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The development and validation of a Bystander and Residential Exposure Assessment Model (BREAM) - PS2005
This project sets out to develop a computational model to predict the potential exposure to pesticides for bystanders and residents in the countryside that can be used as a tool in risk assessments. The work will be concerned with boom sprayers operating over arable crops in a range of conditions relevant to the United Kingdom. Model development and validation will be supported by tests in controlled wind tunnel conditions with the overall model predictions validated against full-scale field trials results.
Three main routes can be identified by which residents and bystanders may become exposed to pesticides, namely:
1. from droplets and vapours released at the time of a pesticide application;
2. from vapours emitted from a treated area post a pesticide application; and
3. from dusts contaminated with pesticide that may be emitted from treated cropped areas particularly during harvesting operations.
The potential exposure of residents and bystanders relates to the downwind distribution of the pesticide both in the air and on to non-target surfaces as well as the behaviour patterns of people in these downwind areas. The model will include a component to account for such behaviour based on data from published sources.
It is proposed to model the transport of droplets and vapours during application using three regions as follows:
(a) a region close to the nozzles where the interaction of air, vapour and droplet flows associated with the operation of the nozzle and air flows arising from the natural wind and forward motion of the sprayer combine to detrain droplets and vapours from the spray and so provide the source for drifting spray;
(b) a region around the spraying vehicle where the flows due the motion of the vehicle will be considered in the initial phases of the downwind transport of airborne droplets and vapours;
(c) a larger region away from the spraying vehicle where the drifting cloud of droplets and vapours may be influenced by features in the terrain such as the presence of hedges, buildings and slope and where the characteristics of the weather conditions may also be important.
The construction of the models will be based on approaches identified from the existing literature together with the results from detailed wind tunnel and field experiments aimed at quantifying pesticide movements close to the nozzle and spraying vehicle. Where possible and appropriate, standard modelling packages will be used particularly in the region furthest from the spraying vehicle. The interfaces between the separate regions in the model will be defined in terms of vertical airborne spray volume profiles as vapour and droplets with defined size distributions and air velocity profiles.
Losses as vapour from treated areas will be determined using simplified relationships based mainly on the properties of the sprayed liquid and the target surface to give a loss rate per unit area over a defined time period of up to 120 hours. The dispersion of this vapour cloud will then be predicted using an established atmospheric dispersion model.
The issue of potential contamination from pesticide contamination of dusts leaving a cropped area will not be included in the initial model development. However, it will be included in the literature reviews associated with the project work so as to obtain an estimate of the extent of this component of potential exposure.
The results from the model will be validated by comparing predictions with measurements made in full-scale field trials in closely monitored conditions and with active pesticide formulations together with results in the published literature. The model will also be used to determine the relative importance of operational factors on the risk of resident and bystander exposure and to examine the effects of using different application technologies on the risks of this exposure. Results of the model development, validation and use of the model will reported in papers submitted for publication in peer refereed journals. Interim results from the first year of the study will be used in conjunction with staff from The Pesticides Safety Directorate to review existing exposure risk assessment methods and to recommend changes if necessary.
The work will be primarily aimed at predicting the pesticide exposure profiles of residents and bystanders in the
countryside as a tool for the regulatory risk assessment process. However, the techniques developed, with small modifications, will also have relevance for estimating the off-target exposure of a range of other organisms (e.g. non-target arthropods) and systems (e.g. surface water).
The primary objective of the work will be to predict the losses of pesticide, both as droplets and vapours, from an application site treated with vehicle mounted boom sprayers operating in the range of conditions likely to be encountered in the UK. Model predictions will define the spatial and temporal loss profiles at three scales:
„« Close to the nozzle so as to be able to predict the effects of different nozzles design and operating parameters;
„« Within 5.0 m of the vehicle so as to account for the disturbance due to the vehicle motion and structure;
„« At a landscape scale up to 1.0 km from an application site to take account of factors such as the presence of buildings, hedges and the slope of the land where the interaction of such structures with the air flow could lead to high local concentrations of airborne pesticide.
The losses of droplets will relate only to the time of application whereas vapour losses will be predicted for a period of up to 120 hours after application, or longer where reliable estimates can be made. For each scale, profiles of droplet size and airborne concentration will be calculated that can be output as necessary.
Potential resident and bystander exposure will be predicted by including elements of human behaviour in the models taken from published literature and assuming perfect collection either by inhalation, dermal contact or a combination of both. Comparative studies will use up to five scenarios defined in conjunction with staff at The Pesticides Safety Directorate.
Field and wind tunnel experiments will be conducted to provide information that can be used in addition to published work to construct the models. An iterative approach is planned in which model developments will be assessed against experimental data particularly as new data is generated. Components of the model will be validated in well controlled wind tunnel experiments and overall model predictions compared with the results from field trials as part of the validation procedure. Results from the study will be reported in papers submitted for publication in peer refereed journals.
Specific objectives relate to:
1.0 Establishing a framework for the development of a predictive model of spray droplet and vapour transport from boom sprayers. This will involve completing:
1.1 A review of previous modelling approaches;
1.2 An examination of the possible methods of modelling spray behaviour:
(a) in the region around each nozzle;
(b) in the immediate vicinity of the spray vehicle and boom assembly;
(c) in an area downwind of an application site so as to account for meteorological and topographical features such as slopes and major obstacles to natural air flows and the wind;
1.3 The collation of the available experimental data that could be used to aid the formulation of components of a spray droplet and vapour transport model relevant to the operation of boom sprayers and/or that would be relevant to the validation of all or parts of the model output;
1.4 A definition of the laboratory/wind tunnel and field experiments required to support the development of sections of a transport model that will need to be formulated;
1.5 A definition of the approaches to be used for the modelling study of each of the zones defined in 1.2 above having evaluated all of the available options;
1.6 Collection of data relating to resident and bystander behaviour to include in exposure estimates within the model;
1.7 Convening a workshop in conjunction with representatives of The Pesticides Safety Directorate to discuss and agree modelling approaches with technical experts from within the European Community particularly including those from research organisations, regulatory bodies and representative of chemical companies where appropriate.
2.0 Preliminary analysis of resident and bystander exposure risks
2.1 Collation of preliminary model outputs.
2.2 Collation of previous field trial data.
2.3 Comparison of preliminary results with existing approaches used by The Pesticides Safety Directorate
3.0 Construction of a spray droplet and vapour transport model for:
3.1 Downwind transport of droplet and vapour movement at the time of application;
3.2 Downwind transport model for vapour losses for periods of up to 120 hours immediately post the application;
3.3 Integrating predicted spatial and temporal dispersion distributions with data relating to resident and bystander behaviour.
4.0 Conducting experiments to generate data required for model formulation, involving:
4.1 Laboratory/wind tunnel studies of spray behaviour close to the nozzle;
4.2 Field studies to obtain information about spray movement on a larger scale around the application vehicle and into downwind areas.
5.0 Validation of the model using:
5.1 Laboratory/wind tunnel experiments to validate key components of the model: this will involve at least one experiment to give data that can be used to validate each new section of the complete model.
5.2 Field trials to validate model predictions at larger scales: at least three full-scale trials will be conducted the results from which can be compared with model predictions as part of the validation of the full model capabilities.
6.0 Use of the model to explore options for the improved management of drift and methods of minimising resident and bystander exposure by adjusting:
6.1 Operational factors and the interaction with application timeliness;
6.2 System variables relating to nozzle design and machine technologies (e.g. air assistance);
6.3 Distances between the edge of treated areas and organisms/systems requiring protection from exposure.
7.1 Preparation of an interim report for Defra/Pesticides Safety Directorate comparing preliminary model predictions with existing approaches to the assessment of pesticide contamination risks to bystander and residents.
7.2 Preparation of two papers for publication in refereed journals relating to model formulation, validation and use;
7.3 Annual progress and a final report to the Pesticides Safety Directorate;
7.4 A final project workshop with relevant participation of technical experts, regulatory representatives and those concerned with generating data for registration purposes.
8.0 Ensuring compliance with The Joint Code of Practice for Research
8.1 Establishing systems that meet the requirements of The Joint Code of Practice for Research.
8.2 Having internal and external audits that enable full compliance with The Joint Code of Practice for Research to be demonstrated.
FRP - Final Report : The development and validation of a Bystander and Resident Exposure Assessment Model (BREAM)
Time-Scale and Cost
Contractor / Funded Organisations
Silsoe Spray Application Unit
Fields of Study