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a Environment Canada, 300-2365 Albert St., Regina, SK, Canada S4P 4K1
b National Water Research Institute, 11 Innovation Blvd, Saskatoon, SK, Canada S7N 3H5
* Corresponding author (allan.cessna{at}ec.gc.ca)
Received for publication December 16, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HPLC, high performance liquid chromatography • LC, liquid chromatography • MS-MS, tandem mass spectrometry • ESI, electrospray ionization • MRM, multiple reaction monitoring
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Knowledge of the fate and behavior of these herbicides in prairie wetlands is essential for a number of reasons. First, millions of small wetlands (less than two hectares) are situated within agricultural areas of the northern Great Plains of North America where herbicides are regularly used (Donald et al., 1999). A large number, therefore, would be susceptible to surface runoff, herbicide drift deposition, and occasional overspraying of sulfonylurea herbicides. Second, sulfonylurea herbicides exhibit increased solubility in alkaline waters (WSSA, 1994) like those characteristic of the prairie landscape. And finally, because sulfonylurea herbicides are highly phytotoxic, environmentally relevant concentrations may significantly reduce primary production or alter plant communities in these highly productive prairie aquatic ecosystems.
Dissipation of sulfonylurea herbicides in aquatic ecosystems may occur in several ways. The sulfonylurea linkage can be cleaved by pH-dependent hydrolysis (reviewed by Sarmah and Sabadie, 2002). Generally, acid-catalyzed hydrolysis is rapid, whereas neutral conditions result in little or no hydrolysis, and base-catalyzed hydrolysis occurs primarily with the pyridine-2-sulfonylurea herbicides. Although sunlight photolysis of sulfonylurea herbicides is known to occur (Choudhury and Dureja, 1996; Pusino et al., 1999; Bhattacherjee and Dureja, 1999; Saha and Kulshrestha, 2002), photodegradation in prairie wetlands may be significantly reduced due to sunlight attenuation by high concentrations (usually > 10 mg L–1) of dissolved organic C characteristic of these water bodies (Arts et al., 2000). Sulfonylurea herbicides can also undergo microbial and abiotic transformations (Sarmah and Sabadie, 2002). All three degradation processes tend to produce the same products; that is, the corresponding sulfonamides and heterocyclic amines.
Possible nondegradation routes of sulfonylurea herbicide dissipation include partitioning into sediments, uptake by aquatic organisms, and loss to pond margins and/or groundwater during hydrological discharge. The latter may be of particular importance in prairie wetlands. Prairie wetlands generally occur in terminal basins (Winter et al., 2001) and one of the major water loss processes determining overall water balance is discharge to wetland margins and/or ground water (LaBaugh et al., 1996). Little is known regarding the dissipation of these herbicides within prairie aquatic ecosystems.
The objectives of this study, then, were twofold: First, to investigate the persistence of three commonly used sulfonylurea herbicides in prairie aquatic ecosystems, and second, to determine whether sulfonylurea herbicide dissipation might be affected by wetland hydrology. Three farm dugouts were selected as wetland surrogates. Each was then fortified with an environmentally relevant concentration of a single sulfonylurea herbicide. The selected herbicides, thifensulfuron-methyl {methyl 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylate}, ethametsulfuron-methyl {methyl 2-[[[[[4-ethoxy-6-(methylamino)-1,3,5-triazin-2-yl]amino]carbonyl]amino]sulfonyl]benzoate}, and metsulfuron-methyl {methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoate} demonstrate a range in soil persistence (Berger et al., 1998). Herbicide persistence in the water column of these dugouts was subsequently monitored until concentrations approached the limit of quantification (10 ng L–1). Additionally, the relationship between chloride ion (a conservative tracer commonly used to delineate hydrological processes in wetland ecosystems) and herbicide concentration was investigated to determine if water loss via hydrological discharge might be a possible herbicide dissipation route.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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8) and can be important staging and nesting areas for native wildlife and for local and migratory birds (Lokemoen, 1973; Hudson, 1983; Bélanger and Couture, 1988, 1989). Dugouts and wetlands are generally located on or immediately adjacent to tilled farmland and, as such, represent a worst case scenario with respect to contamination by pesticides applied in their immediate vicinity. The use of farm dugouts also had the advantage that their morphology improves sample collection consistency while eliminating sediment disturbance during sample collection.
Four farm dugouts south of Regina, Saskatchewan were selected for study. Three were fortified with a single sulfonylurea herbicide while the fourth dugout served as a control. The greatest distance between any two dugouts was 3 km. Detailed physical and chemical characteristics of the dugouts (e.g., dimensions and several water quality parameters) are provided in online supplementary Table S-1 (refer to online supplementary Fig. S-1 and Tables S-1
through S-5).
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For herbicides which persist in soil and carryover to the next growing season, recropping intervals define minimum intervals between herbicide application and the seeding of sensitive crops (e.g., lentil). Based on recommended recropping intervals (SAFRR, 2003), the three sulfonylurea herbicides selected for study represent a range of soil persistence. Thifensulfuron-methyl is the least persistent and requires no recropping interval. Depending on the subsequent crop grown, ethametsulfuron-methyl requires 10- to 22-mo recropping intervals, whereas those for metsulfuron-methyl range from 10 to 48 mo depending on the crop, soil type, and pH of the soil.
Commercially available formulations were used in this study and included Champion Plus: "Plus," 75% thifensulfuron-methyl dry flowable; "Champion FM," 45 g L–1 fenoxaprop-ethyl ((±)-ethyl 2-[4-(6-chloro-2-benzoxazolyl)oxy]phenoxy)propanoate)), 210 g acid equivalent (a.e.) L–1 MCPA [(4-chloro-2-methylphenoxy)acetic acid] iso-octyl ester, and 70 g a.e. L–1 2,4-D [(2,4-dichlorophenoxy)acetic acid] iso-octyl ester as an emulsifiable concentrate, Dupont Canada; Ally Toss-N-Go: 60% metsulfuron-methyl dry flowable, Dupont Canada; and Muster Gold II: "Gold," 75% ethametsulfuron-methyl dry flowable; "Assure/Assure II," 96 g L–1 quizalofop-ethyl (ethyl (RS)-2-[4-[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propionate)/quizalofop-P-ethyl (ethyl (R)-2-[4-[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propionate) as an emulsifiable concentrate, Dupont Canada. From a survey of the farmers on whose land the dugouts were located, none of the three study herbicides were applied to any of the dugout catchments either during the previous growing season (2002) or in the study year (2003).
Fortification Target Concentrations
Inadvertent wetland overspraying using the recommended herbicide application rate would represent a worst case scenario for wetland contamination. For this study, we assumed direct overspray of a small theoretical wetland (0.5-m depth) with no contribution of additional herbicide to the wetland from surface runoff or application drift. The resulting concentration became the target concentration for each sulfonylurea herbicide in its respective fortified dugout.
For example, dugout A, which had a surface area of 817 m2 or 0.0817 ha on the day of application, would require 1.23 g of thifensulfuron-methyl at the recommended application rate of 15 g ha–1. This mass of herbicide divided by the volume of the upper 0.5-m layer (817 m2 by 0.5 m = 408 500 L) yielded a target concentration of 3000 ng L–1. Target concentrations for ethametsulfuron-methyl (dugout B) and metsulfuron-methyl (dugout C) were similarly calculated to be 3000 and 900 ng L–1, respectively.
All of the dugouts, however, were deeper than 0.5 m. Consequently, their volumes were estimated using the formula for water basins described by an elliptic sinusoid (Wetzel, 1975)
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Dugout Fortification
Calculated amounts of each herbicide were mixed into approximately 45 L of water. The aqueous herbicide mixtures were transferred to a hand-operated sprayer equipped with a 15-L tank and a 1.5-m wand and then injected under low pressure into the water column of each dugout from a rubber dingy. To achieve a relatively homogeneous herbicide application, the dingy was pulled using ropes in a grid pattern over the width and length of each dugout while herbicide solution was injected at varying depths by moving the wand tip from just beneath the water surface to approximately 2 m deep. Relatively high wind speeds on the day of application enhanced herbicide mixing. As a result, relatively uniform concentrations were obtained 1 d after application (standard deviations ranged from ± 1.2 to ± 5.6% of mean concentrations; n = 3).
Sample Collection
During ice-free periods, water samples were collected from a dingy positioned at the center of each dugout. An iron-weighted sample bottle holder, which dropped quickly to a 0.5-m depth, was used to collect samples into precleaned 1-L amber glass bottles (VWR International; TraceClean bottles). The bottles were only used once. In winter, samples were similarly collected via a hole drilled through the ice at the center of each dugout.
At regular intervals, a field blank was collected from an artesian spring located near Nokomis, Saskatchewan. According to 14C dating of dissolved inorganic C, water from the spring was at the surface of the earth >40 000 years ago.
On 26 June 2003, a water sample was collected from each of the four dugouts before fortification on 27 June. On 28 June, samples were collected from three different points in each dugout to determine the homogeneity of the sulfonylurea herbicide application. Samples were then collected on 29 and 30 June and on 2, 4, and 7 July, weekly until mid-August and then monthly until mid-October. Following ice formation, samples were collected on 18 November, 16 December, 29 January 2004, and 25 February. After spring snow melt runoff and ice melt, the dugouts were sampled on 6 May 2004 and then monthly until July when sampling was terminated.
Sample Extraction
For sulfonylurea herbicide concentrations >500 ng L–1, sufficient sensitivity for reliable quantitation could be achieved by direct injection (20 µL) of dugout water samples into the liquid chromatography tandem mass spectrometry (LC-MS-MS) system.
For sulfonylurea herbicide concentrations <500 ng L–1, dugout water samples (500 mL) were passed (10 mL min–1) through solid-phase extraction cartridges under a vacuum of 400 mm of Hg. The cartridges contained 225 mg of a copolymer sorbent (60-µm-diam. particle size) designed to have a hydrophilic-lipophilic balance (Oasis HLB extraction cartridges, Waters Corporation, Milford, MA) and were conditioned sequentially with acetone (10 mL), methanol (10 mL), and then water (10 mL). After sample loading, the cartridges were washed with nanopure water (10 mL) and then dried for 1 h under vacuum. The cartridge was then eluted with 95:5 acetone/methanol (10 mL) and the eluate evaporated to dryness using a stream of dry N gas. Resulting residue was redissolved in acetonitrile (1 mL) and transferred to a 2-mL HPLC vial. Nanopure water (0.5 mL) was added to the extract just before LC-MS-MS analysis.
Extract Analysis
Liquid Chromotography Separation
A Waters 2695 Alliance HPLC system was used with a 2.1-mm i.d. by 100-mm Waters Xterra Mass C18 (3.5-µm-diam. particle size) analytical column which was maintained at 30°C. Mobile phase consisted of the following: solvent A, 90:10 (v/v) water/acetonitrile, and solvent B, 90:10 (v/v) acetonitrile/water. Both solvents A and B contained 0.1% formic acid and 2 mM ammonium acetate. Isocratic elution of the column with 70% solvent A and 30% solvent B at a flow rate of 200 µL min–1 resulted in retention times of 4.13, 4.69, and 6.54 min for thifensulfuron-methyl, metsulfuron-methyl, and ethametsulfuron-methyl, respectively. All injection volumes were 20 µL.
Electrospray Ionization Tandem Mass Spectrometry
The sulfonylurea herbicides were quantitated and their presence confirmed using the Micromass Quattro Ultima triple quadrupole mass spectrometer equipped with an electrospray ionization (ESI) interface set to positive ion mode. Ionization and MS-MS conditions were optimized by infusing a 0.5-mg L–1 solution of each sulfonylurea herbicide into the ion source in a 50:50 (v/v) acetonitrile/water solution with a syringe pump. The parent ion for each herbicide (M+H) was selected for collision-induced dissociation using the first quadrupole. The second quadrupole, into which argon gas was introduced, functioned as a collision cell and the third quadrupole was used to monitor the resulting major product ion.
Suitable multiple reaction monitoring (MRM) transitions were chosen from the product ion scans (m/z 338.3:167.2 for thifensulfuron-methyl; m/z 382.2:167.2 for metsulfuron-methyl; m/z 411.3:196.3 for ethametsulfuron-methyl). These transitions corresponded to cleavage of the sulfonylurea linkage between the carbonyl group and the amino N attached to the sulfonyl moeity such that the ion used for quantification of each sulfonylurea herbicide contained the substituted triazine ring. Since the triazine rings of thifensulfuron-methyl and metsulfuron-methyl were substituted similarly with methyl and methoxy substituents (supplementary Fig. S-1), the same fragment ion (m/z 167.2) was monitored for quantification of both herbicides. Instrumental conditions were as follows: source temperature, 90°C; capillary voltage, 4.39 kV; hex 1 voltage, 6.9 V; hex 2 and aperture voltage, 0V; desolvation temperature, 220°C; N desolvation gas flow rate, 488 L h–1; N cone gas flow rate, 145 L h–1; N nebulizer gas flow rate was at maximum flow; multiplier voltage, 650 V; and the interchannel delay was 0.10 s while dwell time ranged from 0.20 to 0.50 s for the three time-dependent MRM channels. Argon was used as the collision gas at a pressure which increased the Pirani gauge reading to 3.53 x 10–4 torr. The cone voltage (12 to 16 V) and collision energy (23 to 26 eV) were dependent on the MRM channel. Resolution was set to achieve unit mass resolution for quadrupole 1 and approximately 2-amu resolution for quadrupole 3.
Laboratory Fortification Experiments
Fortification experiments were performed to determine the recovery of the three sulfonylurea herbicides from deionized water and dugout water. Water (500 mL) fortified with 5 or 50 ng of each sulfonylurea herbicide dissolved in 100 µL of acetonitrile, resulted in concentrations of 10 and 100 ng L–1, respectively. Mean recoveries of thifensulfuron-methyl, ethametsulfuron-methyl, and metsulfuron-methyl from deionized water were 84 ± 4%, 72 ± 13%, and 93 ± 6%, respectively, at 100 ng L–1 (n = 13) with corresponding values of 100 ± 20%, 81 ± 14%, and 100 ± 8% at 10 ng L–1 (n = 13). Corresponding mean recoveries from dugout waters fortified at 100 ng L–1 were 126 ± 40%, 82 ± 11%, and 115 ± 27% (n = 10), whereas those fortified at 10 ng L–1 were 160 ± 30%, 130 ± 20%, and 160 ± 60% (n = 10). The high recoveries from the fortified dugout waters may indicate ionization enhancement in the source of the mass spectrometer due to the relatively high dissolved organic matter content of these waters (supplementary Table S-1). Herbicide recoveries indicated that the solid-phase extraction method was effective for both deionized and dugout waters, and when coupled with ESI tandem mass spectrometric quantification and confirmation, provided reliable recoveries down to 10 ng L–1. The instrument detection limit for each of the three sulfonylurea herbicides was in the order of 10 pg (equivalent to 1 ng L–1).
Normalization of Sulfonylurea Herbicide Concentrations
Not all routes of sulfonylurea herbicide dissipation were investigated in the present study. Consequently, the time required for 50% herbicide dissipation from the water column is expressed as DT50 rather than as a half-life. Half-life generally implies some modification or degradation of the parent molecule such that the parent molecule is no longer present in any compartment of the environment.
Before DT50 determination for each herbicide, sulfonylurea herbicide concentrations were normalized (using chloride ion as a conservative tracer) to account for evaporation effects. For this calculation, the following assumptions were made: increases in chloride ion concentrations in the dugout water were caused by evaporation, increases in chloride ion mass were caused by hydrological recharge from ground water, and decreases in chloride ion mass were caused by hydrological discharge to ground water (Webster et al., 1996). These relationships were also used to investigate whether hydrological processes played a role in herbicide dissipation from the dugout water.
Water volume decreases due to evaporation were considered equivalent to increases in chloride ion concentration relative to that on the day of herbicide application. Normalized herbicide concentrations (Cnorm) were therefore calculated by multiplying the measured concentration (Cmeas) by the evaporation factor (Fevap)
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RESULTS AND DISCUSSION |
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Target Sulfonylurea Herbicide Concentrations
The initial mean concentrations (n = 3; ± standard deviation) detected 1 d after herbicide application were 3160 ± 180, 4630 ± 60, and 990 ± 20 ng L–1, for thifensulfuron-methyl, ethametsulfuron-methyl, and metsulfuron-methyl, respectively. With the exception of the smallest dugout, initial concentrations were within 10% of the target concentrations.
Dugout Hydrology
Details of dugout volume, chloride ion mass and concentration, evaporation factor, measured and normalized herbicide concentrations, volume of dugout water lost by evaporation and hydrological discharge, and mass of herbicide lost during hydrological discharge are presented in supplementary Tables S-2, S-3, and S-4 for thifensulfuron-methyl, ethametsulfuron-methyl, and metsulfuron-methyl, respectively. From June to October, all dugouts experienced marked decreases in water depth. During this period, limited rainfall (90 mm) was not sufficient to generate surface runoff to the dugouts. Consequently, observed water volume decreases were due to evaporation and/or hydrological discharge. Water volume in the largest and mid-sized dugout decreased by 54 and 28%, respectively, whereas the smallest dugout had gone dry by 21 August.
Dugout B showed continuous hydrological discharge, whereas dugout C showed two periods of discharge (herbicide application to mid-July and mid-September to mid-October) and the intervening period with neither discharge nor recharge. In both dugouts, decreases in water volume were accompanied by increases in chloride ion concentration and/or decreases in chloride ion mass. Decreasing volume was inversely correlated with increasing chloride ion concentration (r2 = 0.932 and 0.992, respectively) and directly correlated with decreasing chloride ion mass (r2 = 0.999 and 0.983, respectively). These correlations suggested that chloride ion concentration increased due to evaporative water loss, whereas chloride ion mass decreased because of hydrological discharge. It was estimated that discharge accounted for 87 and 84% of the volume decrease in dugouts B and C, respectively, while corresponding evaporative losses were 13 and 16%. Relatively higher evaporative losses occurred from the smaller dugout most likely because of a higher (1.3 vs. 0.4) surface area to volume ratio. There was no evidence of recharge from ground water.
Dugout A was the most hydrologically complex of the treated dugouts with continuously alternating periods of water loss due to discharge and water gain due to recharge. Because of recharge, the overall water volume decrease was only 28%, and from 4 July to 18 July, the water volume did not change, indicating that recharge plus rainfall (29 mm) were offset by loss due to evaporation.
In some cases, chloride ion concentration increases were accompanied by simultaneous chloride ion mass decreases indicating simultaneous occurrence of evaporation and discharge. Volume decrease between two consecutive sampling dates due to discharge was determined using a chloride ion mass factor (FCl) where FCl was equal to chloride ion mass for a given sampling day divided by the chloride ion mass on the previous sampling day. Multiplying the volume decrease by FCl gave the volume decrease due to discharge. The evaporative volume was then obtained as the difference between the volume decrease between consecutive sampling dates and that due to discharge.
Sulfonylurea Herbicide Persistence
From 28 June to 17 October
Following dugout fortification, water column concentrations of all three sulfonylurea herbicides decreased with time (Fig. 1). Herbicide mass in the water column also decreased continuously during this period, and at 112 d after herbicide application, overall decreases were 99 and 74% for thifensulfuron-methyl and metsulfuron-methyl, respectively. At 45 d after application, just before dugout B went dry, the overall decrease in ethametsulfuron-methyl mass was 95%. Observed decreases in herbicide mass may have been due to destructive processes (microbial degradation, photolysis, hydrolysis) or nondestructive processes involving movement to another environmental compartment (uptake by aquatic organisms, loss during hydrological discharge, partitioning to sediments). None of these processes were measured in this study.
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Over the 112-d study period (June to October), the DT50 of thifensulfuron-methyl and metsulfuron-methyl were 16 and 84 d, respectively. The DT50 of 84 d for metsulfuron-methyl is significantly longer than that (29.1 d) reported for boreal forest lake water by Thompson et al. (1992) and may reflect the higher pH of the dugout water (lake water pH ranged from 6.7 to 7.3 vs. a mean value of 8.5 ± 0.5 for dugout C). The DT50 of ethametsulfuron-methyl (30 d) was determined from the 45-d period between application and 11 August when the last water sample was collected (just before the dugout went dry). The decreasing order of the DT50 values (metsulfuron-methyl>ethametsulfuron-methyl>thifensulfuron-methyl) reflects the relative persistence of the herbicides in soil (see recommended recropping intervals; SAFRR, 2003).
In all three dugouts, herbicide mass and water volume decreases were directly correlated, suggesting that a portion of the decrease in herbicide mass in each dugout was associated with hydrological discharge. Indeed, the strongest correlations were observed for dugouts B (r2 = 0.988) and C (r2 = 0.982) and the weakest for dugout A (r2 = 0.901) which experienced extensive recharge. For dugouts B and C, it was estimated that 53 and 43% of the mass decrease for ethametsulfuron-methyl and metsulfuron-methyl, respectively, was due to discharge, with the remainder of loss of each herbicide due to other routes of dissipation. Since 40 to 50% dissipation of these two herbicides may simply have been movement to other environmental compartments (e.g., sediments, ground water) during discharge, their DT50 values may be an underestimation of their actual environmental half-lives. In dugout A, in which extensive recharge occurred, only 9% of the thifensulfuron-methyl decrease was due to discharge with 91% due to other routes of dissipation.
Under Ice Cover
Herbicide concentrations in the dugout water during winter and following snow melt runoff are provided in supplementary Table S-5. In winter under ice, concentrations of thifensulfuron-methyl and metsulfuron-methyl in the dugouts continued to decrease (Fig. 1). By 5 March, when the last water sample was taken through the ice, concentrations had decreased to 30 and 70 ng L–1, respectively. Field observations suggested that, during ice cover, water volumes (ice plus liquid water) did not change significantly.
During winter, the rate of thifensulfuron-methyl dissipation was markedly reduced (Fig. 1) compared to the open water season. This biphasic dissipation most likely occurred because colder water temperatures would have slowed several dissipation processes, including hydrolysis of the sulfonylurea and methyl ester linkages, microbial degradation, and uptake by aquatic organisms. It is not clear why initial thifensulfuron-methyl concentrations were higher than the October concentration or why dissipation of metsulfuron-methyl was somewhat enhanced under ice cover and colder temperatures.
Thifensulfuron-methyl had a DT90 of approximately 50 d and was more than 90% dissipated from the water column by mid-August, well before ice cover formation in the fall (Fig. 1). In contrast, 90% dissipation of metsulfuron-methyl did not occur until well after ice formation at approximately 200 d.
After Snow Melt Runoff
In May following spring snow melt runoff, measured concentrations of thifensulfuron-methyl and metsulfuron-methyl in the dugout waters were lower than the corresponding concentrations in March. Decreased concentrations were most likely due to dilution with snow melt runoff water; May 2004 water volumes in dugouts A and C increased 62 and 79%, respectively, above October 2003 volumes. When normalized to October volumes (Fig. 1; see also supplementary Table S-5), May 2004 concentrations of thifensulfuron-methyl and metsulfuron-methyl were substantially lower than the respective October concentrations and essentially unchanged from March values. Normalized concentrations of both herbicides generally decreased continuously from May to July when the last water sample was collected.
Interestingly, ethametsulfuron-methyl was again detected when dugout B, which went dry in August 2003, refilled with spring meltwater (Fig. 1; see also supplementary Table S-5). This suggests that some sulfonylurea herbicide was retained in surficial sediments during hydrological discharge and entered the water column after spring runoff. Its persistence in the exposed (aerobic) dugout sediment from late August 2003 until spring runoff in 2004 is supported by the 23-mo recropping interval recommended for sensitive crops (SAFRR, 2003). The concentration of ethametsulfuron-methyl also decreased continuously from May to July.
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CONCLUSIONS |
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Although other routes of dissipation were not investigated in this study, persistence of the three sulfonylurea herbicides in dugout water for more than 12 mo suggests that there was relatively slow partitioning of these compounds into bottom sediments and little photolytic degradation or uptake by aquatic vegetation.
Herbicide dissipation, as indicated by the DT50 values, was also dependent on the chemical characteristics of each sulfonylurea herbicide. The DT50 values determined in this study, which ranged from 16 to 84 d, reflect the relative stability of the corresponding sulfonylurea and methyl ester linkages to hydrolysis under neutral (pH = 7) and weakly alkaline (pH = 10) conditions (Berger and Wolfe, 1996). They also reflect recropping intervals recommended for the Canadian prairies (SAFRR, 2003). Similar relative persistence has been reported for sterilized and nonsterilized aquatic sediments (Berger and Wolfe, 1996) and field soil (Berger et al., 1998).
The persistence of thifensulfuron-methyl, ethametsulfuron-methyl, and metsulfuron-methyl in prairie pothole wetlands will be similar to that observed in the farm dugouts. Because of the high phytotoxicity of these herbicides and their observed persistence in prairie dugout water, environmentally relevant concentrations of these herbicides may have a significant effect on aquatic algae, macrophytes, and food chains in prairie wetlands.
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ACKNOWLEDGMENTS |
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