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Published in J. Environ. Qual. 33:294-301 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Surface Water Quality

Alachlor and Bentazone Losses from Subsurface Drainage of Two Soils

S. Dousset*,a, M. Babutb, F. Andreuxa and M. Schiavonc

a UMR GéoSol-Microbiologie des Sols INRA A111, Université de Bourgogne, Centre des Sciences de la Terre, 6, boulevard Gabriel, 21 000 Dijon, France
b Biologie des Systèmes Aquatiques-CEMAGREF, 3, bis quai Chauveau, 69336 Lyon Cedex 09, France
c Laboratoire Sols et Environnement-ENSAIA-INRA/INPL, BP 172, 54505 Vandoeuvre-lès-Nancy Cedex, France

* Corresponding author (Sylvie.Dousset{at}u-bourgogne.fr).

Received for publication October 18, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Atrazine (6-chloro-N2–ethyl-N4–isopropyl-1,3,5-triazine-2,4-diamine) is frequently detected at high concentrations in ground water. Bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] plus alachlor (2-chloro-2',6'-diethyl-N-methoxymethylacetanilide) is a potential herbicide combination used as a substitute for atrazine. Thus, the objective of this study was to assess the environmental risk of this blend. Drainage water contamination by bentazone and alachlor was assessed in silty clay (Vertic Eutrochrept) and silt loam (Aquic Hapludalf) soils under the same management and climatic conditions. Drainage volumes and concentrations of alachlor and bentazone were monitored after application. Herbicides first arrived at the drains after less than 1 cm of net drainage. This is consistent with preferential flow and suggests that about 3% of the pore volume was active in rapid transport. During the monitoring periods, bentazone losses were higher (0.11–2.40% of the applied amount) than alachlor losses (0.00–0.28%) in the drains of the silty clay and silt loam. The rank order of herbicide mass losses corresponded with the rank order of herbicide adsorption coefficients. More herbicide residues were detected in drainage from the silty clay, probably due to preferential flow and more intensive drainage in this soil than the silt loam. Surprisingly, herbicide losses were higher in the drains of both soils in the drier of the two study years. This could be explained by the time intervals between the treatments and first drainage events, which were longer in the wetter year. Results suggest that the drainage phases occurred by preferential flow in the spring–summer period, with correspondingly fast leaching of herbicides, and by matrix flow during the fall–winter period, with slower herbicide migration.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE INTENSIVE USE OF PESTICIDES, especially the exclusive application of certain types on specific crops, is frequently responsible for the pollution of environmental media. Many studies reveal contamination of surface or subsurface water (Brambilla et al., 1993; Buhler et al., 1993; Moorman et al., 1999) and ground water (Croll, 1986) by atrazine in concentrations higher than the European Community Council limit for water consumption of 0.1 µg L–1 (European Community Council, 1980, 1991). Consequently, the authorized amount of atrazine has been limited in France at 1500 g ha–1 in 1990, then at 1000 g ha–1 in 1997, and its use was banned as of 30 Sept. 2003 (Official Journal of the French Republic, 2001). When used in association, the herbicides alachlor and bentazone can be substituted for atrazine to provide the same spectrum of action on weeds in maize (Zea mays L.) crops. Several laboratory studies have shown that the association presents few risks to water contamination. Alachlor is quickly degraded in soil (Helling et al., 1988; Bowman, 1990) and lysimetric studies show slower migration of alachlor relative to atrazine (Bowman, 1990; Buhler et al., 1993; Wietersen et al., 1993). In a field-scale experiment, Stoller et al. (1975) found little downward movement of bentazone. However, more recent studies have reported bentazone leaching to depths below 1 m (Vischetti et al., 1998; Boesten and Van der Pas, 2000; Spliid, 2002). Few field-scale studies dealing with herbicide losses by drainage in France are available.

The objectives of this study were to assess drainage water contamination by bentazone and alachlor under field conditions in two soils, a silty clay and a silt loam, which were selected for their different textures and structures. The two soils were planted with maize and studied under the same agricultural management practices and climatic conditions for two seasonal cycles.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Site and Sampling
This research was conducted on poorly drained soils at the ENSAIA Experiment Station in Champenoux, France (La Bouzule WGS 84: 48°44'26'' N, 6°19'58'' E; Fig. 1) . The agricultural soils studied were located on a toposequence with a silt loam soil [stagnic luvisol (Food and Agriculture Organization of the United Nations, 1998), Aquic Hapludalf (USDA, 1999)] at the summit and a silty clay soil [vertic cambisol (Food and Agriculture Organization of the United Nations, 1998), Vertic Eutrochrept (USDA, 1999)] on the 5 to 8% slope (Florentin and Novak, 1998). The main soil properties are listed in Table 1. The plot sizes were 1.85 ha for the silty clay soil and 2.83 ha for the silt loam soil. Tillage and planting were performed up and down the slope (Fig. 1). Subsurface drainage systems were installed in 1969. Plastic drain pipes were placed 12 m apart on the silt loam and 8 m apart on the silty clay soil at an average depth of 0.9 m as described by Lesaffre et al. (1984).



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  		Fig. 1. Location map and layout of subsurface drain system in the experimental drainage field.

 

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  		Table 1. Surface soil characteristics (Ap horizons).

 
The drainage from each soil area was sampled and measured in the drainage system pipes from May 1992 to June 1994. Automatic water samplers (Model 900; American Sigma, Loveland, CO) allowed the flow to be measured continuously and the drainage to be sampled on a flow-proportional basis. Composite weekly samples were discharged into glass bottles. They contained no suspended material, probably due to the sampling system in which initially suspended particulates could have settled in the weir.

On the herbicide application date in 1993, 100-cm3 soil samples were collected from the 0- to 20-cm depth increment using 5-cm-long steel cores to evaluate the herbicide degradation. For each soil, 10 samples per depth, collected over the whole plot, were carefully mixed to obtain representative samples of each plot. Soil sampling was repeated every month for four months. The field persistence of the two herbicides was calculated from the changes in herbicide concentrations measured at 0 to 5 cm.

Crop and Chemical Management
Soils were planted with maize over the entire experimental area in the spring of 1992 and 1993. Alachlor (Lasso; Monsanto, St. Louis, MO) was applied on 14 May 1992 at a rate of 2.0 kg a.i. ha–1 and on 5 May 1993 at a rate of 2.5 kg a.i. ha–1. Bentazone (Basamaïs; BASF, Ludwigshafen, Germany) was applied on 14 May 1992 and on 9 June 1993 at 0.8 kg a.i. ha–1. Alachlor and bentazone concentrations in the drainage were monitored periodically from the application date to 20 June 1994.

Alachlor is a cream-colored solid. Its vapor pressure is 2.1 x 10–3 Pa at 20°C and its water solubility is 242 mg L–1 at 25°C (Tomlin, 1997). Its half-life at 20°C in soil is 20 to 40 d (Pothuluri et al., 1990; Walker and Welch, 1991) and its adsorption coefficient (Koc) is 170 L kg–1 (Tomlin, 1997). Bentazone is a colorless crystalline powder. Its vapor pressure is 0.17 x 10–3 Pa at 20°C and its water solubility is 570 mg L–1 at 20°C. Its half-life is greater than 84 d (Thorstensen and Lode, 2001) and average Koc is 42 L kg–1 (Tomlin, 1997).

Laboratory Methodology
For the monitoring period before 11 Apr. 1994, a representative aliquot of 2-L water samples was freeze-dried and redissolved in 2 mL methanol, then analyzed by high performance liquid chromatography (HPLC) as described below. Spike recoveries were 28.8 ± 1.3% for alachlor and 66.2 ± 6.2% for bentazone. Correction for recovery values was made.

To improve recovery rates of the herbicides, after 18 Apr. 1994, 500-mL aliquots of drainage water were passed through Supelclean cartridges (3-mL low-displacement disposable columns; Supelco, Bellefonte, PA) packed with reverse-phase octadecylsilane (C18) bonded silica gel (55-µm particle size). The cartridges were activated by rinsing with 10 mL methanol followed by 10 mL deionized water. Then the water samples were carefully passed through the cartridge at a slow, uniform flow rate of approximately 2 mL min–1. After drying, the residues retained by the cartridge were eluted with 2 mL methanol. The methanol eluate was evaporated to dryness and redissolved in 2 mL methanol. Recovery rates were 110.1 ± 8.2% for alachlor and 42.1 ± 2.9% for bentazone. Correction for recovery values was made.

Soil samples were air-dried and sieved at 2 mm. Subsamples (50 g) were extracted twice at room temperature with methanol by 12-h rotary shaking. The two extracts were mixed, evaporated to dryness at 30°C with a rotovapor, and then dissolved in methanol (2 mL) for analysis. Herbicide loss by volatilization was limited because of the low vapor pressures of alachlor (2.1 x 10–3 Pa) and bentazone (0.17 x 10–3 Pa). Recovery of field-applied bentazone and alachlor measured in the time-zero soil samples was 100 ± 5%.

Pesticide Residues Analysis in Water and Soil
The water samples were analyzed for alachlor and bentazone using a Beckman Coulter (Fullerton, CA) HPLC with a diode array detector. The mobile phase was 70:30 (v/v) acetonitrile and water for alachlor analysis and 60:40 (v/v) acetonitrile and phosphoric acid (0.4%) for bentazone analysis. The flow rate of the mobile phase was 0.8 mL min–1. Ultraviolet detection was made at 220 nm for alachlor and 224 nm for bentazone. The minimum detection limit was 0.1 µg L–1 in water and 0.01 µg kg–1 in soil for both herbicides.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rainfall and Drainage
Rainfall was lower in September and December 1992 than the 29-yr norm, as opposed to September and December 1993, which were very rainy (Fig. 2) . However, November 1992 was much rainier than the 29-yr norm, whereas November 1993 was very dry. The period from January to June 1993 was especially dry. The 1992–1993 monitoring period was drier than the 29-yr average (89%), in contrast to the 1993–1994 monitoring period, which was wetter (110%) (Table 2).



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  		Fig. 2. Monthly precipitation at Champenoux (2 km from the experimental site) during the monitoring period (1992–1994). The solid line shows the 29-yr average from 1972 to 2000 at Tomblaine meteorological station (8 km from the experimental site).

 

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  		Table 2. Rainfall and percentage of 29-yr normal{dagger} and drain discharge during the monitoring periods following the application of bentazone (14 May 1992 and 9 June 1993) and alachlor (14 May 1992 and 5 May 1993).

 
The first rains occurred within the first 4 d (1.8 mm) and 5 d (3.2 mm for alachlor and 5.8 mm for bentazone) after herbicide application in 1992 and 1993, respectively. The first drainage events occurred 25 d after both herbicide treatments in 1992 and 19 d after bentazone application and 59 d after alachlor application in 1993.

In the 1992–1993 monitoring period, the drainage was lower for the silt loam soil (117 mm) than for the silty clay soil (168 mm) (Table 2). Low rainfall in July 1992 (67 mm) resulted in low volumes of drainage from both soils, whereas high rainfall (125 mm) in November 1992 resulted in great drainage events of 55 mm for the silt loam soil and 74 mm for the silty clay soil. A third phase of drainage occurred in January 1993 following a considerable rainfall (74 mm) (Fig. 3) .



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  		Fig. 3. Cumulative rainfall (—) and drain discharge versus time for the silty clay (—{circ}—) and silt loam (—{square}—) soils.

 
For the 1993–1994 monitoring period, which was wetter than the 29-yr norm, discharge was greater for the silt loam soil (202 mm) than for the silty clay soil (166 mm) (Table 2). A small discharge occurred in October 1993 after the large rains of September 1993 (132 mm). Drainage was scarce in November 1993. After the large rains of December 1993 (164 mm) and January 1994 (86 mm) (Fig. 3), the drainage was intense and reached 150 mm for the silt loam soil and 129 mm for the silty clay soil.

Runoff from these hillslope plots probably occurred, especially during the great rainfall events, as observed in a subsequent monitoring study (data not shown). Herbicide losses from both plots could have occurred as a result of the runoff, possibly in particulate-bound forms. However, as no suspended matter was present in the samples collected, this possibility could not be verified. Another consequence of the runoff could have been the contamination of the silty clay soil plot by herbicide-containing flow from the silt-loam plot.

Alachlor Discharge in Drainage
Alachlor was detected in 46% of the drainage samples analyzed in 1992 from the silty clay soil, but only 23% from the silt loam soil. In 1993, the detection frequency decreased to 26% in the silty clay soil drainage and 9% in the silt loam soil drainage (Dousset et al., 1996).

During the first drainage event (approximately 10 mm) in July 1992, large amounts of alachlor reached the drains of the silty clay soil (5000 mg ha–1) and silt loam soil (400 mg ha–1) 50 d after herbicide application (Fig. 4) . A second discharge event for alachlor occurred as a consequence of the high drainage event of November–December 1992 (approximately 80 mm) (Fig. 4), but it was smaller than the first event (500 mg ha–1) and occurred only in the silty clay soil.



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  		Fig. 4. Alachlor (–––) and bentazone (—) discharges vs. cumulative drainage for the two studied soils during the monitoring period (1992–1994).

 
Alachlor was discharged in drains of the silty clay soil in amounts of 160 mg ha–1 in June 1993, 45 d after application, and 250 mg ha–1 in November 1993 during a larger drainage event (approximately 24 mm). Thereafter, alachlor concentrations were lower than the detection limit until June 1994. In the drains of the silt loam soil, alachlor was detected through June 1993, but only once afterward in October 1993 (Fig. 4). During the winter rains, no alachlor was detected in the drains of the silt loam soil in 1992 and 1993, contrary to its behavior in the silty clay soil.

Total alachlor losses in the drainage were greater for the silty clay soil than the silt loam soil. The losses amounted to 0.28 and 0.02% of the applied rate in the silty clay soil and 0.02 and 0.00% in the loam soil over the 1992 and 1993 monitoring periods, respectively (Fig. 5 , Table 3).



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  		Fig. 5. Cumulative alachlor (—{circ}—) and bentazone (—{square}—) losses vs. cumulative drainage of the silty clay and silt loam soils during the monitoring period (1992–1994).

 

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  		Table 3. Herbicide losses in drains of the silty clay and silt loam soils during the monitoring periods.

 
Bentazone Discharge in Drainage
Bentazone was detected in 85% of the drainage samples analyzed in 1992 from the silty clay soil and 92% of the drainage samples from the silt loam soil. The detection frequency decreased to 52% in 1993 in drains of the silty clay soil and 51% in those of the silt loam soil (Dousset et al., 1996).

Huge amounts of bentazone (20000 mg ha–1) reached the drains of the silty clay soil in 1992 after the first drainage event (15 mm) in July 1992, compared with 350 mg ha–1 in the drains of the silt loam soil 15 d after treatment (68 mm). Bentazone was frequently detected in both soil drains through January 1993 with the greatest discharges following a 50-mm drainage event during November–December 1992 (Fig. 4).

After June 1993, bentazone was discharged in amounts of 100 and 50 mg ha–1 in the drains of the silty clay and silt loam soils, respectively. A second, greater discharge occurred in December 1993–January 1994 when 800 and 600 mg ha–1 bentazone were detected in the drains of the silty clay and silt loam soils, respectively.

Bentazone was recovered in greater amounts in the silty clay soil drains than in the silt loam soil drains. The losses were 2.40 and 0.29% of the applied rate in the silty clay soil and 0.11 and 0.19% in the silt loam soil during the 1992 and 1993 monitoring periods, respectively (Fig. 5, Table 3).

Field Persistence and Parent Molecule Dissipation
Alachlor and bentazone degradation usually fit a first-order model (Weed et al., 1998; Thorstensen and Lode, 2001):

where C is the soil concentration of parent molecule at time t (d), C0 is the concentration at time 0, and k is the rate constant (d–1).

Alachlor was more persistent than bentazone in the two soils studied (Table 4). The field persistence of alachlor was greater in the silty clay soil (20.7 d) than in the silt loam soil (17.7 d). Similarly, the field persistence of bentazone was greater in the silty clay soil (12.5 d) than in the silt loam soil (7.5 d). In neither of the two studied soils were herbicide residues detected below 5 cm (data not shown).


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  		Table 4. First-order kinetics parameters for loss of alachlor and bentazone from the two soils.{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Drain discharge occurred mainly in November–December and was greater for the silt loam soil (202 mm) during the wetter period (1993–1994) than during the drier period (1992–1993) (117 mm). Conversely, water flow in the silty clay soil was similar during both dry and wet events with the drain discharge reaching 170 mm during the 1992–1993 and 1993–1994 periods. Thus, regardless of the amount of rain, water flow through the silty clay soil was a function of soil infiltration velocity, which was lower in the silty clay soil than in the silt loam soil during fall and winter. Soils were near saturation and water migration in the silty clay soil was limited, partly due to clay swelling. Therefore, the excess water remained on the soil surface where evaporation or runoff from the plot could occur.

For the two monitoring periods, alachlor and bentazone were detected in spring just after treatment, in accordance with the results of Kladivko et al. (1991) for alachlor. The first herbicides detected reached the drain less than 25 d after application in 1992 and less than 50 d in 1993, with less than 1 cm net drainage from the soil. If the total pore volume of the unsaturated zone had been involved in the migration of these chemicals, approximately 30 cm of drainage would have been required before any surface-applied chemical reached the drain. The early herbicide detection is consistent with preferential-flow concepts and suggests that about 3% of the pore volume was active in rapid transport. Such presumptive evidence for macropore flow was also seen by Isensee et al. (1990) and Gish et al. (1991). Additional evidence for the migration of the herbicides by preferential flow was that both herbicides reached the drain at the same time despite differences in their sorption coefficients. More sorptive compounds would generally be expected to move more slowly and at attenuated concentrations. The same phenomenon has been reported by Kladivko et al. (1991), who showed that pesticides with different sorption coefficients such as alachlor, carbofuran, cyanazine, and atrazine drained from a silt loam soil at the same time.

Indeed, in a recent study, using metolachlor (similar chemical family to alachlor) on the same plots, Novak et al. (2001) showed larger losses of this molecule in the silty clay soil than in the silt loam soil during the spring and summer. The higher drainage of the silty clay soil in this period was explained by the formation of dessication cracks in the shrinking clay, which facilitated faster water flow. In addition, the discharged amounts of bentazone were lower than those of alachlor, except in the case of the silty clay soil in July 1992. Thus, our results are in agreement with the occurrence of preferential flow, and the amounts of alachlor and bentazone measured in the drains were not related to herbicide solubility or sorption coefficient.

During the subsequent drainage event in November–December, the herbicide losses were generally greater in the drains of the silty clay soil than in those of the silt loam soil, despite the lower volume of drainage from the silty clay soil. The reduced drainage from the silty clay soil during the fall–winter period was caused by near-saturation conditions that induced clay swelling and impeded water flow through the soil. The greater amounts of leached herbicide in the drain of the silty clay soil could be explained by the higher herbicide persistence in this soil. The amounts of bentazone in the drainage water, resulting from the winter matrix-flow drainage events, were generally higher than those of alachlor, possibly due to the higher solubility of bentazone (570 mg L–1) compared with alachlor (242 mg L–1) and to its lower sorption coefficient (Koc = 42 L kg–1) compared with that of alachlor (170 L kg–1). During the migration of water and herbicides through the soil matrix, interactions between the herbicides and soil may have occurred, resulting in decreased leaching of the herbicide due to sorption.

Pesticide transport through the soil down to the drains appeared to be event-driven and therefore could not be described as a simple function of time or net drain flow volume. Neither was pesticide transport a simple function of rain intensity (volume over time) because it also depended on factors such as the moisture content of the soil, temperature, and amount of time between rain events. Furthermore, since water samples were collected once a week, it was not possible to relate a specific rainfall event with the herbicides' behavior. In the spring–summer period, herbicide transport occurred by preferential flow, then in the fall–winter period, the herbicides migrated through the soil matrix possibly slowing their movement. During the monitoring periods, the losses of bentazone were higher (from 0.11–2.40% of the applied amount) than the losses of alachlor (from 0.00–0.28%) in the drains of the silty clay and silt loam soil. Comparison of bentazone losses in drains with other sites was not possible due to the lack of available data. The relative losses of alachlor in the silt loam soil were in agreement with the findings of Kladivko et al. (1991) and Jaynes et al. (1999) who showed no alachlor losses at all, either in the drain or in the drainage water of a watershed. Conversely, the 0.28% loss of alachlor recovered in the drain of the silty clay soil is in accordance with the results of Paterson and Schnoor (1992), who recovered 0.3% in drainage from a sandy soil. The higher losses of bentazone relative to alachlor could explain bentazone's shorter field persistence (7.5–12.5 d) compared with alachlor (17.7–20.7 d). It is also possible that the degradation of bentazone in the field was faster than that of alachlor, even though this would be contradictory to laboratory-measured half-lives, which were higher for bentazone than for alachlor (Pothuluri et al., 1990; Walker and Welch, 1991; Thorstensen and Lode, 2001).

In addition, more herbicide residues were detected in drains of the silty clay soil than of the silt loam soil. Working on the same plots with metolachlor, Novak et al. (2001) also showed that the losses of metolachlor were higher in the drains of the silty clay than in those of the silt loam soil. Our results might be explained by (i) greater preferential flow in the silty clay than the silt loam, at least during the spring–summer period, (ii) slower degradation of the herbicides in the finer textured soil, and (iii) drainage events being more significant in the silty clay than in the silt loam.

In the silty clay soil, higher water flow occurred during the first three months after application in 1992 than in 1993. However, the herbicide losses were greater in the drains of both soils in 1992 than in 1993, despite 1993 being wetter. This might be explained by the length of time between treatment and drainage events, which was longer in 1993 (150 d) than in 1992 (60 d), resulting in higher sorption and/or degradation of herbicides. In the 1992 monitoring period, 88% of alachlor and 95% of bentazone losses from the silty clay soil occurred within the three months following the application, compared with losses of only 1% for alachlor and 7% for bentazone losses from the silty clay in 1993. Similar behavior was observed in the silt loam soil. In their studies, Gaynor et al. (1995) and Isensee et al. (1990) also observed that the timing of rainfall relative to the date of herbicide application was critically important to pesticide leaching.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In this field-scale study, alachlor and bentazone reached the drains of the silty clay and the silt loam to different extents. The drainage occurred in two successive phases: (i) by preferential flow during the spring–summer period, with very fast migration of the two herbicide molecules and (ii) by matrix flow during the fall–winter period when the soils were water-saturated, with lower migration of the herbicides, depending on the respective potential interactions of alachlor and bentazone with the soil matrix.

The range of herbicide losses varied from 0.1 to 2.4% for bentazone and 0.00 to 0.28% for alachlor in the drains of the two soils. Thus, the total mass of bentazone reaching the drains was more than 10 times higher than that of alachlor, which was probably due to the lower adsorption coefficient of bentazone. These results would explain the lower field persistence of bentazone (7–12 d) compared with that of alachlor (18–21 d) in the 0- to 5-cm soil layer. In addition, the differences in herbicide losses between the two soils and the two surveyed years could be related to the magnitude of preferential flow and the time elapsed between the treatment and the first drainage events.

These results indicate that the field-scale monitoring of herbicide losses in drains is necessary to compare the behavior of each molecule in different kinds of soils and its reproducibility throughout at least two planting periods. Such an approach allows the leaching risk of each molecule to be reliably assessed. The risk of water contamination was shown to be greater for bentazone than for alachlor. With the September 2003 ban of atrazine in France, the use of the alachlor–bentazone combination for maize protection will probably increase, possibly resulting in a new environmental risk. The threat of contamination would be immediate due to the significant fast leaching of bentazone as well as delayed due to the slow migration of alachlor.


   ACKNOWLEDGMENTS
 
The authors would like to thank A. Jacobson (UMR INRA A111, Dijon, 21) for her valuable comments on the manuscript. We also thank Météo France (Tomblaine, 54) for providing the meteorological data and J.M. Portal and T. Orel of CNRS (Nancy, 54) for their analytical contribution. This work was financially supported in part by the Agence de l'Eau Rhin-Meuse.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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