PECs: Computer Simulation


J. Linders
7th April 2010
 

Predicted (Estimated) Environmental Concentrations: Computer Simulation

Modelling

 

General Process

The application of an agrochemical against certain pests will result in some concentration in an environmental compartment. The environmental compartments soil, water, sediment, and air, may be considered as well as the concentrations in several biota such as in fish tissue or in aquatic plant tissue. If a new active substance has to be evaluated based on its environmental behaviour and potential risks, measured levels of the substance are not available. In some cases an already registered substance has to be re-evaluated and measurements may have been through monitoring programs and can be used in the evaluation as well. A variety of data in the dossier available to governmental authorities to (re-)evaluate an agrochemical are suitable for the use in estimation methods of the potential environmental concentration. Data needed for these estimations include the application data of the substance, e.g. dosage, frequency of use and interval between the applications, as well as physico-chemical data, e.g. molar mass, vapour pressure, solubility, octanol / water partition coefficient, or the logarithm of this coefficient. In addition, the environmental fate parameters of the substance should be included in the dossier. These include degradation rate in the different compartments, soil, water, sediment and air as well as the partition onto soils and sediment particles1,2, 3.

Although necessary, these data are not in themselves sufficient to carry out an estimation of the concentration in the environment. In addition, the receiving environmental compartment has to be defined. In this process the so-called scenario parameters have to be determined by the national authorities. Parameter choices depend on the desired vulnerability of the scenario. Either a normal/average case, a worse case or a worst case scenario may be developed depending on the needs of the assessment. In general, the scenario is defined according to a tiered system, in which a worst case situation may be reserved for Tier I, a worse case scenario for Tier II and a more or less normal case as Tier III. Sometimes, even a Tier IV is defined to estimate a real case in a very specific scenario for typical local situations. In the remainder of this paper the Tier IV situation is not taken into consideration4. However, some information concerning the possibilities of Tier IV calculations may be found at the following web site5.

 

Two approaches are dealt with here in more detail: the PEC estimation according to the US EPA6 … 10 and the process as developed by the European Union11 … 24. (PECs are referred to as Estimated Environmental Concentrations (EEC) in North America.) Of course, there are other authorities that may use mathematical models to determine the PECs/EECs. They are not dealt with here because the intention of the paper is not to be complete but to show the approaches available.

 

The main characteristic of the estimation of the PEC of the approaches dealt with here is the use of mathematical models that are based on the scientific principles of the conservation of mass and the continuity equation. These models do generally not have an analytical solution and have therefore to be solved using a computer with sufficient memory to carry out the sophisticated calculations. However the currently used models all run on the computers of the present status.

 

United States Environmental Protection Agency

 

The USEPA Office of Pesticide Programs uses a number of simulation models in a tiered system as described above. The following is a brief summary of models in use or proposed for use.

 

GENEEC26 The GENeric Estimated Environmental Concentration Version 2 model is a tier 1 model for estimating high exposure pesticide concentrations and is used for aquatic ecological risk assessment. It is based on the results of simulations using the linked PRZM and EXAMS models.

 

FIRST6 The FQPA Index Reservoir Simulation Tool is a tier 1 model for estimating high exposure concentrations and is used for human health drinking water risk assessments for the U.S. Food Quality Protection Act (FQPA). It is also based on the results of simulations using the linked PRZM and EXAMS models.

 

SCIGROW7 The Screening Concentration In GROund Water is a tier 1 ground water model used to estimate upper level pesticide concentrations for use in human health drinking water assessments. SCIGROW is a regression equation based data collected in prospective groundwater monitoring studies. It predicts concentrations based on application rate, number of applications, half-life and organic carbon-normalized partition coefficient (KOC).

 

PRZM8 The Pesticide Root Zone Model is a tier two model which estimates the mass of pesticide leaving a treated field through rain-induced runoff or wind-induced spray drift. It simulates movement of pesticide to a receiving water body which is simulated by the EXAMS model to which it is electronically linked.

 

EXAMS8 The Exposure Analysis Modeling System is a tier two model which simulates a receiving water body. It is electronically linked to the PRZM model which estimates the pesticide entering the EXAMS water body through runoff or spray drift (see PRZM above).

 

MUSCRAT9 The MUltiple SCenario Risk Assessment Tool is a tier three model which consecutively runs multiple PRZM/EXAMS scenarios that are chosen to represent a range of crops, geographic locations and relative vulnerabilities. It is designed to represent a wide range of conditions for use in national-level risk assessments. It has not yet been approved for routine use.

 

WARP10 The WAtershed Regressions for Pesticides is a tier 3 model. It is designed to estimate multiple percentiles of the distribution of pesticide concentrations at any site for which pesticide usage and a few soil and weather parameters are known. It has been used to estimate pesticide concentrations in drinking water at more than 7,000 drinking water intake assessment locations and 65,000 aquatic ecological assessment sites in single runs of the model. It was designed by the U.S. Geological Survey in conjunction with the EPA Office of Pesticide Programs and the U.S. Department of Agriculture and with assistance from the pesticide industry and national drinking water groups. It is currently undergoing testing and review.

 

European Union

 

The development of methods to determine or estimate the Predicted Environmental Concentration (PEC) in the European Union has been carried out by the FOCUS organisation. FOCUS stands for FOrum for the Coordination of pesticide fate models and their USe. FOCUS consists of Steering Committee and several working groups. The Steering Committee determines the work plan and the working groups carry out the specific tasks given to them by the Steering Committee. The chairman of the working group reports back to the Steering Committee. If the task is done the working group is dismissed. The Steering Committee may accept the result of the working group and may propose the report to be considered as a Guidance Document to the legislative body of the European Union. It should be stressed that a Guidance Document is not legal instrument; countries may deviate from the guidance given if it has good reasons to do so. Nevertheless, the intention of the guidance document is to achieve harmonisation of the authorisation process in the EU as there are now 25 different governments in charge. The working groups consist of representatives of registration authorities, governmental research institutions, academia and industry. It is intended to reach consensus on the different topics with a high level of sophistication and a state-of-the-art approach of the PEC estimation taking into account the up-to-date scientific knowledge. FOCUS has a web site at the Joint Research Centre of the EU at Ispra, Italy, where also a helpdesk is available11.

Currently, FOCUS has developed guidance documents on the following topics:

  • Leaching models, overview of models, describing the leaching process to groundwater12;
  • Surface water models, overview of models used for the description of fate processes occurring in surface water, including the main routes of entry like drainage, runoff and drift13; a specific tool for the Tiers I and II is also available14
  • Models to estimate soil concentrations15;
  • Groundwater scenarios using a model selection (4) to estimate the groundwater concentration at 9 different scenarios including the parameterisation of 4 models; the models PEARL16, PELMO17, PRZM18 may be used for all scenarios whilst the model MACRO19 is only parameterised for 1 scenario;
  • Surface water scenarios using a model selection (1 for each process) to estimate the concentration in surface water at 10 different locations including the parameterisation for the scenarios; the input route drift is calculated using continuous equations fitted through data of drift experiments; the model MACRO20 is parameterised for the process drainage at 6 out of 10 scenarios; the model PRZM21 is parameterised for the process runoff at 4 out of 10 scenarios, whilst these 3 models are used to generate the input for the surface water fate model TOXSWA22; the drift calculation is included in the shell around the surface water models (SWASH)23;
  • Air models; an overview of models to estimate the concentration in air; a selection of (an) air model(s) nor the development of scenarios has not (yet) been carried out, although a recommendation has been provided; this guidance document has not yet been released;
  • Degradation kinetics; as the process of degradation was found to be very important in all compartments and registration authorities developed a wide variety of methods to derive and use the critical half life value from the dossier data it was felt necessary to develop also guidance for harmonisation on this very specific topic; this guidance document has not yet been released.

In addition the Steering Committee installed a working group on version control. The main task of this group is to take care of new developments in the scientific knowledge and implement these if considered necessary. Also mistakes are inventoried and solved regularly.

 

Modelling forum

As the information and also the use of the models from the two regions (US and EU) is rather limited to the relevant areas a web site has been set up to make information of pesticide fate models known in a wider context and to a greater audience. The web site is very useful for all persons interested in the development and use of this specific type of models: pesticide fate. The modelling site, called PFMODELS, may be visited at the address given in24; a very important feature of the web site for interested people is to ask advice of known experts in the field to solve a problem in a specific situation under consideration.

 

Disclaimer: The author can not exclude that the internet links mentioned in the references will not be changed if time passes. To the best of the author’s knowledge the references have been included, but we can not take any responsibility for the changes that may occur.

 

 

 

References

 

1.         US Environmental Protection Agency, Pesticides, Test Guidelines for Data Requirements

            http://www.epa.gov/opp00001/science/guidelines.htm

 

2.         European Commission, DG Health and Consumers,  Guidance Documents for the

            implementation of Council Directive 91/414/EEC (Plant Protection Products)       

            http://ec.europa.eu/food/plant/protection/resources/publications_en.htm

 

3.         UK Health & Safety Executive, Chemical Regulations Directorate, Pesticides, Data Requirements            Handbook and Supplementary Guidance

            http://www.pesticides.gov.uk/approvals.asp?id=643

 

4.         Forum for the Co-ordination of  Pesticide Fate Models and their Use (FOCUS) – Surface Water   

            http://viso.ei.jrc.it/focus/sw/index.html

 

5.         Forum for the Co-ordination of  Pesticide Fate Models and their Use (FOCUS) – Landscape and Mitigation

             http://viso.jrc.it/focus , under “Landscape and Mitigation”

 

6.         US Environmental Protection Agency, Pesticides, Water Models - FIRST

             http://www.epa.gov/oppefed1/models/water/index.htm

 

7.         US Environmental Protection Agency, Pesticides, Water Models – SCI-GROW

            http://www.epa.gov/oppefed1/models/water/index.htm

 

8.         US Environmental Protection Agency, Pesticides, Water Models – PRZM/EXAMS

            http://www.epa.gov/oppefed1/models/water/index.htm

 

9.         Hendley, P. and Giddings, J. ECOFRAM Aquatic Report (1999) - MUSCRAT

            http://www.epa.gov/oppefed1/ecorisk/aquareport.pdf

 

10.        Larson, S.J., Crawford, C.G. and Gilliom, R.J., 2004, Development and application of watershed             regressions for pesticides (WARP) for estimating atrazine concentration distributions in streams:        U.S. Geological Survey Water-Resources Investigations Report 03-4047

            http://pubs.usgs.gov/wri/wri034047/wrir034047.pdf

 

11.        Forum for the Co-ordination of  Pesticide Fate Models and their Use (FOCUS) – Home Page

            http://viso.ei.jrc.it/focus/index.html : focus.helpdesk@jrc.it

 

12.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Groundwater

            http://viso.ei.jrc.it/focus/gw/index.html

 

13.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water

            http://viso.ei.jrc.it/focus/sw/index.html

 

14.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water

             http://viso.ei.jrc.it/focus/sw/index.html , under “STEPS_ONE_TWO”

 

15.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Documentation

            http://viso.ei.jrc.it/focus/index.html , under “documentation”

 

16.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Groundwater,     PEARL

            http://viso.ei.jrc.it/focus/gw/index.html , under “PEARL”

 

17.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Groundwater,      PELMO

            http://viso.ei.jrc.it/focus/gw/index.html , under “PELMO”

 

18.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Groundwater,      PRZM

            http://viso.ei.jrc.it/focus/gw/index.html , under “PRZM”

            Remark: there is a difference with address # 20

 

19.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Groundwater,     MACRO

            http://viso.ei.jrc.it/focus/gw/index.html , under “MACRO”

            Remark: there is a difference with address # 19

 

20.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water,   MACRO

            http://viso.ei.jrc.it/focus/sw/index.html , under “MACRO”

            Remark: there is a difference with address # 18

 

21.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water,   PRZM

            http://viso.ei.jrc.it/focus/sw/index.html , under “PRZM”

            Remark: there is a difference with address # 17

 

22.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water,    TOXSWA

            http://viso.ei.jrc.it/focus/sw/index.html , under “TOXSWA”

 

23.        Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS) – Surface Water,   SWASH

            http://viso.ei.jrc.it/focus/sw/index.html , under “SWASH”

 

24.        PFMODELS.org – A site dedicated to pesticide fate modelling

            http://www.pfmodels.org

 

 

 

Last modified April 7th 2010


Date added: 2010-05-10 01:33:44   
Last Updated -0001-11-30 00:00:00   
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