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Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

1 Introduction

This report describes the findings of subproject 5 (see preface overview) in relation to transport and fate of pesticides in surface water. In the report, the experiments performed to evaluate key processes under relatively controlled conditions are described.

Two approaches, subdivided into more specific tasks, were taken:

1. Laboratory experiments for evaluation of the adsorption and degradation process in relation to:

• The usefulness of standard registration data in the registration model
• The applicability of the paradigm of reversible equilibrium/kinetic adsorption
• The paradigm of bioavailability and degradation (only degradation of dissolved fraction)

2. Full scale experiments in ponds using controlled and known substance input in relation to:

• Dissipation time from water for substances having different physicochemical properties
• Evaluation of the influence from macrophytes including the influence on the mixing conditions in the water column
• Evaluation of the vertical mixing conditions
• Generation of data for evaluation of the effective diffusion coefficient

1.1 Laboratory experiments for evaluation of adsorption and degradation processes

The model-based tool (registration model) relies on a suite of predefined scenario runs conducted by the evaluated model(s) (model tools report). The details of the scenarios are given in the “Model tools report”. In order to run the scenarios for a new or already registered pesticide, the model must be parameterised with the data supplied to the Danish EPA (Statutory Order) by the manufacturer in compliance with the approval procedure. However, for pesticide properties, such as those related to biodegradation and sorption, the model is not readily parameterised by the data requested by the Danish EPA. This problem can be solved in two ways. Either by using unspecific general parameter values or by estimating the missing parameters from available data.

In order to evaluate the feasibility of these two approaches, it was necessary to gather precise data on the processes, which was done in the laboratory.The adsorption process is very central in the registration model in relation to both retardation and bioavailability. The sorption to solids in the soil system or in the stream sediment causes the pesticide to move slower than the water that transports it. At the same time, sorption in general reduces the bioavailability, which again reduces degradation. For strongly sorbing pesticides, these processes can easily determine pesticide concentration levels in the streams and special attention was therefore given to the study of sorption processes in the laboratory.

An important question in relation to sorption concerns sorption kinetics. As data on sorption kinetics are not requested by the Danish EPA, and thus not available for modelling, it is important to decide whether the registration model can be made to give realistic results without these data, and whether surrogate data can be derived indirectly from other pesticide properties. The basis for answering these questions is the knowledge on sorption rates and sorption equilibria, which have been studied and reported here, together with knowledge on registration data.

Often data on degradation in stream water and stream-sediment water suspensions have not been made available to the Danish EPA by the registration applicant as is the case with the model pesticides used in this study. As for sorption, it has thus been necessary to obtain experimental data.

The sorption and biodegradation of pesticides are dependent on the conditions of the locality where these processes take place. The sorption and biodegradation experiments were thus conducted with a range of sediments collected at the localities used for calibration of the model and, to some degree, with other sediments. By expanding the data set in this way, relevant catchment-specific data could be derived for evaluation of the model, at the same time as more general conclusions on parameter variability, parameter estimation etc. could be made.

The results from the laboratory experiments are used for a general evaluation of the registration model (Styczen et al., 2002a) and also in a sensitivity analysis of the catchment model, as described elsewhere (Styczen et al., 2002b).

1.2 Full-scale experiments in ponds using controlled and known substance input

The field experiments with ponds were conducted in order to get the knowledge on the processes governing disappearance from the water body of a pond, which is necessary for modelling. In the experiments, the dissipation of different pesticides having different physico-chemical properties was followed.

The depletion of pesticides from the water by adsorption to sediment was studied for three compounds. Influence of macrophytes on the dissipation of pesticides and on hydraulic mixing was studied in directly comparable ponds with and without macrophytes.

The model system assumes that the water column of surface water is vertically well mixed. Another aim of the ponds experiments was thus to evaluate the validity of this assumption.A third important purpose of the field experiments was to generate data for the model for evaluation of the effective diffusion coefficients of pesticides into sediment of the ponds since earlier studies have revealed that the effective diffusion coefficient might exceed the molecular diffusion coefficients by several orders of magnitude (Sørensen et al., 2002).  

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