Analysis and Detection of the New- Corn Herbicide Acetochlor in River Water and Rain PAUL D. CAPEL,*,' LIN MA,* BLAINE R. SCHROYER,' STEVEN J. LARSON,+ AND THERESE A . G I L C H R I S T *
Alachlor Acetoch lo r Metolachlor FIGURE 1. Chemical structure of acetochlor, alachlor, and metolachlor.
Water Resources Division, U.S. Geological Survey, Gray Fresh water Biological Institute, Narvarre, Minnesota 55392, and Gray Freshwater Biological Institute, University of Minnesota, Narvarre, Minnesota 55392
Introduction The use of herbicides on corn has become common and important. In the United States, the herbicides used most often on corn in 1989 were alachlor, atrazine, butylate, cyanazine, dicamba, EPTC, metolachlor, and 2,4-D. The 1989 estimated use in the United States of these eight herbicides on corn was 89 x lo6 kg (1). In March 1994, the U.S. EPA conditionally registered the herbicide acetochlor (2-chloro-N-(ethoxymethyl) -N-(2-ethyl-6-methylphenyl)acetamide) as a partial replacement for many of these other corn herbicides ( 2 ) .As part of the registration criteria, the use of acetochlor must result in a reduction in use of the most common corn herbicides (alachlor, atrazine, butylate, EPTC, metolachlor, and 2,4-D). Total use of these six herbicides on corn must decline by 1.8 x lo6kg by the end of 18 months (November 1995) and by 3.0 x lo6 kg by the end of 5 years (March 1999) based on 1992 use, adjusting for differences in planted acreage. Thus, acetochlor is destined to be a high-use herbicide. Other criteria in its conditional registration are based on concerns of surface water and ground water contamination and include a mandate for industry, state, and Federal participation in surface water and ground water monitoring programs. As an example of these criteria with regard to surface water, the registration will be cancelled if the annual acetochlor concentration is 2 2 pg/L in two large community water supply systems (systems serving over 100 000 people) or in 10 community water supply systems of any size that derive their water primarily from surface water. As an example with regard to groundwater, the registration will be cancelled if the acetochlor concentration is 20.1 pg/L in 220 wells in a state monitoring program followed by two subsequent detections of 20.1 pg/L in monthly sampling of each ofthese wells conducted over the following 6 months (2). * To whom correspondence should be addressed: e-mail address:
[email protected];Fax: 612-471-9070. +
U S . Geological Survey.
3
Present address: Terracon Environmental, Inc., White Bear Lake,
* University of Minnesota. M N 55110.
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FIGURE 2. High-performance liquid chromatogram of commercial acetochlor (Harness+) with the retention times of two other corn herbicides indicated. (Acetochlor and alachlor have identical retention times under these chromatographic conditions.) HPLC conditions: column, Spherisorb ODS2, 25 cm, 5 pin; column temperature, 60 "C; injection volume, 50 ,uL; flow rate, 1.4 mumin; mobile phases, methanol with 50 m M ammonium acetate and water with 50 m M ammonium acetate; gradient, initially 20% methanol, linearly to 60% at 10.5 min, linearly to 100% at 21.0 min, return to 20% at 23.0 min; detection wavelength, 216 nm.
Acetochlor has been used for years in Europe and South Africa ( 3 - 3 , although searches of two computerized databases located no publications describing its environmental behavior and fate other than one study of its sorptive and residual behavior in soil (6).The U.S. EPA registration document (2)suggests that "acetochlor and its degradates are moderately persistent and moderately to very mobile in soil, although this varies depending on the characteristics of the soil where it is applied. As a result, there is a relatively high potential for acetochlor residues to reach ground and surface water". Structurally, acetochlor is a close analog of alachlor and metolachlor (Figure 1) and therefore should be able to be isolated from water and analyzed by methods similar to those for alachlor and metolachlor. Because there were no discussions of analytical methods for acetochlor in environmental water samples found in the literature, there is a need for background information to hasten the incorporation of acetochlor into existing analyticalmethods. This paper addresses the analysis of acetochlor by both gas chromatography/mass spectrometry (GC/MS) and highperformance liquid chromatography/diode array detection (HPLC/DAD). The analysis of acetochlor is discussed in relation to three high-use herbicides: atrazine, alachlor, and metolachlor ( I ) . Concentrations and fluxes of acetochlor in rain and in the Blue Earth River in south-central Minnesota from the spring and early summer of 1994 are
0013-936X/95/0929-1702$09 OO/O
C 1995 American Chemical Society
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FIGURE 3. Total-ion gas chromatogram of commercial acetochlor (Harness+) with the retention times of three other corn herbicides indicated. GCMS conditions: column, DE-5.30 m, 0.25 mm i.d., 0.25 p m film; injection temperature, 270 " C injection volume, 2pL; flow rate, 1.4 ml/min helium; GC temperature gradient, initially 100 "Cfor 5 min, 6.0 " C h i n to 280 "C, total run time 23.0 min; MS mode: scan, MS temperature, 190 "C.
reported and compared with the concentrations and fluxes of alachlor, atrazine, and metolachlor.
Experimental Section In the United States, acetochlor is marketed as Harness+ (Monsanto Co.) and Surpass EC (Zeneca). In this study, Harness+, which is 75.4% active ingredient, was dissolved in methanol for HPLC analysis [Waters: pump (600E),diode array detector (9961, and autosampler (717+)1 and GC analysis [Hewlett-Packard:GC (5890 Series111,MS (5971A), and autosampler (737611. The methanol solution of the commercial mixture was used to determine HPLC and GC retention times, Wspectra, and mass spectra for acetochlor and other compounds present in the commercial mixture. The HPLClDAD and GUMS analyseswere performedwith methods established for analysis of the other corn herbicides. The details of the HPLC and GC methods are described in the captions of Figures 2 and 3, respectively.
An analytical standard of acetochlor (Axact Standards) confirmed the retention time and spectra of acetochlor obtained with the commercial product and established the relative response factor for quantification of environmental samples. River water and rain samples were obtained at a site on the Blue Earth River near Frost, MN. The herbicides were isolated from the water by solid-phase extraction (SPE) [500 mg, octadecyl Empore SPE-disk (3M)l. For both the rain and river samples, the SPE disks were cleaned and preconditionedwith 5 mL of 70:30 (v/v)hexanel2-propano1, 5 mL of methanol, and 10 mL of "organic-free" water. Weekly composited rain samples were collected by a wetonly rain sampler (AeroChemetrics) modified to sample for pesticides (7).The pesticides were isolated in real time during periods of rain with the SPE disks. Butachlor and terbutylazine were added as recovery surrogates. Weekly composited, flow-weighted river water samples were obtained by an automatic stream sampler (ISCO). After butachlor and terbutylazine were added as recovery surrogates, up to 1 L of river water was passed through a 47mm SPE disk preceded by a glass-fiber filter (Whatman GFF). The SPE disks were air dried and extracted with about 15 mL 70:30 (vlv) hexanel2-propanol. The solvent was reduced to 100pL, transferred to an autosampler vial with a low volume insert, spiked with the internal standard 4,4'dibromobiphenyl, and analyzed by GUMS. With a 1-L water sample, this analytical method yielded a detection limit of 10 ng/L for acetochlor, alachlor, atrazine, and metolachlor. Recoveries of these four herbicides spiked into organic-free water at 200 nglL were found to be 66 f 7,67 & 3,72 & 11,and 72 & 7%,respectively. The recoveries of the two surrogates, butachlor and terbutylazine, were 72 f 7 and 67& 9%. Organic-free water blanks processed in the same manner as environmental samples were without contamination at the 10 ng/L detection limit at the retention times of these four herbicides.
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mass/charge FIGURE 4. Mass spectrum of acetochlor under the conditions described in Figure 3. For environmental water samples, the mass spectrometer was operated in the selected-ion monitoring mode. The ions with mass to charge ratios of 146 and 162 were used for quantification and confirmation, respectively.
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Sampllng Period, 1994 FIGURE 5. Concentrations of atrazine, alachlor, metolachlor, and acetochlor in weekly composited rain (A) and river water (B) samples from the Blue Earth River near Frost, MN, during spring and early summer 1994.
Results and Discussion High Performance Liquid Chromatography/DiodeArray DetectionAnalysis. The analysis of the commercial product yielded a peak for acetochlor and other smaller peaks in the chromatogram (Figure2). Acetochlor and alachlor had the identical retention time under these chromatographic conditions, which were developed for atrazine, alachlor, and their metabolites. The small chromatographic peaks were not identified. The UV spectra of acetochlor and alachlor (maximum UV absorbances at 217.8 nm) were nearly identical, as might be expected from the similarity of their chemical structures (Figure 1). These initial results indicate that acetochlor can be identified by HPLC with UV detection with a detection limit of about 200 ng/L (based on a 1-L water sample), but precautions need to be taken to separate alachlor and acetochlor. Because both compounds may be in environmentalwater samples, acetochlor could be mistakenly identified and quantified as alachlor. Gas Chromatography/Mass Spectrometry Analysis. GUMS analysis of the commercial product yielded a peak for acetochlor, one other significant peak, and a number of smaller peaks. Acetochlor (Figure 3) had a retention time between metribuzin and alachlor under these chromatographic conditions, which were developed to separate atrazine, alachlor, metolachlor, and metribuzin. The other peaks were not identified. The mass spectrum of acetochlor is presented in Figure 4. The retention time of acetochlor and its mass spectrum are such that it could be added without complications to many existing GUMS analytical methods that analyze for other corn herbicides. Detections in River Water and Rain Samples. The Blue Earth River drains an area of intensive corn and soybean production in south-central Minnesota. Weekly compos1704
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ited river water and rain samples were obtained as part of an on-going study of the importance of wet deposition of pesticides. The initial data from this study (Figure 5) are the first observations of acetochlor in natural waters. Acetochlor was consistently observed throughout the spring and early summer in the rain and in the river at this site. The concentrations in both rain and river water were in the range of 10-250 ng/L in most samples. This was similar to the concentrations of other common corn herbicides (atrazine,alachlor,and metolachlor) in many ofthe samples. The calculated riverine fluxes for the time period May through mid-Julywere0.7,1.9,4.4,and3.8 kgfor acetochlor, alachlor, atrazine, and metolachlor, respectively. The calculated fluxes in rain to the river basin upstream of the sampling site (495 km2)for the same time period were 15, 29, 110, and 12 kg, respectively. Since this is the first year (1994)of registered use of this compound on corn in the United States, it is largely unknown how much acetochlor was used in the Blue Earth River basin compared to the other established corn herbicides. It was estimated that about 10%of the farmers in this basin purchased acetochlor in the spring of 1994 [Shane Johnson, 1995, Faribault (Minnesota) County Soil and Water Conservation District, personal communication]. The mere presence of acetochlor in rain and surface water, particularly at concentrations and fluxes similar to these other three herbicides, indicates the need for continued monitoring and for studies of its environmental behavior and fate.
Acknowledgments This work was funded by the Minnesota Department of Agriculture's Surface Water Monitoring Program and the US. Geological Survey's National Water-QualityAssessment
Program. The use of brand, firm, or trade names in this paper is for identification purposes only and does not constitute endorsement by the U.S.Geological Survey.
literature Cited (1) Gayness, L. P.; Puffer, C. Herbicide Use in the United States; Resources for the Future: Washington, DC, 1991; 127 pp. (2) United States Environmental Protection Agency, Prevention, Pesticides and Toxic Substances. Questions and Answers, Conditional Registration ofilcetochlor; U.S. EPA Washington, DC, Mar 11, 1994; 18 pp. (3) Cohen, R. Crop Protect. 1992, 1 1 , 181-185. (4) Jablonkai, I.; Hatzios, K. K. Pestic. Biochem. Physiol. 1991, 41, 221-231.
(5) Van Rensburg, E.; Van Dyk, L. P.; De Swardt, G. H. South Afi.I. Plant Soil 1990, 7, 113-116. (6) Weber, J. B.; Peter, C. J. Weed Sci. 1982, 30, 14-20. (7) Capel, P. D. In US. GeologicalSuruey ToxicSubstancesHydrology Program-Proceedings of the technical meeting, Mallard, G. E., Aronson, D. A., Eds.; USGS WRIR-91-4034; U.S. Geological Survey: Denver, CO, 1991; pp 334-337.
Received for review October 24, 1994. Revised manuscript received February 28, 1995. Accepted March 6, 1995. ES9406571
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