Pesticides in streams and subsurface drainage water within two arable catchments in Denmark: Pesticide application, concentration, transport and fate

Pesticides in streams and subsurface drainage water within two arable catchments in Denmark: Pesticide application, concentration, transport and fate

1 Introduction

1.1 Objectives

The main objective of the measuring programme presented is to generate data for validating and testing a new Danish pesticide model developed to predict the transport of pesticides to surface waters and the fate of pesticides in streams within small agricultural catchments. Therefore, we have measured the occurrence, concentration and transport of selected pesticides in stream water and subsurface drainage water during the study years 1999 and 2000. Water samples were mainly collected during spates, as the hypothesis was that precipitation and/or snow-melt would transport dissolved and more hydrophobic pesticides with surface runoff or through the soil to tile drainage runoff and would eventually reach the stream channel. However, water samples were also collected during periods with steady or decreasing discharge conditions. During such periods pesticides from groundwater, and during the spraying season also pesticides from wind drift, are believed to act as the main contributing pesticide pathways. Transport of selected key pesticides is quantified and compared with the application in the catchment in order to achieve an estimate of the loss ratio (pesticide transport divided by pesticide application). Finally, we have measured the fate of pesticides in streams by sampling suspended sediment and streambed sediment at different times of the year.

1.2 Background

In recent years there has been a growing awareness of the occurrence and concentration of pesticides in surface water due to the deterioration of surface water as a drinking water resource and the possible impact of pesticides on biota. Funen County was the first to monitor pesticides in streams in Denmark and link the presence to biota (macroinvertebrates) (Pedersen, 1996; Funen County, 1997, 1999 and 2001).

Other international studies have shown that pesticides applied in agriculture may risk being delivered to surface waters via different hydrological pathways, wind drift, atmospheric deposition or by accidental loss during cleaning of spraying equipment (Bellamie and Gouy, 1992; Kreuger, 1996; Kreuger, 2000). Many investigations have documented the presence of a large number of pesticides in surface water, especially in the spraying season and for certain pesticides all the year round (Harris et al., 1994; Mogensen and Spliid, 1995; Kreuger, 1998; Mogensen and Kronvang, 1999).

The importance of wind drift for pesticide loss to a large stream and the possible ecological consequences were investigated in a controlled field experiment on Zealand in spring 2000 (Bønding, 2001). The experiment was conducted with the insecticide esfenvalerate that was sprayed in normal doses at three different distances to the stream edge (2 m, 10 m and 20 m). The wind direction was changing during the experiment, but mainly along the stream channel with a wind speed of up to 2 m/s. Each experimental reach and an upstream control reach were equipped with automatic water samplers and 12 caves with the macroinvertebrate B. rhodani were installed prior to the experiment. Esfenvalerate was not detected in the stream water at any of the experimental reaches, nor could any significant effects be seen on the macroinvertebrates.

An important source for pesticides to stream water could be point sources due to bad handling of pesticides in the filling and cleaning procedure. The importance of these sources is, however, difficult to measure and interpret (Kreuger, 1998). Kreuger (1998) investigated a culverted stream and measured high concentrations of many pesticides. The author failed, however, to trace the source of pesticides to farm yards using bromide as a conservative tracer. Point sources were believed to play an important role for the high pesticide concentrations measured in culvert water during dry periods, possibly due to spillage of pesticides during filling and cleaning of spraying equipment.

Kreuger (2000) has shown a large decrease in the loss of pesticides from a small Swedish catchment during the period 1990-1998. The reason for this was four-fold: 1) Improved handling of pesticides due to information to farmers, etc.; 2) Change from high-dose pesticides to low-dose pesticides; 3) A small reduction in the number of farmers applying pesticides which could reduce the number of point sources; 4) An increase in the use of glyphosate which was not included in the investigation. The above findings show that point sources could be uncertain in investigations of the dynamics and fate of pesticides in catchments.

The existing knowledge on pesticide occurrence and concentration in surface water is in most cases based on routine monitoring programmes (Aiken, 1999; Ludvigsen, 2000; Iversen et al., 2001). The presence and concentration of pesticides in streams during spates has until now only been investigated in a few number of studies (Liess et al., 1999, Kreuger, 1998, Spalding and Snow, 1989). Results from these investigations have revealed that high concentrations of pesticides are often detected during high-flow periods, especially during spates in the spraying season (Kreuger, 1996). In Denmark, very few studies have focused on the concentration and transport of pesticides in subsurface drainage water and streams. Vilholdt et al. (1999) and Kronvang et al. (2002) have shown that both soluble and more hydrophobic pesticides are transported through the upper soil column during rain events.

The importance of Koc (partition coefficient between pesticide adsorbed to soil organic carbon and water), sprayed area, and dosage for the occurrence of pesticides in Danish streams has been demonstrated by using partial order ranking (Dobel et al., 2000; Iversen et al., 2001). However, the amount of pesticides applied on fields in the catchments has not yet been compared with the resulting concentration and transport in subsurface drainage water and streams in Denmark.

Kreuger and Törnqvist (1998) compared applied amount of pesticides and pesticide physico-chemical properties with the mean pesticide concentration and pesticide transport in a stream draining the Vemmehög catchment. They developed statistically significant relationships revealing log Pow (octanol/water partition coefficient) and applied amount of pesticides to be the most important predictors. The loss ratio (applied/transport) of pesticides has been calculated in a large number of small and large-scale investigations (Kladivko et al., 1991; Frank et al., 1991; Gomme et al., 1991; Battaglin et al., 1993; Albanis et al., 1994, Traub-Eberhard et al., 1994; Harris et al., 1995; Kreuger, 1998).

This study focuses on measurement of concentration, transport and loss ratio (applied/transport) of a range of pesticides in two hydrologically contrasting arable catchments. In this report, we will present and discuss the results from simultaneous measurements of pesticide concentrations and transport in subsurface tile drainage water and stream water within two agricultural catchments with different soil types. Pesticide concentrations in suspended sediment and streambed sediments in the two streams are furthermore presented.