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Pesticide leaching at ESTRUP

Chapter 5, PLAP - Monitoring results May 1999 - June 2002

5.1 Material and methods

5.1.1 Site description and monitoring design
Estrup is located in central Jutland (Figure 1) west of the Main Stationary Line on a hillisland, i.e. a glacial moraine preserved from the Weischselian Glaciation. Estrup has thus been exposed to weathering, erosion, leaching and other geomorphologic processes for a much longer period than that of the other sites. The test field covers a cultivated area of 1.26 ha (105 x 120 m) and is virtually flat. The site is highly heterogeneous with considerable variation in both topsoil and aquifer characteristics (Table 1). Such heterogeneity is quite common for this geological formation, however. Based on three profiles excavated in the buffer zone bordering the field the soil was classified as Abruptic Argiudoll, Aquic Argiudoll and Fragiaquic Glossudalf (Soil Survey Staff, 1999). The topsoil is characterized as sandy loam with a clay content of 10-20% and an organic carbon content of 1.7-7.3%. The site is also characterized by a C horizon of low permeability. The saturated hydraulic conductivity in the C horizon is 10-8 m/s, which is about two orders of magnitude lower than at the other loamy sites (Table 1). The geological structure is complex comprising a clay till core with deposits of different age and composition (Figure 30). 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). Please note that the geological conditions only allowed one of the planned horizontal wells to be installed as drilling in sand proved impossible.

5.1.2 Agricultural management
The field was ploughed on 11 April 2000 whereafter spring barley (cv. Barke) was sown. The barley emerged on 25 April 2000. On 15 May the herbicide metsulfuron-methyl and potassium bromide tracer were applied. The herbicide flamprop-M-isopropyl was applied on 31 May. Combined fungicide and insecticide spraying with propiconazole, fenpropimorph and dimethoate was carried out on 15 June and 5 July. The barley was harvested on 28 August yielding 52.6 hkg/ha of grain (85% dry matter). The low yield is attributable to at least two factors. Firstly, due to the instrumentation work the field had to be ploughed in the spring rather than in the autumn, as would normally be the case on this soil. As a consequence a proper seedbed could not be established, and crop establishment was therefore poor. Secondly, the soil in minor parts of the field had been compacted in autumn and winter 1999 during installation of the monitoring equipment.

Figure 29
Figure 29. Overview of the Estrup 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 30. Geological description of the Estrup site (Lindhardt et al., 2001).

On 13 October 2000, glyphosate was sprayed to combat couch grass. The field was ploughed on 23 October and sown with field pea (cv. Julia) on 2 May. The peas emerged on 13 May. Weeds were sprayed only once using bentazone and pendimethalin on 22 May. The insecticide pirimicarb was sprayed on 26 June. The crop was harvested on 22 August yielding 43.2 hkg/ha of peas (86% dry matter).

Winter wheat (cv. Ritmo) was sown on 19 October 2001, much later than usual due to the very wet weather in August and September. Due to the unusually high temperatures in October, however, the wheat emerged just 12 days later. Weeds were sprayed in autumn with ioxynil and bromoxynil on 20 November and again in spring using amidosulfuron on 25 April and MCPA on 13 May. Propiconazole was sprayed to combat fungi on 27 May and 17 June, while pirimicarb was sprayed to combat pests on 24 June. The winter wheat was harvested 9 August yielding 69.4 hkg/ha (85% dry matter). A higher yield could have been obtained had the crop been sown in due time. Ponding was observed at a small area of the southeastern part of the field near S2. In autumn 2002, this problem had been solved by repairing a drainpipe inadvertently damaged, presumably during installation of the monitoring equipment in the buffer zone (Lindhardt et al,. 2001). Management practice at the site is detailed in Appendix 3 (Table A3.4).

5.1.3 Model set-up and calibration
The MACRO model was applied to the Estrup 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 monitoring period from July 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 measured drainage flow and measured time series of soil water content at one depth (25 cm b.g.s.) from a single soil profile S1 (see Figure 29D). The TDR probes installed at the other depths yielded unreliable data with saturations far exceeding 100% and dynamics with increasing soil water content during the drier summer periods. The data from the soil profile S2 have been excluded due to the above-mentioned problem of water ponding above the TDR probes installed at S2.

Despite the lack of measured time series, a simple calibration procedure was applied that only necessitated minor adjustment of the hydraulic properties of the C horizon and 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).

5.2 Results and discussion

5.2.1 Soil water dynamics and water balances
The model simulations were generally consistent with the observed data, thus indicating a good model description of the overall soil water dynamics in the unsaturated zone (Figure 31).

The model was able to provide a good simulation of the measured groundwater table, including the rapid rise of the groundwater table at the end of October 2000. The simulated groundwater table often fluctuated slightly above the drain depth during periods where drainage flow occurred. The peaks corresponded to larger storm events and resulted in an almost fully saturated soil profile. This is also reflected by the TDR probes located at 25 cm b.g.s., where the soil water saturation approaches 100%.

The simulated accumulated drainage flow matches well with the measured drainage flow for both monitoring years. Moreover, both the drainage flow pattern and the onset of the drainage flow were well captured by the model. The measured drainage flow amounted to as much as 95% of the percolation modelled immediately above the drains (0.85 m b.g.s.). The high drainage runoff was due to the significantly lower permeability of the C horizon than that of the overlying A and B horizons. The percolation rate presumably exceeded the infiltration capacity of the C horizon for long periods, leaving the groundwater table to rise above the drain depth into the B horizon. Following minor adjustment of the hydraulic properties of the C horizon this process now seems to be captured well by the model.

The drainage season varied significantly between the three monitoring periods. Continuous drainage runoff started as early as the beginning of September in 2002 or as late as the end of October in 2001, and continued until mid April in 2002 and mid June in 2000. During the first and the third monitoring periods, drainage runoff amounted to 500 mm during the drainage season (7½ months), whereas the shorter drainage season during the second year (5 months) only resulted in 300 mm of drainage runoff. It should be noted that for the first monitoring period, simulated drainage volume is used because measured drainage runoff is only available from April 2000 (Figure 31C).

Percolation at Estrup is shown at 0.6 m b.g.s. instead of at 1 m b.g.s. because the soil at 1 m b.g.s. was saturated for longer periods (Figure 31). Percolation occurred continuously in the first two years from September to May/June, whereas the third year was characterized by a shorter percolation period with higher percolation rates. Percolation ceased at the end of March followed by minor peaks caused by major storm events in the spring/summer. The percolation pattern the first two years was characterized by a large initial peak at the onset (~30 mm/d) followed by a more stable period with minor peaks, all below 7 mm/d. The third year was characterized by less intense storm events and the absence of an initial large percolation peak.

Figure 31
Figure 31. Soil water dynamics at Estrup: Locally measured precipitation and simulated percolation 0.6 m b.g.s. (A), simulated and measured groundwater level (B), simulated and measured drainage flow (C), and simulated and measured soil 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 (see Figure 29).

Table 9. Annual water balance for Estrup (mm/year). Precipitation is corrected to the soil surface according to the method of Allerup and Madsen (1979).
Table 9

The first monitoring period at Estrup (2000/2001) was dry, whereas the following period was rather wet. The year preceding the monitoring period was also rather wet (Table 9). This pattern is reflected in the accumulated drainage runoff and the estimated groundwater recharge. The high evapotranspiration in 2001/2002 was due to the fact that the crop was winter wheat.

5.2.2 Bromide leaching
At Estrup, total recovery of bromide in the drainage water during the two-year monitoring period amounted to 4.8 kg/ha, indicating that 24% of the applied tracer had leached into the drains. Although concentration levels decreased through the monitoring period, slightly elevated bromide concentrations were detected in both suction cups and drainage water at the end of the monitoring period (Figure 32A and B). This indicates that part of the bromide was still retained in the upper part of the soil profile, probably in the matrix. Retained bromide can therefore be expected to continue to leach for a long period of time.

The majority of the leached bromide probably left the system through drainage runoff as the modelled water balance suggested that 65-70% of the percolating water left through the drainage system. However, the results did show subsequent transport of small amounts of bromide to a depth of 2 and 3.5 m b.g.s. (Figure 32B and D). Slightly elevated concentrations were detected 2 m b.g.s in suction cups as well as in the horizontal well 3.5 m b.g.s. Although the concentration and frequency of detection were very low, slightly elevated concentrations were also detected in the downstream monitoring wells, especially in wells M3 and M4 (Figure 33).

Figure 32
Figure 32. Bromide concentration at Estrup. A and B refer to suction cups located at S1 and S2. The bromide concentration is also shown for drainage runoff (C) and the horizontal monitoring well H1 (D). The green vertical line indicates the date of bromide application.

Figure 33
Figure 33 . Bromide concentration at Estrup. The data derive from the vertical monitoring wells (M2-M12). Screen depth is indicated in m b.g.s. The green vertical line indicates the date of bromide application.

5.2.3 Pesticide leaching
Monitoring began at Estrup in April 2000 and presently encompasses 12 pesticides and 13 degradation products as indicated in Table 10 and Figure 34. It should be noted that precipitation in Table 10 is corrected to the soil surface according to Allerup and Madsen (1979), whereas percolation (0.6 m b.g.s.) refers to accumulated percolation as simulated with the MACRO model (Section 5.2.1).

Table 10. Pesticides analysed at Estrup 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 average leachate concentration in the drainage water. The number of pesticide-positive samples is indicated in parentheses.
Table 10

Figure 34
Figure 34. Pesticide application and precipitation (primary axis) together with simulated percolation 0.6 m b.g.s. (secondary axis) at Estrup. Pesticides applied later than April 2002 is not included.

With metsulfuron, fenpropimorph and dimethoate the leaching risk was found to be negligible at the Estrup site. Apart from one sample containing less than 0.1 µg/l of metsulfuron, triazinamin and fenpropimorph, none of these compounds or the degradation products listed in Table 10 have yet been detected during the two-year monitoring period. Slight leaching of flamprop-M-isopropyl, flamprop (free acid) and propiconazole was observed. All three substances were detected in several drainage water samples, although at concentrations below 0.1 µg/l. Leaching was confined to the 2000/2001 leaching period, with the last sample containing pesticide being detected in March 2001. For further details, see Kjær et al. (2002).

Glyphosate and its degradation product AMPA leached from the root zone at average concentrations considerably exceeding 0.1 µg/l, especially in the case of glyphosate. Thus the average concentration in the drainage water during the 2000/2001 leaching period was 0.54 µg/l, while that of AMPA was 0.17 µg/l (Figure 35). The leaching appeared to be governed by a combination of pronounced macropore flow occurring shortly after application and a limited sorption and degradation capacity. Both compounds leached continuously throughout the whole six-month drainage runoff period in 2000/2001. Leaching continued during the drainage runoff in 2001/2002. Although the concentration level was much lower, continuous leaching of AMPA in particular was observed during the second monitoring period (Table 11 and Figure 36.). Leaching was greatest with glyphosate during the first monitoring period, but with AMPA during the second period. The primary data and a detailed description of the leaching pattern in 2000/2001 are provided in Kjær et al. (2002). Apart from three samples containing 0.01-0.04 µg/l glyphosate, AMPA and glyphosate have not been detected in the groundwater monitoring screens located below the depth of the drainage system. Finally, it should be noted that monitoring of glyphosate and AMPA has not yet been completed, but will continue throughout the next monitoring period.

Table 11. AMPA and glyphosate in drainage water at Estrup during the two monitoring years. Cmean refers to
the weighted average concentration (µg/l), Detection to percent of detection (% of analysed samples) and Cmax
to the maximum concentration found (µg/l).
Table 11

The leaching risk of the pesticides applied in 2001 and 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:

  • Slight leaching of ioxynil, pirimicarb and bentazone was observed. All three compounds were detected in several drainage water samples. In some cases the concentration exceeded 0.1 µg/l, but the average concentrations were relatively low, ranging from 0.01 to 0.03 µg/l (Table 10). The most frequently detected compound was bentazone, which was present in 81% of the analysed drainage water samples (Figure 37). It should be noted that drainage runoff commenced about two and a half months prior to the application of ioxynil and bromoxynil. The weighted concentration of ioxynil and bromoxynil refers to the period from the date of application until 1 July 2002
  • Pendimethalin and bromoxynil was both detected in the drainage water, but only in very few samples. The concentration range was 0.07-0.6 µg/l for pendimethalin and 0.01- 0.04 µg/l for bromoxynil.
Pesticide leaching at Estrup has hitherto been confined to the depth of the drainage system. Thus pesticides have only very rarely been detected in groundwater monitoring screens located below the depth of the drainage system. The bulk of all leached pesticide probably left the system through drainage runoff since the water balance suggests that 65-70% of the percolation ran off through the drainage system (Section 5.2.1). Due to decreased hydraulic conductivity, water and solute transport at Estrup were much slower beneath the drainage system than above it (Lindhardt et al ., 2001). The slower transport time may also allow for dispersion, dilution, sorption and degradation, thereby reducing further transport.

Figure 35
Figure 35. Precipitation (A) together with concentration of glyphosate (B), AMPA (C) and bromide (D) in the drainage runoff in 2000/2001 at Estrup. The green vertical lines indicate the date of application.

Figure 36
Figure 36. Precipitation (A) together with concentration of glyphosate (B), AMPA (C) and bromide (D) in the drainage runoff in 2001/2002 at Estrup. Bromide and glyphosate was applied in April 2000 and October 2000, respectively. Please note that scales used for glyphosate and AMPA differ from those used in Figure 35.

Figure 37
Figure 37. Precipitation (A) together with concentration of bentazone (B), pirimicarb (C) and ioxynil (D) in the drainage runoff at Estrup. The green vertical lines indicate the date of application.

5.3 Summary
At Estrup the leaching risk of pesticides applied during 2000 can be summarized as follows:
  • With metsulfuron, fenpropimorph and dimethoate the leaching risk was found to be negligible.
  • Flamprop-M-isopropyl, flamprop (free acid) and propiconazole were detected in several drain water samples, but only in concentrations below 0.1 µg/l.
  • Glyphosate and its metabolite AMPA leached from the root zone at average concentrations considerably exceeding 0.1 µg/l. Thus the average concentration of glyphosate in the drainage water was 0.54 µg/l during the 2000/2001 leaching period, while that of AMPA was 0.17 µg/l. Leaching has hitherto been confined to the depth of the drainage system, pesticides rarely having been detected in the groundwater monitoring screens located below the depth of the drainage system.
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:
  • Pendimethalin and bromoxynil were detected, but only in very few samples
  • Slight leaching of ioxynil, pirimicarb and bentazone took place, all three compounds having been detected in several drainage water samples. The concentrations sometimes exceeded 0.1 µg/l, but the average concentrations were relatively low, ranging from 0.01 to 0.03 µg/l.
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