Pesticides Research, 63. – Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions

Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions

Summary and conclusions

In connection with registration of new pesticides, the Danish Environmental Protection Agency (Danish EPA) needs to evaluate whether the environment will be affected to an unacceptable degree. Based on the submitted data, the appropriateness of the pesticide is evaluated for Danish conditions. The evaluation is carried out in ”tiers” which is a type of screening system. First, the compound is evaluated under rather unrealistic but simple conditions (tier 1). If the compound is unlikely to damage organisms under these conditions, it can be registered for use. Otherwise the evaluation is carried out again for more realistic conditions (tier 2). More tiers can be used, to substantiate that a compound can be used without risk, perhaps only under special conditions.

The end-product of the project ”Pesticides in Surface Water” is a model tool (PestSurf) that can be used in the registration procedure for new pesticides at tier 2-level or higher. PestSurf is based on models of two existing catchments. It is assumed that the two basic models represent certain well known Danish conditions. On selected points the basic models are modified to make them more appropriate as general risk analysis tools.

To ensure that all relevant transport- and transformation processes were represented in the model, the work was initiated with a literature review. Different working groups have then further addressed the least well described transport and transformation processes. The working groups have specified how their components should be included in the final scenarios: wind drift, dry deposition, deposition at the soil surface and colloid transport.

As far as possible, the recommendations of the FOCUS groups of the EU (working with pesticide registration) were taken into account.

Although the scenarios have to be close to reality, a number of decisions have been taken during the process concerning descriptions and parameter choice that will make a difference for the risk assessment. The steering committee of the project has therefore been involved in all decisions concerning parameter values in cases when the choices were controversial.

A stream scenario was made for each of the two catchments, based on the calibrated models. For the sandy clay-scenario, the piped part of the stream is opened in the scenario, but otherwise the calibrated model and the scenario model are physically identical. In the scenarios, winddrift and dry deposition always take place perpendicular to the stream and follow the average wind conditions. In the scenarios the total agricultural area is covered with the same crop which is sprayed at the same time. This is not realistic for larger catchments, but for some crops or pesticides, the actual coverage of small catchments is large. Existing unsprayed zones along the stream are not sprayed. The user can choose to include unsprayed zones of different widths if wind drift is an important source.

The ponds in the scenarios are artificial, as no measurements were carried out in the project on ponds in the two catchments. No calibration data are therefore available. But already in the inception phase of the project, standard ponds for Danish conditions were defined, and these are implemented to the extent possible in the two catchments. The most common type on sandy soils is directly connected to the groundwater. The other type (on moraine clay) is determined by the slow infiltration to underlying layers. The first type is created in the Odder Bæk catchment, the other in the Lillebæk catchment, and catchments for the ponds are defined, resulting in lakes of 200-500 m², which do not dry out and have a typical variation of water level of 1 m. To be able to take into account the importance of the biological structure of the ponds, and the load of nutrients, it is possible to choose between a macrophyte dominated pond with a low level of nutrient salts and a phytoplankton dominated pond with high levels of nutrients.

The scenarios are built into a user interface that guides the transfer of pesticide data, the choice of crop, the time of spraying and dose and the width of the buffer zone from the interface to the mathematical models. All the water calculations are carried out in advance and cannot be changed by the user. Furthermore, the model contains information about a number of parameters of importance for the fate and transport of pesticides, which are determined through the calibrations.

Standard values follow, to the extent possible, the recommendations given by the FOCUS groups for this type of calculations (FOCUS 2000, 2002). In the EU registration process, a number of separate models are used for the simulation of different sub-components (surface runoff, drain water, groundwater and pond/open drain/stream. These are implemented on hypothetical catchments. The result is that the hydrological description is poorer than what is possible here, and that it is not possible to validate the combined system.

The project was faced with unexpected problems of which, some can be handled through an assessment of uncertainty, and others are of a more general nature. For example the detailed calculations require considerable calculation time, and it was agreed early in the project that the necessary water calculations should be made in advance, and the solute calculations could be carried out ”on top” to save time. However, the required fine resolution in time and space of the results of the water calculations, make them so space consuming that it has been necessary to reduce the simulation period. The simulation period is now 8 years, but only the last 4 years are chosen as basis for the evaluation of results.

The problems observed in connection with the calibrations for the sandy clay and sandy catchments, respectively, will also influence the scenarios. The problems relate particularly to the parameterisation of the macropores and to some extent to the ability to catch the very sharp pesticide peaks in drain water due to too high dilution of drain water by groundwater. Generally, the concentrations in the clay catchment are overestimated, while they are underestimated in the sandy catchment.

With the assumptions made concerning wind drift and sprayed area, the concentrations simulated in the stream become rather high. The wind drift peaks and, in some cases, also dry deposition are by far the largest calculated loads when the buffer zone is small (or 0). This is similar to the findings in the FOCUS-models used in EU for regulatory purposes, but not to what was found in the measuring programme related to the project. However, transport via drains in the simulations was substantial.

A number of uncertainties and errors have been observed when the tool is used for simulations. The developed description of pesticide transport with colloids is imperfect, and does not lead to a level of transport that is as high as observed. This means that the concentration in the stream of highly adsorbing pesticides is underestimated. Furthermore, in the model the drain water is mixed with too much groundwater, leading to too flat and too wide peaks of pesticide entering the stream.

The model has a finer resolution in time than used earlier in risk assessments, and describes the wind drift loads to streams and ponds. However, for drain flow, the size of the intermediate files poses a limit to how detailed the resolution can be, and high concentrations in short duration flow events can be difficult to describe.

While the Lillebæk scenario fulfils the criteria originally defined, some of the assumptions for the choice of Odder Bæk turned out to be wrong. The texture is more clayey than expected, and macropores, which were not supposed to be present in this scenario, seem to have an effect on the simulated concentrations. It is therefore necessary to pose the question whether the scenario fulfils the expectations of the Danish EPA.

To increase the applicability of the model, a further effort should be done to remove the problems identified with process descriptions and time resolution of the simulations.