Use of Isotopes to Identify Sources of 

Ground Water, Estimate Ground-Water-Flow Rates, 

and Assess Aquifer Vulnerability in the 

Calumet Region of Northwestern Indiana and 

Northeastern Illinois

Water-Resources Investigation Report 02-4213

Prepared in cooperation with the

U.S. Environmental Protection Agency

U.S. Department of the Interior

U.S. Geological Survey

U.S. Department of the Interior

U.S. Geological Survey

Use of Isotopes to Identify Sources of 

Ground Water, Estimate Ground-Water-Flow Rates, 

and Assess Aquifer Vulnerability in the 

Calumet Region of Northwestern Indiana and 

Northeastern Illinois

By Robert T. Kay, E. Randall Bayless, and Robert A. Solak1

Prepared in cooperation with the

U.S. Environmental Protection Agency

Water-Resources Investigation Report 02-4213

Indianapolis, Indiana

2002

1

MACTEC Engineering and Consulting, Novi, Michigan
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The 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

5957 Lakeside Boulevard

Indianapolis, IN 46278-1996

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver, CO 80225-0286
Contents

Contents iii

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Description of the Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Physiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Land Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Hydrogeologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Unconsolidated Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Bedrock Geology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Calumet Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Confining Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Silurian-Devonian Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Surface-Water and Ground-Water Interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Study Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Sampling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Laboratory Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Distribution of Isotopes in Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Oxygen and Deuterium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Helium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Recharge and Sources of Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Calumet Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Confining Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Silurian-Devonian Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Ground-Water-Flow Rates and Aquifer Vulnerability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Calumet Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Confining Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Silurian-Devonian Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Figures

1.–12. Maps showing:

1. Location of study area and sampling locations, Calumet region of northwestern Indiana and

northeastern Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Location of important topographic features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. Surficial geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5. Thickness of the Calumet aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Contents—Continued

iv Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Figures–Continued

6.–12. Maps showing:

6. Thickness of the confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Bedrock geology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

8. Elevation of bedrock surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

9. Water-table configuration, June 23–25, 1992 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

10. Direction of vertical hydraulic gradients within the Calumet aquifer, June 23–25, 1992 . . . . . . . . . . 26

11. Potentiometric surface of the Silurian-Devonian aquifer, June 23–25, 1992 . . . . . . . . . . . . . . . . . . . 28

12. Direction of surface-water flow, June 23–25, 1992 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

13.–15. Graphs showing:

13. The hydrogen and oxygen isotopic composition of precipitation relative to monthly air

temperature at Midway Airport, Chicago, Ill., 1962–1979 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

14. Isotopic composition of precipitation at Midway Airport, Chicago, Ill., 1962–1979 . . . . . . . . . . . . . 38

15. Isotopic composition of ground water, precipitation, and lake water, July 1997–February 2001 . . . . 39

16. Map showing the del deuterium, del oxygen-18, and tritium values in water samples from the 

Calumet aquifer, July 1997–February 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

17. Section of hydrogen and oxygen isotopic composition and tritium concentrations of ground water 

through a transect at Bairstow Landfill, Hammond, Ind., July 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

18. Map of the del deuterium, del oxygen-18, and tritium values in water samples from the confining 

unit, July 1997–February 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

19. Graph showing the hydrogen isotope composition and depth for samples from wells, July 1997–

February 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

20. Map showing the del deuterium, del oxygen-18, and tritium values in water samples from the 

Silurian-Devonian aquifer, July 1997–February 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

21. Graph of tritium concentrations and depth for samples from wells, July 1997–February 2001. . . . . . . . 49

22. Map showing apparent ground-water age in the Calumet aquifer, July 1997–February 2001 . . . . . . . . . 52

Tables

1. Well descriptions for sampled sites in northwestern Indiana and northeastern Illinois . . . . . . . . . . . . . . 34

2. Isotope data for sites in the study area, northwestern Indiana and northeastern Illinois. . . . . . . . . . . . . . 41
Contents v

Conversion Factors, Vertical Datum, and Abbreviations

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

following equation:

°F = 1.8(°C) + 32

Vertical Datum: In this report, vertical coordinate information is referenced to the National Geodetic Vertical Datum 

of 1929 (NGVD of 1929).

δ (delta), as used in this report, refers to the stable isotope ratio δx=[(Rx/RSTD)-1] x1000, where Rx and RSTD are the 

2H/1H and 18O/16O values of the sample and reference standard. The value of δ is generally expressed in parts per 

thousand (o/oo, or per mil).

Per mil, as used in this report, is equivalent to parts per thousand.

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

foot per day (ft/d) 0.3048 meter per day (m/d)

tritium unit (TU) 3.2 picocuries per liter (pCi/L)

Abstract 1

Use of Isotopes to Identify Sources of Ground Water, 

Estimate Ground-Water-Flow Rates, and 

Assess Aquifer Vulnerability in the Calumet Region of 

Northwestern Indiana and Northeastern Illinois

By Robert T. Kay, E. Randall Bayless, and Robert A. Solak1

Abstract

Isotope data collected in the Calumet 

Region of northwestern Indiana and northeastern Illinois, one of the most heavily 

industrialized regions of the United States, 

indicated that water in the surficial Calumet 

aquifer is well mixed. The Calumet aquifer is 

recharged areally by precipitation and locally 

may be recharged by surface water. The residence time of ground water in the Calumet 

aquifer is approximately 5 to 15 years. Groundwater-flow rates through the Calumet aquifer 

are estimated to be 400 to 2,300 feet per year. 

The permeable deposits, shallow water table, 

lack of an overlying confining unit, and 

proximity to numerous contaminant sources 

indicate that the Calumet aquifer is vulnerable 

to contamination. 

Isotopic data indicate that ground water 

in the confining unit underlying the Calumet 

aquifer is derived from a variety of sources 

that include Lake Michigan, modern precipitation, and discharge from the basal 

Silurian-Devonian bedrock aquifer. The source 

and apparent age of the water are variable 

and appear to be affected locally by various 

geologic and hydraulic factors. The vertical 

ground-water-flow rate through the unweathered part of the confining unit is about 0.20 feet 

per year and is about 6.3 feet per year through 

the weathered part. The data indicate the 

weathered part of the confining unit may be 

more vulnerable to contamination than the 

unweathered confining unit.

Ground water in the basal SilurianDevonian aquifer is derived from Lake 

Michigan, glacial-age water, and modern 

precipitation. Post-1952 recharge has occurred 

in the vicinity of Stony Island, Ill. Groundwater recharge of the Silurian-Devonian 

aquifer may be occurring near Calumet Harbor. 

The Silurian-Devonian aquifer is vulnerable 

to contamination where the confining unit is 

thin or absent or where the integrity of the 

confining unit has been compromised.

Introduction

The Calumet region of northwestern Indiana 

and northeastern Illinois is one of the most heavily 

industrialized areas of the United States (fig. 1). 

Current and historical industries include steel mills, 

petroleum refineries and tank farms, foundries, railroads, and chemical manufacturing (fig. 2). Two 

hazardous-waste incinerators, at least 11 sanitary 

landfills, numerous uncontrolled waste-disposal 

sites, and approximately 80 accidental-spill sites 

are in the region. Low-lying and moist-soil areas 

throughout the region have been filled with slag, 

municipal and industrial waste, dredging spoil, 

construction and demolition debris, fly ash, and 

natural materials (Kay and others, 1997). Chemicals from these and other sources have degraded 

surface- and ground-water quality in the region 

1

MACTEC Engineering and Consultin, Novi, 

Michigan.
STD4

ST1D
Introduction 3

(U.S. Department of Health, Education and 

Welfare, 1965; and HydroQual, Inc., 1985; Fenelon 

and Watson, 1993; Duwelius and others, 1996). 

Because of the presence of wetlands, the proximity 

to Lake Michigan, the vulnerability of aquifers 

that supply water for much of the region, and the 

existence of many facilities that are subject to environmental regulation, there is a need to understand 

the sources of water and vulnerability of various 

geologic units to contamination and to assess the 

potential for remediation. 

To characterize ground-water recharge and 

flow rates as well as aquifer vulnerability in northwestern Indiana and northeastern Illinois, the 

U.S. Geological Survey (USGS), in cooperation 

with the U.S. Environmental Protection Agency 

(USEPA), collected water samples from an areally 

distributed network of ground-water wells and one 

surface-water sampling site. Water samples were 

analyzed for concentrations of tritium (3H) and 

helium (3

He) and isotopic abundances of hydrogen 

(

2

H/1H) and oxygen (18O/16O). The data were used 

to assess the relative age of water in Lake Michigan 

and the hydrogeologic units in the area, the sources 

of water in the hydrogeologic units in the area, the 

interconnection between the various hydrogeologic 

units, and the factors affecting vulnerability of the 

hydrogeologic units to contamination. The insight 

gained as the result of this investigation can be 

used to assess the migration rates and directions of 

anthropogenic compounds in ground water and the 

effectiveness of ground-water remediation efforts.

Purpose and Scope

This report describes the results of an investigation designed to characterize ground-water 

recharge and flow rates and vulnerability of the 

aquifers to contamination in a heavily industrialized area of northwestern Indiana and northeastern 

Illinois. Water samples were collected from a well 

network distributed across northwestern Indiana 

and northeastern Illinois. Samples were collected 

from 26 wells during July 1997 and from 21 different wells and Lake Michigan between January 2000 

and April 2001. Thirty-six of the sampled wells 

were screened in unconsolidated deposits and 16 

were finished in bedrock. Samples were analyzed 

for concentrations of tritium and helium and isotopic abundances of oxygen and hydrogen. Absolute 

and relative ages of ground water, computed 

from sample results, were used to address project 

objectives. 

Acknowledgments

The authors thank the many state and local 

agencies and corporations that provided the information and (or) access to the data-collection sites 

that made this study possible.

ARCADIS Geraghty & Miller, 

Langhorne, Inc., Chicago, Ill.

BP (formerly Amoco), Whiting, Ind.

Bethlehem Steel Corp., 

Burns Harbor, Ind.

Canonie Environmental Services 

Corporation, Porter, Ind.

E.I. duPont deNemours and 

Company, East Chicago, Ind.

Illinois Department of Natural 

Resources, State Water Survey

Indiana Department of Environmental 

Management

Indiana Department of Natural 

Resources, Division of Water

Indiana Department of Transportation

Indiana Geological Survey

Ispat Inland Inc., (formerly Inland 

Steel Co.), East Chicago, Ind.

LTV Corporation, East Chicago, Ind.

National Steel Corporation, 

Midwest Operations, Portage, Ind.

Navistar International Corporation, 

Chicago, Ill.

Northern Indiana Public Service 

Company (NIPSCO), 

Merrillville, Ind.

USX Corporation (formerly United 

States Steel LLC), Gary, Ind.

Waste Management of Illinois, Inc.,

CID Facility, Calumet City, Ill.

The authors also thank Fredrick A. Micke 

and Douglas Yeskis of the U.S. Environmental 

Protection Agency for their assistance with the 

investigation.
4 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 
Introduction 5

Burns

Harbor

Gary

Harbor

Mineral Springs Road
6 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Description of the Study Area

The study area is in northwestern Indiana 

and northeastern Illinois and includes parts 

of Lake and Porter Counties, Indiana, and Cook 

County, Illinois. The study area is approximately 

bounded by the southern limit of the Little Calumet 

River and Interstates 80/94 and 294 to the south, 

Crawford Avenue to the west, Mineral Springs 

Road to the east, and 80th Street and Lake Michigan to the north.

Physiography

The study area is in the Eastern Lake Section 

of the Central Lowland physiographic province 

defined by Fenneman (1938). The Indiana part of 

the study area is in the Calumet Lacustrine Plain 

subdivision of the Northern Moraine and Lake 

Region (Malott, 1922; Schneider, 1966). The Calumet Lacustrine Plain extends westward into the 

Illinois part of the study area where it is called 

the Chicago Lake Plain subsection of the Great 

Lakes Section of the Central Lowland physiographic province (Leighton and others, 1948).

The study area generally has a low relief 

surface and is largely covered by rivers, lakes, and 

poorly drained swampy wetlands. Topographically 

elevated features including dunes, beach ridges, 

hummocky moraine deposits, and bedrock mounds 

break up this flat surface. The Tolleston Beach 

Ridge rises more than 30 ft above the surrounding 

area and trends east-west through the study area 

(fig. 3). Hundreds of smaller, sandy, beach ridges 

and dunes with heights of 5 to 10 ft above 

the surrounding wetlands are present north of the 

Tolleston Beach Ridge which separates the Little 

Calumet and Grand Calumet Rivers in the central 

part of the study area. These ridges and dunes 

roughly parallel the shoreline of Lake Michigan. 

In addition to the dunes and beach ridges, Blue 

Island and Stony Island rise above the lowlands. 

The Calumet Sag Channel is in a glacial sluiceway, 

or outwash channel, between the Tolleston Beach 

Ridge and Blue Island.

Climate

The climate in the study area is classified 

as temperate continental, with a mean annual 

temperature of about 10oC and a mean annual precipitation of 35.7 in. (National Oceanic and 

Atmospheric Administration, 1982). More than 

half of the average annual precipitation falls from 

April 1 through August 31. Although large variations in precipitation and temperature may occur 

in any year, summers generally are hot and humid 

and the winters are cold. Lake Michigan has a 

moderating local effect on temperature.

The National Oceanic and Atmospheric 

Administration (NOAA) maintained two weather 

stations in the study area—one at the Gary, Ind., 

Regional Airport and the other at the Indiana Dunes 

National Lakeshore. From 1951 to 1980, the mean 

monthly temperature at these stations varied from 

about –5oC in January to about 23oC in July, and 

the mean monthly precipitation varied from 1.5 in. 

in February to 4.0 in. in June. Precipitation at the 

Indiana Dunes National Lakeshore was slightly 

higher than at Gary (National Oceanic and Atmospheric Administration, 1982). 

An estimated 70 percent of the average annual 

precipitation in this area is returned to the atmosphere by evapotranspiration (Mades, 1987). Based 

on that information, the average annual precipitation available for recharge to ground water is no 

greater than 10.7 in. More than three-quarters of 

all evapotranspiration occurs during the growing 

season (U.S. Geological Survey, 1970). During 

the growing season, evapotranspiration normally 

exceeds precipitation by about 1 to 2 in. and 

depletes available soil moisture. During the nongrowing season, precipitation generally exceeds 

evapotranspiration by about 11 in., replenishes soil 

moisture, and recharges ground water. The mean 

annual evaporation from Lake Michigan is 29.5 in., 

or about 83 percent of the average annual precipitation.

Land Use

Land use in the study area is primarily residential and industrial. Large tracts of open water, 
Description of the Study Area 7

natural land, and land for the processing and disposal of wastes also are present. Much of the land 

along Lake Michigan and the Calumet River is or 

was used for steel production. Land used by the 

petrochemical industry for tank farms and petroleum refining is south and west of the steel mills 

in Indiana and at scattered locations along the 

Grand Calumet River, the Calumet Sag Channel, 

and Lake Calumet in Illinois. A variety of other 

industries including automobile assembly, scrap 

processing, and chemical manufacturing are in 

the study area. Several large sanitary landfills, 

wastewater-treatment plants, and unregulated 

waste-disposal sites also are present.

Hydrogeologic Setting

The geology and hydrology of the study area 

have been described by a number of investigators 

(Bretz, 1939, 1955; Rosenshein and Hunn, 1968; 

Willman, 1971; Hartke and others, 1975; Watson 

and others, 1989; Cravens and Zahn, 1990; Kay and 

others, 1996; Kay and others, 1997). Their descriptions include lithologic and hydrologic data and 

form the basis for the following discussion of the 

hydrogeologic setting. 

The geologic deposits of concern to this investigation are bedrock units of the Silurian and 

Devonian Periods and unconsolidated deposits of 

the Quaternary Period. The stratigraphic nomenclature used in this report is from the Illinois State 

Geological Survey (Willman and Frye, 1970; Willman and others, 1975) and the Indiana Geological 

Survey (Shaver and others, 1970, 1986; Brown and 

Thompson, 1995).

Unconsolidated Geology

Most unconsolidated sediments in the study 

area originally were the result of continental glaciation or were lake-bottom and near-shore deposits 

of historic Lake Michigan (Willman, 1971; Hartke 

and others, 1975). Glacial and lacustrine processes 

resulted in the deposition of three types of materials: fluvial and eolian sand, lacustrine silt and clay, 

and glacial till (fig. 4). The thickness of unconsolidated sediments in the study area ranges from less 

than 1 ft in the vicinity of Thornton Quarry to more 

than 200 ft east of Burns Harbor (figs. 5 and 6).

Sand and sand-and-gravel deposits of the 

Parkland Sand, the Dolton and Carmi Members, 

and an unnamed glacial sluiceway generally occur 

at or near land surface and overlie fine-grained 

deposits in the area east of Lake Calumet. The 

thickness of these deposits generally increases from 

west to east, ranging from less than 1 ft in most of 

the study area west of Lake Calumet to about 100 ft 

at the Indiana Dunes National Lakeshore (fig. 5).

The Parkland Sand has been described as a 

well-sorted, medium-grained sand that was blown 

from the glacial outwash and beach deposits into 

dunes and sheet-like deposits around the dunes 

(Willman, 1971). The Parkland Sand is found along 

the Tolleston Beach Ridge and the western flank 

of Blue Island and at the Indiana Dunes National 

Lakeshore. The Parkland Sand is equivalent to the 

dune facies of the Atherton Formation in Indiana 

(Shaver and others, 1970). The Parkland Sand 

(Atherton Formation) forms the upper part of the 

surficial Calumet aquifer.

A glacial sluiceway, eroded into and—in some 

areas—through surficial till deposits along the Calumet Sag Channel, contains fluvial sand-and-gravel 

deposits overlain by muck and silt (fig. 4). The 

maximum thickness of these sands and gravels is 

about 25 ft (fig. 5). Glacial outwash deposits of 

sand and gravel also are found along the path of the 

Little Calumet River in parts of Indiana. In Indiana, 

outwash and sluiceway deposits are part of the 

Martinsville Formation described by Shaver and 

others (1970).

The Equality Formation is subdivided into the 

Dolton and Carmi Members. The Dolton Member 

is equivalent to the beach and shoreline deposits 

of the Atherton Formation (Schneider and Keller, 

1970; Willman, 1971). The Dolton Member is predominantly sand but contains thin, discontinuous 

beds of muck and peat as well as pebbly sand and 

gravel. The sands consist of shore and shallowwater lake deposits and commonly are found in 

ridges that define the former locations of spits and 

beaches. The Dolton Member is at the land surface
8 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Little
Description of the Study Area 9

Description of the Study Area 11
12 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

.
Description of the Study Area 13
14 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Lit tle

.
Description of the Study Area 15
16 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

in most of the area east of the Calumet River and at 

sporadic locations west of Lake Calumet. The Dolton Member underlies the Carmi Member between 

the Illinois/Indiana State line and the eastern shore 

of Lake Calumet and along parts of the Little Calumet River. Together, the Parkland Sand and the 

Dolton member of the Equality Formation (equal 

to the Atherton of Indiana), combine to form the 

Calumet aquifer. 

The Carmi Member consists of finely bedded 

or laminated lake deposits of silt and clay with 

localized peat beds and is the surficial deposit in 

much of the area around Lake Calumet and parts 

of the Little Calumet River. The Carmi Member 

underlies the Dolton Member near the confluence 

of the Calumet, Grand Calumet, and Little Calumet 

Rivers (Woodward-Clyde Consultants, 1984) and 

most of the Indiana part of the study area (Watson 

and others, 1989). The Carmi Member is equivalent 

to the lacustrine facies of the Atherton Formation 

(Schneider and Keller, 1970; Willman, 1971). 

The Wadsworth Till Member overlies the 

Equality Formation (Shaver and others, 1970; 

Willman, 1971) and is comprised primarily of gray 

clayey till. The till is very hard and tends to become 

denser and more consolidated with depth. The 

Wadsworth Till Member is part of the Wedron Formation in Illinois (Willman, 1971) and composes 

part of the Lagro Formation in Indiana (Shaver 

and others, 1970). The Wadsworth Till Member 

is present at land surface at Blue Island and east 

of the Calumet Lagoons. Throughout most of the 

study area, the Wadsworth Till periodically was 

submerged and reworked by wave action (Willman, 

1971; Watson and others, 1989).

Throughout much of the study area, lenticular 

bodies of poorly sorted gravel, sand, and silt overlie 

the bedrock. These deposits informally are known 

as the Lemont Drift in Illinois (Cravens and Zahn, 

1990). The exact age of these deposits is unknown, 

but they appear to have been eroded and weathered 

before being covered by sediments during subsequent glacial advances.

The Lemont Drift, Wadsworth Till Member, 

and Carmi Member (where not underlain by sand) 

constitute a continuous unit of fine-grained unconsolidated material overlying the bedrock in almost 

all of the study area. These fine-grained deposits are 

absent in small areas northeast of Stony Island and 

west of the Little Calumet River in Illinois, but they 

are more than 150 ft thick beneath much of the eastern part of the study area (fig. 6). In the western 

part of the study area, the Lemont Drift and the 

Wadsworth Till Member constitute most of the finegrained material, with the Carmi Member typically 

being less than 15 ft thick (Land and Lakes Co., 

1988). In those parts of the study area where finegrained deposits are within about 10 ft of the 

land surface, the upper part of the unit typically 

is weathered. An extensive network of vertical fractures, macropores, soil joints, and root channels 

characterizes the weathered zone (Ecology and 

Environment, Inc., 1990). The size and number of 

the weathering features decrease with depth, and 

these features appear to be absent below about 30 ft 

(Ecology and Environment, Inc., 1990).

The unconsolidated deposits in the study area 

have been altered extensively by anthropogenic 

activities. Substantial volumes of unconsolidated 

geologic material have been removed during quarrying, tunneling, and excavating for buildings and 

landfills. In addition, the surficial geology also has 

been modified by the deposition of large amounts of 

fill including slag, dredging spoil, construction and 

demolition debris, and municipal wastes. These 

activities have combined to disrupt the spatial continuity of the unconsolidated deposits, particularly 

near the land surface, modifying the surface topography (Kay and others, 1997) and disrupting the 

shallow hydrologic system.

Bedrock Geology

The bedrock that underlies unconsolidated 

deposits in the study area is from the Silurian 

and Devonian Periods (fig. 7). The bedrock units 

comprise primarily dolomite, limestone, and shale. 

The strata are essentially horizontal, except in the 

northeastern part of the study area where the bedrock strata dip slightly toward the northeast.
Description of the Study Area 17

The Upper Devonian Antrim Shale is the 

youngest bedrock unit and is at the bedrock surface 

in the Porter County, Ind., part of the study area. 

The Antrim Shale consists of brown to black, 

noncalcareous shale with gray calcareous shale 

or limestone in the lower part of the formation, and 

it paraconformably overlies the Traverse Formation 

(Shaver and others, 1986).

The lower to middle Devonian deposits 

include the Detroit River and Traverse Formations. These units unconformably overlie the 

Niagaran Series in the eastern part of the study 

area. The Traverse Formation unconformably overlies the Detroit River Formation. The Traverse 

Formation consists of brown to gray, fine- to 

coarse-grained limestone to dolomitic limestone 

(Shaver and others, 1986). The Detroit River Formation varies from a light-colored, fine-grained, 

sandy dolomite near the base of the formation to 

a gray to dark-brown dolomite and limestone with 

thin to massive beds of gypsum and anhydrite in 

the upper part of the formation (Shaver and others, 

1986). Both formations thicken toward the northeast.

The oldest bedrock strata of concern to this 

investigation are Silurian carbonates (dolomites 

and limestones) of the Niagaran Series. The Niagaran carbonates are up to 300 ft thick in the study 

area and are present at the bedrock surface in Illinois and western Indiana. These deposits are known 

as the Wabash Formation in Indiana (Shaver and 

others, 1986) and the Racine Dolomite in Illinois 

(Willman, 1971).

Large reef structures present at the land 

surface at Stony Island and Thornton Quarry characterize the Niagaran carbonates. The reefs are 

made up of a vuggy dolomite with traces of argillaceous material and sand. Away from the reefs, the 

Niagaran deposits consist of dense, cherty, argillaceous dolomite and limestone with local lenses of 

green shale.

The Niagaran carbonates contain an irregularly distributed network of vertical fractures with 

a major trend at N. 47 W. and a minor trend at about 

N. 57 E. (Zeizel and others, 1962). Fractures generally are more abundant near the rock surface, where 

the rock is more weathered, and decrease in number 

with depth as the rock becomes more competent 

(Suter and others, 1959). The reef deposits tend to 

have fewer fractures than the inter-reef deposits.

In addition to fractures, vertical faults have 

been identified in the bedrock in Illinois (fig. 7). 

Faulting has offset the bedrock strata as much as 

30 ft, but displacement does not extend upward into 

the unconsolidated deposits (Keifer and Associates, 

1976). The extent of faulting in Indiana is unknown.

The relief of the bedrock surface varies by 

more than 175 ft in the study area (fig. 8). Bedrock 

highs are present at Stony Island and Thornton 

Quarry. Bedrock lows are present near Burns Harbor, Gary Harbor, and the Indiana Harbor Canal. 

Bedrock highs at Stony Island and Thornton Quarry 

are attributed to the greater erosion resistance of 

the reef deposits in these areas (Bretz, 1939). The 

bedrock valleys may mark the paths of pre-glacialage drainage systems (Bretz, 1939).

The bedrock deposits have been altered extensively by anthropogenic activities in the western 

part of the study area. Substantial volumes of bedrock material have been removed during quarrying 

and tunneling. These activities have combined 

to disrupt the continuity of the bedrock geologic 

deposits and to modify the hydrology in the bedrock aquifer (Kay and others, 1996).

Ground Water

The hydrogeologic units of concern to this 

investigation are the surficial Calumet aquifer, the 

underlying confining unit, and the basal SilurianDevonian aquifer.

Calumet Aquifer

Surficial sands and permeable fill deposits 

compose the Calumet aquifer (Hartke and others, 

1975). The Calumet aquifer is under water-table 

conditions; it is continuous throughout most of 

the area east of Lake Calumet but is present only 

at scattered locations west of Lake Calumet. The 

thickness of the Calumet aquifer increases from 

west to east in the study area, with a maximum

Description of the Study Area 19 Mineral Springs Road
20 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Litt le
Description of the Study Area 21
22 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

thickness of about 70 ft along the Lake Michigan 

shoreline near the Grand Calumet Lagoons. In the 

extreme eastern part of the study area, a confining 

unit is present between the upper and lower parts 

of the aquifer.

The Calumet aquifer is recharged by direct 

infiltration of precipitation and is the primary 

pathway for lateral ground-water flow in the unconsolidated deposits (Watson and others, 1989; 

Cravens and Zahn, 1990; Fenelon and Watson, 

1993; Greeman, 1995). Water in the Calumet aquifer generally flows from topographic highs toward 

topographic lows (fig. 9) (Fenelon and Watson, 

1993; Greeman, 1995; Kay and others, 1996) and 

from higher water-table elevations to lower elevations. Local changes in this pattern are a result of 

perturbations in natural ground-water flow, including ground-water recharge from landfill leachate 

and ponded water; ground-water discharge to sewer 

lines and ditches; and pumping centers at quarries, 

underpasses, and ground-water remediation sites. 

Horizontal ground-water velocities in the Calumet 

aquifer have been estimated at 3.6 to 120 ft/yr (Kay 

and others, 1996).

The depth to the water table in the Calumet 

aquifer ranges from near land surface along the 

shoreline of Lake Michigan to more than 100 ft 

beneath dunes along the lake shore in the eastern 

part of the study area. The depth to water in the 

Calumet aquifer in most of the study area is less 

than 15 ft (Greeman, 1995; Kay and others, 1996). 

Natural discharge from the Calumet aquifer 

primarily is to the rivers, lakes, and wetlands in the 

study area. Evapotranspiration also constitutes a 

major portion of the total discharge during spring 

and summer (Rosenshein and Hunn, 1968; Greeman, 1995). Water from the Calumet aquifer also 

discharges to the underlying confining unit in most 

of the study area. In addition to natural discharge, a 

substantial volume of water from the Calumet aquifer discharges to sewers and storm drains (Fenelon 

and Watson, 1993).

No clear pattern is evident in the direction 

of vertical flow within the Calumet aquifer. Downward flow is indicated by ground-water levels 

measured south of Burns Harbor; on the peninsula 

east of Indiana Harbor; and between the Grand Calumet River, the Indiana Harbor Canal, Gary Harbor, 

and Lake Michigan (fig. 10). Upward flow is indicated along parts of the ground-water divide at the 

Tolleston Beach Ridge between the Grand Calumet 

and Little Calumet Rivers and near Lake Michigan. 

Horizontal flow is indicated in most of the area near 

Wolf Lake. Flow within the Calumet aquifer in the 

area between Lake George, Lake Michigan, and 

the Indiana Harbor Canal is directed upward, downward, or horizontally. Vertical flow in the Calumet 

aquifer appears to be affected primarily by pumping 

and drainage to sewers (Fenelon and Watson, 

1993).

Confining Unit

The Antrim Shale, glacial till, lacustrine silt 

and clay deposits, and low-permeability fill deposits compose the confining unit. The water table 

is in the confining unit in most of the area west of 

Lake Calumet where the Calumet aquifer is absent. 

The confining unit is more than 200 ft thick in 

Porter County and is thin (less than 20 ft thick) 

or absent near Stony Island, Thornton Quarry, and 

in isolated areas south of Blue Island. Except for 

small areas northeast of Stony Island and south 

of Blue Island, the confining unit underlies the 

Calumet aquifer and restricts flow between 

the Calumet aquifer and the underlying SilurianDevonian aquifer (Kay and others, 1996). In those 

parts of the study area where the fine-grained 

deposits are within 10 ft of the land surface, the 

upper part of the confining unit is more weathered 

and is more permeable than at depth. Rosenshein 

(1963, p. 22) estimated the average vertical hydraulic conductivity of the confining unit in Lake 

County at 4.0 x 10-4 ft/d. 

The water-table configuration in the confining 

unit generally is a subdued reflection of surface 

topography. Ground water generally flows from 

topographic highs to topographic lows, typically 

from topographic ridges to the surface-water bodies 

(Kay and others, 1996). Local changes in this pattern result primarily from enhanced recharge to 

ground water beneath landfills and surface-water 

impoundments. 
Description of the Study Area 23

The confining unit is recharged by groundwater flow from the Calumet aquifer and infiltration 

from precipitation where the Calumet aquifer is 

absent. Discharge from the confining unit primarily 

is to the underlying Silurian-Devonian aquifer, with 

some discharge to the rivers, lakes, and wetlands 

in the study area. In the shallow part of the confining unit where the Calumet aquifer is absent, 

evapotranspiration constitutes a major part of the 

discharge during spring and summer (Rosenshein 

and Hunn, 1968). The depth of the water table in 

the confining unit is typically about 15 ft but ranges 

from near land surface around Lake Calumet to 

about 27 ft near some of the landfills (Kay and 

others, 1996). 

The vertical distribution of water levels in the 

study area indicates the potential for flow from 

the water table or the base of the Calumet aquifer 

down into the confining unit in most of the study 

area (Kay and others, 1996). Vertical flow through 

the weathered and unweathered parts of the confining unit is greater than the horizontal flow. The 

horizontal ground-water velocity at the water table 

in the confining unit was calculated to range from 

0.16 to 0.37 ft/yr (Kay and others, 1996). The vertical ground-water velocity through the weathered 

and unweathered parts of the confining unit is estimated to be 13.9 and 0.55 ft/yr, respectively (Kay 

and others, 1996).

Silurian-Devonian Aquifer

The Silurian-Devonian aquifer is composed 

of dolomite and limestone strata that underlie 

the confining unit in most of the study area. The 

Silurian-Devonian aquifer is unconfined at Stony 

Island and Thornton Quarry. In small areas northeast of Stony Island and south of Blue Island, the 

confining unit is absent and the Silurian-Devonian 

aquifer is in direct hydraulic connection with the 

Calumet aquifer. In the rest of the study area, the 

Silurian-Devonian aquifer is semi-confined. Movement of ground water within the Silurian-Devonian 

aquifer is primarily through an interconnected 

network of joints, fissures, faults, bedding-plane 

openings, and solution cavities in the bedrock. 

Prior to industrial and urban development of 

the study area, which began about 1870, water 

levels in the Silurian-Devonian aquifer are thought 

to have been substantially higher than at present; 

discharge from the Silurian-Devonian aquifer 

appears to have been to Lake Michigan through the 

confining unit and the Calumet aquifer. Pumping 

from the Silurian-Devonian aquifer for industrial 

supply and dewatering of deep tunnels in the study 

area has lowered water levels in the aquifer substantially (Rosenshein, 1963; Watson and others, 

1989; Cravens and Zahn, 1990; Fenelon and Watson, 1993; Kay and others, 1996), particularly in 

the vicinity of Lake Calumet. By the early 1980s, 

the Silurian-Devonian aquifer had been abandoned 

as a source for industrial water supply, but dewatering of deep tunnels continues. Recharge to the 

Silurian-Devonian aquifer from the confining unit 

may be a recent (within the past 100 years) phenomena, resulting from overdrafts of the SilurianDevonian aquifer (Rosenshein, 1963).

Water-level measurements in the SilurianDevonian aquifer made in 1992 indicate the 

potentiometric surface is highest at the bedrock 

high near Stony Island and in the southeastern part 

of the study area (fig. 11). The potentiometric highs 

are separated by a depression centered near the confluence of the Little Calumet and Grand Calumet 

Rivers. This depression appears to be centered at 

a drop shaft open to the aquifer that has been used 

to dewater the formation. The depression in the 

potentiometric surface around Thornton Quarry is 

attributed to excavation and pumping at the quarry. 

The average linear ground-water velocity through 

the uppermost 20 ft of the Silurian-Devonian aquifer is calculated to range from 5.1 to 10.6 ft/yr (Kay 

and others, 1996). 

In areas where a confining unit overlies the 

Silurian-Devonian aquifer, water-level data indicate the potential for ground water to flow from 

the Calumet aquifer and the confining unit into 

the Silurian-Devonian aquifer (Kay and others, 

1996). Where the confining unit is absent, recharge 

is from the Calumet aquifer or direct infiltration 

from precipitation.
24 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Little

Calumet

River
Description of the Study Area 25
26 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 
Description of the Study Area 27
28 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 
Description of the Study Area 29
30 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Some ground water may discharge from 

the Silurian-Devonian aquifer to Lake Michigan 

through the confining unit and the Calumet aquifer 

in the eastern half of the study area (Watson and 

others, 1989). Water levels in the Silurian-Devonian 

aquifer, however, are lower than levels in Lake 

Michigan in most of the area between the Indiana 

Harbor Canal and Stony Island, indicating the 

potential for water to flow from the lake into 

the aquifer. 

Surface Water

The Grand Calumet, Little Calumet, and Calumet Rivers are the principal drainages in the study 

area (fig. 12). Surface-water flow in most of the 

Grand Calumet and part of the Little Calumet Rivers is to Lake Michigan through the Indiana Harbor 

Canal and Burns Harbor, respectively. Surfacewater flow in a western part of the Grand Calumet 

River (2 mi east of the Illinois/Indiana State line) 

and the western part of the Little Calumet River 

(4 to 5 mi east of the Illinois/Indiana State line), 

however, is west to the Calumet Sag Channel in 

Illinois. Surface-water flow of the Calumet River 

is spatially and temporally variable, often being 

toward Lake Calumet south of the confluence with 

Squaw Creek and toward Lake Michigan north of 

the confluence. A decline in the level of Lake Michigan by as little as 0.5 ft can produce a hydraulic 

gradient capable of shifting the surface-waterdrainage divide in the Little Calumet and Grand 

Calumet Rivers to the west and reversing flow 

in the Calumet River (U.S. Department of Health, 

Education and Welfare, 1965, p. 60; Greeman, 

1995). Conversely, a rise in lake level could 

increase the amount of flow from Lake Michigan 

into the Calumet River and Lake Calumet and shift 

the flow divides on the Grand Calumet and Little 

Calumet Rivers to the east. 

Surface-water gradients on the Calumet and 

Grand Calumet Rivers and the Indiana Harbor 

Canal averaged less than 0.5 ft/mi in June 1992 

(Kay and others, 1996). Surface-water gradients 

on the Little Calumet River averaged 0.7 ft/mi 

in June 1992 (Kay and others, 1996). The low 

hydraulic gradients on the rivers indicate sluggish 

flow. The flow of the Grand Calumet River primarily is derived from industrial and wastewater 

discharge (HydroQual, 1985). Because Lake Michigan is the primary source of potable water in the 

study area, virtually all wastewater discharge (and 

therefore most of the water in the Grand Calumet 

River) ultimately is derived from Lake Michigan.

With the exception of Lake Michigan, the 

lakes in the study area partially or completely are 

isolated from the surface-water-flow system. Water 

is delivered to Lake Calumet through manmade 

drainage channels, storm sewers, recharge from 

ground water, and flow from and to the Calumet 

River. Water originally pumped from Lake Michigan is discharged to Wolf Lake by industries along 

the northern arm of the lake. Squaw Creek, a shallow, manmade drainage ditch connects Wolf Lake 

to the Calumet River. Therefore, most of the 

water in Wolf Lake and Squaw Creek ultimately 

is derived from Lake Michigan. Wolf Lake, Lake 

George, and the smaller lakes scattered throughout 

the region are either in topographic depressions 

between dune and beach ridge deposits or in pits 

created by mining of sand and clay. 

Surface-Water and Ground-Water Interactions

Interactions between surface water and 

ground water in the study area are complex. Shallow ground-water flow typically is toward the major 

surface-water bodies, indicating the potential for 

ground-water discharge to surface water in most of 

the region. Observation of ground-water levels in 

wells paired with surface-water gages, however, 

indicates the potential for frequent periodic reversals of surface-water/ground-water flow along 

some river reaches in the study area (Greeman, 

1995) and at Lake Michigan near Calumet Harbor. 

Precipitation, variations in the quantity of industrial 

and wastewater discharge to the rivers, evapotranspiration, and fluctuations in the stage of Lake 

Michigan affect the timing and location of the flow 

reversals.

Interpretation of the interaction between surface water and ground water also is complicated 

by the presence of sheet piles driven through the 

Calumet aquifer along much of shorelines of Lake
Study Methods 31

Michigan and Lake Calumet, the Calumet River, 

the Calumet Sag Channel, and the Indiana Harbor 

Canal. The sheet piles form a barrier to the flow 

of surface water and ground water, forcing water to 

move under the wall or through cracks, holes, and 

joints in the sheet piles. Surface-water/groundwater flow across the sheet piles is probably small, 

compared to flow between the Calumet aquifer and 

surface-water bodies where pilings are absent.

Recharge of ground water from the Calumet 

aquifer accounts for approximately 10 percent of 

the water in the Grand Calumet River (Fenelon and 

Watson, 1993). Water in the confining unit has the 

potential to discharge to surface water in most of 

the study area where the confining unit is at or near 

the land surface (Kay and others, 1996). Calculations indicate, however, that recharge from the 

confining unit accounts for less than 1 percent of 

the flow in the rivers in the study area.

Study Methods

Water samples were collected from 26 wells 

during July 1997 and 21 different wells during 

January–February 2000 and April 2001. In addition, one sample was collected from Lake Michigan 

near the Illinois/Indiana State line, south of Calumet Harbor in January 2000 (table 1).

Sampling Methods

All samples from wells open to the Calumet 

aquifer and the Silurian-Devonian aquifer were collected with submersible sampling pumps after at 

least three well volumes were purged from the well. 

The temperature, pH, specific conductance, redox 

potential, and dissolved-oxygen concentration of 

the purge water were measured to ensure parameter 

stabilization prior to sample collection. Wells open 

to the confining unit could not sustain discharge and 

were sampled with low-flow submersible sampling 

pumps. The low-flow submersible pumps were lowered into the screened interval of the well and the 

sample was collected after the volume of water in 

the discharge line was purged at least twice. Samples were collected and stored in accordance with 

standard USGS procedures (Wilde and others, 

1999).

Samples for determination of tritium (3H), 

oxygen (18O/16O), hydrogen (2

H/1H), and tritiogenic helium (3He) initially were collected 

throughout the study area during July 1997. The 

samples from Illinois, however, were damaged 

during shipping and were re-collected during January and February 2000. Additional samples were 

collected in Illinois during April 2001. Additional 

samples for the analysis of 3

H, 18O/16O, and 2H/1H 

were collected in Indiana during February 2001.

Ground-water samples collected in Indiana 

in July 1997 were submitted to the National Risk 

Management Laboratory of the U.S. Environmental Protection Agency in Cincinnati, Ohio, in July 

1998 for analysis. (There is no holding time for 

these samples.) All other samples were analyzed 

by the USGS National Water Quality Laboratory 

in Denver, Colo. The analysis included samples 

collected in Illinois in January and February 2000 

and submitted for analysis in March 2000, samples 

collected in Illinois in April 2001 and submitted 

for analysis in May 2001, and samples collected 

in Indiana during February 2001 and submitted for 

analysis in February 2001. Ground-water samples 

for 3

He analysis were submitted to the LamontDoherty Earth Observatory at Columbia University 

in December 2000. 

Laboratory Methods

All Indiana samples and the Illinois samples 

from wells open to the Calumet aquifer or the water 

table in the confining unit were analyzed based 

on USGS schedule 460, with a detection limit of 

1.8 tritium units (TU). Tritium samples from 

Illinois wells open to the Silurian-Devonian aquifer 

and the lower part of the confining unit were analyzed using USGS schedule 624, with a detection 

limit of 0.78 TU. The 2

H/1H and 18O/16O analyses 

were performed using USGS schedule 1142, and 

the 3He analyses were performed using USGS 

schedule 1033.
32 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 
Study Methods 33
34 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Table 1. Well descriptions for sampled sites in northwestern Indiana and northeastern Illinois

[DUP, quality-assurance replicate sample; NA, not applicable; CA, Calumet aquifer; MCU, mid-depth confining unit; WTCA, water table; 

CA, Calumet aquifer; SD, Silurian-Devonian bedrock aquifer; SW, surface water; SL, slag]

Well name

Date 

of 

sample 

collection

Latitude/

longitude

U.S. Geological Survey 

 site-identification 

number

Landsurface 

altitude 

(NGVD of 

 1929)

Well 

open 

interval 

(feet 

below 

land 

surface)

Hydrologic 

unit

A20 7/09/1997 413503/871935 413503087193501 614 21 -24 CA

B7 7/15/1997 413617/872252 413617087225202 595 8 -11 CA

B8 7/15/1997 413617/872252 413617087225201 595 32 -37 CA

B8 (DUP) 7/15/1997 413617/872252 413617087225201 595 32 -37 CA

B10 7/09/1997 413544/872337 413544087233700 606 17 -20 CA

BH1D 1/18/2000 414238/874011 414238087401102 667 75 -90 MCU

BH1S 1/18/2000 414238/874011 414238087401101 667 5 -15 WTCA

BH7D 1/18/2000 413910/873358 413910087335803 587 69 -79 SD

BH7I 1/18/2000 413910/873358 413910087335802 587 19 -29 MCU

BH7S 1/18/2000 413910/873358 413910087335801 587 7 -17 WTCA

BH13 7/09/1997 413548/872040 413548087204001 603 9 -19 CA

BH16D 1/10/2000 414250/873624 414250087362402 599 68 -78 SD

BH18D 1/18/2000 414316/873131 414316087313103 595 58 -68 SD

BH18D (DUP) 1/18/2000 414316/873131 414316087313103 595 58 -68 SD

BH18I 1/18/2000 414316/873131 414316087313102 594 35 -45 MCU

BH18S 1/18/2000 414315/873131 414315087313101 594 8 -18 WTCA

BH31 7/16/1997 413947/873025 413947087302501 598 15 -28 SL&CA

BH32D 7/16/1997 413949/873019 413949087301901 597 36 -38.5 CA

BH32D (DUP) 7/16/1997 413949/873019 413949087301901 597 36 -38.5 CA

BH32I 7/16/1997 413949/873019 413949087301902 597 27.5-30 CA

BH32S 7/14/1997 413949/873019 413949087301903 597 21 -23.5 CA 

BH32SL 7/14/1997 413949/873019 413949087301904 597 17.5- 2 SL

BH33I 7/16/1997 413951/873019 413951087301901 585 13.5- 1 CA

BH33S 7/16/1997 413951/873019 413951087301902 585 7.5-10 CA

BH33SL 7/16/1997 413951/873019 413951087301903 585 2.5- 5 SL
Table 1. Well descriptions for sampled sites in northwestern Indiana and northeastern Illinois—Continued

Study Methods 35

BR1 7/10/1997 413328/872023 413328087202301 595 145 SD

BR2 2/07/2001 413437/871506 413437087150601 600 147 SD

BR3 7/11/1997 413419/873017 413419087301701 595 140 SD

BR4 20/7/2001 413716/872326 413716087232601 595 147 SD

BR5 2/08/2001 413732/872558 413732087255801 587 156 SD 

C1 7/17/1997 413830/872600 413830087260000 586 4.5- 9.5 CA

C3 7/17/1997 413828/872513 413828087251301 595 23 - 28 CA

D10 7/18/1997 414043/872908 414043087290802 588 27.5- 32.5 CA

D11 7/18/1997 414043/872908 414043087290801 588 22 - 27 CA

D21 7/17/1997 413941/872926 413941087292600 584 18 - 23 CA

D67 7/10/1997 413647/872852 413647087285202 588 4 - 7 CA

D68 7/10/1997 413647/872852 413647087285201 588 18 - 23 CA

D68 (DUP) 7/10/1997 413647/872852 413647087285201 588 18 - 23 CA

D75 7/10/1997 413435/872919 413435087291901 600 5 - 8 CA

FILO102-5 1/18/2000 414023/873634 414023087363402 593 73 - 83 SD

FILO102D 1/18/2000 414021/873634 414021087363402 600 80 - 90 SD

GO5D 1/17/2000 413829/873423 413829087342301 592 83 - 93 SD

G213 1/17/2000 413834/873433 413834087343301 593 10 - 19 SD

G218 1/17/2000 413823/873429 413823087342903 585 66 - 75 SD

IC 1/10/2000 413929/873522 413929087352201 590 53 - 58 SD

IC (DUP) 1/10/2000 413511/873522 413511087352201 590 53 - 58 SD

IP 1/10/2000 414132/873659 414132087365901 591 72 - 77 SD

R251 1/17/2000 413824/873411 413824087341101 589 35 - 45 MCU

R253 1/17/2000 413823/873429 413823087342902 586 32 - 42 MCU

ST1D 4/04/2001 414106/873355 414106087335502 588 98 -115 SD

ST4D 4/04/2001 414104/873431 414104087343102 590 80 -103 SD

WS1 2/16/2000 414238/873320 414238087332002 588 20 - 25 MCU

WS2 2/16/2000 414158/873316 414158087331601 588 4 - 15 WTCA

Lake Michigan1 1/18/2000 414315/873130 414315087313001 582 NA SW

Well name

Date 

of 

sample 

collection

Latitude/

longitude

U.S. Geological Survey 

 site-identification 

number

Landsurface 

altitude 

(NGVD of 

 1929)

Well 

open 

interval 

(feet 

below 

land 

surface)

Hydrologic 

unit

1

Lake Michigan site sampled from the lake surface.
36 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

Quality Assurance

Duplicate samples were collected from five 

wells to help verify the reproducibility of the sampling techniques. Tritium concentrations were 

identical in duplicate samples. The δ18O values 

varied by less than 0.3 per mil (0.003 percent) 

between samples. The δ2H values varied by less 

than 5 per mil (0.05 percent). Replicate estimates 

of ground-water age at a single well site, based on 

3

H/3He measurements, were 9.90 and 9.95 years.

Distribution of Isotopes in Ground Water

The structure of an atom is defined by the 

number of protons (which defines the element) and 

the number of neutrons (which defines the isotope 

of the element). The sum of the protons and the 

neutrons for a given element is its atomic weight. 

Oxygen and Deuterium

Most oxygen molecules have 8 protons and 

8 neutrons, for an atomic weight of 16 (16O). About 

0.20 percent of oxygen atoms have 10 neutrons, for 

an atomic weight of 18 (18O) (Freeze and Cherry, 

1979). Hydrogen (1

H) has one proton in its nucleus. 

About 0.016 percent of hydrogen atoms are in the 

form of deuterium (2

H), which has a proton and a 

neutron in its nucleus (Freeze and Cherry, 1979). 

The 16O, 18O, 1

H, and 2H isotopes are the main 

isotopes that make up the water molecule. These 

isotopes of oxygen and hydrogen are stable and 

do not disintegrate by radioactive decay. 

Stable isotopes of oxygen and hydrogen in the 

water molecule are measured as the ratio of the two 

most abundant isotopes of a given element, 2

H/1H

and 18O/16O, relative to a reference standard. These 

ratios are expressed in delta units (δ) as parts per 

thousand (per mil, written as o/oo). The general 

expression for stable isotope notation is

δx = [(Rx/RSTD)-1] x 1000,

where Rx and RSTD are the 2H/1H and 18O/16O 

of the sample and reference standard, 

respectively. 

The reference standard (RSTD) for oxygen and 

hydrogen isotopes is Standard Mean Ocean Water 

(Freeze and Cherry, 1979). For example, an oxygen 

sample with an δ18O value of –50 o/oo is depleted in 

18O by 5 percent or 50 o

/oo relative to the standard.

The difference in the mass of oxygen and 

hydrogen isotopes in water results in distinct partitioning of the isotopes (fractionation) as a result 

of evaporation, condensation, freezing, melting, 

or chemical and biological reactions. For example, 

δ2

H and δ18O values in precipitation are isotopically lighter in areas with lower mean annual 

temperature. Strong seasonal variations are expected at any given location (Dansgaard, 1964). 

Average annual values of δ2H and δ18O in precipitation, however, show little variation at any one 

location.

The International Atomic Energy Agency 

monitored the oxygen and hydrogen isotopic composition of precipitation at Chicago’s Midway 

Airport from January 1962 through October 1979; 

the data are accessible through the World Wide 

Web at:

ftp://ftp.iaea.org

Midway Airport is approximately 10 mi 

northwest of Lake Calumet. The isotopic composition of precipitation in the study area is assumed to 

be identical to that of Midway. The δ2

H and δ18O 

in precipitation at Midway Airport are lowest in 

January (mean values of –13.5 and –95.3 o/oo, 

respectively), when air temperatures are coolest, 

and highest in July (–2.4 and –17.2 o/oo, respectively), when air temperatures are warmest (fig. 13). 

The values of δ2H and δ18O in precipitation show 

a significant (R2 [root mean square] of 0.80 or 

greater) direct correlation with the average monthly 

air temperature. During 1962 through 1978, the 

δ18O values for the precipitation samples at Midway Airport ranged from 2.5 to –22.6 o

/oo, and 

the δ2H values ranged from 4.9 to –165.3 o/oo. The 

weighted mean δ18O value for all precipitation 

was –6.9 o/oo, and the weighted mean δ2H value 

was –43 o/oo (Harvey, 2001). Plotting values of 

δ2H with δ18O in precipitation at Midway Airport 

(fig. 14) shows a significant (R2 value of 0.96) 

direct covariance in the hydrogen and oxygen
Distribution of Isotopes in Ground Water 37

isotopic composition of precipitation in the study 

area, described by the equation

δ2H = 6.9δ18O + 0.08 o/oo. 

This equation describes the local meteoric water 

line (LMWL) and provides a reference with which 

the isotopic composition of the ground water in the 

region can be evaluated.

In shallow ground-water systems with temperatures less than 50oC, such as are present in the 

study area, the isotopic compositions of δ2H and 

δ18O in water are not affected by water/rock interactions (Perry and others, 1982), and these stable 

isotopes can be used as natural tracers. Differences 

in the isotopic composition of ground water and 

precipitation, therefore, can be used to detect differences in the source water.

Plotting values of δ2H with δ18O for ground 

water and Lake Michigan samples shows that 

most of the samples plot near the LMWL (fig. 15). 

The δ2

H and δ18O values for Lake Michigan at 

Calumet Harbor (–43.2 and –5.7 o/oo, respectively) 

are similar to the values of –45 o

/oo for δ2H

and –5.9 o/oo for δ18O reported by Machavaram 

and Krishnamurthy (1995) from southeastern Lake 

Michigan. These values also are similar to the 

weighted mean value in precipitation measured 

at Midway Airport.

The δ2H values in samples from wells open 

to the water table ranged from –41.9 to –71.3 o/oo

(table 2, fig. 16). The δ18O values in samples from 

wells open to the water table ranged from –5.4 

to –11.6 o/oo. The geometric mean of the δ2H 

(–48.1 o/oo) and δ18O (–7.0 o/oo) values in the 

samples from wells open to the water table in 
the western part of the study area are substantially 

heavier than the geometric mean of the δ2H 

(–66.9 o/oo) and δ18O (–9.9 o/oo) values in the 

samples from the water-table wells in the eastern 

part. 

The δ2H values in samples from wells open 

to the middle or the base of the Calumet aquifer (all 

wells open to the middle or the base of the Calumet 

aquifer are west of Wolf Lake and designated as 

Hydrologic unit “CA” in table 1) ranged from –63.4 

to –76.3 o/oo (table 2, fig. 16). The δ18O values 

in samples from wells open to the middle or the 

base of the Calumet aquifer ranged from –9.0 to 

–9.7 o/oo (fig. 16). The geometric means of δ2H and 

δ18O for samples open to the middle and base of the 

aquifer were –66.3 o/oo and –9.4 o/oo, respectively.

The geometric mean of the δ2H and δ18O 

values in the samples from wells open to the water 

table in the Calumet aquifer in the eastern part of 

the study area are nearly identical to the geometric 

mean of the δ2H and δ18O values in the samples 

from wells open to the middle and the base of the 

Calumet aquifer in the eastern part of the study area 

(fig. 16). 
Distribution of Isotopes in Ground Water 39

A transect of nested wells at the Bairstow 

landfill were selected for the measurement of δ2

H 

and δ18O values. The Bairstow site is part of a 

former steel-slag landfill near the central part of 

the study area. With one exception, the isotopic 

measurements occurred within a limited range 

of values (–65.3 to –71.3 o/oo δ2H and –8.4 

to –10.2 o/oo δ18O) (fig. 17). The δ2H at one site 

(BH32S) was –76.3 o

/oo.

All wells open to the middle or the base of the 

confining unit are in the western part of the study 

area. The δ2

H values in samples from wells open to 

the middle or the base of the confining unit ranged 

from –43.9 to –74.2 o

/oo (fig. 18). Values of δ18O in 

samples from wells open to the middle or the base 

of the confining unit ranged from –5.7 to –12.2 o/oo. 

Values of δ2

H and δ18O in these samples show a 

range along the meteoric water line.

Values of δ2H and δ18O in ground water from 

the confining unit (fig. 15) show a covariance in 

the hydrogen and oxygen isotopic composition described by the equation

δ2H = 6.95δ18O + 0.009 o/oo.

The equation is similar to that of the LMWL.

Distribution of Isotopes in Ground Water 41

Table 2. Isotope data for sites in the study area, northwestern Indiana and northeastern Illinois

[δ, del notation; 2H, hydrogen-2; 18O, oxygen-18; 3H, hydrogen-3; 3He, helium-3; per mil, parts per thousand; --, no data; 

<, less than; DUP, quality-assurance replicate sample; U, unsuccessful attempt to measure ground-water age]

Well name

3

H 

(tritium units)

δ2H 

(per mil)

δ18O 

(per mil)

3

H/3He age 

 (years at date of 

 sample collection)

3

H/3He 

age error 

 (years)

A20 8.1 -69.4 -9.5 7.16 0.39

B7 8.3 -70.0 -9.4 1.71 .45

B8 8.9 -66.4 -9.4 6.71 .32

B8 (DUP) 8.9 -67.5 -9.6 5.96 .31

B10 8.6 -68.4 -8.5 3.59 .39

BH1D 10 -49.5 -8.0 U --

BH1S 13 -52.7 -8.0 -- --

BH7D <.78 -103 -14.2 -- --

BH7I 8.9 -45.2 -6.6 U --

BH7S 21 -41.9 -5.4 -- --

BH13 8.0 -68.9 -11 6.20 .33

BH16D 4.0 -90.3 -12 -- --

BH18D <.78 -58.1 -7.4 -- --

BH18D (DUP) <.78 -57.3 -7.5 -- --

BH18I .90 -43.9 -5.7 U --

BH18S 20 -46.2 -6.4 -- --

BH31 9.1 -71.3 -9.3 -- --

BH32D 8.8 -65.7 -9.4 -- --

BH32D (DUP) 8.8 -70.2 -9.4 -- --

BH32I 10 -68.3 -9.5 -- --

BH32S 9.7 -76.3 -9.5 -- --

BH32SL 8.0 -68.3 -8.4 -- --

BH33I 8.1 -63.4 -9.4 -- --

BH33S 9.4 -68.1 -9.5 -- --

BH33SL 8.8 -65.3 -10 -- --
Table 2. Isotope data for sites in the study area, northwestern Indiana and northeastern Illinois— 

Continued

42 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

BR1 1.5 -120 -14 -- --

BR2 .19 -102 -14 -- --

BR3 1.4 -117 -17 -- --

BR4 .78 -150 -20 -- --

BR5 .31 -140 -19 -- --

C1 8.6 -53.2 -10 U U

C3 8.9 -64.4 -9.4 15.89 .22

D10 8.6 -64.6 -9.0 -- --

D11 8.1 -70.4 -9.6 U --

D21 9.4 -64.2 -9.7 U U

D67 8.8 -65.4 -9.7 U U

D68 9.1 -66.5 -9.7 9.95 .29 

D68 (DUP) -- -- -- 9.90 .30

D75 10 -65.3 -11 U --

FILO102-5 2.0 -82.2 -11 -- --

FILO102D <.78 -80.8 -11 -- --

GO5D 1.3 -86.8 -12 -- --

G213 18 -50.5 -8.0 -- --

G218 <.78 -105 -14 -- --

IC <.78 -80.2 -11 -- --

IC (DUP) <.78 -78.0 -11 -- --

IP <.78 -94.7 -13 -- --

R251 <.78 -74.2 -10 -- --

R253 1.2 -71.3 -10 -- --

ST1D 4.2 -99.0 -13 -- --

ST4D 6.9 -63.6 -9.5 -- --

WS1 2.8 -46.6 -6.5 -- --

WS2 10 -50.1 -7.4 -- --

Lake Michigan 22 -43.5 -5.7 -- --

Well name

3

H 

(tritium units)

δ2H 

(per mil)

δ18O 

(per mil)

3

H/3He age 

 (years at date of 

 sample collection)

3

H/3He 

age error 

 (years)


Distribution of Isotopes in Ground Water 45

The δ2H and δ18O values in ground water 

from unconsolidated wells indicated a possible 

inverse relation with well depth (fig. 19). Isotopic 

values in wells in the eastern and central parts of 

the study area generally showed a slight decrease 

in δ2H and δ18O with depth. Isotopic values showed 

a greater decrease with increasing well depth for 

ground water in the western part of the study area.

The δ2H values in samples from wells open 

to the top of the Silurian-Devonian aquifer ranged 

from –58.1 o

/oo near Lake Michigan at the Illinois/

Indiana State line, to –150.4 o

/oo near Lake Michigan between Gary Harbor and Indiana Harbor 

(table 2, fig. 20). The δ18O values ranged from 

–7.45 o/oo to –20.06 o/oo in the same wells, respectively. Values of δ2

H and δ18O in ground-water 

samples from the Silurian-Devonian aquifer show 

a significant (R2 value of 0.96) direct covariance 

described by the equation

δ2H = 7.32δ18O + 0.917.

Tritium

Tritium (3H) is a radioactive form of the 

hydrogen atom that contains a proton and two neutrons and has a half-life of 12.3 years (Freeze and 

Cherry, 1979, p. 82). Tritium concentrations are 

measured by counting the rate of beta (electron) 

emissions associated with neutron decay and are 

expressed as absolute concentrations, using tritium 

units (TU). A tritium unit is the equivalent of 1 tritium atom for every 1018 atoms of hydrogen. For 

1 liter of water, 1 TU is equivalent to 0.12 disintegrations per second. 

Tritium is produced naturally by the bombardment of nitrogen in the atmosphere by cosmic 

radiation (Grosse and others, 1951). Atmospheric 

tritium and oxygen then combine to form water, 

which enters the ground-water system as precipitation. Although few measurements are available, 

it is estimated that the natural concentration of 

tritium in precipitation is between 5 and 20 TU 

(Kauffman and Libby, 1954). 

Tritium also was created as a by-product 

of atmospheric testing of thermonuclear bombs 

from 1951 through 1980, particularly from 1951 

through 1962 (Clark and Fritz, 1997). Tritium concentrations in precipitation have been measured 

monthly at a site in Ottawa, Canada, since 1952. 

Tritium concentrations at Ottawa varied on a 

monthly basis but showed a general increase from 

about 12 TU in early 1953 to about 6,000 TU in 

1963 (Plummer and others, 1993). Tritium concentrations in precipitation at the Ottawa site 

have decreased since 1963. By about 1990, anthropogenic tritium had been removed from the 

atmosphere and tritium concentrations in precipitation had returned to near background levels. 

Tritium concentrations in precipitation were 

measured at Midway Airport in Chicago from 

January 1962 through December 1979. Tritium 

concentrations at Midway Airport were greater than 

700 TU for most of 1962 and reached a maximum 

of 3,572 TU in August 1963. Tritium concentrations in precipitation were less than 50 TU in 1979.

Changes in the amount of tritium in precipitation through time, radioactive decay, hydrodynamic 

dispersion, and mixing water of different ages in 

the subsurface typically preclude the use of this isotope for quantitatively estimating ground-water 

residence times. In many cases, qualitative observations may be the best use of tritium data (Clark and 

Fritz, 1997). The most accurate use of the data is to 

indicate pre- or post-1952 ground-water recharge. 

For a well-defined hydrologic system, analysis of 

the tritium-input function may provide sufficient 

information to make quantitative age estimates for 

ground water in the system. More-complex flow 

systems generally require more-complex models 

to quantitatively use the tritium data (Plummer 

and others, 1993). Time-sequence data and multidepth samples also are required to accurately date 

ground-water horizons using tritium data. The 

3H/3

He method was developed to remove the ambiguity associated with tritium-age estimation.

To maximize the usefulness of the tritium 

data collected for this study, ground-water ages 

were estimated with general guidelines provided 

by Clark and Fritz (1997). The guidelines assume 

that a piston-flow model (Clark and Fritz, 1997) is 

applicable to the hydrologic system and, correlatively, no dispersion or mixing occurs (Plummer 

and others, 1993). For this study, the validity of 


48 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

these assumptions varied across sites and between 

hydraulic units. Where tritium data are used to 

estimate ground-water age, the hydrologic setting 

also is described so that the appropriateness of the 

assumptions can be assessed.

The Clark and Fritz (1997) guidelines indicate that ground water underlying regions with 

continental climates containing less than 0.8 TU 

recharged the water table prior to 1952. Water with 

0.8 to 4 TU may represent a mixture of water that 

contains components of recharge from before and 

after 1952. Tritium concentrations from about 5 to 

15 TU may indicate recharge after about 1987. Tritium concentrations between about 16 and 30 TU 

are indicative of recharge since 1953 but cannot be 

used to provide a more specific time of recharge. 

Water with more than 30 TU probably is from 

recharge in the 1960s or 1970s, whereas water with 

more than 50 TU predominately is from recharge 

in the 1960s. 

The highest tritium concentration (22 TU) 

measured during this study was in water from the 

surface of Lake Michigan near the Indiana/Illinois 

State line (table 2). Water from Lake Michigan 

is a composite from various water sources with a 

variety of ages. 

Tritium concentrations in samples from wells 

open to the water table were 8.0 TU or greater in the 

study area. Tritium concentrations in wells open to 

the water table in the eastern part of the study area 

ranged from 8.0 to 10.2 TU (fig. 21) and may indicate ground water recharged since 1987. Tritium 

concentrations in wells open to the middle and 

deeper parts of the Calumet aquifer ranged from 8.1 

to 10.3 TU (fig. 21), virtually identical to the range 

at water table for this area. Tritium concentrations 

in wells open to the water table in the western part 

of the study area ranged from 10 to 21 TU, indicating water with substantially varying ages (fig. 21).

Tritium concentrations in samples from wells 

open to the middle or base of the confining unit in 

the western part of the study area ranged from less 

than the detection limit to 10 TU (fig. 21). Tritium 

concentrations did not show a direct correlation 

with depth in the confining unit; however, tritium 

concentrations in wells screened below 40 ft are 

generally lower than tritium concentrations in wells 

screened above 40 ft.

Tritium concentrations in samples from wells 

open to the Silurian-Devonian aquifer ranged from 

less than the detection limit to 4.0 TU (fig. 21). 

Tritium concentrations in the Silurian-Devonian 

aquifer were 4 TU or greater in the three samples 

collected in the approximate area between Stony 

Island and the eastern part of Lake Calumet.

Helium

Radioactive decay of tritium produces the 

noble gas helium-3 (3

He). Determination of 

the 3H/3He ratio, therefore, can be used to estimate the apparent date when a sample of water 

entered the aquifer as recharge. Because these 

substances virtually are inert in ground water, 

unaffected by ground-water chemistry, and not 

derived from contamination from most of the typical anthropogenic sources, H3/He3

 dating can be 

applied to a wide range of hydrologic investigations 

(Schlosser and others, 1989; Aeschbach-Hertig and 

others, 1998; Ekwurzel and others, 1994; Solomon 

and others, 1995; Sheets and others, 1998; Stute 

and others, 1997; Dunkle-Shapiro and others, 

1998). The 3

H/3He method has been used to corroborate the results of other age-dating methods, such 

as chlorofluorocarbons (Ekwurzel and others, 1994; 

Szabo and others, 1996) and krypton 85 (85Kr) 

(Ekwurzel and others, 1994). The 3H/3He method 

also has been used to provide information for 

site characterization (Solomon and others, 1995; 

Aeschbach-Hertig and others, 1998) and to assist 

with the calibration and interpretation of groundwater-flow models (Sheets and others, 1998; Szabo 

and others, 1996).

The 3H/3He method was used in an attempt 

to measure apparent age dates of water from the 

Calumet aquifer at 12 sites in Indiana and from 

the confining unit at 3 sites in Illinois. Analytical 

evidence indicated that data were unreliable at 5 of 

the 12 sites in Indiana and all of the sites in Illinois. 

Apparent ground-water ages at the eight remaining 

sites from the Calumet aquifer ranged from 1.71 

to 15.9 years (fig. 22 and table 2). The median 

50 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

and mean ages were 6.00 and 6.71 years, respectively. The helium results are consistent with the 

interpretations made from the analysis of tritium 

concentrations.

Apparent ground-water ages in the Calumet 

aquifer between the Grand Calumet River and 

the Little Calumet River in Indiana vary from 

1.71 years in water from a 9-ft well near the Grand 

Calumet River (B7) to 9.95 years at a 21-ft well 

near the Grand Calumet River (D68). Intermediate 

values are present at various points along the flowpath. Intermediate-age values were measured in 

water from wells B8 (6.71 years) and B10 (3.59 

years); wells B7, B8, and B10 are along a possible 

ground-water flowpath. The apparent age of ground 

water in the Calumet aquifer in the one sample collected between the Grand Calumet River and Lake 

Michigan was 15.9 years in water from a depth of 

26 ft near Lake Michigan. 

The complex ground-water hydrology of the 

study area makes delineating a complete flowpath a 

difficult task. Fenelon and Watson (1993) identified 

leaky sewers, pump-and-treat systems, industrial 

discharges, and inflow from the bedrock aquifer 

as factors that made calibration of a numerical 

model impossible in the study area. Ground-water/

surface-water interactions along the Lake Michigan 

shoreline, ship canals, streams, and ditches likely 

also cause perturbations to ground-water flowpaths. 

The flowpaths used to examine the 3

H/3He ages 

were based on ground-water levels measured during June 1992 (Kay and others, 1996) and may be 

affected by seasonal changes and other factors 

discussed above.

Although water near discharge points in the 

Calumet aquifer tends to have a younger apparent 

age than water near the ground-water divides, there 

is no straightforward relation between apparent 

ground-water age and distance along the apparent 

flowpath. Various factors could explain this relation, including sparse data density, complicated 

flow systems, unidentified sources of tritium, and 

mixing of shallow and deep ground water. Data 

density would have to be increased throughout the 

study area to take full advantage of dating methods. 

Some of the wells sampled in the eastern part 

of the study area (such as A20-BH13-B8-B7 or 

B10-B8-B7) could be along a ground-water flowpath, but the well depths do not conform to a single 

ground-water flowpath. Wells must be located specifically along a probable flowpath to allow the 

examination of ground-water transport and waterquality evolution; the well network used for this 

data-collection effort was not designed to serve 

this purpose.

Recharge and Sources of Ground Water

The isotopic compositions of ground water in 

the hydrogeologic units of concern were examined 

to determine the recharge and sources of water 

to each unit. In so doing, some information was 

gleaned about the flowpath the ground water 

used to reach that destination. 

Calumet Aquifer

The concentrations of tritium and helium 

and the oxygen and hydrogen isotopic composition 

indicate that water in the Calumet aquifer is derived 

from modern (post-1952) precipitation. The similarity of δ2H and δ18O and tritium values in water 

from wells screened at various intervals in the 

Calumet aquifer indicates a uniform source of water 

and a relatively rapid flow through the aquifer in 

the eastern part of the study area. Some of the variability in the isotopic composition in the aquifer 

may result from differences in the amount of 

recharge from Lake Michigan and other surfacewater bodies, the seasonal timing of recharge from 

precipitation, flow rates, and flow patterns. 

A transect of nested piezometers located 

immediately south of Lake George (fig. 17) illustrates the distribution of isotopes in the Calumet 

aquifer (figs. 16 and 17). Well BH31 and the BH32 

and BH33 well clusters are open to various depths 

in the Calumet aquifer and are aligned along a 

single short flowpath near Lake George. Water samples from well clusters BH32 and BH33 have δ2H 

values approximately –65 o

/oo, with the exception 

of well BH32SL that has a δ2H value of –76.3 o

/oo. 

The δ18O values in samples from the same wells 

are about –9.45 o/oo, with the exception of BH32S 
Recharge and Sources of Ground Water 51

(–8.4513 o/oo) and BH33S (–10.22 o/oo). Tritium 

concentrations similarly are uniform in water from 

the Lake George transect wells. The similarity of 

isotope values in ground water at the Lake George 

transect indicates that water in the Calumet aquifer 

is well mixed but that geologic heterogeneities (in 

combination with small fluctuations in the isotopic 

composition of recharge) may affect the measured 

isotope values.

The isotopic data indicate that some parts of 

the Calumet aquifer may be affected by recharge 

from adjacent surface-water bodies in the western 

part of the study area. Wells BH7S and BH18S are 

within 50 ft of the Calumet River and Lake Michigan, respectively. The concentration of tritium 

and the δ2H and δ18O values in the ground-water 

samples from wells BH7S and BH18S are similar 

to that of Lake Michigan and are noticeably different from almost every other sample from 

the Calumet aquifer. These patterns indicate 

that ground water at wells BH18S and BH7S is 

comprised primarily of water recharged to the 

ground-water system from Lake Michigan and 

the Calumet River.

The isotopic composition of well G213 varies 

from that in the remainder of the study area. Well 

G213 is open to the Calumet aquifer and is in the 

western part of the study area near the confluence 

of the Calumet and Grand Calumet Rivers. The tritium concentration in well G213 is elevated, which 

is consistent with recharge from surface water in 

this area. The δ2H and δ18O values of water from 

this well, however, are isotopically lighter than 

those in the water from Lake Michigan. It is possible that the high tritium at this well is the result 

of leachate from a landfill in the vicinity. Landfill 

leachate often contains elevated concentrations of 

tritium (Hackley and others, 1999).

Analysis of the isotopic composition of well 

WS2 varies from that elsewhere in the study area. 

The tritium concentration in this well is consistent 

with values measured in the Calumet aquifer in the 

rest of the study area, but the δ2H and δ18O values 

are substantially heavier than elsewhere in the aquifer. The reason for these differences is not obvious. 

Confining Unit

Potential sources of water to the confining 

unit include recharge from precipitation where the 

Calumet aquifer is absent, inflow from the Calumet 

aquifer and the Silurian-Devonian aquifer, and 

recharge from surface water. The concentration of 

tritium and the oxygen and hydrogen isotopic compositions indicate that water in the confining unit 

is derived from modern (post-1952) precipitation, 

pre-modern (before 1952 but less than 10,000 years 

before present) and perhaps “glacial-age” water 

from the Pleistocene Epoch (10,000 to 40,000 years 

before present). The variability of tritium concentrations and δ2

H and δ18O values of water from 

wells screened at various depths in the confining 

unit indicates the potential for mixed sources of 

water and relatively slow rates of ground-water 

flow in the confining unit. 

Most of the water in the confining unit contains more than 1 TU, which indicates the presence 

of modern water. With the exception of well 

BH1D, tritium concentrations in the confining 

unit decrease with increasing depth, indicating a 

decreasing percentage of modern water with depth 

in the confining unit. The sample from well R251 

did not contain detectable tritium, indicating a premodern source of water. 

The δ2H and δ18O values for water from well 

BH18I are virtually identical to the isotopic composition of the sample from Lake Michigan (table 2), 

indicating that the confining unit may be recharged 

locally by Lake Michigan. The presence of less 

than 1 (0.90) TU of tritium in the sample from well 

BH18I would indicate that the recharge from Lake 

Michigan of the confining unit occurred in 1952 or 

shortly thereafter. It seems probable that flow from 

Lake Michigan into the confining unit occurred 

sometime after 1952, given the fact that precipitation with elevated concentrations of tritium 

would require some time to accumulate in the lake. 

Recharge from Lake Michigan to the confining unit 

is likely a response to dewatering of the SilurianDevonian aquifer and subsequent declines in the 

potentiometric surface in the confining unit. 
52 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 
Recharge and Sources of Ground Water 53
54 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

The tritium concentration in ground water at 

well BH1D indicates that water has recharged the 

confining unit since 1987. Well BH1D is near a 

ground-water divide in the western part of the study 

area. A large component of vertical flow, common 

near ground-water divides, may explain the occurrence of relatively young ground water at depth in 

the confining unit at BH1D. 

The δ2H and δ18O values in the middle and 

deeper parts of the confining unit generally are 

lighter than at the water table in the western part 

of the study area and show a clear tendency to decrease with depth. Samples collected from wells 

with a depth less than about 25 ft tend to have an 

isotopic composition similar to that of the water 

table in the western part of the study area and are 

likely to be completely modern water. Samples collected from wells deeper than 30 ft have δ2H and 

δ18O values that are substantially lighter than the 

values from water-table wells in the western part 

of the study area. These results are consistent with 

data from Hackley and others (1999), who examined δ2H and δ18O values at a cluster of wells 

in the northeastern part of Lake Calumet. 

Tritium concentrations and δ2H and δ18O 

values indicate an increasing component of glacialage water with depth in most of the confining unit. 

The confining unit may have received recharge 

primarily from the underlying Silurian-Devonian 

aquifer in the past. The prevailing flow direction, 

however, probably has been reversed by declining 

hydraulic heads in the Silurian-Devonian aquifer 

and the confining unit now is recharged primarily 

from precipitation, the Calumet aquifer, and surface water.

Silurian-Devonian Aquifer

Previous investigators examining δ2H and 

δ18O trends in the Cambrian-Ordovician aquifer 

system in north-central Illinois have attributed 

light δ2H and δ18O values found in the Chicago 

area to glacial-age recharge from the Pleistocene 

Epoch (Perry and others, 1982; Siegel and 

Mandle, 1984). In a study of Lake County SilurianDevonian bedrock wells by Hasenmueller and 

others (2001), the δ2H (–148 to –90 o/oo) and δ18O

(–19 to –12 o/oo) values were interpreted as indicating the presence of glacial-age water. The 

comparatively light δ2H and δ18O values that characterize much of the Silurian-Devonian bedrock 

aquifer in the study area probably reflect the presence of glacial-age water. Similarly light values 

were not observed in the Calumet aquifer or most 

of the confining unit.

In some wells open to the Silurian-Devonian 

aquifer, the aquifer appears to contain a large fraction of water with glacial-age origins. Water from 

wells BR4 (–150 o

/oo δ2H and –20 o/oo δ18O) and 

BR5 (–140 o

/oo δ2H and –19 o/oo δ18O) isotopically 

is depleted relative to other Silurian-Devonian 

wells in the network and probably contains a substantial amount of glacial-age recharge. Tritium 

values confirm that the water in the SilurianDevonian aquifer beneath most of the study area 

was recharged prior to 1952. 

In most wells open to the Silurian-Devonian 

aquifer, the δ2

H and δ18O values of ground water 

generally are heavier than those measured in BR4 

and BR5 and probably indicate a mixture of 

glacial-age water, post-glacial-age water, and 

modern recharge. The intermediate δ2H and δ18O 

values range from –80 to –105 o

/oo and from –11 

to –14 o/oo, respectively, in the eastern and central 

part of the study area and from –102 to –120 o/oo

(δ2

H) and –14.1 to –17.4 o/oo (δ18O) in the western 

part. Data from this study indicate that samples 

from wells with intermediate δ2

H and δ18O values 

most commonly occur where the confining unit 

is absent and may be related to leakage of stormwater runoff from the Water Reclamation District 

of Greater Chicago tunnel and reservoir project 

(TARP), improperly abandoned water-supply 

wells, natural mixing in response to hydraulichead gradients, and landfill excavations into the 

Silurian-Devonian aquifer. Lake Michigan also 

may recharge the bedrock aquifer near Calumet 

Harbor. 

The isotopic composition of bedrock well 

BH18D is similar to that of Lake Michigan and 

indicates that recharge to the bedrock through the 
Ground-Water-Flow Rates and Aquifer Vulnerability 55

confining unit may be occurring near this well. 

The absence of detectable concentrations of tritium 

in well BH18D indicates that the recharge is postglacial but pre-modern. 

The isotopic information indicates that some 

bedrock wells may contain a component of modern 

recharge. The presence of more than 1 TU of tritium and elevated values of δ2H and δ18O at wells 

BH16D, ST1D, ST4D, FILO102-5, and GO5D, for 

example, indicates the presence of primarily modern recharge. These wells are south (hydraulically 

downgradient) of Stony Island, and precipitation 

probably is directly recharging the aquifer where 

the confining unit is absent.

In addition to the presence of recharge from 

precipitation, intermediate δ2H and δ18O values 

and elevated tritium concentrations in the SilurianDevonian aquifer at wells ST1D, ST4D, FILO102-

5, and G05D may indicate facilitated transport 

through the confining unit. Well ST4D is near a 

landfill; the δ2

H and δ18O values are –9.5 o/oo and 

–63 o/oo, respectively, and the tritium concentration 

is 6.9 TU. The isotopic composition clearly indicates a modern component of ground water and 

may indicate enhanced recharge as a result of landfill excavation. Well ST1D is approximately 500 ft 

east of well ST4D and has similar isotopic characteristics. Bedrock well GO5D, near the Little 

Calumet River and a sanitary landfill, also shows 

a component of modern recharge. A nearby, improperly abandoned water-supply well may have 

affected the hydrology and isotopic values at well 

FILO102-5 (Ecology and Environment, Inc., 1990). 

Some δ2H and δ18O values may indicate mixing of ground waters of various ages in response to 

flow along hydraulic-head gradients. The clustering 

of δ2

H data in plots of δ2H with depth may indicate 

that ground water from some Silurian-Devonian 

wells in the western part of the study area is a 

mixture of water from shallower unconsolidated 

deposits and deeper, isolated bedrock aquifers like 

the Silurian-Devonian aquifer in the eastern and 

central part of the study area. Potentiometic data 

for the Silurian-Devonian aquifer may support the 

existence of an east-west flow component in some 

parts of the formation.

Hasenmueller and others (2001) also identified isotopic evidence that modern precipitation has 

mixed with glacial-age water in some parts of the 

bedrock aquifer. Ground water with isotopic signatures indicative of mixed Silurian-Devonian and 

Calumet aquifer ground water was identified near 

a regional ground-water divide in southern Lake 

County, Ind., farther to the east near Lake Michigan, and in areas with unconsolidated deposits that 

are underlain by the Antrim Shale (Hasenmueller 

and others, 2001). 

Ground-Water-Flow Rates 

and Aquifer Vulnerability

The ground-water-isotope data were examined in the spatial context of the study area. In 

the classic model by Toth (1962), regional groundwater flowpaths in an aquifer system (without 

pumping or other human stresses) originate near a 

basin divide with a significant downward vertical 

component of flow. With distance from the divide, 

the vectors develop a dominantly horizontal component as water flows through the aquifer and 

beneath the basin. The flowpaths end with a substantial upward vertical component as ground water 

discharges to rivers, lakes, and wetlands. It should 

be expected, therefore, that the age of ground water 

in the Calumet aquifer generally increases with 

depth and distance along the flowpath.

Calumet Aquifer

Hydraulic and isotopic data indicate that water 

in the Calumet aquifer and at the water table in 

the confining unit is derived from precipitation 

that entered the ground-water system since 1987. 

The residence time for ground water in the Calumet 

aquifer probably is from 5 to 15 years, based on 

3H/3

He ages (5.27, 9.41, and 15.5 years) for samples collected from three wells near local discharge 

points. If the age of the ground water at the three 

wells near the surface-water bodies is divided by 

the distance from the ground-water divide to the 

well (2.3, 2.1, and 1.2 miles, respectively), ground-
56 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

water velocity through the Calumet aquifer is 

estimated to be 1,200 and 2,300 ft/yr between the 

Tolleston Beach Ridge and the Grand Calumet 

River and 400 ft/yr in the area defined by the 

Indiana Harbor Canal, Lake Michigan, and Gary 

Harbor. These values are more than an order of 

magnitude greater than the ground-water velocities 

calculated from solution of the Darcy equation by 

Kay and others (1996). 

The rapid ground-water-flow rates described 

above, the shallow water tables, the lack of an overlying confining unit, and the proximity to numerous 

sources of contamination, indicate that the Calumet 

aquifer is vulnerable to contamination. The relatively rapid ground-water-flow rates and short 

residence times in the aquifer, however, also demonstrate the potential for remediation in the event 

of aquifer contamination. 

Confining Unit

Hydraulic and isotopic data indicate that water 

in the confining unit is derived from a variety of 

sources, including precipitation that has entered 

the aquifer since 1952, surface-water recharge, 

post-glacial and glacial-age water. At well BH7I, 

the confining unit is overlain by the Calumet aquifer, and tritium concentrations indicate that water in 

the confining unit is derived from (1) precipitation 

that recharged the aquifer since about 1987 and has 

not been subject to extensive dispersion or mixing, 

or (2) a mixture of ground waters in which at least 

one end member contains some post-1952 recharge. 

Elevated (above background) tritium concentrations in the overlying Calumet aquifer (BH7S), 

however, indicate recharge from the Calumet River. 

The ground-water-flow rate at BH7I was not calculated because the piston-flow assumption was 

not valid.

The confining unit also is overlain by the 

Calumet aquifer at well WS1 and is likely to be 

unweathered at this location. Tritium concentrations (2.8 TU) in water from the well indicate that 

water in the confining unit contains a component 

of post-1952 precipitation. Tritium concentrations 

in the Calumet aquifer near well WS1 indicate 

(1) water derived from either precipitation that 

recharged the aquifer since about 1952 and has not 

been subject to extensive dispersion or mixing, or 

(2) a mixture of ground water in which at least one 

end member contains some post-1952 recharge. 

The minimum vertical ground-water-flow rate for 

the unweathered part of the confining unit is estimated to be 0.28 ft/yr, assuming that water in well 

WS1 flowed through about 15 ft of the Calumet 

aquifer and 7 ft of the confining unit and that water 

in the Calumet aquifer and confining unit have maximum ages of 13 (1987 to 2000) and 48 years (1952 

to 2000). This flow rate for the unweathered part of 

the confining unit is about one-half of the vertical 

velocity (0.55 ft/yr) estimated from hydraulic-head 

data and the Darcy equation by Kay and others 

(1996). 

Isotopic data indicate that water from BH18I, 

which is open to the unweathered confining unit, 

contains a component of post-1952 recharge. 

Assuming piston flow, the water in well BH18I 

has flowed through approximately 30 ft of fill and 

10 ft of the confining unit in no more than 48 years. 

Because of the high permeability of the fill deposits, it is assumed that the time required for ground 

water to flow through the fill deposits is negligible 

and the average vertical velocity through the confining unit is estimated at 0.21 ft/yr. This value is 

similar to the vertical velocity calculated for the 

confining unit at well WS1. The estimated vertical 

velocities indicate that the unweathered confining 

unit is only moderately vulnerable to contamination, provided the Calumet aquifer has not been 

removed completely or reduced substantially in 

thickness by quarrying or landfill excavations.

Well BH1D is near a ground-water divide and 

the Calumet aquifer is absent. Tritium data indicate 

that water from this well represents precipitation 

that recharged the aquifer since about 1987 and has 

not been subject to extensive dispersion or mixing 

or a mixture of ground waters in which at least 

one end member contains some post-1952 recharge. 

The hydrogeologic setting indicates a likelihood of 

substantial vertical flow through the confining unit. 

The approximate vertical ground-water-flow rate 

near well BH1D is 6.3 ft/yr, assuming an age of 

about 13 years (piston-flow conditions), uniform 
Summary and Conclusions 57

geology, and vertical ground-water flow. This 

velocity is about half the vertical velocity calculated for the upper part of the confining unit, 

based on hydraulic-head data and the Darcy equation (13.9 ft/yr); it exceeds the estimated vertical 

velocity through the lower unweathered part of the 

confining unit (0.55 ft/yr) by more than a factor 

of 10 (Kay and others, 1996). The flow velocity 

calculated with the isotope data may represent a 

composite of flow through weathered and unweathered parts of the confining unit in the vicinity of 

well BH1D, or the piston-flow assumption may not 

be applicable in this case. The computed flow rates 

indicate that the weathered part of the confining 

unit may be more vulnerable to contamination than 

the deeper unweathered part.

Isotopic data indicate that the lower, unweathered, part of confining unit near wells R251, 

R253, C6PI, and C6PD may contain a mixture of 

glacial-age water derived from upward flow under 

pre-development conditions and modern (post1952) water. The mixing of ground waters with 

different apparent ages invalidates the piston-flow 

assumption. If post-1952 recharge is present at 

these wells, then it has flowed approximately 37 ft 

through the confining unit in no more than 48 years, 

for a minimum velocity of 0.77 ft/yr. This value is 

similar to the vertical velocity of 0.55 ft/yr calculated for the unweathered part of the confining unit 

from solution of the Darcy equation. Facilitated 

movement through the confining unit (landfill excavation, vertical fractures, and other features) may 

be indicated. Facilitated transport would increase 

the vulnerability of the confining unit to contamination.

Silurian-Devonian Aquifer

The hydraulic and isotopic data indicate that 

the hydrology of the Silurian-Devonian aquifer has 

been affected by historical and current withdrawals 

in the vicinity of Lake Calumet. The aquifer may 

be vulnerable to contamination where the overlying 

confining unit is thin or absent or where integrity 

of the confining unit has been compromised. The 

Silurian-Devonian aquifer appears to be substantially less vulnerable to contamination in the 

eastern and central parts of the study area than in 

the western part.

Summary and Conclusions

The area of northwestern Indiana and 

northeastern Illinois is one of the most heavily 

industrialized regions of the United States. To 

characterize ground-water recharge, flow rates, 

and aquifer vulnerability, the U.S. Geological 

Survey, in cooperation with the U.S. Environmental 

Protection Agency, collected water samples from 

48 wells during July 1997 through April 2001. 

Samples were analyzed for 3H, δ2H, and δ18O. 

The date of ground-water recharge of some samples 

was determined with the 3

H/3He method.

Ground water in the study area is derived 

from modern and pre-modern precipitation, 

surface-water recharge, and glacial-age recharge 

residing in deeper aquifers. Isotopic values from 

the surficial Calumet aquifer in the eastern part 

of the study area fall within a limited range and 

indicate a well-mixed aquifer with active flow. 

Residence time of water in the Calumet aquifer 

in the eastern and central parts of the study area 

is approximately 5 to 15 years. Ground-water-flow 

rates are estimated at 400 to 2,300 ft/yr. Vulnerability of the Calumet aquifer to contamination is 

indicated by the data, but remediation may similarly be possible.

In the western part of the study area, the 

Calumet aquifer and weathered confining unit contain noticeably heavier isotopic values than those 

measured in shallow ground water in the east. The 

explanation for these differences may be attributed 

in part to recharge from Lake Michigan and its 

connected waterways.

In the western part of the study area, isotope 

values in wells open to the middle and base of the 

confining unit indicate multiple sources of water, 

including recharge from Lake Michigan near Calumet Harbor, modern precipitation near Blue Island, 

and glacial-age water in much of the surrounding 

area. The source and age of the water is affected by 

the presence or absence of the overlying Calumet 

aquifer, location along the ground-water flowpath, 
58 Use of Isotopes, Calumet Region, Northwestern Indiana and Northeastern Illinois 

and natural and anthropogenic mechanisms that 

affect ground-water flow. Subject to the applicability of certain assumptions, the isotopic data indicate 

that the vertical ground-water flow in the confining 

unit ranges from 0.2 to 6 ft/yr. 

The vulnerability of ground water in the confining unit to contaminants at land surface ranges 

from moderately vulnerable to invulnerable. The 

vulnerability of the confining unit to contamination 

is increased in areas where the unit is thin, absent, 

weathered, and affected by anthropogenic activities 

that include excavations and improperly abandoned 

wells. Remediation of contamination in the confining unit would be significantly more difficult than 

for the Calumet aquifer.

Isotopic values in ground water from wells 

open to the basal Silurian-Devonian aquifer indicate multiple sources of water in the aquifer, 

including recharge from Lake Michigan near the 

Calumet Harbor; modern precipitation near Stony 

Island; primarily glacial-age water near Lake 

Michigan in the eastern part of the study area; and 

a mixture of modern, pre-modern, and glacial-age 

water in the remainder of the study area. The source 

and age of the water is affected by the presence or 

absence of the overlying confining unit, location 

along the ground-water flowpath, pumping effects, 

and the presence of facilitated transport through 

the confining unit. The Silurian-Devonian aquifer 

is vulnerable to contamination in the western part 

of the study area, where the confining unit is thin or 

absent or where the integrity of the confining unit 

has been compromised. The Silurian-Devonian 

aquifer in the central and eastern part of the study 

area is less vulnerable to contamination.

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