POSTFLOOD OCCURENCE


OF


SELECTED AGRICULTURAL CHEMICALS


AND


VOLATILE ORGANIC COMPOUNDS


IN NEAR-SURFACE


UNCONSOLIDATED AQUIFERS


IN THE 

UPPER MISSISSIPPI 

RIVER BASIN 

9 9 3 

U.S. GEOLOGICAL SURVEY CIRCULAR 1120-G

Front cover-A scene from Minnesota shows the problems 

caused by the 1993 floods (Bernard 

Volker, Herman, Minnesota). 

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) 
POSTFLOOD OCCURRENCE OF SELECTED 

AGRICULTURAL CHEMICALS AND VOLATILE 

ORGANIC COMPOUNDS IN NEAR-SURFACE 

UNCONSOLIDATED AQUIFERS IN THE UPPER 

MISSISSIPPI RIVER BASIN, 1993 

By Dana W. Kolpin and E. Michael Thurman 

Floods in the Upper Mississippi River Basin, 1993 

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

BRUCE BABBITT, Secretary 

U.S . GEOLOGICAL SURVEY 

GORDON P. EATON, 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 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1995 

Free on application to the


US Geological Survey


Information Services


Box 25286, 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 traumawas unmatched by any other flood disaster in United States history. Property damage 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, during, 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 nation 

wide. 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 ; precipitation ; water-qual 

ity 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 floodplain inundation ; and documentation of geomorphologic changes. 

G`6x ' /° ar~r 

Director 

January 11, 1995 

Foreword III 
Contents 

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

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

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

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

Results of previous studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Study design and methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Data-collection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Quality assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Statistical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Extent of flooding problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Chemical occurrence and distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Herbicides and metabolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Nutrients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 

Nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 

Volatile organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 

FIGURES 

1-4. Photographs showing: 

1 . An example of damage caused when the Mississippi River breached a levee near the southern tip 

of Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2. Flood conditions along the Missouri River in west-central Missouri, July 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

3. Aerial view of the Iowa River in eastern Iowa after floodwaters had receded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

4. An inundated municipal well along the Cedar River in eastern Iowa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

5 . Graph showing water level during the 1993 water year for a well completed in the Mississippi River 

alluvium in eastern Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

6. Map showing location of study region in the upper Mississippi River Basin and the 110 wells sampled 

for this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

7-8 . Photographs showing: 

7. A farmstead in Minnesota, July 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

8. Water problems caused by heavy rains in 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

9-12. Graphs showing relation between: 

9. Severity of 1993 flooding and change in total herbicide concentration in water samples collected 

during 1991 or 1992 and those collected for this study during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 

10. Changes in dissolved-oxygen concentration and change in total herbicide concentration in water samples 

collected during 1991 or 1992 and those collected for this study during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 

11 . Well depth below land surface and change in total herbicide concentration in water samples collected 

during 1991 or 1992 and those collected for this study during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 

12. Percentage of land in corn and soybean production and change in total herbicide concentration in water 

samples collected during 1991 or 1992 and those collected for this study during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 

13. Map showing location of wells completed in selected alluvial aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 

14. Graph showing relation of categories of flooding to changes in nitrate concentration between the samples 

collected during 1991 or 1992 and those collected for this study during 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 

15. Map showing changes in nitrate concentration between the samples collected during 1991 or 1992 

and those collected for this study during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 
TABLES 

1 . Summary of 1993 herbicide and herbicide metabolite data for the 110 wells completed in near-surface 

unconsolidated aquifers, upper Mississippi River Basin, sampled for this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

2. Summary of previous (1991 and 1992) water-quality data by routine analytical methods for the 110 wells 

completed in the near-surface unconsolidated aquifers, upper Mississippi River Basin, sampled 

during 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

3. Summary of 1993 nutrient data for the 110 wells completed in the near-surface unconsolidated 

aquifers, upper Mississippi River Basin, sampled for this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

4. Summary of 1993 volatile organic compound data for the 110 wells completed in the near-surface 

unconsolidated aquifers, upper Mississippi River Basin, sampled for this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

CONVERSION FACTORS AND ABBREVIATED WATER-QUALITY UNITS 

Multiply By To obtain 

Length 

foot (ft) 0 .3048 meter 

Area 

acre 0 .4047 hectares 

Milligram per liter (mg/L) is a unit expressing the concentration of a chemical constituent in solution as weight€

(milligrams) of solute per unit volume (liter) of water.€

Microgram per liter (pg/L) is a unit expressing the concentration of a chemical constituent in solution as weight€

(micrograms) of solute per unit volume (liter) of water.€

VI Contents 
Postflood Occurrence of Selected Agricultural Chemicals and 

Volatile Organic Compounds in Near-Surface Unconsolidated 

Aquifers in the Upper Mississippi River Basin, 1993 

By Dana W. Kolpin and E. Michael Thurman 

Abstract 

The historic stream flooding and intense 

rainfall across the upper Mississippi River Basin 

during summer 1993 had an immediate effect on 

near-surface unconsolidated aquifers by raising 

the water levels closer to the land surface . The 

objective of this study was to determine if this 

flooding also had immediate effects on groundwater quality . Water samples were collected during September and October 1993 from 110 wells 

completed in near-surface unconsolidated aquifers and were analyzed for herbicides, herbicide 

metabolites, inorganic nutrients, and volatile 

organic compounds. The results of these samples 

were compared with those obtained during summer 1991 or 1992. The difference was not statistically significant in the frequency of herbicide 

detection, total herbicide concentration, nitrate 

concentration, or the frequency of volatile 

organic compound detection between water samples collected in 1991 and 1992 and those collected in 1993 when all 110 wells were considered collectively . However, water samples from 

the Missouri River alluvial aquifer had a fourfold 

increase in the frequency of herbicide detection . 

There also appears to be a relation between 

increases in total herbicide concentration and the 

occurrence of stream flooding near a well. Water 

samples from wells that had at least a 20-percent 

increase in dissolved-oxygen concentration had 

the greatest frequency of substantial changes in 

total herbicide concentration and substantial 

increases in nitrate concentration . Increased dissolved-oxygen concentration could indicate 

areas where recharge has increased as a result of 

extensive stream flooding and intense rainfall . 

An inverse relation was determined between 

well depth and changes (increase or decrease) in 

total herbicide concentration . Water in shallow 

wells more quickly reflect changes in water quality in response to changes in recharge. Significantly more urban residential and industrial land 

use was within a 30-meter radius of the well for 

wells in which volatile organic compounds were 

detected. Because water moves more slowly 

along ground-water flow paths compared with 

surface-water runoff, additional information is 

required to determine long-term effects of the 

1993 flood on ground-water quality. 

INTRODUCTION 

Flooding was severe across large parts of the 

upper Mississippi River Basin during summer 1993 

(fig . 1) . This flooding was unprecedented in terms of 

its extent, long duration, and extensive damage. At 45 

U.S . Geological Survey (USGS) streamflow-gaging 

stations in 8 Midwestern States, peak discharges 

exceeded 100-year recurrence intervals (Parrett and 

others, 1993). At 41 USGS streamflow-gaging stations, the peak discharge was greater than the previous maximum discharge . This record flooding during 

summer 1993 was facilitated by a wetter-than-normal 

spring that kept much of the soils in the upper Midwest saturated and a persistent weather pattern that 

created intense rainfall from June through August. 

Rainfall during July 1993 was more than 150 percent 

of normal (1961-90) over much of the upper MidPost-Flood Occurence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 1 
Figure 1 . An example of damage caused when the Mississippi River breached a levee near the southern tip of Illinois . 

west and more than 400 percent of the normal in several areas (Hillaker, 1993 ; Wahl and others, 1993). 

The 1993 flood also affected the quality of surface waters in the upper Mississippi River Basin. Data 

collected from the Mississippi River and its tributaries 

during the flood showed that the large volumes of 

water did not have the anticipated dilutional effect on 

the concentrations of agricultural chemicals that were 

being transported in the river, but simply flushed 

increased amounts into the river systems (Goolsby and 

others, 1993). The concentrations of agricultural chemicals in 1993 were similar to those measured at much 

lower flows during 1991 and 1992, but the corresponding daily chemical loads that were being transported 

were substantially larger . The total atrazine load transported to the Gulf of Mexico from April through 

August 1993 was about 80 percent higher than that for 

the same period in 1991 and about 235 percent higher 

than that in 1992 (Goolsby and others, 1993). 

Ground water is the major source of drinking 

water in the upper Midwest, and conventional watertreatment practices do not remove most agricultural 

chemicals. The large increases in the load of agricultural chemicals that were being transported in surface 

waters in the upper Mississippi River Basin from the 

1993 flood raised the concern that transport of these 

2 Introduction 

chemicals to ground water, particularly alluvial aquifers, also could be increasing. Possible modes of 

chemical transport to ground water from flooding 

include reversed hydraulic gradients due to prolonged 

high river stages (fig . 2), infiltration of water from 

overbank flooding (fig . 3), and direct flow down the 

well casing through inundation of a well (fig . 4). 

The 1993 flood increased ground-water 

recharge and caused an immediate increase in water 

levels in unconsolidated aquifers . For example, the 

increase in water level in a well completed in the Mississippi River alluvium in eastern Iowa, is shown in 

figure 5. About 40 percent of the USGS water-level 

monitoring wells completed in unconsolidated aquifers in Iowa had the highest water level for their 

period of record (D. Sneck-Fahrer, U.S . Geological 

Survey, oral commun., 1994). 

Purpose and Scope 

The purpose ofthis study was to determine 

whether the 1993 flood affected the water quality of 

near-surface aquifers in the upper Mississippi River 

Basin. This report presents and summarizes information 

on the concentrations of agricultural chemicals and volatile organic compounds in water samples from near-
Figure 2. Flood conditions along the Missouri River in west-central Missouri, July 1993. 

Figure 3. Aerial view of the Iowa River in eastern Iowa after floodwaters had receded (September 1993). The recent sediment 

deposition clearly denotes the maximum extent of floodwater inundation . Note the amount of water left behind in the low-lying 

areas. 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 3 
Z 

Figure 4. An inundated municipal well along the Cedar River 

in eastern Iowa. 

surface unconsolidated aquifers in the upper Mississippi 

River Basin collected during September and October 

1993, which was shortly after the worst of the flooding . 

Ground water moves slowly compared with 

surface water. The rate of ground-water movement 

can vary greatly depending on the materials through 

which the water passes, the hydraulic conductivity of 

the unsaturated zone, and the distance from the land 

surface to the saturated zone. Consequently, additional information is required to determine long-term 

effects of the 1993 flood on the water quality of nearsurface aquifers . For example, changes in the nitrate 

concentrations of ground water lagged behind 

changes in the amounts of nitrogen fertilizer usage by 

about 4 to 19 months (Hall, 1992) . 

Results of Previous Studies 

During 1991, the USGS performed a reconnaissance study to determine the hydrogeologic, seasonal, and geographic distribution of herbicides and 

nitrate in near-surface aquifers of the midcontinental 

4 Introduction 

10 

J

W 

m

F
W
W
W 12
 z


J
W


W
J


CC 14
 W
F
Q


16 

Figure 5. Water level during the 1993 water year for a well 

completed in the Mississippi River alluvium in eastern Iowa. 

United States (Kolpin and Burkart, 1991 ; Burkart and 

Kolpin, 1993 ; Kolpin, Burkart, and Thurman, 1993, 

1994). Near-surface aquifers were defined as having 

the top of the aquifer material about 15 mbelow land 

surface. Astatistical design was used to select 303 

wells from which water samples were collected in 

March or April (preplanting) and July or August 

(postplanting) 1991 . However, seven of these wells 

could only be sampled once during the 1991 study 

and 10 water samples could not be analyzed because 

of lost or broken bottles . Samples were analyzed for ,, 

l l herbicides, 2 triazine herbicide metabolites, and 4' 

inorganic nutrients. 

Herbicides or triazine metabolites were 

detected in about 24 percent of the 589 samples collected for analysis during 1991 . Results of this study 

showed that water from unconsolidated aquifers is 

more likely to contain herbicides than water from bedrock aquifers (frequency of detection 34 percent compared with 18 percent) . Recharge rates generally are 

faster and recharge sources are in closer proximity to 

the unconsolidated aquifers than to the bedrock aquifers because of such differences as aquifer geometry 

and the likelihood of confining conditions (Kolpin and 

others, 1994) . Ground-water samples from wells 

located within 30 mof a stream had more than twice 

the frequency of herbicide detection than of wells with 

no streams nearby (48 percent compared with 22 percent). Because almost all sampled wells within 30 m 
of a stream were completed in alluvial aquifers, this 

difference in frequency of herbicide detection could 

be caused by a hydraulic connection between aquifers 

and streams. Frequencies of detection and concentrations of herbicides are much larger in streams than in 

aquifers of the midcontinent (Thurman and others, 

1992; Goolsby and Battaglin, 1993). Therefore, 

recharge to an aquifer by a stream could be a source of 

herbicide contamination to the aquifer (Squillace and 

others, 1993) . 

During 1992, the USGS conducted a followup study to investigate why herbicides were not 

detected in greater than 70 percent of the samples 

from near-surface aquifers sampled in 1991 . By using 

a stratified-random procedure, 101 wells were 

selected from the 1991 study network and were 

resampled once during July or August 1992. The 

same 13 herbicide compounds and 4 inorganic nutrients were analyzed as in the 1991 study, as well as an 

additional 45 herbicides, insecticides, and metabolites . A subset of these 101 wells also was sampled 

for 63 volatile organic compounds. 

Apesticide or pesticide metabolite was 

detected in ground water from about 62 percent of the 

101 wells sampled (Kolpin, Goolsby, and others, 

1993) . The greater frequency of detection during 1992 

was the result of an increased number of pesticides and 

metabolites analyzed and a more sensitive analytical 

method that had reporting limits that were about an 

order of magnitude less than those used in 1991 . 

Ground water from 117 wells were analyzed 

for tritium concentrations during either the 1991 or 

1992 study. The combination of tritium's short half 

life (12.3 years), low rate of natural production, and 

large inputs caused by nuclear testing makes it a useful tool for determining water that has recharged aquifers since 1953 (Bradbury, 1991) . For this study, no 

pesticide or pesticide metabolite concentrations were 

determined (at 0.05 pg/L reporting limits) in wells 

containing "old" water. Because agricultural chemicals, such as atrazine, have been used to enhance crop 

yields only during the last 40 years, the absence of 

pesticide detection in wells that contain water 

recharged before 1953 ("old" water) was expected. 

STUDY DESIGN AND METHODS 

Because the greatest effect on water quality of 

near-surface aquifers was expected in the areas with 

the greatest rainfall and stream flooding, the study 

region was defined as the area with more than 150 

percent of normal rainfall from April 1 to July 31, 

1993, (Climate Analysis Center, National Weather 

Service, written commun., 1993) (fig . 6) . Information 

from the 1991-1992 studies was used to select wells 

that would have the greatest potential for determining 

the immediate effects of the 1993 flood on groundwater quality. The initial well selection identified 114 

wells from the network that were completed in unconsolidated aquifers that contained "new" (post-1953) 

water and were located in the high-rainfall region . 

However, six of these wells could not be sampled as 

a result of either the well owner declining to participate in the study or continued inundation by flood 

water; two of these wells were replaced with alternative sampling wells known to be in an area with flood 

problems during 1993. One of these alternative wells 

happen to reside outside the study region . The 110 

wells selected for study were sampled during September or October 1993 . 

Data-Collection Methods 

Methods to collect and process the water samples were the same as those used for the 1991 and 

1992 studies (Kolpin and Burkart, 1991; Kolpin, 

Goolsby, and others, 1993). All samples were collected by USGS personnel with equipment constructed of materials, such as glass and stainless steel, 

that would not leach or adsorb organic compounds. 

Decontamination procedures were implemented to 

prevent cross-contamination of water between wells 

and samples. Wells were purged before sampling 

until pH, water temperature, and specific conductance stabilized . Where possible, water levels were 

measured before purging. However, water levels 

could be measured in only 25 of the 110 wells sampled . Well owners were interviewed to determine 

how the flooding and intense rainfall may have 

affected the area that surrounds the sampled wells. 

The concentrations of 11 herbicides (alachlor, 

ametryn, atrazine, cyanazine, metolachlor, metribuzin, prometon, propazine, prometryn, simazine, 

and terbutryn) and 2 triazine herbicide metabolites 

[deethylatrazine (DEA), deisopropylatrazine (DIA)] 

were determined by extraction on disposable C-18 

solid-phase extraction cartridges followed by gas 

chromatography/mass spectrometry (GC/MS) (Thurman and others, 1990; Meyer and others, 1993). The 

analytical reporting limit for these herbicides and herPostflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 5 
0 100 200 KILOMETERS 

Figure 6. Location of study region in the upper Mississippi River Basin and the 110 wells sampled for this study. The 

shaded region represents an area with greater than 150 percent of normal rainfall from April 1 to July 31, 1993 (Climate Analysis Center, National Weather Service, written commun., 1993). 

bicide metabolites was 0.05 pg/L. The total herbicide 

concentration, which was defined here as the sum of 

the concentrations of the 11 herbicides and 2 triazine 

metabolites determined by GC/MS, was calculated 

for each sample . In addition, an alachlor metabolite, 

ethanesulfonic acid (ESA), was extracted and isolated 

by solid-phase extraction and analyzed by immunoassay (Aga and others, 1994). The analytical reporting 

limit for ESA was 0.10 ~tg/L. 

The concentrations of dissolved nitrite, nitrite 

plus nitrate (hereafter referred to as "nitrate"), ammonium, and orthophosphate were determined by colori 

metric methods (Fishman and Friedman, 1989). The 

analytical reporting limit for these nutrient compounds 

was 0.01 mg/L, except for nitrate which was 0.05 mg/L. 

The concentrations of the 63 volatile organic 

compounds were analyzed in unacidified water samples 

according to U.S . Environmental Protection Agency 

(USEPA) method 524.2 . All these water samples were 

analyzed within 14 days of the time of collection . 

Quality Assurance 

A quality-assurance program was implemented to determine the effect, if any, of field and 

laboratory equipment and procedures on the herbi 

cide, nutrient, and volatile organic compound concentrations detected in water samples . Field blanks, 

which are made from organic-free water, were submitted as water-quality samples from seven predetermined sites. Field blanks were subject to the same 

sample processing, handling, and equipment as the 

regular samples. Field duplicate samples were collected at seven predetermined sites to determine the 

reproducibility of sample processing and analytical 

6 Study Design and Methods 
methods. Field duplicates were water samples collected along with the regular samples and processed 

as if they had been obtained at a specific site . Sixteen 

blind spike samples of knownherbicide concentration were submitted to the laboratory ; concentrations 

were 0.07 and 0.70 p,g/L. 

Although results of the field blanks indicated 

no reportable concentrations of nitrate, concentrations of chloroform, methylenechloride, and xylene 

were reported in two of the blank samples, and 

alachlor was reported in two blank samples. The 

cause of the volatile organic compounds in the two 

field blanks was determined to be the result of laboratory-grade organic-free water not being used in producing the blanks as directed. The cause of the 

alachlor detection in two of the field blanks was determined not to be the result of the analytical methods 

used. The back-up herbicide sample (all regular and 

quality-assurance herbicide samples were collected in 

duplicate) for the two herbicide blanks in question 

were analyzed and also were found to contain 

alachlor. 

The results of the field duplicates indicated 

that no concentrations differed by more than 20 percent from the regular sample for nitrate or volatile 

organic compounds . In only 5 of the 98 herbicide 

determinations did the duplicate sample differ from 

that of the regular sample by more than 20 percent. 

Of these five cases, four had concentrations near the 

reporting limit; for example, 0.07 pg/L of metolachlor in the regular sample and 0.10 ~tg/L of metolachlor in the duplicate sample . 

Median percent recoveries for the blind herbicide spikes were as follows: alachlor (143), atrazine 

(128), cyanazine (144), DEA (102), DIA (98), meto 

lachlor (150), metribuzin (128), prometon (109), and 

simazine (114). ESA was below the reporting limit 

for all alachlor spike samples submitted for analysis . 

Statistical Methods 

Nonparametric statistical methods were used 

to analyze the data collected. These methods were 

appropriate because the data were not normally dis 

tributed, and a large percentage of the data were censored; that is, many of the concentrations were less 

than the reporting limit. Nonparametric statistics have 

the advantage over parametric statistics of not being 

overly affected by outliers in the data because the 

ranks of the data are used in the analysis rather than 

the actual concentrations . The nonparametric statistical methods included the Wilcoxon signed-rank and 

Kruskal-Wallis tests (Helsel and Hirsch, 1992) . The 

Wilcoxon signed-ranked test is used to determine 

whether the median difference between paired observations equals zero . Because this study was undertaken to determine if the flooding increased the 

transport of agricultural chemicals to unconsolidated 

aquifers, a one-tailed Wilcoxon signed-rank test was 

used. The Kruskal-Wallis test can be used to analyze 

for statistical differences of two or more groups . 

Asignificance level of 0.05 was applied for all 

statistical tests in this study. This acceptable probability of error ((x = 0.05) means that the chance is 1 in 

20 that results of the statistical test indicated a significant difference when one did not exist. Reporting the 

p value associated with a test shows the level of certainty that a statistical difference exists . The smaller 

the p value, the greater the certainty that a reported 

statistical difference resulted . 

EXTENT OF FLOODING PROBLEMS 

Well owners were interviewed to determine 

how flooding and intense rainfall may have affected 

the area that surrounds the sampled wells (figs. 7, 8) . 

Of the 110 wells sampled during 1993, 30 were 

reported to be unaffected by flooding or intense rainfall ; 15 were somewhat affected by nearby stream 

flooding (floodwaters nearby, but not in the immediate vicinity of the well) ; 30 were severely affected by 

stream flooding (12 of which were inundated at least 

to the base of the well) ; and 35 were not affected by 

stream flooding, but were affected by standing water 

Figure 7. A farmstead in Minnesota, July 1993. The 1993 

flood affected many rural areas in the upper Midwest (Bernard Volker, Herman, Minnesota) . 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 7 
from intense rainfall . Of the 25 wells where a waterlevel measurement was made, 17 were found to have 

had a rise in water level since the previous measurement. Because the 25 wells were not spatially distributed across the study area and were not representative 

of any specific type of flooding, the changes reported 

might not represent the entire study area . 

A 

B 

Figure 8. Water problems caused by heavy rains in 1993. 

Agricultural area in central Iowa under normal conditions (A) 

and during July 1993 (8) . 

CHEMICAL OCCURRENCE AND 

DISTRIBUTION 

Concentrations of 11 herbicides, 3 herbicide 

metabolites, 4 nutrients, and 63 volatile organic compounds were determined by using water samples col 

lected during 1993 from 110 wells from the well 

network utilized in the 1991 and 1992 studies. 

Herbicides and Metabolites 

The results of the herbicide analyses from the 

110 wells sampled during September or October 

8 Chemical Occurrence and Distribution 

1993 are summarized in table 1 . The most frequently 

detected herbicide compound was ESA followed by 

atrazine, DEA, and DIA. A reportable concentration 

of at least one herbicide compound was found in 55 .4 

percent of the wells (37.3 percent if ESA is not 

included). One sample contained a concentration of 

alachlor (4 .27 p,g/L) that exceeded the USEPA maximum contaminant level (MCL) for treated drinking 

water (U.S . Environmental Protection Agency, 1992). 

Of the almost 800 samples that have been collected 

since the ground-water studies began in 1991, this 

was the first and only sample in which a herbicide 

concentration was greater than an MCL for drinking 

water. Water from this same well exceeded one-half 

of the MCL for alachlor several times during the previous studies. 

The frequencies of detection from previous 

water-quality samples collected from the same 110 

wells are summarized in table 2 and generally are simi 

lar to the results from this study (table 1) . Differences 

for either the frequency of herbicide detection (p = 

0.395 ; one-tailed Wilcoxon signed-rank test) or the 

total herbicide concentration (p = 0.164 ; one-tailed 

Wilcoxon signed-rank test) were not significant 

between the previous samples (summer 1991 or, where 

available, summer 1992) and those from this study. 

The total herbicide concentrations measured in 

this study (postflood) were compared with the data 

collected in either 1991 or 1992 (preflood) from each 

of the 110 wells. A difference of 20 percent or more 

was used to define a substantial change in total herbicide concentration of each well . This threshold was 

selected because an examination of the quality-assurance results indicated that differences of greater than 

20 percent are beyond that caused by analytical error 

(Kolpin and others, 1994). Total herbicide concentration did not substantially change in 81 wells, 63 of 

which had no herbicide detection in the postflood or 

the preflood samples. Of the remaining 29 wells, total 

herbicide concentration substantially increased in 16 

wells and substantially decreased in 13 wells. 

Because information is not available on herbicide 

concentrations in the water that recharges the aquifers, increases or decreases in total herbicide concentration cannot be determined in any given well . 

When the data were separated according to the 

severity of flooding effects (somewhat affected by 

stream flooding, severely affected by stream flooding, affected by intense rainfall, or not affected by 

stream flooding or intense rainfall), samples from 10 
---

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Table 1 . Summary of 1993 herbicide and herbicide metabolite data for the 110 wells completed in near-surface unconsolidated aquifers, upper Mississippi River Basin, sampled for this study 

[gg/L, micrograms per liter, - - -, no data or none available] 

Frequency Reporting Maximum Maximum Health 

of limit concen- contamin- advisory Chemical compound€ detection 

(percent) 

Alachlor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 

Ametryn. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 

Atrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.6 

Cyanazine . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 

Deethylatrazine (DEA) . . . . . . . . . . . . . . . . . 23 .6 

Deisopropylatrazine (DIA) . . . . . . . . . . . . . . 13 .6 

Ethanesulfonic acid (ESA) . . . . . . . . . . . . . . 40.0 

Metolachlor . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 

Metribuzin . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .8 

Promeryn . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 

Prometryn . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 

Propazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 

Simazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .8 

Terbutryn . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 

1 U.S Environmental Protection Agency (1992) . 

of the 16 wells that had a substantial increase in total 

herbicide concentration were found to be severely 

affected by stream flooding (fig . 9) ; inundation by 

floodwater was noted for four of these wells. Thus, 

the relation may be direct between the occurrence of 

stream flooding in the immediate vicinity of a sampled well and increases in total herbicide concentration in the aquifer. The flood waters could be sources 

of herbicides in the aquifer as a result of reversed 

hydraulic gradients caused by prolonged high river 

stages (fig . 2), from infiltration of herbicide-laden 

floodwater through the soil matrix (fig . 3), or by 

direct flow down the well casing if the well became 

inundated (fig . 4) . Because these flood waters are 

composites of water from the drainage basin at that 

point, the area of land that could affect the water quality in the aquifer is greatly increased, thereby increasing potential sources of herbicide contamination. No 

tration ant level' level' 

(wg/L) (wg/L) (wg/L) (wg/L) 

0.05 4.27 2 

.05 2,000 

.05 1 .80 3 3 

.05 .88 1 

.05 1 .67 

.05 .48 

.10 7.31 

.05 .59 100 

.05 .22 200 

.05 1 .07 100 

.05 

.05 10 

.05 .07 4 4 

.05 

relation is apparent between changes in total herbicide concentration and the remaining three types of 

flooding effects (fig . 9) . 

Substantial changes in total herbicide concentration do appear to be related to other factors. Water 

from wells with a substantial increase in dissolved 

oxygen concentration also generally had a substantial 

change (increase or decrease) in total herbicide concentration (fig . 10). A substantial increase in dissolved-oxygen concentration might indicate areas 

with increased recharge from stream flooding or 

intense rainfall . Increased recharge could decrease 

the time available for biotic processes that consume 

02 to occur and allow oxygenated water to travel farther along flow paths . Whether increased recharge 

causes increased or decreased total herbicide concentrations in unconsolidated aquifers depends on herbicide concentrations in the recharging waters . Factors 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 9 
---

- - -

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Table 2. Summary of previous (1991 and 1992) water-quality data by routine analytical methods for the 110 wells completed in 

the near-surface unconsolidated aquifers, upper Mississippi River Basin, sampled during 1993 

[The compounds ametryn, prometryn, propazine, and terbutryn also were analyzed, but never detected above the 0.05-pg/L report limit. gg/

L, micrograms per liter, - - -, no data or none available] 

March-April 1991 July-August 1991 July-August 1992 

Chemical 

Report- Maxi- Maxi- Maxi­ ing 

compound limit Number Frequency of mum Number Frequency of mum Number Frequency of mum 

of detection concen- of detection concen- of detection concen­ (wg/L) samples (percent) tration samples (percent) tration samples (percent) tration 

(gg/L) (wg/L) (wg/L) 

Herbicides 

Alachlor . . . . . 0.05 108 1.8 1.05 102 0 28 7.1 1.00 

Atrazine . . . . . .05 108 21 .3 1.08 102 31 .4 1 .01 28 32.1 1 .03 

Cyanazine . . . . .05 108 .9 .21 102 2.0 .68 28 0 

Deethylatrazine-

(DEA). .05 108 17 .6 1 .41 102 31 .4 2.32 28 32.1 1 .79 

Deisopropylatrazine 

(DIA). .05 108 5.6 .59 102 13 .7 1 .17 28 17.8 .28 

Ethanesulfonic acid 

(ESA). .10 0 0 28 46.4 2.51 

Metolachlor . . .05 108 4.6 .90 102 4.9 .71 28 7.1 .76 

Metribuzin . . . .05 108 0 102 2.0 .09 28 0 

Prometon . . . . .05 108 5.6 .31 102 6.9 .58 28 10.7 1 .35 

Simazine . . . . . .05 108 .9 .06 102 2.0 .10 28 3.6 .09 

Inorganic nutrients (reporting limits and maximum concentrations in milligrams per liter) 

Ammonium. . . 0.01 106 74 .5 2.1 110 83 .6 4.0 35 77 .1 2.0 

Nitrate . . . . . . . .05 109 72 .5 23 110 74 .5 25 35 88 .6 27 

Nitrite . . . . . . . .01 106 14 .2 .12 110 40.9 .18 35 14.3 .04 

Orthopho 

sphate . .01 106 49.1 .42 110 50 .0 .43 35 77 .1 .45 

Volatile organics compounds (detections only) 

Chloroform . . . 0.2 0 

Tetrachloroethylene. .2 0 

that could affect herbicide concentrations in recharging water are the amount of chemical application in 

the recharge area during 1993 and the quality of flood 

waters that might have inundated the recharge area. 

0 21 14.3 0.8 

0 21 4.8 .9 

An inverse relation was noted between the frequency of occurrence of wells with a substantial 

change in total herbicide concentration and classes 

(0-29 ft, 30-59 ft, 60-89 ft, 90 ft and greater) of well 

1 0 Chemical Occurrence and Distribution 
70 50 

60 ® Well somewhat affected by stream flooding 

well intensely affected by stream flooding 

O

0 

w 50 

aU 

Z_ 

2

H 40 

J 

w 

W 30 

O 

w 

Q 

zZ 

w 20 

U 

m 

W 

d 

10 

0 

20-percent Less than 20-percent 

or more 20-percent or more 

decrease decrease or increase 

(N=13) increase (N=16) (N=81) 

CHANGE IN TOTAL HERBICIDE CONCENTRATION 

Figure 9. Relation between severity of 1993 flooding and 

change in total herbicide concentration in water samples collected during 1991 or 1992 and those collected for this study 

during 1993 from 110 wells completed in near-surface 

unconsolidated aquifers, upper Mississippi River Basin. 

depth (p = 0.018 ; Kruskal-Wallis test). As the well 

depths increased, changes (increase or decrease) in 

total herbicide concentration in the aquifer between 

the preflood and postflood samples were less (fig . 

11). Because the age of the ground water in near-surface aquifers generally is older at greater depths 

(D.W. Kolpin and D.A. Goolsby, U.S . Geological 

Survey, unpub. data, 1994); water in shallow wells is 

more representative of recent recharge and more 

quickly reflects changes in water quality in response 

to the change in the quality of recharge than water at 

greater depth. 

The percentage of land in corn and soybean production within a 400-m radius of a sampled well also 

was significantly (p = 0.019 ; Kruskal-Wallis test) 

related to changes in total herbicide concentration. As 

the amount of this land increased, the frequency of 

occurrence of wells with a substantial increase in total 

herbicide concentration also increased (fig . 12). This 

40 

W 

0 z z0 

Q Z = W

FU 30 

Z 

O

U

J W W 

3: U

Om 

ww 20

0 

J 

z F W 

Q

W? 

o10 

20-percent Lessthan 20-percent 

or more 20-percent or more 

decrease decrease or increase 

(N=42) increase (N=19)

(N=49) 

CHANGE IN DISSOLVED-OXYGEN CONCENTRATION 

Figure 10. Relation between changes in dissolved-oxygen concentration and change in total herbicide concentration in water 

samples collected during 1991 or 1992 and those collected for 

this study during 1993 from 110 wells completed in near-surface unconsolidated aquifers, upper Mississippi River Basin. 

trend is likely source related. As percentage of land in 

corn and soybean production near the well increases, 

the corresponding increases in the amount and area of 

herbicide application increase the chances of herbicide 

transport to the aquifer. 

About 83 percent of the samples from wells 

that had a substantial increase or decrease in total herbicide concentration also had a substantial increase or 

decrease in nitrate concentration. Only about 22 percent of the wells in which herbicides were detected, 

but had no substantial change in total herbicide concentration, had a substantial change in nitrate concentration. Changes in nitrate concentration may indicate 

environments where the potential for a change in the 

total herbicide concentration could also result . 

Subregional spatial patterns in herbicide detections and concentrations were apparent for parts of 

the study area. The frequency of herbicide detection 

in samples from eight wells that are completed in the 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 1 1 
_j 

w

50 

40 

w

(7 z

z0 

U

< 

¢ Z x w 30 

F= U ~zO 

J U

w W 

LU 

O °° x ww 20 

F J 

z~ 

Q O 

w z

d 

10 

N Number of wells in category 

u to Zy 3u to 59 60 to 89 Greater than 

(N=13) (N=52) (N=24) or equal to 90 

(N=21) 

WELL DEPTH, IN FEET BELOW LAND SURFACE 

Figure 11. Relation between well depth below land surface 

and change in total herbicide concentration in water samples collected during 1991 or 1992 and those collected for this 

study during 1993 from 110 wells completed in near-surface 

unconsolidated aquifers, upper Mississippi River Basin. 

Missouri River alluvium (fig . 13) increased from 12.5 

percent in the previous studies to 50 percent in this 

study; triazine herbicides were not detected . The herbicides detected in these 8 wells during this study 

were alachlor (37.5 percent of samples), metribuzin 

(25 percent), and metolachlor (12.5 percent) . These 

herbicides can be used on soybeans for weed control 

(alachlor and metolachlor can be used on corn and 

soybeans). 

The cool, wet conditions that persisted during

spring and early summer 1993 delayed or prevented 

the planting of corn in areas of the upper Mississippi 

River Basin. The number of acres planted in corn was 

greater than 2 million less than planned (U.S Department of Agriculture, 1993, 1994) in the nine States 

where the sampling for this study took place. Some 

of these fields that were intended to be planted with 

corn were instead planted with soybeans because of 

1 2 Chemical Occurrence and Distribution 

the much shorter time to crop maturity, whereas others remained fallow for the entire 1993 growing season. This could have changed not only the timing of 

chemical application, but also the normal chemicaluse patterns in many areas . More research is needed 

to determine if the increase in the frequency of herbicide detection in water from the Missouri River alluvium in 1993 is from increased recharge, changes in 

the timing of chemical application, or changes in 

chemical use. 

Increases in the frequency of herbicide detection were not found in other alluvial aquifers sam70 

Concentration always below 0.05 microgram 

per liter reporting limit 

Less than 20-percent change 

z 

O 60 

Fz 

w

U 

z 

0

U 50 w

0 

U 

m 

x 

w 

x 

a 

O 40 

H 

z 

w

0 

z 

a 

x

U 30 

x 

rn 

w 

20 O 

w 

0 

z 

w 

U 

w

d 10 

Less than 25 25 to 75 More than 75 

PERCENTAGE OF LAND IN CORN AND SOYBEAN 

PRODUCTION WITHIN A 400-METER RADIUS OF SAMPLED WELL 

Figure 12. Relation between percentage of land in corn and 

soybean production and change in total herbicide concentration in water samples collected during 1991 or 1992 and 

those collected for this study during 1993 from 110 wells 

completed in near-surface unconsolidated aquifers, upper Mississippi River Basin. 
R5~ 

Aquifer in which well is completed 

Missouri River alluvium 

Platte River alluvium 

o Mississippi River alluvium 

o Nishnabotna River alluvium 0 100 200 KILOMETERS 

Figure 13. Location of wells completed in selected alluvial aquifers . 

pled. Herbicides were detected in all seven wells 

completed in the Platte River alluvium (fig . 13) in the 

samples collected for previous studies and this study. 

Herbicides were detected in one of four wells in the 

Mississippi River alluvium in the samples collected 

for previous studies and this study. Water in this well 

had a substantial increase in total herbicide concentration . The six wells completed in the Nishnabotna 

River alluvium (includes the East and the West 

Nishnabotna Rivers in southwestern Iowa) were similar to those in the Mississippi River alluvium in that 

the frequency of herbicide detection (33 percent) did 

not change and the total herbicide concentration 

increased in the two wells that contained herbicides. 

Nutrients 

The results of the nutrient analyses for samples from the 110 wells sampled are summarized in 

table 3. The most frequently detected nutrient was 

ammonium followed by orthophosphate, nitrate, and 

nitrite. Although nitrate was the third most frequently 

detected nutrient, it had the highest median concentration (0 .82 mg/L). Water from 33.6 percent of the 

wells contained nitrate concentrations that were equal 

to or greater than 3 .0 mg/L; this level has been suggested as a division between natural and anthropogenic sources of nitrate (Madison and Brunett, 1985, 

p. 93). Nitrate concentrations exceeded the MCL of 

10.0 mg/1L (U.S . Environmental Protection Agency, 

1992) in 10 percent of the samples. The nutrient 

results from preflood water-quality samples 

collected for the same 110 wells are summarized in 

table 2 . 

When the nutrient analyses from this study 

(postflood) were compared statistically with those of 

samples collected from the same wells in either sum 

mer 1991 or 1992 (preflood), ammonium (p = 0.028 ; 

one-tailed Wilcoxon signed-rank test) and orthophosPostflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 13 
- -

- -

Table 3. Summary of 1993 nutrient data for the 110 wells completed in the near-surface unconsolidated aquifers, upper 

Mississippi River Basin, sampled for this study 

[mg/L, milligrams per liter ; - - -, no data or none available] 

Frequency 

of Chemical compound detection 

(percent) 

Ammonium (dissolved as nitrogen) . . . . . . . . . 93 .6 

Nitrite (dissolved as nitrogen). . . . . . . . . . . . . . 9.1 

Nitrate (dissolved as nitrogen) . . . . . . . . . . . . . 72.7 

Orthophosphate (dissolved as phosphorus) . . . 77 .3 

1 U.S Environmental Protection Agency (1992) . 

phate (p < 0.001 ; one-tailed Wilcoxon signed-rank 

test) concentrations were found to be significantly 

higher in the postflood samples. Nitrite (p = < 0.001 ; 

one-tailed Wilcoxon signed-rank test) concentrations 

were found to be significantly lower in the postflood 

samples. However, 78 percent of the increases in 

ammonium concentration, 88 percent of the increases 

in orthophosphate concentration, and 97 percent of 

the decreases in nitrite concentrations were less than 

or equal to 0.05 mg/L. These low-level changes could 

occur randomly as a function of laboratory holding 

times or similar variations . Nitrate (p = 0.471 ; onetailed Wilcoxon signed-rank test) concentrations 

were not found to be significantly different between 

the preflood and postflood samples . 

Nitrate 

To determine possible temporal differences in 

nitrate concentration, analyses from the postflood 

samples were compared with the preflood nitrate anal 

yses at each well . In most cases, these were the water 

samples collected during summer 1991, but water 

samples collected during summer 1992 were used 

where available. A 20-percent difference in concentration between the preflood and postflood samples 

was the threshold to define a substantial change in 

nitrate . This threshold was selected because an examination of preflood quality-assurance results (Kolpin 

and others, 1994) indicated that differences of greater 

than 20 percent should be greater than that caused by 

analytical error. By using this threshold, 19 wells 

Reporting 

limit 

(mg/L) 

Maximum 

concentration 

(mg/L) 

Maximum 

contaminant level' 

(mg/L) 

0.01 4.2 -

.01 .06 

.05 21 10 

.01 .48 

Health 

advisory 

level' 

(mg/L) 

were determined not to have a nitrate concentration 

above reporting limits, 29 had no substantial change, 

31 had a substantial decrease, and 31 had a substantial increase in nitrate concentration. When the data 

were separated according to the severity of flooding 

effects, the number of samples from wells in which 

the nitrate concentrations increased or decreased 

from the preflood sample remained almost equal for 

each category (fig . 14) . 

Because information was not available on 

nitrate concentrations in the water that recharged the 

aquifers, an increase or decrease in nitrate concentra 

tion could not be determined for any given well . The 

increased mass transport of nitrate into the Mississippi River and its large tributaries during 1993 flood 

(Goolsby and others, 1993) may indicate the 

expected effects on ground-water quality, but these 

surface-water-quality results might not be representative of ground-water quality within the unconsolidated aquifers sampled for this study. Factors that 

could affect nitrate concentrations in water that 

recharged the aquifers include the amount of fertilizer 

applied in the recharge area during 1993 and the quality of any flood waters that inundated the recharge 

area . Thus, possible increased recharge as a result of 

flooding could cause an increase or decrease in 

nitrate concentration in ground water depending on 

these factors . Another complicating factor is the 

much greater and more variable response times of 

ground water to the possible water-quality effects 

from the 1993 flood than surface water. Ground 

water moves slowly compared with surface water. 

The rate of ground-water movement can vary greatly 

1 4 Chemical Occurrence and Distribution 
50 

40 

0

0

W 

a

U

Z 30


F


J


W


0 20
 W
0


F
Z
W
U


W 

a 

10 

0 

0.011 ; median water level = 3 .6 mbelow land surface) 

than those that had either no change (median water 

level = 6.1 m) or a substantial increase (median water 

level = 6.7 m) in nitrate concentration. One possible 

cause for these decreases in nitrate is denitrification, 

which takes place under anaerobic conditions in the 

presence of labile organic carbon and denitrifying bacteria. Previous research has found that denitrification 

tends to occur in aquifers that have a shallow water 

table (Starr and Gillham, 1993) . The dissolved-oxygen 

concentrations detected indicate the possibility of suitable conditions for denitrification. For example, water 

from wells that had a substantial decrease in nitrate 

concentration also had a smaller median dissolved-oxygen concentration (0.5 mg/L) than those that had either 

no change (0 .96 mg/L) or a substantial increase (1 .83 

mg/L) in nitrate concentration. 

Amap of the temporal differences in nitrate concentration show possible subregional spatial patterns 

(fig. 15) . Water in a cluster ofwells in central Illinois 

generally had a substantial increase in nitrate concentration, and another cluster in western Iowa, western Minnesota, and southeastern South Dakota generally had a 

substantial decrease in nitrate concentration. 

Volatile Organic Compounds 

The postflood results of the analyses for volatile organic compounds from the samples collected 

from the 110 wells are summarized in table 4. During 

this study, 15 volatile organic compounds were 

detected . Only 1,2-dichloroethane and benzene concentrations in one sample exceeded the USEPAMCL for 

treated drinking water. The three most frequently 

detected compounds, in decreasing order, were chloroform, dichlorobromomethane, and chlorodibromomethane. Apossible source of these compounds is 

the chlorination treatment of raw water to obtain drinking water (Howard, 1990). These compounds were 

detected in eight wells, of which four were public-supply wells, three were domestic wells, and one was an 

industrial well . In this study, water samples were collected to be representative of the aquifer; thus, samples 

should have been obtained before any water-treatment 

processes. However, of these public-supply wells, two 

had no access points to sample water before treatment 

and required chlorinators to be turned off 24 hours 

before sampling, and one was flushed and disinfected 

before sampling to reduce potential bacterial contamination as a result of recent floodwater inundation . 

20-percent Less than 20-percent or more 20-percent or more decrease decrease or increase (N=13) increase (N=16) 

(N=81) 

CHANGE IN TOTAL NITRATE CONCENTRATION 

Figure 14. Relation of categories of flooding to changes in 

nitrate concentration between the samples collected during 

1991 or 1992 and those collected for this study during 1993 

from 110 wells completed in near-surface unconsolidated 

aquifers, upper Mississippi River Basin. 

depending on the materials through which the water 

passes, the hydraulic conductivity of the unsaturated 

zone, and the distance from the land surface to the saturated zone. Thus, more time might pass before 

effects of the flood on the quality of water in near-surface unconsolidated aquifers become apparent. 

Wells that had a substantial increase in dissolved-oxygen concentration had a greater frequency 

of substantial increases in nitrate concentration (53 per 

cent) than those that had either no substantial change 

(26 percent) or a substantial decrease (19 percent) in 

dissolved-oxygen concentration. As was discussed 

above, an increase in the dissolved-oxygen concentration may indicate areas with increased recharge from 

either extensive stream flooding or intense rainfall . 

Wells that had a substantial decrease in nitrate 

concentration had significantly higher water levels (p = 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 15 
1

85° 

QOo 

y. `\c'0 ' :rL:NOIS 1 INDIANA , I 

< . i.? 

EXPLANATION 

Changes in nitrate concentrations 

No change 

V Decrease greater than 20 percent 0 100 200 MILES ® Increase greater than 20 percent 

0 100 200 KILOMETERS 

Figure 15. Changes in nitrate concentration between the samples collected during 1991 or 1992 and those collected for 

this study during 1993 from 110 wells completed in near-surface unconsolidated aquifers, upper Mississippi River Basin. 

The few wells in this study with available background data on volatile organic compounds make identification of temporal differences tenuous . Whether the 

subset of wells with background data are representative of the entire population of wells sampled for this 

study cannot be determined; however, the frequency of 

volatile organic compound detection was almost identical between the samples collected during 1992 and this 

study. Volatile organic compounds were detected in 

water from 14.3 percent (3 of 21) of the wells sampled 

during summer 1992 and 14.5 percent (16 of 110) of 

those sampled for this study. None of the wells that 

were sampled during 1992 had concentrations that 

exceeded 1 .0 gg/L, whereas 62.5 percent (10 of 16) of 

the samples from the wells in which volatile organic 

compounds were detected had concentrations that 

exceeded 1.0 ltg/L for this study. 

The difference (p = 0.5446 ; Kruskal-Wallis 

test) was not significant in the frequency of volatile 

organic compound detection when the data were separated according to the severity of flooding effects . 

Only 1 of 12 samples from wells inundated by flood 

waters had a volatile organic compound detection . 

Land use, however, may be a more important factor 

in determining the frequency of volatile organic compound detection . Significantly higher amounts of 

urban-residential (p = 0.037) and industrial (p = 

0.011) land use were within a 30-m radius of the 

wells in which volatile organic compounds were 

detected compared to those in which volatile organic 

compounds were not detected. 

SUMMARY 

Water samples were collected during September and October 1993 from 110 wells to determine 

the immediate effects the 1993 flood had on the 

16 Summary 
---

---

---

---

-- ---

---

--

--- ---

--- ---

--

---

Table 4. Summary of 1QQ3volatile organic compound data foxthe110wells completed in the near-surface unconsolidated 

aquifers, upper Mississippi River Basin, sampled for this study 

[gg/I, micrograms per no data or none available] 

Frequency Reporting Maximum Maximum Health 

of concen- contamin- advisory 

Chem I compound detection limit tretimn ant level' level' 

(percent) (gg/L) (p,gIL) (ltg/L) (gg/L) 

.l,1,1, ~ ~ . . . . . . . O 0,2 70 

1,1,1-Tticbhrocthauc . . ~ . . . . . . . 2.7 2 1 .3 200 200 

l .l . . . . . . . 0 2 '-- -

1, 1 . . . . . . . . . . . . . . ~ 0 .2 5 3 

l .lJ/ichlor"cthuoc . . . . . . . . . . . . 27 2 4 

l . l . . . . . . . . . .


l .l . . . ~ . . . . . . . V 2


1 0 2 

l . . . . . . . . . . . . .9 2 2.3 '- 40 

l 0 2 70 70


! . . . . . . . . . . . V .2


l,2-Chlonxuluenc, . . . . . . . . . . ~ . . . . . . 0 ~Z l00


l _ - - - - - - - 0 2 - -


l ~~ . . . . . . . ~ . . . . . . 0 .2 600 600


l . . ~ . . . . . . . . . . . . . . 2.7 ~ 6.9 5 

1~. . ~ ~ ~ ~ . . . . . . . . . . . ~ 0 2 - 5 

l . . 0 2 - l00 l00 

l . 

l . . . . . . . . 0 2 600 600 

l 0 .2 -' 

l,4fhlorowluone . . . . . . . . . . . . . . . . . . . . . . 0 .2 '-- 100 

l 0 2 75 75 

__ - .--_-'--,_- ~~ . . . . . . . 0 ~2 - '­

thylvinylethc . . . . . . . 0 l -

Acaddo . . . . . . . 0 20 -

Acryloudrile . . . . . . . . . . . . . . . . . . . . . ~ 0 20 

Benzene . . . . . . . . . . . . . . 17 2 78 5 

Bromvhxozuon . . . . ~~ . . . . . . . 0 .2 

Postflw Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Umcossolidated Aquifers 1 7 
--- ---

- - -

--- - - -

--- --- ---

--- --- ---

- - -

--- --- - - -

--- --- - - -

--- - - -

--- --- - - -

- - -

--- ---

--- --- - - -

--- ---

--- --- ---

--- - -

--- --- - - -

--- - - -

--- --- - - -

--- --- - - -

--- --- ---

--- ---

--- --- - - -

--- --- ---

---

--- --- ---

---

Table 4. Summary of 1993 volatile organic compound data for the 110 wells completed in the near-surface unconsolidated 

aquifers, upper Mississippi River Basin, sampled for this study-Continued 

Chemical compound 

Frequency 

of 

detection 

Reporting 

limit 

Maximum 

concentration 

Maximum 

contaminant level' 

Health 

advisory 

level' 

(percent) (gg/L) (wg/L) (wg/!_) (wg/L) 

Bromochloromethane . . . . . . . . . . . . . . . . . . . 0 .2 90 

Bromoform . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 0.2 1 .7 100 

Carbontetrachloride . . . . . . . . . . . . . . . . . . . . 0 .2 5 

Chlorobenzene . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Chlorodibromomethane . . . . . . . . . . . . . . . . . 2.7 .2 5.7 100 60 

Chloroethane . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Chloroform . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 .2 51 100 

Cis-1,2-dichloroethene . . . . . . . . . . . . . . . . . . 0 .2 

Cis-1,3-dichloropropene . . . . . . . . . . . . . . . . . 0 .2 

Dibromochloropropane . . . . . . . . . . . . . . . . . . 0 1 .2 

Dibromomethane . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Dichlorobromomethane . . . . . . . . . . . . . . . . . 4.5 .2 25 100 

Dichlorodifluoromethane . . . . . . . . . . . . . . . . 0 .2 1,000 

Trichlorotrifluoroethane . . . . . . . . . . . . . . . . . 0 .5 

Ethylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . .9 .2 .9 700 700 

Hexachlorobutadiene . . . . . . . . . . . . . . . . . . . 0 .2 

Isopropylbenzene . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Methylbromide . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 -

Methylchloride . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Methylene chloride . . . . . . . . . . . . . . . . . . . . . .9 .2 .4 

Methyltertbutylether . . . . . . . . . . . . . . . . . . . . 0 1 

N-Butylbenzene . . . . . . . . . . . . . . . . . . . . . . . 0 .2 

N-propylbenzene. . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Naphthalene . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 20 

P-isopropyltoluene . . . . . . . . . . . . . . . . . . . . . 0 .2 

Sec-butylbenzene . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Styrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .2 100 100 

Tert-butylbenzene . . . . . . . . . . . . . . . . . . . . . . 0 .2 

Tetrachloroethylene . . . . . . . . . . . . . . . . . . . . .9 .2 .6 5 

Toluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 .2 12 1,000 1,000 

1 
--- --- ---

---

--- ---

---

Table 4. Summary of 1993 volatile organic compound data for the 110 wells completed in the near-surface unconsolidated 

aquifers, upper Mississippi River Basin, sampled for this study-Continued 

Frequency 

of Chemical compound detection 

(percent) 

Trans- 1,3-dichloropropene . . . . . . . . . . . . . . 0 

Trichloroethylene. . . . . . . . . . . . . . . . . . . . . . .9 

Trichlorofluoromethane . . . . . . . . . . . . . . . . . 0 

Vinyl chloride . . . . . . . . . . . . . . . . . . . . . . . . 0 

Xylenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 

VS Environmental Protection Agency (1992) . 

occurrence, concentration, and transport of agricultural chemicals and volatile organic compounds in 

near-surface unconsolidated aquifers of the upper 

Mississippi River Basin. Of the 110 wells sampled, 

30 were reported to be unaffected by flooding or 

intense rainfall, 15 were somewhat affected by 

nearby stream flooding (flood waters nearby, but not 

in the immediate vicinity of the well), 30 were 

severely affected by stream flooding (12 of which 

were inundated at least to the base of the well), and 

35 were affected not by stream flooding, but by standing water from rainfall . Results from these samples 

were compared with those obtained from the same 

wells that were sampled during 1991 or 1992. There 

was no statistically significant difference in either the 

frequency of herbicide detection or total herbicide 

concentration between the previous samples and 

those collected for this study. However, water from 

about 63 percent of the wells that had at least a 20-

percent increase in total herbicide concentration also 

were severely affected by stream flooding . No direct 

relation was determined between the historic flooding 

and intense rainfall during 1993 for either the nitrate 

or the volatile organic compound concentrations . 

However, the substantial increases in dissolved-oxygen concentration in some wells may indicate areas 

of increased recharge. The extensive stream flooding 

and intense rainfall is one possible explanation for 

such an increase . Water from wells that had at least a 

20-percent increase in dissolved-oxygen concentration had the greatest likelihood of substantial 

increases or decreases in total herbicide concentration 

Maximum Maximum Health Reporting concen- contamin- advisory 

(gg/L) (4g/L) (wg/L) (wg/L) 

. 2 

0.2 0.4 5 

.2 2,000 

.2 ___ 2 

.2 5.3 10,000 10,000 

limit tration ant level' level' 

and substantial increases in nitrate concentration. 

Water in shallow wells more quickly reflect changes 

in water quality in response to changes in recharge. 

The frequency of herbicide detection 

increased from 12.5 percent in previous samples to 

50 percent in those collected from this study for the 

eight wells that were completed in the Missouri River 

alluvium. The herbicides detected in samples from 

these wells were alachlor, metribuzin, and metolachlor, all of which are used to control weeds in soybeans. The 1993 flood could have caused this 

increase in the frequency of herbicide detection by 

increasing the chemical transport to ground water, 

changing the normal timing of chemical application, 

and changing the normal patterns in crop types and 

chemical use. 

These results document only the immediate 

effects the 1993 rainfall and stream flooding had on 

ground-water quality of near-surface unconsolidated 

aquifers . Because ground-water moves much more 

slowly than surface runoff, additional data will be 

required to determine if the 1993 flood has long-term 

effects on ground-water quality. 

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in water by solid-phase extraction and enzymelinked immunosorbent assay: Analytical Chemistry, 

v. 66, p 1495-1499 . 

Postflood Occurrence of Selected Agricultural Chemicals and Volatile Organic Compounds in Near-Surface Unconsolidated Aquifers 1 9 
Bradbury, K.R., 1991, Tritium as an indicator of groundwater age in central Wisconsin: Ground Water, v. 29, 

p. 398-404. 

Burkart, M.R., and Kolpin, D.W., 1993, Hydrologic and 

land-use factors associated with herbicides and 

nitrate in near-surface aquifers : Journal of Environ 

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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. 

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 Unites States : U.S . Geological Survey 

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

Goolsby, D .A., Battaglin, W.A., and Thurman, E.M., 

1993, Occurrence and transport of agricultural chemicals in the Mississippi River Basin, July through 

August, 1993 in Floods in the upper Mississippi 

River Basin, 1993 : U.S . Geological Survey Circular 

1120-C, 22 p . 

Hall, D.W., 1992, Effects of nutrient management of 

nitrate levels in ground water near Ephrata, Pennsylvania: Ground Water, v. 30, no. 5, September 

October 1992, p. 720-730. 

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods 

in water resources : New York, Elsevier, 522 p. 

Hillaker, H., 1993, The great Iowa floods of 1993 : An analysis of the weather events of 1993 and what they por 

tend for 1994: Iowa Groundwater Quarterly, v. 4, 

no. 4, December 1993, p. 1, 12-13. 

Howard, P.H., ed., 1990, Handbook of Environmental Fate 

and Exposure Data for Organic Chemicals: 

Chelsea, Michigan, Lewis Publishers, 546 p. 

Kolpin, D.W., and Burkart, M.R., 1991, Work plan for 

regional reconnaissance for selected herbicides and 

nitrate in near-surface aquifers of the mid-continen 

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Open-File Report 91-59, 18 p. 

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Hydrogeologic, water-quality, and land-use data for 

the reconnaissance for herbicides and nitrate in near 

surface aquifers of the midcontinental United States, 

1991: U.S . Geological Survey Open-File Report 93-

114, 61 p. 

Kolpin, D.W., Goolsby, D.A., Aga, D.S, Iverson, J .L., and 

Thurman, E.M., 1993, Pesticides in near-surface 

aquifers-Results of the midcontinental United 

States ground-water reconnaissance, 1991-92, in 

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

agricultural chemicals in water resources of the mid20 References Cited 

continental United States : U.S . Geological Survey 

Open-File Report 93-418, p. 64-74. 

Kolpin, D.W., Burkart, M.R., and Thurman, E.M., 1994, 

Herbicides and nitrate in near-surface aquifers in the 

midcontinental United States, 1991 : U.S . Geological Survey Water-Supply Paper 2413, 34 p. 

Madison, R.J ., and Brunett, J .O., 1985, Overview of the 

occurrence of nitrate in ground water of the United 

States, in National water summary 1984-Hydrologic events, selected water-quality trends, and 

ground-water resources : U.S . Geological Survey 

Water-Supply Paper 2275, p. 93-105. 

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 

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A, 14 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 

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1719-1729 . 

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organic carbon availability in two aquifers : Ground 

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

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

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