Envlron. Sci. Technol. 1993, 27, 1542-1552
Nonpoint Source Contamination of the Mississippi River and Its Tributaries by Herbicides Wllfred E. Perelra’ and Frances D. Hostettler U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025
A study of the Mississippi River and its tributaries during July-August 1991, October-November 1991, and AprilMay 1992 has indicated that the entire navigable reach of the river is contaminated with a complex mixture of agrochemicals and their transformation products derived from nonpoint sources. Twenty-three compounds were identified, including triazine, chloroacetanilide, thiocarbamate, phenylurea, pyridazine, and organophosphorus pesticides. The upper and middle Mississippi River Basin farm lands are major sources of herbicides applied to corn, soybeans, and sorghum. Farm lands in the lower Mississippi River Basin are a major source of rice and cotton herbicides. Inputs of the five major herbicides atrazine, cyanazine, metolachlor, alachlor, and simazine to the Mississippi River are mainly from the Minnesota, Des Moines, Missouri, and Ohio Rivers. Ratios of desethylatrazine/atrazine potentially are useful indicators of groundwater and surface water interactions in the Mississippi River. These ratios suggested that during baseflow conditions, there is a significant groundwater contribution to the river. The Mississippi River thus serves as a drainage channel for pesticide-contaminated surface and groundwater from the midwestern United States. Conservative estimates of annual mass transport indicated that about 160 t of atrazine, 71 t of cyanazine, 56 t of metolachlor, and 18t of alachlor were discharged into the Gulf of Mexico in 1991.
Introduction Nonpoint-source (NPS) contamination of the United States surface water and groundwater has emerged as an important environmental problem in the last decade (1). Although significant advances have been made in controlling point-source pollution, little progress has been accomplished in the area of NPS pollution of surface waters and groundwaters. This is because of the seasonality, inherent variability, and multiplicity of NPS pollution. The US. EPA has determined that agricultural activities such as tillage practices and animal waste management are the primary factors contributing to NPS pollution nationwide (2,3). Pesticide contamination of groundwaters and surface waters from agricultural use has been well documented (2,4-8). In addition, some herbicides are used on roadsides and utility rights-of-way. For select herbicides, this can exceed 10 % of the amount applied to land for agriculture, increasing nonpoint source pollution. The midwestern United States is one of the largest agricultural regions in the country, accounting for 75 % of the nation’s corn and 60% of the nation’s soybean production. Millions of kilograms of herbicide active ingredients are used on farm lands in the midwest for crop protection and to improve crop yields (9). Studies conducted by the U S . Geological Survey have shown that
* Author to whom all correspondence should be addressed. 1542 Environ. Scl. Technol., Voi. 27, No. 5 , 1993
large concentrations of herbicides are flushed from crop land in the midwestern United States and are transported through surface water as pulses in response to late spring and early summer rainfall (5). It has been further shown that many of these compounds are transported into tributaries of the Mississippi River and the lower Mississippi River, and ultimately into the Gulf of Mexico (IO). Pesticide usage on major crops in a 14-state region draining to the Mississippi River, along with their selected physicochemical properties, are shown in Table I. The major crops include corn, soybeans, sorghum, rice, and cotton. While corn, soybeans, and sorghum are grown mainly in the upper and middle Mississippi River Basin, rice, cotton, and soybeans are grown mainly in the lower Mississippi River Basin. Important factors that determine the distribution and fate of pesticides in the Mississippi River include (a) biogeochemical properties of pesticides; (b) geographic location of crop type and amount and time of pesticide application; ( c ) soil type and sorptive capacity; (d) tillage practice; and (e) variations in climatic, seasonal, and hydrologic conditions (10). In general, compounds that are relatively water-soluble and have relatively long soil half-lives are transported in agricultural runoff or groundwater to the Mississippi River. The Mississippi River is the largest river in the United States and flows 3772 km (2344 mi) from its headwaters in Lake Itasca, MN, to the Gulf of Mexico. The Mississippi River Basin is the largest watershed in the United States and drains an area of about 2.9 million km2 in 31 states and two Canadian provinces. The river is delineated into the lower Mississippi River, which extends from the Gulf of Mexico northward to the mouth of the Ohio River at Cairo, IL, and the upper Mississippi River, which extends northward from Cairo, IL, to its source in northern Minnesota. The upper Mississippi River is impounded by 29 locks and dams that are operated by the US.Army Corps of Engineers. The lower Mississippi, below St. Louis, is an unimpounded segment of the river. The Mississippi River discharges an average of 1.4 X lo9m3 of water per day to the Gulf of Mexico (11).The Mississippi River is extensively used by a large segment of the population for municipal, industrial, recreational, and agricultural purposes. The Mississippi River and its tributaries are the sources of 23 % of the public surfacewater supplies for the United States, providing water to about 70 cities. This valuable resource, however, also receives untreated and partially treated industrial and domestic wastes along its course (12). However, NPS pollution by agrochemical contributions from overland runoff and groundwater discharge may be the most significant factor responsible for deterioration of water quality in the Mississippi River and its tributaries. Although many studies of the water quality of the Mississippi River have been reported in the literature (IO, 12,13-20),there have been no comprehensive studies of inputs, distributions, and transport of pesticides in the entire river reach of the Mississippi River. In an attempt
This article not subject to U.S. Copyright.
Published 1993 by the Amerlcan Chemical Society
Table I. Physicochemical Properties, Soil Half-Life, and Major Crop Use of Selected pesticides Used in Mississippi River Basin (raf 34)
pesticide atrazine ametryn alacblor cyanazine earbofuran diazinon diuron fluometuron hexazinone molinate
metribuzin metolachlor methylparathion norflurazon prometon prometryn p!opyil simazine thiabencarb a
water solubility (mg/L) 33 185 240 170 351
60 42 110
33 m 970 1220 530 60 28 720 33 200 6.2 28
soil sorption coeff, K, (gimL)
annual usage in soil half life (day)
100 300 170 190 22 1 m 480 100 54 190 60 200 5100
600 150 400 149 130 900
14-state area (ref 9) (million8 of kg AIVyr)
50 40 90 85 90 21 40 90 5 90 500 60 1 60 21
major crop use corn, sorghum corn corn, soybeans, sorghum corn, cotton rice (insecticide) crop (insecticide) alfalfa, cotton cotton pine forests (nonnop) rice soybeans corn, soybeans, sorghum rice (insecticide) cotton weeds (noncrop) cotton rice corn rice
0.1 0.9 1.0 0.5
17 0.5 0.3 2.9 0.4 0.2
AI = active inmedient.
to evaluate the extent of nonpoint-source contamination of the Mississippi River and ita tributaries by pesticides, a water-quality study of the entire navigable reach of the Mississippi River was started in 1991. This study is an extension of an earlier study of the distributions and transport of pesticides in the lower Mississippi River and its tributaries during 1987-1989 (IO). Because the Mississippi River and ita tributaries drain about 41% of the contiguous United States, the water quality of the Mississippi River may be indicative of the quality of streams and rivers in a large portion of the nation. This report is apreliminaryassessmentof inputs,distributions, and mass transport of selected pesticides in the Mississippi River and its tributaries. Experimental Section
SampleCollection. Water sampleswere collected from the Mississippi River and its tributaries during three sampling cruisesin July-August 1991,OctoberNovember 1991,and April-May 1992. Thesesamplingcruises covered the entire navigable Mississippi River, starting above St. Anthony Falls, MN, at upper MississippiRiver mile 858.3, and concluding below Belle Chasse, LA, at lower Mississippi River mile 72.8; a total distance of 1739.3 river miles. Twelve sites from the main channel of the Mississippi River and 14 sites in the tributaries were sampled during each sampling cruise. Sampling sites are shown in Figure 1. About 32 samples including field blanks were analyzed during each sampling cruise. Collection of water samples from the lower Mississippi River and its major tributaries during 1987-1989 is described in an earlier report (IO).A summary of hydrologic characteristics for the sampling sites is shown in Table 11. Estimates of travel time based on water velocity estimates are shown in Table 111. Samples from the Mississippi River mainstream and some of the larger tributaries were collected off the research vessel Acadiana with a hydraulic winch and collapsihlebag sampler containing FEP or PFA-Teflon bags, by previouslypublishedmethods (21-23). (Theuse of brand or product names in this article is for identification purposes only and does not constitute endorsement by
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Figure 1. Sampling skes along the Mlsslssippl River and its tributaries.
the US. Geological Survey.) The basic sampling scheme used the equal-transit-rate(equal-width-increment)depthintegration method, with 1-17 verticals per cross-section. Discharge measurements were made by the method of Moody and Troutman (21). During all sampling cruises, a discharge-weightedpumping method was used to collect water samples (23) at certain sampling sites where velocities were low. Because of logistical reasons, water samplescollected from the smaller tributaries were surfacegrab samples and were not depth-integrated. Sampling EWM~. SCI. ~ h n ~ ivol. . . 27. NO. 8. is93
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Table 11. Drainage Basins, Area, a n d Mean Annual Discharges (ref 35) (from w a t e r resources data for each state for 1989)
basin
drainage (km2)
Mississippi River above St. Paul, MN Minnesota River above Jordan St. Croix River above St. Croix Falls Mississippi River above Prescott, WI Chippewa River above Durand, WI Wisconsin River above Muscoda Mississippi River above McGregor, IA Rock River above Joslin, IL Mississippi River above Clinton, IA Iowa River above Wapellow, IA Mississippi River above Keokuk, IA Des Moines River above St. Francisville Illinois River above Valley Center, IL Missouri River above Hermann, MO Mississippi River above St. Louis, MO Mississippi River above Thebes, IL Ohio River above Metropolis, IL White River above Clarendon, AR Arkansas River above Murray Dam Mississippi River a t Vicksburg, MS
49 500 42 000 17 700 116 000 23 300 26 900 174 800 24 700 221700 32 400 308200 36 400 69 700 1358 000 1805 000 1847000 528300 60 700 4093000 2 954 000
mean annual discharge (m3/s) 225 107 131 488 216 246 1000
174 1350 197 1810 165 635 2 190 5 150 5600 7720 738 1190 17 360
Includes upper Arkansas Basin, which contributes almost no water to the Mississippi River. (I
Table 111. Estimates of Water Travel Time i n t h e Mississippi River (ref 35) reach Minneapolis to St. Louis St. Louis to Baton Rouge Baton Rouge to Gulf of Mexico
travel time (day) longest median shortest 47 18 4
31 15 3
14 8 2
sites and discharge measurements for three sampling cruises on the Mississippi River and its tributaries in 19911992 were shown in Table IV. Samples collected by the collapsible-bag sampler or by the pumping method were passed through a 63-pm nickel-mesh sieve to remove sand and debris. Surface-grab samples were not sieved. Typically, 1-Lgrab-samples of water were collected from the smaller tributaries, and 50-100 L of water were collected for the discharge-weighted samples. Water samples were composited in 20-L churn splitters (usually three churn splitters per composite). Proportional fractions from the 20-L splitters were combined into 8-L splitters from which aliquots were drawn for various analyses. For the pesticide analyses, 1 L of sample was drawn into a clean Teflon bottle and preserved with 5 drops of chloroform. Samples were refrigerated and analyzed within 2-3 weeks of sample collection. All sampling equipment was washed and rinsed with methanol to prevent cross-contamination of samples from site to site. Sample Preparation and Analysis. A decision was made to filter water samples, since the herbicides were found in an earlier study to be predominantly dissolved rather than bound to particulates (IO). Water samples were filtered through glass fiber filters (Gelman Sciences, Type A/E; nominal pore size 0.45 pm). A 1-L sample was adjusted to pH 8.5 with a small amount of 10% KOH solution. After the addition of 10g of NaCl and an internal standard solution containing the atrazine analogue, ter1544 Envlron. Scl. Technol., Vol. 27, No. 8 , 1993
buthylazine, the sample was extracted with three volumes of methylene chloride (75,50, and 50 mL). The combined methylene chloride extracts were dried over anhydrous Na2S04 and concentrated to a volume of about 5 mL in a Kuderna-Danish apparatus, using benzene as the “keeper” solvent. 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 (241,using a Finnigan-Mat ITS-40 iontrap mass spectrometer. The gas chromatograph was maintained at 50 OC for 4 min and programmed at 6 “C/ min to 300 O C . The injector temperature was 280 OC. The capillary column (30 m, 0.25-mm i.d., containing a 0.25pm bonded phase of DB-5) was interfaced directly to the ion source of the mass spectrometer. All compounds were confirmed by comparing their mass spectra and retention times to authentic standards analyzed under identical conditions. In addition, the ITS-40 ion-trap mass spectrometer provided full-scanmass spectra on as little as 50-100 pg of analyte, thereby confirming the identity of trace components. 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. The only pesticide found at trace levels in the field blanks was deet (an insect repellant). Contamination of samples by trace levels of deet may have occurred during sampling. However, there was no use of deet reported aboard the R.V. Acadiana. Recoveries of pesticides and degradation products from Mississippi River water samples spiked at a concentration of 0.5 pg/L are shown in Table V. Concentration values reported are uncorrected for percent recovery. The lower limit of detection of the analytes are as follows: desisopropylatrazine, norflurazon, and desmethylnorflurazon, 10ng/L; cyanazine and cyanazineamide, 25 ng/L; all other analytes, 5 ng/L; with a signal to noise ratio greater than 5/1. Results and Discussion Triazine Herbicides. Seasonal Variations;Concentration Differences of Main S t e m vs Tributaries. Concentrations of triazine herbicides and degradation products in the dissolved phase in the Mississippi River and its tributaries during the July-August 1991 sampling cruise are shown in Table VI. The July-August and OctoberNovember sampling trips were designed so as to coincide with the possibility of pre- and postemergent application. The triazine herbicides included ametryn, atrazine, degradation products desethylatrazine and desisopropylatrazine, cyanazine and its transformation product cyanazineamide (25),metribuzin, hexazinone,prometon, prometryn, and simazine. Three major triazine herbicides, in order of decreasing concentrations, are atrazine, cyanazine, and simazine. Concentrations Compared to Regulatory Contaminant Levels. Concentrations of atrazine in the entire navigable reach of the Mississippi River and its tributaries during the three sampling cruises in 1991-92 are shown in Figure 2. Because agricultural runoff from crop lands is greatest in late spring and early summer, concentrations were greatest following the spring application and diminished significantly in fall and winter. The high summer concentrations of atrazine in the lower Mississippi River are due to contributions of atrazine from the Iowa, Des Moines, Missouri, Illinois, and Ohio Rivers. Concentrations of atrazine in the upper Mississippi River generally were
Table IV. Sampling Sites a n d Discharge Measurements for T h r e e Sampling Cruises on Mississippi River a n d Its Tributaries 1991-1992 water water river miles water discharge date discharge discharge date from head date 1991 :ref 35) (m3/s) 1991 (ref 35) (m%) 1992 (ref 35) (m%) of pass river and site 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
Mississippi River above St. Anthony Falls, MN Minnesota River a t mile 3.5,MN Mississippi River at Hastings, MN St. Croix River at mile 0.5,WI Mississippi River near Pepin, WI Chippewa River at mile 1.7,WI Mississippi River a t Trempealeau, WI Mississippi River below Lock and Dam 9,WI Wisconsin River a t mile -1.0, WI Mississippi River a t Clinton, IA Rock River at mile 1.0,IL Iowa River a t mile -1.0, IA Mississippi River at Keokuk, IA Des Moines River a t mile 1.0, IA Mississippi River near Winfield, MO Illinois River at Hardin, IL Missouri River at St. Charles, MO Kaskaskia River a t mile 1.5,IL Mississippi River at Thebes, IL Ohio River a t Olmsted, IL White River a t mile 1.2,AR Arkansas River at mile 0.0, AR Yazoo River a t mile -3.0, MS Mississippi River below Vicksburg, MS Mississippi River near St. Francisville, LA Mississippi River below Belle Chasse, LA .~
-
-
1812 1798 1766 1765 1719 1717 1668 1594 1585 1474 1433 1388 1317 1315 1193 1172 1149 1071 998 954 598 581 437 433 266 73
Table V. Recovery of Pesticides a n d Degradation Products from Mississippi River Water: T h r e e Replicates Spiked at 0.5 wgIL
compound
mean recovery (%)
SDof themean
RD, %
atrazine ametryn alachlor 2-chloro-2’,6’-diethylacetanilide carbofuran cyanazine cyanazineamide deet desethylatrazine desisopropylatrazine desmethylnorflurazon diazinon fluometuron hexazinone 2-hydroxy-2’,6’-diethylacetanilide 4-ketomolinate metolachlor metribuzin molinate norflurazon prometone prometryn simazine thiobencarb
91 92 94 94 101 114 40 84 75 59 122 94 96 90 113 87 93 99 80 106 91 87 92 87
6.1 2.3 4.5 3.1 3.5 5.9 8.9 2.6 3.8 3.2 9.1 6.4 3.8 6.0 2.6 3.0 3.5 3.8 7.2 2.9 1.5 4.6 3.2 4.0
6.7 2.5 4.8 3.3 3.5 5.2 22 3.1 5.1 6.5 7.5 6.8 4.0 6.7 2.3 3.4 3.8 3.8 9.0 2.7 1.6 5.3 3.5 4.6
smaller than the lower Mississippi River. A recent timeseries study has shown that the concentration of atrazine exceeded the maximum contaminant level (MCL) of 3 qg/L for drinking water in the Missouri River at Hermann, MO, throughout the month of June and at two sites on the Mississippi River during parts of May and June 1991 (26). Ranges of concentrations of the three major triazine herbicides, atrazine, cyanazine, and simazine, in the Mississippi River between 1991 and 1992 are shown in Figure 3. The greatest concentrations of these compounds
7-05 7-06 7-08 7-08 7-10 7-10 7-12 7-15 7-15 7-18 7-20 7-20 7-21 7-22 7-24 7-25 7-27 7-28 7-29 7-30 8-01 8-01 8-02 8-03 8-05 8-07
470 600 980 260 1340 170 1440 1590 145 1850 70 200 2050 620 2730 260 1100 7 4390 2410 370 480 640 8750 6190 4340
10-07 10-08 10-10 10-10 10-13 10-12 10-15 10-18 10-19 10-22 10-24 10-25 10-27 10-28 10-30 10-31 11-03 11-04 11-05 11-06 11-08 11-08 11-10 11-09 11-11 11-13
220 130 350 95 510 150 663 694 160 939 80 70 1410 80 1230 520 1350 10 3870 2480 1210 1620 540 10690 8950 8840
4-07 4-09 4-10 4-11 4-12 4-12 4-14 4-17 4-17 4-19 4-20 4-22 4-23 4-24 4-26 4-27 4-29 4-30 5-01 5-03 5-05 5-05 5-07 5-06 5-08 5-10
310 260 570 320 950 300 1330 1590 889 2320 340 680 4220 731 5070 860 3560 30 10480 6150 920 710 70 21750 15130 14540
are found near the confluences of the Iowa, Des Moines, Missouri, Illinois, and Ohio Rivers with the Mississippi River, During all sampling cruises, the concentrations of the triazine herbicides did not exceed the maximum contaminant level (MCL) for drinking water or the Health Advisory Levels for these compounds in the Mississippi River. During the July-August 1991and April-May 1992 cruises, the concentration of atrazine exceeded the MCL for drinking water in the Kaskaskia River, a tributary of the Mississippi River, which drains agricultural areas in western Illinois. However, the Kaskaskia River is a very small river with low water discharges (
