O C C U R R E N C E 

AND 

TRANSPORT 

OF 

AG RICULTURAL 

CHEMICALS 

IN THE 

M I S S I S S I P P I 

RIVER BASIN 

JULY THROUGH AUGUST 

9 9 3 

U.S . GEOLOGICAL SURVEY CIRCULAR 1120-C 
Front Cover-View of Highway 67, West Alton, Missouri, 

(Srenco Photography, St . Louis, Mo.) 

Back Cover-View of Spirit of St . Louis Airport, 

Chesterfield, Mo. (Srenco Photography, 

St . Louis, Mo.) 

Field Hydrologist making streamflow 

measurements (U.S . Geological Survey) 
OCCURRENCE AND TRANSPORT OF AGRICULTURAL 

CHEMICALS IN THE MISSISSIPPI RIVER BASIN, 

JULY THROUGH AUGUST 1993


By Donald A. Goolsby, William A. Battaglin, and E. Michael Thurman 

Floods in the Upper Mississippi River Basin, 1993


U.S. GEOLOGICAL SURVEY CIRCULAR 1120-C 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S . GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U. S . Government 

First Printing September 24, 1993


Second Printing (with corrections) October 18, 1993


UNITED STATES GOVERNMENT PRINTING OFFICE: 1993


Free on application to the


U. S. Geological Survey


Map Distribution


Box 25286, MS 306


Denver Federal Center


Denver, CO 80225

FOREWORD 

During spring and summer 1993, record flooding inundated much of the upper Mississippi 

River Basin. The magnitude of the damages-in terms of property, disrupted business, and personal trauma-was unmatched by any other flood disaster in United States history. Property dam 

age alone is expected to exceed $10 billion. Damaged highways and submerged roads disrupted 

overland transportation throughout the flooded region. The Mississippi and the Missouri Rivers 

were closed to navigation before and after the flooding . Millions of acres of productive farmland 

remained under water for weeks during the growing season. Rills and gullies in many tilled fields 

are the result of the severe erosion that occurred throughout the Midwestern United States farmbelt. The hydrologic effects of extended rainfall throughout the upper Midwestern United States 

were severe and widespread. The banks and channels of many rivers were severely eroded, and 

sediment was deposited over large areas of the basin's flood plain. Record flows submerged many 

areas that had not been affected by previous floods. Industrial and agricultural areas were inundated, which caused concern about the transport and fate of industrial chemicals, sewage effluent, 

and agricultural chemicals in the floodwaters . The extent and duration of the flooding caused 

numerous levees to fail . One failed levee on the Raccoon River in Des Moines, Iowa, led to flooding of the city's water treatment plant. As a result, the city was without drinking water for 19 days . 

As the Nation's principal water-science agency, the U.S. Geological Survey (USGS) is in a 

unique position to provide an immediate assessment of some of the hydrological effects of the 

1993 flood. The USGS maintains a hydrologic data network and conducts extensive water 

resources investigations nationwide. Long-term data from this network and information on local 

and regional hydrology provide the basis for identifying and documenting the effects of the flooding. During the flood, the USGS provided continuous streamflow and related information to the 

National Weather Service (NWS), the U.S . Army Corps of Engineers, the Federal Emergency 

Management Agency (FEMA), and many State and local agencies as part of its role to provide 

basic information on the Nation's surface- and ground-water resources at thousands of locations 

across the United States . The NWS has used the data in forecasting floods and issuing flood warnings. The data have been used by the Corps of Engineers to operate water diversions, dams, locks, 

and levees . The FEMA and many State and local emergency management agencies have used 

USGS hydrologic data and NWS forecasts as part of the basis of their local flood-response activities . In addition, USGS hydrologists are conducting a series of investigations to document the 

effects of the flooding and to improve understanding of the related processes. The major initial 

findings from these studies will be reported in this Circular series as results become available . 

U.S. Geological Survey Circular 1120, Floods in the Upper Mississippi River Basin, 1993, 

consists of individually published chapters that will document the effects of the 1993 flooding . 

The series includes data and findings on the magnitude and frequency of peak discharges; precipi 

tation ; water-quality characteristics, including nutrients and man-made contaminants ; transport of 

sediment; assessment of sediment deposited on flood plains ; effects of inundation on ground-water 

quality ; flood-discharge volume; effects of reservoir storage on flood peaks ; stream-channel scour 

at selected bridges ; extent of flood-plain inundation; and documentation of geomorphologic 

changes. 

L~~ /,~. #,,x 

Acting Director 

September 24, 1993 

Foreword III 
1

2

3

4

5

6

Contents 

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Sources and properties of selected agricultural chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Drinking-water standards and aquatic-life guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Previous studies of agricultural chemicals in the Mississippi River Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Occurrence and transport of agricultural chemicals during the 1993 flood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Data-collection methods . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Occurrence and distribution of herbicides and nitrate-nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Relation to drinking-water standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Transport of herbicides and nitrate-nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Calculation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Transport of herbicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Transport of nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

FIGURES 

1. Map showing harvested cropland for each county in the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2. Graph showing estimated annual herbicide use in the United States and the Mississippi River 

Basin from 1987 through 1989 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

3. Graphs showing streamflow and temporal distribution oftriazine herbicide concentrations as 

determined by immunoassay analysis in 1990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4.-8. Graphs showing streamflow and temporal distributions in the concentrations of: 

4. Several commonly used herbicide concentrations in the Mississippi River at Thebes, Illinois . . . . . . . . . . . 8 

5. Atrazine at three sites on the Mississippi River from April 1991 through August 1993 . . . . . . . . . . . . . . . . . . . . . . 13 

6. Alachlor at three sites on the Mississippi River from April 1991 through August 1993 . . . . . . . . . . . . . . . . . . . . . 14 

7. Cyanazine at three sites on the Mississippi River from April 1991 through August 1993. . . . . . . . . . . . . . . . . . . 15 

8. Nitrate-nitrogen at three sites on the Mississippi River from April 1991 through August 1993 . . . . . . . . . . 16 

9.-10 . Graphs showing streamflow and temporal distribution of daily loads of : 

9. Atrazine at three sites on the Mississippi River from April 1991 through August 1993 . . . . . . . . . . . . . . . . . . . . . . 18 

10 . Nitrate at three sites on the Mississippi River from April 1991 through August 1993 . . . . . . . . . . . . . . . . . . . . . . . . 20 

TABLES 

. Estimated annual use of atrazine, alachlor, cyanazine, metolachlor from 1987 through 1989 and commercial 

nitrogen fertilizer during 1991 upstream from selected locations in the Mississippi River Basin . . . . . . . . . . . . . . . . . . . . . 4 

. Physiochemical properties, health-based limits for drinking water, and Canadian aquatic-life guidelines 

for selected commonly used herbicides and nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

. Streamflow, nitrate-nitrogen, and herbicide data for three sites on the Mississippi River during the 

1993 flood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

. Streamflow, nitrate-nitrogen, and herbicide data for three sites on the Mississippi River tributaries 

during the 1993 flood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

. Three largest daily load estimates for atrazine, alachlor, cyanazine, and metolachlor at three sampling 

sites on the Mississippi River in 1991 and 1992 and during the 1993 flood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

. Three largest daily load estimates for nitrate-nitrogen at three sampling sites on the Mississippi River 

in 1991 and 1992 and during the 1993 flood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

Contents V 
Occurrence and Transport of Agricultural Chemicals in the 

Mississippi River, July Through August 1993 

By Donald A. Goolsby, William A. Battaglin, and E . Michael Thurman 

Abstract 

Heavy rainfall and severe flooding in the 

upper Mississippi River Basin from mid-June 

through early August 1993 flushed extraordinar 

ily large amounts of agricultural chemicals (herbicides and nitrate) into the Mississippi River, 

many of its tributaries, and, ultimately, the Gulf 

of Mexico. Even though extremely high streamflows were recorded during the flood in 1993, 

concentrations of herbicides, such as atrazine, 

alachlor, cyanazine, and metolachlor, were similar to the maximum concentrations measured during spring and summer 1991 and 1992. It was 

anticipated that the higher streamflows during 

the flood would dilute the concentrations of herbicides that are usually flushed into streams in 

late spring and summer. Instead, concentrations 

were similar to those measured during much 

lower flows, but the daily loads of herbicides 

transported in some reaches of the Mississippi 

River were higher than those measured in 1991 

and 1992. The total atrazine load transported to 

the Gulf of Mexico from April through August 

1993 (539,000 kilograms) was about 80 percent 

higher than that for the same period in 1991 and 

235 percent higher than for the same period in 

1992. The concentrations of atrazine and cyanazine in a few individual samples exceeded healthbased limits for drinking water. However, 

because drinking-water regulations are based on 

the average of at least four quarterly samples, the 

annual average concentrations in the Mississippi 

River probably will not exceed these limits for 

1993 . Nitrate concentrations were similar to 

those measured during spring and summer 1991 

and 1992. The loads of nitrate-nitrogen transported into the Gulf of Mexico during July and 

August 1993 were as much as 5,734 metric tons 

per day. These loads generally are similar to 

those measured in spring 1991 and 1992 but 

larger than those measured in summer 1991 and 

1992. The total nitrate-nitrogen load transported 

to the Gulf of Mexico from April through August 

1993 (827,000 metric tons) was about 37 percent 

larger than that for this same period in 1991 and 

112 percent larger than that for the same period in 

1992. The transport of extraordinarily high loads 

of nitrate and large amounts of fresh-water into 

the Gulf of Mexico during midsummerwhen primary production is highest could increase phytoplankton biomass and affect the gulf ecosystem 

along the Louisiana coast. 

INTRODUCTION 

Flooding was severe throughout parts of the 

upper Mississippi River Basin from mid-June 

through early August 1993 as a result of extraordi 

nary amounts of rainfall . Rainfall during July was 

more than 200 percent of normal (1961-90) July rainfall over parts of nine upper Midwestern States (Wahl 

and others, 1993), and record peak discharges were 

recorded at more than 40 U.S . Geological Survey 

(USGS) streamflow-gaging stations (Parrett and 

others, 1993). The cool, wet conditions that preceded 

the flooding delayed planting by several weeks in 

some areas (Iowa Agricultural Statistics, 1993). The 

heavy rainfall and flooding began shortly after most 

crops were planted and agricultural chemicals, such 

as herbicides, were applied to cropland. Previous 

studies by the USGS (Goolsby and others, 1991 ; 

Thurman and others, 1991, 1992; Goolsby and 

Introduction 1 
Battaglin, 1993) have shown that large amounts of the 

herbicides applied to recently planted crops can be 

flushed from cropland during storm runoff. Consequently, the heavy rainfall that produced the record 

flooding could flush large amounts of herbicides into 

streams and ground water. Nitrate concentrations 

also can be high in some streams during periods of 

heavy rainfall in spring and early summer (Goolsby 

and Battaglin, 1993 ; Lucey and Goolsby, 1993). 

Therefore, the upper Mississippi River Basin flooded 

at the time of the year when the potential for adverse 

effects on the quality of surface and ground water 

from agricultural chemicals was greatest . High concentrations of agricultural chemicals in streams are of 

concern because they can affect the suitability of the 

water for use as public water supplies and can have 

adverse effects on stream biota. 

Purpose and Scope 

The purpose of this report is to present and 

summarize information on the concentrations and 

transport of agricultural chemicals (herbicides and 

nitrate) in the Mississippi River during the peak and 

recession stages of flooding in July and August 

1993. Pertinent results from USGS regional studies 

of agricultural chemicals in streams in the Mississippi 

River Basin during 1989 and 1990 are discussed, as 

well as background information on results from a 

study of the distribution and transport of these chemicals in the Mississippi River from April 1991 through 

September 1992. Information on the usage and properties of agricultural chemicals frequently detected in 

streams in the Mississippi River Basin also is presented. The concentrations and loads of herbicides 

and nitrate during the 1993 flood are compared with 

those from previous studies . The 1993 flood data presented in this paper are limited to the chemical and 

hydrologic data available at the end of August 1993. 

Sources and Properties of Selected 

Agricultural Chemicals 

The Mississippi River Basin contains the largest and most intensive agricultural region in the country. This region, which includes about 65 percent of 

the total harvested cropland in the Nation (fig . 1 ; U.S . 

Department of Commerce, 1989), produces more 

than 80 percent of the corn and soybeans grown in 

the country and much of the cotton, rice, sorghum, 

and wheat. To increase yields from these crops, large 

amounts of pesticides and nitrogen fertilizers are 

used. On the basis of the latest available estimates 

(1987 through 1989 for pesticides, 1991 for nitrogen 

fertilizer), it is estimated that more than 100,000 metric tons (t) of pesticides (herbicides, insecticides, fungicides) and about 6.3 million t of nitrogen fertilizers 

(as nitrogen) were applied to cropland in the Mississippi River Basin in 1991 (U.S . Environmental Protection Agency, 1990; Gianessi and Puffer, 1991 ; 

U.S . Department of Agriculture, 1992; J.J . Fletcher, 

West Virginia University, written commun., 1992). 

Herbicides, which are used to control weeds 

and grasses, account for more than 80 percent of the 

agricultural use of pesticides in the United States (U.S. 

Department of Agriculture, 1992). The 20 most 

heavily used herbicides and the estimated amounts 

used annually in the United States and in the Mississippi River Basin are shown in figure 2. Herbicides, 

such as atrazine, alachlor, cyanazine, and metolachlor, 

used in the production of corn, soybeans, and sorghum 

are among the most heavily used herbicides. The estimated annual use of these herbicides upstream from 

selected locations on the Mississippi River and several 

major tributaries is listed in table 1 . The annual estimates for herbicide use shown in figure 2 and table 1 

are for 1987 through 1989 (Gianessi and Puffer, 

1991). The actual usage for these herbicides in 1993 

could be higher or lower than the estimates listed in the 

table; for example, the nationwide use of cyanazine 

increased nearly 25 percent between 1989 and 1991 

(Dr. Patti Tillotson, Dupont Agricultural Products, 

written commun., 1993). For atrazine, the annual use 

probably has decreased in recent years because of a 

reduction in the recommended application rate (U.S. 

Department of Agriculture, 1993). 

Nitrogen fertilizers are used extensively on 

crops in the Mississippi River Basin to increase 

yields . Many forms of commercial fertilizers are 

used, including anhydrous ammonia, ammonium 

nitrate, ammonium sulfate, and urea. Other sources 

of nitrogen in the Mississippi River Basin include animal wastes (cattle, hogs, poultry), domestic sewage, 

legumes, and mineralization of vegetation and soil 

organic matter. Very little specific information is 

available to quantify nitrogen sources other than commercial nitrogen fertilizer. The estimated use of commercial nitrogen fertilizer in 1991 upstream from 

selected locations on the Mississippi River and several major tributaries is listed in table 1 . 

2 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Herbicides and nitrogen derived from point 

and nonpoint sources in the Mississippi River Basin 

can be transported into the Mississippi River in run 

off from agricultural and urban areas, discharge from 

reservoirs and aquifers, and atmospheric deposition 

(Goolsby, Thurman, and others, 1993). The physiochemical properties of the herbicides, as well as such 

factors as usage, rainfall patterns and intensity, and 

farming practices, are important in governing the 

amounts and concentrations of these chemicals in 

streams . Several important physiochemical properties of the most widely used and most frequently 

detected herbicides in the Mississippi River basin are 

listed in table 2. These properties include water solubility, soil sorption coefficient (Koc), and soil halflife. Water solubility determines how easily herbicides wash off soil and crop residues and leach 

through the soil . Herbicides with solubilities of 

greater than about 30 milligrams per liter (mg/L) are 

considered to be soluble and are likely to wash off 

40' 

EXPLANATION 

Percent of county as 

harvested cropland 

Missing or 0 

1to5 

6to15 

16 to 30 

31 to 60 

More than 60 

"" "Boundary of Mississippi River Basin 

the soil during storms (Becker and others, 1989).€

Those with solubilities of less than about 1€

mg/L are likely to remain on the soil surface . The€

Koc is a measure of the tendency of a herbicide to€

attach to soil particles . The larger the Koc value, the€

more strongly the herbicide will be adsorbed to soil€

particles . Those with Koc values of less than about€

500 milliliters per gram (mL/g) tend to be transported€

primarily in the dissolved phase (Becker and others,€

1989). Those with Koc values of more than 1,000€

mL/g are transported primarily on suspended-sedi-€

ment particles . Soil half-life is the length of time€

required for herbicides to degrade in the soil to one-€

half of their previous concentration . The longer the€

soil half-life, the more persistent the herbicide is and€

the longer it will be available to wash off in storm€

runoff. Once herbicides are in surface water, the half-€

life generally is much longer than in soil because sur-€

face water contains much less organic matter and€

fewer microorganisms to degrade the herbicides.€

200 400 MILES 

I I 200 400 KILOMETERS 

Figure 1 . Harvested cropland for each county in the United States, expressed as a percentage of the total area of each 

county (U.S . Department of Commerce, 1989) . 

Introduction 3 
Atrazine 

Alachlor 

Metolachlor 

EPTC€

2,4-D€

Trifluralin€

Cyanazine€

Butylate€

Pendimethalin€

0 

U Glyphosate 

m Dicamba 

Bentazon 

Propanil 

MSMA 

Metribuzin 

Molinate = 

MCPA 

Propazine 

Propachlor 

Simazine 

I 

0 10,000 

Entire United States 

Mississippi River Basin 

I I 

20,000 30,000 

ESTIMATED ANNUAL HERBICIDE USE, IN METRIC TONS 

Figure 2. Estimated annual herbicide use in the United 

States and the Mississippi River Basin from 1987 through 

1989 (Gianessi and Puffer, 1991). 

Some herbicides can persist for long periods of time 

in reservoirs where runoff from late spring and summer storms is often stored for many months 

(Goolsby, Battaglin, and others, 1993). However, little specific information is available on herbicide halflives in natural water bodies. 

The physiochemical properties listed in table 2 

indicate that soluble and mobile herbicides, such as 

atrazine, alachlor, cyanazine, and metolachlor, are 

transported primarily in the dissolved phase. This 

group includes the majority of herbicides in use today. 

However, the soil half-life and such factors as intensity 

of use; application methods ; timing, intensity, and 

duration of rainfall ; and antecedent soil moisture conditions also are important in determining the amounts of 

herbicides that can be transported into streams . 

Much of the nitrogen transported by the Mississippi River is primarily nitrate and various forms of dissolved and particulate organic nitrogen. Small 

amounts of ammonium and nitrite also are usually 

present, but these compounds generally are intermediate products in the oxidation of nitrogen compounds to 

nitrate in well-oxygenated streams . Nitrate is highly 

soluble and mobile in streams and also is a plant nutrient, which can be readily assimilated in the reproduction and growth of aquatic plants . However, if plant 

growth and primary production are minimal, as they 

typically are in streams with high suspended-sediment 

concentrations and little light penetration, such as the 

Mississippi River, then nitrate can behave as a conservative solute. 

Table 1 . Estimated annual use of atrazine, alachlor, cyanazine, and metolachlor from 1987 

through 1989, and commercial nitrogen fertilizer during 1991 upstream from selected locations 

in the Mississippi River Basin 

[t, metric tons ; km2 , square kilometers . Source of herbicide data-Gianessi and Puffer (1991) ; Source of 

fertilizer data-U.S . Environmental Protection Agency (1990) ; J.J . Fletcher, West Virginia University, written 

commun., 19921 

Location 

Mississippi River at 

Clinton, Iowa. 

Illinois River at Valley 

City, Illinois . 

Missouri River at 

Hermann, Missouri . 

Mississippi River at 

Thebes, Illinois . 

Ohio River at GrandChain, Illinois . 

Mississippi River at 

Upstream Estimated application (t) 

drainage 

area, Atrazine Alachlor Cyanazine Metolachlor Nitrogen 

(km2) 

222,000 1,440 2,040 1,590 1,630 607,000 

69,000 1,960 1,880 715 1,760 450,000 

1,357,000 6,280 4,660 1,970 3,490 1,930,000 

1,847,000 13,400 12,200 6,210 11,000 4,055,000 

526,000 5,060 4,900 1,440 3,440 1,133,000 

2,914,000 20,600 18,500 8,170 15,800 6,263,000 

Baton Rouge, 

Louisiana. 

4 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
- -

- -

Table 2. Physiochemical properties, health-based limits for drinking water, and Canadian 

aquatic-life guidelines for selected commonly used herbicides and nitrate 

[mg/L, milligrams per liter; mUg, milliliters per gram; wg/L, micrograms per liter; --, no data or none available] 

Soil 

Chemical Water sorption 

compound solubility' coefficient 

(mg/L) (KJI 

(mUg) 

Alachlor . . . . . . . . . . . 240 170 

Atrazine . . . . . . . . . . . 33 100 

Cyanazine . . . . . . . . . 170 190 

Metolachlor . . . . . . . 530 200 

Metribuzin . . . . . . . . 1,220 41 

Simazine . . . . . . . . . . . 6.2 138 

Nitrate . . . . . . . . . . . . . Very large. . 

'Becker and others (1989) . 

'U .S . Environmental Protection Agency (1992) . 

'Trotter and others (1990) . 

'Pauli and others (1991a) . 

Drinking-Water Standards and Aquatic-Life 

Guidelines 

Maximum contaminant levels (MCL), or 

drinking-water standards, have been established by 

the U. S. Environmental Protection Agency (1992) 

for several herbicides and nitrate (table 2). MCL's 

for herbicides are based on annual average concentrations and are legally enforceable under the Safe 

Drinking Water Act (1986) . Annual average concentrations of herbicides are determined from analyses 

of a minimum of four quarterly samples per year, 

and the standards are violated only if the average 

concentration in these samples exceeds the MCL. 

Further, MCL's for herbicides apply to water delivered at the tap and not the raw water source. However, conventional water treatment removes little of 

the water-soluble herbicides currently (1993) used in 

the Mississippi River Basin. At present (1993), 

MCL's have been established only for individual 

compounds and do not address the possible effects 

of complex mixtures of herbicides and their degradation products . 

Unlike herbicides, the MCL for nitrate is 

based on a single sample and not an annual average. 

If a water sample from a public water supply exceeds 

10 mg/L, then a second sample is collected to confirm the original sample. If the average of these two 

samples exceeds 10 mg/L, then the water supply is 

not in compliance with the Safe Drinking Water Act. 

Health advisory levels (HA) have been established by the U.S .Environmental Protection Agency 

Health Canadian Soil Maximum 

half-life contaminant advisory aquatic-life 

(days)' level' level' guidelines 

(hg/L) (1910 

15 ' leg/L - -

60 3 ftg/L 3 32 

14 - - 1 42 

20 - - 100 5 8 

30 - - 200 6 1 

75 4 Ag/L 4 '10 

- - 10 mg/L - - - -

'Kent and others (1991) . 

'Pauli and others (1990) . 

7Pauli and others (1991b). 

(USEPA) for several herbicides that do not have 

MCL's. HA'S are the recommended maximum concentrations in drinking water for lifetime exposure 

and are not legally enforceable . However, in time, 

the lifetime HA' S for some herbicides could become 

enforceable MCL's. HA'S for a few selected herbicides are listed in table 2. 

Some herbicides can have adverse effects on 

aquatic life, but at present (1993), the USEPA has 

established standards or criteria for aquatic life for 

few of the current generation herbicides . However, 

the Canadian Government (Environment Canada) has 

established water-quality guidelines for specific 

water uses in Canada that include drinking water, 

aquatic life, and agricultural uses of water (Wong and 

Kent, 1988) . The guidelines are numerical concentrations recommended to support and maintain a designated water use. Because no U.S . guidelines or 

criteria for aquatic life have been established for most 

herbicides in current use, the Canadian herbicide 

water-quality guidelines, where available, have been 

included in table 2 as a point of reference for herbicide concentrations measured in the Mississippi 

River Basin . 

Previous Studies of Agricultural Chemicals 

in the Mississippi River Basin 

Several regional studies of herbicides, insecticides, and nitrate in surface waters of the Mississippi 

River Basin have been done since 1989. These studies show that the occurrence and transport of herbiIntroduction 5 
herbicides is seasonal; concentrations are highest 

within a period of about 3 months after application . 

A 1989 study of 147 streams that drain areas of from 

about 200 to more than 100,000 square kilometers 

(km2) showed that herbicide concentrations increased 

from near analytical reporting limits [0.05 microgram 

per liter (gg/L)] before planting to more 100 Rg/L in 

a few streams during storm runoff after planting 

(Thurman and others, 1991, 1992; Goolsby and Battaglin, 1993). Median concentrations of alachlor, atrazine, and cyanazine were 0.92, 3 .8, and 0.97 pg/L, 

respectively, during the postplanting sampling 

period. Median concentrations of these three herbicides before planting and in the fall were less than 0.3 

gg/]-. 

Frequent sampling (two to four times per day 

during storms) of nine streams during 1990 by using 

automatic samplers indicated that for periods of several weeks to about 3 months following application of 

herbicides, elevated herbicide concentrations persisted 

during storm runoff (Goolsby and Battaglin, 1993). 

Herbicide concentrations were highest during runoff 

from the first storms after application (fig . 3). 

Between storms, as overland flow ceased and streamflow decreased, herbicide concentrations decreased 

and commonly approached preplanting levels . Peak 

300 

z 

O

U 250 w

U 

w

a 

200 

w 

w 

150 U 

m 

U 

Z

100 

O

J 

a 

50 W 

F

U 

0 

w 50€ F€

J€

w€

a€

40 

Q

O 

O 

U 30 

Z 

Z 

O€

F 20€

W€

F€Z€

W€

U€

O

z 10€

U€

W 

Z 

N

Q 

0 F April May 

w 50 F 

J 

W

W 

a 

40 

Q 

0

O 

U 30 

Z 

z 

O 

20 

F-€

z€W€

U tv€

10 

U 

w 

z 

N 

¢ 

0 F April May 

June July August 

1990 

300 

Z€

O€

250 w€

w 

a 

200 U

Q

w

F 

w 

150 U 

m 

U 

Z 

100 

June July August 

1990 

Figure 3. Streamflow and temporal distribution of triazine herbicide concentrations as determined by immunoassay analysis in 1990. A, Sangamon River in Illinois ; B, West Fork Big Blue 

River in Nebraska . 

Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
herbicide concentrations in these streams tended to 

decrease with runoff from each subsequent storm 

(fig . 3) . Concentrations of triazine herbicides were 

determined by immunoassay, which is an analytical 

method that uses antibodies to detect and quantify 

specific compounds or groups of compounds in water 

and other materials . The triazine immunoassay primarily measures atrazine but also can indicate the 

presence of other triazine herbicides, such as 

simazine and propazine . 

The seasonal pattern in herbicide concentrations in the Mississippi River is similar to that 

described above (Goolsby and others, 1991 ; Goolsby 

and Battaglin, 1993). The temporal changes in concentrations of several commonly used herbicides and 

streamflow in the Mississippi River at Thebes, Illinois, for April 1991 through March 1992 are shown 

in figure 4. The herbicide concentrations were highest (greater than about 1 pg/L for atrazine) at this site 

from early May to early July 1991 . Low, but detectable, concentrations of atrazine, alachlor, and metolachlor were found year round at this site (fig . 4A, B), 

and cyanazine was detected in about 96 percent of the 

samples (fig . 4A). The year-round occurrence of herbicides in the Mississippi River indicates that they 

are stored in surface reservoirs (Goolsby, Battaglin, 

and others, 1993) and in aquifers (Squillace and others, 1993) . The herbicides are slowly discharged into 

streams in quantities large enough to produce detectable concentrations year round in the Mississippi 

River. 

Previous studies indicate that nitrate concentrations in streams in the Midwestern States have a seasonal pattern . This pattern, however, is distinctly 

different from that for herbicides. Unlike herbicides, 

which are transported primarily in overland flow, 

nitrate appears to be transported in overland and subsurface flows (Goolsby and Battaglin, 1993 ; Lucey 

and Goolsby, 1993). Nitrate concentrations are 

higher during late fall, winter, and spring and are 

lower during summer. The lower concentrations in 

summer are attributed, in part, to assimilation of 

nitrate by aquatic and terrestrial plants during the 

growing season, decreased streamflow and groundwater discharge to streams, and greater evapotranspiration, all of which contribute to less leaching of 

nitrate from the soil and unsaturated zones to streams 

(Goolsby and Battaglin, 1993). 

OCCURRENCE AND TRANSPORT OF 

AGRICULTURAL CHEMICALS DURING 

THE 1993 FLOOD 

Heavy rainfall and flooding began in many 

parts of the upper Mississippi River Basin in June 

1993, shortly after most crops had been planted . The 

cool, wet conditions that prevailed over much of the 

upper Midwest during spring and early summer 1993 

delayed or prevented planting of corn in some areas. 

Because of this delay, more soybeans and less corn 

were planted than normal in some areas, such as Iowa 

(Dana Kolpin, U.S. Geological Survey, oral commun., 1993). This could have resulted in the application of less-than-normal amounts of corn herbicides, 

such as atrazine. The timing and intensity of the rainfall that followed planting was such that large quantities of agricultural chemicals, such as herbicides that 

are typically applied at the time of planting, could 

potentially be flushed into streams. However, 

because of the large volumes of floodwater, it was 

anticipated that herbicide concentrations would be 

lower than typically are measured at this time of year 

because of dilution. To determine the effects of 

flooding on the occurrence, concentrations, and transport of agricultural chemicals in the Mississippi 

River, water sampling was begun during early to midJuly at six locations where data had been collected in 

the 1991-92 study. Three of the sampling locations 

were on the mainstem of the Mississippi River, and 

three were on large tributaries. Samples were collected once to twice weekly. At present (early September 1993), the sampling is still underway. The 

locations of the six sampling sites, in down-river 

order; the drainage areas; and the date sampling 

began are as follows : 

Date 

Site Name of Drainage sampling 

(km') (1993) 

05420500 Mississippi River at 222,000 July 16 

Clinton, Iowa. 

05586100 Illinois River at Valley 69,000 Do. 

City, Illinois . 

06345000 Missouri River at 1,357,000 July 17 

Hermann, Missouri . 

07022000 Mississippi River at 1,847,000 July 20 

Thebes, Illinois . 

03612500 Ohio River at Grand 526,000 July 22 

Chain, Illinois . 

07374000 Mississippi River at 2,914,000 July 7 

number sampling site area began 

Baton Rouge, 

Louisiana . 

Occurrence and Transport of Agricultural Chemicals During the 1993 Flood 
-----

w 

Ir 

w 

a 

In 

C7 

O

Q 

U_ 

z 

Z 

O 

F 

It 

zZ 

w

U 

Z

O

U 

w 

w B H 

J 

w 0.8 Alachlor a 

Desethylatrazine 

°°°°°° Simazine O 0.6
 O


Ir


z 

0.4 z I , ~1 

O II 1 I A 

II 

Q .I 1~1 I ~ .1 1 I 

tr 

zz 0.2 

\

1 

//I

II

I j1 I 1 

w t I I I I I \w 

U

Z v II 

O C 

U . , , ,. . ., . . . .. . . .. . . . . . . . . .. . . . . . . . . 

A M J J A F 

1991 1992 

14,000 0 

z 

O


U


w 12,000


In 

w 

a 10,000 

w 

ww 8,000 

U 

m 

6,000 U 

Z 

4,000 O

J 

LL 

w 2,000 

tY 

F 

Figure 4. Streamflow and temporal distributions of several major herbicides in the Mississippi River at Thebes, Illinois . A, Atrazine, cyanazine, and metolachlor concentrations ; 8,

Alachlor, simazine, and desethylatrazine concentrations ; C, Streamflow. 

8 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Except for the Illinois River, these sites had 

been sampled at approximately weekly intervals from 

April 1991 through July 1992. Sampling of the Illi 

nois River was discontinued at the end of March 

1992. Sampling at the Baton Rouge, Louisiana, site 

continued through September 1992. The data collected on herbicides and nitrate in the 1991-92 study 

provide a basis for examining the effects of the 1993 

flood on the concentrations and mass transport of 

these agricultural chemicals in the Mississippi River. 

Data-Collection Methods 

Methods used to collect, process, and analyze 

water samples are the same as those used in the 1991-

92 study and are described in detail elsewhere 

(Goolsby and others, 1991). What follows is a brief 

summary of the methods. Depth-integrated water 

samples were collected at five or more locations 

across the river at all sites, except Baton Rouge. 

Because previous studies indicated dissolved solutes 

in the river at Baton Rouge were well mixed, a single 

integrated sample from the upper 7 meters of the 

water column was collected there. Samples from the 

river cross-section locations were composited and filtered through glass-fiber filters for herbicides or 

membrane filters for nitrate and several other dissolved nutrients . All materials used for herbicide 

sampling were stainless steel, glass, aluminum, or 

Teflon. The concentrations of 11 herbicides and 2 

herbicide metabolites were determined by using gas 

chromatography/mass spectrometry following extraction on disposable C-18 solid-phase extraction cartridges (Thurman and others, 1990; Meyer and 

others, 1993). The herbicides analyzed were 

alachlor, ametryn, atrazine, cyanazine, metribuzin, 

metolachlor, prometon, propazine, prometryn, 

simazine, and terbutryn . The herbicide metabolites 

analyzed were desethylatrazine and desisopropylatrazine, which are primarily metabolites of atrazine . 

The analytical reporting limits for all herbicides and 

metabolites analyzed were 0.05 [tg/L. In addition, an 

alachlor soil metabolite, ethanesulfonic acid (ESA), 

was extracted and isolated by solid-phase extraction 

and analyzed (reporting limit, 0.1 pg/L) by immunoassay (D .S . Aga, U .S . Geological Survey, written 

commun., 1993). About 10 percent of the samples 

analyzed were duplicates or blanks. 

The concentrations of dissolved nitrite and 

nitrite plus nitrate were determined by automated 

colorimetric methods (Fishman and Friedman, 

1989). The concentration of nitrate was calculated as 

the difference between these two determinations . Dissolved ammonium and orthophosphate also were 

determined by colorimetric methods (Fishman and 

Friedman, 1989). 

Additional data on herbicide and nitrate concentrations in the lower Mississippi River [sampling 

site located about 200 kilometers (km) downstream 

from the USGS sampling site at Baton Rouge] for 

early March through mid-August 1993 were obtained 

from the Jefferson Parish Water Quality Laboratory 

near New Orleans, Louisiana (Wayne Koffskey, written commun., 1993). These samples were weekly 

composites collected continuously at two sampling 

points on opposite banks of the Mississippi River by 

using peristaltic pumps and storing the samples at 4 

degrees Celsius . The herbicide samples were analyzed by using the USEPA's Method 505 (U.S. Environmental Protection Agency, 1988). Although the 

location of the sampling site is close to New Orleans 

and the sample collection and analytical methods are 

different from those used by the USGS, these data are 

still useful in constructing the temporal distribution 

pattern for herbicides, such as atrazine, in the lower 

Mississippi River during 1993. Data from the two 

weekly composite samples were averaged and used to 

construct a 6-month sequence of atrazine concentrations and loads during 1993 for comparison with previous data from Baton Rouge. The same comparison 

was made for nitrate . 

Streamflow data for all sites, except Baton 

Rouge, were obtained from USGS offices in Iowa, 

Illinois, Kentucky, and Missouri. Streamflow data 

for Baton Rouge were obtained from the U.S. Army 

Corps of Engineers, New Orleans District (John 

Miller, U.S. Army Corps of Engineers, written commun., 1993). These data are for a site about 129 km 

upstream from Baton Rouge but should closely represent the streamflow at Baton Rouge. The Corps of 

Engineers also provided data on streamflow diverted 

from the Mississippi River to the Atchafalaya River, 

which is a distributary of the Mississippi River. This 

diversion is approximately 30 percent of the streamflow at Baton Rouge. The sum of the streamflow at 

Baton Rouge and the Atchafalaya diversion closely 

represents the total discharge from the Mississippi 

River Basin to the Gulf of Mexico. 

Occurrence and Transport of Agricultural Chemicals During the 1993 Flood 
Occurrence and Distribution of Herbicides 

and Nitrate-Nitrogen 

Streamflow, herbicide and nitrate concentrations, and several other chemical and physical data 

collected at the six sites on the Mississippi River and 

its tributaries during the 1993 flood are listed in 

tables 3 and 4. The range and median concentrations 

for selected herbicides, herbicide metabolites, and 

nitrate determined in water samples collected at the 

six sites are as follows : 

Number Concentration (Ag/L) Compound of

samples Median Range 

Herbicide: 

Herbicide 

Alachlor . . . . . . . . . . . . . . . . . . 26 0.20 <0.05-0.38 

Atrazine . . . . . . . . . . . . . . . . . . 26 2.22 1 .27-3 .31 

Cyanazine. . . . . . . . . . . . . . . . 26 1.18 .45-1 .91 

Metolachlor. . . . . . . . . . . . . . 26 .81 .49-1 .90 

Desisopropylatrazine . . . 

ESA . . . . . . . . . . . . . . . . . . . . . . . 

20 

20 

.31 

2.0 

.18- .57 

.5 -4 .0 

metabolite : 

Desethylatrazine . . . . . . . . 20 .51 .28- .76 

Nitrogen : 

Nitrate . . . . . . . . . . . . . . . . . . . . 44 2.03 mg/L .64-3.89 mg/L 

Atrazine, ESA, and cyanazine generally had the 

highest concentrations followed by metolachlor, the 

two atrazine metabolites, and alachlor. Except for 

ESA, for which previous data are few, this order is Similar to that observed in previous studies and is a reflection of the usage and short-term stability of these 

compounds in water. The concentrations of herbicides 

in water samples from the three sites on the Mississippi River were similar to or slightly higher than the 

maximum concentrations determined at the same three 

sites during late spring and early summer 1991 and 

1992 (see figs. 5-7). For example, at the Thebes site, 

the concentration of atrazine in four samples collected 

during July 1993 ranged from about 2.2 to about 3 .0 

pg/L (table 3) . The four highest atrazine concentrations determined at this site ranged from 3.3 to 4.2 

pg/L in 1991 and from 1 .3 to 2.6 pg/L in 1992 (fig . 

5). The only other known data that define the seasonal 

patterns of herbicides in this reach of the Mississippi 

River were collected in 1975 and 1976 by the Ciba 

Geigy Corporation (the manufacturer of atrazine ; Tierney, 1992). These data show that the maximum atrazine concentrations in the Mississippi River at St. 

Louis, Missouri, which is about 215 km upstream from 

the Thebes site, were from 3 to 4 ~tg/L in 1975 and 

from 1.5 to 4 gg/L in 1976. 

At the Baton Rouge site during July 1993, concentrations of atrazine, alachlor, and cyanazine were 

similar to the maximum concentrations during 1991 

but were higher than those during 1992 (figs . 5-7). 

In addition to data collected as part of this study, data 

on the concentrations of several herbicides for early 

March through mid-August 1993 were obtained from 

the Jefferson Parish Water Quality Laboratory 

(Wayne Koffskey, written commun., 1993) . The atrazine data from this laboratory, which represent 

weekly composite samples collected at a site 200 km 

downstream from the Baton Rouge site, are shown in 

figure 5, along with the atrazine data from the Baton 

Rouge site . These data show that the temporal pattern of atrazine during spring and summer 1993 was 

similar to that measured in 1991. These data also 

indicate that the concentration of atrazine at Baton 

Rouge began to decrease in mid- to late July (fig . 5). 

From 1978 through 1989, the Ciba Geigy Corporation collected data at Vicksburg, Mississippi, that 

showed the seasonal patterns in atrazine concentrations in the lower Mississippi River (Tierney, 1992). 

Data on atrazine concentrations from this site, which 

are approximately equivalent to that at the Baton 

Rouge site, show seasonal patterns that are similar to 

those from 1991 through 1993 . The summer maximum concentrations of atrazine reported from 1975 

through 1989 varied from year to year but generally 

were from 1 .5 to 3 ltg/L, with a few higher values 

that ranged from 4 to 6 pg/L (Tierney, 1992). 

The concentrations of alachlor were less than 1 

pg/L and cyanazine ranged from less than 1 to about 2 

gg/L at the three Mississippi River sites during July 

1993 . These concentrations generally were similar to 

maximum concentration previously measured at the 

sites in 1991 (figs . 6, 7) . The same generally was true 

for most other herbicides and metabolites, including 

metolachlor, desethylatrazine, and desisopropylatrazine. The concentrations of ESA ranged from 1 .3 to 4 

pg/L at the three Mississippi River sites and were similar to those of atrazine. However, no previous data on 

this compound are available with which the 1993 

results can be compared. 

In general, herbicide concentrations were 

slightly lower at the Clinton, Iowa, site (table 3) than 

elsewhere . This is the most upstream site and drains 

an area where less land is devoted to crops than farther downstream. Concentrations of herbicides in the 

three tributaries-the Illinois, the Missouri, and the 

Ohio Rivers-were not substantially different from 

those at the Thebes and the Baton Rouge sites 

(tables 3, 4). 

10 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
- - - -

- - - -

-- --

- - - -

- - - -

-- --

- -

- -

-- -- -- --

- - - -

- - - -

- -

- -

The large volumes of water being discharged 

by the upper Mississippi River and its tributaries 

were expected to dilute herbicides and other chemi 

cals transported into the river. However, as noted 

above, this was not the case. Apparently, flooding 

and high flows simply flushed larger amounts of herbicides into the rivers than did the lower flows in previous years . This resulted in herbicide concentrations 

that were typical for the spring runoff period of the 

year. A possible explanation for the transport of 

larger amounts of herbicides into rivers is the timing 

and intensity of the rainfall in the upper Midwest during 1993 . Intense and sustained rain fell shortly after 

planting in many parts of the area and near the time 

when the most concentrated amounts of herbicides were 

on the soil . Under these conditions, a larger fraction of 

herbicides would be transported into rivers than would 

have been with less rainfall or with rain falling later after 

herbicide application. A previous study showed that in 

1991, less than 2 percent of the herbicides applied that 

year in the Mississippi River Basin were transported into 

large rivers (Battaglin and others, 1993; Goolsby and 

Battaglin, 1993) . Because of the heavy rainfall and 

flooding, it is expected that the fraction of herbicides 

transported into the Mississippi River in 1993 will be 

much larger than in 1991 . 

Nitrate concentrations were less than 3 mg/L as 

nitrogen in all samples collected in July and August 1993 

Table 3. Streamflow, nitrate-nitrogen, and herbicide data for three sites on the Mississippi River during the 1993 flood 

[m'/s, cubic meters per second ; p,S, microsiemens per centimeter at 25 degrees Celsius ; pg/L, micrograms per liter; mg/L, milligrams per 

liter; kg/d, kilograms per day; t/d, metric tons per day; ESA, ethanesulfonic acid ; - -, data not yet available] 

Nitrate- Specific Desiso- Cyana- ESA Metol- Atrazine nitrogen Sampling Flow conduc- Nitrate- Alachlor Atrazine Desethyl- propyldate atrazine zine achlor load 

(1993) (m'/s) 

July 16 . . . . . . . . . . . . 6,170 

July 20 . . . . . . . . . . . . 5,830 

July 23 . . . . . . . . . . . . 5,240 

August 4 . . . . . . . . . 3,340 

August 13 . . . . . . . . . 2,920 

August 26 . . . . . . . . . 3,170 

July 16 . . . . . . . . . . . . 23,510 

July 18 . . . . . . . . . . . . 23,730 

July 20. . . . . . . . . . . . 25,290 

25,290 

August 16 . . . . . . . . . 21,630 

August 25 . . . . . . . . . 17,670 

July 7 . . . . . . . . . . . . . 23,870 

July 9 . . . . . . . . . . . . . 23,650 

July 12 . . . . . . . . . . . . 24,020 

July 15 . . . . . . . . . . . . 25,770 

July 19 . . . . . . . . . . . . 27,160 

July 22. . . . . . . . . . . . 28,120 

July 26 . . . . . . . . . . . . 29,110 

July 29 . . . . . . . . . . . . 29,850 

August 2 . . . . . . . . . 30,810 

August 5 . . . . . . . . . 31,440 

August 6 . . . . . . . . . 31,580 

August 9 . . . . . . . . . 32,200 

August 12 . . . . . . . . 32,820 

August 16 . . . . . . . . . 33,190 

August 19 . . . . . . . . . 32,740 

August 23 . . . . . . . . . 32,000 

August 26 . . . . . . . . . 31,320 

tance 

(AS) (mg/L) (Ag/L) (t/d) 

Station 05420500-At Clinton, Iowa 

409 2.80 0.33 1.47 0.34 0.18 1 .40 4.00 0.71 780 1,457 

435 - - .27 1.46 .36 .22 1 .44 3 .10 .64 740 

436 2.54 .19 1.27 .28 .18 1 .09 3.00 .49 580 1,151 

500 2.28 __ __ __ 659 

nitrogen (AgIL) (hgIL) (Rg/L) 

atrazine 

(wg/L) (Ag/L) (AS/L) (kg/d) load 

__ 

495 2.07 522 

496 2.26 619 

Station 07022000-At Thebes, Illinois 

.32 2 .79 .53 .31 .83 3.10 1 .03 5,670 

.38 3 .04 .74 .47 1 .82 1 .80 1 .17 6,240 

335 2 .38 .26 2.18 .56 .39 1 .27 2.00 .81 4,770 5,205 

.32 2.56 1 .91 .98 5,599 

392 

441 

1 .78 

1 .87 

3,330 

2,858 

Station 07374000-At Baton Rouge, Louisiana' 

351 2.50 .36 3.23 

345 2.40 .37 3.31 

418 2.50 .32 2.99 

414 2.40 .31 3.19 

357 2.20 .23 2.66 

347 2.00 .17 2.08 

344 2.10 .15 1 .80 

345 2.00 .14 1.70 

360 2.00 .12 1.60 

380 2.00 

394 2.10 

378 1.90 

358 1.70 .09 1 .50 

364 1.70 

381 1 .70 

406 1 .70 - -

395 1 .50 - -

.40 .29 1.42 2.20 1 .04 6,750 5,222 

.46 .31 1.53 1.90 1 .07 6,770 4,908 

.52 .31 1.20 2.80 1 .07 6,280 5,253 

.50 .35 1 .34 2.50 .97 7,110 5,348 

.52 .33 1 .15 1 .70 .87 6,250 5,167 

.43 .27 .84 1 .30 .64 5,060 4,864 

- - 1 .00 .60 4,530 5,286 

- - .75 .60 4,388 5,162 

- - - - .73 .50 4,279 5,348 

__ __ __ __ 5,438 __ 5,734 

__ 5,292 

- - - - .45 .50 4,257 4,825 

__ __ __ __ 4,879 __ 4,813 __ 4,704 

__ 4,063 

'Strearnflow and atrazine and nitrate loads include the amounts diverted from the Mississippi River into the Atchafalaya River . 

Occurrence and Transport of Agricultural Chemicals During the 1993 Flood 
-- --

- -

- -

- - - -

- - -- --

- -

- -

- -

- - - -

- -

-- --

from the three sites on the Mississippi River (table 3) 

and less than 4 mg/L in all samples collected in July 

and August 1993 from the tributary sites (table 4). 

These concentrations are similar to those measured in 

summer 1991 and 1992 (fig. 8). At the three Mississippi River sites, nitrate-nitrogen concentrations generally decreased from mid-July through the end of 

August (fig . 8) . Concentrations of nitrate are typically lower during the summer than during the 

remainder of the year. Because of the high streamflow and prolonged flushing of the upper Mississippi 

River Basin, nitrate-nitrogen concentrations during 

the remainder of 1993 are not expected to be as low 

as those measured during the preceding 2 years. 

Additional data on nitrate concentrations in the Mississippi River are needed to answer this question. 

Relation to Drinking-Water Standards 

As discussed in the section "Drinking-Water 

Standards and Aquatic-Life Guidelines," the MCL's 

for herbicides are based on the average concentrations in a minimum of four quarterly samples . Individual samples that exceed these limits do not 

necessarily constitute noncompliance with the Safe 

Drinking Water Act. Similarly, HA'S are based on 

annual average concentrations. During July and 

August 1993, the concentration of atrazine exceeded 

the MCL of 3 pg/L in 4 of 26 samples (tables 3, 4), 

and cyanazine exceeded the HA of 1 pg/L in 17 of 26 

samples (tables 3, 4) . On the basis of previous studies of these herbicides in the Mississippi River Basin, 

the annual average concentrations for these herbicides probably will not exceed the MCL's or HA'S in 

1993. During 1991 and 1992, the annual average concentrations for these herbicides were far below the 

health-based limits (Goolsby and Battaglin, 1993). 

None of the other herbicides measured during July 

and August 1993 had concentrations that exceeded 

the MCL's or HA's. Furthermore, the concentration 

of nitrate in all samples analyzed was well below the 

nitrate MCL of 10 mg/L. 

Table 4. Streamflow, nitrate-nitrogen, and herbicide data for three sites on the Mississippi River tributaries during the 1993 flood 

[m'/s, cubic meters per second; gS, microsiemens per centimeter at 25 degrees Celsius; /Lg/L, micrograms per liter; mg/L, milligrams per 

liter; kg/d, kilograms per day; t/d, metric tons per day; ESA, ethanesulfonic acid ; <, less than ; --, data not yet available] 

Specific Nitrate- DesisoFlow conduc- nitrogen Alachlor Atrazine Desethyl- propyl- Cyanaatrazine zine Nitrate- Metol- Atrazine ESA hlor load nitrogen 

(Fig) ac load (ug/L) (kg/d) (t/d) 

2.10 0.92 350 478 

2.40 .86 350 486 

2.40 .81 340 526 

2.00 .59 340 553 __ __ __ 314 

299 

272 

1 .90 2,910 

888 

1 .10 .62 1,710 

1 .90 .83 440 386 

1 .50 .60 510 443 

.50 .59 580 

224 

166 

247 

198 

Sampling 

date 

(1993) 

July 16 . . . . . . . . . . . . 

July 20. . . . . . . . . . . . 

July 22 . . . . . . . . . . . . 

July 26 . . . . . . . . . . . . 

August 4 . . . . . . . . . 

August 12 . . . . . . . . . 

August 19 . . . . . . . . . 

August 26 . . . . . . . . . 

August 31 . . . . . . . . . 

July 17 . . . . . . . . . . . . 

July 24 . . . . . . . . . . . . 

July 27 . . . . . . . . . . . . 

August 6 . . . . . . . . . . . 

August 25 . . . . . . . . . 

July 22 . . . . . . . . . . . . 

July 29 . . . . . . . . . . . . 

August 3 . . . . . . . . . 

August 10 . . . . . . . . 

August 19 . . . . . . . . 

August 25 . . . . . . . . . 

(m'/s) tance 

(AS) 

1,420 505 

1,560 492 

1,600 505 

2,000 467 

1,510 437 

1,305 576 

1,277 565 

593 

612 

13,450 263 

9,100 331 

11,020 

2,270 424 

3,740 362 

3,740 362 

2,920 291 

1,480 390 

3,970 361 

3,310 292 

(mg/L) (Ag/L) (wg/L) (ug/L) atrazine (4g/L) 

(Fig/L) 

Station 05586100-Illinois River at Valley City, Illinois 

3.89 0.06 2 .87 0.70 0.51 1 .30


3.61 .14 2 .62 .76 .57 1 .51


3.80 .12 2 .47 .73 .57 1 .48


3.20 .09 1 .95 .66 .47 1 .17


2.41 __


2.65 - -


2.46 - -


2.74 - -


2.63


Station 06934500-Missouri River at Hermann, Missouri 

- - .33 2.50 - - - - 1 .30 

1.13 

.11 1 .80 .40 .30 .79 

1 .39 

1.40 

Station 03612500-Ohio River at Grand Chain, Illinois 

1 .97 .21 2 .27 .68 .29 .82 

1.37 .10 1 .59 .49 .33 .69 - - < .05 1 .79 .41 .26 .63 

.84 

1 .29 - -

.72 

.72 

1 2 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
0


z


50 O


U


W


to


W 

0-

40 (n 

w 

w 

U 

30 m 

U 

w 

O 

to


0


20 z
rn 

O 

x 

z 10 -

0

J 

0 ¢ 

3 ~II~ \ 30 

z FI . t rn o ~ ;1 o 

2 Streamflow 20 

w 

\f . 

10 

O 

a 0 0 

A M J J A S O N D J F M A M J J A S O N D J F M A M J J A 

1991 1992 

Samplesfrom


New Orleans, Louisiana.


Atrazine analysis made by


the Jefferson parish, Louisiana,


WaterQuality Laboratory


0 100 200 MILES 

0 100 200 KILOMETERS 

O 

EXPLANATION a Clinton ¢ 

Sampling site and w station location z 

0 

U 

1993 

Ct 

z 

50	 O 

U 

w 

1993 flood 

w 

o_ 

40 ¢ 

f 

U 

30 m 

U 

O 

to 

0 

20 a 

O 

x 

Pt

V 10 

O 

w 

w 0 

Figure 5. Streamflow and temporal distribution in the concentrations of atrazine at three sites on the Mississippi River from 

April 1991 through August 1993. A, Clinton, Iowa; B, Thebes, Illinois ; C, Baton Rouge, Louisiana . 

Occurrence and Transport of Agricultural Chemicals During the 1993 Flood 1 3 
0.2 -, ;10 

"1993 floc O 

a 0 0 s
F-
 AMJJAS0NDJFMAMJJASONDJFMAMJJA to


1992 1993 

Gul f of Mex0 100 200 MILES 

I 

0 100 200 KILOMETERS 

EXPLANATION 

Clinton 

Sampling site and 

station location 

Figure 6. Streamflow and temporal distribution in the concentrations of alachlor at three sites on the Mississippi River from 

April 1991 through August 1993. A, Clinton, Iowa; B, Thebes, Illinois; C, Baton Rouge, Louisiana . 

1 4 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Figure 7. Streamflow and temporal distribution in the concentrations of cyanazine at three sites on the Mississippi River from 

April 1991 through August 1993. A, Clinton, Iowa; B, Thebes, Illinois ; C, Baton Rouge, Louisiana . 

Occurrence And Transport Of Agricultural Chemicals During The 1993 Flood 1 5 
Figure 8. Streamflow and temporal distribution in the concentrations of nitrate-nitrogen at three sites on the Mississippi River 

from April 1991 through August 1993. A, Clinton, Iowa; 8, Thebes, Illinois ; C, Baton Rouge, Louisiana. 

1 6 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Transport of Herbicides and Nitrate 

The transport of agricultural chemicals (herbicides and nitrate) by rivers is a function of the concentration of these compounds and the volume of water 

that flows in the river. During the 1993 flood, concentrations of agricultural chemicals were similar to those 

measured at the same time of the year in 1991 and 

1992; however, the volume of water transported by the 

upper Mississippi River was much larger than normal 

for midsummer. The resulting estimates of the daily 

loads of agricultural chemicals transported by the Mississippi River are large relative to estimates calculated 

from data collected in 1991 and 1992. 

Calculation Methods 

The amount (loads) of herbicides or nitrate transported each day by rivers was calculated by multiplying 

daily mean streamflow by herbicide or nitrate concentra 

tion. The resulting estimates of daily agricultural chemical loads (kilograms or metric tons) can then be 

compared with estimates of daily agricultural chemical 

loads calculated from data collected in other studies. 

The daily load at Baton Rouge was estimated from concentrations of agricultural chemicals measured at Baton 

Rouge and the combined streamflow for Baton Rouge 

and the Atchafalaya diversion . The daily load estimate 

for Baton Rouge, therefore, closely represents the total 

load of agricultural chemicals from the Mississippi 

River to the Gulf of Mexico. 

In earlier studies, estimates of seasonal or 

annual loads of agricultural chemicals were calculated 

by multiplying daily mean streamflow by measured or 

estimated daily herbicide or nitrate concentration and 

then accumulating these estimates for a period of interest, such as 1 year (Battaglin and others, 1993; 

Goolsby and Battaglin, 1993). Estimates of the loads 

of atrazine and nitrate transported by the Mississippi 

River from April through August 1993 were calculated 

for comparison with loads transported from April 

through August 1991 and 1992. 

Transport of Herbicides 

The combination ofhigh herbicide concentrations, which are typical during late spring and summer, 

and high streamflow resulted in large loads of herbicides 

in the upper Mississippi River during the 1993 flood. 

Estimates of the loads of atrazine from April 1991 

through August 1992 and from mid-July through midAugust 1993 at three sites on the Mississippi River are 

shown in figure 9. The three largest daily load estimates 

for atrazine, alachlor, cyanazine, and metolachlor in 

1991 and 1992 and during the 1993 flood are listed in 

table 5. Also shown in figure 9 are the atrazine load estimates at New Orleans from March through August 1993. 

At the Thebes site, the maximum daily atrazine 

load during the 1993 flood was 6,240 kilograms (kg) 

(fig. 9; table 5), or approximately 1 .7 times that of 1991 

and more than three times that of 1992. The maximum 

daily cyanazine load during the 1993 flood (4,180 kg) 

was about 1 .5 times the maximum observed in 1991 and 

about five times that in 1992 (fig. 9; table 5). For metolachlor, the maximum daily load during the 1993 flood 

(2,400 kg) was approximately 1 .3 times that in 1991 and 

about six times that of 1992 (fig . 9; table 5). Finally, the 

maximum daily load of alachlor during the 1993 flood 

(780 kg) was the same as that of 1991 and about five 

times that of 1992. Similar proportions among maximum daily herbicide load during the 1993 flood and 

those during 1991 and 1992 are noted at the Clinton site 

(fig . 9; table 5). At the Baton Rouge site, large maximum daily herbicide loads were estimated during the 

1993 flood. The total amount of atrazine discharged to 

the Gulf of Mexico from April through August 1993 is 

estimated to be 539,000 kg compared with 296,000 kg 

during this same period in 1991 and 160,000 kg in 

1992. The 1993 atrazine load for this 5-month period 

was 80 and 235 percent larger, respectively, than in 

1991 and 1992. The much larger transport in 1993 is 

attributed to extensive flushing of atrazine from the 

upper Mississippi River Basin during the 1993 flood. 

Transport of Nitrate 

Near-normal nitrate-nitrogen concentrations of 

2 to 3 mg/L during July and August 1993 combined 

with high streamflows resulted in the transport of large 

amounts of nitrate in the upper Mississippi River during the 1993 flood. The maximum daily loads of 

nitrate-nitrogen during the 1993 flood at the Thebes 

and the Baton Rouge sites were more than 5,000 t. 

These loads were larger than those measured in 1992 

but similar to those measured in spring and early summer 1991 (fig . 10; table 6). The annual load of nitratenitrogen transported by the Mississippi River during 

1993 is expected to be much larger than that estimated 

for 1991 or 1992 because large loads of nitrate-nitrogen are being transported in the summer. For example, 

the nitrate-nitrogen load transported to the gulf of Mexico from April through August 1993 was 827,000 t 

compared with 603,000 t in 1991 and 390,000 t in 

Occurrence And Transport Of Agricultural Chemicals During The 1993 Flood 17 
f 

O 

Y 

Z_ 

9	 O 

6

O

J 

z_ 

F 

S 

T9TFS, . 

MINNESOTA 

I l i 

5 

Jxi WISCONSIN ( MICHIGAN 

IOWA J 

Clinton , \ C ic o 

z 

4 

40° ILLINOIS INDIANA ] 

ftiue valley city . 

Hermann $, L~(iis MISSOURI 

P 

Thebes 

G 

35°1 . ._.. ... . _~" Memphis ARKA A 

f 

3 

Y 

zu Q ]onto 0 

TUCKY z 

Chain 

a 

TENNESSEE i 

i 

MISSISSIPPI 

f 

LO 

Baton Ro se 30' 

Orleans 

Gul f of Mexico 

EXPLANATION Clinton 

Sampling site and 

station location 

9,000 

8,000 

7,000 

6,000 

5,000 

4,000 

3,000 

2,000 

1,000 

,000 

8,000 

7,000 

6,000 

5,000 

4,000 

3,000 

2,000 

1,000 

A M J J A S O N DIJ F M A M J J A S O N DIJ F M A M J J A 

1991 1992 1993 

1993 flood I 

z 50 O 

U

W 

U) 

w 

o. 

40 

w 

f 

U 

m 

U 

W 

30 

20 

U 

30 °° 

U 

O 

0 

20 

i 

z 10 -

O

J

W 

0 

N 

z 

50 

_ 

Atrazine loads at New Orleans, w Louisiana. Atrazine load data from ~14 - a 

Jefferson Parish, Louisiana, \ ' 40 

Water Quality Laboratory I I- W 

, I Iz 

d 

a

0 

w

Z_ 

a 

Figure 9. Streamflow and temporal distribution of daily load of atrazine at three sites on the Mississippi River from April 1991 

through August 1993. A, Clinton, Iowa; 8, Thebes, Illinois ; C, Baton Rouge, Louisiana . 

1 8 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Table 5 . Three largest daily load estimates for atrazine, alachlor, cyanazine, and metolachlor at three sampling sites on 

the Mississippi River in 1991 and 1992 and during the 1993 flood 

[in kilograms per day] 

Herbicide 

Atrazine . . . . . . . . . . . . . . . . . . . . 

Alachlor . . . . . . . . . . . . . . . . . . . . 

Cyanazine . . . . . . . . . . . . . . . . . . 

Metolachlor . . . . . . . . . . . . . . . . 

Atrazine . . . . . . . . . . . . . . . . . . . 

Alachlor . . . . . . . . . . . . . . . . . . . . 

Cyanazine . . . . . . . . . . . . . . . . . . 

Metolachlor. . . . . . . . . . . . . . . . 

Atrazine . . . . . . . . . . . . . . . . . . . . 

Alachlor . . . . . . . . . . . . . . . . . . . . 

Cyanazine . . . . . . . . . . . . . . . . . . 

Metolachlor . . . . . . . . . . . . . . . . 

First 

530 

260 

310 

190 

3,620 

780 

2,980 

1,830 

6,060 

840 

2,930 

2,280 

1991 1992 

Second Third First Second 

Station 05420500-At Clinton, Iowa 

430 180 96 75 

230 110 27 26 

240 220 100 59 

190 170 46 45 

Station 07022000-At Thebes, Illinois 

3,600 3,240 1,780 1,160 

740 700 160 150 

2,610 2,540 890 800 

1,730 1,630 430 410 

Station 07374000-At Baton Rouge, Louisiana' 

5,860 4,910 2,330 2,310 

720 680 210 200 

2,800 2,700 980 770 

2,260 1,910 1,080 780 

1993 flood 

Third First Second Third 

73 780 740 580 

15 180 140 86 

53 750 730 490 

27 380 320 220 

910 6,240 5,670 5,600 

130 780 700 650 

680 4,180 3,730 2,780 

370 2,400 2,140 2,090 

2,290 7,110 6,770 6,750 

170 760 750 690 

660 3,130 2,990 2,970 

630 2,250 2,190 2,170 

'Includes the amounts of herbicides diverted from the Mississippi River into the Atchafalaya River. 

1992. The 1993 load for this period was 37 and 112 

percent, respectively, larger than in 1991 and 1992. 

Nitrate-nitrogen loads generally are much lower in the 

summer months than in the remainder of the year (Battaglin and others, 1993; Goolsby and Battaglin, 1993). 

The transport of extraordinarily large loads of 

nitrate from the Mississippi River Basin into the Gulf 

of Mexico during the summer months of 1993 could 

affect the gulf ecosystem along the Louisiana coast 

by stimulating algal growth. The National Oceanic 

and Atmospheric Administration has monitored the 

concentrations of nitrate, dissolved oxygen, and other 

characteristics in the gulf to determine the effects of 

the flooding . The data indicate that during July 1993, 

salinity of surface water in the gulf shelf was lower 

than average for this time of year and that phytoplankton biomass was higher than average (Michael Dowgiallo, National Oceanic and Atmospheric 

Administration, written commun., 1993). These 

effects could be attributed to larger-than-average seasonal inflows of freshwater and nutrients, such as 

nitrate. 

SUMMARY 

Water samples for the analysis of herbicides 

and nitrate were collected during July and August 

1993 to determine the effects of record flooding on the 

concentrations and transport of these common agricultural chemicals in the Mississippi River Basin. Samples were collected at three sites on the mainstem of 

the Mississippi River and at sites near the mouths of 

three large tributaries-the Illinois, the Missouri, and 

the Ohio Rivers. Results from these samples were 

compared with those from a previous study of the Mississippi River conducted in 1991 and 1992. The 

results show that the heavy rainfall and severe flooding 

from mid-June through early August 1993 flushed 

extraordinarily large amounts of agricultural chemicals 

into the Mississippi River, many of its tributaries, and, 

ultimately, the Gulf of Mexico. Concentrations of herbicides, such as atrazine, alachlor, cyanazine, and metolachlor, were similar to the maximum concentrations 

measured during spring and summer 1991 and 1992, 

even though high streamflows were recorded. Consequently, the record flooding did not dilute the concentrations of herbicides as was anticipated . Instead, 

larger-than-average amounts of herbicides were 

flushed into streams and concentrations of herbicides 

remained about the same as typical for this time of the 

year. The maximum daily load of atrazine transported 

by the Mississippi River in the vicinity of Thebes during the flood of 1993 was as much as 70 percent higher 

than that measured in 1991 . The total load of atrazine 

discharged to the Gulf of Mexico from April through 

Summary 19 
9 

Z 50	 O 

U 

w 

f/) 

w 

a 

40 

w

r 

w 

U 

30 

U 

O 

N 

O 

20 a 

N 

O 

x

r


z
 10 -


3

O 

J 

0 r

N 

Clinton 

Sampling site and 

station location 

1991 

Figure 10. Streamflow and temporal distribution of daily load of nitrate at three sites on the Mississippi River from April 1991 

through August 1993. A, Clinton, Iowa; 8, Thebes, Illinois; C, Baton Rouge, Louisiana . 

20 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993 
Table 6. Three largest daily load estimates for nitrate-nitrogen at three sampling sites on the Mississippi River in 1991 and 

1992 and during the 1993 flood 

[In metric tons per day] 

1991 1992 1993 flood 

Site 

First Second Third First Second Third First Second Third 

Station 05420500 at Clinton 1,070 990 960 1,100 1,040 910 1,495 1,151 660 

Iowa. 

Station 07022000 at Thebes, 4,890 4,760 4,340 3,110 2,900 2,510 5,205 3,330 2,858 

Illinois . 

Station 0737400 at Baton 5,900 5,670 5,630 4,500 4,420 4,380 5,734 5,437 5,348 

Rouge, Louisiana .' 

'Includes amounts of nitrate diverted from the Mississipi River into the Atchafalaya River. 

August 1993 (539,000 kg) was about 80 percent larger 

than that for the same period in 1991 and 235 percent 

larger than that for this same period in 1992. The concentrations of atrazine and cyanazine in a few samples 

exceeded health-based limits for drinking water. However, compliance with drinking-water regulations is 

based on the average of at least four quarterly samples; 

the annual average concentrations of these herbicides 

in the Mississippi River probably will not exceed these 

limits for 1993. Nitrate concentrations also remained 

typical for this time of the year, although nitrate loads 

generally were higher than those measured in spring 

and summer 1991 and 1992. It is estimated that 

827,000 t of nitrate-nitrogen was discharged to the 

Gulf of Mexico from April through August 1993. This 

amount is about 37 percent larger than that for the 

same period in 1991 and about 112 percent larger than 

that for the same period in 1992. The discharge of 

extraordinarily high loads of nitrate and large amounts 

of freshwater into the Gulf of Mexico during midsummer when primary production is highest could increase 

phytoplankton biomass and affect the gulf ecosystem 

along the Louisiana coast. 

REFERENCES CITED 

Battaglin, W.A., Goolsby, D.A., and Coupe, R.H., 1993, 

Annual use and transport of agricultural chemicals in 

the Mississippi River, 1991-92, in Goolsby, D.A., 

and others, eds., Selected papers on agricultural 

chemicals in water resources of the Midcontinental 

United States : U.S . Geological Survey Open-File 

Report 93-418, p. 26-38. 

Becker, R. L., Herzfeld, D., Ostlie, K. R., and StammKatovich, E. J., 1989, Pesticides-Surface runoff, 

leaching, and exposure concerns: Minneapolis, 

University of Minnesota, Minnesota Extension 

Service AG-BU-3911, 32 p. 

Fishman, M.J ., and Friedman, L.C ., 1989, Methods for the 

determination of inorganic substances in water and 

fluvial sediments: U.S . Geological Survey Tech 

niques of Water Resources Investigations, book 5, 

chap. A1, 545 p. 

Gianessi, L.P ., and Puffer, C.M., 1991, Herbicide use in 

the United States : Washington, D.C ., Resources for 

the Future, December 1990 (revised April 1991), 

128 P. 

Goolsby, D.A., and Battaglin, W.A., 1993, Occurrence, distribution, and transport of agricultural chemicals in 

surface waters of the Midwestern United States, in 

Goolsby, D.A., and others, eds., Selected papers on 

agricultural chemicals in water resources of the Midcontinental United States : U.S . Geological Survey 

Open-File Report 93-418, p. 1-24. 

Goolsby, D.A., Battaglin, W.A., Fallon, J.D ., Aga, D.S ., 

Kolpin, D.W., and Thurman, E.M., 1993, Persistence 

of herbicides in selected reservoirs in the Midwestern United States-Some preliminary results, in 

Goolsby, D.A., and others, eds., Selected papers on 

agricultural chemicals in water resources of Midcontinental United States : U.S . Geological Survey 

Open-File Report 93-418, p. 51-63. 

Goolsby, D.A., Coupe, R.H., and Markovchick, D.J ., 1991, 

Distribution of selected herbicides and nitrate in the 

Mississippi River and its major tributaries, April 

through June 1991 : U.S . Geological Survey WaterResources Investigations Report 91-4163, 35 p. 

Goolsby, D.A. Thurman, E.M., Pomes, M.L ., Meyer, M., 

and Battaglin, W.A., 1993, Occurrence, deposition, 

and long range transport of herbicides in precipitation in the Midwestern and Northeastern United 

States, in Goolsby, D.A ., and others, eds., Selected 

papers on agricultural chemicals in water resources 

of the Midcontinental United States : U.S . Geological Survey Open-File Report 93-418, p. 75-87. 

References Cited 21 
Iowa Agricultural Statistics, 1993, Iowa crops and 

weather: Des Moines, Iowa Agricultural Statistics, 

v. 93, no. 16, 1 p. 

Kent, R.A., Pauli, B.D., Trotter, D.M., and Gareau, J., 

1991, Canadian water quality guidelines for metolachlor: Ottawa, Environment Canada, Scientific 

Series 184, 34 p. 

Lucey, K.J ., and Goolsby, D.A., 1993, Effects of climatic 

variations over 11 years on nitrate-nitrogen concentrations in the Raccoon River, Iowa: Journal of Envi 

ronmental Quality, v. 22, p. 38-46. 

Meyer, M.T ., Mills, M.S ., and Thurman, E.M., 1993, Automated solid-phase extraction of herbicides from 

water for gas chromatography/mass spectrometry 

analysis : Journal of Chromatography, v. 629, p. 55­

59. 

Parrett, Charles, Melcher, N.B . and James, R.W., Jr ., 1993, 

Flood discharges in the upper Mississippi River 

Basin, in Floods in the upper Mississippi River 

Basin, 1993: U.S . Geological Survey Circular 1120-

A, 14 p. 

Pauli, B.D ., Kent, R.A ., and Wong M.P ., 1990, Canadian 

water quality guidelines for metribuzin : Ottawa, 

Environment Canada, Scientific Series 179, 44 p. 

Pauli, B.D., Kent. R.A ., and Wong, M.P ., 1991a, Canadian 

water quality guidelines for cyanazine: Ottawa, 

Environment Canada, Scientific Series 180, 26 p. 

Pauli, B.D., Kent, R.A ., and Wong, M.P ., 1991b, Canadian 

water quality guidelines for simazine: Ottawa, Environment Canada, Scientific Series 187, 29 p. 

Squillace, P.J ., Thurman, E.M., and Furlong, E.T ., 1993, 

Groundwater as a nonpoint source of atrazine and 

deethylatrazine in a river during base flow condi 

tions: Water Resources Research, v. 29, no. 6, p. 

1719-1729. 

Thurman, E.M., Meyer, M.T., Ponies, M. L., Perry, C.E ., 

and Schwab, A.P ., 1990, Enzyme-linked inimunosorbent assay compared with gas chromatography/mass 

spectrometry for the determination of herbicides in 

water: Analytical Chemistry, v. 62, p. 2043-2048 . 

Thurman, E.M., Goolsby, D.A., Meyer, M.T., and Kolpin, 

D.W., 1991, Herbicides in surface waters of the Midwestern United States-The effect of spring flush: 

Environmental Science and Technology, v. 25, p. 

1794-1796. 

1992, A reconnaissance study of herbicides and 

their metabolites in surface water of the Midwestern 

United States using immunoassay and gas chromatography/mass spectrometry : Environmental Science and Technology, v. 26, no. 12, p. 2440-2447. 

Tierney, D.P ., 1992, A review of historical surface water 

monitoring for atrazine in the Mississippi, Missouri, 

and Ohio Rivers, 1975-1991 : Greensboro, N.C ., 

Ciba Geigy Corporation, Technical Report 6-92, 69 

P. 

Trotter, D.M., Baril, A., Wong, M.P., and Kent, R.A., 

1990, Canadian water quality guidelines for atrazine : Ottawa, Environment Canada, Scientific Series 

168, 106 p. 

U.S . Department of Agriculture, 1992, Agricultural 

resources-Situation and outlook report: U.S . 

Department of Agriculture, Economic Research Service, AR-25, 66 p. 

1993, Agricultural resources-Situation and outlook report: U.S . Department of Agriculture, Economic Research Service, AR-30, p. 42-45. 

U.S . Department of Commerce, 1989, Census of agriculture, 1987-Final county file: U.S . Department of 

Commerce, Bureau of Census, [machine-readable 

data file]. 

U.S . Environmental Protection Agency, 1988, Methods for 

the determination of organic compounds in drinking 

water: U.S . Environmental Protection Agency, EPA600/4-88/039, p. 109-141 . 

1990, County-level fertilizer sales data : U.S . Environmental Protection Agency, Office of Policy, Planning, and Evaluation, PM-221 . 

1992, Drinking Water Regulations and Health 

Advisories, U.S . Environmental Protection Agency, 

Office of Water, 13 p. 

Wahl, K.L., Vining, K.C ., and Wiche, G.J ., 1993, Precipitation in the Upper Mississippi River Basin, January 1 

through July 31, 1993, in Floods in the upper Missis 

sippi River Basin, 1993: U.S . Geological Survey Circular 1120-B, 10 p. 

Wong, M.P ., and Kent, R.A., 1988, Developing Canadian 

water quality pesticide guidelines for the protection 

of aquatic life : Water Pollution Control Research 

Journal of Canada, v. 23, no. 4, p. 500-509. 

22 Occurrence and Transport of Agricultural Chemicals in the Mississippi River, July Through August 1993