Environ. Sci. Technol. 1990, 2 4 , 1400-1406
Sevon, M. Nevada Department of Wildlife, Fallon, NV, personal communication, 1988. Thommes, M. M.; Lucas, H. F., Jr.; Edgington,D. N. Proc. 15th Conf. Great Lakes Res. 1972, 192-197. Chau, Y. K.; Saitoh, H. Int. J. Environ. Anal. Chem. 1973,
Holland Biomedical Press: Amsterdam, 1979; pp 161-173. (56) Lindqvist, 0.; Jernelov, A.; Johansson, K.; Rodhe, H. Mercury in the Swedish Environment, Global and Local Sources. Report submitted to the National Swedish En-
3, 133-139.
(57) Temmerman, E.; Dumarey, R.; Dams, R. Anal. Lett. 1985, 18, 203-216. (58) Nojiri, Y.; Otsuki, A.; Fuwa, K. Anal. Chem. 1986, 58, 544-547. (59) Uchino, E.; Kosuga, T.; Konishi, S.; Nishimura, M. Enuiron. Sci. Technol. 1987, 21, 920-922.
Potter, L.; Kidd, D.; Standiford, D. Environ. Sci. Technol. 1975, 9, 41-45.
Schell, W. R.; Barnes, R. S. In Aqueous Environmental Chemistry of Metals; Rubin, A. J., Ed.; Ann Arbor Science Publication: Ann Arbor, MI, 1976; pp 129-165. Turner, R. A.; Lindberg, S. E. Enuiron. Sci. Technol. 1978, 12, 918-923.
Gardner, D. Water Res. 1978, 12, 573-575. Fitzgerald, W. F. In The Biogeochemistry of Mercury in the Enuironment; Nriagu, J. O., Ed.; Elsevier/North
vironmental Protection Board, Solna, Sweden, 1984.
Received for review June 13,1989. Accepted April 23,1990. This work was supported by the California State Water Resources Control Board, Contract 5-246-250-0 and by the Electric Power Research Institute, Contract RP2020-9.
Occurrence, Distributions, and Transport of Herbicides and Their Degradation Products in the Lower Mississippi River and Its Tributaries Wilfred E. Pereira” and Colleen E. Rostad
U.S. Geological Survey, Box 25046, Mail Stop 408, Denver Federal Center, Denver, Colorado 80225-0046
The Mississippi River and its tributaries drain extensive agricultural regions of the midcontinental United States, where large amounts of herbicides are applied as weed control agents on crops such as corn and soybeans. Studies being conducted by the U S . Geological Survey along the lower Mississippi River and its major tributaries, representing a 1930-km river reach, have confirmed that several triazine and chloroacetanilide herbicides and their degradation products are present in this riverine system. These herbicides include atrazine, and its degradation products, desethyl- and desisopropylatrazine; cyanazine; simazine; metolachlor; and alachlor and its degradation products, 2-chloro-2’,6’-diethylacetanilide, and 2-hydroxy-2’,6‘-diethylacetanilide. Loads of these compounds were determined at 17 different sampling stations under various seasonal and hydrologic conditions, during five sampling trips from July 1987 to June 1989. Stream loads of herbicides were relatively small during the drought of 1987 and 1988. Stream loads were much greater during the relatively wet year of 1989. Trace levels of atrazine, cyanazine, and metolachlor also were associated with suspended sediments. Distribution coefficients (K,) of these compounds varied considerably between sites and were much larger than K , values reported in the literature. The annual transport of atrazine into the Gulf of Mexico was estimated to be less than 2% of the amount of atrazine applied each year in the midwest.
Introduction Synthetic organic agrochemicals are applied annually on agricultural soils in the United States to improve crop yields. New compounds are continually being introduced for agricultural use as insecticides, herbicides, and fungicides, as older generation compounds are phased out of agricultural practice. Many synthetic organic agrochemicals applied to crops eventually are transported to surface waters by various mechanisms, such as nonpoint source runoff, groundwater discharge, or atmospheric deposition. It has been estimated that pesticide losses in runoff from agricultural fields generally are -0.5% or less of the amounts applied, unless severe rainfall conditions occur within 1-2 weeks after application (1). 1400
Environ. Sci. Technol., Vol. 24, No. 9, 1990
Because millions of kilograms of relatively water soluble herbicides such as atrazine, simazine, cyanazine, alachlor, and metolachlor are applied each year in the United States as pre- and postemergent weed-control agents on crops such as corn and soybeans (2))it is reasonable to assume that substantial amounts of these compounds and their degradation products may be present in surface waters draining agricultural areas of the country. About 75% of the nation’s corn and 60% of the nation’s soybeans are grown in the midcontinental United States. The Mississippi River basin drains these agricultural areas. Therefore, nonpoint source pollution of the Mississippi River by synthetic organic agrochemicals and their degradation products is a major factor to be considered in water quality assessments. The Mississippi River is the largest river in the United States. It flows 3732 km from its source at Lake Itasca in northern Minnesota to the Gulf of Mexico. The Mississippi River and its tributaries drain -41% of the contiguous United States, discharge an average of 18400 m3 of water/s, and transport several hundred million tons of suspended sediment to the Gulf of Mexico each year. Seventy-five percent of the sediment in the Mississippi River is derived from the Missouri River, whereas nearly 50% of the total water discharged a t the mouth of the Mississippi River comes from the Ohio River, and less than 15% of the total water comes from the Missouri River. Many herbicides and their degradation products that enter the Mississippi River from the tributaries are transported down the lower Mississippi River and, eventually, are discharged into the Gulf of Mexico. Studies being conducted by the U.S Geological Survey at 17 different sites along the lower Mississippi River and its major tributaries, representing a 1930-km river reach, have confirmed that many of these compounds are distributed in water and suspended sediments. An earlier study also reported the presence of herbicides in the Mississippi River (3). This report presents the preliminary results of a study to determine (a) occurrence and distributions of synthetic organic agrochemicals, (b) mixing and redistribution of agrochemicals downstream from major river confluences, (c) processes that affect the fate
Not subject to U S . Copyright. Published 1990 by the American Chemical Society
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Figure 2. Distribution coefficients (K,) for atrazine, cyanazine, and metolachlor and suspended sediment concentrations at sampling sites (May-June 1988). Table I. Physicochemical Properties and Soil Half-Life of Herbicidesa
compound
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Figure 1. Sampling sites along the lower Mississippi River and its major tributaries.
and transport of these compounds in the lower Mississippi River and its tributaries.
Experimental Section Sample Collection. Representative depth-integrated water samples were collected in July-August 1987, November-December 1987, May-June 1988, March-April 1989, and May-June 1989 at 17 sites along the lower Mississippi River and its major tributaries by the methods of Nordin et al. ( 4 ) and Meade (5). The basic sampling scheme used the equal transit rate (equal width increment) depth integration method, using 10-30 verticals per cross section. Discharge measurements were made according to the procedure described by Nordin ( 4 ) . The Lagrangian sampling strategy was used (6, 7). The vessel from which the samples were collected traveled downstream at mean river velocity, so that the same parcel of water was sampled at each site in a sequential manner. However, because of logistical reasons, true Lagrangian sampling was never achieved. The low-water cruise of May-June 1988 was the closest attempt to Lagrangian sampling. Samples were collected off the research vessel Acadiana with a hydraulic winch and collapsible bag sampler containing an FEP-Teflon bag. Samples were composited in a Teflon-coated churn splitter, and subsamples were collected in clean FEP-Teflon bottles containing chloroform as preservative. Samples were refrigerated and were analyzed within 1-2 weeks of collection. Suspended sediments were collected in May-June 1988 by pumping a large volume of water (300-500 L) at each vertical along a cross section, from a point at middepth, through a Sharples Model AS-12 continuous-flow super centrifuge provided with an FEP-Teflon liner. Samples were preserved with chloroform and refrigerated until analysis. Details of these sampling and collection procedures have been described by Leenheer (8).
simazine atrazine alachlor metolachlor cyanazine
solubility, rg/mL
K,, g/mL
soil half-life, days 60
240
138 100 170
530
200
20
170
190
14
6.2
33
75 15
a Reference: Soil Conservation Service, 1980. Water Quality Workshop: Integrating Water Quality and Quantity into Conservation Planning. SCS National Technical Center, Ft. Worth, TX.
Sample Preparation and Analysis. Water samples were filtered through glass fiber filters (Gelman Sciences, type A-E). (Any trade names are for descriptive purposes only and do not constitute endorsement by the U S . Geological Survey.) A l-L aliquot of sample was adjusted to pH 8.5 with a small amount of 10% KOH solution. After the addition of 10 g of NaCl and an internal standard solution containing the atrazine analogue terbuthylazine, the sample was extracted with three volumes of methylene chloride (75, 50, and 50 mL). The combined methylene chloride extracts were dried over anhydrous Na2S04and concentrated in a Kuderna-Danish apparatus to a volume of approximately 8 mL. The extract was concentrated further to a volume of 100 pL under a slow stream of dry N2. The extract was then analyzed by gas chromatography-ion trap mass spectrometry (9). All compounds were confirmed by comparing their mass spectra and retention indexes to authentic standards analyzed under identical conditions. Replicate samples were analyzed at several sites during each sampling trip. Field equipment and laboratory blanks were found to be devoid of the analytes under investigation. Average percent recovery of the surrogate analyte terbuthylazine was greater than 99%. Recoveries of the analytes of interest spiked into three replicates of Mississippi River water at -0.5 pg/L are as follows: simazine, 82% (RSD, 36%); atrazine, 117% (RSD, 12%); cyanazine, 98% (RSD, 19%); alachlor, 68% (RSD, 8%); metolachlor, 94% (RSD, 870); 2,6-diethylaniline, 40% (RSD, 11% ); desisopropylatrazine, 34% (RSD, 2%); desethylatrazine, 74% (RSD, 15%); 2hydroxy-2',6'-diethylacetanilide, 102% (RSD, 18%). Concentration values reported are uncorrected for percent recovery. The lower limit of detection of analytes in Mississippi River water samples was 5 ng/L, with a signal to noise ratio greater than 511. Environ. Sci. Technol., Vol. 24, No. 9, 1990 1401
Atrazine
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WATER DISCHARGE, I N CUBIC METERS PER SECOND
Figure 3. Loads of herbicides and degradation products (kilogramslday)in the lower Mississippi River and its major tributaries (May-June 1988;
Suspended sediments were dewatered by centrifugation and dried under N P . The samples were uniformly ground with a glass mortar and pestle. The sample (15 g) was spiked with an internal standard solution (terbuthylazine), equilibrated for 15 min, and extracted in a centrifuge tube with acetone plus four drops of 10% KOH solution (2 X 20 mL), followed by hexane (5 mL), using a Teckmar sonic disruptor. The organic extracts were separated by centrifugation, combined, and concentrated to a small volume in a Kuderna-Danish apparatus. The extract was then fractionated on a small column of neutral alumina. Four fractions were collected with the following eluants: (a) 10 mL of hexane, (b) 10 mL of benzene, (c) 10 mL of methylene chloride, (d) 10 mL of methlene chloride-methanol (l:l, v/v). Fraction d was analyzed for herbicides and degradation products by gas chromatography-positive ion chemical ionization tandem mass spectrometry (IO). Average percent recovery of terbuthylazine was 81%. Results and Discussion Hydrology. Sampling sites for this study along the lower Mississippi River and its major tributaries are shown 1402
Environ. Sci. Technol., Vol. 24, No. 9, 1990
in Figure 1. Sampling sites were selected a t strategic locations, so that maximum information could be obtained about contaminant transport above and below major river confluences. Because of drought conditions in the midwest, the Mississippi River and its major tributaries generally were at low flow stage during the first three sampling trips in 1987 and 1988. The river stage at Memphis, TN, reached an all-time record low flow in July 1988. Maximum measured river discharge was 10400 m3/s in December 1987 at Vicksburg, MS. In 1989, the water discharge at Vicksburg, MS, was -26500 m3/s in March, and 24800 m3/s in June (J. Moody, US. Geological Survey, written communication). Transport of Herbicides by Suspended Sediments. One of the objectives of this study was to investigate mixing, movement, and storage of sediment-associated contaminants in the Mississippi River. Suspended sediments collected in May-June 1988 from different sites were analyzed for herbicides and their degradation products. Only trace amounts of atrazine, cyanazine, and metolachlor were found to be associated with suspended sediments.
-
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WATER DISCHARGE, IN CUBIC METERS PER SECOND
numbers indicate loads in kilograms/day).
Distribution coefficients (Kd) of atrazine, cyanazine, and metolachlor were calculated from the concentrations of these compounds in the dissolved and suspended phases at different sampling sites. Percent organic carbon of each suspended sediment was determined, and Kd values were normalized to the organic carbon content to give K, values (organic carbon values ranged from 3 to 5 % ). From the Kd values and the suspended sediment concentrations, it was determined that 99.5% of these compounds was in the dissolved phase and less than 0.5% in the suspended phase. Suspended sediment concentrations, and K, values for atrazine, cyanazine, and metolachlor, at each sampling site are shown in Figure 2. Experimentally determined K , values differed considerably between sites and are much greater than K, values reported in Table I. K , values reported in the literature are usually determined on agricultural soils under equilibrium conditions. Suspended sediment concentrations in the Mississippi River in May-June 1988 were abnormally low. The larger K , values reported in this study are probably due to nonu-
-
niform mixing of suspended sediment and water resulting in disequilibrium. In an dynamic system such as the Mississippi River, which receives herbicides from various tributaries, equilibrium conditions are rarely attained. Transport of Herbicides and Their Degradation Products in the Dissolved Phase. Several triazine and chloroacetanilide herbicides and their degradation products were identified in water samples collected during all five sampling trips. These compounds included atrazine and its degradation products, desethyl- and desisopropylatrazine; simazine; cyanazine; alachlor; and metolachlor. In addition to these compounds, three possible degradation products of alachlor, 2-chloro-2’,6‘-diethyland 2,6acetanilide, 2-hydroxy-2’,6’-diethylacetanilide, diethylaniline were confirmed and quantified by GC-ion trap mass spectrometry. These five parent compounds are the major herbicides used in the midwest (2). Concentrations of these compounds in water for the May-June 1988 sampling trip are shown in Table 11. Data in Table I1 indicate that the Missouri, Illinois, and Ohio Rivers are Environ. Sci. Technol., Vol. 24, No. 9, 1990
1403
T a b l e 11. Concentrations (ng/L) of Herbicides a n d T h e i r Degradation P r o d u c t s i n Water Samples from t h e Mississippi River a n d Its T r i b u t a r i e s (May-June 1988s.
Stationb 1. 2. 3. 4.
Mississippi River near Winfield, MO Illinois River below Meredosia, IL Missouri River at Herman, MO Mississippi River at St. Louis, MO‘
5. Mississippi River a t Thebes, IL 6. Ohio River at Olmsted, IL‘ 7. Mississippi River below Hickmann, KY 8. Mississippi River a t Fulton, TN‘ 9 Mississippi River a t Helena, AR 10. White River at mile 11.5, AR 12. Mississippi River above Arkansas City, AR 13. Yazoo River a t mile 10, MS 14 Mississippi River below Vickburg, MS 15. Old River Outflow Channel nar Knox Landing 16. Mississippi River at St. Francisville, LA 17. Mississippi River at Belle Chase, LA a
2-chloro2-hydroxy2,g-diethyl- desisopropyl- desethyl- 2’,6’-diethyl- sima- atra- 2’,6’-diethyl- ala- metol- cyanaaniline atrazine atrazine acetanilide zine zine acetanilide chlor achlor zine 6 ND ND 924 843 805 183 ND ND ND 253
8 16 23 32 20 23 22 27 24 45 14
71 82 43 65 67 69 67 40 33 40 58
ND 41 ND 208 185 147 350 ND ND
202 127 165 94 ND 64
19 17 22 12 17 16
66 56 66 48
ND 36
10 21
15
113
19 66 96 79 27 76 34 78 81 80 59
256 694 616 513 474 498 521 261 255 266 439
15 ND ND 16 19 52 28 6 9 ND ND
147 388 137 799 723 723 900 36 35 43 387
164 224 273 225 239 228 216 72 79 98 157
33 647 556 405 446 424 357 43 80 93 213
240 233 253 181 6 119
57 59 130 46 ND 50
501 499 487 483 67 507
10 20 ND 6 ND 9
542 565 597 514 6 387
157 167 166 155 46 171
145 173 195 163 ND 209
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6
6
82
41
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35 18
41 337
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17 135
15
54
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51
485
6
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211
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ND, not detected. bSite 11, Arkansas River not sampled. “Triplicate samples collected.
significant contributors of herbicides and their degradation products to the lower Mississippi River. In general, concentrations of herbicides were greatest in the summer months, after spring application, and decreased significantly in winter. One of the objectives of the study was to determine mixing and redistribution of synthetic organic agrochemicals downstream from major river confluences. Loads were determined by multiplying the concentration of the analyte by the measured river discharge at the time of sampling at each site. Calculation of stream loads at each sampling station provides valuable information about transport of pollutants. This information is useful for (a) providing estimates of pollutant transport past each sampling station on a daily or annual basis, (b) validating sampling techniques, (c) distinguishing point sources of pollutants from nonpoint sources, and (d) providing information about the conservative or nonconservative behavior of pollutants. Loads of herbicides and their degradation products (kilograms/day) in the lower Mississippi River and its tributaries in May-June 1988 are shown as a function of discharge and river kilometers in Figure 3. 2,6-Diethylaniline has been reported to be a degradation product of alachlor (21). It is also used as a starting material in the synthesis of alachlor as well as several other organic chemicals. Loads of 2,6-diethylaniline indicate that this compound is generated at a point source near St. Louis. Because 2,6-diethylaniline was not found in any of the tributaries, it probably is unique to this point source near St. Louis. Similarly, alachlor and its degradation products 2-chloro-2’,6’-diethylacetanilideand 2-hydroxy-2‘,6‘-diethylacetanilide also are generated a t a point source near has been reSt. Louis. 2-Chloro-2’,6’-diethylacetanilide ported to be a degradation product of the herbicides alachlor (11,121 and antor (13). The presence of alachlor, 2-chloro-2’,6’-diethylacetanilide,and 2-hydroxy-2‘,6‘-diethylacetanilide in the tributaries indicates that these compounds also are derived from nonpoint sources. 1404
Environ. Sci. Technol., Vol. 24, No. 9, 1990
If a compound is conservative and there are no localized inputs, then the sum of the loads in each tributary above its confluence with the Mississippi River must be equal to the load below their confluences with the Mississippi River. Loads of atrazine and its degradation products desethyl- and desisopropylatrazine; simazine; cyanazine; and metolachlor indicate that the sum of the loads of these compounds in the Missouri, upper Mississippi, and Illinois Rivers are in reasonable agreement with their loads at St. Louis. A similar finding is observed further downstream, where the sum of the loads of these compounds in the Mississippi River at Thebes and the Ohio River at Olmsted are in reasonable agreement with their loads in the Mississippi River at Hickman. According to federal law, 30% of the water from the Mississippi and Red Rivers is diverted through the Old River Outflow Channel, to the Afchafalaya River. Figure 3 shows that the loads of these conservative herbicides and their degradation products in the Old River Outflow Channel are approximately 2630% of the loads of these compounds a t Vicksburg, indicating a good materials balance. This agreement in mixing loads below major river confluences attest to the validity of the equal transit rate (equal width increment) depth integration sampling and analytical protocols used in this study. The data also indicate that these compounds exhibit relatively conservative behavior as compared with other hydrophobic compounds in the lower Mississippi River. Seasonal Variations in Herbicide Transport. The effect of seasonal variations and hydrologic conditions on transport of herbicides was examined for five sampling trips conducted in 1987 through 1989. Stream loads of atrazine, simazine, cyanazine, alachlor, and metolachlor are shown in Figure 4a and b. Although normal amounts of herbicides were applied in 1987 and 1988, due to severe drought conditions there was very little agricultural runoff in the midwest. This resulted in low stream loads shown in Figure 4a and b. In 1989, which was a relatively wet year, significant amounts of stored herbicides and their degradation products were flushed out of agricultural soils
2.500
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Figure 4. Streams Loads of atrazine, simazine, cyanazine, alachlor, and metolachlor for five sampling trips in the Mississippi River and its tributaries (1987-1989): site numbers correspond to sites in Figure 1 and Table 11.
by rainfall and snow melt, resulting in the larger stream loads in the Mississippi River. Atrazine is degraded by soil microorganisms by mechanisms involving dealkylation, deamination, dehalogenation, and hydroxylation (14, 15). Desethyl- and desisopropylatrazine are major degradation products of atrazine. Ratios of desethylatrazine/atrazine were examined as an index of the effect of seasonal variations on biodegradation of atrazine. These ratios are shown in Figure 5. The March-April 1989 sample collection was conducted during the herbicide preapplication period, and the May-June 1989 sample collection was completed during the herbicide postapplication period in the midwest. The higher ratios of desethylatrazine/atrazine in the Mississippi River during the herbicide preapplication period in March and April probably are due to the longer residence time of atrazine in soils between the herbicide application period in 1988 and the herbicide preapplication period in 1989. This longer residence time of atrazine in soils resulted in greater biodegradation of atrazine to desethylatrazine. The annual loads of herbicides and their degradation products transported in the dissolved phase from the Mississippi River into the Gulf of Mexico was estimated from the average herbicide loads at Belle Chasse, LA (the last sampling station before the Gulf of Mexico), for each
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Flgure 5. Ratios of desethylatrazine to atrazine in the Mississippi River and its tributaries (March-April 1989) and MayJune 1989; site numbers correspond to sites in Figure 1 and Table 11.
sampling period in 1987 and 1989. These transport numbers are shown in Table 111. The 1987 transport number for atrazine represents -0.4% and the 1989 number represents -1.7% of the total amount of atrazine applied annually in the midwest. It is interesting to note that although the usage of atrazine and alachlor in the midwestern United States is about the same, in 1989,4 times as much atrazine was transported to the Gulf of Mexico. These transport data are consistent with the environmental Environ. Sci. Technol., Vol. 24, No. 9, 1990
1405
Table 111. Estimated Annual Transport of Selected Herbicides and Their Degradation Products into the Gulf of Mexico* compound atrazine metolachlor alachlor simazine desethylatrazine desisopropylatrazine 2-chloro-2’,6’-diethylacetanilide Values in metric tons.
1987
1989
105
429 212 129
26 4 6
7
68 47
2
9 6
ndb
nd, not determined.
persistence differences of atrazine and alachlor shown in Table 1. In a large dynamic river such as the Mississippi, these seemingly large annual loads reported in Table I11 only represent sub part per billion concentrations in the dissolved phase. Nevertheless, the environmental effects of low concentrations of herbicides and their degradation products on the off-shore environment of the Gulf of Mexico are not well understood and merit further study.
Conclusions This report clearly demonstrates that the lower Mississippi River and its major tributaries are contaminated by herbicides and their degradation products. The upper Mississippi, Illinois, and Ohio Rivers are significant contributors to the herbicide loads in the lower Mississippi River. These compounds are transported mainly in the dissolved phase. Seasonal and hydrologic conditions control the loads of herbicides and their degradation products in the Mississippi River. Loads of herbicides a t 17 different sampling sites ranged from hundreds of kilograms per day during 1987 and 1988 to thousands of kilograms per day in 1989. Stream loads provided valuable information about transport of herbicides above and below major river confluences. In addition, point sources of these compounds were distinguished from nonpoint sources. Mass balances of the loads of atrazine, desethylatrazine, simazine, cyanazine, and metolachlor above and below major river confluences were in reasonable agreement, indicating their conservative behavior. These compounds were derived mainly from nonpoint sources. Alachlor and its degradation products, 2-chloro-2’,6’-diethylacetanilide and 2-hydroxy-2’,6’-diethylacetanilide,were generated mainly at a point source near St. Louis. Sampling and analytical strategies described are essential in understanding the transport and fate of herbicides in large river systems. Registry No. P,B-Diethylaniline, 91-66-7; desisopropylatrazine, 82623-63-0; desethylatrazine, 6190-65-4; 2-chloro-2’,6’-diethylacetanilide, 6967-29-9; simazine, 122-34-9; atrazine, 1912-24-9; 2-hydroxy-2’,6‘-diethylacetanilide, 52559-52-1;alachlor, 15972-60-8; metolachlor, 5 1218-45-2; cyanazine, 2 1725-46-2. Literature Cited (1) Wauchope, R. D. The content of surface water draining from
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Received for review August 17,1989. Revised manuscript received April 9, 1990. Accepted May 2, 1990.
