magnitude and persistence of herbicide residues in farm dugouts and ponds in the canadian prairies

638

Environmental Toxicology and Chemistry, Vol. 16, No. 4, pp. 638–643, 1997q 1997 SETAC

Printed in the USA0730-7268/97 $6.00 1 .00

MAGNITUDE AND PERSISTENCE OF HERBICIDE RESIDUES IN FARM DUGOUTSAND PONDS IN THE CANADIAN PRAIRIES

RAJ GROVER,† DON T. WAITE,*‡ ALLAN J. CESSNA,† WALLY NICHOLAICHUK,§ DON G. IRVIN,\LORNE A. KERR† and KELLY BEST§

†Agriculture & Agri-Food Canada, Research Station, Regina, Saskatchewan S4P 3A2, Canada‡Environment Canada, 300-2365 Albert Street, Regina, Saskatchewan S4P 4K1, Canada

§National Hydrology Research Center, Saskatoon, Saskatchewan S7N 3H5, Canada\Toxicology Research Center, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0, Canada

(Received 8 March 1996; Accepted 27 August 1996)

Abstract—Farm ponds or dugout waters were monitored for residues of seven major herbicides used in the Canadian prairies fromfall of 1987 to spring of 1989. The frequencies of confirmed detection of herbicides in water samples, depending on the time ofsampling, in decreasing order were: 2,4-dichlorophenoxyacetic acid (2,4-D; 93–100%), diclofop (46–95%), bromoxynil (50–85%),4-chloro-2-methyl-phenoxyacetic acid (MCPA; 33–70%), triallate (28–63%), dicamba (17–55%), and trifluralin (0–18%). Thecorresponding frequencies of quantifiable residues ($0.05 mg/L) were lower, ranging from 75 to 86% for 2,4-D to 0 to 7% fordicamba. Median residues in all water samples were near or below the quantification limits of 0.05 mg/L. Maximum residues variedwidely and were (mg/L): trifluralin (not detectable [ND]–0.11), bromoxynil (0.27–0.33), dicamba (ND–11.2), triallate (0.05–0.87),MCPA (0.12–1.97), 2,4-D (0.64–2.67), and diclofop (0.27–3.47). Maximum residues were seasonal and declined to near or belowdetection limits by the following sampling time. Median values were two to three orders of magnitude less than the correspondingmaximum allowable concentration and interim maximum allowable concentration guidelines for drinking water in Canada and theUnited States. Maximum values were also less than these guidelines. Only the maximum values for residues of MCPA and 2,4-Dapproached the guidelines for these herbicides in water used for irrigation.

Keywords—Water quality Herbicides Farm ponds Dugouts

INTRODUCTION

On-farm ponds, both natural and man-made, are commonin agricultural lands across North America. In the Canadianplains alone, more than 100,000 man-made farm ponds, calleddugouts, have been constructed since 1935 [1]. In the UnitedStates, more than two million farm ponds had been constructedacross agricultural lands by 1980 [2]. In Canada, these dugoutstypically receive water from spring snowmelt or from infiltra-tion from shallow (surficial) aquifers. These ponds or dugoutsprovide a valuable and often sole source of water for humanactivities on the farms. Such activities include human waterconsumption, livestock watering, and irrigation. In addition,these ponds are an important source of water for native wildlifeand local and migratory bird populations.

In the Canadian prairies, dugouts range in size from ;100m2 to .6,000 m2 in surface area, receiving water from thesurrounding drainage areas varying in size from ,10 ha to.1,000 ha. Farm ponds or dugouts are located on or imme-diately adjacent to tilled farmland. As such they represent a‘‘worst-case’’ scenario with respect to potential contaminationfrom pesticides used in their immediate vicinity.

Considerable data have been accumulated on the magnitudeof contamination of lakes and rivers with pesticides due toagricultural activities in surrounding watersheds, as well asinformation on associated mechanisms of contamination [3,4].However, analogous data for small bodies of water, such asfarm dugouts or ponds, are minimal, notwithstanding theirfrequency of occurrence across the agricultural landscape and

* To whom correspondence may be addressed.

the multifunctional purpose for which these bodies of waterare used [5,6]. There are also situations where point-sourcecontamination of farm dugout waters may be significant, re-sulting in residues above irrigation water quality guidelinesand potential for damage to crops. Documented cases of phen-oxy-sensitive crop damage from 2,4-dichlorophenoxyaceticacid (2,4-D)-contaminated irrigation water obtained from farmponds are not uncommon [5].

Considering that more than 20 million kilograms of her-bicides alone are used annually across the Canadian prairies[7] and the large number of farm ponds within this region,studies were initiated to determine the frequency of occur-rence, magnitude, and persistence of herbicide residues in thesedugouts. This study summarizes data on herbicide residues inwater samples from farm dugouts in Saskatchewan, wheremore than half of the annual herbicide use in the Canadianprairies occurs [7].

MATERIALS AND METHODS

Study sites and cropping practices

Dugouts in four regions in Saskatchewan, based on soilassociations and soil zones, were selected in the fall of 1987.The Regina and Balgonie regions are in the brown–black soilzone in southern Saskatchewan and the Melfort and Tisdaleregions are in the black–gray zones in central Saskatchewan(Table 1 and Fig. 1). Five to six on-farm dugouts were selectedin each of these regions.

The pH of the dugout waters varied during the season andalso between different dugouts, ranging from 7.2 to 10.2 (Table2). The dugout surface areas ranged from 130 to 6,201 m2.

Magnitude and persistence of herbicide residues in farm dugouts Environ. Toxicol. Chem. 16, 1997 639

Table 1. Soil classification, association(s), series, and texture classes of dugout drainage areas

Region Classification Association Series Texturea

BalgonieReginaMelfortTisdale

ChernozemicChernozemicSolonetzicSolonetzic

Oxbow/WeyburnSceptreMelfortTisdale/Arborfield/

Eldersely

Orthic dark brown to blackSego/orthic brownOrthic blackOrthic gray to dark gray

slhcsil–sicl–sicsic–cl

a l 5 loam, sl 5 sandy loam, c 5 clay, cl 5 clay loam, hc 5 heavy clay, sil 5 silty loam, sicl 5 siltyclay loam, sic 5 silty clay.

Fig. 1. Map of southern and central Saskatchewan showing soil zones selected for the dugout study.

The drainage areas in the immediate vicinity of the dugoutswere defined and ranged from 7 to 99 ha. All dugouts, exceptone in the Regina area, received runoff water in the spring of1988, ranging from 12 to more than 6,000 m3.

Cropping practices and herbicide use in the farmland ad-jacent to the dugouts were determined by interviewing eachowner in 1987 and 1988. The major crops grown in two south-ern regions were wheat, canary seed, and barley, whereas thetwo central regions grew canola, wheat, barley, flax, and avariety of other special crops (Table 3). All of the seven majorherbicides being monitored (bromoxynil, 2,4-D, dicamba, di-clofop, 4-chloro-2-methyl-phenoxyacetic acid (MCPA), trial-late, and trifluralin) were used in the four regions. However,MCPA, dicamba, and trifluralin were used most often in thecentral regions and 2,4-D and triallate were used most oftenin the southern regions. Crop rotation practices determined theoverall uses from year to year. Documentation of herbicideapplications to land adjacent to the dugouts provided infor-mation on potential spray drift, accidental spills that mighthave occurred near the dugouts, and local or point-source con-tamination from runoff.

Sampling procedure

Dugout water samples for herbicide analysis were collectedbefore seeding in April, 1988 and 1989, after herbicide ap-plications in July 1988, and after harvest in late August toOctober in 1987 and 1988. The 1987 samples were collected

during the initial survey operations in the fall of 1987. Becauseof early freeze-up, the 1988 fall samples from the Melfort andTisdale sites were collected in January 1989, prior to springthaw. Two water samples were collected from each of the fiveor six dugouts in each region, thus providing 10 to 12 samplesper region at each time of sampling.

Water samples were collected into new, 3.8-L amber glassbottles. The bottles, weighted and suspended on a rope acrossthe dugout, collected water samples at about 0.5 m below thewater surface near the center of the dugout. The 1.5- to 2-Lwater samples were transported to the laboratory in coolers,where they were maintained at 2108C until extraction. Thebottles were stored on their side to prevent breakage duringfreezing.

Analytical procedure

Each water sample was shaken to suspend sediments, ifany, and an unfiltered aliquot (500 ml) was extracted followingestablished methods of liquid–liquid extraction [8]. Followingadjustment of the pH to 12, the neutral herbicides were ex-tracted into n-hexane. The pH was then adjusted to ,2 andthe acidic herbicides extracted into diethyl ether. The concen-trated ether extract was then methylated using diazomethane.Following concentration, the neutral herbicides and the meth-ylated acid extracts were individually subjected to Florisil col-umn clean-up prior to quantitation by gas chromatography.

Gas chromatographic analysis was performed using a Hew-

640 Environ. Toxicol. Chem. 16, 1997 R. Grover et al.

Table 2. Numbers and variations in pH, size, drainage area, andspring runoff volumes of on-farm dugouts monitored in the study

Region No.

Range of

pH

Dugoutsurface

area(m2)

Drainagearea(ha)

Springrunoffa

(m3)

BalgonieReginaMelfortTisdale

6555

7.2–9.47.7–10.27.2–9.47.2–9.4

154–664130–545770–6,201360–2,204

8–187–998–35

25–41

54–2020–12

275–1,287446–2,083

a March–April 1988.

Table 3. Major crops and herbicide uses in 1987–88 in the immediate vicinity of the selected dugouts

Region Year Cropsa Herbicidesb

Balgonie 1987 Wheat, barley, oats Diclofop, bromoxynil, MCPA, dicamba, 2,4-D

1988 Wheat, barley 2,4-D, diclofop, bromoxynil, triallate, dicam-ba, MCPA

Melfort 1987 Wheat, barley, peas, flax MCPA, dicamba, 2,4-D1988 Canola, barley, wheat,

flax, peasMCPA, dicamba, trifluralin, bromoxynil

Regina 1987 Wheat, canary seed,lentils

2,4-D, MCPA, bromoxynil, triallate, dicamba

1988 Wheat, canary seed Triallate

Tisdale 1987 Barley, wheat, canola,flax, beans, peas

MCPA, dicamba, bromoxynil, trifluralin

1988 Canola, barley, wheat,flax, peas

MCPA, dicamba, trifluralin, bromoxynil

a In order of decreasing hectarage seeded.b In order of decreasing hectarage treated (only herbicides, being monitored are listed). 2,4-D 5 2,4-

dichlorophenoxyacetic acid, MCPA 5 4-chloro-2-methylphenoxyacetic acid.

lett Packard model 5890 gas chromatograph equipped with anelectron capture detector (ECD) and a Hall 1000 electrolyticconductivity detector (ELCD), operated in the halogen mode.The column employed for both detectors was a Hewlett-Pack-ard HP-1 fused-silica, cross-linked, 25 m 3 0.53-mm i.d.column with a film thickness of 0.88 mm. The oven temper-ature was initially held at 708C for 1 min and then temperatureprogrammed at 58C/min to 2708C and held for 5 min. Thecarrier gas for both columns was helium (ultra high purity,UHP) at a flow rate of 6.5 ml/min. The ECD make-up gaswas nitrogen (UHP) at 70 ml/min, with detector temperatureset at 3508C. The Hall detector reaction gas flow rate washydrogen (UHP) at 25 ml/min, with the purge make-up gas(helium UHP) set at 20 ml/min. The conductivity solventflow rate was maintained at 0.5 ml/min with glass distilledgrade n-propanol. The reaction furnace temperature was setat 8108C. All injections were cool-on-column, using a Hew-lett Packard UHP model 7673A auto injector. Confirmationwas performed with a Hewlett Packard model 5970 massselective detector (MSD), operated in the selected ion mon-itoring mode (SIM).

All data on acidic herbicides are expressed as acid equiv-alents. The minimum quantitation level was set at 0.05 mg/Lin water for all herbicides, with percent recoveries establishedat 1 and 0.1 mg/L. The percent recoveries varied from 73 to112%, depending upon the herbicide and the level of fortifi-cation. All residue levels at and above 0.05 mg/L were bothconfirmed by gas chromatography (GC) MSD and quantified

by GC ECD or GC ELCD. Residue levels below 0.05 mg/Lwere either not confirmed (designated not detected [ND]) or,if confirmed by GC MSD operated in the SIM, are markedwith a superscript b representing trace amounts only (,0.05b).Ion area ratios used for confirmation were 1.85 (291/276), 1.53(234/236), 1.53 (203/205), 1.43 (253/255), 0.51 (155/214),0.24 (128/268), and 1.30 (306/264) for bromoxynil, 2,4-D,dicamba, diclofop, MCPA, triallate, and trifluralin, respec-tively. The presence of the selected ions (indicated in brackets)being monitored and ratios within 630% of the standard ionratios were considered confirmatory.

RESULTS AND DISCUSSION

For purposes of discussion, two terms should be defined.‘‘Confirmed detections’’ refer to the sum of herbicide residues,0.05 mg/L and those .0.05 mg/L that were confirmed to bepresent in the dugout water samples on the basis of ion arearatios generated using the MSD. ‘‘Quantified detections’’ referonly to the herbicide residues .0.05 mg/L and similarly con-firmed to be present.

Frequency of detection

The frequency of confirmed detections of the seven her-bicides monitored in dugout waters from the four regions inSaskatchewan varied with the herbicides, both regionally andseasonally (Figs. 2 and 3).

Postemergence herbicides. 2,4-Dichlorophenoxyaceticacid was the most frequently detected (93–100%) herbicide indugout water, irrespective of the sampling region or the season.The long history of its use on the Canadian prairies (.50 years)may account for this high frequency of detection and may alsohave overshadowed the local use patterns described in Table3. The next most frequently detected herbicide was diclofop(56–78%), followed by bromoxynil (50–71%), MCPA (35–70%), and dicamba (22–50%). With the exception of MCPA,each of these herbicides showed maximum frequencies of de-tection in the summer sampling following applications, usuallyfrom late May to early July. The maximum frequency of de-tection of MCPA occurred with the spring samples and mayreflect snowmelt inputs from fall applications of MCPA towinter wheat.

From the Tisdale region, approximately twice as many wa-


Fig. 2. Frequency of confirmed detection of seven herbicides in farmdugout waters located in four soil regions of Saskatchewan.

Table 4. Number of samples, median and maximum levels ofresidues, and frequency of confirmed quantification for seven

herbicides in farm dugout waters in Saskatchewana

Herbicide

Spring1988 and

1989Summer

1988

Fall1987 and

1988

Bromoxynil No. of samplesMedian (mg/L)Maximum (mg/L)Frequency (%)c

80,0.05b

0.27b

8

40,0.05b

0.28b

13

70ND

0.33b

32,4-D No. of samples

Median (mg/L)Maximum (mg/L)Frequency (%)

810.07b

2.67b

75

400.09b

0.96b

85

700.07b

0.64b

86Dicamba No. of samples


81ND11.2b

7

40,0.05b

,0.05b

0

70ND

0.26b

7Diclofop No. of samples


80ND

0.27b

28

400.08b

3.47b

78

700.07b

1.36b

58MCPA No. of samples

Median (mg/L)80

,0.05b40ND

30d

NDMaximum (mg/L)Frequency (%)

1.97b

390.12b

200.28b

33Triallate No. of samples


81ND

0.87b

14

40ND

0.05b

3

70,0.05b

0.19b

21Trifluralin No. of samples


83ND

0.11b

11

42ND

,0.050

70ND

0.08b

3

a 2,4-D 5 2,4-dichlorophenoxyacetic acid, MCPA 5 4-chloro-2-meth-ylphenoxyacetic acid, ND 5 not detected (,0.05 mg/L and not con-firmed).

b Includes all confirmed detections.c Frequency of confirmed and quantified detections only.d In fall 1988 only.

Fig. 3. Frequency of confirmed detection of seven herbicides in farmdugout waters during spring, summer, and fall in Saskatchewan.

ter samples had detectable MCPA residues (70%), comparedto 35 to 40% for the other regions (Fig. 2), reflecting the greateruse of MCPA in the vicinity of these dugouts in the Tisdaleregion during the study period (Table 3). Detections of MCPAvaried from 33 to 55% overall with the seasons (Fig. 3). Onthe other hand, dicamba residues were detected more fre-quently in Melfort and Regina dugouts (49–50%), comparedto 22 to 25% in water samples from the Balgonie and Tisdaleregions (Fig. 2), again reflecting use patterns within theregions.

Frequencies of quantified detections of herbicides (Table 4;i.e., residues $0.05 mg/L) were substantially lower than fre-quencies of confirmed detections for all five postemergenceherbicides, and also showed wide variation between herbicidesand seasons. As with confirmed detections, 2,4-D accountedfor the maximum frequency of quantified detections, with aseasonal variation of 75 and 86%. Frequencies of quantifieddetections for the other postemergence herbicides (listed indecreasing order) were diclofop (28–78%), MCPA (20–39%),bromoxynil (3–13%), and dicamba (0–7%). The low quantifieddetections of bromoxynil and dicamba probably reflect themuch greater susceptibility of bromoxynil to photodegradation

[9] and the relatively lower application rates and use of di-camba [10]. Diclofop, the only herbicide not applied in thefall, showed the expected pattern of maximum quantified de-tections after the June applications, fewer in the fall, and thefewest the following spring.

Preemergence (soil-applied) herbicides. Of the two pre-emergence, soil-incorporated herbicides, triallate was morefrequently detected (63%) in dugouts near Regina, the regionof highest use, compared to 38 to 41% in the other regions(Fig. 2). Because triallate is also applied in the fall, maximumdetection frequency of this herbicide (60%, Fig. 3) occurredin samples collected in the fall. Trifluralin is used extensivelyin canola production in central Saskatchewan (Table 3) andmaximum frequencies of detection of trifluralin were observedfor the Tisdale (18%) and Melfort (11%) regions, areas of hightrifluralin use. Trifluralin was detected only in samples of dug-out water collected in the spring and fall and these detectionsmost likely originated from fall applications of the herbicide.Like bromoxynil, trifluralin is also very susceptible to dissi-pation by photodegradation [9] and this property probably ex-plains the relatively low detection frequency of this herbicidein dugout waters in all regions and why, following even springapplications, no trifluralin residues were detected in samplescollected during the summer.

The decreasing order of frequencies of confirmed herbicide

642 Environ. Toxicol. Chem. 16, 1997 R. Grover et al.

Table 5. Canadian Water Quality Guidelines for drinking andirrigation water and maximum residues observed in this study

Herbicidea

Drinking water

MACb

recom-mended(mg/L)

IMACc

(mg/L)

Irrigationwater(mg/L)

Maximumresiduesobserved(mg/L)

Bromoxynil2,4-DDicambaDiclofopd

MCPATriallateTrifluralin

——

1209

—230—

5100—————

—4

——

1–2e

—45

0.332.67

11.203.471.970.870.11

a 2,4-D 5 2,4-dichlorophenoxyacetic acid, MCPA 5 4-chloro-2-meth-ylphenoxyacetic acid.

b Maxium acceptable concentration.c Interim maximum acceptable concentration.d Methyl ester.e Proposed.

detections, for both the post- and preemergence herbicides,reflects a combination of factors, such as the extent of long-term herbicide use, seasonal herbicide use patterns, the amen-ability of the herbicides to transport/deposition into dugoutwaters, and the subsequent persistence of the herbicides inthese waters. The high frequency of confirmed detections for2,4-D, irrespective of the site or the time of sampling, mostlikely reflects its pervasive use in each of the four region overthe last 50 years. Similarly, the relative long-term use of otherherbicides [7] may have also played a role in their frequenciesof confirmed detections.

Residue levels and persistence

All herbicide residue data for water samples, whether thevalues were below or above the limit of quantification, wereincluded in the determination of corresponding median values(Table 4). Median residue levels in dugout waters for all sam-pling seasons and all four regions, which represented four soiltypes, were generally near or below the limit of quantificationfor all seven herbicides (0.05 mg/L). Median values rangedfrom ND for trifluralin to ND to 0.08 mg/L for diclofop and0.07 to 0.09 mg/L for 2,4-D (Table 4).

Maximum detected residues among the seven herbicidesvaried widely and in general their pattern paralleled the medianvalues. Lowest residues were detected for trifluralin (ND–0.11mg/L), followed by bromoxynil (0.27–0.33 mg/L), dicamba(ND–11.2 mg/L), triallate (0.05–0.87 mg/L), and MCPA (0.12–1.97 mg/L) in increasing order, with highest values for diclofop(0.27–3.47 mg/L) and 2,4-D (0.64–2.67 mg/L) (Table 4). Max-imum residues of 2,4-D, MCPA, dicamba, triallate, and triflu-ralin were detected in dugout water samples collected in thespring following snowmelt, suggesting that such residues re-sulted from applications of these herbicides during the pre-vious fall. Transport of fall-applied herbicides in spring snow-melt has been reported. In a 6-year study [11], the averageloss of 2,4-D in spring snowmelt was 4.1% of the amountapplied to 4- to 5-ha watersheds the previous fall. The rela-tively high dicamba (11.7 mg/L) and 2,4-D (2.67 mg/L) resi-dues detected in the spring samples from dugouts in the Melfortregion resulted from a fall application of dicamba and 2,4-Dto ditches draining into the dugout. 2,4-Dichlorophenoxyaceticacid and dicamba are often used in the fall to control weeds,such as winter annuals and thistles. However, with the excep-tion of 2,4-D, quantified detections of the four postemergenceherbicides in the dugout waters generally declined to levelsnear or below 0.05 mg/L by the end of the growing season.This would suggest that their persistence in quantifiableamounts is relatively short-lived or seasonal. Quantified de-tections of triallate in spring or fall samples decreased to #0.05mg/L the following summer (Table 4), whereas those for tri-fluralin decreased to nondetectable levels, most likely as aconsequence of the susceptibilty of trifluralin to photodegra-dation [9].

The median concentrations (Table 4) may represent a gen-eral contamination level across the prairies due, in part, tolong-term herbicide use, and also to nonpoint-source inputs,such as runoff from snowmelt and rainfall and probably to alesser extent from atmospheric deposition within the region.Residues of all seven herbicides have been reported in springrunoff [11,12] as well as in the atmosphere samples collectedin the region [13,14]. On the other hand, the maximum ob-served concentrations may reflect local agricultural practicesrather than regional pesticide use patterns. This is highlighted

by the maximum concentrations observed for dicamba and2,4-D, which resulted from localized runoff incidents. It shouldalso be kept in mind that essentially no data are availableregarding the magnitude of herbicide residues present in thesediments of prairie dugouts. Residues in sediments, generallyconsidered to be in equilibrium with those in the water abovethe sediments, could also affect the magnitude of herbicideresidues, especially the median observed values, in these waterbodies.

In general, the highest frequency of detection and the quan-tifiable herbicide residues in the dugout waters were detectedduring and following the application periods. For the fourpostemergence herbicides other than 2,4-D, the residue levelsin dugout waters declined to levels at or below limits of quan-tification by the end of the growing season or the followingspring, suggesting that their persistence in quantifiableamounts is short-lived or seasonal (Fig. 3 and Table 4). Of thetwo preemergence herbicides triallate and trifluralin, the high-est frequency of detection and the quantifiable residues wereagain detected following the application periods in spring orfall; their residues in dugout waters were near or below quan-tification detection limits during the summer sampling period.The relatively low residues of trifluralin in dugout waters inall regions, when compared to triallate, are no doubt due toits rapid photolysis in aqueous environments [9].

Residues and water quality

Maximum acceptable concentrations (MACs) and interimmaximum acceptable concentrations (IMACs) as published un-der the Canadian Water Quality Guidelines [15] and U.S. En-vironmental Protection Agency Guidelines [16] are very sim-ilar, but both are available for only five of the seven herbicidesmonitored in this study (Table 5). The observed median con-centrations for these five herbicides (ND–0.09 mg/L) were twoto three orders of magnitude less than the corresponding MACand IMAC values. As indicated earlier, it is important to notethat the median residue levels may be an indicator of thegeneral contamination of dugout waters by herbicides withinthe prairie region. The maximum observed residues (Tables 4and 5), which probably represent worst-case scenarios for her-bicide contamination of farm dugouts (as discussed previ-ously), were also 3- (diclofop), 10- (dicamba), 15- (bromox-


ynil), 38- (2,4-D), and 235-fold (triallate) less than the cor-responding MAC and IMAC drinking water standards. Al-though detected herbicide residues were less than drinkingwater guidelines in all 21 dugouts studied, all of the dugoutshad confirmed detections of one or more of the seven herbi-cides monitored at each sampling time. Thus, based on thethree sampling times used in this study, none of the dugoutwaters was ever free of herbicide contamination on a season-to-season basis. It should also be noted that, internationally,Canada and the United States have some of the highest drink-ing water guidelines for pesticides. For example, the EuropeanCouncil of Agricultural Ministers recently adopted 0.1 mg/Lfor all agricultural pesticides as the MAC for drinking or po-table waters [17].

The observed median and maximum residue levels in dug-out waters were also compared to the Canadian Irrigation Wa-ter Quality Guidelines (Table 5) [18]. The median observedresidues were 20- to 900-fold less for the three available guide-lines (i.e., for MCPA, 2,4-D, and trifluralin). However, themaximum observed residues for MCPA and 2,4-D were, re-spectively, equal to and just below corresponding irrigationwater guidelines.

Acknowledgement—We thank the Canadian Network of ToxicologyCenters for partial financial support from the Green Plan Funds.

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magnitude and persistence of herbicide residues in farm dugouts and ponds in the canadian prairies

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