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a Environment Canada, 300-2365 Albert St., Regina, SK, Canada S4P 4K1
b Research Center, Agriculture and Agri-Food Canada, Lethbridge, AB, Canada T1J 4B1
c Research Station, Agriculture and Agri-Food Canada, Regina, SK, Canada
d Research Station, Agriculture and Agri-Food Canada, Regina, SK, Canada
e Faculty of Engineering, University of Regina, Regina, SK
* Corresponding author (Don.Waite{at}ec.gc.ca)
Received for publication May 3, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMETHODSRESULTS AND DISCUSSIONGENERAL DISCUSSION AND...REFERENCES |
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Abbreviations: PUF, polyurethane foam
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONGENERAL DISCUSSION AND...REFERENCES |
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Although 2,4-D may be applied as a low-volatile (iso-octyl or butoxyethyl) ester or amine (dimethylamine) salt, these are eventually hydrolyzed to the free acid in the environment. 2,4-D and triallate exhibit different chemical and physical properties and, consequently, different behavior in the environment. Triallate is less soluble than 2,4-D but has a much higher vapor pressure (Table 1). Triallate also has a longer field half-life than 2,4-D (82 d for triallate versus 10 d for 2,4-D), and is more strongly adsorbed to the soil than 2,4-D (Koc = 2400 mL g-1 for triallate versus 20–60 mL g-1 for 2,4-D; Wauchope et al., 1992). The 2,4-D amine salts are essentially nonvolatile but much more water soluble than the acid (Table 1), and the low-volatile esters are more volatile but less water soluble than the acid. The form in which 2,4-D is present will determine its behavior in the environment.
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Previous Saskatchewan studies have reported pesticide concentrations in air, water, and bulk atmospheric deposits (e.g., Cessna et al., 2000; Grover, 1991; Waite et al., 1992, 1995, 1999). It is, however, also important to consider the role of surface film in the dynamic movement of pesticides from the atmosphere into aquatic ecosystems (Waite et al., 2000). The organic surface film that floats on top of surface water bodies acts as an interface between the atmosphere above and the water below. Contaminants entering surface waters from the atmosphere must first pass this layer before partitioning or mixing into the underlying water. Various authors have reported a number of contaminants from surface films, including nutrients (Eisenreich and Armstrong, 1977; Eisenreich et al., 1978); heavy metals (Eisenreich et al., 1978; Hardy et al., 1985; Westernhagen et al., 1987); and anthropogenic organic compounds including pesticides (Larsson et al., 1974; Eisenreich et al., 1978; Westernhagen et al., 1987; Maguire and Tkacz, 1988; Davey et al., 1990; Muir et al., 1991; Chernyak et al., 1996; Liu and Dickhut, 1997). It is of interest, then, to compare the quantities of herbicides in surface film with the quantities deposited to the water surface and with the concentrations in the underlying water.
The purpose of this study was to make this comparison with herbicides that have different physico–chemical properties and markedly different behavior in the environment. Using two prairie farm dugouts, quantities of 2,4-D and triallate were measured in air, bulk (wet plus dry) atmospheric deposits, surface film, and water to investigate links between atmospheric concentrations and bulk deposition rates and their relationships with quantities measured in surface film and water. These two dugouts were surrounded for hundreds of kilometers by agricultural land to which both herbicides were applied. Regardless of direction, wind action could potentially transport the herbicides to the region of the dugout. For this reason the study concentrated on the relative concentrations of the herbicides in the four environmental compartments reported rather than on atmospheric trajectories.
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METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONGENERAL DISCUSSION AND...REFERENCES |
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Sampling Methods
Ambient Air
Duplicate high-volume air samplers (Model PS-1; General Metals Works, Village of Cleves, OH), designed to collect airborne organic chemicals including herbicides, were used in this study. The air samples were collected 1 m above the ground. The air flow through the samplers (8.33 m3 h-1) was calibrated in the field using a Sierra-Misco (Berkeley, CA) orifice head and was monitored daily during operation by integral Venturi/Magnahelic assemblies. The samplers were operated for a 12-h period daily (0500 to 1700 h) for 7 d each week for a total air flow of about 700 m3 wk-1.
The sampling unit in the high-volume PS-1 samplers consisted of a 102-mm-diameter borosilicate microfibre filter (Micro Filtration Systems, Dublin, CA) in front of a polyurethane foam (PUF)/XAD-2 resin composite cartridge. The cartridge contained 25 mL of XAD-2 adsorbent resin (Caledon Laboratories Ltd., Georgetown, ON) sandwiched between 50-mm (upstream) and 25-mm (downstream) foam (polyester-type PUF) plugs. The PUF/XAD-2 resin sandwich was housed in a 60-mm-i.d. by 125-mm-long glass cylinder with a stainless steel wire mesh support screen. The filter and the composite cartridge collected particulate and vapor components of the air, respectively. Prior to assembly of the sampling cartridges for field use, the PUF plugs and the XAD-2 adsorbent were solvent-cleaned by Soxhlet extraction and monitored for background interferences. Filters and PUF/XAD-2 resin cartridges were analyzed separately.
Bulk Atmospheric Deposition
Bulk atmospheric deposition samples were collected using duplicate pan samplers, as described by Waite et al. (1995). Each sampler consisted of a 1- by 1-m sheet metal (galvanized steel) pan, with 15-cm-high sides and a centrally located drain hole, supported on a wooden enclosure that elevated the sampler surface about 60 cm above ground level. During rainfall events, the pans drained directly into 22.7-L glass carboys, carrying the rainfall sample as well as dry surface deposits that may have accumulated during the sampling period. The glass carboys were wrapped with aluminum foil to eliminate photolysis losses. Rain gauges were located beside the samplers and precipitation was recorded weekly. When there was <4 mm rainfall during the sampling period, the pan was rinsed with 4 L of distilled water to obtain the bulk deposition sample for analysis. When rainfall exceeded 4 mm during the sampling period, the total bulk deposition sample was collected from the carboy, without rinsing the collector pan, and the volume recorded.
Surface Film
Weekly sampling was carried out from a boat positioned centrally in each dugout. Surface film was sampled, following the method of Muir et al. (1991) and Waite et al. (2000), using a 20- by 20-cm Pyrex glass plate with a handle fastened to one side by means of silicon caulking. The lower plate surface was touched, horizontally, onto the surface of the water and then lifted and held vertically, one corner down, over a glass funnel placed in a 500-mL, wide mouth, amber glass bottle. Using a wash bottle, the glass plate was rinsed, into the funnel, with approximately 25 mL of dichloromethane (CH2Cl2). Five such subsamples were collected for each composite sample. The bottles were sealed with aluminum foil, capped, placed in a cooler, and returned to the laboratory where they were stored at -20° C until analysis. Two composite samples were collected, for each sampling day, from each dugout.
In 1990, two sets of duplicate surface film samples were collected from Dugout 1. One set was collected as described above and the second set by using two rafts to stabilize the dugout surface and quench wave action. The rafts were constructed of 55 x 100 x 5 cm (thick) rigid foam insulation board in which five holes, 25 x 25 cm, were cut. The walls of the holes were lined with glass but were left open to the air and the water. Surface film samples were collected, as described above, by dipping the glass sampling plate sequentially into each hole. The rafts were anchored in the center of the dugout.
Dugout Water
Water samples were collected directly into new, 4.5-L amber glass bottles by plunging the bottle to a depth of 0.25 m and then holding it while it filled with 1 to 2 L of water. The bottles were sealed with aluminum foil, capped, and stored, on their sides, at -20° C until analysis.
Extraction Methods
Ambient Air (Filter and PUF/XAD-2 Resin Cartridge Samples)
The glass fiber filters were Soxhlet-extracted with 300 mL acetone for 6 h. The assembled PUF/XAD-2 resin cartridges were similarly extracted with 700 mL acetone for 16 h. The individual acetone extracts were concentrated to <10 mL using a rotary evaporator, and transferred to 50-mL graduated centrifuge tubes along with four 4-mL acetone rinses of the rotary evaporator flask. The individual extracts were further concentrated to <1 mL with a stream of nitrogen and then methylated with ethereal diazomethane. The methylated extracts were subjected to Florisil column (Fisher Scientific, Edmonton, AB, Canada) cleanup prior to gas chromatographic analysis.
Bulk Deposition and Dugout Water Samples
Aqueous bulk deposition samples and dugout water samples were extracted in the same manner. Each sample was thawed and shaken to suspend any sediments, and an unfiltered aliquot (500 mL) was extracted following established methods of liquid–liquid extraction (Cessna et al., 1985; Grover et al., 1997).
Surface Film Samples
Extraction methods for surface film samples have been described in detail by Waite et al. (2000) and will be summarized here. Surface film samples consisted of approximately 125 mL of CH2Cl2 and 35 mL of water. Each sample was transferred to a 250-mL separatory funnel. The water volume was brought to 100 mL with deionized (nanopure) water and the pH adjusted to 13 to 14 by the addition of 1 mL of 20% sodium hydroxide solution. An additional 25 mL of CH2Cl2 was added, the flask shaken for 2 min to extract the neutral herbicide triallate, and the CH2Cl2 was passed through anhydrous sodium sulfate to remove residual water. The extraction of the aqueous phase was repeated with a second portion of CH2Cl2 (25 mL) and the aqueous phase retained for extraction of 2,4-D. The volume of the combined CH2Cl2 extracts, together with a 25-mL CH2Cl2 rinse of the sodium sulfate, was reduced to approximately 2 mL using a rotary evaporator and transferred to a 50 mL graduated centrifuge tube along with four 2-mL rinses of the evaporating flask. The extract volume was then reduced to approximately 2 mL using a gentle stream of dry nitrogen gas. Isooctane (2 mL) was added and the volume again reduced to 2 mL. This step was repeated two more times. The isooctane extract was transferred to a Florisil (deactivated with 5% water) clean-up column and the column eluted with 0.5% acetone in hexane (Cessna et al., 1985). The column eluate (10 to 30 mL) was concentrated to less than 1 mL and then taken to volume (1 mL) with hexane prior to gas chromatographic analysis.
The aqueous phase was acidified to pH < 2, by addition of 1 mL of concentrated H2SO4, for extraction of 2,4-D. Sodium chloride (20 g) was added to the acidified aqueous phase, which was then extracted twice with diethyl ether (50 mL). The combined ether extracts were passed through anhydrous sodium sulfate and, together with a 25-mL diethyl ether rinse of the sodium sulfate, concentrated just to dryness using a rotary evaporator. Acetone (25 mL) was added and the solution evaporated just to dryness to azeotropically remove any residual water. The residue was dissolved in diethyl ether, methylated with etherial diazomethane, subjected to Florisil column clean-up, and concentrated to 1 mL prior to gas chromatographic analysis using the procedures of Grover et al. (1997).
Gas Chromatography
Gas chromatographic analysis of all extracts was performed using a Hewlett Packard (Palo Alto, CA) Model 5890 gas chromatograph (GC) equipped with a 63Ni electron capture detector. The gas chromatographic system was operated using the GC column and operating conditions described previously (Grover et al., 1997).
All herbicide residue detections were confirmed using a Hewlett Packard Model 5890 GC interfaced to a Model 5970 mass selective detector (MSD) operated in the selected ion monitoring mode. The ions monitored were triallate 128, 268 and 2,4-D 234, 236. The GC–MSD system used the same column and GC operating conditions used above and the capillary interface between the GC and MSD was maintained at 280°C throughout each extract analysis. Prior to analysis, sample extracts were concentrated to 100 µL.
Quality Assurance–Quality Control
Filters and PUF/XAD-2 resin cartridges were extracted in batches of eight, along with four quality control samples (12 samples total). Field filter and PUF/XAD-2 resin control samples were obtained by placing a filter and cartridge into a non-operating high-volume sampler, located at one of the field sites, for each sampling period. Randomly selected filters, cartridges, and solvent blanks were also extracted and analyzed as laboratory controls. With each batch of field samples, then, four control samples consisting of a fortified sample, a field control, a background control, and a solvent blank were generally extracted. Extracts of all field and laboratory control samples contained less than the minimum quantitation concentration of both herbicides.
Bulk deposition, surface film and dugout water samples were extracted in batches of eight along with one fortified sample and one background control sample (10 samples total). Bulk deposition samples for fortification were collected from a pan sampler at the research station during one single precipitation event. Samples of dugout water and surface film for fortification were collected from a dugout on the Agriculture and Agri-Food Canada research station near Regina, Saskatchewan.
Ambient Air (Filter and PUF/XAD-2 Resin Cartridge Samples)
The glass fiber filters were fortified with 10 or 1 µg each of 2,4-D (acid) and triallate by applying an acetone solution of the herbicides over the surface of the filters in two 0.5-mL portions. The filter was then placed in a fume hood for 15 min after each application to permit evaporation of the acetone. The upstream PUF plug of the PUF/XAD-2 resin cartridge was similarly fortified with 10 or 1 µg each of 2,4-D and triallate by applying 1 mL of acetone solution of the herbicides to the upstream surface of the foam plug and then placing the cartridge in a fume hood for 1 h to permit evaporation of the acetone. The filters and the PUF/XAD-2 resin cartridges were then Soxhlet-extracted individually and the extracts analyzed as described above. The amount of each herbicide applied to the filter or the cartridge was equivalent to a 700 m3 sample of air with a concentration of 14.3 and 1.4 ng m-3 of each herbicide for the 10 and 1 µg fortification levels, respectively. Recoveries of 2,4-D and triallate from the filters were 94.1 ± 3.3% and 98.5 ± 4.4% (respectively) and from the PUF/XAD-2 cartridges 90.4 ± 20.2% and 97.4 ± 12.3% (respectively). The minimum quantitation level was taken to be 40 pg m-3 of air of 3% of the lowest fortification level.
Surface Film
Surrogate surface film samples (
25 mL) were fortified with a mixture of 0.5 µg of each herbicide dissolved in 100 µL of acetone (equivalent to 2500 ng m-2) and then extracted as described above and corrected for background concentrations. Mean recoveries of the herbicides from the surrogate surface film samples were 82 ± 7% and 91 ± 3% for 2,4-D and triallate, respectively (n = 3). Minimum quantitation level was taken as 5 ng per sample for surface film samples. The area measured by a surface film sample was 2000 cm2 (five subsamples of 20 x 20 cm each) so the minimum quantitation level could also be expressed as 25 ng m-2.
Bulk Deposition and Dugout Water Samples
Bulk deposition and dugout water samples were fortified with 2,4-D and triallate at 1.0 and 0.1 µg L-1 by the addition of 1.0 and 0.1 µg of the herbicides dissolved in 1 mL of methanol. Unfortified samples were analyzed to determine background concentrations. Mean recoveries, after correcting for background concentrations, were: bulk deposition 84.9 ± 21.9% and 88.0 ± 10.2% (n = 14); dugout water 98.3 ± 12.0% and 105 ± 3.7% (n = 14) for 2,4-D and triallate, respectively. Minimum quantitation levels were taken as half of the lowest fortification level, that is, 50 ng L-1 for water samples. For bulk deposition samples, with the minimum sample volume of 4 L, obtained during a week without rain when the sample consisted of a 4-L rinse of the sampler, the 0.1 µg L-1 fortification level was equivalent to 0.4 µg m-2 of bulk deposition for each herbicide.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONGENERAL DISCUSSION AND...REFERENCES |
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Atmospheric Concentrations
2,4-D
2,4-D was detected in 63% of atmospheric samples (filter plus PUF/XAD-2 resin cartridge) collected at both dugouts in 1989 and 53% of the samples collected at Dugout 1 in 1990 (Table 2). Detections in 1989, when no 2,4-D was applied in the vicinity of the dugout, probably resulted from medium- to long-range atmospheric transport, which may explain why similar mean concentrations of 2,4-D (0.67 and 0.77 ng m-3) were detected at both of these dugouts. However, the mean concentration measured at Dugout 1 in 1990 was of the same order of magnitude (0.49 ng m-3) even though 2,4-D was applied in the vicinity.
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Triallate
Triallate, which was applied in the vicinity of the dugouts in both years, was detected in 100% of the samples collected at Dugout 1 for both 1989 and 1990 and in 94% of the samples from Dugout 2, 1989 (Table 3). Atmospheric concentrations were approximately an order of magnitude higher than those for 2,4-D, with mean values of 12.82 and 9.29 ng m-3 in 1989 (Dugouts 1 and 2, respectively) and 8.38 ng m-3 at Dugout 1 in 1990.
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Bulk Deposition
2,4-D
In both 1989 and 1990, bulk atmospheric deposition of 2,4-D was generally greatest during the month of June when the majority of the herbicide was applied in the region (Fig. 3)
. The maximum bulk atmospheric deposition rate, measured at both dugouts in 1989, was 3550 ng m-2 d-1, and at Dugout 1 in 1990 was 1550 ng m-2 d-1 (Table 4, Fig. 3). 2,4-D was detected in 69 to 75% of the samples collected for both dugouts, for both years. Median concentrations ranged from 180 ng m-2 d-1 (Dugout 1, 1990) to 450 ng m-2 d-1 (Dugout 2, 1989). The mean deposition rates were the same (900 ng m-2 d-1) at Dugouts 1 and 2 in 1989 and somewhat lower (440 ng m-2 d-1) at Dugout 1 in 1990.
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Triallate
As with 2,4-D, bulk atmospheric deposition of triallate was generally greatest during the application period (Fig. 4)
. Most of the triallate bulk deposition occurred from mid-May until mid-June, with only occasional detections afterward. Although atmospheric concentrations of triallate were much higher than those of 2,4-D (Tables 2 and 3, Fig. 1 and 2), triallate was detected in only 44 to 54% of the bulk deposition samples compared with 69 to 75% for 2,4-D. While the maximum deposition rates (730 to 2300 ng m-2 d-1) were only slightly lower than for 2,4-D, the median (0 to 60 ng m-2 d-1) and mean (120 to 320 ng m-2 d-1) deposition rates were considerably lower (Table 5).
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Relationship between Bulk Deposition and Atmospheric Concentrations
Organic contaminants, such as pesticides, can exist in the atmosphere as vapors or in association with particulates. Rainfall may affect bulk deposition of pesticides by washout of particles and gasses of local origin (Dickhut and Gustafson, 1995) or those present through long-range atmospheric transport (Barrie, 1992). In situations where both local and long-range transport contribute to bulk atmospheric deposition, atmospheric concentrations of the contaminant, measured at a 1-m height using high-volume samplers, may not reflect quantities deposited.
2,4-D
While bulk atmospheric deposition rates of 2,4-D do not appear to be directly correlated with atmospheric concentrations (Fig. 5) , they do appear to be related to rainfall. Using Dugout 2 (1989) as an example, there was a positive correlation (r = 0.7260) between weekly rainfall and bulk deposition during the weeks of 18 May to 19 July, when most of the bulk deposition occurred. The weeks of 18, 24, and 31 May, when 2,4-D was present in bulk deposition but was not in atmospheric samples collected at a 1-m height, suggest that long-range transport may have contributed to the bulk deposits. Both 18 and 31 May had similar deposition rates and similar rainfall but bulk deposition and precipitation were both much higher for the week of 24 May. It is possible that this 2,4-D may have been transported, by the rain clouds, from other regions. Thus, it would not have been collected by the 1-m high-volume air sampler. A contributing factor may also have been the deposition of large soil particles eroded from neighboring fields during tilling and planting. These may have been undersampled by the PS-1 sampler, which is not efficient in collecting larger particles.
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Triallate
Triallate was continuously detected in the atmosphere at Dugout 2 (1989) at a 1-m height from the beginning of the application season (mid-May) through to the week of 19 July (Fig. 6)
. In contrast to 2,4-D, none of the triallate detected was associated with atmospheric particulates (Fig. 2). There was a positive but weaker correlation (r = 0.4996) between weekly rainfall and bulk deposition of triallate during this period in which all of the bulk deposition of the herbicide occurred. Similar trends were found for Dugout 1 (1989 and 1990). As with 2,4-D, however, the contributions of environmental concentrations at a 1-m height to bulk deposits were not clearly defined.
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2,4-D
Maximum quantities of 2,4-D ranged from 308 ng m-2 (Dugout 1, 1989) to 714 ng m-2 (Dugout 1, 1990) (Table 4) with detections in 27 to 87% of the samples collected. In 1989, at both dugouts, all of the detections were found from 18 May to 21 June (Fig. 7) . In 1989, when 2,4-D was not applied in the immediate vicinities of the dugouts, 2,4-D was detected prior to spring application of the herbicide and for only the first half of the application season, which normally extends from the first week in June to the first week in July. In 1990, when 2,4-D was applied in the vicinity of Dugout 1, the herbicide was detected prior to the spraying season and throughout the spring application period and at higher concentrations than in 1989. In addition, because of the different sampling protocol in 1990, 2,4-D was also detected in the fall (mid-August to mid-September) samples.
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Relationship between Surface Film Quantities and Bulk Deposition
2,4-D
Bulk deposition of 2,4-D was generally detected at each dugout from the beginning of the sampling period until the end of July, with greater amounts being deposited in 1989 (Fig. 3). The greater deposition in 1989 was not, however, reflected in the quantities of 2,4-D detected in the surface film samples (Fig. 7). In addition, 2,4-D quantities in surface film were not correlated with bulk deposition rates. For example, in 1989, 2,4-D was not detected in surface film after the third week in June although deposition occurred until the end of July. In the fall of 1990, 2,4-D was detected in surface film but not in bulk deposits.
Triallate
Bulk deposition rates of triallate were greater in 1989 than in 1990 (Fig. 4) but it was detected, in very low quantities, in only one 1989 surface film sample (Table 5). In contrast, in 1990 triallate was detected in surface film samples throughout the application season, although bulk deposition rates were lower. Triallate quantities in surface film were not, then, correlated with bulk deposition rates.
Dugout Water
2,4-D
In 1989, 2,4-D was detected in all water samples collected from both dugouts with maximum concentrations of 290 and 240 ng L-1, respectively (Table 4). The pattern of concentrations was different for the two dugouts (Fig. 8)
. The highest concentrations in Dugout 1 were measured in the spring to mid-summer period while, for Dugout 2, higher concentrations were detected later in the sampling season.
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In contrast to Dugout 1, Dugout 2 was drained and dredged in 1988 and newly filled with spring snowmelt runoff in 1989. As a result, it is unlikely that chemical persistence in the sediments contributed to the concentrations of 2,4-D found in water samples from 1989 but that this contamination resulted from herbicide transported by spring runoff from snowmelt. Concentrations were higher on 14 and 21 June (Fig. 8), coinciding with a period of high atmospheric concentrations (Fig. 1) and bulk deposition (Fig. 3). Concentrations were, however, lower on 28 June and 5 July and then rose to a peak on 16 August. Atmospheric 2,4-D was found from 2 to 16 August but concentrations were much lower than during the spraying season in June. Atmospheric concentrations and bulk deposition rates, then, cannot provide an explanation for increases in 2,4-D in the water of Dugout 2. As with Dugout 1, it is possible that evaporation of water from the dugout may have resulted in an increase in 2,4-D concentration during the summer.
In 1990 the concentrations of 2,4-D in Dugout 1 were much lower, overall, than in 1989. The highest concentration was measured on 22 May when residues were also found in the atmosphere and in bulk deposits. The concentration in water declined to trace levels until 19 June, when concentrations again increased. High levels were also recorded in the ambient air and in bulk deposits for that period, coinciding with regional application of the herbicide. The correlation is not precise, however, as high atmospheric concentrations and bulk deposition rates were also found for the week ending 12 June 1990, when only a trace of 2,4-D was detected in the dugout water.
Triallate
Triallate, which has a lower water solubility than 2,4-D (Table 1), was detected in 27 to 53% of the dugout water samples collected in 1989 and 1990 (Table 5, Fig. 9)
, a much lower rate of detection than for 2,4-D. Although the maximum concentration of triallate (500 ng L-1; Dugout 1, 1990) was higher than the maximum concentrations of 2,4-D, mean and median values were lower. In Dugout 1, for both 1989 and 1990, the highest concentrations were measured in the early spring, coinciding with the highest atmospheric concentrations (Fig. 2) and bulk deposition rates (Fig. 4). Thereafter, triallate appeared only sporadically in water samples. Dugout 2 (1989) had the highest detection rate, with quantifiable concentrations of triallate measured for the first seven weeks of the sampling season, although atmospheric concentrations were similar to those of Dugout 1. There were no obvious reasons for the differences between the two dugouts. It is possible that some of the triallate was transported into the dugouts in spring runoff. Waite et al. (1992) frequently found triallate residues in Saskatchewan spring runoff, from snowmelt, with concentrations as high as 980 ng L-1.
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Because the actual volume of surface film could not be quantified, it is difficult to express the amount of herbicide in the surface film as a concentration that might be compared with the concentrations found in dugout water. A very rough estimate can be obtained, however, by using the volume of water, approximately 35 mL, which was collected with the surface film samples. This would be equal to 175 mL for a 1-m2 surface film sample. If the quantity of 2,4-D found in Dugout 1 surface film on 31 May 1989 (308 ng) is assumed to be present in 175 mL of water, the concentration can be calculated to be 1760 ng L-1. The maximum concentrations of 2,4-D in surface film, then, ranged from 1760 to 4080 ng L-1 with mean concentrations from 274 to 623 ng L-1. This may be compared with concentrations in the dugout water of 120 to 290 ng L-1 (maxima) and 45 to 143 ng L-1 (mean) (Table 4).
While the surface film concentrations were high, it must be considered that the quantity of herbicide present in the surface film, if mixed evenly in the dugout, would not significantly raise the total concentration in the dugout water. For example, the maximum quantity of 2,4-D, 714 ng m-2 (Dugout 1, 1990), if mixed into the 2.1-m deep dugout, would increase the concentration in the water by 0.03 ng L-1. The biological significance of herbicides in the surface film may be to animals such as frogs, water striders, beetles, and plants such as duckweed (Lemna spp.), which are associated with the surface region of ponds.
Triallate
In 1989, even though triallate was detected in water samples from both dugouts, the herbicide was not found in surface film samples. In 1990, when water concentrations were similar to those in 1989, triallate was detected in surface film samples. Thus, as with 2,4-D, there is no apparent correlation between triallate quantities in surface film and concentrations in dugout water.
Concentrations of triallate in surface film were calculated, similarly to those of 2,4-D, as 0 to 1211 ng L-1 (maxima) and 0 to 194 ng L-1 (mean). As with 2,4-D, surface film concentrations were much higher than those in dugout water (110 to 500 ng L-1 maxima and 17 to 48 ng L-1 mean).
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GENERAL DISCUSSION AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONGENERAL DISCUSSION AND...REFERENCES |
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(ii) Both herbicides were found in bulk (wet + dry) atmospheric deposition. Generally, higher concentrations of 2,4-D were found than of triallate. On the basis of Henry's Law constants for these compounds, 2,4-D vapor, relative to triallate, would have favored partitioning into falling rain droplets (Grover, 1991). Because 2,4-D appeared in the atmosphere associated with particles, it would also have been subject to physical washout from the atmosphere. The concentration of chemicals in precipitation is not, however, necessarily dependent only on local atmospheric concentrations. Barrie (1992) reviewed atmospheric scavenging ratios and discussed the complex factors affecting contaminant concentrations in rainwater. Since both local atmospheric conditions and long-range atmospheric transport from other regions can contribute to the quantity of chemicals found in precipitation, bulk deposition may not be related to local atmospheric concentrations. Rain clouds, for example, may transport pesticides from distant locations (Barrie, 1992).
(iii) The surface film on the dugouts was found to contain both 2,4-D and triallate. Surface films consist of decomposition products of aquatic plants and animals (autochthonous material), allochthonous materials from runoff (Sondergren, 1987), and anthropogenically generated pollutants such as petroleum products (Baier, 1972). Surface films or microlayers have been examined for their natural composition (Harvey, 1966; Baier, 1972; Larsson et al., 1974; Sondergren, 1987) and found to consist of proteins, carbohydrates, and lipids in various proportions. The presence of hydrophilic compounds, such as carbohydrates, would promote the absorption of more water-soluble pesticides, whereas hydrophobic compounds such as lipids would promote adsorption of the less water-soluble pesticides. The greater presence of 2,4-D in the surface films may, then, result from a greater proportion of hydrophilic compounds therein.
Biological, chemical, and photolytic degradation would affect the accumulation of herbicides in the surface layer. High degradation rates would reduce the concentration of chemicals even though large quantities were deposited. Differential degradation rates for 2,4-D and triallate could result in higher concentrations of the more stable compound even though the less stable chemical was deposited at higher rates. Cessna and Muir (1991) reported that both 2,4-D and triallate may undergo photolysis in aqueous solution. There appears to be no information in the literature on biological degradation rates in natural surface films. Similarly, there is no information on the chemical degradation of these herbicides in natural surface films that potentially have a variable chemical composition.
Finally, herbicide dissipation from surface film may also occur through volatilization to the atmosphere, mixing or partitioning into the dugout water, or adsorption to the sides of the dugout. Triallate has a higher vapor pressure than 2,4-D and might be expected to volatilize more quickly from the surface film. An equilibrium between the atmosphere and the surface film would only occur if atmospheric concentrations remained constant, which, as shown by the present data, did not occur. Lower atmospheric concentrations for a day or two before surface film sampling might result in significant volatilization from the very thin surface film. Wind action would cause mixing with the dugout water and contact with the shore of the dugout, both of which would reduce herbicide concentrations in the surface film. It was not possible to estimate the effect that these perturbations might have caused.
(iv) Dugout water samples contained both 2,4-D and triallate with generally higher concentrations of the more soluble 2,4-D being found. Concentrations early in the sampling season may have resulted from carryover from the previous year or transport via spring runoff from snowmelt. Increased concentrations during the sampling season may have resulted from atmospheric deposition or from concentrations caused by evaporation of the dugout water.
(v) The concentrations of both herbicides in all environmental compartments sampled tended to be highest during the time period when they were applied to local cropland. Precise correlations relating the varying concentrations in the four matrices were, however, not found. This may result from the many unquantifiable environmental factors affecting concentrations of these herbicides.
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