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Establishment of a basis for administrative use of PestSurf

Annex 2

2 Comparison of risk assessment data produced by spray drift assessments, FOCUS SW and PestSurf

Table 2.1. Overview of chemical properties of bentazon and the parameters used in the simulations.
Tabel 2.1. Oversigt over bentazons kemiske egenskaber og parametrene brugt i simuleringerne.

2.2 Concentration generated by spray

2.3 Concentrations generated by FOCUS SW

2.3.1 D3 - Ditch

2.3.2 D4 – Pond

2.3.3 D4 –Stream

2.3.4 Conclusion – FOCUS SW

The highest concentration is generated in the ditch (D3). It is caused by wind drift and the concentration becomes 95.9 µg/l. The pond scenario shows the highest concentration in the sediment, 2.80 µg/kg. For all scenarios, the concentrations are lower than what is generated by the simpler assessments.

2.4 PestSurf

2.4.1 Sandy Catchment, stream

The distribution of concentrations were assessed in several steps. First, the maximum concentrations at each calculation point were listed, and the dates for the occurrence of the maximum was assessed. The points, for which the maximum value also represents a local maximum were selected for further analysis. The relevant values are listed in Table 2.2.

Table 2.2. Maximum concentrations (ng/l) of bentazon simulated for each calculation point in the sandy catchment.
Tabel 2.2. Maximumskoncentrationer (ng/l) af bentazon simuleret for hvert beregningspunkt i det sandede opland.

The pattern over time was dominated by wind drift for all calculation points. A baseflow contribution is, however, visible. The pattern is identical all through the stream, see Figure 2.1, Figure 2.2 and Figure 2.3. The baseflow contribution is mainly seen late in the drainage season, particularly in 2003, which represents the end of a very wet winter.

Figure 2.1. Concentration pattern over time for bentazon at the top of the sandy catchment.
Figur 2.1. Koncentrationsmønster som funktion af tid for bentazon i det sandede opland.

Figure 2.2. Concentration pattern over time for bentazon in the middle part of the sandy catchment.
Figur 2.2. Koncentrationsmønster som funktion af tid for bentazon i midten af det sandede opland.

Figure 2.3. Concentration pattern over time for bentazon at the bottom of the sandy catchment.
Figur 2.3. Koncentrationsmønster som funktion af tid for bentazon i den nedstrøms ende af det sandede opland.

Figure 2.4 and Figure 2.5 show logitudinal profiles of the stream on the dates of maximum concentrations for the upper, middle and lower parts of the stream in the sandy catchment. The thin black line represents the concentration, while the thick black line shows the maximum concentrations obtained during the simulations. In addition, the outline of the stream is shown.

Figure 2.4. Concentrations of bentazon in the sandy catchment on 13. May 1998, where maximum concentrations are reached in the upper end and lower end.
Figur 2.4. Koncentrationer af bentazon i det sandede opland den 13. maj 1998, hvor maximumskoncentrationer nås i den øvre og nedre del.

Figure 2.5. Concentrations of bentazon in the sandy catchment on 8. May 2001, where maximum values are reached in the middle part, 1.5 to 2.5 hours after the end of the spray event shown above.
Figur 2.5. Koncentrationer af bentazon i det sandede opland den 8. maj 2001, hvor maximumskoncentrationer nås i den midterste del af oplandet, 1.5 til 2.5 timer efter den viste drifthændelse er afsluttet.

In order to present the data in a similar fashion to the FOCUS SW-results, data were extracted and recalculated for the time series marked in Table 2.2. The global maxima and time weighted concentrations (up to 7 days) were extracted and are reported in Table 2.3. Note that the unit is ng/l.

Bentazon does not sorb strongly, and the concentrations on macrophytes are relatively small in comparison, 115 ng/l (Figure 2.6), compared to 39.3 µg/l in the water phase. The concentrations in pore water and adsorbed on sediment are shown in Figure 2.7 and Figure 2.8. The concentration in porewater reaches its maximum, 28.5 ng/l, 56 m from the upstream point of Odder Baek, while the concentration in sediment reaches 80 ng/kg at the maximum point. The pore water and sediment concentrations are highest during the year with low flow, and the concentrations thus seem more affected by the base flow contribution than by the drift.

Table 2.3. Instantaneous and time weighted concentrations (ng/l) of bentazon at selected points in the sandy catchment.
Tabel 2.3. Beregnet og tidsvægtet concentration af bentazon, ng/l, på udvalgte lokaliteter i det sandede vandløb.

Figure 2.6. Concentration of bentazon adsorbed to macrophytes. As the sorption properties of bentazon are poor, the values are low compared to concentrations in the water phase.
Figur 2.6. Koncentration af bentazon asorberet på makrofytter. Da sorptionsegenskaberne af bentazon er dårlige, er værdierne lave sammenlignet med koncentrationerne i vandfasen.

Figure 2.7. Pore water concentration of bentazon in the upstream part of the sandy catchment. The maximum pore water concentration of 28.5 ng/l is reached 56 m from upstream.
Figur 2.7. Porevandskoncentration for bentazon i den opstrøms del af sandede opland. Den maximale porevandskoncentration på 28.5 ng/l opnås 56 m fra den opstrøms ende.

Figure 2.8. Bentazon adsorbed to sediment. The concentration is in µg/g sediment and not µg/m³ as stated. The maximum concentration of 80 ng/kg is reached 56 m from the upstream end.
Figur 2.8. Bentazon sorberet til sediment. Koncentrationen er i µg/g sediment og ikke i µg/m³ som angivet. Maximumskoncentrationen på 80 ng/kg opnås 56 m fra den opstrøms ende.

The global maximum value calculated by PestSurf for the sandy catchment in the water phase is 39.3 µg/l. This is considerably more than what is found in the D3-ditch scenario (9.95 µg/l), and it is due to the fact that a long stretch of stream is sprayed at the same time. 112 m from the upstream end of the stream, the concentration only reaches 1.89 µg/l. The sorption to macrophytes is small compared to the concentration in the water phase. The concentration in the sediment is lower than the concentration in the FOCUS SW-D3-ditch, 80 ng/kg compared to 1.56 µg/kg.

Figure 2.9 shows the output of the PestSurf Excel template. The template works with pre-defined data extraction points. The plot requires specification of a “lowest detection value” (ldc), which defines when a pesticide occurrence is defined as an event. The time series plot is identical to the time series shown earlier, and the graph in the upper right corner resembles the plots in Figure 2.4 and Figure 2.5, but takes into account a longer period of time. A curve is generated when a downstream point reaches a concentration higher than the ldc. The programme then tracks the highest concentration for each calculation point in the stream within the last 24 hours. The plot in the lower right corner shows how many events have concentrations higher than a given toxicity value for the selected monitoring points. The curves show some variation from year to year.

Table 2.4 shows the output of the PestSurf Excel template generated by the PestSurf Excel sheet based on the ldc-value. The selected table shows the point along the stream (of the pre-defined points) with the highest concentration. The highest concentration extracted by the PestSurf Excel sheet was 21.3 µg/l. This is only about half the maximum value of the simulation, which was 39.3 µg/l.

Click here to see Figure 2.9.

Figure 2.9. Overview for bentazon in the sandy catchment generated by the PestSurf excel template. The max concentrations generated over the 24 hours are similar to the overviews in Figure 2.4 and Figure 2.5. The lowest detection value was set to 100 ng/l.
Figur 2.9. Oversigt for alpha-cypermethrin i det sandede opland genereret med PestSurf-excel-skabelonen. Den maximale koncentration genereret over 24 timer svarer til oversigterne i Figure 2.4og Figure 2.5. Detektionsgrænsen var sat til 100 ng/l.

Table 2.4. Part of the result sheet generated by the PestSurf Excel sheet. The selected table shows the pre-defined point along the stream with the highest concentration recorded. Lowest detection value is set to 100 ng/l, toxicity to fish, daphnies and algae are set to 200, 400 and 1000 ng/, respectively. The recorded peaks can be seen in Figure 2.9.
Tabel 2.4. Uddrag af resultatpresentationen genereret af PestSurf-Excel-arket. Den udvalgte tabel viser det fordefinerede punkt langs med åen med højest koncentration. Detektionsgrænsen er sat til 1 ng/l mens toxicitetsværdierne for fisk, dafnier og alger er henholdsvis 200, 400 og 1000 ng/l. De viste hændelser kan ses i Figur 2.9.

Click here to see Table 2.4.

2.4.2 Sandy Catchment, pond

The concentration pattern is evaluated in the middle of the pond only, see Figure 2.10. The pond receives the major contribution from groundwater. The drift peaks are hardly visible. The maximum concentration is quite high, 3.1 µg/l, and occurs at the end of the drainage season of a very wet winter.

Figure 2.10. Concentrations of bentazon in the sandy pond.
Figur 2.10. Koncentration af bentazon i det sandede vandhul.

In Table 2.5, global maxima and time weighted concentrations (up to 7 days) were extracted.

Table 2.5. Maximum concentrations (ng/l) of bentazon for the sandy pond.
Tabel 2.5. Maximumskoncentrationer af bentazon (ng/l) det sandede vandhul.

The sorption to macrophytes is shown in Figure 2.11. The concentration is considerably lower than the concentration in the water phase. The porewater concentration in the sediment reaches a concentration of 429 ng/l, coinciding with the highest concentration in the pond, see Figure 2.12. The amount of bentazon sorbed to sediment is shown in Figure 2.13. The concentration in sediment reaches 0.40 µg/kg.

Compared to the FOCUS D3-ditch, the concentration in the PestSurf sandy pond is lower, 3.08 µg/l compared to 9.95 µg/l. The sediment concentration is also significantly lower, 0.400 µg/kg compared to 1.56 µg/kg.

Figure 2.11. Sorption of bentazon to macrophytes in the sandy pond.
Figur 2.11. Sorption af bentazon til makrofytter i det sandede vandhul.

Figure 2.12. Pore water concentration of bentazon in the sandy pond.
Figur 2.12. Porevandskoncentration af bentazon i det sandede vandhul.

Figure 2.13. Sorption of bentazon to sediment in the sandy pond. The concentration is in µg/g sediment and not µg/m³ as stated.
Figur 2.13. Sorption af bentazon til sediment id et sandede vandhul. Koncentrationen er i µg/g sediment og ikke µg/m³ som angivet.

Figure 2.14 shows the output of the PestSurf Excel template. The template works with one pre-defined data extraction point for the pond (center of the pond). The plot requires specification of a “lowest detection value” (ldc) which defines when a pesticide occurrence is defined as an event. The time series plot is identical to the time series shown earlier. The plot to the right shows how many events have concentrations higher than a given toxicity value for the selected monitoring points.

Table 2.6 shows part of the result sheet generated by the PestSurf Excel sheet based on the ldc-value.

Click here to see Figure 2.14.

Figure 2.14. Overview for bentazon in the sandy pond generated by the PestSurf excel template. The time series shown is identical to the one in Figure 2.10. The lowest detection value is set to 250 ng/l.
Figur 2.14. Oversigt for bentazon i det sandede opland genereret med PestSurf-excel-skabelonen. Den viste tidsserie er mage til den i Figur 2.10 Detektionsgrænsen er sat til 250 ng/l.

Table 2.6. Part of the result sheet generated by the PestSurf Excel sheet. Lowest detection value is set to 250 ng/l, toxicity to fish, daphnies and algae are set to 500, 1000 and 2000 ng/, respectively. The recorded peaks are shown in Figure 2.14.
Tabel 2.6. Uddrag af resultatpresentationen genereret af PestSurf-Excel-arket. Detektionsgrænsen er sat til 250 ng/l mens toxicitetsværdierne for fisk, dafnier og alger er henholdsvis 500, 1000 og 2000 ng/l. De tabellerede hændelser er vist i Figur 2.14.

Click here to see Table 2.6.

2.4.3 Sandy Loam Catchment, stream

The distribution of concentrations was assessed in several steps. First, the maximum concentrations at each calculation point were listed, and the dates for the occurrence of the maximum was assessed, see Table 2.7. The points, for which the maximum value also represents a local maximum were selected for further analysis.

Some points were disregarded in the first analysis. If more points peaked on the same date, generally only one was analyzed.

Table 2.7. Maximum concentrations (ng/l) of bentazon simulated for each calculation point in the Sandy loam catchment.
Tabel 2.7. Maximumskoncentrationer (ng/l) af bentazon simuleret for hvert beregningspunkt i morænelersoplandet.

All maximum concentrations are caused by drift events. The concentration pattern for the upper and lower part of the stream is shown in Figure 2.15 and Figure 2.16. The upstream ends of the system is totally dominated by the drift events, but some drainage contribution with a rather peaky pattern is visible in wet periods. Drift also dominated the lower part of the stream, but the extreme rainfall event around 16-17. September 1998 is visible in several of the time series. It stands out least clearly in the very upstream ends of the tributaries. Due to the relatively high degradation rate of bentazon, there is no buildup in the groundwater visible at the lower end of Lillebaek.

The high concentrations caused by drift is due to a buildup of concentrations over a long stretch receiving bentazon within 30 minutes, reaching a maximum value of 106 µg/l. At the same time, the water depth is small, particularly in the upstream part of the stream. The water depth during the event of 4. May 2001 is 6 cm 330 m from the upstream end and 13 cm in the maximum point “Nedrelillebaek 1279”.

The extreme event in September 1998 causes the maximum drainage concentration of around 11 µg/l.

Other high drainage concentrations (5.5 µg/l) are found in Steensbaek, which is not a permanent stream - it could be viewed as a drainage ditch. The highest concentration here occurs during the wet winter. Apart from the drift events and the mentioned drainage events, the maximum concentrations are in the range 1.5-2.0 µg/l.

Figure 2.15. Concentration pattern for bentazon in the upstream part of the sandy loam catchment.
Figur 2.15. Koncentrationsmønster for bentazon i den opstrøms ende af morænelersoplandet.

Figure 2.16. Concentration pattern for bentazon in the lower end of the sandy loam catchment.
Figur 2.16. Koncentrationsmønster for bentazon i den nedstrøms ende af morænelersoplandet.

The concentrations in the sandy loam catchment on selected dates (18. September 1998 and 4. May 2001) are shown in Figure 2.18 and Figure 2.18. The thin black line represents the concentration, while the thick black line shows the maximum concentrations obtained during the simulations. In addition, the outline of the stream is shown. The patterns shown for the drainage event differ significantly from the pattern of the drift event.

Figure 2.17. Concentrations in the sandy loam catchment on 18. September 1998, due to drainage.
Figur 2.17. Koncentrationer i morænelersoplandet den 18.September 1998. Koncentrationerne skyldes drænvand.

Figure 2.18. Concentrations in the sandy loam catchment on 4. May 2001, due to drift.
Figur 2.18. KOncentrationer i morænelersoplandet den 4. maj 2001. Koncentrationerne skyldes drift.

To be able to extract comparable values to FOCUS SW, the global maxima and time weighted concentrations (up to 7 days) were extracted when these were meaningful. They are shown in Table 2.8.

The concentrations of bentazon sorbed to macrophytes are shown in Figure 2.19 and Figure 2.20 for the upper and lower part of the sandy loam catchment, respectively. The concentration reaches 1.21 µg/l and does not significantly influence the concentration in the water phase. The pattern of bentazon in porewater is shown in Figure 2.21. The maximum value reached in the catchment was 165 ng/l in Steensbaek 125, and the maximum value in the main stream was 80 ng/l (Ovrelillebaek 1187). The concentration of bentazon adsorbed to sediment is shown in Figure 2.22. The maximum concentration reached is about 0.13 µg/kg in Steensbaek and 65 ng/kg in the main stream.

The maximum concentration level generated by PestSurf is much higher (106 µg/l) than the concentration generated by the FOCUS SW-stream-scenario for D4-stream (8.25 µg/l). This is due to the fact that the whole agricultural area is sprayed simultaneously. An additional factor is the very shallow water depth in the stream at the time of spraying. A depth of 6 cm will lead to concentrations at least 5 times the concentration at 30 cm’s depth. The concentration 100 m from the upstream end is 55 µg/l, 6-7 times the value found in FOCUS SW.

As mentioned earlier, an additional high value is found during the extreme event. With respect to sediment, the PestSurf value of 65 ng/kg in the main stream or 130 ng/kg in a tributary is considerably lower than the FOCUS SW-value of 1.1 µg/kg. This is due to the fact that the presence of high concentrations is very short in PestSurf, and the diffusion into porewater is slow.

Figure 2.23, Figure 2.24, Table 2.9 and Table 2.10 show the results generated by the PestSurf templates. The maximum value generated by the templates for the upper part of the stream is 38.5 µg/l and 106 µg/l for the lower part. The maximum value generated in the upstream part of the catchment is 67.3 µg/l in OvreLillebaek (290) while the maximum in the spreadsheet and in the main stream is 106 µg/l. The templates thus did catch the maximum value of the simulation.

Figure 2.19. Concentration of bentazon on macrophytes in the upstream part of the sandy loam catchment.
Figur 2.19. Koncentration af bentazon på makrofytter i den opstrøms del af morænelersoplandet.

Figure 2.20. Concentration on macrophytes in the downstream part of the sandy loam catchment.
Figur 2.20. Koncentration på makrofytter i den nedstrøms del af morænelersoplandet.

Figure 2.21. Maximum pore water concentration of bentazon in the sandy loam catchment.
Figur 2.21. Maximum porevandskoncentration af bentazon i morænelersoplandet.

Figure 2.22. Maximum concentration of bentazon sorbed to sediment in the sandy loam catchment. The concentration is in µg/g sediment and not µg/m³ as stated.
Figur 2.22. Maximumskoncentration af bentazon sorberet på sediment i morænelersoplandet. Koncentrationen er i µg/g sediment og ikke µg/m³ som angivet.

Table 2.8. Instantaneous and time weighted concentrations (ng/l) of bentazon in the sandy loam catchment.
Tabel 2.8. Beregnet og tidsvægtede koncentrationer (ng/l) af bentazon i morænelersoplandet.

Click here to see Table 2.8.

Click here to see Figure 2.23.

Figure 2.23. Overview for bentazon in the sandy loam catchment generated by the PestSurf excel template for the upstream part of the catchment. The limiting value was set to 150 ng/l.
Figur 2.23. Oversigt for Bentazon i morænelersoplandet genereret med PestSurf-excel-skabelonen for den opstrøms del af oplandet. Detektionsgrænsen var sat til 150 ng/l.

Click here to see Figure 2.24.

Figure 2.24. Overview for bentazon in the sandy loam catchment generated by the PestSurf excel template for the downstream part of the catchment. The limiting value was set to 150 ng/l.
Figur 2.24. Oversigt for bentazon i morænelersoplandet genereret med PestSurf-excel-skabelonen for den nedstrøms del af oplandet. Detektionsgrænsen var sat til 150 ng/l.

Table 2.9. Part of the result sheet generated by the PestSurf Excel sheet for the upstream part of the sandy loam catchment. Lowest detection value is set to 150 ng/l, toxicity to fish, daphnies and algae are set to 1000, 5000 and 10000 ng/, respectively. The recorded peaks are shown in Figure 2.23.
Tabel 2.9. Uddrag af resultatpresentationen genereret af PestSurf-Excel-arket for den opstrøms del af morænelersoplandet. Detektionsgrænsen er sat til 150 ng/l, toxicitetsværdierne for fisk, dafnier og alger er henholdsvis 1000, 5000 og 10000 ng/l. Hændelserne er vist i Figur 2.23.

Click here to see Table 2.9.

Table 2.10. Part of the result sheet generated by the PestSurf Excel sheet for the downstream part of the sandy loam catchment. Lowest detection value is set to 150 ng/l, toxicity to fish, daphnies and algae are set to 1000, 5000 and 10000 ng/, respectively. The recorded peaks are shown in Figure 2.24.
Tabel 2.10. Uddrag af resultatpresentationen genereret af PestSurf-Excel-arket for den nedstrøms del af morænelersoplandet. Detektionsgrænsen anvendt i simuleringen er 150 ng/l, toxicitetsværdierne for fisk, dafnier og alger er henholdsvis 1000, 5000 og 10000 ng/l. Hændelserne er vist i Figur 2.23.

Click here to see Table 2.10.

2.4.4 Sandy Loam Catchment, pond

The concentration pattern is evaluated in the middle of the pond only, see Figure 2.25. The pond receives contributions through drift and drain flow, in good correspondence with the fact that it is situated in the upper part of the sandy loam catchment. The maximum concentration is 1493 ng/l. The largest contribution occurs during the wet winter between 1998 and 1999.

Figure 2.25. Concentrations of bentazon for the sandy loam pond.
Figur 2.25. Koncentration af bentazon i morænelersvandhullet.

Figure 2.26 shows the concentration of bentazon on macrophytes in the pond. It follows the concentration pattern in the water phase, the maximum concentration reaching around 33 ng/l. This does not influence the concentration in the water phase significantly. The concentration in pore water reaches 167 ng/l, see Figure 2.27 at the time where the water concentration in the pond is at maximum. The amount of bentazon sorbed to sediment is shown in Figure 2.28. The maximum concentration reached is 155 ng/kg.

In Table 2.11, global maxima and time weighted concentrations (up to 7 days) are extracted.

Figure 2.26. Bentazon sorbed to the macrophytes in the sandy loam pond.
Figur 2.26. Bentazon sorberet til makrofytter i morænelersvandhullet.

Figure 2.27. Pore water concentration of bentazon in the sandy loam pond.
Figur 2.27. Porevandskoncentration af bentazon i morænelersvandhullet.

Figure 2.28. Bentazon sorbed to sediment in the sandy loam pond. Note that the concentration is in µg/g and not µg/m³ as indicated.
Figur 2.28. Bentazon sorberet til sediment i morænelersvandhullet. Bemærk at koncentrationen er i µg/g og ikke µg/m³ som angivet.

Table 2.11. Actual and time weighted concentrations (ng/l) of bentazon in the sandy loam pond.
Tabel 2.11. Beregnede og tidsvægtede koncentrationer (ng/l) af bentazon i morænelersvandhullet.

Figure 2.29 and Table 2.12 show output from the PestSurf template, with a time series identical to Figure 2.25.

The concentration generated in the FOCUS SW-scenario D4-pond and the PestSurf sand loam pond are comparable. D4 generates a concentration of 2.17 µg/l while PestSurf reaches 1.49 µg/l. The sediment concentration generated in PestSurf is, however, somewhat lower (0.155 µg/kg) than the concentration reached in the corresponding FOCUS SW-scenario (2.80 µg/kg).

Click here to see Figure 2.29.

Figure 2.29. Overview for bentazon in the sandy loam pond generated by the PestSurf excel template. The time series shown is identical to the one in Figure 2.25. The limiting value is set to 100 ng/l.
Figur 2.29. Oversigt for bentazon i morænelersoplandet genereret med PestSurf-excel-skabelonen. Den viste tidsserie er mage til den i Figur 2.25. Detektionsgrænsen er sat til 100 ng/l.

Table 2.12. Part of the result sheet generated by the PestSurf Excel sheet. Lowest detection value is set to 100 ng/l, toxicity to fish, daphnies and algae are set to 500, 1000 and 2000 ng/, respectively.
Tabel 2.12. Uddrag af resultatpresentationen genereret af PestSurf-Excel-arket. Detektionsgrænsen er sat til 100 ng/l mens toxicitetsværdierne for fisk, dafnier og alger er henholdsvis 500, 1000 og 2000 ng/l.

Click here to see Table 2.12.

Table 2.13. Summary of simulation results for bentazon.
Tabel 2.13. Opsummerede resultater for bentazon.

Click here to see Table 2.13.

2.5 Summary of simulations

The maximum actual concentrations for all simulations are recorded in Table 2.13.

The sandy pond generates lower concentrations than the D3-ditch, which produce lower concentrations than the sandy stream. The sandy pond is influenced by groundwater, while the D3 ditch and stream are mainly influenced by drift, although the stream receives some drainage contribution.

The major reason for the high stream concentration of 39.3 µg/l is the fact that a very long stretch of water is sprayed at the same time. The theoretical maximum for a stationary situation in the point of maximum concentration reaches a concentration of 55.5 µg/l. If the concentration is extracted 112 m from the upstream end of the sandy catchment, it only reaches 1.89 µg/l, compared to 9.95 µg/l reached in the D3-ditch. However, this is a result of a difference of exposure of due to presence of 20 m wide buffer zones along this part of the stream (a factor of 14.7) and a water depth at the time of spraying of about 9-10 cm at the time of spraying.

The concentration of the D4-pond is about 30 % higher than for the sandy loam pond. Both ponds are mainly influenced by drainage. The fact that more soil types and groundwater levels are present in PestSurf, probably results in the lower concentration level.

The concentration in the sandy loam stream is 106 µg/l compared to 8.25 in the FOCUS D4-stream. Both values are generated by drift. With the same reservation on the maximum value as mentioned for alphacypermethrin, the maximum concentration on the stretch, where the stream is permanent and upstream of the groundwater influence is extracted. It is 51.6 µg/l, 625 m from the upstream end.

The major reason for the high stream concentration is the fact that a very long stretch of water is sprayed at the same time. If the concentration is extracted 112 m from the upstream end of the sandy catchment, it only reaches 1.9 µg/l, compared to 9.9 µg/l reached in the D3-ditch. However, this is a result of a difference of exposure of due to presence of 20 m wide buffer zones along this part of the stream (a factor of 14.7) and a water depth at the time of spraying of about 9-10 cm at the time of spraying.

For the sandy loam catchment, the concentration 125 m from the upstream end is 55.3 µg/l, and thus still much higher than the 8.25 µg/l found in the FOCUS D4-stream. The water depth at this stretch is about 4.5 cm. The difference in water depth thus explains the difference in concentrations.

The macrophytes do not excert a strong influence on the concentration in the water phase in any of the scenarios scenarios due to the low potential for adsorption of bentazon.

With respect to sediment concentrations, all of the concentrations generated by PestSurf are lower than the concentrations generated by FOCUS SW. With respect to the streams, the diffusion description differ between FOCUS and PestSurf and that could be part of the explanation for the difference. The diffusion coefficient used in the two models is similar. Another difference could be lower contact time between the high concentrations and the porewater due to higher flow velocities in PestSurf. For the ponds, however, the diffusion description is similar in the two models. The diffusion coefficient in PestSurf ponds stem from dam experiments.

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Version 1.0 December 2006, © Danish Environmental Protection Agency