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Pesticide leaching at SLAEGGERUP

Chapter 7, PLAP - Monitoring results May 1999 - June 2002

7.1 Materials and methods

7.1.1 Site description and monitoring design
The Slaeggerup test site is located on Zealand near the village of Slaeggerup northeast of Roskilde (Figures 1 and 46). The test field area is 2.2 ha (130 x 165 m). The ground surface within the test field slopes gently (1-4°) towards the northeast, the difference in altitude
between highest and lowest levels being around 4.5 m. Three soil profiles were excavated on the site, all of which are classified as Typic Argiudoll (Soil Survey Staff, 1999). The topsoil content of clay within the three profiles was 19-24%, whereas the organic matter content was 1.8-2.4%. The sediments penetrated when drilling the piezometers and monitoring wells could be subdivided into three lithological units (Figure 47). The upper unit was generally up to 2.5 m thick. Its uppermost part (0-0.65 m) consisted of meltwater clay with numerous desiccation cracks and biopores. Further down, the unit consisted of sandy meltwater gravel and then gravely meltwater sand. Within these two parts there were only small vertical and horizontal fractures. The middle unit consisted of up to 4 m of clay till with numerous horizontal and vertical fractures. The largest of these fractures traversed the entire unit and ended at the lowest unit consisting of sand till. The sand had no fractures. The content of clay decreased with depth from around 55% in the meltwater clay of the upper unit to 16.3% in the sand till of the lowest unit. A brief description of the sampling procedure is provided in Appendix 2. The monitoring design and test site are described in detail in Lindhardt et al. (2001) and the analysis methods in Kjær et al . (2002).

7.1.2 Agricultural management
Herbicide spraying was carried out on 9 May 2000 using metsulfuron-methyl, on 5 June using flamprop-M-isopropyl and on 14 June using tribenuron methyl. Fungicide spraying was carried out on 9 June and 26 June with propiconazole and fenpropimorph. The pesticide dimethoate was sprayed on 9 June. The crop was harvested on 8 August yielding just 39.8 hkg/ha of grain and 10.2 hkg/ha of straw (85% and 100% dry matter, respectively), which is about half of the normal yield for the location. The low yield is probably attributable to the fact that installation of monitoring equipment had prevented autumn ploughing, and the field was instead ploughed in the spring. As a consequence seedbed establishment was poor, as reflected in the very low final plant number (only 142 plants/m2). The harvested field was ploughed in November 2000.

Figure 46
Figure 46. Overview of the Slaeggerup site. The innermost white area indicates the cultivated land, while the grey area indicates the surrounding buffer zone. The positions of the various installations are indicated, as is the direction of groundwater flow (by an arrow).

Figure 47
Figure 47. Geological description of the Slaeggerup site (Lindhardt et al., 2001).

Field peas were sown on 11 April 2001. Weeds were spayed with bentazone and pendimethalin on 1 May and pests with pirimicarb. It was intended that fluazifop-P-butyl should be sprayed to combat wild oats, but this was erroneously omitted. Due to heavy infestation, the wild oats had to be weeded out by hand. From the beginning of June the field was heavily invaded by wood pigeons (L. Columba palumbus ). According to an official from the Danish Forest and Nature Agency, problems with wood pigeons are widespread on Zealand. In this particular year, the late sowing caused by rainy conditions further aggravated the problem caused by the wood pigeons. At the time they need large amounts of food for their young, the height of the pea plants will normally keep them from landing in the field. This was not the case, however. In spite of considerable effort to control bird damage using advanced scarecrows, balloons painted as birds of prey, and culling, pea yield at harvest on 19 August was only 26.6 hkg/ha (86% dry matter), which is around half of the normal yield.

On 26 September 2001 the field was sprayed with glyphosate in the form of Roundup Bio (using 4.0 l/ha), at which time it still had not been possible to remove the pea residues. On 10 October it was decided to shred the residues and on 13 October the field was ploughed. Two days later the field was sown with winter wheat (cv. Bill). The wheat emerged on 1 November. One week later, ioxynil and bromoxynil were sprayed to combat weeds. Weeds were sprayed again on 22 April using amidosulfuron and on 15 May using flamprop-Misopropyl. Fungicide spraying was carried out on 31 May and 14 June using propiconazole, and pests were sprayed using pirimicarb on 14 June. The winter wheat was harvested on 20 August 2002 yielding 72.3 hkg/ha of grain (85% dry matter). The yield was lower than normal for the location, probably due to the late sowing caused by the wet weather of autumn 2001. Management practice at the site is detailed in Appendix 3 (Table A3.6).

7.1.3 Model set-up and calibration
The MACRO model was applied to the Slaeggerup site covering the soil profile to a depth of 5 m b.g.s., always including the groundwater table. The model was used to simulate the water flow in the unsaturated zone during the full monitoring period April 2000-June 2002 and to establish an annual water balance.

The model was calibrated to the observed groundwater table measured in the piezometers located in the buffer zone as well as to three time series of soil water content measured at 25, 60 and 110 cm b.g.s. in the two profiles S1 and S2 (see Figure 46). A simple calibration procedure was applied that only necessitated adjustment of the empirical BGRAD parameter regulating the boundary flow and the drain depth, which was determined by the groundwater level during drainage periods. All remaining parameters were based on measured data or literature/default values. For a detailed description of data acquisition, model set-up and calibration procedures, see Kjær et al . (2002).

7.2 Results and discussion

7.2.1 Soil water dynamics and water balances
The model simulations were generally consistent with the observed data during the whole monitoring period, thus indicating that the model describes the overall soil water dynamics in the unsaturated zone reasonably well. The model was able to match the measured groundwater table, and the dynamics of the soil water as determined from the TDR probes was well captured by the model.

Measured drainage flow during the first winter period was very low (11 mm). The next winter the flow was ten-fold greater (110 mm). The model simulation yielded similar figures (7 and 116 mm, respectively), but the modelled drainage flow was delayed compared to the measured drainage flow in the first monitoring year. The measured drainage flow started to accumulate in mid December 2000, at which time the groundwater table was located 2 m b.g.s. Thus it was not possible to match the dynamics of the measured drainage flow without an unreasonable increase in the groundwater level or an unreasonably low drain depth. A better description of the drainage dynamics was obtained the following year, when the two major flow events are well captured. The overall trends in soil water content as measured by the TDR probes were successfully modelled (Figure 48D, E and F).

Table 14. Annual water balance for Slaeggerup (mm/year). Precipitation is corrected to the soil surface according to the method of Allerup and Madsen (1979).
Table 14

The first monitoring period (July 2000-June 2001) was close to normal at Slaeggerup, whereas the following year was wet (Table 14). The previous year was very dry with 17% less precipitation than normal. The modelled, accumulated drainage flow corresponded well to the measured drainage flow for both years. The simulated groundwater recharge differed significantly between the years, ranging from 119 mm in 1999/2000 to 328 mm in 2000- 2001, probably because of the large precipitation deficit during 1999/2000 and the limited drainage flow in 2000/2001. The high evapotranspiration in 2001/2002 is due to the crop (winter wheat) and the wet soil conditions during the growing season in the spring and early summer 2002. As shown by the modelled percolation 1 m b.g.s., spring and summer 2002 were characterized by continued percolation until the end of the monitoring period at the end of June 2002 (Figure 48A). In the previous two years, in contrast, percolation ceased at the beginning of June.

Bromide tracer studies could not be carried out at Slaeggerup because the water supply authorities refused permission due to the presence of a large municipal drinking water supply in the vicinity. Hence, no bromide data are available to verify water transport patterns.

Figure 48
Figure 48. Soil water dynamics at Slaeggerup: Locally measured precipitation and simulated percolation at 1 m b.g.s. (A), simulated and measured groundwater level (B), simulated and measured drainage flow (C), and simulated and measured soil water saturation (SW sat.) at three different soil depths (D, E and F). The measured data in B derive from piezometers located in the buffer zone. The measured data in D, E and F derive from TDR probes installed at S1 and S2 (see Figure 46).

7.2.2 Pesticide leaching
Monitoring at Slaeggerup began in April 2000 and presently encompasses 12 pesticides and 8 degradation products (Figure 49 and Table 15). It should be noted that precipitation in Table 15 is corrected to the soil surface according to Allerup and Madsen (1979), whereas percolation (1 m b.g.s.) refers to accumulated percolation as simulated with the MACRO model. It should also be noted that as tribenuron methyl (applied here as Express) degrades rapidly, the leaching risk is more associated with its degradation product, triazinaminmethyl. For the same reason it is the degradation product and not the parent compound that is monitored in the PLAP (Table 15.

Table 15. Pesticides analysed at Slaeggerup with the product used shown in parentheses. Degradation products are in italics. Precipitation and percolation are accumulated from date of first application (App. date) until 1 July 2002. 1 st month percolation refers to accumulated percolation within the first month after application. Cmean refers to weighted average concentration in the drainage runoff. The number of pesticide-positive samples is indicated in parentheses.
Table 15

Figure 49
Figure 49. Pesticide application and precipitation (primary axis) together with simulated percolation 1 m b.g.s. (secondary axis) at Slaeggerup. Pesticides applied later than April 2002 is not included.

With metsulfuron-methyl, propiconazole, triazinamin-methyl, dimethoate and fenpropimorph, the leaching risk was found to be negligible at the Slaeggerup site. Apart from one drainage sample containing 0.25 µg/l fenpropimorphic acid, none of these compounds or their degradation products listed in Table 15 have yet been detected during the two-year monitoring period. All of these compounds were applied during summer 2000, when precipitation input was close to normal and almost counterbalanced by actual evapotranspiration such that there was little percolation during the first month after application. The detection of fenpropimorphic acid at a concentration of 0.25 µg/l occurred in connection with a major storm event on 5 September 2000 (58 mm of precipitation) (Figure 49).

Flamprop-M-isopropyl and flamprop (free acid) were detected at Slaeggerup, but only in a few water samples. Following the major storm event on 5 September, they were detected at a concentration of 0.02 µg/l and 0.35 µg/l, respectively, in a single flow-proportional drainage water sample. In addition, flamprop-M-isopropyl was detected in five drainage water samples at concentrations of 0.014-0.035 µg/l. For further details, see Kjær et al. (2002).

Figure 50
Figure 50. Precipitation (A) together with the drainage water concentration of glyphosate (B) and AMPA (C) at Slaeggerup. The green vertical line indicates the date of application.

The leaching risk of pesticides used on both the pea crop in 2001 and the winter wheat crop in 2002 will not be evaluated until the 2003 monitoring results become available, i.e. when two years of monitoring data have been collated. The preliminary findings are that:

  • On 16 May, just 16 days after application, bentazone was detected at a concentration of 0.01 µg/l in soil water sampled 1 m b.g.s. at S2 as well as in the two uppermost screens of the vertical well M6. The following autumn, minor leaching of bentazone was detected. Bentazone was thus found in one flow-proportional sample of drainage water collected on 24 October 2001 (0.024 µg/l) and in five time-proportional samples collected between 6 February and 12 March 2002 (0.01-0.03 µg/l). The average concentration in the drainage water during the 2002/2003 leaching period was 0.02 µg/l.
  • On 27 November 2001, 13 days after application, ioxynil and bromoxynil were detected in a time-proportional drainage water sample at concentrations of 0.18 µg/l and 0.14 µg/l, respectively. Moreover, ioxynil was detected at a concentration of 0.02 µg/l in a time-proportional sample collected on 4 December 2001 and at a concentration of 0.046 µg/l in a flow-proportional sample collected on 1 March 2002.
  • August and in particular September 2001 were rainier than usual, causing drainage flow earlier than usual for the location (Figure 48; Appendix 4). When the field was sprayed with Roundup Bio (4.0 l/ha) on 26 September 2001, drainage flow had been occurring for a good week, although at very low levels (0.1 to 0.3 mm ha/day). When glyphosate and AMPA initially appeared in the drainage water just six days after application and after 34 mm of precipitation, the level of drainage flow was low. When the concentrations subsequently started to increase, the flow was still low due to a dry November and December (Figure 50; Appendix 4). Despite the very high concentrations detected (5.1 µg/l glyphosate and 5.4 µg/l AMPA), the amount leached during this period was thus very small. When runoff eventually started to increase due to the wetter than normal weather in January and February (Appendix 4), the concentration of both glyphosate and AMPA had decreased to low levels (Figure 50; Appendix 9). During the 2001/2002 leaching season, the average drainage water concentration of glyphosate was 0.04 µg/l, while that of AMPA was 0.06 µg/l. Apart from a single sample containing 0.017 µg/l AMPA, glyphosate and AMPA were not detected in samples from the groundwater monitoring wells.
No evidence was found to indicate leaching of the other pesticides applied in 2001 and 2002 since they were only detected in two samples, one containing 0.01 µg/l pirimicarb and one containing 0.01 µg/l pendimethalin (Table 15).

7.3 Summary
At Slaeggerup, the leaching risk of pesticides applied in 2000 can be summarized as follows:
  • With metsulfuron-methyl, propiconazole, triazinamin-methyl fenpropimorph and di-methoate, the leaching risk was found to be negligible.
  • Flamprop-M-isopropyl and flamprop (free acid) were detected, but only in very few samples and only in one case at a concentration exceeding 0.1 µg/l.
The leaching risk of pesticides applied in 2001 and 2002 cannot be fully evaluated at present as the potential leaching period extends beyond the current monitoring period. The preliminary findings are that:
  • There was no evidence to indicate leaching of pentimenthalin and pirimicarb, both compounds only being detected in a single sample in very low concentrations (0.01 µg/l).
  • Bromoxynil, ioxynil, flamprop-M-isopropyl and dimethoate were detected, but only in very few water samples.
  • Bentazone, glyphosate and AMPA did leach from the root zone, but not at unacceptable levels. During the 2001/2002 leaching period the average concentration of bentazone in the drainage water was 0.02 µg/l, while that of glyphosate and AMPA was 0.04 and 0.06 µg/l, respectively. Bentazone was only detected in three samples from the groundwater monitoring wells (0.01 µg/l), while AMPA was detected in one sample (0.017 µg/l).
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