U.S. Department of the Interior

U.S. Geological Survey

Ground-Water Quality and Vulnerability to

Contamination in Selected Agricultural Areas

of Southeastern Michigan, Northwestern

Ohio, and Northeastern Indiana

By Mary Ann Thomas

Water-Resources Investigations Report 00-4146
U.S. Department of the Interior

BRUCE BABBITT, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply

endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

6480 Doubletree Avenue

Columbus, OH 43229-1111

Copies of this report can be purchased from:

U.S.Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

2000 
Contents iii

CONTENTS

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

Introduction .......................................................................................................................................................................... 2

Study design and methods .................................................................................................................................................... 2

Agricultural land-use study ........................................................................................................................................ 2

Subunit survey ............................................................................................................................................................. 6

Description of study area....................................................................................................................................................... 6

Land use....................................................................................................................................................................... 6

Hydrogeology .............................................................................................................................................................. 6

Ground-water quality............................................................................................................................................................. 8

Ground-water age ........................................................................................................................................................ 10

Water type.................................................................................................................................................................... 10

Pesticides and pesticide degradates ............................................................................................................................. 10

Nitrate .......................................................................................................................................................................... 13

Constituents that exceeded secondary drinking-water standards ................................................................................ 14

Chloride ............................................................................................................................................................. 16

Sulfate................................................................................................................................................................ 16

Dissolved solids................................................................................................................................................. 16

Iron..................................................................................................................................................................... 16

Manganese......................................................................................................................................................... 17

Vulnerability of ground water to contamination ................................................................................................................... 17

Hydraulic connection to the land surface ................................................................................................................... 17

Shallow ground water........................................................................................................................................ 17

Deep ground water............................................................................................................................................. 18

Geochemical properties of the till ............................................................................................................................... 18

Heterogeneity of the till............................................................................................................................................... 19

Summary and conclusions .................................................................................................................................................... 19

References cited .................................................................................................................................................................... 20

ILLUSTRATIONS

Figure 1–2. Maps showing:

 1. Location of NAWQA well networks in agricultural areas of the Lake Erie-Lake St. Clair Basin ......... 3

 2. Factors used to define agricultural land-use study area: land use, surficial geology, and

 bedrock geology ............................................................................................................................... 4

 3. Simplified section showing relation of monitor and domestic wells to hydrogeologic setting ...................... 6

 4. Drillers’ logs and core description from the domestic and monitor wells co-located at a site in Williams

County, Ohio ............................................................................................................................................ 9

 Figure 5-10. Graphs showing:

 5. Relation of tritium concentrations to well depth .................................................................................... 10

 6. Piper diagrams of waters from domestic and monitor wells .................................................................. 11

 7. Pesticides most heavily applied in the Lake Erie-Lake St. Clair Basin relative to those

 detected in ground water .................................................................................................................... 12

 8. Relation of pesticide detections to well depth and ground-water age .................................................... 13

 9. Relation of nitrate concentrations to well depth and ground-water age ................................................. 14

 10. Relation of concentrations of constituents that exceeded SMCL’s to well depth and ground-

 water age ............................................................................................................................................... 15
iv Contents

TABLES

 1. Characteristics of ground-water-quality studies and wells ............................................................................. 5

 2. Pesticides analyzed in samples from monitor and domestic wells ................................................................. 7

 3. Pesticide detection frequencies and maximum concentrations ...................................................................... 13

 4. Nitrate detection frequencies and concentrations........................................................................................... 14

5. Nitrate and pesticide results from the northwestern part of the Lake Erie-Lake St. Clair Basin 

relative to those of comparable ground-water studies .................................................................................... 18

CONVERSION FACTORS AND ABBREVIATIONS

Temperature: Temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by use of the 

following equation:

°F = 1.8 (°C) + 32

Abbreviated water-quality units used in this report: Chemical concentrations and water temperature are given in metric units. 

Chemical concentration in water is given in milligrams per liter (mg/L), micrograms per liter (μg/L), or milliequivalents per liter (meq/

L). Milligrams per liter is a unit expressing the concentration of chemical constituents in solution as weight (milligrams) of solute per 

unit volume (liter) of water. One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less than 

7,000 mg/L, the numerical value is approximately the same as for concentrations in parts per million. Milliequivalents per liter is 

derived from the concentration in milligrams per liter, multiplied by the reciprocals of the combining weights of the ions of interest. 

Concentration of organic carbon in till is given in milligrams of constituent per kilogram of sample (mg/kg).

Radioactivity is expressed in picocuries per liter (pCi/L). A picocurie is one-trillionth (1x10-12) the amount of radioactivity 

represented by a curie (Ci). A curie is the amount of radioactivity that yields 3.7x1010 radioactive disintegrations per second. A 

picocurie yields 2.22 disintegrations per minute. Tritium concentration is expressed in tritium units (TU). One tritium unit is equal 

to one tritium atom per 1018 hydrogen atoms; in terms of radioactivity, it is equivalent to 3.24 picocuries per liter. 

Multiply By To obtain

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile 2.590 square kilometer

acre 0.4047 square hectometer (hectare)

ton 0.4536 kilogram

gallon per minute (gal/min) 3.785 liter per minute

cubic foot per second (ft3/s) 0.02832 cubic meter per second

cubic foot per day (ft3/d) 0.02832 cubic meter per day

million gallons per day (Mgal/d) 0.04381 cubic meter per second
Abstract 1

Ground-water quality and vulnerability to contamination in 

selected agricultural areas of southeastern Michigan, 

northwestern Ohio, and northeastern Indiana

by Mary Ann Thomas

Abstract

Ground-water quality was assessed in the northeastern part of the Corn Belt, where tile-drained 

row crops are underlain by fractured glacial till. 

Data were collected from 30 shallow monitor 

wells and 18 co-located domestic wells as part of 

the U.S. Geological Survey’s National WaterQuality Assessment in the Lake Erie-Lake St. 

Clair Basin.

Pesticides or pesticide degradates were 

detected in 41 percent of the monitor wells and 6 

percent of the domestic wells. The pesticides 

detected closely correspond to those most heavily 

applied—herbicides used on corn and soybeans. 

Pesticide degradates were detected three times 

more frequently, and at higher concentrations, 

than were parent compounds. No pesticide concentration exceeded a USEPA Maximum Contaminant Level (MCL), but MCL’s have not been 

established for 9 of the 11 compounds detected.

Thirty-seven percent of monitor-well samples had nitrate concentrations indicative of 

human influences such as fertilizer, manure or 

septic systems. Nitrate was the only chemical 

constituent detected at a concentration greater 

than an MCL. The MCL was exceeded in 7 percent of samples from monitor wells which were 

too shallow to be used as a source of drinking 

water. 

Pesticide and nitrate concentrations in the 

study area are low relative to other agricultural 

areas of the Nation. Several authors have suggested that ground water in parts of the Upper 

Midwest is minimally contaminated because it is 

protected by the surficial glacial till or tile drains. 

These ideas are examined in light of the relations 

between concentration, well depth, and groundwater age in the study area. 

Most of the shallow ground water is hydraulically connected to the land surface, based on the 

observations that 83 percent of waters from monitor wells were recharged after 1953, and 57 percent contained a pesticide or an elevated nitrate 

concentration. Fractures or sand-and-gravel 

stringers within the till are the probable pathways.

In some areas, deeper parts of the groundwater-flow system are also hydraulically connected to the land surface. Almost half the waters 

from wells 50 to 100 feet deep were recharged 

after 1953. Anthropogenic constituents were 

detected in samples from three domestic wells 60 

to 121 feet deep, in areas where the till is relatively coarse-grained. 

The hydrogeologic system has several 

geochemical characteristics conducive to transformations or sorption of nitrate or pesticides: (1) the 

till is clay-rich, has a high organic-carbon content, 

and contains an abundance of pyrite-rich shale 

fragments, (2) the ground water has low dissolved-oxygen concentrations, and (3) iron and 
2 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

manganese oxides and oxyhyroxides line the 

faces of fractures in the unsaturated zone. 

Although the aquifer system appears be protected from contamination in some areas, the fact 

that the surficial till is heterogeneous and of variable thickness suggests that the protection is not 

uniform. The protection can be breached by fractures or sand-and-gravel stringers, which are 

apparent in core samples but not noted on domestic-well logs. 

Introduction

The effects of agricultural land use on water quality 

have been investigated as part of the U.S. Geological 

Survey (USGS) National Water-Quality Assessment 

(NAWQA) program. The Lake Erie-Lake St. Clair 

Basin is the focus of one of these NAWQA studies. 

The northwestern part of the basin (fig. 1) includes 

part of the midwestern Corn Belt, where rates of fertilizer and pesticide use are among the highest in the 

country (Mueller and Helsel, 1996; Barbash and others, 1999). In this area, the potential for agrichemicals 

to affect ground-water quality is a concern because 

domestic wells are the primary source of drinking 

water. Although heavy use of agrichemicals is a logical basis for concern, preliminary data from NAWQA 

studies throughout the country indicate that ground 

water in parts of the Upper Midwest is minimally contaminated by nitrate and pesticides, presumably 

because ground water is protected by the surficial glacial till and (or) widespread tile drainage (Mueller and 

others, 1995; Mueller and Helsel, 1996; Barbash and 

others, 1999; U.S. Geological Survey, 1999). 

This report is based on data from two NAWQA 

studies done in the Lake Erie-Lake St. Clair Basin in 

1998. The report has three goals. The first is to 

describe ground-water quality at a range of depths and 

locations within an agricultural area underlain by till. 

The focus is on comparing ground-water quality to 

U.S. Environmental Protection Agency (USEPA) 

drinking-water standards and to state well-depth 

requirements. 

The second goal is to present evidence as to 

whether the study area is linked to those parts of the 

Upper Midwest where the ground water is minimally 

contaminated by agrichemicals. This will be accomplished by comparing nitrate and pesticide detection 

frequencies in the study area to those of comparable 

studies in other agricultural areas.

The third goal is to use the relations between 

concentration, well depth, and ground-water age to 

refine our understanding of how vulnerable ground 

water is to the effects of agricultural land use. 

Study design and methods

Water quality was assessed in shallow ground water 

from monitor wells and in deeper ground water from 

domestic wells. NAWQA guidelines were followed 

for defining the extent of the study area (Gilliom and 

others, 1995); selecting sampling sites within the study 

area (Scott, 1990); installing monitor wells (Lapham 

and others, 1995); and collecting water-quality, quality-control, and ancillary data (Koterba and others, 

1995).

Agricultural land-use study

The goal of a NAWQA agricultural land-use study is 

to determine how agricultural land use affects shallow 

ground-water quality. NAWQA guidelines state that 

land-use studies should be located in areas that are relatively homogenous in terms of factors that affect 

water quality. In the northwestern part of the Lake 

Erie-Lake St. Clair Basin, those factors were judged to 

be land use, surficial geology, and bedrock geology. 

Therefore, the extent of the study was limited to areas 

that meet all of the following criteria: (1) the land use 

is predominantly agricultural, and the primary crops 

are corn and soybeans (fig. 2A), (2) the glacial sediment is more than 100 ft thick, and consists of till at 

the land surface (fig. 2B), and (3) the underlying bedrock is interbedded shale and sandstone of Mississippian age (fig. 2C). These criteria are met in a 

noncontiguous, 1,647-mi2 area of southeastern Michigan, northwestern Ohio, and northeastern Indiana. 

Within the study area as defined, sites were randomly selected for installation of monitor wells. At 

each potential well site, field reconnaissance verified 

that the land use was predominantly agricultural, and 

domestic well logs were used to verify that the surficial sediment was till.

A network of 30 monitor wells was installed 

during the summer of 1997. The goal was to screen the 

wells just below the water table. To accomplish this, 
Study design and methods 3

Figure 1. Location of NAWQA well networks in agricultural areas of the Lake Erie-Lake St. Clair Basin

Detroit

Toledo

Ann 

Arbor

Pontiac

OAKLAND

LIVINGSTON

WAYNE WASHTENAW

MACOMB

LAPEER

LENAWEE MONROE

JACKSON

ST. CLAIR

LUCAS FULTON

WILLIAMS

HILLSDALE

SANILAC

MICHIGAN

OHIO

INDIANA

5 10 15 20 25 MILES

0 5 10 15 20 25 KILOMETERS

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972 0

Albers Equal-Area Conic Projection

Standard parallels 29O

 30' and 45O 30', central meridian 83O

Ind. Ohio

Penn.

Mich.

N.Y.

Canada

Map Area

EXPLANATION

Lake Erie-Lake St. Clair Basin

Monitor wells

Domestic wells

42o

43o

CANADA

UNITED STATES

Lake Erie

Lake 

St. Clair

85o 84o 83o
4 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

Figure 2. Factors used to define agricultural land-use study area: (A) land use, (B) surficial geology, and (C) bedrock geology.

A B

C

Ind. Ohio

Penn.

Mich.

N.Y.

Canada

Surficial sediment consisting of glacial till

Mississipian shale and sandstone

bedrock 

Lake Erie-Lake St. Clair Basin

Agricultural land use

Map Area

EXPLANATION
Study design and methods 5

boreholes were drilled through the unsaturated surficial till, and well screens were set in the shallowest 

saturated sand-and-gravel stringer estimated to be 

capable of producing a volume of water sufficient for 

sampling. These coarse-grained stringers were commonly only a few inches thick and were not identified 

on domestic well logs. At all sites, a thin stringer of 

sand and (or) gravel was present less than 20 ft below 

the top of the saturated zone. The median depth of the 

30 monitor wells is 18 ft, and the range is 9 to 34 ft 

(table 1). Sediment cores were collected during well 

installation. Four-inch-diameter cores were retrieved 

from depths of 4 ft below land surface to the bottom of 

the well. Lithology was described, and representative 

samples were analyzed for organic carbon content.

Three casing volumes were purged from the 

wells before sampling. Approximately two-thirds of 

the wells were slow to recharge; therefore, these wells 

were purged on a series of visits spaced over a period 

of several weeks or months. In addition, at least one 

casing volume was purged immediately before sampling. The well network was sampled in the early summer of 1998. Samples were analyzed for major ions,

 Table 1. Characteristics of ground-water-quality studies and wells in the northwestern part of the Lake Erie-Lake St. Clair Basin, 1998 

[ft, feet; VOC’s, volatile organic compounds; PVC, polyvinyl chloride]

Agricultural land-use study Domestic wells from agricultural 

areas of the subunit survey 

Study characteristics 

Goal of study Assess how agricultural land use affects shallow 

ground-water quality.

Assess the quality of ground water used for drinking 

water in an agricultural area.

Sampling network 30 monitor wells installed by USGS following 

NAWQA protocols (Lapham and others, 1995). 

18 domestic wells co-located with the monitor wells 

sampled for the Agricultural Land-Use Study.

Depth of wells Median: 18 ft 

Range: 9 to 34 ft 

Median: 90 ft 

Range: 31 to 158 ft

Horizon sampled Shallowest saturated sand-and-gravel stringer within 

glacial till. 

Sand-and-gravel aquifer overlain by glacial till. 

Sampling date May to July 1998 May to August 1998

Sample collection Collected with submersible pump. Followed 

NAWQA protocols for collection, filtration, and 

preservation (Koterba and others, 1995). 

Collected from outside tap before water softener. 

Followed NAWQA protocols for collection, 

filtration, and preservation (Koterba and others, 

1995). 

Laboratory, analyses, and 

analytical methods

USGS National Water Quality Laboratory: 

– Major ions (Fishman and Friedman, 1989) 

– Nutrients (Fishman and Friedman, 1989) 

– Pesticides (Zaugg and others, 1995; Werner and 

 others, 1996) 

– Tritium (Ostland and Dorsey, 1977)

– Dissolved organic carbon (Brenton and Arnett,

 1993) 

USGS Organic Chemistry Lab: 

– Pesticides and degradates (Zimmerman and

 Thurman, 1999; Hostetler and Thurman, 2000)

USGS National Water Quality Laboratory: 

– Major ions (Fishman and Friedman, 1989)

– Nutrients (Fishman and Friedman, 1989)

– Pesticides (Zaugg and others, 1995)

– Tritium (Ostland and Dorsey, 1977)

– Dissolved organic carbon (Brenton and Arnett,

 1993) 

– VOC’s (Connor and others, 1998) 

– Radon (American Society for Testing and

 Materials, 1996)

USGS Organic Chemistry Lab:

– Pesticides and degradates (Zimmerman and 

 Thurman, 1999; Hostetler and Thurman, 2000)

Well characteristics

Year of construction 1997 1969-98

Length of screen 5 ft 3 to 8 ft

Casing material PVC PVC (14); galvanized steel (4) 

Grouted? Yes Yes (11); no (7)
6 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

nutrients, tritium, dissolved organic carbon, pesticides, 

and pesticide degradates (tables 1 and 2).

Subunit survey

The goal of a NAWQA subunit survey (SUS) is to 

assess the water quality of an aquifer that is an important source of drinking water. In the northwestern part 

of the Lake Erie-Lake St. Clair Basin, thick lenses or 

layers of sand and gravel within the glacial deposits 

are the primary source of water to domestic wells. 

The SUS sampling network consists of 28 

domestic wells. The study area for the SUS was 

defined to coincide with that of the agricultural landuse study in terms of surficial sediment (fig. 2B) and 

bedrock geology (fig. 2C). Land use was not a factor 

used to define the extent of the SUS; however, approximately two-thirds of the SUS study area consists of 

row-crop agriculture. In agricultural areas, the SUS 

domestic wells were co-located with the randomly 

selected monitor wells installed for the agricultural 

land-use study. Of the 28 domestic wells sampled, 18 

are co-located with monitor wells in agricultural areas 

(fig. 1, fig. 3).

Figure 3. Simplified section showing relation of monitor and 

domestic wells to hydrogeologic setting.

The domestic wells were sampled during the 

summer of 1998. A minimum of three casing volumes 

were purged from the wells immediately prior to sampling. Samples of untreated water were collected and 

analyzed for major ions, nutrients, tritium, dissolved 

organic carbon, pesticides, pesticide degradates, volatile organic compounds (VOC’s), and radon (tables 1 

and 2).

Description of study area

The study area is part of the Eastern Lake Section of 

the Central Lowlands Physiographic Province, where 

the topography ranges from nearly flat-lying land to 

low-relief rolling hills (Casey and others, 1997). Mean 

annual air temperature is approximately 45 to 48°F, 

and mean annual precipitation is approximately 28 to 

38 in. (Casey and others, 1997). Precipitation is highest in May, June, and July and lowest in January and 

February (Casey and others, 1997). 

Land use

The predominant land use is row-crop agriculture, and 

the predominant crops are corn, soybeans and small 

grains, which are planted in rotation. Small, scattered 

herds of livestock, farmhouses, rural homes, and small 

towns also occur in the study area. 

Almost all of the cultivated land is artificially 

drained by a widespread, long-standing network of tile 

drains and ditches. Subsurface tile drains, typically set 

about 3 ft deep, discharge to streams or drainage 

ditches. Some of the natural streams are managed as 

agricultural drainage ditches. 

Hydrogeology

Domestic wells are typically 50 to 150 ft deep and produce water from discontinuous layers of sand and 

gravel that are present at various depths within the glacial till (fig. 3). Collectively, these layers of sand and 

gravel are considered to be an aquifer system that is 

variably confined (Fleming, 1994). The aquifer system 

is recharged primarily by infiltration of precipitation. 

Ground water discharges to wells, tile drains, agricultural ditches, streams, wetlands, and small lakes. 

Regional ground-water discharge is to major rivers 

and ultimately to Lake Erie. 

The aquifer system is overlain by till, an 

unsorted glacial deposit. The study area includes tills 

from two glacial lobes; Erie Lobe tills are typically 

clayey, and those of the Saginaw Lobe can be sandy or 

contain small lenses of sand and gravel (Fleming and 

others, 1994). At most of the monitor-well sites (24 of 

30), the till consists of a dense mixture of silt, clay, and 

gravel interspersed with one or more thin stringers of 

sand and gravel. The surficial till is saturated at depths 

of approximately 5 to 15 ft below land surface, and the 

unsaturated till contains fractures, root traces, and 

50 

100 

150 

Depth below

land surface

0 

200 

EXPLANATION

Sand-and-gravel

aquifer

Glacial till

Tile drain

Water table

Domestic well

Monitor well

(feet)

Not to scale
Description of study area 7

Table 2. Pesticides analyzed in samples from monitor and domestic wells, northwestern part of the Lake Erie-Lake St. Clair Basin, 

1998

[MDL, Method detection limit in micrograms per liter; degradates are in italics]

Compound MDL Compound MDL Compound MDL

Pesticides, monitor and domestic wells

USGS National Water Quality Laboratory: 

Acetochlor 0.002 Eptam 0.002 Pendimethalin 0.004

Alachlor .002 Ethalfluralin .004 Permethrin .005

Atrazine .001 Ethoprop .003 Phorate .002

Benfluralin .002 Fonofos .003 Prometon .018

Butylate .002 HCH, alpha .002 Pronamide .003

Carbaryl .003 Lindane .004 Propachlor .007

Carbofuran .003 Linuron .002 Propanil .004

Chlorpyrifos .004 Malathion .005 Propargite .013

Cyanazine .004 Methyl azinphos .001 Simazine .005

Dacthal .002 Methyl parathion .001 Tebuthiuron .01

DDE .006 Metolachlor .006 Terbacil .007

Deethylatrazine .002 Metribuzin .002 Terbufos .013

Diazinon .002 Molinate .004 Thiobencarb .002

Dieldrin .001 Napropamide .004 Triallate .001

Diethylaniline,2’6- .003 Parathion .003 Trifluralin .002

Disulfoton .017 Pebulate .004

USGS Organic Chemistry Laboratory:

Acetochlor 0.05 Deethylatrazine 0.05 Metribuzin 0.05

Acetochlor ESA .20 Deisopropylatrazine .05 Prometon .05

Acetochlor OA .20 Hydroxyatrazine .05 Prometryn .05

Alachlor .05 Cyanazine .20 Propachlor .05

Alachlor ESA .20 Cyanazine amide4 .05 Propazine .05

Alachlor OA .20 Metolachlor .05 Simazine .05

Amethryn .05 Metolachlor ESA .20 Terbutryn .05

Atrazine .05 Metolachlor OA .20

Additional pesticides, monitor wells only 

USGS National Water Quality Laboratory:

2,4,5-T 0.035 Clopyralid 0.05 Methiocarb 0.026

2,4-D .035 Dacthal, MA .017 Methomyl .017

2,4-DB .035 Dicamba .035 1-Naphthol .007

Acifluorfen .035 Dichlobenil .02 Neburon .015

Aldicarb .016 Dichlorprop .032 Norflurazon .024

Aldicarb sulfone .016 Dinoseb .035 Oryzalin .019

Aldicarb sulfoxide .021 Diuron .02 Oxamyl .018

Bentazon .014 DNOC .035 Picloram .05

Bromacil .035 Esfenvalerate .019 Propham .035

Bromoxynil .035 Fenuron .013 Propoxur .035

Carbofuran, 3-hyd .014 Fluometuron .035 Silvex .021

Chloramben .011 MCPA .05 Triclopyr .05

Chlorothalonil .035 MCPB .035
8 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

horizontal partings. At 6 of the 30 sites, the till is visibly more coarse grained, and fractures in the unsaturated zone were not observed in 4-in.-diameter cores. 

Recharge to the aquifer system occurs through 

the surficial till by (1) migration along near-vertical 

fractures, (2) diffusion through the till matrix, (3) 

migration through sand-and-gravel deposits hydraulically connected to the land surface, or (4) any combination of these processes. Characteristics of the till 

influence the chemistry of the water and the efficiency 

of the hydraulic connection between the land surface 

and ground water. 

Drillers’ logs from co-located domestic and 

monitor wells in Williams County, Ohio, are shown in 

figure 4. The domestic well is screened in a thick layer 

of gravel at a depth of 57 ft (fig. 4A). The monitor well 

is screened in a sand-and-gravel stringer within the till 

at a depth of 30 ft (fig. 4B). 

Figure 4C is a description of the sediment from 

a 4-in.-diameter core retrieved during installation of 

the monitor well. The characteristics of the glacial till 

at this site are representative of the till at most of the 

sites in the study area (24 of 30). In addition, the till is 

similar to that in southwestern Ontario (Ruland and 

others, 1991), northeastern Indiana (Fleming, 1994), 

and western Ohio (Strobel, 1993). 

The upper part of the till is weathered. Below 

the soil zone, the till matrix is light orangeish brown. 

In southwestern Ontario, this color has been attributed 

to the alteration of chlorite (Ruland and others, 1991). 

Near-vertical fractures are prominent and are lined 

with a pale-gray alteration halo described as a calcareous material (Ruland and others, 1991), gypsum (A.H. 

Fleming, Indiana Geological Survey, oral commun., 

1997), and caliche (Strobel, 1993). 

At a depth of 7 ft, the till is slightly moist, and 

the matrix color is darker than above. Iron oxyhydroxides occur on the gray alteration haloes lining the fractures. Oxidized iron also is present along root traces 

and horizontal partings, and within the matrix as mottling. At a depth of approximately 11 ft, the matrix is 

grayish brown, and the gray alteration haloes are less 

distinct. Black, dendritic manganese oxides and two 

shades of iron oxyhydroxides are visible on the fracture faces. 

The top of the unweathered zone occurs at 

approximately 12 ft. Below this depth, the till matrix is 

dark gray, sticky, and dense, and it shows no evidence 

of fracturing or mineralization. Fragments of black 

shale are a common component of the tills except in 

the extreme northern part of the study area. The fragments are presumably derived from the pyrite-rich 

Antrim Shale that subcrops along the eastern boundary 

of the study area (Fleming, 1994).

Ruland and others (1991), Strobel (1993), and 

Fleming (1994) used hydraulic data to demonstrate 

that (1) hydraulically active fractures are present in the 

unweathered zone despite the fact that they are not visible in cores, (2) fractures are interconnected, either 

directly, or by intersection with sand-and-gravel 

stringers, (3) a zone of active ground-water flow 

exends from the land surface to depths of 15-35 ft, and 

the flow direction is predominantly horizontal, and (4) 

fracture density decreases with depth below the land 

surface. Ruland and others (1991) concluded that less 

densely spaced fractures that are hydraulically active 

may extend deeper than 30 ft in some cases and that 

ground-water flow in these deeper, more isolated fractures may be mainly vertical in response to seasonal 

changes in hydraulic gradient. 

The hydrogeology is complex because of several factors: (1) heterogeneity of the aquifer system, 

(2) heterogeneity of the till, (3) fracturing in the surficial till, and (4) the network of tile drains that affects 

ground-water recharge and discharge. The influence of 

each factor, and the interaction between factors, is not 

completely understood. 

Ground-water quality 

The complete data sets from the agricultural land-use 

study (30 monitor wells) and the subunit survey (28 

domestic wells) are published elsewhere (Shindel and 

others, 1999). The pages that follow focus on data 

from the 30 monitor wells and the subset of 18 domestic wells located in agricultural areas. The goal is to 

describe ground-water quality at a range of depths and 

locations within an agricultural area underlain by glacial till. No assumptions are made regarding the 

hydraulic connection between the co-located monitor 

and domestic wells; well pairs are not assumed to lie 

along the same ground-water flowpaths. It is assumed, 

however, that the monitor wells were recharged in 

agricultural areas, an assumption that is consistent 

with the study design for NAWQA land-use studies.
Ground-water quality 9

Figure 4. Drillers’ logs and core description from the domestic and monitor wells co-located at a site in Williiams 

County, Ohio.

0

70

5

10

15

20

25

30

35

40

45

50

55

60

65

WELL DEPTH, IN FEE

T

0

32

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

WELL DEPTH, IN FEE

T

 

EXPLANATION

Well screen 

Depth of water in well casing

 

DOMESTIC WELL MONITOR WELL 

 Driller’s log

Core description

Driller’s log

A B

Soil

Yellow clay

Gray clay

Gravel

Silty sand and gravel, saturated.

Sticky, dense,

gray silt and clay.

Scattered gravel

and black shale fragments.

No visible fractures or mineralization.

3 very thin layers of sand and gravel.

Dark-brown

silty sediment

Orange-brown

silt and clay.

Scattered gravel

and black shale

fragments.

Saturated

gray silt and clay.

Scattered gravel

and black shale

fragments.

Sand and

gravel

Orange-brown matrix.

Roots, iron mottles.

Near-vertical fractures with gray 

alteration haloes.

Slightly moist.

Gray-brown matrix.

On fractures: iron and manganese oxides; alteration halo less prominent.

Saturated.

C

Dark-brown cuttings

Brown matrix. Intense iron mottles.

On fractures: gray alteration haloes and iron oxides.
10 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

Ground-water age

The age of the ground water was estimated by means 

of tritium age-dating, which is based on the fact that 

tritium concentrations in precipitation increased after 

1953 because of atmospheric testing of atomic weapons (Plummer and others, 1993). Waters with tritium 

concentrations of less than 1 Tritium Unit (TU, equivalent to 3.2 picocuries per liter) are interpreted to have 

been recharged before 1953 and are hereafter referred 

to as “old waters.” Waters with tritium concentrations 

greater than or equal to 1 TU are interpreted to have 

been recharged after 1953, and are hereafter referred 

to as “young waters.” This is a simplification of the 

real system, where young and old ground waters can 

mix in the subsurface. Nonetheless, waters with more 

than 1 TU are assumed to contain some fraction of 

young water. 

The relation between ground-water age and well 

depth is shown in figure 5. All of the samples from 

wells less than 20 ft deep were young waters. All samples from wells greater than 100 ft deep were old 

waters. Samples from wells between 20 and 99 ft deep 

were 46 percent young and 54 percent old.

Water type 

The relative proportions of major ions in samples from 

domestic and monitor wells are illustrated in figure 6. 

Samples from domestic wells have a median dissolved 

solids (DS) concentration of 387 mg/L, and most samples are calcium-magnesium-bicarbonate-type waters. 

Relatively dilute calcium-magnesium-bicarbonatetype waters are characteristic of recently recharged 

ground water in the humid East (Back and others, 

1993). Domestic-well samples with higher than 

median dissolved solids concentrations are enriched in 

chloride or sulfate or both. 

Monitor-well samples have a significantly 

higher median dissolved solids concentration 

(500 mg/L) than domestic-well samples. Samples 

from monitor and domestic wells are similar in that 

(1) samples with low dissolved solids concentrations 

are calcium-magnesium-bicarbonate-type waters, and 

(2) samples with high dissolved solids concentrations 

are enriched in chloride or sulfate or both. Possible 

sources of chloride and sulfate are discussed in a later 

section.

Figure 5. Relation of tritium concentrations to well depth 

in the northwestern part of the Lake Erie-Lake St. Clair 

Basin, 1998.

Pesticides and pesticide degradates

Rates of pesticide use in the study area are among the 

highest in the country (Barbash and others, 1999; U.S. 

Geological Survey, 1999). The five most heavily used 

pesticides in the Lake Erie Basin during 1994-95 were 

metolachlor, atrazine, cyanazine, acetochlor, and 

alachlor (Brody and others, 1998). All are herbicides 

used on corn and soybeans (Thelin, 1998). Samples 

from monitor and domestic wells were analyzed for 

parent compounds (the original active ingredient) and 

selected degradates (breakdown products) (table 2). 

Hereafter in this report, the term “pesticide” refers to 

either a parent compound or a degradate. 

Pesticides detected in the ground water closely 

correspond to those most heavily used in the Lake Erie 

0.5 1 10 100 200

TRITIUM CONCENTRATION,

IN PICOCURIES PER LITER 

175

0

25

50

75

100

125

150

WELL DEPTH, IN FEE

T

Tritium concentration used to differentiate

Domestic well

Monitor well

Old

waters

Young

waters

EXPLANATION

Solid symbol: old water

<20 feet:

 young water

20 to 100 feet:

 54% old waters

 46% young waters

>100 feet:

 old waters

Open symbol: young water

 between waters recharged before and

 after 1953 (3.2 picocuries per liter =1

 Tritium Unit)
Ground-water quality 11

PERCENT OF TOTAL, IN MILLIEQUIVALENTS PER LITER

PERCENT OF TOTAL, IN MILLIEQUIVALENTS PER LITER

Domestic wells

Dissolved solids concentrations:

 Median: 316 milligrams per liter

 Range: 250 to 404 milligrams per liter

Monitor wells

Dissolved solids concentrations:

 Median: 466 milligrams per liter

 Range: 82 to 1840 milligrams per liter

EXPLANATION

Waters recharged before 1953

Waters recharged after 1953

Figure 6. Piper diagrams of waters from domestic and monitor wells, northwestern part of the Lake Erie-Lake St. Clair Basin,

1998.
12 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

Figure 7. Pesticides most heavily applied in the Lake Erie Basin relative to those detected in ground water, northwestern 

part of the Lake Erie-Lake St. Clair Basin, 1998.

0 10 20 30 0

 

0 10 20 30

0.01

6.02

3.27

0.11

0.05

3.53

1.04

0 10 20 30 0

0.009E

0.02E

1.52

0.97

0.12

 

 

 

 

PESTICIDES USED PESTICIDES DETECTED

27 monitor wells 18 domestic wells

METOLACHLOR

ATRAZINE

CYANAZINE

ACETOCHLOR

ALACHLOR

OTHER

Percentage of total pesticide use,

Lake Erie Basin, 1994-95

(Data from Brody and others, 1998)

METOLACHLOR

ATRAZINE

BROMOXYNIL

Metolachlor ESA

Metolachlor OA

Deethylatrazine

Cyanazine amide

Acetochlor ESA

Acetochlor OA

Alachlor ESA

Alachlor OA

Percentage of samples

in which compound was detected

4 4

EXPLANATION

Maximum concentration

detected is shown to the 

right of each bar.

E: estimated concentration

OA: oxalic acid

ESA: ethanesulfonic acid

Degradates are in italics.

Parent compounds are in

capital letters.

Analysis from the USGS

Organic Chemistry Lab

Analysis from the USGS

National Water Quality 

Lab

Basin (fig. 7). Six pesticides were detected in the study 

area—metolachlor, atrazine, cyanazine, acetochlor,

alachlor, and bromoxynil. All are herbicides used on 

corn; metolachlor and alachlor are also used on soybeans, and bromoxynil is also used on wheat and other 

small grains (Thelin, 1998). Five of the six pesticides 

were the most heavily applied pesticides in the Lake 

Erie Basin during 1994-95 (Brody and others, 1998). 

Samples were not analyzed for glyphosate, which 

ranked sixth in terms of the amount applied in the 

Lake Erie Basin during 1994-95 (Brody and others, 

1998).

 Pesticides were detected in 41 percent (11 of 

27) of the monitor wells and 6 percent (1 of 18) of the 

domestic wells. In monitor wells, degradates were 

detected three times more frequently, and at higher 

concentrations, than were parent compounds (fig. 7; 

table 3). Metolachlor ESA (ethanesulfonic acid) was 

the compound that was detected in the highest concentrations (1.1 to 6.0 μg/L). Alachlor ESA was the most 

frequently detected compound in the study area, even 

though it ranked fifth in terms of use during 1994-95. 

The discrepancy may be due to heavier use of alachlor 

in the past; for example, alachlor was ranked second in 

terms of use during 1989-90 in Williams County, Ohio 

(Battaglin and Goolsby, 1994).

No pesticide was detected at a concentration 

greater than a USEPA Maximum Contaminant Level 

(MCL), which is an enforceable drinking-water standard based on health criteria. However, MCL’s have 

not been established for 9 of the 11 compounds 

detected in the study area. 

Mixtures of two or more pesticides were found 

in half of the wells with a pesticide detection (6 of 12). 

One sample contained a mixture of five pesticides. 

The health effect of mixtures of pesticides is unknown 

(U.S. Geological Survey, 1999).

Relations between pesticide detections, well 

depth, and ground-water age are shown in figure 8. 

Most of the pesticide detections (75 percent) were in 

young waters from monitor wells less than 25 ft deep, 

the minimum casing-depth required for wells in Michigan (Part 127, Public Act 368, 1978, revised 1994) 

and Ohio (Ohio Department of Health, 2000). Two 
Ground-water quality 13

samples containing pesticides were from wells greater 

than 25 ft deep. One was a 121-ft-deep domestic well 

that produced old water with an estimated deethylatrazine (DEA) concentration of 0.009 μg/L. DEA is a 

degradate of atrazine that is more mobile than its parent compound (Barbash and Resek, 1996). This sample also contained the maximum concentrations of 

ammonia (0.46 mg/L) and nitrite (0.11 mg/L) detected 

in any domestic well. The well is in an area where the 

till is unusually coarse grained relative to most other 

parts of the study area.

Trace concentrations of degradates of pesticides 

introduced after 1953 (atrazine and metolachlor) were 

detected in two waters classified as old. This finding 

indicates that these samples are actually a mixture of 

old and young waters (Kolpin and others, 1995). 

Nitrate 

Nitrogen occurs naturally in soil organic matter and 

the atmosphere. Anthropogenic sources of nitrogen 

include chemical fertilizers, manure, septic-system 

effluent, and fossil-fuel combustion products. In the 

study area, fertilizer application rates are reported to 

be among the highest in the country (Mueller andHelsel, 1996); therefore, agricultural fertilizers are 

assumed to be the major source of nitrogen to the 

ground water. Nitrate is the form of nitrogen that 

causes human-health concerns in drinking water and is 

therefore the focus of this report. The term “nitrate” is 

used as shorthand for “nitrate plus nitrite, reported as 

nitrogen.” 

Detection frequencies and concentrations of 

nitrate in monitor and domestic wells are summarized 

in table 4. In monitor wells, the median nitrate concentration was 0.17 mg/L, and the maximum value was 

14.8 mg/L. In domestic wells, nitrate concentrations 

were significantly lower. In 17 of 18 domestic wells, 

nitrate was not detected with a minimum reporting 

level of 0.05 mg/L. In one domestic-well sample, the 

nitrate concentration was 1.56 mg/L. 

In ground water, nitrate concentrations greater 

than 2 mg/L are considered to be the result of human 

influence (U.S. Geological Survey, 1999). Thirtyseven percent of samples from monitor wells (11 of 

1 2 3 4 5

0

175

25

50

75

100

125

150

Minimum

casing-depth

requirement

WELL DEPTH, IN FEET

NUMBER OF PESTICIDES DETECTED

0 6

EXPLANATION

Domestic well

Monitor well

Solid symbol: old water

Open symbol: young water

Figure 8. Relation of pesticide detections to well 

depth and ground-water age in the northwestern part of the Lake Erie-Lake St. Clair Basin, 

1998.

 Table 3. Pesticide detection frequencies and maximum 

concentrations in ground-water samples, northwestern part of 

the Lake Erie-Lake St. Clair Basin, 1998

[Detection frequencies are based on varied minimum reporting limits;

μg/L, micrograms per liter; E, estimated concentration]

Summary statistic

27 

monitor 

wells

18 

domestic 

wells

Pesticide parent compounds: 

Detection frequency (%)

Maximum concentration (μg/L) 

 11

 0.11 

0

N/A

Pesticide degradates: 

Detection frequency (%) 

Maximum concentration (μg/L) 

33

6.02

6

 0.009E

Pesticide parent compounds or degradates:

Detection frequency (%) 41 6
14 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

30) had nitrate concentrations greater than 2 mg/L. In 

this report, the term “elevated nitrate concentration” is 

used as shorthand for “nitrate concentration greater 

than the estimated background concentration of 

2 mg/L”. 

Nitrate was the only constituent detected at a 

concentration greater than an MCL. The MCL for 

nitrate, 10 mg/L, was exceeded in samples from 7 percent of the monitor wells (2 of 30) but in none of the 

samples from domestic wells.

The relations between nitrate concentration, 

well depth, and ground-water age are shown in figure 

9. The two exceedances of the MCL were in samples 

from monitor wells 19 to 20 ft deep. The MCL was not 

exceeded in wells deeper than the minimum welldepth requirement; however, elevated nitrate concentrations indicative of human activities were detected in 

samples from three monitor wells 28 to 31 ft deep. 

Nitrate was detected in one domestic-well sample at a 

concentration of 1.56 mg/L, a concentration that is 

probably high enough to be the result of human activities. This sample was from a 60-ft-deep well in an area 

where the surficial till is clayey but is only 20 ft thick. 

Between depths of 21 and 60 ft, the sediment is predominantly sand and gravel.

Constituents that exceeded secondary drinking-water 

standards 

USEPA Secondary Maximum Contaminant 

Levels (SMCL’s) are non-enforceable drinking-water 

standards based on esthetic criteria such as taste, odor, 

and staining. SMCL’s were exceeded with respect to 

five inorganic constituents: chloride, sulfate, dissolved 

solids, iron, and manganese (fig. 10). Chloride concentrations greater than the SMCL are interpreted to 

be the result of human activities. In contrast, iron concentrations greater than the SMCL are probably 

largely due to natural factors. High concentrations 

of sulfate, dissolved solids, and manganese could be 

due to either natural sources, human activities, or both. 

Chloride. Chloride concentrations in young, 

shallow waters were significantly higher than those in 

old, deep waters, according to the results of a ranksum test with a 95-percent confidence level. The 

SMCL for chloride (250 mg/L) was exceeded in samples from 10 percent of the monitor wells (3 of 30). 

0 2 4 6 8 10 12 14 16

0

175

25

50

75

100

125

150

Minimum

casing-depth

requirement

MCL

Background

concentration

WELL DEPTH, IN FEET

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

EXPLANATION

Domestic well

Monitor well

Solid symbol: old water

Open symbol: young water

MCL–Maximum Contaminant Level

 (U.S. Environmental Protection Agency)

18

Figure 9. Relation of nitrate concentrations to well 

depth and ground-water age in the northwestern 

part of the Lake Erie-Lake St. Clair Basin, 1998.

Table 4. Nitrate detection frequencies and concentrations in 

ground-water samples, northwestern part of the Lake Erie-Lake 

St. Clair Basin, 1998

[mg/L, milligrams per liter; detection frequencies are based on a 

minimum reporting limit of 0.05 ug/L]

Summary statistic

30 

Monitor 

wells

18 

Domestic 

wells

Percentage of samples with 

concentration >2 mg/La

a

 2 mg/L is background concentration for ground water (U.S. 

Geological Survey, 1999)

37 0

Percentage of samples with 

concentration >10 mg/Lb 7 0

Median concentration (mg/L) 0.17 <0.05

Maximum concentration (mg/L) 14.8 1.56

b

 10 mg/L is the U.S. Environmental Protection Agency 

Maximum Contaminant Level
Ground-water quality 15

0 100 200 300 400 500

0

175

25

50

75

100

125

150

0 1, 250 500 750 1,000 250

0

175

25

50

75

100

125

150

500 1,500 2,500 3,500

0

175

25

50

75

100

125

150

0.01 2 0.1 1 10 0

0

175

25

50

75

100

125

150

0.001 0.01 0.1 1 5

0

175

25

50

75

100

125

150

 

0

 

CHLORIDE SULFATE

DISSOLVED SOLIDS IRON

MANGANESE

WELL DEPTH, IN FEET WELL DEPTH, IN FEET WELL DEPTH, IN FEET

CONCENTRATION,

IN MILLIGRAMS PER LITER

CONCENTRATION,

IN MILLIGRAMS PER LITER

SMCL SMCL

SMCL SMCL

SMCL

EXPLANATION

Domestic well

Monitor well

Solid symbol: old water

Open symbol: young water

Minimum casing-depth requirement

SMCL–Secondary Maximum Contaminant Level

 (U.S. Environmental Protection Agency)

Figure 10. Relation of concentrations of constituents that exceeded SMCL’s to well depth and 

ground-water age, northwestern part of the Lake Erie-Lake St. Clair Basin, 1998.
16 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

Possible sources of elevated chloride concentrations are anthropogenic (potash fertilizer, road salt, 

septic-system effluent) and natural (upward migration 

of deep-seated brine, dissolution of halite). In the 

study area, it is unlikely that elevated chloride concentrations are from natural sources because (1) relations 

between concentration, well-depth and ground-water 

age are not consistent with upward migration of old, 

deep, high-chloride waters (brine), and (2) halite has 

not been identified in the glacial sediment or near-surface bedrock (Mozola, 1953; Coen, 1989; Fleming and 

others, 1994). Therefore, elevated chloride concentrations are likely to be from human activities. 

One sample from a 92-foot-deep domestic well 

had a chloride concentration of 138 mg/L and a chloride/bromide ratio of 1,056. In a nearby study area, 

similar values were interpreted to be an indication of 

human influence (Thomas, 2000). This well is in an 

area where the till is coarse grained relative to the 

study area as a whole. 

Sulfate. Sulfate concentrations are significantly higher in young, shallow waters than in old, 

deep waters, according to the results of a rank-sum test 

(95-percent confidence level). The SMCL for sulfate 

(250 mg/L) was exceeded in 6 of 30 monitor wells. In 

domestic-well samples, none of the sulfate concentrations were greater than the SMCL, but concentrations 

in young waters were higher than those in old waters 

at similar depths. In general, the relations between 

concentration, well depth, and ground-water age are 

consistent with a source of sulfate at or near the land 

surface (fig. 10). 

Possible sources of sulfate are anthropogenic 

(fertilizer, coal combustion) and natural (mineral reactions, organic matter in soil). In the study area, fertilizer is one possible source of sulfate in shallow ground 

water; ammonium sulfate was the second most heavily 

used form of nitrogen fertilizer in Michigan during 

spring 1997 (R. Pigg, Michigan Department of Agriculture, written commun., 1998). Potassium sulfate, 

potassium-magnesium sulfate, and calcium sulfate 

(gypsum) also were used as fertilizer in Michigan (R. 

Pigg, written commun., 1998). 

Mineral reactions in the shallow subsurface are 

another possible source of high sulfate concentrations 

in shallow ground water. Sulfides (for example, pyrite) 

present in the till at the time of deposition can be oxidized to soluble forms of sulfur (for example, gypsum) 

in the upper weathered zone of the till (Hendry and 

Wassenaar, 2000). Dissolution of gypsum can also 

increase sulfate concentrations in ground water. 

On the basis of data from the current study, it is 

not possible to differentiate between the three possible 

sources of sulfate—fertilizer, pyrite oxidation, and 

gypsum dissolution. The statistical correlation 

between sulfate and calcium (Kendall’s tau = 0.60) is 

one of the strongest between any two chemical constituents in the study. However, this correlation could be 

due to either gypsum dissolution or pyrite oxidation 

(Kolle and others, 1985). In addition, the correlation 

between sulfate and calcium does not disprove ammonium sulfate fertilizer as a source of sulfate in the shallow ground water. 

Dissolved solids. Concentrations of dissolved 

solids are significantly higher in young, shallow 

waters than in old, deep waters, according to the 

results of a rank-sum test (95-percent confidence 

level). The SMCL for dissolved solids (500 mg/L) was 

exceeded in 50 percent of the monitor wells and 17 

percent of the domestic wells (fig. 10). Concentrations 

of dissolved solids are correlated with calcium, sulfate, magnesium, or chloride concentrations. 

Iron. In contrast to chloride, sulfate, and dissolved solids, concentrations of iron are significantly 

lower in young, shallow waters than in old, deep 

waters, according to the results of a rank-sum test (95-

percent confidence level).The SMCL for iron (0.3 mg/

L) was exceeded in 33 percent of the monitor wells 

and 100 percent of the domestic wells. 

Iron solubility is related to the oxidation-reduction potential and pH of ground water (Hem, 1985). 

All waters from domestic wells had low concentrations of dissolved oxygen (the median was 0.14 mg/L 

and the maximum was 0.8 mg/L). Therefore, high iron 

concentrations in domestic-well samples may be the 

result of reductive dissolution of iron oxyhydroxides 

or iron-rich silicate minerals. Another possible source 

is corrosion of steel well casings (Hem, 1985), but 

samples from the four domestic wells with steel casings did not have unusually high iron concentrations. 

In samples from monitor wells, possible additional sources of dissolved iron are (1) oxidation of 

iron-rich clay minerals such as chlorite (Ruland and 

others, 1991), (2) oxidation of pyrite-rich black shale 

fragments, or (3) reductive dissolution of iron oxyhydroxides that occur along fractures, as mottles within 

the till matrix, or on the surface of mineral grains. In 

fig. 4C, the two different shades of iron oxyhydroxides 

just above the saturated zone could be evidence of sev-
Vulnerability of ground water to contamination 17

eral episodes of reduction and oxidation in response to 

fluctuating water levels.

Manganese. The results of a rank-sum test (95-

percent confidence level) indicate that manganese concentrations in young, shallow waters are not significantly different from those in old, deep waters. The 

SMCL for manganese (0.05 mg/L) was exceeded in 

samples from 60 percent of monitor wells and 22 percent of domestic wells (fig. 10).

Manganese is a component of some fertilizers 

that are applied in relatively small amounts in Michigan (R. Pigg, written commun., 1998). Another possible source of dissolved manganese is mineral 

reactions. Manganese exchanges with iron, magnesium, or calcium in lattices of silicate minerals (Hem, 

1985). Manganese solubility is sensitive to oxidationreduction potentials, but less so than iron (Hem, 1985). 

Mineral reactions that could increase dissolved manganese concentrations in shallow ground water are (1) 

oxidation of pyrite, clays, or other minerals containing 

reduced manganese species or (2) reductive dissolution of manganese oxides, including those that occur 

just above the water table on the faces of the fractures 

(fig. 4C). 

Vulnerability of ground water to contamination

National synthesis of data from NAWQA water-quality studies done during 1993-95 indicates that ground 

water in parts of the Upper Midwest is minimally contaminated by nitrate and pesticides, despite high rates 

of agrichemical use (U.S. Geological Survey, 1999). 

Data from the current study are consistent with this 

conclusion. As table 5 shows, detection frequencies 

and median concentrations of nitrate and pesticides 

from the current study are low when compared to the 

Midwest as a whole (Kolpin and others, 1996) and to 

the Nation as a whole (Mueller and Helsel, 1996; U.S. 

Geological Survey, 1999). Ground water in the study 

area appears to be protected, to some degree, from the 

effects of agricultural land use. Several authors have 

suggested that ground water in parts of the Upper Midwest is protected by the surficial glacial till and (or) 

tile drains (Mueller and others, 1995; Mueller and 

Helsel, 1996; Nolan and others, 1997; Barbash and 

others, 1999; U.S. Geological Survey, 1999). Presumably, the surficial layer of glacial till could protect the 

ground water by (1) physically impeding movement of 

water and solutes from the land surface, (2) facilitating 

biogeochemical processes that convert nitrate and pesticides to other compounds, or (3) both. Presumably, 

tile drains could (1) intercept water and solutes before 

they reach the water table, (2) facilitate the discharge 

of water and solutes from the shallow ground water 

and thereby prevent them from migrating to deeper 

parts of the ground-water-flow system, or (3) both. 

Data from the current study do not confirm or disprove 

any of these hypotheses. However, inferences about 

the vulnerability of ground water in the study area can 

be based on the relations between concentration of 

anthropogenic constituents, well depth, and groundwater age.

Hydraulic connection to the land surface 

Shallow ground water. Movement of water or 

solutes from the land surface to the shallow ground 

water is not completely impeded, as indicated by the 

observations that 83 percent of the waters from monitor wells were recharged after 1953, and 57 percent 

contained either a pesticide or an elevated nitrate concentration. These data support the conclusions of 

hydraulic studies in nearby fractured tills—that an 

active ground-water-flow system exists between the 

land surface and depths of 15 to 35 ft (Ruland and others, 1991; Strobel, 1993; Fleming, 1994; Fleming and 

others, 1994). Moreover, Fleming and others (1994) 

concluded that, in northern Indiana, shallow ground 

water in the upper part of fractured, clayey till is one 

of the hydrogeologic settings most vulnerable to contamination.

Tile drains are typically installed at a depth of 

3 ft, and pesticides or elevated nitrate concentrations 

were detected in monitor wells 10 to 31 ft deep. Therefore, tile drains do not intercept all agrichemicals 

before they reach the water table. Over most of the 

study area, the surficial till contains near-vertical fractures that provide a direct pathway from the land surface to the water table. The possibility that tile drains 

would intercept agrichemicals before they reach the 

water table seems most likely (1) where tile drains 

intersect fractures or (2) where the till is relatively 

coarse-grained and therefore unfractured. Over most 

of the study area, tile drains probably function as a network of small discharge areas, thereby exacerbating 

the shallow nature of the local ground-water-flow system. Presumably, contaminants enter the shallow 

ground water but then discharge through tile drains 

rather than migrate to deeper parts of the groundwater-flow system. 
18 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

Table 5. Nitrate and pesticide results from the northwestern part of the Lake Erie-Lake St. Clair Basin relative to those of comparable 

ground-water studies

[NWQL, USGS National Water Quality Laboratory; OCL, USGS Organic Chemistry Laboratory; mg/L, milligrams per liter; -, no data]

Constituent Summary statistic

Results 

 Current study

Midwest 

Pesticide 

Study, 1992a

NAWQA 

Studies, 

1993-95b

Shallow ground water from monitor wells 

Nitrate Percentage of samples with concentration >10 mg/L 7 - 15

Median concentration (mg/L) 0.17 - >3

Pesticides Percentage of samples with at least one pesticide 

based on pesticides analyzed at NWQL (table 2) 10 59

Ground water from domestic-supply wells

Nitrate Percentage of samples with concentration >10 mg/L 0 8 12

Percentage of samples with concentration >3 mg/L 0 33 - 

Median concentration (mg/L) <0.05 - 2 

Pesticides Percentage of samples with at least one pesticide 

based on pesticides analyzed at NWQL and OCL (table 2) 6 62 -

Percentage of samples with at least one pesticide 

based on pesticides analyzed at NWQL (table 2) 6 - 33c

a

 Midwest Pesticide Study, 1992 (Kolpin and others, 1994); includes water-quality data from shallow aquifers in agricultural areas of 

12 Midwest states. b NAWQA studies, 1993-95 include data from agricultural areas of 20 Study Units throughout the country (Mueller and Helsel, 1996; 

U.S. Geological Survey, 1999). c

 Statistic is from areas of mixed land use. 

Deep ground water. The hydraulic connection 

between the shallow, dynamic ground-water-flow system in fractured till and the underlying aquifer system 

is not well understood. Fleming and others (1994) 

speculated that the shallow and deep ground-waterflow systems in northern Indiana are not well connected, if at all. Ruland and others (1991) concluded 

that hydraulically isolated fractures could extend 

beyond a depth of 30 ft in southwestern Ontario. In 

western Ohio, the potential for fracture flow exists to 

depths of at least 60 ft (M.L. Strobel, U.S. Geological 

Survey, written commun., 2000). 

In the current study, almost half (6 of 13) of the 

waters between depths of 50 and 100 ft were recharged 

after 1953 (fig. 5). In addition, anthropogenic constituents were detected in domestic wells at depths of 121 

ft (pesticide metabolite), 92 ft (elevated chloride concentration and chloride/bromide ratio), and 60 ft (elevated nitrate concentration). These wells are in areas 

where the till is relatively coarse grained, or the surficial till is clayey but thin. These observations indicate 

that parts of the aquifer system are hydraulically connected to the land surface. 

In the study area, the minimum casing depth for 

water-supply wells is 25 ft. If active ground-water 

flow extends to depths of 25 to 35 ft, water in some of 

the shallowest domestic wells could be vulnerable to 

the effects of land use.

Geochemical properties of the till. 

The observations that pesticides were detected in the 

ground water, but that most were degradates rather 

than parent compounds, indicates that pesticides can 

physically reach the shallow ground water but that 

they are chemically broken down somewhere along 

the way. The clay-rich tills have several properties 

conducive to the transformation of nitrate or pesticides 

to other compounds: 
Summary and conclusions 19

• Clays, which have a negative surface charge, promote sorption of pesticides.

• Organic carbon also sorbs pesticides. In addition, 

organic carbon is used as a substrate for microbes 

that mediate processe that chemically transform 

nitrogen and pesticides. Sediment collected from 

monitor-well cores had fairly high organic-carbon 

content; the median organic carbon content was 

10 mg/kg, and the range was 5 to 27 mg/kg. 

• Low dissolved-oxygen concentrations promote 

denitrification. Dissolved-oxygen concentrations 

less than 1 mg/L were detected in samples from 

100 percent of the domestic wells and 43 percent 

of the monitor wells. 

• Iron and manganese oxides, which occur along 

fractures in the weathered till, are chemically 

reactive with organic compounds and nitrate. 

• Reduced sulfur compounds (pyrite or organic sulfur) in unweathered till can serve as the electron 

donor for denitrification (Kolle and others, 1985; 

Korom, 1992; Robertson and others, 1996). Kolle 

and others (1985) documented that nitrate was 

reduced by pyrite oxidation in a reaction mediated 

by Thiobacillus denitrificans. The reaction products were gaseous nitrogen, sulfate, and dissolved 

iron. 

Heterogeneity of the till 

The presence of coarse-grained sediment in the till is 

likely to increase the vulnerability of ground water to 

contamination. This assumption is supported by the 

observation that domestic-well samples containing 

anthropogenic constituents were from areas where the 

till is relatively coarse grained. Presumably, the 

coarser grained sediment could increase the hydraulic 

connection to land surface, decrease biogeochemical 

transformations, or both.

A sequence of till studied by Hendry and Wassenaar (2000) was chosen to be a homogeneous, clayrich till without stringers of sand and gravel. Age dating indicated that waters deeper than 60 ft were thousands of years old. In the study area, by contrast, 

waters from similar depths were substantially 

younger; 6 of 13 samples from wells 50 to 100 ft deep 

were recharged after 1953. The striking difference in 

the age of waters between the two areas could be 

partly due to the fact that till in the study area is heterogeneous and contains numerous sand stringers that 

are potential pathways for the migration of young 

waters. 

Although the aquifer system may be protected 

from contamination in some areas, the fact that the 

surficial till is heterogeneous and of varied thickness 

suggests that protection is not uniform. Moreover, it is 

not always possible to predict where the ground water 

is protected and where it is not. Thin sand-and-gravel 

stringers or fractures are rarely noted on driller’s logs, 

partly because they are not always visible in well cuttings. In Figure 4, for example, the driller’s log for the 

domestic well shows that the aquifer is overlain by 

more than 50 ft of homogeneous clay. From this 

description, one might assume that the underlying 

aquifer system is not vulnerable to contamination. 

However, the core from the monitor well shows potential pathways for contaminants: (1) near-vertical fractures that are visible from the land surface to the water 

table, but that probably extend to deeper depths 

(Ruland, 1991; Strobel, 1993; Fleming, 1994), and (2) 

at least four sand stringers that extend for unknown 

distances. Indeed, the sand and gravel at a depth of 31 

ft contained concentrations of nitrate (2.6 mg/L) and 

chloride (350 mg/L) indicative of human influence, 

and the water in the aquifer was recharged after 1953. 

Summary and conclusions

Ground-water quality was assessed in the northeastern 

part of the Corn Belt, where rates of pesticide and fertilizer use are among the highest in the country. Data 

were collected from 30 shallow monitor wells and 18 

co-located domestic wells in areas where tile-drained 

row crops are underlain by glacial till. The study was 

part of the U.S. Geological Survey’s National WaterQuality Assessment of the Lake Erie drainage basin.

Pesticides or pesticide degradates were detected 

in samples from 41 percent of the monitor wells and 6 

percent of the domestic wells. The pesticides detected 

closely correspond to those most heavily applied in the 

Lake Erie-Lake St. Clair Basin—herbicides used on 

corn, soybeans, or wheat. Degradates were detected 

more frequently, and at much higher concentrations, 

than parent compounds. Concentrations were generally low; no pesticide, was detected at a concentration 

greater than a USEPA Maximum Contaminant Level 

(MCL). However, MCL’s do not exist for 9 of the 11 

pesticide compounds detected. 

Nitrate was the only chemical constituent 

detected at a concentration greater than an MCL. The 

MCL was exceeded in 7 percent of samples from monitor wells. In 37 percent of monitor-well samples, 
20 Ground-Water Quality and Vulnerability to Contamination in Selected Agricultural Areas

nitrate was detected at concentrations greater than 

2 mg/L, a level indicative of human influence. The primary source of nitrate is probably agricultural fertilizer, but other possible sources are manure and septicsystem effluent.

Secondary MCL’s, which are based on esthetic 

criteria, were exceeded with respect to chloride, sulfate, dissolved solids, iron, and manganese. High concentrations of chloride are interpreted to be the result 

of human inputs; possible sources are potash fertilizer, 

road salt, and septic-system effluent. In contrast, high 

concentrations of iron are interpreted to be from mineral reactions. High concentrations of sulfate, dissolved solids, and manganese could be due to either 

human activities or mineral reactions in the shallow 

subsurface. 

Detection frequencies of nitrate and pesticides 

are low in comparison to the Midwest as a whole and 

the Nation as a whole. Preliminary data from NAWQA 

studies indicates that ground water in parts of the 

Upper Midwest is minimally contaminated by pesticides and fertilizers, presumably because it is protected by the surficial glacial till and (or) tile drains. 

In the study area, most of the shallow ground 

water is hydraulically connected to the land surface, as 

indicated by the observations that 83 percent of waters 

from monitor wells were recharged after 1953, and 57 

percent contained a pesticide or an elevated nitrate 

concentration. Potential pathways are provided by 

near-vertical fractures and (or) sand-and-gravel stringers within the till. Water-quality data from the current 

study are consistent with hydraulic data from other 

studies that indicate a dynamic local flow system to 

depths of 25 to 35 ft. The minimum casing depth for 

water-supply wells is 25 ft, therefore, water in some of 

the shallowest domestic wells could be vulnerable to 

the effects of land use. 

In some areas, deeper parts of the ground-waterflow system are also hydraulically connected to the 

land surface. Almost half the waters from wells 50 to 

100 ft deep were recharged after 1953. Anthropogenic 

constituents were detected in samples from three 

domestic wells 60 to 121 ft deep. These three wells are 

in areas where the till is coarse grained relative to the 

study area as a whole. 

Over most of the study area, tile drains do not 

intercept all agrichemicals before they reach the water 

table. Near-vertical fractures in the till provide a direct 

pathway for migration of solutes from the land surface 

to the water table. Presumably, tile drains could facilitate the discharge of agrichemicals from the shallow 

ground water and thereby prevent their migration to 

deeper parts of the ground-water-flow system. 

The hydrogeologic system has several 

geochemical characteristics conducive to transformations or sorption of nitrate or pesticides: (1) the till is 

clay-rich, has a high organic-carbon content, and contains an abundance of pyrite, (2) the ground water has 

low dissolved-oxygen concentrations, and (3) iron and 

manganese oxides and oxyhyroxides occur on fractures in the unsaturated zone. 

The aquifer system appears to be protected from 

contamination relative to aquifers beneath most other 

agricultural areas of the country. The nature of the protection may be both physical and geochemical. However, the surficial glacial till is heterogeneous, and 

therefore the protection is not likely to be uniform. 

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