Environ. Sci. Technol. 1994,28, 2228-2232
RESEARCH COMMUNICATIONS Herbicides in the Great Lakes Shawn P. Schottler and Steven J. Eisenreich' Gray Freshwater Biological Institute and Department of Civil Engineering, University of Minnesota, P.O. Box 100, Navarre, Minnesota 55392
Introduction The herbicides atrazine, alachlor, and metolachlor are intensively used on croplands in the Great Lakes basin. The Laurentian Great Lakes, representing 20% of the earth's freshwater resource are potentially threatened by loadings of these herbicides, yet little data exist on their occurrence and importance in the Great Lakes. Limited measurements made in the eastern basin of Lake Ontario in 1990 yielded concentrations of about 90 ng/L atrazine. Based on these measurements, an intensive 2-year research program was initiated to quantify the occurrence and environmentalbehavior of selected high-use herbicides in the Great Lakes. Atrazine and desethylatrazine (DEA) were present in all samples. Average concentrations of atrazine ranged from about 20-35 ng/L in Lakes Huron and Michigan to 70-110 ng/L in Lakes Ontario and Erie (Table 1). Inventories of atrazine plus metabolites in the Great Lakes may exceed 600 000 kg, with water column residence times on the order of years. Current use of atrazine in the U.S. Great Lakes basin is estimated a t greater than 2700 t annually (1). Atrazine and other high-use herbicides occur in rivers (2-7), groundwaters (8),and precipitation (9, IO) at concentrations exceeding 1000-10 000 ng/L. Agricultural rivers in the Lake Erie basin deliver annual flow weighted mean concentrations of atrazine, alachlor, and metolachlor between 500-2400 ng/L and maximum concentrations greater than 20 000 ng/L ( 2 , 3 ) . However, little data exist on the presence of these herbicides within the Great Lakes. The objective of this research was to quantify the concentrations, sources, and fate of selected high-use herbicides in Lakes Michigan, Huron, Erie, and Ontario. Methodology Our strategy utilized the oceanographic approach of constructing synoptic vertical profiles of atrazine [and transformation products desethylatrazine (DEA) and deisopropylatrazine (DIA)], alachlor, and metolachlor in all the lakes for two successive years. Water column profiles of herbicide concentrations representing 4-10 depths per site were constructed for 10 sites in Lake Michigan, four sites in Lake Huron, five sites in Lake Erie, and 7 sites in Lake Ontario (Figure 1). A combined total of 490 samples were collected from the four lakes in September 1991 and August 1992. In addition, two sites in central Lake Erie were revisited in early and late June ~
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Environ. Scl. Technol.. Vol. 28,No. 12, 1994
1993 to more closely examine the potential for seasonal variations. Although Lake Superior was not a focus of this study, two sites in the western basin in 1990 and one site in the eastern basin in 1993 were sampled. Four-liter water samples were collected using a General Oceanics (Model 1015) rosette sampler on board the U.S. EPA Research Vessel L u k e Guardian. Herbicides were isolated from 2-L unfiltered samples using 5-g C-18 solidphase extraction cartridges (Varian Corp.) and extracted with diethyl ether. At sample volumes greater than 2-L, DEA and DIA exhibit about 40 % and 90 % breakthrough from the extraction cartridges, respectively. To compensate for this, 35 % of the samples was processed in duplicate using a 350-mL sample volume, thus achieving greater than 75% retention of the metabolites. Extracts were concentrated to 100pL under agentle stream of purified Nz and analyzed by injection of 2 pL onto a gas chromatograph (GC-HP 5890) coupled to a mass spectrometer (MS-HP 5971A). The GC was equipped with a 30-m DB-5 capillary column with 0.25 mm i.d. and 25 pm film thickness. The MS was operated in selective ion monitoring mode, with at least one confirmation ion used for all herbicides. Details of analytical procedures are presented by Schottler et al. (7). Limits of detection for atrazine, alachlor, and metolachlor were -0.01 ng injected, requiring sample concentrations of a t least 5 ng/L. Detection limits for DEA and DIA required a sample concentration of 10 ng/L for DEA (based on a 2-L sample), and 30 ng/L for DIA (based on a 350-mL sample volume). For Lake Superior samples, 4-L of water was passed through the extraction cartridge, and extracts were concentrated to less than 75 pL. Using this technique, it was possible to lower the detection limit for atrazine, alachlor, and metolachlor in Lake Superior samples to -3 ng/L. However, using these large volumes of water for the Lake Superior samples result in greater than 80 % breakthrough of DEA and DIA, effectively increasing the detection limit to greater than 100ng/L. Recoveries (and relative standard deviations, RSD) for the analytes of interest spiked in deionized water at concentrations of about 100 ng/L were as follows: atrazine 98% (RSD 9%),alachlor 93% (RSD lo%), metolachlor 97% (RSD 22%), DEA 96% (RSD 18%),and DIA 75% (RSD 28%). Procedural recoveries, measured by the recovery of terbutylazine and butachlor in each sample as surrogates for the triazines and acid amides respectively ranged from 60 to 120% . All samples were corrected for surrogate recovery.
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0013-936X/94/0928-2228$04.50/0
0 1994 Amerlctnn Chemical Society
Table 1. Concentrations and Inventories of Herbicides in the Great LakesP Erie Michigan
Huron
atrazine, 1991 atrazine. 1993 DEA, 1991 DEA, 1993 metolachlor, 1991 metolachlor, 1992 Alachlor, 1991 Alachlor, 1992
34 37 16 24 Tr-5 Tr-5 Tr Tr
21 23 18 26 Tr Tr ND Tr
atrazine atrazine + DEA
175 275
77 155
(western basin) Concentrations (ng/L) 40 75 27 47 10 20 Tr-5 Tr-5 Inventory (T)
(eastern basin)
Ontario
Superior
110
93 39 67 28 25 Tr-5 Tr-5
83 97 43 67 15 15 Tr Tr
3 3 ND ND ND ND ND ND
45b 68b
145 235
36 >36
0 Atrazine and DEA were detected in all samples collected from Lakes Michigan, Huron, Erie, and Ontario; metolachlor was present in >80% samples. Alachlor was confirmed in less than 40% of the samples with concentrations at or just below (trace concentration) the method detection limit of 5 ng/L. (Tr-5 denotes that the herbicide was detected in the range of trace to 5 ng/L.) DIA was detected at trace levels (
