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Introduction

Chapter 1, PLAP - Monitoring results May 1999 - June 2002

There is growing public concern in Denmark about pesticide contamination of our surface waters and groundwater. Pesticides and their degradation products have increasingly been detected in the groundwater during the past decade and are now present in much of the Danish groundwater. Under the Danish National Groundwater Monitoring Programme (GRUMO), pesticides and their degradation products have so far been detected in 40% of all screens monitored (Jørgensen, 2002).

The increasing detection of pesticides in groundwater over the past 10 years has raised doubts as to the adequacy of the existing approval procedure for pesticides. A main issue in this respect is that the EU assessment and hence also the Danish assessment of the risk of pesticide leaching to the groundwater is largely based on data from laboratory or lysimeter studies. However, these types of data may not suffice to adequately characterize the leaching that may occur under actual field conditions. A major limitation is that the laboratory and lysimeter studies do not include the spatial variability of the soil parameters (hydraulic, chemical and microbiological soil properties) affecting pesticide leaching. This is of particular importance for silty and loamy soils, where preferential transport may have a major impact on pesticide leaching. In fact, various field studies suggest that considerable preferential transport of several pesticides occurs to a depth of 1 m under conditions comparable to those pertaining in Denmark (Kördel, 1997).

The inclusion of field studies, i.e. test plots exceeding 1 ha, in risk assessment of pesticide leaching to the groundwater is considered an important improvement in risk assessment procedures. For example, the US Environmental Protection Agency (US-EPA) has included field-scale studies in its risk assessments since 1987. Pesticides that potentially may leach to the groundwater are required to be included in field studies as part of the registration procedure. Over the past decade the US-EPA has therefore conducted field studies of more than 50 pesticides (US Environmental Protection Agency, 1998). A similar concept has also been adopted within the European Union (EU), where Directive 91/414/EEC, Annexe VI (Council Directive 97/57/EC of 22 September 1997) enables field study results to be included in the risk assessments.

1.1 Objective
In 1998, the Danish Government initiated the Pesticide Leaching Assessment Programme (PLAP), an intensive monitoring programme aimed at evaluating the leaching risk of pesticides under field conditions. The PLAP is intended to serve as an early warning system providing decision makers with advance warning if approved pesticides leach to the groundwater in unacceptable concentrations. The programme focuses on pesticides used in arable farming, and monitors leaching at six agricultural test sites representative of Danish conditions.

The objective of the PLAP is to improve the scientific foundation for decision making in the Danish registration and approval procedures for pesticides. The specific aim is to analyse whether pesticides applied in accordance with current regulations leach to the groundwater at levels exceeding the maximum allowable concentration of 0.1 µg/l.

1.2 Structure of the PLAP
The pesticides included in the PLAP were selected by the Danish Environmental Protection Agency on the basis of expert judgement. At present, 27 pesticides and 17 of their degradation products are included in the PLAP. All the compounds analysed are listed in Appendix 1. The reasons for selecting the specific pesticides are detailed in Lindhardt et al. (2001).

Figure 1
Figure 1. Location of the six PLAP sites Tylstrup, Jyndevad, Silstrup, Estrup, Faardrup and Slaeggerup.

Soil type and climatic conditions are considered to be some of the most important parameters controlling pesticide leaching. The PLAP therefore encompasses six test sites representative of the dominant soil types and the climatic conditions in Denmark (Figure 1). The groundwater table at all six sites is shallow, thereby enabling a rapid groundwater response to pesticide leaching (Table 1). Cultivation of the PLAP sites is in line with conventional agricultural practices in the vicinity. The pesticides are applied in the maximum permitted dosage and in the manner specified in the regulations. Hence any occurrence of pesticides or degradation products in the groundwater downstream of the sites can be related to the current approval conditions pertaining for the individual pesticides.
The PLAP was initiated in autumn 1998. During 1999, the six test sites were selected and established. Monitoring was initiated in 1999 at Tylstrup, Jyndevad and Faardrup, and in 2000 at Silstrup, Estrup and Slaeggerup (See Table 1).

Site characterization and monitoring design are described in detail in Lindhardt et al. (2001). This report presents the results of the monitoring period May 1999-June 2002. Preliminary results covering part of the monitoring period (May 1999-June 2001) have been published previously (Kjær et al. , 2002). The present report should therefore be seen as a continuation of the latter report, with the main focus on the last year of the monitoring period (July 2001-June 2002). For detailed description of the first part of the monitoring period (May 1999-June 2001), see Kjær et al . (2001) and Kjær et al . (2002).

Table 1. Characteristics of the six PLAP sites (modified from Lindhardt et al., 2001).
Table 1
Within the PLAP, the evaluation of pesticide leaching risk is based upon at least two years of monitoring data. For some pesticides the present report must be considered preliminary because they have been monitored for an insufficient period of time. A more complete evaluation of the data, including model simulation of the pesticide transport and transformation processes, will thus be made once a more comprehensive data set covering the entire leaching period becomes available.

Hydrological modelling of the unsaturated zone at each PLAP site supported the monitoring data. The MACRO model (version 4.2) was used to describe the soil water dynamics at each site during the full monitoring period July 1999-June 2002. In addition, bromide transport was simulated at the two sandy sites Tylstrup and Jyndevad.

The risk of pesticide leaching is highly dependent on the degradation and sorption processes occurring in the root zone. To improve interpretation of the data, sorption and degradation studies have been conducted on selected combinations of pesticides and soil types representative of the PLAP. The methodology and results are presented in Section 8.

Scientifically valid methods of analysis are essential for the integrity of the PLAP. The field monitoring work has therefore been supported by intensive quality assurance entailing continuous evaluation of the analyses employed. The quality assurance methodology and results are presented in Section 9.

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