S., Mazzini, F., Otto, S., Rapagnani, M.R., Zaghi, C., Zanin, G. 2017. Misure di mitigazione del rischio per la riduzione della contaminazione dei corpi idrici superficiali da deriva e ruscellamento, Documento di orientamento, Ministero della Salute, Document Doc.MinSal-luglio2009_rev1-15 marzo 2017

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