Application of Hydrogeology and Tritium-Based 

Groundwater-Age Estimates to Assess the Travel Time 

of Groundwater at the Site of a Landfill to the Mahomet 

Aquifer near Clinton, Illinois

Scientific Investigations Report 2015–5159

U.S. Department of the Interior

U.S. Geological Survey

Application of Hydrogeology and TritiumBased Groundwater-Age Estimates to 

Assess the Travel Time of Groundwater 

at the Site of a Landfill to the Mahomet 

Aquifer near Clinton, Illinois

By Robert T. Kay and Paul M. Buszka

With a section on Regional Indications of Recharge to the Mahomet Aquifer 

from Previously Collected Tritium and Pesticide Data 

By Paul M. Buszka and William S. Morrow

Scientific Investigations Report 2015–5159

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Kay, R.T., and Buszka, P.M., 2016, Application of hydrogeology and groundwater-age estimates to assess the travel 

time of groundwater at the site of a landfill to the Mahomet Aquifer, near Clinton, Illinois, with a section on Regional 

Indications of Recharge to the Mahomet Aquifer from Previously Collected Tritium and Pesticide Data, 

by Buszka, P.M. and Morrow, W.S.: U.S. Geological Survey Scientific Investigations Report 2015–5159, 54 p., http://

dx.doi.org/10.3133/sir20155159.

ISSN 2328-0328 (online)
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................5

Regional Hydrogeologic Setting and Indications of Recharge to the Mahomet Aquifer 

from Prior Data ........................................................................................................................6

Topographic Setting of the Clinton Site.............................................................................................6

Methods of Data Collection and Analysis for the Clinton Site ...............................................................7

Water Sample Collection at the Clinton Site....................................................................................7

Laboratory Analysis of Groundwater Samples From the Clinton Site..........................................7

Interpretation of Recharge Using Tritium Data..............................................................................10

Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time 

at the Clinton Site ...........................................................................................................................12

Hydrogeology and Estimates of Groundwater Age at the Clinton Site......................................12

Continuity of Deposits at the Clinton Site ..............................................................................12

Shallow Sand Deposits.............................................................................................................19

Shallow Peat Deposit................................................................................................................19

Upper Radnor Till Sand .............................................................................................................19

Lower Radnor Till Sand.............................................................................................................20

Organic Soil Unit ........................................................................................................................20

Glasford Formation Below the Organic Soil Unit and the Banner Formation Tills..........21

Mahomet Aquifer at the Clinton Site ......................................................................................21

Two-Part Mixing Model Evaluation of Tritium-Based Groundwater Age Estimates 

at the Clinton Site .........................................................................................................21

Assessment of the Groundwater Travel Time at the Site of the Proposed Chemical 

Waste Unit..............................................................................................................................26

Vertical Groundwater Travel Time Estimated at the Clinton Site Using Tritium-Age 

Based Groundwater Velocity .....................................................................................26

Vertical Groundwater Travel Time Using Estimates of Effective Porosity and Darcy 

Velocity...........................................................................................................................27

Scenario One, Vertical Hydraulic Conductivity Equal to Geometric Mean 

of all Site Triaxial Permeability Tests ...............................................................31

Scenario Two, Vertical Hydraulic Conductivity Equal to Geometric Mean of Site 

Triaxial Permeability Tests Above the Upper Radnor Till Sand and Below 

the Roxana Silt-Robein Silt Member................................................................31

Scenario Three, Vertical Hydraulic Conductivity Equal to Values for Tills of the 

Wedron Group and Glasford Formation from Regional Groundwater Flow 

Model ....................................................................................................................32

Scenario Four, Vertical Hydraulic Conductivity Equal to Value for Glasford 

Formation Tills From Regional Groundwater Flow Model ............................33

Scenario Five, Vertical Hydraulic Conductivity Equal to Value for Banner 

Formation Tills from Regional Groundwater Flow Model .............................34

Summary of Hydrogeology and Recharge Interpretations from Clinton Site Data..................35
iv

Regional Indications of Recharge to the Mahomet Aquifer from Previously Collected 

Tritium and Pesticide Data............................................................................................................37

Regional Tritium Data.........................................................................................................................39

Regional Pesticide Data ....................................................................................................................40

Indications of Regional Heterogeneity in Recharge to the Confined Part of the Mahomet 

Aquifer.....................................................................................................................................42

Data Limitations............................................................................................................................................43

Summary and Conclusions.........................................................................................................................43

References Cited..........................................................................................................................................46

Figures

1. Map showing location of the Clinton site, near Clinton, Illinois ...........................................3

2. Map showing location of solid waste units and sampled monitoring wells, Clinton 

site, near Clinton, Illinois .............................................................................................................4

3. Map showing location of the Mahomet aquifer and selected bedrock valleys in eastcentral Illinois................................................................................................................................5

4. Map showing well and boring locations and the elevations of sand deposits, within 

predominantly silt and clay deposits, and in the Mahomet aquifer in the vicinity 

of Clinton landfill unit #3, near Clinton, Illinois .........................................................................9

5. Graphs showing concentrations of tritium in precipitation and decay-corrected, 

annual average concentrations of tritium in precipitation, decay corrected to the 

date of sampling (May 16, 2012)...............................................................................................11

6. Graphs showing glacial stratigraphy of Quaternary-aged deposits and pertinent 

features beneath Clinton landfill unit #3, near Clinton, Illinois............................................13

7. Map showing location of lines of geologic section, Clinton land fill unit #3, near 

Clinton, Illinois .............................................................................................................................14

8. Geologic cross section showing line of geologic section A–A’, Clinton landfill 

unit #3, near Clinton, Illinois ......................................................................................................15

9. Geologic cross section showing line of geologic section B–B’, Clinton landfill 

unit #3, near Clinton, Illinois ......................................................................................................16

10. Geologic cross section showing line of geologic section C–C’, Clinton landfill 

unit #3, near Clinton, Illinois ......................................................................................................17

11. Geologic cross section showing line of geologic section D–D’, Clinton landfill 

unit #3, near Clinton, Illinois ......................................................................................................18

12. Graphs showing mixing computations indicating differences and similarities 

between tritium in the sample from well G53S in the vicinity of Clinton landfill 

unit #3, near Clinton, Illinois and hypothetical mixtures of pre-1953 recharge and 

1953–2002 recharge....................................................................................................................23

13. Conceptual diagram of features and data used in the solution of the Darcy velocity 

equation according to scenario one for the deposits at Clinton landfill unit #3, 

near Clinton, Illinois....................................................................................................................28

14. Conceptual diagram of features and data used in the solution of the Darcy velocity 

equation according to scenario two for the deposits at Clinton landfill unit #3, 

near Clinton, Illinois....................................................................................................................32

15. Conceptual diagram of features and data used in the solution of the Darcy velocity 

equation according to scenario three for the deposits at Clinton landfill unit #3, 

near Clinton, Illinois....................................................................................................................33
v

16. Conceptual diagram of features and data used in the solution of the Darcy velocity 

equation according to scenario four for the deposits at Clinton landfill unit #3, near 

Clinton, Illinois .............................................................................................................................34

17. Conceptual diagram of features and data used in the solution of the Darcy velocity 

equation according to scenario five for the deposits at Clinton landfill unit #3, near 

Clinton, Illinois .............................................................................................................................35

18. Maps showing locations of domestic wells in regional Mahomet aquifer network 

in east-central Illinois that previously were sampled by the U.S. Geological Survey 

for pesticide and tritium concentrations, 1996–2011 ............................................................38

Tables

1. Well data and results of tritium analyses, Clinton site, near Clinton, Illinois, May 16, 

2012. ................................................................................................................................................8

2. Estimates of effective porosity computed using the Darcy velocity equation with a 

tritium-based vertical groundwater velocity and vertical hydraulic conductivity 

values from the Clinton site, near Clinton, Illinois and from Roadcap and others 

(2011).............................................................................................................................................29

3. Estimates of vertical groundwater velocity and groundwater travel time for the 

Clinton site, near Clinton, Illinois by solution of the Darcy velocity equation using 

selected values of vertical hydraulic conductivity and equivalent porosity from 

scenarios in table 2 and vertical hydraulic conductivity from Roadcap and others 

(2011).............................................................................................................................................30

4. Results of U.S. Geological Survey sampling from domestic wells open to the 

Mahomet aquifer in east-central Illinois, 1996 to 2011.........................................................50

5. Minimum vertical groundwater flow velocities and maximum time of travel from 

land surface to the Mahomet aquifer for water samples collected by 

U.S. Geological Survey from domestic wells open to the Mahomet aquifer in eastcentral Illinois, 1996 to 2011, with detections of a pesticide and tritium-based 

groundwater age classification. ..............................................................................................53
vi

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88) unless otherwise noted. Horizontal coordinate information is referenced to the 

North American Datum of 1983 (NAD 83), unless otherwise noted. Altitude, as used in this 

report, refers to distance above the vertical datum.

Supplemental Information

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L), micrograms per liter (μg/L), or picograms per liter (pg/L). Chemical concentrations and water temperature are given in metric (SI) units.

A milligram per liter (mg/L) is a unit expressing the concentration of chemical constituents in 

solution as weight (milligrams) of solute per unit volume (liter) of water. A concentration of 

1,000 micrograms per liter (μg/L) is equivalent to 1 milligram per liter. A concentration of 1,000 

nanograms per liter (ng/L) is equivalent to 1 microgram per liter. At dissolved solids concentrations of about 7,000 mg/L or less, nanogram per liter is used interchangeably in this report with 

the related term part per trillion (ppt). A concentration of 1 million picograms per liter (pg/L, also 

picograms per kilogram or pg/kg) is equivalent to 1 milligram per liter.

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

gallon (gal) 3.785 liter (L)

Flow rate

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

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

Mass

ounce, avoirdupois (oz) 28.35 gram (g)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

Hydraulic conductivity

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

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8
vii

Abbreviations

< less than

+/- plus or minus

CWU chemical waste unit

CLU#3 Clinton landfill unit #3

3

H tritium or hydrogen-3

ne effective porosity (dimensionless)

Kh horizontal hydraulic conductivity

Kv vertical hydraulic conductivity

NAVD 88 North American Vertical Datum of 1988

NWIS National Water Information System

NWQL National Water Quality Laboratory

PVC polyvinyl chloride

PCBs polychlorinated biphenyls

QA quality assurance

TSCA Toxic Substances Control Act

TU tritium units

US United States

USGS U.S. Geological Survey

Vv vertical groundwater velocity

One tritium unit (TU) is equivalent to a concentration of 3.2 picocuries per liter and is equal a 

concentration of one atom of tritium per 1018 hydrogen atoms (tritium/hydrogen ratio = 10-18). 

A picocurie is a unit of radioactivity; 1 picocurie is defined as 3.7 × 10-2 decays per second; or in 

International System of Units (SI) terminology, 3.7 × 10-2 becquerel.

Horizontal hydraulic conductivity is given in units of feet per day. Vertical hydraulic conductivity 

is given in units of feet per year.

Abstract 

The U.S. Geological Survey used interpretations of 

hydrogeologic conditions and tritium-based groundwater age 

estimates to assess the travel time of groundwater at a landfill 

site near Clinton, Illinois (the “Clinton site”) where a chemical 

waste unit (CWU) was proposed to be within the Clinton landfill unit #3 (CLU#3). Glacial deposits beneath the CWU consist predominantly of low-permeability silt- and clay-rich till 

interspersed with thin (typically less than 2 feet in thickness) 

layers of more permeable deposits, including the Upper and 

Lower Radnor Till Sands and the Organic Soil unit. These glacial deposits are about 170 feet thick and overlie the Mahomet 

Sand Member of the Banner Formation. The Mahomet aquifer 

is composed of the Mahomet Sand Member and is used for 

water supply in much of east-central Illinois.

Eight tritium analyses of water from seven wells were 

used to evaluate the overall age of recharge to aquifers beneath 

the Clinton site. Groundwater samples were collected from 

six monitoring wells on or adjacent to the CLU#3 that were 

open to glacial deposits above the Mahomet aquifer (the upper 

and lower parts of the Radnor Till Member and the Organic 

Soil unit) and one proximal production well (approximately 

0.5 miles from the CLU#3) that is screened in the Mahomet 

aquifer. The tritium-based age estimates were computed with a 

simplifying, piston-flow assumption: that groundwater moves 

in discrete packets to the sampled interval by advection, without hydrodynamic dispersion or mixing. 

Tritium concentrations indicate a recharge age of at least 

59 years (pre-1953 recharge) for water sampled from deposits below the upper part of the Radnor Till Member at the 

CLU#3, with older water expected at progressively greater 

depth in the tills. The largest tritium concentration from a 

well sampled by this study (well G53S; 0.32 ± 0.10 tritium 

units) was in groundwater from a sand deposit in the upper 

part of the Radnor Till Member; the shallowest permeable 

unit sampled by this study. That result indicated that nearly all 

groundwater sampled from well G53S entered the aquifer as 

recharge before 1953. Tritium was detected in a trace concentration in one sample from a second monitoring well open to 

the upper part of the Radnor Till Member (well G07S; 0.11 ± 

0.09 tritium units), and not detected in samples collected from 

two monitoring wells open to a sand deposit in the lower part 

of the Radnor Till Member, from two samples collected from 

two monitoring wells open to the Organic Soil unit, and in 

two samples collected from a production well screened in the 

middle of the Mahomet aquifer (a groundwater sample and a 

sequential replicate sample). The lack of tritium in five of the 

six groundwater samples collected from the shallow permeable units beneath CLU#3 site and the two samples from the 

one Mahomet aquifer well indicates an absence of post-1952 

recharge. Groundwater-flow paths that could contribute post1952 recharge to the lower part of the Radnor Till Member, the 

Organic Soil unit, or the Mahomet aquifer at the CLU#3 are 

not indicated by these data.

Hypothetical two-part mixtures of tritium-dead, pre-1953 

recharge water and decay-corrected tritium concentrations in 

post-1952 recharge were computed and compared with tritium 

analyses in groundwater sampled from monitoring wells at 

the CLU#3 site to evaluate whether tritium concentrations in 

groundwater could be represented by mixtures involving some 

post-1952 recharge. Results from the hypothetical two-part 

mixtures indicate that groundwater from monitoring well 

(G53S) was predominantly composed of pre-1953 recharge 

and that if present, younger, post-1955 recharge, contributed 

less than 2.5 percent to that sample. The hypothetical twopart mixing results also indicated that very small amounts of 

post-1952 recharge composing less than about 2.5 percent of 

the sample volume could not be distinguished in groundwater 

samples with tritium concentrations less than about 0.15 TU.

The piston-flow based age of recharge determined from 

the tritium concentration in the groundwater sample from 

monitoring well G53S yielded an estimated maximum vertical 

velocity from the land surface to the upper part of the Radnor 

Till Member of 0.85 feet per year or less. This velocity, if 

Application of Hydrogeology and Tritium-Based 

Groundwater-Age Estimates to Assess the Travel Time 

of Groundwater at the Site of a Landfill to the Mahomet 

Aquifer near Clinton, Illinois

By Robert T. Kay and Paul M. Buszka 
2 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

assumed to apply to the remaining glacial till deposits above 

the Mahomet aquifer, indicates that recharge flows through the 

170 feet of glacial deposits between the base of the proposed 

chemical waste unit and the top of the Mahomet aquifer in a 

minimum of 200 years or longer. Analysis of hydraulic data 

from the site, constrained by a tritium-age based maximum 

groundwater velocity estimate, computed minimum estimates 

of effective porosity that range from about 0.021 to 0.024 for 

the predominantly till deposits above the Mahomet aquifer.

Estimated rates of transport of recharge from land surface 

to the Mahomet aquifer for the CLU#3 site computed using 

the Darcy velocity equation with site-specific data were about 

260 years or longer. The Darcy velocity-based estimates 

were computed using values that were based on tritium data, 

estimates of vertical velocity and effective porosity and available site-specific data. Solution of the Darcy velocity equation indicated that maximum vertical groundwater velocities 

through the deposits above the aquifer were 0.41 or 0.61 feet 

per year, depending on the site-specific values of vertical 

hydraulic conductivity (laboratory triaxial test values) and 

effective porosity used for the computation. The resulting 

calculated minimum travel times for groundwater to flow from 

the top of the Berry Clay Member (at the base of the proposed 

chemical waste unit) to the top of the Mahomet aquifer ranged 

from about 260 to 370 years, depending on the velocity value 

used in the calculation. In comparison, plausible travel times 

calculated using vertical hydraulic conductivity values from a 

previously published regional groundwater flow model were 

either slightly less than or longer than those calculated using 

site data and ranged from 230 to 580 years. 

Tritium data from 1996 to 2011 USGS regional sampling 

of groundwater from domestic wells in the confined part of 

the Mahomet aquifer—which are 2.5 to about 40 miles from 

the Clinton site—were compared with site-specific data from 

a production well at the Clinton site. Tritium-based groundwater-age estimates indicated predominantly pre-1953 recharge 

dates for USGS and other prior regional samples of groundwater from domestic wells in the Mahomet aquifer. These 

results agreed with the tritium-based, pre-1953 recharge age 

estimated for a groundwater sample and a sequential replicate 

sample from a production well in the confined part of the 

Mahomet aquifer beneath the Clinton site.

The regional tritium-based groundwater age estimates 

also were compared with pesticide detections in samples from 

distal domestic wells in the USGS regional network that are 

about 2.5 to 40 miles from the Clinton site to identify whether 

very small amounts of post-1952 recharge have in places 

reached confined parts of the Mahomet aquifer at locations 

other than the Clinton site in an approximately 2,000 square 

mile area of the Mahomet aquifer. Very small amounts of post1952 recharge were defined in this analysis as less than about 

2.5 percent of the total recharge contributing to a groundwater sample, based on results from the two-part mixing 

analysis of tritium data from the Clinton site. Pesticide-based 

groundwater-age estimates based on 22 detections of pesticides (13 of these detections were estimated concentrations), 

including atrazine, deethylatrazine (2-Chloro-4-isopropylamino-6-amino-s-triazine), cyanazine, diazinon, metolachlor, 

molinate, prometon, and trifluralin in groundwater samples 

from 10 domestic wells 2.5 to about 40 miles distant from the 

Clinton site indicate that very small amounts of post-1956 to 

post-1992 recharge can in places reach the confined part of the 

Mahomet aquifer in other parts of central Illinois. The relative 

lack of tritium in these samples indicate that the amounts of 

post-1956 to post-1992 recharge contributing to the 10 domestic wells were a very small part of the overall older groundwater sampled from those wells. 

The flow process by which very small amounts of pesticide-bearing groundwater reached the screened intervals of the 

10 domestic wells could not be distinguished between wellintegrity related infiltration and natural hydrogeologic features. Potential explanations include: (1) infiltration through 

man-made avenues in or along the well, (2) flow of very small 

amounts of post-1956 to post-1992 recharge through sparsely 

distributed natural permeable aspects of the glacial till and 

diluted by mixing with older groundwater, or (3) a combination of both processes. 

Presuming the domestic wells sampled by the USGS in 

1996–2011 in the regional study of the confined part of the 

Mahomet aquifer are adequately sealed and produce groundwater that is representative of aquifer conditions, the regional 

tritium and pesticide-based groundwater-age results indicate 

substantial heterogeneity in the glacial stratigraphy above 

the Mahomet aquifer. The pesticide-based groundwater-age 

estimates from the domestic wells distant from the Clinton 

site also indicate that parts of the Mahomet aquifer with the 

pesticide detections can be susceptible to contaminant sources 

at the land surface. The regional pesticide and tritium results 

from the domestic wells further indicate that a potential exists 

for possible contaminants from land surface to be transported 

through the glacial drift deposits that confine the Mahomet 

aquifer in other parts of central Illinois at faster rates than 

those computed for recharge at the Clinton site, including 

CLU#3. This analysis indicates the potential value of submicrogram-per-liter level concentrations of land-use derived 

indicators of modern recharge to indicate the presence of very 

small amounts of modern, post-1952 age recharge in overall 

older, pre-1953 age groundwater.

Introduction

The U.S. Geological Survey (USGS) applied interpretations of hydrogeology and tritium-based groundwater age 

estimates to assess the travel time of groundwater at a landfill 

site near Clinton, Illinois (the “Clinton site”; fig. 1) where a 

chemical waste unit (CWU) was proposed to be within the 

Clinton landfill unit #3 (CLU#3; fig. 2). The Clinton site is 

approximately 2.1 miles (mi) south of the center of the city 

of Clinton, in De Witt County, Illinois (fig. 1). CLU#3 has 

been permitted for the disposal of non-hazardous wastes. An 
Introduction 3

application for a Toxic Substances Control Act (TSCA) permit 

to operate a chemical waste unit (CWU) within the Clinton 

landfill unit #3 (CLU#3) was previously submitted to the 

U.S. Environmental Protection Agency. The application, that 

has since been withdrawn, proposed to use the CWU part of 

CLU#3 for the disposal of polychlorinated biphenyls (PCBs) 

and PCB-related items in accordance with the TSCA. 

The proposed elevation of the base of the CWU is about 

660 feet (ft) above the North American Vertical Datum of 

1988 (NAVD 88). The USGS estimated the travel time of 

groundwater through the geologic deposits at the location of 

the proposed CWU, with emphasis on the travel time through 

the deposits that are between the base of the proposed CWU 

and the top of the underlying Mahomet aquifer.

0 1 MILE

0 1 KILOMETER

Base from U.S. Geological Survey 1:24,000-scale

and 1:100,000-scale digital data,

Albers Equal-Area Conic projection,

standard parallels 33° and 45°,

central meridian −89°

-88°59' -88°58' -88°57' -88°56' -88°55' -88°54'

40°10'

40°09'

40°08'

40°07'

40°06'

ILLINOIS

Map

area

De Witt County

Clinton

Weldon 

Springs 

Lake

Salt Creek

Coon

Creek

Clinton

landfill

54

54

10

10

51

Figure 1. Location of the Clinton site, near Clinton, Illinois.
4 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Figure 2. Location of solid waste units and sampled monitoring wells, Clinton site, near Clinton, Illinois.

720

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680

680

720

640

660

Salt

Creek

W−10

EX−6

G53S

EX−4

G06D

G07S

G07D

Heritage Rd

51

Wilmax Rd

E 350 N Rd

1050 E Rd

Clinton

landfill

unit #1

Clinton

landfill

unit #2

Clinton

landfill

unit #3

(CLU#3)

Proposed

chemical

waste unit

Base from U.S. Geological Survey 1:24,000 

and 1:100,000 digital data,

Albers Equal-Area Conic projection,

standard parallels 33o

 and 45o,

central meridian -89o

0 0.2 MILE

0 0.2 KILOMETER

Meadowbrook Rd

-88°58' -88°57'40" -88°57'20" -88°57' -88°56'40"

40°07'

40°06'40"

40°06'20"

40°06'

EXPLANATION

Clinton landfill unit

Landfill unit #3 excavation boundary

Natural topographic contour, in feet

 above NAVD 88

W−10 Sampled well and name
Introduction 5

The Mahomet aquifer is about 170 ft below the proposed 

CWU. The Mahomet aquifer is the water supply for numerous 

municipal, residential, commercial, irrigation, and industrial users in east-central Illinois (Roadcap and others, 2011; 

Kempton and others, 1991), including the City of Clinton. 

Approximately 170 ft of till composed predominantly of 

silt and clay with intermittent sand layers is present beneath 

the proposed CWU. The till deposits confine the underlying Mahomet aquifer. The part of the Mahomet aquifer that 

includes the Clinton area has been described as a glacial drift 

aquifer in a deep buried valley (the Mahomet Buried Bedrock 

Valley in the Illinois River Basin, shown on fig. 3, p. 5 in 

Morrow, 2001) that is hydraulically confined beneath 100 ft 

or more of clays and silts in the Wisconsinan, Illinoian, and 

pre-Illinoian drift (Morrow, 2001). This part of the Mahomet 

aquifer is referred to in the rest of this report as the “confined 

part of the Mahomet aquifer” and extends from the Illinois 

River Basin boundary in Champaign County westward to the 

western boundary of the Mahomet Buried Bedrock Valley in 

the Illinois River Basin in McLean and Logan Counties 

fig. 3). 

Purpose and Scope

This report summarizes the results of a scientific investigation to assess the travel time of groundwater through the 

geologic deposits that are beneath CLU#3, which includes the 

proposed CWU. This report describes site-specific data on the 

hydrogeology of the Mahomet aquifer and overlying glacial 

deposits covering an area of approximately 0.6 square miles 

at the Clinton site. Data collection, analysis, and interpretive 

Figure 3. Location of the Mahomet aquifer and selected bedrock valleys in east-central Illinois (compiled and modified 

from Kempton and others, 1991; and Roadcap and others, 2011).

Pekin

Eureka

Morton

Normal Paxton

Havana

Urbana

Watseka

Rantoul

Clinton Mahomet

Lincoln

Mackinaw

Danville

Hoopeston

Champaign

Monticello

Beardstown Petersburg

Bloomington

Gibson

City

Springfield

Decatur

Mackinaw Valley

Kenney

Valley

Onarga

Valley

Mahomet Valley

Clinton

landfill

Sangamon

Sangamon

River

River

Illinois

River

Mackinaw River

Vermilion

Salt

Creek

River

Kaskaskia River

Embarras R.

Iroquois

River

MCLEAN

IROQUOIS

LOGAN

CHAMPAIGN

PIATT

VERMILION

MACON

MASON

CASS

SANGAMON

TAZEWELL

DE WITT

WOODFORD

MENARD

FORD

ILLINOIS

INDIANA

88°

88°30'

89°30'

90°

90°30'

89°

41°

40°30'

40°

0 10 20 30 40 MILES 50

0 10 20 30 40 KILOMETERS 50

EXPLANATION

Mahomet aquifer present

Approximate area of enhanced aquifer recharge near Paxton and Monticello

Centerline of Mahomet Valley

Centerline of tributary valley

Western boundary of Mahomet Buried Bedrock Valley in Lower Illinois River

 Basin as used in Morrow (2001) 

Illinois River Basin

Base from U.S. Geological Survey

1:100,000-scale digital data,

Albers Equal-Area Conic projection,

standard parallels 33° and 45°,

central meridian −89°

ILLINOIS
6 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

approaches used to understand groundwater travel times also 

are described. Tritium and water-level data collected during a 

sampling event on May 16, 2012, from six monitoring wells 

open to overlying deposits and one production well open to 

the Mahomet aquifer at and near CLU#3 were used to estimate the age of the recharge contributing to groundwater in 

these deposits and to estimate the vertical rate of groundwater 

flow and travel time through the deposits. The age of recharge 

estimates were prepared using data from monitoring wells 

and a nearby production well approximately 0.5 miles from 

the CLU#3. Additional analysis of groundwater velocity and 

travel time based on solution of the Darcy velocity equation, 

using site-specific data checked against regional information, 

also was prepared for this report. Assumptions used in the 

analyses, and the qualitative impact of these assumptions on 

the velocity estimates are discussed in this report.

Tritium-based estimates of recharge age at the Clinton 

site were computed assuming there was no mixing of groundwater with different ages between recharge and sampling. 

Hypothetical two-part mixtures of tritium-dead, pre-1953 

recharge and decay-corrected tritium concentrations in post1952 recharge were computed and compared with tritium 

analyses in groundwater sampled from wells at the CLU#3 

site to evaluate whether tritium concentrations in groundwater 

could be represented by mixtures involving some post-1952 

recharge. 

Estimates of groundwater age from the prior USGS and 

other regional sampling programs also were evaluated to 

understand whether post-1952 recharge has reached confined 

parts of the Mahomet aquifer outside of the vicinity of the 

CLU#3 site and to understand the degree of heterogeneity in 

the overlying confining glacial deposits. This report provides interpretations of the presence or absence of post-1952 

recharge in the confined part of the Mahomet aquifer covering 

an area of approximately 2,000 square miles in east-central 

Illinois using the tritium and pesticide results from previously 

collected samples of groundwater. The tritium and pesticide 

results were from a prior 1996 to 2011 USGS sampling of preexisting domestic wells in the confined part of the Mahomet 

aquifer. The pre-existing domestic wells are 2.5 to about 

40 miles from the CLU#3 site and were selected to represent 

groundwater conditions in the Mahomet Buried Bedrock Valley in the Illinois River Basin for the USGS National Water 

Quality Assessment Program (fig. 3 in Morrow, 2001, p. 5). 

Pre-2002 tritium data reported by Hackley (2002) also were 

used for this evaluation. 

Regional Hydrogeologic Setting and Indications 

of Recharge to the Mahomet Aquifer from Prior 

Data

The Mahomet aquifer occupies an east-west trending buried bedrock valley that extends in Illinois from an area on the 

east near Watseka, Hoopeston, and Danville westward to the 

Illinois River near Havana (fig. 3). Numerous smaller tributary 

bedrock valleys also are present, including the Kenney Valley 

near the Clinton site and the Mackinaw Valley, which unite 

with the Mahomet Valley in Tazewell County (fig. 3). The 

various valley-fill deposits that compose most of the Mahomet 

aquifer consist primarily of sand and gravel deposits that are 

30 to 100 ft thick and directly overlay the bedrock. These 

sands and gravels are overlain primarily by silt-and-clay rich 

till deposits with interspersed sand and gravel deposits that 

typically are thin and of limited areal extent (Soller and others, 

1999). The deposits of the Mahomet aquifer and the overlying (predominantly) till deposits are as much as 400 ft thick 

(Hackley and others, 2010).

Groundwater flow in the Mahomet aquifer in the Clinton 

area is affected by a potentiometric high about 20 miles southeast of the Clinton site in the vicinity of the Sangamon River 

in Piatt County (Roadcap and others, 2011, recharge area 

shown in fig. 3). This potentiometric high, mapped by Hackley 

and others (2010, p. 1051), has been interpreted to be an area 

of “enhanced” recharge from land surface to the Mahomet 

aquifer, potentially because of the presence of hydraulically 

interconnected sand deposits above the Mahomet aquifer in 

central Piatt County (Panno and others, 1994; Mehnert and 

others, 2004; Hackley and others, 2010). The term “enhanced” 

was used by those authors to indicate that greater amounts of 

natural recharge reached the Mahomet aquifer in that area. 

Groundwater flow in the vicinity of this enhanced recharge 

area is east toward Urbana-Champaign and west through the 

Clinton area to the Illinois River. The Mahomet aquifer in 

western Piatt, De Witt, and McLean counties also appears to 

receive some recharge from the underlying bedrock through 

upward vertical groundwater flow (Panno and others, 1994; 

Hackley and others, 2010).

Topographic Setting of the Clinton Site

The natural surface topography in the vicinity of CLU#3 

typically is between 710 and 730 ft NAVD 88 (fig. 2); however, much of the CLU#3 area has been excavated to about 

660 ft NAVD 88 to accommodate waste disposal (fig. 2). As 

of 2013, topography decreases along a north-south trending 

natural drainage in the eastern one-third of CLU#3, and at the 

western edge of CLU#3. South of CLU#3, the flood plain of 

Salt Creek has an elevation of about 640 to 650 ft NAVD 88. 

The footprint of the proposed CWU also has been excavated, 

but the CWU excavation is north of the bluff in the southwestern part of CLU#3, where surface topography ranges from 

about 680 ft NAVD 88 next to the CWU to about 645 ft in the 

flood plain.
Methods of Data Collection and Analysis for the Clinton Site 7

Methods of Data Collection and 

Analysis for the Clinton Site

The naming convention of some of the wells and borings 

was consistent with those used by and later revised by previous investigators at the Clinton site (PDC Technical Services, 

2005; PDC Technical Services, 2006; Joyce Day, PDC Technical Services, Inc., written commun., 2012). Monitoring well 

EX-21S (fig. 4) was renamed well G53S (fig. 2, table 1). Monitoring well EX-24 (fig. 4) was renamed well G06D (fig. 2, 

table 1). Monitoring wells EX-23S and EX-23D (fig. 4) were 

renamed wells G07S and G07D (fig. 2, table 1). To maintain 

consistency with the terminology used by these investigators, 

the lithologic information presented in subsequent figures in 

this report uses the names of the original borings with which 

the lithologic descriptions are associated, whereas figure 2 

uses the names of the monitoring wells from which the waterlevel and water-chemistry data were collected. Boring location 

names and well names are listed in table 1.

The hydrogeology of the CLU#3 area, including the 

CWU, has been interpreted by USGS analysis of lithologic 

logs from about 45 borings (including clustered borings) 

drilled in the vicinity of the CLU#3 (fig. 4), and from about 

46 borings drilled in the vicinity of Clinton landfill unit #2 

(CLU#2; fig. 2). The boring logs from CLU#3 are presented 

in an appendix to the application for the CWU compiled by 

PDC Technical Services, Inc. (2005). The boring logs from the 

vicinity of CLU#2 were provided by Joyce Day (PDC Technical Services, Inc., written commun., 2012). For consistency, 

this document uses the geologic terminology (Upper and 

Lower Radnor Till Sands, Organic Soil unit) and stratigraphic 

interpretations used by PDC Technical Services, Inc (2005). 

In addition, the results of hydraulic tests (slug tests, triaxial 

permeability tests, water-level measurements) done by PDC 

Technical Services (2005) are referenced in this report. However, the description of the presence, elevation, and extent of 

the geologic deposits, as well as other aspects of the hydrogeology in the CLU#3 area (including groundwater velocity and 

effective porosity [ne

]) are based solely on analysis done by 

the USGS.

Water Sample Collection at the Clinton Site

Unfiltered water samples were collected for this investigation from monitoring wells EX-6, G53S, EX-4, G06D, 

G07S, and G07D; and from production well W-10 on May 

16, 2012 (table 1 and fig. 2). Samples were collected from 

the monitoring wells by use of dedicated bladder pumps with 

intakes positioned within the screened interval of the well.

The monitoring wells were purged of three well volumes 

8 days before the sampling event. The initial purging on 

May 8 was done to remove standing water from the monitoring wells to help ensure the presence of formation water in the 

wells when the samples were collected on May 16 (Andrew 

Whelpley, PDC Technical Services, Inc., oral commun., 2012). 

The monitoring wells were subjected to additional purging and 

monitoring of field-measured water-quality properties (pH, 

temperature, oxidation-reduction potential, dissolved oxygen, 

and specific conductance) as part of sample collection. During 

the sampling event, monitoring wells EX-6, EX-4, G07S, and 

G07D were purged and sampled using a standard low-flow 

technique (Puls and Barcelona, 1996).

Monitoring wells EX-6, G53S, EX-4, G06D, G07S, and 

G07D exhibited less than 0.30 ft of drawdown during the May 

16 purging and samples were collected after field-measured 

water-quality properties were stable for at least three consecutive readings collected at 5 minute intervals. Monitoring well 

G53S had more than 0.30 ft of drawdown during purging 

and was pumped dry after approximately two well volumes 

were purged. The water level in monitoring well G53S was 

allowed to return to its hydrostatic condition before the sample 

was collected. Hydrostatic conditions were achieved within 

about 30 minutes of the well being pumped dry. Monitoring 

well G06D measured more than 0.30 ft of drawdown during 

purging and was purged for three well volumes before sample 

collection. Production well W-10 was purged for 30 minutes at 

a rate of 16.7 gallons per minute 

(gal/min) by discharging water from a faucet approximately 

120 ft from the well. A sample and duplicate sample were 

then collected at the faucet. Water in the W-10 sample was not 

subjected to softening or other forms of treatment.

Groundwater samples for tritium analysis were collected 

in USGS laboratory-approved 1-liter (L) plastic bottles and 

shipped to the USGS Isotope Tracer Laboratory in Menlo 

Park, California within 2 days of collection. Samples were 

contained, stored, and shipped in accordance with USGS protocols (Koterba and others, 1995).

Laboratory Analysis of Groundwater Samples 

From the Clinton Site

Groundwater samples were analyzed for tritium at the 

laboratory in mid-September 2012, using an electrolytic 

enrichment and liquid scintillation analytical method similar to 

that described in USGS method R-1172-76 (Thatcher and others, 1977, p. 73). Tritium data from this method are reported as 

concentrations in tritium units (TU) with a 1-sigma combined 

standard uncertainty, consistent with the method described in 

McCurdy and others (2008, p. 8–9), with the exception that 

the data are not censored to a method reporting limit. One TU 

is equal to a concentration of one tritium atom per 1018 hydrogen atoms (tritium atom to hydrogen atom ratio equals 10-18) 

(Thatcher and others, 1977, p. 8). A liquid scintillation counter 

measures the number of disintegrations per minute when 

tritium in the sample decays by beta emission to stable helium. 

This count rate is related to the count rate of a sample of 

known concentration to determine the tritium concentration of 

the sample. The count rate has a Poisson distribution and the 

longer the count time, the better the accuracy. The counting 
8 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater Table 1.

Well data and results of tritium analyses, Clinton site, near Clinton, Illinois, May 16, 2012.

[NAVD 88, North American Vertical Datum of 1988; the Upper and Lower Radnor Till Sand and Organic Soil are locally named and identified by PDC Technical Services, Inc., 2005; A tritium concentration less than the sample specific critical level is referred to in the report as a very low, near zero tritium concentration. A very low, near zero tritium concentration indicates that tritium derived decay counts yielded from the sample analysis could not be distinguished from the analytical background concentation of zero tritium units]

Well 

name, as 

shown 

on 

figure 2

Boring 

name

USGS well 

identifier

Latitude, 

in degrees, 

minutes, 

seconds

Longitude, 

in degrees, 

minutes, 

seconds

Land 

surface 

elevation, in feet 

above 

NAVD 

88

Measuring point 

elevation, in 

feet 

above 

NAVD 88

Elevation 

range 

of well 

screen, 

in feet 

above 

NAVD 88

Hydrogeologic unit 

monitored

Waterlevel 

altitude 

on May 

16, 2012, 

in feet 

above 

NAVD 88

Tritium 

concentration and 

one-sigma 

uncertainty 

value, in 

tritium units

Sample 

specific 

critical 

level 

(detection 

limit), in 

tritium 

units

Tritiumbased 

groundwater age 

classification1

Groundwater samples

G53S EX-21S 400631088570101 40°06'31.4" 88°57'01.7" 700.4 703.14 643–653 Upper Radnor Till Sand 661.54 0.32 ±0.10 0.10 Post-1952

G07S EX-23S 400642088564701 40°06'41.8" 88°56'47.6" 711.0 713.28 642–652 Upper and Possibly 

Lower Radnor Till Sand

673.33 0.11 ±0.09 0.09 Pre-1953

EX-4 EX-4 400703088565301 40°07'03.2" 88°56'53.3" 724.5 727.33 631–641 Lower Radnor Till 

Sand

690.43 -0.03 ±0.10 0.10 Pre-1953

EX-6 EX-6 400634088565401 40°06'34.2" 88°56'54.2" 694.2 696.67 636–646 Lower Radnor Till 

Sand

671.22 0.00 ±0.11 0.11 Pre-1953

G06D EX-24 400654088564701 40°06'54.3" 88°56'47.4" 727.4 729.73 630–640 Organic Soil 675.03 -0.08 ±0.09 0.09 Pre-1953

G07D EX-23D 400642088564702 40°06'41.9" 88°56'47.5" 710.8 713.41 623–633 Organic Soil 671.33 0.03 ±0.11 0.11 Pre-1953

W-10 W-10 400705088575101 40°07'05.5" 88°57'51.2" 721.0 722.96 440–444 Mahomet aquifer 601.14 -0.04 ±0.10 0.10 Pre-1953

Quality-assurance sample (sequential replicate collected immediately after groundwater sample)

W-10 W-10 400705088575101 40°07'05.5" 88°57'51.2" 721.0 722.96 440–444 Mahomet aquifer 601.14 -0.03 ±0.10 0.10 Pre-1953

1Tritium-based groundwater age classification reference: Clark and Fritz (1997), adapted to represent samples collected in 2012.
Methods of Data Collection and Analysis for the Clinton Site 9

EX–29/EX–30

718

640-641

517-519

402-489

374

EXPLANATION

Boring location and name (”/” indicates shared well location)

N

Elevation of bottom of boring, in feet above NAVD 88

Upper Radnor Till Sand

Lower Radnor Till Sand

Elevation of sand unit(s), in feet above NAVD 88*

Elevation range in blue text is that of sand and gravel of

 Mahomet aquifer, in feet above NAVD 88

Elevation of land surface, in feet above NAVD 88

0

0

0.05

0.05

0.1 MILE

0.1 KILOMETER

* Elevation range is given for deposits with thicknesses greater than 1 foot 

(U)

(L)

EX–2

721

685-699

640 (L)

612

EX–3

712

700-703

694-688

677-685

651-658 (U)

642-643 (L)

639

614

612

EX–5

728

659

653 (U)

637-641 (L)

636

EX–6

694

651-653 (U)

648

636-640 (L)

635

EX–7

691

668

648

635-641 (L)

630

EX–10

711

662

EX–11

706

668

631-633

630

EX–12D/EX–12S

705

660-673

643-644 (L)

633-637

628

EX–15

731

664-672

639-642 (L)

636

EX–16

717

669

639 (L)

617

EX–17

715

675-677

667

634-635

628

EX–18

725

697

656-676

630-631

625

EX–19

718

666

640 (L)

637-638

634

EX–20

674

655-656

632-634

630

EX–9

648

640-644 (L)

619-629

603

EX–22D/EX–22S

667

648-654 (U)

629-632

626

EX–23D/EX–23S

711

650-652 (U)

641-645 (L)

637

627-631

622

EX–24

727

629

EX–25

717

692-697

652-656 (U)

638-640 (L)

638

EX–26

707

664-668

650-651 (U)

638

EX–27

724

705-706

697

667

640

EX–28

724

710-712

703-705

650 (U)

644

EX–29/EX–30

718

640-641 (L)

634

626

592-594

587-589

576

571

569

543-547

538

522

517-519

402-489

374

Proposed 

chemical

waste unit

Clinton landfill

unit #3

EX–4

724

718

703-706

699

685-691

635-638 (L)

630

EX–1

723

697-700

629-631

620-623

613

EX–13

723

681

668

628

EX–14

727

665-666

633

EX–8

654

634-640 (L)

616-618

609

EX–21D/EX–21S

701

694

676-677

647-650 (U)

637-640 (L)

634

Figure 4. Well and boring locations and the elevations of sand deposits, within predominantly silt and clay deposits, and in the 

Mahomet aquifer in the vicinity of Clinton landfill unit #3, near Clinton, Illinois.
10 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

(analytical) uncertainty is indicated by the 1-sigma precision 

estimate in table 1, which may be read as “plus or minus” the 

counting error value. The true concentration of tritium in a 

sample has an approximately 68 percent probability (based 

on one standard deviation of the radioactivity count) of being 

within the 1-sigma uncertainty value of the measured value 

(McCurdy and others, 2008). For example, there is a 

68 percent probability that the true concentration of tritium in 

a sample with a reported tritium concentration of 1.0 TU and a 

1-sigma uncertainty of ±0.25 TU is between 0.75 and 1.25 TU. 

In cases when the tritium concentration, minus its 

1-sigma uncertainty, is equal to or less than zero, that tritium 

concentration is reported in table 1, but is considered to be 

not significantly different from zero. In cases when the tritium 

concentration, minus its 1-sigma uncertainty, is greater than 

zero, that tritium concentration is reported and is regarded as 

the true concentration (McCurdy and others, 2008).

Interpretation of Recharge Using Tritium Data

Tritium (3H) is a radioactive form of the hydrogen atom 

that contains a proton and two neutrons. Tritium is produced 

naturally by the bombardment of atmospheric nitrogen by cosmic radiation (Grosse and others, 1951). Atmospheric tritium 

and oxygen then combine to form tritiated water, which enters 

the groundwater system as recharge from precipitation. The 

natural concentration of tritium in precipitation is estimated to 

be between 5 and 20 tritium units (TU; Kauffman and Libby, 

1954; Payne, 1972). The value used in this report to describe 

initial, pre-1953 atmospheric testing concentration of tritium 

in precipitation was 8 TU, as described by Thatcher (1962). 

Tritium also was created as a by-product of atmospheric testing of thermonuclear bombs from 1951 through 1980, with 

particularly large amounts produced from 1951 through 1962 

(Clark and Fritz, 1997). Tritium concentrations in precipitation 

have been measured monthly at a site in Ottawa, Canada, from 

1953 to at least 2002 (International Atomic Energy Agency, 

2006). Tritium concentrations generally increased from about 

30 TU in late 1953 to about 6,000 TU in 1963, returning to 

near natural levels (about 19 TU at Ottawa, for example) by 

about 1993 (fig. 5A).

Radioactive decay of tritium leads to decreases in tritium 

concentration with time (fig. 5B). For example, recharge from 

precipitation with a tritium concentration of 8 TU in 1951 

(Thatcher, 1962) would have a tritium concentration of about 

0.3 TU in 2012 according to the equation:

c(t) = c(t

0

) * e-(0.693t/12.33) (1)

Where:

c(t) is the tritium concentration at the time of 

sample collection and analysis (2012),

t is the time in years between the start time and 

the time being considered (t=61 years from 

1951 to 2012),

c(t

0

) is the starting tritium concentration when 

tritium in precipitation entered the 

groundwater system as recharge (8 TU),

t

0 is the time when tritium in precipitation 

entered the groundwater system as 

recharge (in this example 1951),

e is a constant equal to 2.71828, and

12.33 is the half-life of tritium in years (Lucas and 

Unterweger, 2000).

Mixing and reaction of waters are complexities that are 

ignored in simple models of tritium concentrations in groundwater. A basic assumption made in an analysis of tritium-based 

indications of groundwater age is that the groundwater is 

undergoing uniform or “piston” flow. Piston flow is a simplified and idealized concept of groundwater flow in which 

groundwater moves in discrete packets by advection, without 

hydrodynamic dispersion or mixing (Shapiro and others, 

2012). In piston flow, preferential flow along secondary flow 

paths does not substantially contribute to the hydrologic system. With these assumptions, the tritium concentration in the 

groundwater sample represents the original tritium concentration in recharge from infiltrating precipitation, modified only 

by radioactive decay.

Groundwater that contained less than about 0.8 TU 

in 1997 (Clark and Fritz, 1997, p. 185) and about 0.3 TU 

or less in 2012 is assumed to be “submodern” and free of 

“bomb tritium” and to have entered groundwater as recharge 

before about 1953. Those values are based on a pre-1953, 

pre-atmospheric testing concentration of tritium in precipitation of 8 TU and assume that tritium concentrations in water 

subsequently changed only because of radioactive decay. 

Groundwater that contained about 0.8 TU or more tritium in 

1997 and more than 0.3 TU in 2012 is assumed to contain 

some component of bomb tritium or “modern” recharge and 

indicates that most of the recharge contribution to the groundwater sample was after about 1952. For the purposes of this 

report, water that recharged groundwater before about 1953 

also will be referred to as “submodern” or pre-1953 recharge 

and water that recharged during or after 1953 will be referred 

to as “modern” or post-1952 recharge.

A piston-flow model consistent with that described in 

concept by Shapiro and others (2012) was used to explain 

the Clinton sitefield data and represent the overall movement 

of groundwater and tritium in the subsurface. An alternate 

two-part mixture hypothesis was included in the data analysis 

to evaluate whether tritium results are affected by the mixing of older, pre-1953 and younger, post-1952 water during 

groundwater flow between recharge and sampling. Hypothetical two-part mixtures were computed by varying the assumed 

percentage of tritium-dead recharge from before 1953 and 

decay- modified recharge from a year between 1953 and 2002. 

Decay-modified values of tritium in precipitation used for 

these estimates were based on values computed from Ottawa, 

Canada precipitation between 1953 and 2002.
Methods of Data Collection and Analysis for the Clinton Site 11

Tritium in Ottawa, Canada precipitation

EXPLANATION

0.1

1

10

100

1,000

10,000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Tritium concentration in precipitation, in tritium units 

Precipitation sample collection date, in years

1950 1960 1970 1980 1990 2000 2010

Tritium concentration in precipitation, in tritium units, 

corrected for radioactive decay to May 16, 2012 

Year of averaged tritium value

A

B

0.1

1

10

100

1,000

10,000

Figure 5. Concentrations of tritium in precipitation and decay-corrected, annual average concentrations of tritium in 

precipitation, decay corrected to the date of sampling (May 16, 2012). A, concentrations of tritium in precipitation from 

Ottawa, Canada, 1952 to 2002 and B, decay-corrected, annual average concentrations of tritium in precipitation. Data 

from B were used to estimate tritium concentrations in 1953 to 2002 recharge to groundwater for two-part mixture 

computations.
12 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Hydrogeology, Estimates of 

Groundwater Age, and Assessment of 

Groundwater Travel Time at the Clinton 

Site

Recharge and vertical groundwater flow to the Mahomet 

aquifer in areas overlain predominantly by glacial till is 

affected by several factors, including 

• the horizontal continuity and hydraulic character of the 

glacial till deposits, 

• horizontal and vertical continuity of water-bearing 

units within the glacial till, 

• the presence or absence of interstitial and secondary 

porosity and the character of vertical permeability 

through the units, and

• the direction of groundwater flow, as indicated by 

water-level gradients vertically between the glacial till 

units and the underlying Mahomet aquifer. 

This section of the report discusses these elements of 

the groundwater flow system beneath the Clinton site, with 

particular attention to techniques that provide indications of 

recharge and rates of vertical groundwater flow beneath the 

CLU#3 site and the proposed CWU. 

Hydrogeology and Estimates of Groundwater 

Age at the Clinton Site

The Quaternary-aged deposits beneath CLU#3 at the 

Clinton site consist of the Banner Formation deposits, which 

are divided into the Mahomet Sand Member and the overlying 

deposits of the Hillary Till Member and Tilton Till Member 

(fig. 6; Soller and others, 1999; Daniel Drommerhausen, Shaw 

Environmental, Inc., written commun., 2013). The Mahomet 

aquifer is composed of the Mahomet Sand Member and is 

used for water supply in much of east-central Illinois (Roadcap and others, 2011). The Banner Formation is overlain by 

the Glasford Formation, which consists of the Smithboro Till 

Member, the Vandalia Till Member, the Radnor Till Member, 

and the Berry Clay Member. The Glasford Formation is overlain by the Roxana Silt and Robein Silt Member of the Mason 

Group (referred to in this report as the Roxana Silt-Robein Silt 

Member), the Tiskilwa Formation of the Wedron Group, and 

the surficial Peoria Silt. At the Clinton site, the deposits above 

the Berry Clay Member have been excavated at CLU#3 so that 

the base of the proposed CWU will be directly overlying the 

Berry Clay Member.

Assessment of hydrogeologic conditions throughout the 

glacial stratigraphy at the Clinton site was limited by the fact 

that almost all of the available site borings at CLU#3 terminate 

more than 100 ft above the top of the Mahomet aquifer 

(figs. 4, 7–11). Boring EX-30 is the only boring at CLU#3 to 

penetrate the Mahomet aquifer. Boring CSM-1, about 750 ft 

west of the western boundary of CLU#3 (fig. 7), also penetrates the Mahomet aquifer. Boreholes EX-30 and CSM-1 

were abandoned before this study so no monitoring wells are 

open to the Mahomet aquifer at the landfill.

Continuity of Deposits at the Clinton Site

The Quaternary-aged geologic deposits beneath CLU#3 

at the Clinton site consist predominantly of low- permeability 

silt-and-clay rich till with thin interspersed sands (typically 

less than 2 ft) and peat (figs. 5, 6, 8–11). When present, sand 

deposits, particularly laterally or vertically continuous sand 

deposits, have the potential to be pathways for lateral off-site 

flow and vertical flow of groundwater. For example, Hackley 

and others (2010) describe how stacked sand deposits near 

a river valley may have created a hydrologic window in the 

glacial deposits above the Mahomet aquifer at one distal 

location in Piatt County (over 20 miles from the Clinton site). 

Assessment of the location and potential interconnectedness of 

sand deposits therefore was a focal point of this investigation. 

For the purposes of this report a sand deposit was classified as 

substantially continuous if it was present at a similar elevation 

beneath a part, though not all of the CLU#3 or the CWU. For 

example, a sand deposit classified as substantially continuous 

may be present within the eastern one-half of CLU#3, but not 

in smaller areas beneath CLU#3 where till or peat may be at 

the elevation of the sand deposit in one or more borings.

Assessment of the lateral and vertical continuity of the 

sand deposits was based on correlations interpreted from an 

examination of lithologic logs from borings and cross sections 

(figs. 5, 7–11). Vertically continuous sand deposits were not 

identified above the Mahomet aquifer beneath CLU#3 at the 

Clinton site. Subsequent discussion is of the lateral continuity of the sand deposits unless vertical continuity is expressly 

described. Horizontally continuous till, sand or other deposits are considered to be present when at similar elevations 

or along a spatial trend in elevation, as described in lithologic logs from borings at CLU#3 and the surrounding area. 

Because this discussion focuses on hydrogeologic characteristics of the Clinton site, the geologic deposits are discussed in 

order of increasing depth below land surface.

Although this discussion focuses on the continuous sand 

deposits, the silt-and-clay rich till deposits, within which the 

sands are vertically and horizontally interspersed, are both 

laterally and vertically continuous beneath CLU#3. Roadcap 

and others (2011, p. 24) indicate that regionally, groundwater 

does not readily flow through the fine-grained Quaternary 

deposits (silt, clay, or till) above the Mahomet aquifer, in part, 

because the pore spaces are small and not well interconnected. 

Secondary permeable features, if present, have the potential to 

create paths of hydraulic interconnection of sand units across 

glacial till deposits. Subsurface evidence of secondary permeable features previously described as possible in till deposits 
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 13

in other areas of the confined part of the Mahomet aquifer by 

Roadcap and others (2011, p. 27–28) were not identified in 

logs of borings drilled at the Clinton site that were reviewed 

for this study. These secondary permeable features described 

as possible in other areas of the Mahomet aquifer included 

vertically interconnected sands deposited during downcutting 

of streams during interglacial periods or by upward injection 

of overpressured sands from buoyancy failures of till during 

Period Formation Member or 

unit

Approximate 

elevation,

in feet above North 

American Vertical 

Datum of 1988

Description Pertinent features

Mason No formation 

defined Peoria Silt unit Typically less 2 feet thick

Modern soil, 

silty loess

Wedron Tiskilwa No member 

defined

Typically about 720

to 671

Till with 

interspersed 

sands

Mason No formation 

defined

Roxana SiltRobein Silt

Member

663–670 Peat and organic

silt

Berry Clay 658–666 Blue clay

Radnor Till 631–657

Till with 

interspersed 

sands

Vandalia Till 539–630

Till with 

interspersed 

sands

Smithboro Till 522–538

Till with 

interspersed 

sands

Tilton Till 503–521

Till with 

interspersed 

sands

Hillary Till 490–502 Till

Mahomet Sand 402–489 Sand and gravel

Mahomet aquifer is 

composed of the Mahomet 

Sand Member. No 

permeability tests at Clinton

site from this formation.

1

1

Site permeability test information from PDC Technical Services, Inc. (2005).

2

Regional model data from Roadcap and others (2011). Quaternary

GroupNo group defined

Deposits excavated to 

accommodate landfill. 

Vertical fractures likely in 

at least part of these 

deposits. Vertical hydraulic

conductivity

0.024 feet per year from site

triaxial permeability tests.1

Vertical hydraulic 

conductivity of Wedron 

Group 0.37 feet per year 

from regional model.2

Glasford

Berry Clay directly beneath 

landfill excavation. Upper 

Radnor Till Sand at 647- 

650 feet at well G53S.

Vertical hydraulic 

conductivity 0.035 feet per 

year from site triaxial 

permeability tests1

 and

0.037 feet per year from 

regional model2

. Vertical 

fracturing not described in 

borings at Clinton landfill

unit #3.1

Banner

Vertical hydraulic 

conductivity 0.015 feet per 

year from regional model. 

No site triaxial permeability

tests from this deposit.

1

Figure 6. Glacial stratigraphy of Quaternary-aged deposits and pertinent features beneath Clinton landfill unit #3, near Clinton, 

Illinois.
14 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Clinton landfill

unit #3

Proposed

chemical

waste 

unit

A

B

C

C ́

B ́

D D ́

A ́

EX–1

EX–2

EX–3

EX–4

EX–5

EX–6

EX–7

EX–10

EX–11

EX–12D/

EX–12S EX–13

EX–14

EX–15

EX–16

EX–17

EX–18

EX–19

CSM–1

EX–20

EX–9

EX–8

EX–22D/

EX–22S

EX–21D/

EX–21S

EX–23D/

EX–23S

EX–24

EX–25

EX–26 EX–27

EX–28

EX–29/

EX–30

EXPLANATION

EX–1

Line of geologic section and identifier

Boring location and name

A A ́

N

0

0

0.05

0.05

0.1 MILE

0.1 KILOMETER

Figure 7. Location of lines of geologic section, Clinton landfill unit #3, near Clinton, Illinois.
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 15 Figure 8. Geologic cross section showing line of geologic section A–A’, Clinton landfill unit #3, near Clinton, Illinois (see fig. 7 for location of geologic section).

?

?

?

?

? ? ? ?

?

?

?

?

? ?

?

?

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

CSM–1

no data

EX–17

665.5

EX–18

no data

EX–29/

EX–30

no data

EX–5

672.8

EX–24

675.8

Mahomet aquifer

URTS

LRTS

bedrock

A A ́

WEST EAST

FEET

750 750700 700650 650600 600550 550500 500450 450400 400

EX–5

672.8

NOT TO SCALE

DATUM IS NAVD 88

00 150 300 METERS

500 1,000 FEET

EXPLANATION

Silt and claySandPeatContact—Dashed where approximately locatedThin sandBoring location with name and water-level elevation in well on November 18, 2004, from PDC Technical Services, Inc., 2005Organic SoilUpper Radnor Till SandLower Radnor Till SandExtent of geologic material unknown

URTS

LRTS

?
16 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater EX–4 691.43 NOT TO SCALE DATUM IS NAVD 88

00 150 300 METERS

500 1,000 FEET

EXPLANATION

Silt and claySandPeatContact—Dashed where approximately locatedThin sandBoring location with name and water-level elevation in well on November 18, 2004, from PDC Technical Services, Inc., 2005Organic SoilLower Radnor Till SandExtent of geologic material unknown

LRTS

?

B B ́

WEST EAST

FEET

750 750700 700650 650600 600550 550

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

EX–12d

663.9

and

EX–12s

665.2

EX–10

no data

EX–14

678.6 EX–4

691.43

EX–13

669.5

EX–1

no data

EX–15

691.6

LRTS LRTS

No data

?

? ?

?

?

? ? ?

Figure 9. Geologic cross section showing line of geologic section B–B’, Clinton landfill unit #3, near Clinton, Illinois (see fig. 7 for location of geologic section).
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 17 Figure 10. Geologic cross section showing line of geologic section C–C’, Clinton landfill unit #3, near Clinton, Illinois (see fig. 7 for location of geologic section).

?

?

?

?

?

C C ́

WEST EAST

FEET

750 750700 700650 650600

550

600550

550 550

Approximate extent of proposed chemical waste unit

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

EX–19

642

EX–26

no data

EX–25

no data

EX–27

no data

EX–23d

673.81

EX-23s

674

LRTS

LRTS

URTS

No data

EX–19

642

NOT TO SCALE

DATUM IS NAVD 88

00 150 300 METERS

500 1,000 FEET

EXPLANATION

Silt and claySandPeatContact—Dashed where approximately locatedThin sandBoring location with name and water-level elevation in well on November 18, 2004, from PDC Technical Services, Inc., 2005Organic SoilUpper Radnor Till SandLower Radnor Till SandExtent of geologic material unknown

URTS

LRTS

?
18 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater Figure 11. Geologic cross section showing line of geologic section D–D’, Clinton landfill unit #3, near Clinton, Illinois (see fig. 7 for location of geologic section).

? ? ?

?

?

?

?

D D ́

WEST EAST

FEET

750 750700 700650 650600

550

600550

500 500

BEND IN

SECTION

BEND IN

SECTION

EX–9

no data

EX–21d

658.29

EX-21s

662.48

EX–20

659.25 EX–8

646.13

LRTS

URTS

No data

NOT TO SCALE

DATUM IS NAVD 88

00 150 300 METERS

500 1,000 FEET

EXPLANATION

Silt and claySandPeatContact—Dashed where approximately locatedThin sandBoring location with name and water-level elevation in well on November 18, 2004, from PDC Technical Services, Inc., 2005Organic SoilUpper Radnor Till SandLower Radnor Till SandExtent of geologic material unknown

EX–20

659.25

URTS

LRTS

?
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 19

glacial ice readvances and smaller scale vertical conduits from 

fracturing of till deposits during the loading and unloading of 

the glacial ice (Roadcap and others 2011, p. 27–28). The relative abundance or lack of vertical permeable interconnections 

of the permeable units through the till deposits are indirectly 

evaluated later in this report through tritium-based groundwater age estimates prepared using analyses of groundwater 

sampled from these permeable units at the Clinton site.

Shallow Sand Deposits

The shallowest substantially continuous sand deposit 

at CLU#3 is present at variable elevations, potentially being 

present at multiple elevations at the same boring, but typically 

is above about 675 ft NAVD 88 (figs. 5, 7–11). Most or all of 

this deposit has been removed by excavation done within the 

footprint of CLU#3, but sand remains in areas that have not 

been excavated. Before excavation, this deposit typically was 

about 2–15 ft thick. The deposit was present at borings EX-1, 

EX-3, EX-4, EX-21, EX-25, EX-27, and EX-28 and may be 

present at EX-17 and EX-18. This sand deposit was identified at elevations between 675 and 705 ft in several borings 

surrounding CLU#2, indicating that the deposit is spatially 

extensive where it has not been excavated. CLU#2 has been 

excavated to an elevation of about 625 ft NAVD 88 (Joyce 

Day, PDC Technical Services, Inc., written commun. 2012), 

so this sand deposit is absent within the waste footprint of 

CLU#2. No wells monitor this deposit at CLU#3 but lithologic logs compiled during drilling describe saturated sands 

at an elevation of about 700 ft at borings EX-1, EX-3, EX-4, 

EX-25, and EX-28 and “wet to saturated” sands at about 705 

ft at boring EX-27.

A second, substantially continuous sand deposit appears 

to be present at an elevation of about 666–668 ft NAVD 88 

along the western edge of CLU#3 at borings EX-7, EX-11, 

EX-12, EX-17, and EX-19 (fig. 4). Sand was observed at 

about this elevation beneath much of the northern and central 

parts of CLU#3 at borings EX-13, EX-14, EX-15, EX-16, 

EX-18, EX-26, and EX-27 (fig. 4). This deposit also has been 

removed by excavation in much of the CLU#3 footprint, 

including the CWU, but it may remain beneath the excavation 

in parts of the CLU#3 footprint and in areas outside the excavation. Before excavation, this deposit typically was less 

than 0.5 ft thick beneath CLU#3, but was as much as 

20 ft thick at boring EX-18. Several borings in the vicinity of 

CLU#2 intersected sand deposits as thick as 20 ft that included 

the 666–668 ft interval, indicating that the sand thickens to the 

west. This deposit also has been excavated within the waste 

footprint of CLU#2 and was at least partly saturated near 

CLU#2. Because shallow groundwater typically flows toward 

the nearest surface-water body, flow in these shallow sands in 

the vicinity of the CWU was expected to be south toward Salt 

Creek.

Shallow Peat Deposit

A substantially continuous peat deposit is present beneath 

much of the CLU#3 area at an elevation of about 660–670 ft 

NAVD 88, as indicated on geologic sections A–A’, B–B’ and 

C–C’ (figs. 8–10). This peat deposit was absent beneath parts 

of the proposed CWU along geologic section D–D’ (fig. 11) 

and was not identified in borings EX-6 and EX-7 (fig. 7; boring logs in PDC Technical Services, Inc., 2005). The deposit 

also is absent in the east-central part of CLU#3 at EX-5 and 

EX-28, although organic material was described at an elevation of about 673 ft NAVD 88 in both of these borings. The 

peat deposit also is absent in local areas where the sand 

deposit (described in the previous paragraph) is present at this 

elevation (EX-12, EX-14, EX-15, EX-18). Where present, 

the peat deposit ranges in thickness from about 2 to 8 ft. This 

deposit also has been removed by excavation beneath much of 

CLU#3, but remains in unexcavated areas.

Upper Radnor Till Sand

The next deepest substantially continuous sand deposit 

is in the upper part of the Radnor Till Member in the southeastern part of CLU#3 at an elevation of about 651 ft NAVD 

88 (fig. 4, 8–11). This deposit has been named the Upper 

Radnor Till Sand by PDC Technical Services, Inc. (2005) 

and is the uppermost deposit monitored by wells at CLU#3. 

The Upper Radnor Till Sand ranges in thickness from about 

0.25 to 3 ft, with the thickness generally increasing from 

north to south. The Upper Radnor Till Sand as identified at 

borings EX-3, EX-5, EX-6, EX-21, EX-22, EX-23, EX-25, 

EX-26, and EX-28, which are in the south-southeastern part of 

CLU#3, including part of the CWU. Based on elevation, the 

Upper Radnor Till Sand also may be present at boring EX-20. 

Single-well aquifer tests (commonly called slug tests) done 

by PDC Technical Services, Inc., in monitoring wells G53S/

EX-21S, EX-22S, and G07S/EX-23S, which are open to the 

Upper Radnor Till Sand, yielded horizontal hydraulic conductivity (Kh) values of 0.46, 0.31, and 0.027 feet per day (ft/d), 

respectively (units converted from PDC Technical Services, 

Inc., 2005).

Water levels measured in three monitoring wells open 

to the Upper Radnor Till Sand during 2003 and 2004 ranged 

from about 661 to 677 ft and indicate a southwesterly flow 

direction (PDC Technical Services, Inc., 2005). The water 

levels measured in 2003–04 were consistent with the water 

levels measured by the USGS at the time of sampling (table 1) 

and indicate that the monitoring wells sampled from the Upper 

Radnor Till Sand for this investigation were hydraulically 

upgradient of CLU#2 and were not affected by groundwater 

flow from CLU#2.

The tritium concentration in the sample from monitoring well G53S (0.32 ± 0.10 TU; table 1) was similar to the 

tritium concentration in 2012 (0.3 TU) that represents recharge 
20 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

at the beginning of atmospheric testing in 1953 (as described 

in the section of this report titled “Interpretation of Recharge 

Using Tritium Data”). This tritium concentration indicates 

that groundwater in the sample from this monitoring well 

was submodern and entered the subsurface as recharge before 

the advent of bomb-influenced tritium in precipitation, about 

1953 or before (Clark and Fritz, 1997); therefore, groundwater in the Upper Radnor Till Sand at this well infiltrated into 

the ground 59 years or more before sample collection. The 

Upper Radnor Till Sand at monitoring well G53S is at a depth 

of 50 ft below land surface; the elevation of the land surface 

at well G53S is 700 ft above NAVD 88 (table 1), and the top 

of the Upper Radnor Till Sand is 650 ft above NAVD 88 at 

well G53S/boring EX21-S (fig. 4). If the assumption of piston 

groundwater flow is valid, the vertical rate of water movement 

from land surface to the Upper Radnor Till Sand, including 

flow through the (likely thin) unsaturated zone, is calculated to 

be 50 ft in a minimum of 59 years, or no more than 0.85 ft/yr.

The trace concentration of detectable tritium in the 

sample collected from the second on-site well screened in the 

Upper Radnor Till Sand, monitoring well G07S (0.11 ± 0.09 

TU, table 1), indicates the water in this sample was submodern 

and recharged groundwater before 1953. Groundwater in this 

monitoring well had an age greater than 59 years at the time of 

sampling. Monitoring well G07S is open to the Upper Radnor 

Till Sand at an elevation of about 650–652 ft NAVD 88 and 

a second lower sand deposit at an elevation of 641 to 645 ft 

NAVD 88 (table 1 and fig. 5). The sample was a mixture of 

water from the Upper Radnor Till Sand and the Lower Radnor 

Till Sand. The small tritium concentrations from the two wells 

that produce water from the Upper Radnor Till Sand (G53S 

and G07S; table 1) relative to the estimated tritium concentrations in recharge (fig. 5B) indicate very slow rates of vertical recharge to the Upper Radnor Till Sand. These data also 

indicate that vertical pathways of groundwater flow from land 

surface to these wells in the Upper Radnor Till Sand at the 

Clinton site, if present, are sparse in their distribution. 

Lower Radnor Till Sand

Below the Upper Radnor Till Sand, the next deepest substantially continuous sand deposit is in the lower part of the 

Radnor Till Member. This deposit has been named the Lower 

Radnor Till Sand by PDC Technical Services, Inc. (2005)

and is present beneath most of CLU#3 and the western part 

of CLU#2 at an elevation of about 640 ft NAVD 88 (fig. 4, 

8–11). This deposit ranges in thickness from about 0.25 to 

5 ft and tends to be thinnest in the center and northwestern 

parts of the CLU#3. The Lower Radnor Till Sand was identified at borings EX-2, EX-3, EX-4, EX-5, EX-6, EX-7, EX-8, 

EX-9, EX-12, EX-15, EX-16, EX-19, EX-21, EX-23, EX-25, 

and EX-29. Slug tests done by PDC Technical Services, Inc., 

in six monitoring wells open to the Lower Radnor Till Sand 

yield a geometric mean horizontal hydraulic conductivity of 

1.5 ft/d (units converted from PDC Technical Services, Inc., 

2005).

Water levels in nine monitoring wells open to the Lower 

Radnor Till Sand during 2003 and 2004 ranged from about 

641 to 691 ft and indicated a predominantly west-southwest 

groundwater flow direction in the Lower Radnor Till Sand 

(PDC Technical Services, Inc., 2005). These values were consistent with the water levels measured by the USGS during the 

May 16, 2012, sampling event (table 1) and indicate that the 

monitoring wells sampled from the Lower Radnor Till Sand 

for this investigation were hydraulically upgradient of CLU#2. 

At monitoring well cluster EX-21S (open to the Upper Radnor 

Till Sand) and EX-21D (open to the Lower Radnor Till Sand) 

water levels indicate a downward vertical hydraulic gradient 

and the potential for downward flow from the Upper Radnor 

Till Sand to the Lower Radnor Till Sand (PDC Technical Services, Inc., 2005). The high water level in the Lower Radnor 

Till Sand at monitoring well EX-4 (table 1) is in an area where 

the Upper Radnor Till Sand is absent and cannot be used to 

assess vertical flow directions between these two deposits.

Tritium was not detected in the samples from the Lower 

Radnor Till Sand at monitoring wells EX-4 and EX-6 (table 1). 

This lack of detectable tritium in the samples indicates that the 

water in these wells entered the subsurface as recharge before 

1953 (Clark and Fritz, 1997), making the age of the water in 

these wells more than 59 years at the time of sampling.

Organic Soil Unit

The deepest monitoring wells at CLU#3 are open to 

the Organic Soil unit identified by PDC Technical Services, 

Inc. (2005). The Organic Soil unit appears to be a paleo-soil 

consisting of a mixture of silt and clay with organic material. 

The Organic Soil unit is present beneath most of CLU#3 but 

is absent or not penetrated at several borings (figs. 8–11). This 

deposit typically is 1–2 ft thick and at an elevation of about 

631–635 ft NAVD 88 beneath CLU#3. Slug tests done by PDC 

Technical Services, Inc. in eight monitoring wells open to the 

Organic Soil unit yielded a geometric mean Kh of 0.15 ft/d 

(units converted from PDC Technical Services, Inc., 2005).

Water levels measured in eight monitoring wells open 

to the Organic Soil unit during 2003 and 2004 ranged from 

elevations of about 659 to 680 ft NAVD 88 and indicate a 

predominantly west-southwest flow direction (PDC Technical Services, Inc., 2005). These values and directions were 

consistent with the water levels measured by the USGS at the 

time of sampling (table 1) and indicate that the Organic Soil 

unit monitoring wells sampled for the current investigation 

were hydraulically upgradient of CLU#2. At well cluster EX12S (open to the Lower Radnor Till Sand) and EX-12D (open 

to the Organic Soil unit) and at well cluster G07S (open to the 

Upper Radnor Till Sand) and G07D (open to the Organic Soil 

unit) water levels indicated downward vertical hydraulic 

gradients and the potential for downward flow at these 

monitoring wells (PDC Technical Services, Inc., 2005) 

(table 1). At well cluster EX-22S (open to the Upper Radnor 

Till Sand) and EX-22D (open to the Organic Soil unit) water 
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 21

levels indicated upward vertical hydraulic gradients and the 

potential for upward flow at these monitoring wells (PDC 

Technical Services, Inc., 2005).

Tritium was not detected in groundwater sampled from 

the Organic Soil unit at monitoring wells G06D and G07D 

(table 1). This lack of detection indicates the water in these 

monitoring wells entered the ground as recharge before 1953 

(Clark and Fritz, 1997), making the age of the water in these 

wells more than 59 years at the time of sampling.

Glasford Formation Below the Organic Soil Unit 

and the Banner Formation Tills

No wells monitor the Glasford Formation deposits below 

the Organic Soil unit or the till members of the Banner Formation at CLU#3 and tritium samples from those deposits could 

not be collected. Geologic interpretation of the deposits below 

the Organic Soil unit is based on lithologic logs for borings 

EX-29/EX-30 and CSM-1 (PDC Technical Services, Inc., 

2005). These logs indicate the presence of numerous sand 

deposits at between elevations of about 562 and 626 ft NAVD 

88 at borings CSM-1 and EX-30 (fig. 4, 8). These deposits 

typically are less than 2 ft thick, but can be as much as about 

10 ft thick. At least two of these sand deposits appear to be 

substantially continuous between borings CSM-1 and EX-30. 

An additional four sand deposits were identified at boring 

EX-30 between elevations of about 515 and 545 ft NAVD 88; 

however, the additional four sand deposits were not detected at 

boring CSM-1.

Mahomet Aquifer at the Clinton Site

The Mahomet aquifer at the Clinton site is composed of 

the Mahomet Sand Member of the Banner Formation (fig. 6). 

The top of the Mahomet aquifer is at an elevation of about 

489 ft NAVD 88 and the bottom is at an elevation of about 

402 ft NAVD 88 at boring EX-30 in the vicinity of the Clinton 

site fig. 4, 6, 8). The Mahomet aquifer was identified in boring 

EX-30 at a depth of about 230–317 ft below land surface. The 

area of the proposed CWU has been excavated to an elevation 

of about 660 ft NAVD 88 and there is approximately 170 ft of 

geologic material between the base of the CWU and the top of 

the Mahomet aquifer. Of this 170 ft of material, about 155 ft 

is silt, clay, or peat, with the remaining 15 ft composed of 

12 separate sand deposits, as in boring EX-30. The thickest of 

the 12 separate sand deposits in boring EX-30 is about 7 ft at 

an elevation range from 651–658 ft NAVD 88 at EX-30.

The water-level elevation in the Mahomet aquifer at 

production well W-10 was measured to be about 601 ft 

NAVD 88 during the May 16, 2012, sampling event (table 1). 

This elevation is consistent with regional assessments of the 

potentiometric surface for the Mahomet aquifer in the Clinton, 

Ill. area (Roadcap and others, 2011), and was at least 60 ft 

lower than the water levels in the wells open to the Organic 

Soil unit, Lower Radnor Till Sand, or Upper Radnor Till 

Sand deposits (table 1). Although well W-10 is approximately 

0.5 mile from the CLU#3 site (fig. 2), the large downward 

vertical hydraulic gradient between wells in the Organic Soil 

unit, Lower Radnor Till Sand and Upper Radnor Till Sand 

and W-10 in the Mahomet aquifer indicates the existence of 

a considerable degree of hydraulic separation between these 

deposits and the Mahomet aquifer but also the potential for 

downward flow from these deposits to the aquifer. The vertical head difference between (1) the Organic Soil unit, Lower 

Radnor Till Sand and Upper Radnor Till Sand and (2) the 

Mahomet aquifer at the Clinton site additionally indicates an 

overall resistance to vertical flow from these deposits to the 

Mahomet aquifer; this resistance could be explained by the 

low vertical permeability of the predominantly fine grained 

glacial till deposits between these units and the Mahomet 

aquifer. These data indicate that the hydraulic interconnection 

between the Upper Radnor Till Sand, the Lower Radnor Till 

Sand and the Organic Soil unit through glacial till deposits to 

the Mahomet aquifer is limited.

Tritium was not detected in concentrations at or greater 

than reporting levels in the two samples collected from the 

Mahomet aquifer at well W-10 (table 1). Production well W-10 

is open to the middle of the aquifer at 440–444 ft NAVD 88, 

about 45 ft below the top of the aquifer. These data indicate 

that the groundwater at the open interval of well W-10, and 

likely in the Mahomet aquifer beneath CLU#3 and the CWU, 

was likely recharged before 1953 (Clark and Fritz, 1997) and 

has an age of more than 59 years.

Two-Part Mixing Model Evaluation of TritiumBased Groundwater Age Estimates at the Clinton 

Site

The validity of the piston-flow assumptions used to 

estimate groundwater age since recharge was evaluated by 

comparing the only notable tritium detection in groundwater 

from the 8 wells sampled at the Clinton site (0.32 ± 0.10 TU; 

monitoring well G53S), with hypothetical tritium concentrations computed from simulated two-part mixtures of pre-1953, 

tritium-dead recharge and post-1952 recharge containing 

tritium. The hypothetical concentrations were computed as if 

they had originated as a two-part mixture of submodern pre1953, tritium-dead recharge combined with 1 percent or more 

of modern recharge containing tritium concentrations from 

precipitation (fig. 5A) corrected for radioactive decay (fig. 5B) 

to the date the samples were collected (May 16, 2012).

Groundwater-age dates also were compared with estimates computed using a two-part model of recharge sources 

and ages of hypothesized two-part flow paths to the screened 

interval in the well and the hydraulic conductivity and porosity characteristics of an aquifer (International Atomic Energy 

Agency, 2006). The two-part or “binary” mixing model 

assumes that a water sample from a well represents a mixture 
22 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

of groundwater from two distinct flow paths and age dates 

that intercept the well screen (Plummer and others, 1993). In 

comparison, the piston-flow model assumes that groundwaterflow paths contributing flow to the well essentially have the 

same recharge source, flow path, and age. The two-part mixing 

model is an idealized conceptual model used to represent the 

tritium concentration of a sample as a mixture of groundwater 

from pre-1953 and post-1952 recharge.

The following processes were used to evaluate sitespecific data with hypothetical two-part mixtures. If the tritium 

concentration in a sample from a CLU#3 site well, such as 

that from monitoring well G53S (table 1), is similar to the 

hypothetical tritium concentration estimated from one or more 

two- part mixtures, those similarities can indicate the potential 

for recharge from multiple flow paths and ages at the well in 

amounts similar to those predicted in the two-part mixtures. 

Binary mixture results also were compared with samples containing very low, “near-zero” tritium concentrations to identify 

the smallest contribution of post-1952 groundwater that could 

be resolved.

Estimated tritium concentrations of hypothetical mixtures 

(Tmixture) were computed by varying percentages of submodern, pre-1953 (tritium dead) recharge and modern, post-1952 

recharge simulated using decay corrected annual mean tritium 

concentrations at Ottawa, Canada from the years 1953 to 

2002, according to the following equation.

Tmixture = (f

old/100) x Told + fyoung/100) x Tyoung (2) 

Where:

f

old is the percentage of old, tritium-dead recharge 

in the hypothetical mixture, ranging from 0 

to 99 percent,

Told is the tritium concentration of submodern, 

tritium-dead recharge (0.0 tritium units 

based on the data from monitoring wells 

EX-4, EX-6, G06D, G07D, and production 

well W-10 in table 1) in the hypothetical 

mixture,

f

young is the percentage of young, modern recharge 

in the hypothetical mixture, derived from 

precipitation in a single assumed year 

ranging from 1953 to 2002, in percentages 

ranging from 1 to 100, 

Tyoung is the concentration of tritium in modern 

recharge in the hypothetical mixture 

derived from precipitation in a single 

assumed year ranging from 1953 to 2002 

and adjusted for radioactive decay to May 

16, 2012.

About 50 hypothetical mixtures were computed in this 

manner between the submodern, tritium dead recharge and 

recharge from each of the annual average, decay-corrected tritium concentrations for years from 1953–2002 (figs. 12 A–E).

The two-part mixing models indicate that groundwater 

sampled by this study from monitoring well G53S was predominantly pre-1953 recharge and that if present, post-1955 

recharge was less than 5 percent of groundwater sampled from 

that well (figs. 12 A–E). The reported tritium concentration 

in the sample from monitoring well G53S was similar to two 

estimated tritium concentrations based on two-part mixtures of 

recharge: one mixture involved 20-percent 1953 recharge/

80-percent pre-1953 recharge, and a second mixture involved 

10-percent 1955 recharge/90- percent pre-1953 recharge 

(fig. 12A). The youngest age of recharge that could have 

substantially contributed (5 percent or more) to hypothetical 

mixtures and yield an estimated tritium concentration similar 

to that from monitoring well G53S was from 1955. That age 

differed by about 3 years from the piston flow-defined age of 

recharge for this sample of no later than 1952. Hypothetical 

mixtures involving 5 percent or more of recharge from 1956-

2002 with submodern, tritium-dead water had tritium concentrations greater than the tritium concentrations from the G53S 

sample.

The overall lack of detectable tritium in the six samples 

from this study with near-zero tritium concentrations is best 

explained as a physical absence of a post-1952 recharge contribution to groundwater in the lower part of the Radnor Till 

Member, the Organic Soil unit, or the Mahomet aquifer 

(table 1) and indicates the lack of groundwater-flow paths, 

fractures and macropores that could introduce post-1952 

recharge to these units. Monitoring well G07S, screened in 

the Upper Radnor Till Sand and a deeper sand that may be the 

Lower Radnor Till Sand with a top of well screen depth of 

about 59 ft below land surface, had a reported tritium concentration of 0.11±0.09 TU (table 1), a very low concentration that indicates that recharge to this well dates from before 

1953. The remaining four sampled monitoring wells EX-4, 

EX-6, G06D, and G07D, and production well W-10, all from 

stratigraphically deeper (by elevation) deposits than the Upper 

Radnor Till Sand, yielded samples with very low, near-zero 

tritium concentrations (TU values at or below the sample 

specific critical level, or levels below the laboratory analytical background, as indicated in table 1). With the exception 

of production well W-10, all groundwater samples from the 

Clinton site were collected from shallow permeable units that 

are within 70 feet of the surface at the Clinton site and within 

approximately 20 feet below the base of the Upper Radnor Till 

Sand. 

Despite their proximity to the surface, the mixing analysis for all of the Lower Radnor Till Sand, the Organic Soil 

unit, as well as the Mahomet aquifer, indicates that groundwater contributing to these samples was pre-1953 recharge and 

was below the capability of the mixing analysis to indicate 

the potential presence of post-1952 recharge in the sample. 

For example, hypothetical mixtures of 2.5 percent of recharge 

from 1953 and 1955 yielded estimated tritium concentrations 

from about 0.1 to about 0.15 TU (figs. 12 A–E). All other 
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 23

hypothetical mixtures of 2.5 percent of recharge from 1954 

or from each year from 1956–2002 with tritium dead water 

yielded estimated tritium concentrations that were greater than 

about 0.15 TU (figs. 12 A–E). When the 1-sigma precision of 

the tritium analyses was added to the tritium concentration 

from monitoring wells EX-4, EX-6, G06D, and G07D, and 

production well W-10 to represent the potential upper range 

of tritium concentration because of analytical variability, the 

upper range values of the 5 samples were less than the 0.15 

TU threshold. This indicates that the presence of 2.5 percent 

or more by volume of recharge from 1954 or from 1956–2002 

in these samples is not possible. The similarity of tritium 

concentrations in samples from monitoring wells EX-4, EX-6, 

G06D, and G07D, and production well W-10 to the 1-percent 

mixtures, and to the 2.5 percent mixtures involving 1953 and 

1955 recharge also indicates that the mixing models could 

not distinguish the presence of very small amounts of modern 

recharge that are less than 2.5 percent of the total volume of 

groundwater samples with tritium concentrations that are less 

than 0.15 TU or essentially zero.

Tritium concentration of hypothetical mixture (T mixture ) was computed as follows:

Tmixture = (fold100) x Told + (fyoung /100) x Tyoung

where: 

fold is the fraction of old recharge in the mixture,

Told is the tritium concentration of "old, pre-1953" recharge (0.0 tritium units),

fyoung is the fraction of young water in the mixture from recharge in an 

assumed year, and 

Tyoung is the concentration of tritium in precipitation (Ottawa, Canada)

from an assumed year from 1952 to 2002, decayed to 2012.

Tritium concentration in sample from well G53S, 0.3±0.1 TU

Tritium concentration in sample from well G53S (0.3 tritium units (TU)) was 

similar to two hypothetical tritium concentrations in two-part mixtures: 

(1) 20-percent 1953 recharge/80-percent pre-1953 recharge. 

(2) 10-percent 1955 recharge/90-percent pre-1953 recharge.

Two mixtures with 5-percent 1953- or 1955-recharge and 95-percent

pre-1953 recharge have tritium concentrations between that of

well G53S and tritium dead water.

0.01

0.1

10

100

1,000

0 10 20 30 40 50 60 70 80 90 100

Tritium concentration of hypothetical two-part mixture, in tritium units (TU)

Percentage of tritium from post-1952 precipitation in two-part mixture, by year

A 

1

EXPLANATION

1953

1954

1955

1956

1957

1958

1959

1960

Age of precipitation in

hypothetical mixture 

with pre-1953 water

Figure 12. Mixing computations indicating differences and similarities between tritium in the sample from 

well G53S in the vicinity of Clinton landfill unit #3, near Clinton, Illinois and hypothetical mixtures of pre-1953 

recharge and 1953–2002 recharge. Plots used to indicate the likelihood of modern recharge in the sample: A, 

mixtures with 1953-1960 recharge; B, mixtures with 1961–69 recharge; C, mixtures with 1970–79 recharge; D, 

mixtures with 1980–89 recharge; and E, mixtures with 1990–2002 recharge.
24 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Tritium concentrations of hypothetical mixtures with 5-percent or more

recharge from 1961-69 were greater than and differed from the tritium

concentration in the May 16, 2012 sample from well G53S (0.3 tritium units (TU)).

Mixtures with 5-percent or more of recharge from 1970-79 had different 

tritium concentrations than pre-1953 recharge (tritium = 0.0 TU).

Tritium concentration in sample from well G53S, 0.3±0.1 TU

Tritium concentration in sample from well G53S, 0.3±0.1 TU

Tritium concentrations of hypothetical mixtures with 5-percent or more

recharge from 1961-69 were greater than and not similar to the tritium

concentration in the May 16, 2012 sample from well G53S (0.3 tritium units (TU)).

Mixtures with 5-percent or more of recharge from 1961-69 had different 

tritium concentrations than pre-1953 recharge (tritium = 0.0 TU).

0.01

0.1

1

10

100

1,000

0 10 20 30 40 50 60 70 80 90 100

Tritium concentration of hypothetical two-part mixture, in tritium units (TU)

Percentage of tritium from post-1952 precipitation in two-part mixture, by year

B 

C 

0.01

0.1

1

10

100

1,000

10,000 EXPLANATION

EXPLANATION

Age of precipitation in

hypothetical mixture 

with pre-1953 water

Age of precipitation in

hypothetical mixture 

with pre-1953 water

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1976

1977

1978

1979

Figure 12. Mixing computations indicating differences and similarities between tritium in the sample from 

well G53S in the vicinity of Clinton landfill unit #3, near Clinton, Illinois and hypothetical mixtures of pre-1953 

recharge and 1953–2002 recharge. Plots used to indicate the likelihood of modern recharge in the sample: A, 

mixtures with 1953-1960 recharge; B, mixtures with 1961–69 recharge; C, mixtures with 1970–79 recharge; D, 

mixtures with 1980–89 recharge; and E, mixtures with 1990–2002 recharge.—Continued
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 25

Figure 12. Mixing computations indicating differences and similarities between tritium in the sample from 

well G53S in the vicinity of Clinton landfill unit #3, near Clinton, Illinois and hypothetical mixtures of pre-1953 

recharge and 1953–2002 recharge. Plots used to indicate the likelihood of modern recharge in the sample: A, 

mixtures with 1953-1960 recharge; B, mixtures with 1961–69 recharge; C, mixtures with 1970–79 recharge; D, 

mixtures with 1980–89 recharge; and E, mixtures with 1990–2002 recharge.—Continued

0 10 20 30 40 50 60 70 80 90 100

Tritium concentration of hypothetical two-part mixture, in tritium units (TU)

Percentage of tritium from post-1952 precipitation in two-part mixture, by year

D 

E 

0.01

0.01

0.1

0.1

1

1

10

10

100

100

Tritium concentrations of hypothetical mixtures with 5-percent or more

recharge from 1990-2002 were greater than and differed from the tritium

concentration in the May 16, 2012 sample from well G53S (0.3 tritium units (TU)).

Mixtures with 5-percent or more of recharge from 1990-2002 had different 

tritium concentrations than pre-1953 recharge (tritium = 0.0 TU).

Tritium concentration in sample from well G53S, 0.3±0.1 TU

Tritium concentration in sample from well G53S, 0.3±0.1 TU

Tritium concentrations of hypothetical mixtures with 5-percent or more

recharge from 1980-89 were greater than and differed from the tritium

concentration in the May 16, 2012 sample from well G53S (0.3 tritium units (TU)).

Mixtures with 5-percent or more of recharge from 1980-89 had different 

tritium concentrations than pre-1953 recharge (tritium = 0.0 TU).

EXPLANATION

EXPLANATION

Age of precipitation in

hypothetical mixture 

with pre-1953 water

Age of precipitation in

hypothetical mixture 

with pre-1953 water

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002
26 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Assessment of the Groundwater Travel Time at 

the Site of the Proposed Chemical Waste Unit

Analysis of geologic and hydraulic data collected for 

a previous investigation of the Clinton site (PDC Technical 

Services, 2005) combined with the hydraulic and tritium data 

collected during this study indicate that low-permeability siltand-clay rich till deposits limit the downward flow of water 

from the land surface to the more permeable deposits within 

approximately 100 ft of the land surface beneath the CWU 

and CLU#3. Estimates of the vertical groundwater velocity 

(Vv) and travel time through the geologic deposits between 

the base of the CWU and the top of the Mahomet aquifer were 

computed using (1) extrapolation of the tritium-based groundwater age at well G53S and (2) solution of the Darcy velocity 

equation (Gibb and others, 1984) for vertical groundwater 

velocity based on a variety of scenarios using site and regional 

data. The tritium-based estimate of vertical groundwater 

velocity through the section of glacial till between the base of 

the CWU and the Upper Radnor Till Sand was used with site 

and regional hydraulic conductivity data to estimate values of 

effective porosity (ne

) in that interval. The ne estimates computed using site-specific and regional data also were compared 

with literature values of ne that represent the range of conditions described for glacial tills (Sara, 2003; Walton, 1984). The 

site-specific estimates and literature based estimate of ne were 

extrapolated across the entire thickness of glacial till and other 

deposits and used to compute estimates of vertical groundwater velocity and travel time from the base of the CWU to 

the Mahomet aquifer. These estimates of travel time were 

compared with indications of travel time from prior USGS 

widespread regional sampling of pesticide and tritium concentrations to understand potential limitations of this approach.

Solution of the Darcy velocity equation included scenarios using vertical hydraulic conductivity (Kv) values derived 

from triaxial permeability tests done on lithologic samples 

collected at CLU#3. These site-specific tests previously 

reported by PDC Technical Services (2005) were considered 

to provide the best available data for Kv from the CLU#3 site 

for this analysis at the time of this report. The Vv and travel 

time calculations using the triaxial permeability tests, along 

with the tritium-based values were used to estimate the range 

of vertical groundwater flow rates at CLU#3.

Estimates of regional Kv values from Roadcap and others 

(2011) were used as comparisons to assess the suitability of 

the ne and Vv estimates computed from the Kv values derived 

from site triaxial permeability tests (PDC Technical Services, 

Inc., 2005), to identify potential data gaps, and to qualitatively 

supplement the assessment of hydrogeologic conditions 

below the CLU#3 site. Estimates of Vv also included scenarios using Kv values for the till deposits of the Banner 

Formation (0.015 ft/yr), Glasford Formation (0.037 ft/yr), 

and Wedron Group (0.37 ft/yr) that were from a regional 

groundwater-flow model developed by Roadcap and others 

(2011). The regional Kv values have not been demonstrated to 

be valid at the CLU#3 site. The Vv and travel time estimates 

based on regional values were applied to yield estimates of 

vertical groundwater flow rates for comparison to travel times 

computed with site-specific data.

Groundwater velocity calculations assume that groundwater flow was directed downward throughout the deposits above the Mahomet aquifer. Although water-level data 

indicate the potential for downward flow from the Upper 

Radnor Till Sand, Lower Radnor Till Sand, and the Organic 

Soil unit to the Mahomet aquifer beneath all of CLU#3, and 

from the Upper Radnor Till Sand to the Lower Radnor Till 

Sand to the Organic Soil unit beneath most of the CLU#3 

area, the potential for upward flow from the Organic Soil unit 

to the Upper Radnor Till Sand has been demonstrated by the 

water-level data southeast of CLU#3 at the EX-22 well cluster. 

A temporary or transient vertically upward flow would delay 

downward flow from the proposed location of the CWU to the 

Mahomet aquifer by increasing the overall travel time across 

that interval.

Vertical Groundwater Travel Time Estimated 

at the Clinton Site Using Tritium-Age Based 

Groundwater Velocity

The low tritium detection from one of the two monitoring 

wells screened in the Upper Radnor Till Sand indicates that 

recharge takes a minimum of about 59 years to flow from land 

surface to a comparatively shallow (50 ft from land surface) 

part of the Upper Radnor Till Sand. From this information it 

is concluded that the Vv through the deposits above the Upper 

Radnor Till Sand was 0.85 ft/yr or less. These deposits include 

the sand deposits and the till deposits above the Upper Radnor 

Till Sand, but the Vv is likely to be most affected by the large 

transit time through the till. 

If the maximum Vv of 0.85 ft/yr determined above also 

is assumed to uniformly apply beneath the CWU and CLU#3, 

a minimum of 200 years is needed for water to flow the 

approximately 170 ft from the top of the Berry Clay Member 

(the top of which is immediately below the CWU and at an 

altitude of about 660 ft NAVD 88) to the top of the Mahomet 

aquifer at about 489 ft NAVD 88 (PDC Technical Services, 

Inc., 2005). This calculated minimum time of travel assumes 

the Vv through the deposits above the Upper Radnor Till Sand 

is equal to or less than the Vv through the deposits from the 

Berry Clay Member to the Mahomet aquifer.

Tritium was at near zero concentrations in samples from 

the second well screened in the Upper Radnor Till Sand and 

from wells that were open to the Lower Radnor Till Sand, the 

Organic Soil unit, or the Mahomet aquifer in the vicinity of 

CLU#3. The Lower Radnor Till Sand and the Organic Soil 

unit are within approximately 20 feet of the base of the Upper 

Radnor Till Sand, whereas the top of the Mahomet aquifer is 

approximately 160 feet below the Upper Radnor Till Sand. 

The data indicate submodern water in the Upper Radnor Till 

Sand, Lower Radnor Till Sand, Organic Soil unit, and the 

Mahomet aquifer. The lack of tritium in groundwater samples 
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 27

from the Lower Radnor Till Sand, the Organic Soil unit, and 

the Mahomet aquifer is consistent with the 200-year travel 

time estimate from the top of the Berry Clay Member to the 

top of the Mahomet aquifer. The low tritium detection in one 

of the two samples collected from the Upper Radnor Till Sand, 

the very low, near zero tritium concentration in the second 

Upper Radnor Till Sand sample and the near absence of 

detectable tritium in all the deeper units indicates a uniformity 

in the hydraulic behavior in the till and supports the use of the 

piston flow model to represent vertical groundwater flow at the 

Clinton site.

Vertical Groundwater Travel Time Using 

Estimates of Effective Porosity and Darcy 

Velocity

A second method used to estimate the time of travel from 

the base of the proposed CWU to the top of the Mahomet 

aquifer was through the solution of the Darcy velocity equation (3) using site hydrogeologic data. Application of this 

method to estimate travel time at the CLU#3 and the CWU 

involved two steps.

Step 1 of this approach computed estimates of effective porosity (ne

) using the maximum vertical velocity of 

groundwater from land surface to the Upper Radnor Till Sand, 

as estimated from the tritium-based analysis of the sample 

from G53S and extrapolated to the entire section above the 

Mahomet aquifer. A rearranged form of the Darcy velocity 

equation (3) was applied to estimate the ne of various units 

using parameter values that included the maximum vertical 

velocity (VvT), measured vertical hydraulic gradients (dh/

dl) and measured and reported values of vertical hydraulic 

conductivity (Kv). 

ne = Kv (1/VvT) * (dh/dl) (3) 

Where

ne is the effective porosity of the deposit through 

which flow is transmitted, 

VvT is the maximum vertical velocity of 

groundwater, as estimated from the tritiumbased analysis of the sample from G53S,

Kv is the vertical hydraulic conductivity of the 

deposit through which flow is transmitted,

dh is the change in water level over the vertical 

distance being considered, and

dl is the vertical distance over which dh is 

measured. 

In step 2, the vertical groundwater velocity (Vv) was then 

recalculated specifically for the glacial units between the base 

of the proposed chemical waste unit (Berry Clay Member) and 

the top of the Mahomet aquifer using a rearranged version of 

the Darcy velocity equation (4). The Vv was computed using 

a range of values for Kv and ne from the site-specific estimates 

and published literature.

Vv = Kv(1/ne)(dh/dl) (4)

Where

Vv is the vertical velocity of groundwater

 and the other parameters are the same as 

defined for equation (3).

Step 1, solving equation 3 for the ne of the deposits above 

the Upper Radnor Till Sand, involves the following assumptions and data.

• The Vv in equation 4 was assumed to be 0.85 ft/yr for 

the deposits above the Upper Radnor Till Sand. This 

was the maximum vertical groundwater velocity based 

on the groundwater age in the Upper Radnor Till Sand 

determined from the concentration of tritium in the 

Upper Radnor Till Sand at monitoring well G53S. The 

actual range of Vv values in deposits above the Upper 

Radnor Till Sand is likely to be comparable or lower 

than the assumed Vv.

• The pre-excavation water table elevation in this area 

is 700 ft NAVD 88 based on data from lithologic logs 

(PDC Technical Services, Inc., 2005). The water level 

used to represent the Upper Radnor Till Sand was 

661.54 ft NAVD 88 from monitoring well G53S (table 

1). These values yield dh = 700–661.54.

• The vertical distance over which the water levels were 

measured (dl) is the difference between the approximate water table (700 ft NAVD 88) and the elevation 

of the top of the Upper Radnor Till Sand at boring 

EX21S/well G53S of 650 ft NAVD 88 (fig. 4), or 

dl=700–650. The elevation of the Upper Radnor Till 

Sand was used for the estimate because the water level 

in monitoring well G53S is likely to represent the 

hydraulic head in the most permeable deposit intersected by the well screen (the Upper Radnor Till Sand), 

rather than the hydraulic head in the less permeable 

silt- and-clay rich till within the rest of the well screen, 

according to the principle described by Sokol (1963).

Values used to represent the Kv of the deposits above the 

Upper Radnor Till Sand in computations of ne

 include the 

following: 

• A geometric mean of Kv of 0.027 ft/yr computed from 

all triaxial test data on sediment cores of silt and clay 

deposits from the site (PDC Technical Services, Inc., 

2005),

• A geometric mean of Kv of 0.034 ft/yr computed from 

triaxial test data on sediment cores from the Berry 

Clay Member and Glasford Formation at the site (PDC 

Technical Services, Inc., 2005), and
28 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

• A Kv of 0.37 ft/yr used by Roadcap and others (2011) 

to represent the Wedron Group tills in their regional 

simulation of groundwater flow in the Mahomet 

aquifer. This Kv was used for ne estimation because 

the tritium data used to evaluate recharge age included 

vertical flow through the Wedron Group tills. The Kv 

of Wedron Group tills was not used to evaluate vertical 

velocity and travel time in later computations because 

the Wedron Group tills were excavated to accommodate the landfill (fig. 6).

• Because the Vv used in this analysis (0.85 ft/yr) was 

the maximum value based on the analysis of the available tritium concentrations, the ne

 values calculated 

using step 1 outlined above were considered to be the 

minimum values for any given Kv value.

Step 2, solving equation 3 for the Vv through the till 

deposits between the Upper Radnor Till Sand and the top of 

the Mahomet aquifer (table 2; fig. 13), involved the following 

assumptions and data:

• Effective porosity values for the deeper till deposits 

between the Upper Radnor Till Sand and the top of the 

Mahomet aquifer were assumed to equal the ne values 

calculated in step 1 for the shallower deposits above 

the Upper Radnor Till Sand. 

• The water level in monitoring well G53S on July 12, 

2012 was used to represent water levels in the Upper 

Radnor Till Sand and the water level in production 

well W-10 on the same date was used to represent 

water levels at the top of the Mahomet aquifer. This 

Kv of interval 0.029 ft/yr from

triaxial permeability tests 

FEET FEET

700 700

650 650

600 600

550 550

500 500

450 450

Tiskilwa Formation of Wedron Group

Roxana Silt-Robein Silt Member

Berry Clay 

Member of 

Glasford 

Formation

Radnor Till Member 

of Glasford Formation

Vandalia Till Member 

of Glasford Formation 

Tilton Till Member of Banner Formation

Hillary Till Member of Banner Formation

Approximate pre-excavation 

Well G53S/ water table 700 feet

Boring EX-21S

Water level 

in well G53S 

661.54 feet on 

May 16, 2012

Potentiometric surface of Mahomet 

aquifer 601.14 feet on May 16, 2012 

Smithboro Till Member 

of Glasford Formation

Upper Radnor Till Sand

Land surface in the vicinity of well G53S. Exceeds 720 feet in 

parts of area. Peoria Silt typically less than 2 feet thick in landfill area.

EXPLANATION

Depth of chemical 

waste unit excavation

Mahomet aquifer

Kv 0.029 ft/yr Vertical hydraulic conductivity of deposit,

 and value, in feet per year (ft/yr)

DATUM IS NAVD 88 NOT TO SCALE

Figure 13. Conceptual diagram of features and data used in the solution of the Darcy velocity equation according to scenario one 

for the deposits at Clinton landfill unit #3, near Clinton, Illinois.
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 29

result yielded dh = 661.54-601.14. Monitoring well 

G53S was chosen for this analysis because data from 

this well was used in the calculation of ne, it is the sampled well that is closest to the CWU, and it monitors 

the Upper Radnor Till Sand. The Upper Radnor Till 

Sand is the monitored deposit nearest the Berry Clay 

Member and the base of the CWU. This use of the 

water level in production well W-10 to represent conditions at the top of the Mahomet aquifer was based on a 

simplifying assumption of a minimal vertical hydraulic 

gradient and predominantly horizontal flow in the aquifer. The assumption was made in the absence of local 

data on vertical flow in the Mahomet aquifer.

• The elevation of the upper hydraulic unit being measured (l1) is 650 ft NAVD 88, based on the altitude of 

the Upper Radnor Till Sand at boring EX-21D/well 

G53S (fig. 4). The elevation used to represent the top 

of the Mahomet aquifer/bottom of the till (l2) was 

489 ft NAVD 88 from boring EX-30 (fig. 4). This 

result yielded dl= 650–489 ft.

Various values were used for Kv of the till deposits from 

the Berry Clay Member to the top of the Mahomet aquifer. 

These included the two Kv values derived from triaxial test 

data previously described in Step 1, and additional Kv values 

of 0.037 ft/yr used to represent Glasford Formation tills and 

0.015 ft/yr used to represent the Banner Formation tills in a 

regional simulation of the Mahomet aquifer by Roadcap and 

others (2011). Once the Vv through the till deposits between 

the Upper Radnor Till Sand and the top of the Mahomet 

aquifer was calculated, travel time through the deposits below 

the CWU (Berry Clay Member to top of Mahomet aquifer) 

was determined by dividing the thickness of the till deposits 

in this interval (155 ft) by the calculated Vv. Each travel time 

is presented in the report as a minimum number of years, as 

computed and rounded to the nearest two significant figures. 

Because the tritium-based Vv for the deposits above the Upper 

Radnor Till Sand was considered to be a maximum value, the 

ne

 values based on this Vv were therefore minimum values. 

The true Vv from the Berry Clay to the top of the Mahomet 

aquifer is likely to be lower than the calculated value, with a 

Table 2. Estimates of effective porosity computed using the Darcy velocity equation with a tritium-based vertical groundwater velocity 

and vertical hydraulic conductivity values from the Clinton site, near Clinton, Illinois and from Roadcap and others (2011).

[Underline denotes calculated value, normal text denotes measured value; NAVD 88, North American Vertical Datum of 1988]

Parameter in calculation

Calculation of effective porosity (ne

)

Equivalent 

Porosity Estimate 

One: Using vertical hydraulic 

conductivity (Kv) 

from geometric 

mean of all site 

triaxial-test data

Equivalent Porosity Estimate Two:

Using vertical 

hydraulic conductivity from geometric mean of site 

triaxial-test data 

from the Tiskilwa 

Formation, Roxanna 

Silt-Robein Silt 

Member, and Berry 

Clay Member

Equivalent 

Porosity 

Estimate Three:

Using vertical hydraulic 

conductivity for 

Wedron Group 

tills (Roadcap 

and others, 

2011)

Vertical groundwater velocity (Vv) from tritium-based estimate of groundwater 

age at point l2, in feet/year

0.85 0.85 0.85

Vertical hydraulic conductivity (Kv), in feet/year 0.027 0.023 0.37

Elevation of water level in upper part of flow path (h1), in feet above NAVD 881 700.00 700.00 700.00

Elevation of water level in lower part of flow path (h2), in feet above NAVD 882 661.54 661.54 661.54

Elevation of upper hydraulic unit being measured (l1), in feet above NAVD 881 700 700 700

Elevation of lower hydraulic unit being measured (l2), in feet above NAVD 883 650 650 650

Difference of h1–h2, in feet 38.46 38.46 38.46

Difference of l1–l2, in feet 50 50 50

Ratio of h1–h2 and l1–l2 [(h1–h2)/(l1–l2)], dimensionless 0.77 0.77 0.77

Effective porosity (ne

), expressed as (percent/100) 0.024 0.021 0.33

1

Elevation of water level in upper part of flow path (h1) and elevation of upper hydraulic unit being measured (l1) values of 700 feet above NAVD 88 based 

on approximate altitude of water table.

2

Elevation of water level in lower part of flow path (h2) value of 661.54 feet above NAVD 88 based on water level in well G53S on May 16, 2012.

3

Elevation of lower hydraulic unit being measured (l2) value of 650 feet above NAVD 88 based on altitude of upper Radnor Till sand at well G53S.
30 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Table 3. Estimates of vertical groundwater velocity and groundwater travel time for the Clinton site, near Clinton, Illinois by solution 

of the Darcy velocity equation using selected values of vertical hydraulic conductivity and equivalent porosity from scenarios in table 2 

and vertical hydraulic conductivity from Roadcap and others (2011).

[Underline denotes calculated value, normal text denotes measured value; Bold values denote values used to represent groundwater NAVD 88, North American 

Vertical Datum of 1988; groundwater travel time estimates are rounded to the nearest two significant figures]

Parameter in calculation

Calculation of vertical groundwater velocity and groundwater travel time 

Scenario One:

From the top of 

the Berry Clay 

Member to the 

top of Mahomet 

aquifer using 

equivalent 

porosity from 

Scenario one 

(table 2) and 

vertical hydraulic conductivity 

from geometric 

mean of all site 

triaxial-test 

data1

Scenario Two: From 

the top of the Berry 

Clay Member to the 

top of the Mahomet aquifer using 

equivalent porosity from Scenario 

two (table 2) and 

vertical hydraulic 

conductivity from 

geometric mean 

of site triaxial-test 

data from the Berry 

Clay Member and 

Glasford Formation1

Scenario 

Three: From the 

top of the Berry 

Clay Member 

to the top of 

the Mahomet 

aquifer using 

equivalent 

porosity from 

Scenario 

three (table 2) 

and vertical 

hydraulic 

conductivity 

of the Glasford 

Formation tills2

Scenario Four:

From the top of 

the Berry Clay 

Member to the 

top of the Mahomet aquifer 

using lowest 

equivalent 

porosity calculated from site 

triaxial-test 

data (Scenario 

two, table 2) 

and vertical 

hydraulic 

conductivity 

of the Glasford 

Formation tills2

Scenario Five:

From the top 

of the Berry 

Clay Member 

to the top of 

the Mahomet 

aquifer using 

lowest lowest 

equivalent 

porosity calculated from site 

triaxial-test 

data (Scenario 

two, table 2) 

and vertical 

hydraulic 

conductivity 

of the Banner 

Formation tills2

Vertical hydraulic conductivity (Kv), in feet/

year

0.027 0.034 0.037 0.037 0.015

Elevation of water level in upper part of 

flow path (h1), in feet above NAVD 883

661.54 661.54 661.54 661.54 661.54

Elevation of water level in lower part of 

flow path (h2), in feet above NAVD 884

601.14 601.14 601.14 601.14 601.14

Elevation of upper hydraulic unit being 

measured (l1), in feet above NAVD 885

650 650 650 650 650

Elevation of lower hydraulic unit being 

measured (l2), in feet above NAVD 886

489 489 489 489 489

Difference of h1–h2, in feet 60.4 60.4 60.4 60.4 60.4

Difference of l1–l2, in feet 161 161 161 161 161

Ratio of h1–h2 and l1–l2 [(h1–h2)/(l1–l2)], 

dimensionless

0.38 0.38 0.38 0.38 0.38

Effective porosity (ne

), expressed as (percent/100)0.024 0.021 0.33 0.021 0.021

Vertical groundwater velocity (Vv), in feet/

year

0.41 0.61 0.041 0.66 0.27

Estimated groundwater travel time through 

155 feet of silt and clay (years)

370 260 3,700 230 580

1

Triaxial permeability test data from PDC Technical Services, Inc. (2005).

2

Vertical hydraulic conductivity data from Roadcap and others (2011).

3

Elevation of water level of 661.54 feet above NAVD 88 in upper part of flow path (h1) based on water level in well G53S on May 16, 2012.

4

Elevation of water level in lower part of flow path (h2) value of 601.14 feet above NAVD 88 based on water level in well W-10 in Mahomet aquifer on 

May 16, 2012.

5

Elevation of upper hydraulic unit being measured (l1) value of 650 feet above NAVD 88 based on altitude of upper Radnor Till sand at well G53S.

6

Elevation of lower hydraulic unit being measured (l2) value of 489 feet above NAVD 88 based on altitude of top of Mahomet aquifer beneath site.
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 31

proportionally greater travel time. A small additional travel 

time represented in days also was computed to represent vertical groundwater flow through the combined 15 ft of sand and 

gravel deposits between the Berry Clay Member and the top of 

the Mahomet aquifer. 

Analysis of hydraulic data and triaxial test data from the 

Clinton site, constrained by a tritium-age based maximum 

groundwater velocity estimate, computed minimum estimates 

of effective porosity (ne) that range from about 0.021 to 0.024 

for the predominantly till deposits above the Mahomet aquifer 

(table 2). The three estimates of ne derived by this study 

(0.024, 0.021 and 0.33; table 2) from Clinton site data and the 

regional value of Kv from Roadcap and others (2011) were 

within or slightly greater than the published ranges of 

ne values in sediments with similar textures (Sara, 2003, 

table 5–8, p. 315). Sara (2003) reports ne values for clay that 

range from 0.01 to 0.2, silt that range from 0.01 to 0.3 and 

glacial sediments that range from 0.05 to 0.2 (Sara, 2003; Walton 1984). Smaller values of ne

 are considered conservative 

for the purpose of estimating vertical groundwater velocity 

because ne is inversely proportional to Vv in the Darcy velocity equation. By comparison, the total porosity values reported 

for 9 tests of the Berry clay from the Clinton site ranged from 

0.235 to 0.379 with a mean value of 0.325 and for 9 tests of 

the Radnor Till from the Clinton site ranged from 0.194 to 

0.446 with a mean value of 0.248 (PDC Technical Services, 

Inc., 2005). These total porosity values from the Clinton site 

were larger than the estimates one and two of ne derived by 

this study (0.024 and 0.021; table 2). Sara (2003, table 5–8, 

p. 315) reports published ranges of total porosity of 0.34 to 

0.6 for clay and 0.34 to 0.61 for silt; no comparable range 

was reported by Sara (2003) for glacial sediments. Effective 

porosity (ne) is typically less than total porosity because ne

represents interconnected pore space through which water and 

fluids can pass and does not include the part of total porosity 

that describes static water and fluids that are bound to mineral surfaces in pore spaces (Sara, 2003, p. 313). Effective 

porosity, in general, can be more than one order of magnitude 

smaller than total porosity with the largest difference in fractured materials (Domenico and Schwartz, p. 25). This study 

therefore uses the estimates of ne

 in table 2 derived by this 

study to estimate the possible range of vertical groundwater 

velocity at the Clinton site.

Scenario One, Vertical Hydraulic Conductivity Equal to 

Geometric Mean of all Site Triaxial Permeability Tests

The geometric mean value of Kv from the triaxial permeability tests done on 29 silt and clay samples collected from 

623 through 703 ft NAVD 88 at CLU#3 was 0.027 feet per 

year (ft/yr; units converted from PDC Technical Services, Inc., 

2005). Using this value of Kv, and the parameters given for 

solution of the Darcy velocity equation previously outlined 

in step 1 (fig. 13), ne was calculated to be no less than 0.024 

for the predominantly silt-and-clay rich till deposits above the 

Upper Radnor Till Sand (table 2). This calculated ne

 value is 

less than the lower end of reported values for glacial till (0.05 

to 0.2) but generally consistent with the lower end of reported 

values for silt (0.01 to 0.3) and clay (0.01 to 0.2) according to 

Sara (2003) and Walton (1984). A smaller value of ne

 yields 

a higher estimate of vertical groundwater velocity. A smaller 

value of ne therefore provides a more conservative estimate of 

groundwater travel time.

Triaxial permeability tests, however, can underestimate 

the field-scale Kv of till deposits, primarily because of the 

small scale of the triaxial permeability measurements in comparison to the field scale of permeable features (Stephenson 

and others, 1988; Bradbury and Muldoon, 1990; Herzog and 

Morse, 1986). Because the values of Vv and dh/dl in equation 3 were independently known (fixed) in the application of 

step 1, any variation in the value of Kv will result in a directly 

proportionate variation in the value of ne. If the true Kv of glacial till and other deposits above the Upper Radnor Till Sand 

at CLU#3 is greater than the Kv indicated by the geometric 

means computed from triaxial permeability tests, the true ne of 

these deposits would be greater than the minimum estimates of 

ne of 0.021 and 0.024.

Using the ne of 0.024, a Kv of 0.027 ft/yr, and the parameters given for solution of the Darcy velocity equation as previously outlined in step 2 (fig. 13), the Vv between the Upper 

Radnor Till Sand and the Mahomet aquifer was calculated to 

be no more than 0.41 ft/yr (table 3). If this Vv is representative of the deposits below the CWU, it should take a minimum 

of about 370 years for water to flow through the 155 ft of silt 

and clay present from the Berry Clay Member to the top of the 

Mahomet aquifer (fig. 13; table 3). Note that if the actual ne

of the deposits above (and below) the Upper Radnor Till Sand 

is greater than the 0.024 assumed Vv, the true Vv will be less 

than 0.41 ft/yr and the true time of travel from the Berry Clay 

Member to the Mahomet aquifer will exceed the estimated 

minimum of 370 years.

The additional groundwater-transit time required for the 

15 ft of sand between the top of the Berry Clay Member and 

the top of the Mahomet aquifer was calculated by solving 

equation 3 for Vv and assuming values for dh and dl based on 

data for monitoring well G53S and the top of the Mahomet 

aquifer previously outlined; a Kv of 1.2 x104

 ft/yr (Roadcap 

and others, 2011), and an ne of 0.2 (Walton, 1984). The calculated Vv using these parameters exceeded 24,000 ft/yr and the 

total transit time through the sand deposits was calculated to 

be less than 1 day.

Scenario Two, Vertical Hydraulic Conductivity Equal to 

Geometric Mean of Site Triaxial Permeability Tests Above 

the Upper Radnor Till Sand and Below the Roxana SiltRobein Silt Member

Because the age of groundwater indicated by the concentration of tritium in the Upper Radnor Till Sand at monitoring 

well G53S provided the foundation for the maximum vertical velocity (VvT) estimate, and because this water migrates 

through the Tiskilwa Formation, the Roxana Silt-Robein Silt 

Member, and the Berry Clay Member before entering the 
32 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Upper Radnor Till Sand, equation 3 also was solved for ne

using a Kv equal to the geometric mean value of 19 triaxial 

permeability tests of samples collected from the deposits 

above the Upper Radnor Till Sand (0.023 ft/yr; units converted 

from PDC Technical Services, Inc., 2005) at CLU#3 (fig. 14). 

The ne was calculated to be 0.021 (table 2).

The Vv from the top of the Berry Clay Member to the 

top of the Mahomet aquifer then was calculated by solving 

equation 3 using the ne of 0.021 and Kv equal to the geometric 

mean value of 19 triaxial permeability tests of samples collected from the Berry Clay Member, the Radnor Till Member, 

and the Vandalia Till Member at CLU#3 (0.034 ft/yr; units 

converted from PDC Technical Services, Inc., 2005; table 3). 

No triaxial permeability tests were done using deposits below 

the Vandalia Till Member. The calculated (maximum) Vv was 

0.61 ft/yr, indicating an estimated minimum of 260 years for 

groundwater to flow from the top of the Berry Clay Member to 

the top of the Mahomet aquifer (table 3).

Scenario Three, Vertical Hydraulic Conductivity Equal 

to Values for Tills of the Wedron Group and Glasford 

Formation from Regional Groundwater Flow Model

The till deposits of the Wedron Group at CLU#3 are composed of the Tiskilwa Formation and constitute most of the 

deposit above the Upper Radnor Till Sand (figs. 6, 15). Solving equation 3 for ne

 of the deposits above the Upper Radnor 

Till Sand using the model-derived Kv (Roadcap and others, 

2011) value for the till deposits of the Wedron Group resulted 

in an ne value of 0.33 (table 2). This value exceeds the range 

of laboratory-measured total porosity values (0.23 to 0.28) 

for seven samples collected from the Tiskilwa Formation of 

FEET FEET

700 700

650 650

600 600

550 550

500 500

450 450

DATUM IS NAVD 88 NOT TO SCALE

Tiskilwa Formation of Wedron Group

Roxana Silt-Robein Silt Member

Berry Clay 

Member of 

Glasford 

Formation

Radnor Till Member 

of Glasford Formation

Vandalia Till Member 

of Glasford Formation 

Tilton Till Member of Banner Formation

Hillary Till Member of Banner Formation

Approximate pre-excavation 

Well G53S/ water table 700 feet

Boring EX-21S

Water level 

in well G53S 

661.54 feet on 

May 16, 2012

Potentiometric surface of Mahomet 

aquifer 601.14 feet on May 16, 2012 

Smithboro Till Member 

of Glasford Formation

Upper Radnor Till Sand

Land surface in the vicinity of well G53S. Exceeds 720 feet in 

parts of area. Peoria Silt typically less than 2 feet thick in landfill area.

EXPLANATION

Depth of chemical 

waste unit excavation

Mahomet aquifer

Kv 0.024 ft/yr Vertical hydraulic conductivity of deposit,

 and value, in feet per year (ft/yr)

Kv of interval 

0.035 ft/yr from 

triaxial

permeability tests 

Kv of interval 

0.024 ft/yr from 

triaxial

permeability tests 

Figure 14. Conceptual diagram of features and data used in the solution of the Darcy velocity equation according to scenario two 

for the deposits at Clinton landfill unit #3, near Clinton, Illinois.
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 33

the Wedron Group at CLU#3 (PDC Technical Services, Inc., 

2005) and literature-based values of ne for glacial till, clay, and 

silt (Walton, 1984). Because ne typically is much less than the 

total porosity, an ne of 0.33 is considered improbably high 

for these deposits, indicating that the model-derived Kv for 

the till deposits of the Wedron Group (at least as a representation of all the deposits above the Upper Radnor Till Sand) 

also is improbably high at CLU#3. The calculated Vv of 

0.041 ft/yr from the top of the Berry Clay Member to the 

top of the Mahomet aquifer, based on this ne and the modelderived Kv for the till in the Glasford Formation (fig. 15; 

table 2) is therefore considered a low estimate. 

 The Kv of the Glasford Formation was used in the 

calculation of Vv and travel time because it constitutes most 

of the till between the CWU and the Mahomet aquifer. The 

calculated (maximum) Vv was 0.041 ft/yr, indicating that with 

this set of conditions it would take an estimated minimum of 

3,700 years for groundwater to flow through from the top of 

the Berry Clay Member to the top of the Mahomet aquifer 

(table 3). Because of the limitations indicated above, the calculated vertical velocity and travel time in Scenario three were 

considered unrealistically long and were not used to represent 

travel times.

Scenario Four, Vertical Hydraulic Conductivity Equal 

to Value for Glasford Formation Tills From Regional 

Groundwater Flow Model

The ne of 0.021 based on the Kv from site triaxial permeability tests of the Tiskilwa Formation, Roxana Silt-Robein 

Silt Member, and Berry Clay Member (PDC Technical 

Figure 15. Conceptual diagram of features and data used in the solution of the Darcy velocity equation according to scenario three 

for the deposits at Clinton landfill unit #3, near Clinton, Illinois.

FEET FEET

700 700

650 650

600 600

550 550

500 500

450 450

DATUM IS NAVD 88 NOT TO SCALE

Tiskilwa Formation of Wedron Group

Roxana Silt-Robein Silt Member

Berry Clay 

Member of 

Glasford 

Formation

Radnor Till Member 

of Glasford Formation

Vandalia Till Member 

of Glasford Formation 

Tilton Till Member of Banner Formation

Hillary Till Member of Banner Formation

Approximate pre-excavation 

Well G53S/ water table 700 feet

Boring EX-21S

Water level 

in well G53S 

661.54 feet on 

May 16, 2012

Potentiometric surface of Mahomet 

aquifer 601.14 feet on May 16, 2012 

Smithboro Till Member 

of Glasford Formation

Upper Radnor Till Sand

Land surface in the vicinity of well G53S. Exceeds 720 feet in 

parts of area. Peoria Silt typically less than 2 feet thick in landfill area.

EXPLANATION

Depth of chemical 

waste unit excavation

Mahomet aquifer

Kv 0.037 ft/yr Vertical hydraulic conductivity of deposit,

 and value, in feet per year (ft/yr)

Kv of tills in Wedron Group 0.37 ft/yr 

(Roadcap and others, 2011) 

Kv of Glasford Formation tills 0.037 ft/yr 

(Roadcap and others, 2011)
34 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

Services, Inc., 2005) was used to represent the Glasford 

Formation (table 2); an approach that was consistent with that 

used for scenario two. Using the Kv value of the Glasford Formation to solve equation 3 for the ne

 of the deposits above the 

Upper Radnor Till Sand is not appropriate because most of the 

deposits above the Upper Radnor Till Sand are not part of the 

Glasford Formation (figs. 6 and 16). This is the lowest ne value 

determined for the Clinton site, which will result in the highest 

calculated Vv and the shortest travel time. The model-derived 

regional Kv (Roadcap and others, 2011) for the till of the 

Glasford Formation (0.037 ft/yr; table 3) is similar to the geometric mean Kv value of the triaxial permeability tests done 

in samples collected from the Glasford Formation at CLU#3 

(0.034 ft/yr), resulting in a similar calculated Vv (maximum 

0.66 ft/yr, table 3). The similarity in Kv values computed from 

Scenarios Two and Four indicates that the Clinton site triaxial 

permeability tests may provide a representative value for the 

Kv of the Glasford deposits at CLU#3. The calculated (maximum) Vv was 0.66 ft/yr, indicating it would take an estimated 

minimum of 230 years for groundwater to flow through from 

the top of the Berry Clay Member to the top of the Mahomet 

aquifer (table 3).

Scenario Five, Vertical Hydraulic Conductivity Equal 

to Value for Banner Formation Tills from Regional 

Groundwater Flow Model

Using the Kv value of the till deposits in the Banner Formation to solve equation 3 for the ne

 of the deposits above the 

Upper Radnor Till Sand is not appropriate because the Banner 

Formation is below the Upper Radnor Till Sand (figs. 6, 17). 

Therefore, the ne

 of 0.021 based on the Kv from Clinton site 

FEET FEET

700 700

650 650

600 600

550 550

500 500

450 450

DATUM IS NAVD 88 NOT TO SCALE

Tiskilwa Formation of Wedron Group

Roxana Silt-Robein Silt Member

Berry Clay 

Member of 

Glasford 

Formation

Radnor Till Member 

of Glasford Formation

Vandalia Till Member 

of Glasford Formation 

Tilton Till Member of Banner Formation

Hillary Till Member of Banner Formation

Approximate pre-excavation 

Well G53S/ water table 700 feet

Boring EX-21S

Water level 

in well G53S 

661.54 feet on 

May 16, 2012

Potentiometric surface of Mahomet 

aquifer 601.14 feet on May 16, 2012 

Smithboro Till Member 

of Glasford Formation

Upper Radnor Till Sand

Land surface in the vicinity of well G53S. Exceeds 720 feet in 

parts of area. Peoria Silt typically less than 2 feet thick in landfill area.

EXPLANATION

Depth of chemical 

waste unit excavation

Mahomet aquifer

Kv 0.037 ft/yr Vertical hydraulic conductivity of deposit,

 and value, in feet per year (ft/yr)

Kv of Glasford Formation tills 0.037 ft/yr 

(Roadcap and others, 2011)

Figure 16. Conceptual diagram of features and data used in the solution of the Darcy velocity equation according to scenario four 

for the deposits at Clinton landfill unit #3, near Clinton, Illinois.
Hydrogeology, Estimates of Groundwater Age, and Assessment of Groundwater Travel Time at the Clinton Site 35

triaxial permeability tests of the Tiskilwa Formation, Roxana 

Silt-Robein Silt Member, and Berry Clay Member (PDC Technical Services, Inc., 2005) was used. Again, this is the lowest 

ne value determined for the Clinton site, which will result 

in the highest calculated Vv and the shortest travel time. 

Use of the model-derived Kv for the Banner Formation tills 

(0.015 ft/yr; Roadcap and others, 2011) to calculate Vv from 

the Berry Clay Member to the top of the Mahomet aquifer 

results in a low Vv value (0.27 ft/yr) in comparison to Vv 

values in scenarios one, two, and four (table 3). This scenario 

indicates that if the Kv of the Banner Formation till deposits 

at CLU#3 and the CWU is lower than that calculated from the 

triaxial permeability tests of the overlying Glasford Formation (triaxial tests were not reported for any of the Banner 

Formation deposits; PDC Technical Services, Inc., 2005), the 

Vv calculated from the triaxial permeability tests in scenarios 

one and two may underestimate the actual minimum transport 

time from the top of the Berry Clay Member to the top of the 

Mahomet aquifer. The calculated (maximum) Vv was 0.27 ft/

yr indicated that groundwater would take an estimated minimum of 580 years to flow through from the top of the Berry 

Clay Member to the top of the Mahomet aquifer (table 3).

Summary of Hydrogeology and Recharge 

Interpretations from Clinton Site Data

At the Clinton site, the Upper Radnor Till Sand, the 

Lower Radnor Till Sand and the Organic Soil unit have limited 

hydraulic interconnection with the Mahomet aquifer and have 

limited potential to communicate post-1952 recharge through 

overlying glacial till to the Mahomet aquifer, as indicated by 

Figure 17. Conceptual diagram of features and data used in the solution of the Darcy velocity equation according to scenario five for 

the deposits at Clinton landfill unit #3, near Clinton, Illinois.

FEET FEET

700 700

650 650

600 600

550 550

500 500

450 450

DATUM IS NAVD 88 NOT TO SCALE

Tiskilwa Formation of Wedron Group

Roxana Silt-Robein Silt Member

Berry Clay 

Member of 

Glasford 

Formation

Radnor Till Member 

of Glasford Formation

Vandalia Till Member 

of Glasford Formation 

Tilton Till Member of Banner Formation

Hillary Till Member of Banner Formation

Approximate pre-excavation 

Well G53S/ water table 700 feet

Boring EX-21S

Water level 

in well G53S 

661.54 feet on 

May 16, 2012

Potentiometric surface of Mahomet 

aquifer 601.14 feet on May 16, 2012 

Smithboro Till Member 

of Glasford Formation

Upper Radnor Till Sand

Land surface in the vicinity of well G53S. Exceeds 720 feet in 

parts of area. Peoria Silt typically less than 2 feet thick in landfill area.

EXPLANATION

Depth of chemical 

waste unit excavation

Mahomet aquifer

Kv 0.015 ft/yr Vertical hydraulic conductivity of deposit,

 and value, in feet per year (ft/yr)

Kv of Banner Formation tills 0.015 ft/yr 

(Roadcap and others, 2011) 
36 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

several characteristics and computed times of groundwater 

travel. 

• The presence of a low tritium detection in one of the 

two samples collected from the Upper Radnor Till 

Sand (well G53S at 0.32 +/- 0.10 TU); 

• The absence of detectable tritium in the other six 

groundwater samples and one sequential replicate 

sample collected from the Clinton site, including a well 

in the Mahomet aquifer; 

• The thickness and lateral extent and continuity of lowpermeability till deposits beneath CLU#3 (estimated at 

approximately 155 feet); and

• The large differences in the water levels between the 

Upper and Lower Radnor Till Sands, and the Organic 

Soil unit and the Mahomet aquifer. 

The travel time for groundwater to flow from the top of 

the Berry Clay Member (at the bottom of the proposed CWU) 

to the top of the Mahomet Aquifer was estimated to be 

200 years or more, using a maximum vertical groundwater velocity of less than 0.85 feet per year derived from the 

interpreted age of a groundwater sample from monitoring well 

G53S. This travel time was less than travel time estimates 

across the same interval that ranged from 230 to 580 years 

that were derived from Darcy velocity computations computed 

using site and literature derived hydraulic data and properties.
 37

This report section interprets previously collected 

USGS tritium and pesticide data from the confined part of 

the Mahomet aquifer in the Illinois River Basin (fig. 3) to 

evaluate the heterogeneity of recharge characteristics of the 

overlying glacial till through the detection of pre-1953 and 

post-1952 recharge. The regional tritium-based groundwater age estimates were compared with pesticide detections 

in samples from distal domestic wells in the USGS regional 

network that are about 2.5 to 40 miles from the Clinton site 

to identify whether very small amounts of post-1952 recharge 

have in places reached confined parts of the Mahomet aquifer 

at locations other than the Clinton site in an approximately 

2,000 square mile area of the Mahomet aquifer. Very small 

amounts of post-1952 recharge were defined in this analysis as 

less than about 2.5 percent of the total recharge contributing 

to a groundwater sample, based on results from the two-part 

mixing analysis of tritium data. Previous interpretations of 

hydrogeologic and water-chemistry data from an approximately 3,940 square mile area of Central Illinois (figs. 1 and 

4 in Roadcap and others, 2011) indicated the potential for 

some regional heterogeneity of recharge characteristics in 

the glacial till that overlies the confined part of the Mahomet 

aquifer (Roadcap and others, 2011; Mehnert and others, 2004). 

These interpretations of previously collected data, referenced 

as “regional results”, are compared to the interpretations in 

the section titled “Summary of Hydrogeology and Recharge 

Interpretations from Clinton Site Data”. Those interpretations 

indicated limited hydraulic interconnection through the predominantly glacial till deposits overlying the Mahomet aquifer 

and limited potential to communicate post-1952 recharge 

through overlying glacial till deposits to the Mahomet aquifer 

from the much smaller Clinton site. 

Specific estimates of the range of travel times from the 

land surface to the Mahomet aquifer at the proposed CWU site 

were compared with regional indicators of recharge in samples 

collected by the USGS from 1996 to 2011 from domestic 

wells scattered throughout the confined part of the Mahomet 

aquifer in the Illinois River Basin surrounding the Clinton 

site, an area of approximately 60 by 25 miles or about 

2,000 square miles (fig. 18A). The pre-existing domestic 

wells are 2.5 to about 40 miles from the CLU#3 site and were 

originally selected to represent groundwater conditions in the 

Mahomet Buried Bedrock Valley for the USGS National Water 

Quality Assessment Program (Morrow, 2001 and fig. 18A). 

Heterogeneity in the overlying glacial deposits can result in 

substantial variations in the groundwater flow rates from location to location. Pesticide detections from a 1996–98 USGS 

regional sampling of the Mahomet aquifer at the 30 domestic wells that are 2.5 to about 40 miles from the Clinton site 

(table 4, at the back of this report, fig. 18B) previously have 

been interpreted as indicating that some modern recharge has 

reached the confined part of the Mahomet aquifer in the vicinity of these wells (Mehnert and others, 2004, p. 6–7). Mehnert 

and others (2004) also interpreted that pesticide detections in 

water samples from 10 of the 30 domestic wells indicated a 

limited amount of modern recharge contribution to an overall 

older, pre-1953 groundwater age in those samples. 

Because of geological heterogeneity, indications of 

recharge and time of travel from the regional sampling results 

may be difficult to relate to results from the Clinton site. 

Assuming the domestic wells in the regional study were properly constructed and that the sampling results reflect the water 

quality in the Mahomet aquifer, the data indicate that heterogeneity exists in the recharge characteristics of glacial materials overlying the confined part of the Mahomet aquifer in the 

area shown in figures 18A and 18B. Spatial variability in the 

amount of silt, clay and sand in the glacial materials above the 

Mahomet aquifer has been suggested as a characteristic that 

regionally affects recharge to the Mahomet aquifer (Roadcap 

and others, 2011; Panno and others, 1994). In other words, the 

water quality in a sample collected from a well screened in the 

Mahomet aquifer may be affected by the local glacial stratigraphy in the overlying materials. 

Interpretation of the pesticide and tritium concentrations 

reported from the regional well network depends on the integrity of wells used to collect the water samples. It is possible 

that pesticide detections used to interpret recharge age in the 

regional well network could be affected by well construction and human-affected local infiltration of contaminants 

as well as by natural avenues of sparsely distributed vertical 

permeability near the well. The wells previously sampled by 

the USGS (table 4, fig. 18B) were existing domestic wells 

that were installed by water-well driller representatives of 

the private landowner. Construction documentation for the 

off-site domestic wells was limited to that recorded by the 

driller in the log for each well and as archived by the Illinois 

State Geological Survey (2014). Because of available time 

and documentation, the quality of the well construction was 

assessed by (1) selecting domestic wells constructed in 1979 

or later to reflect more recent construction and documentation 

practices, (2) an inspection of visible characteristics at each 

wellhead at the time of initial well selection, pre-sampling 

development, and water sampling for indications of nearby 

pesticide storage or use and indications of potential well 

bore or annular transport of pesticides from the surface to the 

screened interval in the well and (3) a review of driller records 

Regional Indications of 

Recharge to the Mahomet 

Aquifer from Previously 

Collected Tritium and 

Pesticide Data 

By Paul M. Buszka and William S. Morrow
38 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

for those wells as reported to the State of Illinois (Illinois State 

Geological Survey, 2014). The regional wells were selected 

to be sampled using criteria initially described by Lapham 

and others (1995) and further defined by Lapham and others 

(1997). The regional wells were pumped to evacuate at least 

three well volumes of water and ensure stability of fieldmeasured water-quality properties in the produced water as 

described in (Koterba and others, 1995) before sampling to 

minimize the effect of short-term (days to months) humanaffected well-bore leakage and enhance the likelihood that the 

sample represented ambient groundwater quality. 

Tritium and pesticide results from prior groundwater 

sampling have been used as regional indicators of modern 

(post-1952) recharge to the Mahomet aquifer in Illinois. The 

interpretation of tritium concentrations in groundwater from 

the Mahomet aquifer has been provided by Hackley and others 

88°

89°30'

90°

90°30'

89°

41°

40°30'

40°

88°30'

0 10 20 30 40 MILES 50

0 10 20 30 40 KILOMETERS 50

Sangamon

Sangamon

River

River

Illinois

River

Mackinaw River

Vermilion

Salt

Creek

River

Kaskaskia River

Embarras R.

Iroquois

River

MCLEAN

IROQUOIS FORD

LOGAN

PIATT

VERMILION

MACON

MASON

CASS

SANGAMON

TAZEWELL

DE WITT

WOODFORD

MENARD

CHAMPAIGN

Pekin

Eureka

Morton

Normal Paxton

Havana

Urbana

Watseka

Mahomet Clinton

Lincoln

Mackinaw

Danville

Hoopeston

Champaign

Monticello

Beardstown Petersburg

Bloomington Gibson

City

Springfield

Decatur

Rantoul

Mackinaw Valley

Kenney

Valley

Onarga

Valley

Mahomet Valley

Clinton

landfill

Map area shown in figure 18B

EXPLANATION

A

Mahomet aquifer present

Approximate area of enhanced aquifer recharge near Paxton and Monticello

Centerline of Mahomet Valley

Centerline of tributary valley

Western boundary of Mahomet Buried Bedrock Valley in Lower Illinois River

 Basin as used in Morrow (2001)

Illinois River Basin

U.S. Geological Survey Mahomet aquifer well with well identifier from table 4

 

 Previously sampled well

 Previously sampled well with detections of pesticide in water samples in 1996, 1998, 2005, or 2007

 Previously sampled well with tritium detection of 5.7 tritium units (TU) in 2007

Base from U.S. Geological Survey

1:100,000-scale digital data,

Albers Equal-Area Conic projection,

standard parallels 33° and 45°,

central meridian −89°

Figure 18. Locations of domestic wells in regional Mahomet aquifer network in east-central Illinois that previously 

were sampled by the U.S. Geological Survey for pesticide and tritium concentrations, 1996–2011. A. Sampled wells 

relative to the confined part of the Mahomet aquifer between the eastern boundary of the Illinois River Basin and 

the western boundary of the Mahomet Buried Bedrock Valley and B. Sampled wells in an inset area centered on the 

confined part of the Mahomet aquifer. 
Regional Indications of Recharge to the Mahomet Aquifer from Previously Collected Tritium and Pesticide Data 39

(2010). Samples analyzed for tritium (29 wells) and pesticides 

(30 wells) were collected on various dates by the USGS from 

1996 to 2011 from a network of 30 domestic wells open to 

the Mahomet aquifer (table 4; fig. 18B; USGS National Water 

Information System, 2014); interpretations using a subset 

of those data collected in 1996 and 1998 were reported by 

Mehnert and others (2004, p. 6–7). This discussion presents 

background information about regional tritium and pesticide 

data and the interpretation approaches used by these authors.

Regional Tritium Data

Tritium, major ions, stable isotopes of oxygen and 

hydrogen and water-level data were interpreted to indicate that 

89° 88°30'

40°15'

40°

0 10 20 MILES

0 10 20 KILOMETERS

Sangamon

River

Kaskaskia

River

Embarras R.

Salt

Creek

Clinton 

Lake

LOGAN

MCLEAN

CHAMPAIGN

MACON PIATT

DE WITT

Urbana

Rantoul

Mahomet

Champaign

Monticello

Clinton

400422088575101

401238088444201

401910089120201 402029088172001

401740089022401

401058088521001

401246088585101

400939088574301

401106088300801

400827088332601

400805088531501

400433088495801

400817088593401

400357088444401

400317088512701 400134088433101

400057088550101

400009088484601

400603088300901

401041088242901

401131088255701

401504088295001

401811088274801

400935089062301

400842089065701

395853088441301

401443088230101

401753088114201

402248088150501

402122088161101

Clinton landfill

Base from U.S. Geological Survey 1:100,000-scale digital data,

Albers Equal-Area Conic projection, standard parallels 33° and 45°,

central meridian −89°

EXPLANATION

B

Mahomet aquifer present

Approximate area of enhanced aquifer recharge near Paxton and Monticello

Centerline of Mahomet Valley

Centerline of tributary valley

Western boundary of Mahomet Buried Bedrock Valley in Lower Illinois River

 Basin as used in Morrow (2001)

Illinois River Basin

U.S. Geological Survey Mahomet aquifer well with well identifier from table 4

 

 Previously sampled well with identifier

 Previously sampled well with detections of pesticide in water samples in 1996, 1998, 2005, or 2007, with identifier

 Previously sampled well with tritium detection of 5.7 tritium units (TU) in 2007, with identifier

400357088444401

400317088512701

401443088230101

Figure 18. Locations of domestic wells in regional Mahomet aquifer network in east-central Illinois that previously 

were sampled by the U.S. Geological Survey for pesticide and tritium concentrations, 1996–2011. A. Sampled wells 

relative to the confined part of the Mahomet aquifer between the eastern boundary of the Illinois River Basin and 

the western boundary of the Mahomet Buried Bedrock Valley and B. Sampled wells in an inset area centered on the 

confined part of the Mahomet aquifer.—Continued
40 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

groundwater in the Mahomet aquifer in western Piatt, De Witt 

(where the CWU is proposed), and McLean counties is relatively more isolated from direct surficial recharge as compared 

with the area of enhanced recharge in central Piatt County 

(Panno and others, 1994; Hackley and others, 2010). Tritium 

was detected in groundwater in 2 of 27 samples from wells 

in the Mahomet aquifer (Hackley and others, 2010). The two 

samples with detectable tritium were from wells in the vicinity 

of an area of enhanced recharge in central Piatt County (Fig. 

18A). Tritium was not detected in samples from the aquifer in 

the rest of east-central Illinois (De Witt, Macon, Champaign, 

McLean Counties and the rest of Piatt County) (Hackley and 

others, 2010). The tritium samples described in Hackley and 

others (2010) were collected before 2002, based on their 

previous sampling and reporting by Hackley (2002). The 

two tritium detections had concentrations of 0.62 and 

0.73 tritium units (TU). These data were interpreted as indicating little to no “modern” or post-1952 recharge was present 

in the Mahomet aquifer outside of an enhanced recharge area 

in central Piatt County as proposed by Hackley and others 

(2010). This interpretation is consistent with the classification 

system of Clark and Fritz (1997); groundwater with tritium 

concentrations less than about 0.8 TU (as of about 1997) is 

“submodern” and was substantially composed of pre-1953 

recharge and groundwater with tritium concentrations greater 

than about 0.8 TU (as of about 1997) is “modern” and contains 

substantial amounts of post-1952 recharge. 

Groundwater-age estimates from the USGS regional data 

indicated largely pre-1953 recharge dates for groundwater 

sampled from the confined part of the Mahomet aquifer in the 

area shown in Figures 18A and 18B. Tritium concentrations 

that were less than or equal to 0.2 TU in USGS samples from 

10 domestic wells sampled in 2007 indicated that they were 

substantially composed of submodern (pre-1953) recharge 

(table 4). A negative tritium concentration is equivalent 

to zero for reporting purposes; a negative value originates 

from tritium derived decay counts yielded from analysis 

of the sample that was less than the analytical background. 

In addition, samples collected in 1996 from two domestic 

wells (400827088332601 and 400817088593401, fig. 18B) 

had tritium concentrations of <0.3 TU (table 4); indicating 

that they were substantially composed of submodern water. 

A search of the USGS National Water Information System 

(NWIS) database identified one domestic well sampled in 

2007 (well 401443088230101) about 30 miles east-northeast 

of the Clinton area in Champaign County north of the City of 

Mahomet with a tritium concentration of 5.7 TU that indicated 

the sample was substantially composed of modern, post-1952 

recharge (table 4, fig. 18B). Most tritium data indicate submodern water in the Mahomet aquifer outside of an enhanced 

recharge area in central Piatt County proposed by Hackley 

and others (2010) and away from the sole USGS tritium-based 

indication of modern recharge in Champaign County described 

earlier in this paragraph. 

USGS water samples from 16 wells sampled from the 

Mahomet aquifer in 1997–98 had inconclusive tritium results 

relative to the submodern/modern tritium age classification. 

These 16 wells had water samples that were analyzed for 

tritium using a method with a reporting level (<1.8 TU) 

that was too high to indicate whether modern, post-1952 

recharge was substantially absent (table 4). A reporting level 

of <1.8 TU is greater than the <0.8 TU concentrations that 

would indicate submodern, pre-1953 recharge in samples 

collected during 1997 and 1998 (Clark and Fritz, 1997). With 

a <1.8 TU reporting level, there could be a small component 

of modern, post-1952 recharge in those groundwater samples 

from the confined part of the Mahomet aquifer.

Regional Pesticide Data 

In an effort to determine whether additional analytes 

beyond tritium in groundwater may be useful in identifying 

potential areas of recharge, the USGS compiled pesticide 

data from a prior 1996–2011 sampling of a network of 

30 domestic wells shown in figure 18B that are screened in the 

confined part of the Mahomet aquifer (covering approximately 

2,000 square miles) in the Illinois River Basin. A search of the 

USGS National Water Information System (NWIS) database 

identified 14 water samples collected from 10 wells in the network of 30 domestic wells from 1996 to 2011 that contained 

detectable concentrations of one or more pesticides (table 4). 

Twenty of the 30 domestic wells sampled the confined part 

of the Mahomet aquifer by USGS from 1996 to 2011 yielded 

water samples that had no detectable pesticide concentrations.

All pesticide detections in the regional Mahomet aquifer 

groundwater samples were at sub-microgram per liter concentrations (nanograms per liter or parts per trillion), just above 

the detection limits for the analytical methods used. Thirteen 

of 22 pesticide detections were reported as estimated concentrations, indicating that the compounds were detected in 

the samples but a larger statistical uncertainty was associated 

with the concentrations reported for those analytical results 

(Oblinger-Childress and others, 1999; table 4). The pesticides 

detected included atrazine, cyanazine, diazinon, metolachlor, 

molinate, prometon, and trifluralin and a pesticide breakdown 

product, deethylatrazine. Six wells had one sample each with 

detections of at least one or more pesticides including atrazine, 

metolachlor, diazinon, prometon, cyanazine, metribuzin, and 

molinate that were not reproduced in a subsequent sample 

(6 samples with pesticide detections and 11 samples with 

no detections) (table 4; National Water Information System, 

2014). The concentrations of the pesticides were at or 

near their analytical reporting levels (table 4). The highest 

pesticide concentration was measured in a 1996 sample from 

well 401058088521001, which had a molinate detection at 

0.019 μg/L.

Four wells had detections of the same pesticide in samples from successive sampling periods. Two atrazine detections in the 1998 samples were from wells 402029088172001 
Regional Indications of Recharge to the Mahomet Aquifer from Previously Collected Tritium and Pesticide Data 41

(0.002 μg/L estimated concentration) and 401041088242901 

(0.011 μg/L; table 4). Diazinon was detected in the 1998 

sample from well 400422088575101 (0.0138 μg/L) that had 

a prior detection of the same pesticide in a 1996 sample 

(0.025 μg/L estimated concentration) (table 4). One pesticide (prometon) detection in the 2007 (0.007 μg/L) samples 

was from well 400805088531501 with a prior detection 

of the same pesticide in a 2005 sample (0.005 μg/L) (table 

4). Metolachlor was detected in a 1998 sample from well 

401041088242901 (0.007 μg/L) and in a 1996 from well 

400817088593401 (0.004 μg/L) and estimated concentrations 

of metolachlor were measured in samples collected in 1998 

from three other regional wells (table 4). Deethylatrazine and 

trifluralin were detected at estimated values in two wells in 

samples collected in 1996 (table 4). Cyanazine was detected in 

a 1998 sample from well 401041088242901 (0.013 μg/L). The 

highest pesticide concentration was detected in a 1996 sample 

from well 401058088521001, which had a molinate detection 

with a concentration of 0.019 μg/L. 

Detections of pesticide compounds previously have 

been compared with their date of registration to indicate a 

contribution of modern recharge to a groundwater sample 

(Barbash and others, 2001). Among the pesticides detected 

in the domestic well network in the Mahomet aquifer are the 

following: 

• Diazinon was registered in the United States in 1956, 

• Atrazine was registered in the United States in 1958, 

• Prometon was registered in the United States in 1959,

• Trifluralin was registered in the United States in 1962,

• Cyanazine was registered in the United States in 1971, 

• Metolachlor was registered in the United States in 

1976, and

• Molinate was registered in the United States in 1992; 

as referenced in U.S. Environmental Protection Agency 

documents (2014, 2008, 2006a, 2006b, 1996, 1995, 

1984, table 5, at the back of this report). 

Conceptually, the detection of prometon in groundwater from a well, if representative of aquifer conditions and 

collected from a properly-constructed well, indicates that a 

component of the affected groundwater sample was composed 

of 1959 or younger recharge. Therefore, the longest possible 

travel time from land surface to the Mahomet aquifer of some 

part of the water in a sample from a well with a pesticide 

detection can be approximated as the difference of the year 

the water was sampled and the year of registration of the 

compound. 

Maximum times of travel from land surface to the 

Mahomet aquifer for a small part of the water sampled from 

wells with pesticide detections (table 5) ranged from 4 to 

46 years with minimum estimated vertical velocities from 5 to 

84 ft/yr. The following are examples summarized from table 5.

• About 4 years or less for well 401058088521001 with 

a total depth to the top of the producing interval of 

336 feet and a minimum vertical groundwater velocity 

of 84 ft/yr estimated from a detection of molinate in a 

1996 sample at 0.019 μg/L (tables 4 and 5; molinate 

registered in 1992, U.S. Environmental Protection 

Agency, 2014);

• About 20 years or less for well 400817088593401 with 

a total depth to the top of the producing interval of 

269 feet and a minimum vertical groundwater velocity 

of 13 ft/yr estimated from a detection of metolachlor in 

a 1996 sample at 0.004 μg/L (tables 4 and 5);

• About 22 years or less for well 402029088172001 with 

a total depth to the top of the producing interval of 

197 ft and a minimum vertical groundwater velocity of 

9 ft/yr with a detection of metolachlor in 1998 at 0.002 

μg/L estimated concentration (tables 4 and 5; metolachlor registered in 1976, U.S. Environmental Protection Agency, 1995);

• About 22 years or less for well 400422088575101 with 

a total depth to the top of the producing interval of 

276 feet and a minimum vertical groundwater velocity 

of 13 ft/yr estimated from a detection of metolachlor in 

a 1998 sample at 0.002 μg/L estimated concentration 

(tables 4 and 5);

• About 38 years or less for well 402122088161101 with 

a total depth to the top of the screen at 249 ft and a 

minimum vertical groundwater velocity of 7 ft/yr with 

a detection of atrazine in 1996 at 0.002 μg/L estimated 

concentration (tables 4 and 5; atrazine registered in 

1958, U.S. Environmental Protection Agency, 2006b); 

and

• About 46 years or less for well 400805088531501 with 

a total depth to the top of the producing interval of 

242 feet and a minimum vertical groundwater velocity 

of 5 ft/yr estimated from a detection of prometon in a 

2005 sample at 0.005 μg/L (tables 4 and 5; prometon 

registered in 1959, U.S. Environmental Protection 

Agency, 2008).

For the wells with pesticide detections, the screened interval in the Mahomet aquifer is overlain by 166 to 291 feet of 

till-dominated glacial material. The relative lack of tritium in 

these samples indicates that the amounts of post-1956 to post1992 recharge contributing to the 10 domestic wells were a 

very small part of the overall older groundwater sampled from 

the wells. A very small amount of post-1952 recharge was 

previously defined in the section of this report titled “Two-Part 

Mixing Model Evaluation of Tritium-Based Groundwater Age 

Estimates at the Clinton Site” as less than about 2.5 percent of 

the total recharge contributing to a groundwater sample. For 

example, if the pesticide contamination migrated vertically 
42 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

through natural features in the tills proximal to the affected 

wells and the maximum transit times for the pesticides can 

be determined from pesticide registration dates, the pesticidebased ages of groundwater from wells 400817088593401 

and 400317088512701 were younger than what would be 

predicted from the groundwater age that was computed using 

tritium data (table 5). For example, well 400817088593401 

has a total depth below land surface to the top of the well 

screen of 269 feet below land surface and a maximum travel 

time of 20 years for the pesticide metolachlor that was registered in 1976 and detected in 1996 at a concentration of 0.004 

μg/L (U.S. Environmental Protection Agency, 1995; data in 

tables 4 and 5). In contrast, a tritium-based age date from the 

same sampling event for this well indicated that the sample 

was composed predominantly of pre-1953 age recharge. The 

pesticide detections in samples from wells 400817088593401 

and 400317088512701 indicate the presence of very small 

amounts of modern, post-1952 age recharge in the overall 

older, pre-1953 age groundwater indicated by tritium concentrations in samples from those wells.

The detection of the pesticide molinate in a 1996 sample 

from well 401058088521001 indicated the fastest maximum 

time of travel of recharge to a well in the confined part of the 

Mahomet aquifer. Well 401058088521001 has a total depth 

below land surface to the top of the well screen of 336 feet 

below land surface and a maximum travel time of 4 years for 

the pesticide molinate that was registered in 1992 and detected 

in 1996 at a concentration of 0.019 μg/L (U.S. Environmental 

Protection Agency, 2014; data in tables 4 and 5). This results 

in a computed vertical velocity through the overlying 282 feet 

of till-dominated glacial material to the top of the well screen 

at 336 feet of no less than approximately 84 ft/yr; nearly 

100 times higher than the maximum vertical groundwater 

velocity estimated for glacial till at the Clinton site using 

the tritium data (0.85 ft/yr). As four years is the maximum 

transit time available, the vertical velocity computed using 

this transit time is a minimum vertical groundwater velocity. 

Consequently, if molinate was not used in the vicinity of this 

well until 1994, the maximum travel time would be reduced to 

two years and the vertical velocity would be twice as high or 

approximately 170 ft/yr. Minimum vertical velocities computed from the other 9 wells with pesticide detections ranged 

from 5 to 13 ft/yr (table 5). The flow processes by which 

molinate and the other pesticides migrated vertically through 

more than 166 to 291 feet of till-dominated glacial material could not be distinguished using the available data from 

processes related to well integrity or to characteristics of the 

local hydrogeology. Potential explanations include: (1) infiltration through man-made avenues in or along the well, (2) flow 

of very small amounts of post-1956 to post-1992 recharge 

through sparsely distributed natural permeable aspects of the 

glacial till and diluted by mixing with older groundwater, or 

(3) a combination of both processes.

The potential for well-integrity related migration of one 

or more pesticides along an annular space or through the well 

bore could not be ruled out. For example, back-siphoning 

down the well through the plumbing systems associated with 

pesticide and fertilizer use has been described as a potential 

cause of the presence of contaminants in water from production wells in other agricultural areas outside Illinois (Kross 

and others, 1992; Sievers and Fulhage, 1992). The off-site, 

regional pesticide data were collected by prior USGS efforts 

in 1996 through 2011 from a series of domestic wells drilled 

after 1978 and as described earlier, details of their construction 

were limited. These concerns are described in the emphasis 

given to well construction and well construction documentation in technical literature on wells used to sample water 

quality (Lapham and others, 1997; Driscoll, 1986; Yeskis and 

Zavala, 2002). As examples, six of the seven wells used in the 

Clinton site study were groundwater monitoring wells drilled, 

constructed and documented to conform with protocols that 

better assure the integrity of samples provided by those wells. 

Indications of Regional Heterogeneity in 

Recharge to the Confined Part of the Mahomet 

Aquifer

Assuming the domestic wells in the regional study were 

properly constructed and that the sampling results reflect the 

water quality in the Mahomet aquifer, these results indicate 

the glacial drift stratigraphy above the confined part of the 

Mahomet aquifer exhibits substantial heterogeneity and that 

parts of the aquifer with the pesticide detections can be susceptible to contaminant sources at the land surface. Ten of 

the 30 domestic wells in the regional study network were associated with pesticide detections in groundwater from 

the confined part of the Mahomet aquifer (table 4). Because 

the confined part of the Mahomet aquifer covers about 

2,000 square miles of glacial terrain, such a conclusion is not 

surprising. Some heterogeneity in the vertical distribution of 

glacial deposits over the Mahomet aquifer previously has been 

described (Roadcap and others, 2011). The pesticide-based 

groundwater-age results, when used with tritium data, indicate 

the potential value of sub-microgram-per-liter level concentrations of land-use derived indicators of modern recharge, such 

as pesticides in agricultural areas, to indicate presence of very 

small amounts of modern, post-1952 age recharge in overall 

older, pre-1953 age groundwater. 

The Clinton site groundwater-age dates derived from 

tritium results (table 1) do not conform with the rapid recharge 

rates indicated at the 10 domestic wells in the regional network associated with pesticide detections (table 5, fig. 18B). 

In De Witt County where the Clinton site is located, 6 of 

11 domestic wells in the regional network were associated 

with pesticide detections (table 5, fig. 18B). In contrast, 

permeable deposits within the Lower Radnor Till Sand and 

Organic Soil unit and at the one well in the Mahomet aquifer contain pre-1953 age recharge that indicate low rates of 

vertical groundwater flow at the Clinton site. The low rates 

of vertical groundwater flow and the vertical head differences 

between permeable deposits within the Lower Radnor Till 
Summary and Conclusions 43

Sand and Organic Soil units and the underlying Mahomet 

aquifer at the Clinton site further indicates a lack of the types 

of “enhanced permeability” hydrogeologic features such as 

vertically interconnected sands that have been proposed by 

Roadcap and others (2011) to explain recharge to the confined 

part of the Mahomet aquifer. 

Data Limitations

A more precise assessment of the time of travel to 

the Mahomet aquifer from the Clinton site—including the 

CLU#3—is limited by the available ne and Kv values and the 

small number of borings with detailed geologic descriptions of 

the deposits between the Organic Soil unit and the Mahomet 

aquifer. The ne values used to represent till units for estimates 

of vertical velocity and time of travel were derived from a 

single tritium analysis of one groundwater sample. The lack of 

precision in ne and Kv values can affect travel time estimates.

 Estimates of vertical travel time through the glacial till at 

the Clinton site in this report were based on single groundwater-level measurements at observation wells. Analysis of previously collected groundwater-level data at these wells could be 

used to represent the likely variability of vertical groundwater 

velocity in response to seasonal and annual changes in groundwater levels and how that variability affects vertical velocity 

and travel time estimates.

The estimates of vertical groundwater flow velocity 

and travel time included assumptions that geologic deposits 

below the Organic Soil unit had the same permeability and 

ne characteristics as those above the unit. Hydrogeologic data 

from wells and borings were less available at CLU#3 for 

deposits beneath the Organic Soil unit to the Mahomet aquifer 

than from land surface to the Organic Soil unit. Additional Kv 

and ne data from the geologic deposits below the Organic Soil 

unit would improve the understanding of the precision and 

variability of Vv and travel time estimates calculated from the 

Darcy velocity equation. 

This report also identified several questions that could 

not be answered by available data related to the routes by 

which pesticides detected in wells sampled by the USGS 

infiltrated through the glacial till or along well-related connections to reach parts of the Mahomet aquifer that are distant 

from the Clinton site. Specifically, additional hydrogeologic, 

well condition, and land-use data are needed to evaluate the 

presence or absence of local conditions that would contribute 

to the migration of pesticide and land-use related compounds 

to the Mahomet aquifer as reported for the 10 wells in the 

regional network. This validation of processes responsible for 

detections of anthropogenic compounds is particularly needed 

at sites where pesticides and land-use related anthropogenic 

contaminants have been detected. Data that could assist with 

this validation include but are not limited to the following: 

• Re-evaluation of the USGS sampled domestic wells in 

the Mahomet aquifer related to the following:

• Well-construction documentation, 

• Sampling-related observations of well condition, 

land use, and pesticide storage and use, and 

• Field and laboratory quality assurance data compiled 

with the pesticide detections; 

• A regional evaluation of field notes and literature on 

the history of the use and storage of pesticides in the 

area over the confined part of the Mahomet aquifer, 

• Collection of additional hydrogeologic data through 

drilling, sampling, and hydraulic testing to identify the 

presence and distribution of natural avenues of vertical 

permeability described for glacial tills in other regions 

(McKay and others, 1993; Abichou and others, 2002; 

Herzog and Griffin, 1990; Herzog and Morse, 1986; 

Ecology and Environment, Inc., 1990), such as the 

following:

• Interconnected sand lenses, 

• Open or infilled fractures, 

• Weathered zones, or 

• Former root paths or burrows, and 

• Installation and sampling of observation wells at 

sites over the Mahomet aquifer that could be paired 

with existing wells with repeat detections of pesticides and land-use related contaminants; these are 

the sites where data most strongly indicate the potential for vertical flow through the glacial till. 

For example, Herzog and Morse (1986) and Haefner (2000) described how testing in till deposits improved 

evaluation of vertical hydraulic conductivity in glacial tills. 

Incorporating pesticide use estimates derived from county and 

regional surveys (Stone, 2013) could assist with this evaluation by indicating where certain pesticides were more likely to 

have been used.

Summary and Conclusions

The U.S. Geological Survey used interpretations of 

hydrogeologic conditions and tritium-based groundwater age 

estimates to assess the travel time of groundwater at a landfill 

site near Clinton, Illinois (the “Clinton site”) where a chemical waste unit (CWU) was proposed to be within the Clinton 

landfill unit #3 (CLU#3). The glacial deposits underlying a 

proposed chemical waste unit (CWU) within Clinton landfill 

unit #3 (CLU#3) consist predominantly of low-permeability 

silt-and-clay rich till interspersed with thin (typically less than 

2 feet in thickness) layers of more permeable deposits, including the Upper and Lower Radnor Till Sands and the Organic 

Soil unit. These glacial deposits are about 170 feet thick and 
44 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

overlie the Mahomet Sand Member of the Banner Formation. The Mahomet aquifer is composed of the Mahomet Sand 

Member and is used for water supply in much of east-central 

Illinois.

At the Clinton site, the Upper Radnor Till Sand, the 

Lower Radnor Till Sand and the Organic Soil unit have limited 

hydraulic interconnection with the Mahomet aquifer, as indicated by the following characteristics: 

• The presence of a low tritium detection in one of the 

two samples collected from the Upper Radnor Till 

Sand (well G53S at 0.32 ± 0.10 TU);

• The absence of detectable tritium in the other six 

groundwater samples and one sequential replicate 

sample collected from the site, including from a well in 

the Mahomet aquifer; 

• The thickness and lateral extent and continuity of lowpermeability till deposits beneath CLU#3 (estimated at 

approximately 155 feet); and

• The large differences in the water levels between the 

Upper and Lower Radnor Till Sands, and the Organic 

Soil unit and the Mahomet aquifer. 

Tritium concentrations were analyzed in water samples 

from six monitoring wells and one production well sampled 

by this study in the vicinity of the CLU#3 at the Clinton site. 

The tritium-based age estimates were computed with a simplifying, piston-flow assumption: that groundwater moves in 

discrete packets to the sampled interval by advection, without 

hydrodynamic dispersion or mixing. Tritium concentrations 

indicate a recharge age of at least 59 years (pre-1953 recharge) 

for water sampled from deposits below the upper part of the 

Radnor Till Member at the CLU#3, with older water indicated 

at progressively greater depth in the tills. The sampling results 

from well G53S indicate that nearly all groundwater in that 

well entered the ground surface as recharge before 1953. Well 

G53S is open to a sand deposit in the upper part of the Radnor 

Till Member that is at an elevation of 650 feet and a depth of 

about 50 feet below ground surface. 

Tritium was detected at a trace concentration in a sample 

from a second monitoring well (G07S; 0.11 ± 0.09 tritium 

units) open to the sand unit of the Upper Radnor Till Sand and 

possibly the Lower Radnor Till Sand. There was an absence 

of tritium in samples from two monitoring wells (EX-4 and 

EX-6) open to a sand deposit in the Lower Radnor Till Sand, 

from two monitoring wells (G06D and G07D) open to the 

Organic Soil unit, and from a production well (W-10) screened 

in the middle of the Mahomet aquifer. The Lower Radnor 

Till Sand and the Organic Soil unit are within approximately 

20 feet of the base of the Upper Radnor Till Sand, whereas the 

Mahomet aquifer is approximately 160 feet below the base of 

the Upper Radnor Till Sand. The trace concentration of tritium 

in the sample from the second well screened in the Upper Radnor Till Sand (G07S), and the absence of tritium in the Lower 

Radnor Till Sand and the Organic Soil unit (EX-4, EX-6, 

G06D, G07D) and the Mahomet aquifer (W-10) indicated 

recharge ages of at least 59 years or more (pre-1953 recharge) 

in these units in the vicinity of the CLU#3. The lack of tritium 

in five of the six groundwater samples collected from the shallow permeable units beneath CLU#3 site and the two samples 

from the one Mahomet aquifer well indicates an absence of 

post-1952 recharge and that groundwater-flow paths that could 

contribute post-1952 recharge to the lower part of the Radnor 

Till Member, the Organic Soil unit, or the Mahomet aquifer at 

the CLU#3 are not indicated by these data.

Hypothetical two-part mixtures of tritium-dead, pre-1953 

recharge water and decay-corrected tritium concentrations in 

post-1952 recharge were computed and compared with tritium 

analyses in groundwater sampled from monitoring wells at 

the CLU#3 site to evaluate whether tritium concentrations in 

groundwater could be represented by mixtures involving some 

post-1952 recharge. Results from the hypothetical two-part 

mixtures indicate that groundwater from monitoring well 

(G53S) was predominantly composed of pre-1953 recharge 

and that if present, younger, post-1955 recharge, contributed 

less than 5 percent to that sample.

A maximum vertical groundwater velocity from land 

surface to the Upper Radnor Till Sand of less than 0.85 feet 

per year was derived from the interpreted age of groundwater 

from monitoring well G53S. The tritium result and associated vertical groundwater velocity indicate an estimated travel 

time of a minimum of 200 years or more for recharge to flow 

through the 170 feet of glacial deposits from the top of the 

Berry Clay Member (at the base of the proposed CWU) to the 

top of the Mahomet Aquifer.

The piston-flow based age of recharge determined from 

the tritium concentration in the groundwater sample from 

monitoring well G53S yielded an estimated maximum vertical 

velocity from the land surface to the upper part of the Radnor 

Till Member of 0.85 feet per year or less. This velocity, if 

assumed to apply to the remaining glacial till deposits above 

the Mahomet aquifer, indicates that recharge flows the 

170 feet of glacial deposits between the base of the proposed 

chemical waste unit and the top of the Mahomet aquifer in a 

minimum of 200 years or longer. Analysis of hydraulic data 

from the site, constrained by a tritium-age based maximum 

groundwater velocity estimate, computed minimum estimates 

of effective porosity that range from about 0.021 to 0.024 for 

the predominantly till deposits above the Mahomet aquifer. 

Solution of the Darcy velocity equation using these effective 

porosity values, water-level data, and vertical hydraulic conductivity data from triaxial permeability tests of samples collected at the CLU#3 site indicated maximum vertical groundwater velocities from the Berry Clay Member to the top of the 

Mahomet aquifer of 0.41 or 0.61 feet per year, depending on 

the site-specific values used in the equation. These velocities 

indicate an estimated travel time for recharge to flow from the 

top of the Berry Clay Member (at the bottom of the CWU) to 

the top of the Mahomet Aquifer to be a minimum of 260 or 

370 years, depending on which velocity value is used. A more 
Summary and Conclusions 45

precise assessment of the time of travel of groundwater from 

land surface to the Mahomet aquifer at the Clinton site, including CLU#3, is limited by the available effective porosity and 

vertical permeability values and the small number of borings 

with detailed geologic descriptions of the deposits between the 

Organic Soil unit and the Mahomet aquifer.

The Darcy velocity equation also was solved to quantify 

travel times using vertical hydraulic conductivity data from a 

previously published regional groundwater flow model. Plausible travel times estimated using the previously published, 

regional-model derived vertical hydraulic conductivity values 

ranged from 230 to 580 years; these travel time estimates were 

either comparable to or of longer duration than those calculated using Clinton-site tritium data (200 years or longer).

Tritium data from 1996 to 2011 USGS regional sampling of the Mahomet aquifer of groundwater from domestic 

wells in the confined part of the Mahomet aquifer, which are 

2.5 to about 40 miles from the Clinton site, were compared 

with site-specific data from a production well at the Clinton 

site. Groundwater-age estimates indicated largely pre-1953 

recharge dates for 30 USGS-sampled domestic wells and other 

prior regional sampling of groundwater from the Mahomet 

aquifer; tritium was detected in concentrations indicating 

largely post-1952 recharge in only one well, a USGS-sampled 

domestic well about 30 miles from the Clinton site vicinity. 

These results agreed with the pre-1953 recharge age estimated 

for a groundwater sample and a sequential replicate sample 

from a production well in the confined part of the Mahomet 

aquifer beneath the Clinton site.

The regional tritium-based groundwater age estimates 

also were compared with the pesticide detections in samples 

from distal domestic wells that are about 2.5 to 40 miles from 

the Clinton site to identify whether very small amounts of 

post-1952 recharge have in places reached the confined part 

of the Mahomet aquifer at locations other than the Clinton site 

in an approximately 2,000 square mile area of the Mahomet 

aquifer. A very small amount of post-1952 recharge was previously defined in the section of this report titled “Two-Part 

Mixing Model Evaluation of Tritium-Based Groundwater Age 

Estimates at the Clinton Site” as less than about 2.5 percent 

of the total recharge contributing to a groundwater sample. 

Groundwater-age estimates were computed for 10 domestic 

wells screened in the Mahomet aquifer that were 2.5 to 

about 40 miles from the CLU#3 site and that had pesticide 

compounds detected. Groundwater-age estimates based on 

22 detections of pesticides or a pesticide degradation product (13 of these detections were estimated values), including 

atrazine, deethylatrazine, cyanazine, diazinon, metolachlor, 

molinate, prometon, and trifluralin in water from 10 domestic 

wells in the regional network indicate that very small amounts 

of post-1956 to post-1992 recharge can reach the confined 

part of the Mahomet aquifer in other parts of central Illinois. 

The relative lack of tritium in these samples indicate that the 

amounts of post-1956 to post-1992 recharge contributing to 

the 10 domestic wells were a very small part of the overall 

older groundwater sampled from the wells. 

The flow process by which very small amounts of 

pesticide-bearing groundwater reached the screened interval in 

domestic wells in the confined part of the Mahomet 

aquifer through 166 to 291 feet of till dominated glacial 

material (in the case of the molinate detection in well 

401058088521001, the total depth to the top of the well screen 

was 336 feet) could not be distinguished between wellintegrity related and natural hydrogeologic features. Potential explanations include: (1) infiltration through man-made 

avenues through or along the well, (2) flow of very small 

amounts of post-1956 to post-1992 recharge through sparsely 

distributed natural permeable aspects of the glacial till and 

diluted by mixing with older groundwater, or (3) a combination of both processes. This study identified no observations 

or data that indicated the presence of these vertical permeable 

features in glacial till below the Upper Radnor Till Sand at the 

Clinton site. A detection of tritium in one domestic well and 

the detections of pesticides in samples from 10 other domestic wells indicate that characteristics that favor transport of 

recharge and potential contaminants from land surface to the 

Mahomet aquifer were present in about 33 percent of the wells 

in the 30-well regional network.

Presuming the off-site domestic wells sampled by the 

USGS in 1996–2011 in the regional study are adequately 

sealed and produce groundwater that is representative of 

aquifer conditions, the regional tritium and pesticide results 

indicate the following: 

• The glacial stratigraphy above the Mahomet aquifer 

exhibits substantial heterogeneity,

• That parts of the Mahomet aquifer with the pesticide 

detections can be susceptible to contaminant sources at 

the land surface, and 

• That a potential exists for possible contaminants from 

land surface to be transported through the glacial 

drift deposits that confine the Mahomet aquifer in 

other parts of central Illinois at faster rates than those 

computed for recharge at the Clinton site, including 

CLU#3. 

The regional pesticide and tritium results from the 

domestic wells indicate the potential value of submicrogramper-liter level concentrations of land-use derived indicators 

of modern recharge to indicate the presence of very small 

amounts of modern, post-1952 age recharge in overall older, 

pre-1953 age groundwater.

The conclusions of this investigation are based on 

assumptions and sources of uncertainty in the analyses presented in this report, including hydrogeologic characteristics 

(substantial continuity of sand deposits, continuity of till 

deposits, uniform groundwater velocity, uniform permeability, 

and uniform effective porosity, based on a tritium determination) and conceptualization of vertical flow (piston flow, which 

is supported by analysis of hypothetical two-part mixing models, and predominantly downward flow). Applications of these 
46 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater

assumptions are explained with the appropriate analysis in the 

body of the report.

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Tables 4 and 5
50 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater 402248088150501 240 NPD 1998 <1.8 TU Indetermi- nate NPD NS NPD NS NS NPD NS NS 401238088444201 200 NPD 1997 <1.8 TU Indetermi- nate NPD NS NPD NS NS NPD NS NS 402122088161101 260 249 1998 <1.8 TU Indetermi- nate PD, atrazine, 0.002E NS NPD, atrazine <0.001 NS NS NPD, atrazine <0.007 NS NS 402029088172001 201 197 1998 <1.8 TU Indetermi- nate PD, atrazine, 0.003E, deeth- ylatrazine 0.003E, metolachlor <0.002 NS PD, atrazine 0.002E, deeth- ylatrazine <0.002, meto- lachlor 0.002E NPD, atrazine <0.007, deeth- ylatrazine <0.006, meto- lachlor <0.013 NPD, atrazine <0.007, deeth- ylatrazine <0.006, meto- lachlor <0.006 NPD, atrazine <0.007, deeth- ylatrazine <0.014, meto- lachlor <0.010 NPD, atrazine <0.007, deeth- ylatrazine <0.014, meto- lachlor <0.014 NPD, atrazine <0.008, deeth- ylatrazine <0.006, meto- lachlor <0.020 401910089120201 320 NPD 1997 <1.8 TU Indetermi- nate NPD NS NS NS NS NPD NS NS 401753088114201 200 NPD NS NS NS NPD NS NS NS NS NPD NS NS 401811088274801 218 NPD 2007 -0.03 TU Pre-1953 NPD NS NS NS NS NPD NS NS 401740089022401 333 NPD 1997 <1.8 TU Indetermi- nate NPD NS NS NS NS NPD NS NS 401504088295001 266 NPD 2007 -0.16 TU Pre-1953 NPD NS NS NS NS NPD NS NS 401443088230101 205 NPD 2007 5.7 TU Post-1952 NPD NS NS NS NS NPD NS NS 401246088585101 365 357 1997 1998 <1.8 TU, <1.8 TU Indetermi- nate PD, atrazine, 0.004E NS NPD, atrazine <0.001 NPD, atrazine <0.007 NPD, atrazine <0.007 NPD, atrazine <0.007 NPD, atrazine <0.007 NPD, atrazine <0.008

401131088255701 205 NPD 2007 -0.06 TU Pre-1953 NPD NS NS NS NS NPD NS NS

Table 4. Results of U.S. Geological Survey sampling from domestic wells open to the Mahomet aquifer in east-central Illinois, 1996 to 2011.[<, less than, indicates constituent not detected and detection limit; TU, tritium unit; NPD, no pesticides detected; NS, not sampled; PD, one or more pesticides detected; E, estimated concentration; concentrations of pesticide; A negative tritium concentration is referred to in the report as a very low, near zero tritium concentration. A negative tritium concentration indicates that tritium derived decay counts yielded from the sample analysis were less than the analytical background concentation of zero tritium units]

Color coding of table cells and type of concentration reported in sample: 

5.7 TU indicates tritium detection, with well identifier and concentration in bold text if greater than 0.03 TU

PD, atrazine, 

0.003E indicates pesticide detected in at least two samples from a well, with well identifier and compound detected and concentration in micrograms per liter in bold text

PD, atrazine, 

0.002E pesticide detected in one sample from a well, with well identifier and compound detected and concentration in micrograms per liter in bold, italicized text

USGS well 

identifier

Well 

Depth 

(feet)

Depth to 

top of well 

screen for 

wells with 

pesticide 

detections 

(feet)

Tritium samples Pesticides detected, by year of sampling, with concentration in micrograms per liter

Year of 

tritium 

sample 

collection 

Tritium 

concentration, in tritium units 

(TU)

Tritiumbased 

groundwater age 

classification2

1996 1997 1998 2002 2005 2007 2009 2011
Table 4 51

Table 4. Results of U.S. Geological Survey sampling from domestic wells open to the Mahomet aquifer in east-central Illinois, 1996 to 2011.—Continued[<, less than, indicates constituent not detected and detection limit; TU, tritium unit; NPD, no pesticides detected; NS, not sampled; PD, one or more pesticides detected; E, estimated concentration; concentrations of pesticide; A negative tritium concentration is referred to in the report as a very low, near zero tritium concentration. A negative tritium concentration indicates that tritium derived decay counts yielded from the sample analysis were less than the analytical background concentation of zero tritium units]

Color coding of table cells and type of concentration reported in sample: 

5.7 TU indicates tritium detection, with well identifier and concentration in bold text if greater than 0.03 TU

PD, atrazine, 

0.003E indicates pesticide detected in at least two samples from a well, with well identifier and compound detected and concentration in micrograms per liter in bold text

PD, atrazine, 

0.002E pesticide detected in one sample from a well, with well identifier and compound detected and concentration in micrograms per liter in bold, italicized text

USGS well 

identifier

Well 

Depth 

(feet)

Depth to 

top of well 

screen for 

wells with 

pesticide 

detections 

(feet)

Tritium samples Pesticides detected, by year of sampling, with concentration in micrograms per liter

Year of 

tritium 

sample 

collection 

Tritium 

concentration, in tritium units 

(TU)

Tritiumbased 

groundwater age 

classification2

1996 1997 1998 2002 2005 2007 2009 2011

401041088242901 210 202 1998 <1.8 TU Indeterminate PD, atrazine, 0.003E,

metolachlor <0.002, cyanazine <0.004

NS PD, atrazine 

0.011, metolachlor 0.007,cyanazine 0.013

NS NS NPD, 

atrazine 

<0.007, 

metolachlor 

<0.010,

cyanazine 

<0.004

NS NS

401058088521001 340 336 1998 <1.8 TU Indeterminate PD, atrazine, 0.005,

molinate 0.019

NS NPD, 

atrazine <0.001,molinate <0.004

NS NS NPD, 

atrazine 

<0.007,

molinate < 

0.003

NS NS

400935089062301 293 NPD 2007 0.03 TU Pre-1953 NPD NS NS NS NS NPD NS NS400939088574301 335 NPD 1997 <1.8 TU Indeterminate NPD NS NS NS NS NPD NS NS401106088300801 275 NPD 2007 0.03 TU Pre-1953 NS NS NS NS NS NPD NS NS400842089065701 234 NPD 1997 <1.8 TU Indeterminate NPD NPD NS NS NS NPD NS NS

400827088332601 208 NPD 1996 <0.3 TU Pre-1953 NPD NS NS NS NS NPD NS NS400805088531501 250 242 1997 <1.8 TU Indeterminate NPD, prometon 

<0.018

NS NS NPD, 

prometon 

<0.015

PD, 

prometon 

0.005

PD, 

prometon 

0.007

NPD,

prometon 

<0.012

NPD, 

prometon <0.012

400603088300901 238 234 1998 <1.8 TU Indeterminate NPD, atrazine <0.001, 

metolachlor <0.002

NS PD, 

atrazine 0.001E, metolachlor 0.002E

NS NS NPD, 

atrazine 

<0.007, 

metolachlor 

<0.010

NS NS
52 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater 400817088593401 273 269 1996 1998 <0.3 TU <1.8 TU Pre-1953 PD, metolachlor 0.004 NS NPD, meto- lachlor <0.002 NS NS NPD, meto- lachlor <0.010 NS NS 400433088495801 273 NPD 2007 0 TU Pre-1953 NPD NS NS NS NS NPD NS NS 400422088575101 280 276 1997 <1.8 TU Indetermi- nate PD, atrazine <0.001, metolachlor <0.002, diazinon 0.0025E NS PD, atrazine 0.001E, meto- lachlor 0.002E, diazinon 0.0138 NS NS NPD, atrazine <0.007, meto- lachlor <0.010, diazinon <0.0050 NS NS 400357088444401 258 NPD 2007 0.2 TU Pre-1953 NPD NS NS NS NS NPD NS NS 400317088512701 257 253 2007 -0.03 TU Pre-1953 PD, trifluralin 0.003E NS NS NS NS NPD, trifluralin <0.009 NS NS 400134088433101 228 NPD 2007 0.05 TU Pre-1953 NPD NS NS NS NS NPD NS NS 400057088550101 270 NPD 1997 <1.8 TU Indetermi- nate NPD NS NS NPD NPD NPD NPD NPD 400009088484601 210 NPD 1997 <1.8 TU Indetermi- nate NPD NS NS NPD NPD NPD NPD NPD 395853088441301 220 NPD 2007 0.05 TU Pre-1953 NPD NS NS NS NS NPD NS NS

1Pesticide registration date references: U.S. Environmental Protection Agency (1995, 1996, 2006a, 2006b, 2008, 2014).2Tritium-based groundwater age classification reference: Clark and Fritz (1997).3Molinate did not have a reported use in Illinois, as indicated in Stone (2013) and as displayed in maps at http://water.usgs.gov/nawqa/pnsp/usage/maps/. 

Table 4. Results of U.S. Geological Survey sampling from domestic wells open to the Mahomet aquifer in east-central Illinois, 1996 to 2011.—Continued[<, less than, indicates constituent not detected and detection limit; TU, tritium unit; NPD, no pesticides detected; NS, not sampled; PD, one or more pesticides detected; E, estimated concentration; concentrations of pesticide; A negative tritium concentration is referred to in the report as a very low, near zero tritium concentration. A negative tritium concentration indicates that tritium derived decay counts yielded from the sample analysis were less than the analytical background concentation of zero tritium units]

Color coding of table cells and type of concentration reported in sample: 

5.7 TU indicates tritium detection, with well identifier and concentration in bold text if greater than 0.03 TU

PD, atrazine, 

0.003E indicates pesticide detected in at least two samples from a well, with well identifier and compound detected and concentration in micrograms per liter in bold text

PD, atrazine, 

0.002E pesticide detected in one sample from a well, with well identifier and compound detected and concentration in micrograms per liter in bold, italicized text

USGS well 

identifier

Well 

Depth 

(feet)

Depth to 

top of well 

screen for 

wells with 

pesticide 

detections 

(feet)

Tritium samples Pesticides detected, by year of sampling, with concentration in micrograms per liter

Year of 

tritium 

sample collection 

Tritium 

concentration, in tritium units 

(TU)

Tritiumbased 

groundwater age 

classification2

1996 1997 1998 2002 2005 2007 2009 2011
Table 5 53

402248088150501 240 NPD NPD NPD NPD 1998 <1.8 TU Indeterminate

401238088444201 200 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate

402122088161101 260 249 7 atrazine (1958), detected in one 

sample

38 years or less based on single 

atrazine detection

1998 <1.8 TU Indeterminate

402029088172001 201 197 9 atrazine (1958), detected in 

successive samples and metolachlor (1976) detected in one sample

38 years or less based on two 

detections of atrazine, 22 years or less based on one detection of metolachlor

1998 <1.8 TU Indeterminate

401910089120201 320 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate

401753088114201 200 NPD NPD NPD NPD NS NS NS

401811088274801 218 NPD NPD NPD NPD 2007 -0.03 TU Pre-1953

401740089022401 333 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate

401504088295001 266 NPD NPD NPD NPD 2007 -0.16 TU Pre-1953

401443088230101 205 NPD NPD NPD NPD 2007 5.7 TU Post-1952

401246088585101 365 357 9 atrazine (1958), detected in one 

sample

38 years or less based on single 

atrazine detection

1997–1998 <1.8 TU

 <1.8 TU

Indeterminate

401131088255701 205 NPD NPD NPD NPD 2007 -0.06 TU Pre-1953

401041088242901 210 202 9 atrazine (1958) detected 

in successive samples, metolachlor (1976) and cyanazine (1971) detected in one sample

38 years or less based on two 

detections of atrazine, 20 years or less based on one detection of metolachlor, and 27 years or 

less based on one detection of cyanazine

1998 <1.8 TU Indeterminate

401058088521001 340 336 84 atrazine (1958), molinate 

(1992) detected in one sample3

38 years or less, based on single 

atrazine detection; 4 years or less, based on one molinate detection3

1998 <1.8 TU Indeterminate

400935089062301 293 NPD NPD NPD NPD 2007 0.03 TU Pre-1953

400939088574301 335 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate

Table 5. Minimum vertical groundwater flow velocities and maximum time of travel from land surface to the Mahomet aquifer for water samples collected by U.S. Geological Survey from domestic wells open to the Mahomet aquifer in east-central Illinois, 1996 to 2011, with detections of a pesticide and tritium-based groundwater age classification.[<, less than, indicates constituent not detected and detection limit; TU, tritium unit; NPD, no pesticides detected; NS, not sampled; A negative tritium concentration is referred to in the report as a very low,near zero tritium concentration. A negative tritium concentration indicates that tritium derived decay counts yielded from the sample analysis were less than the analytical background concentation of zero tritium units]

Color coding of table cells and type of concentration reported in sample: 

5.7 TU indicates tritium detection, with well identifier and concentration in bold text if greater than 0.03 TU

atrazine (1958) indicates pesticide detected in at least two samples from a well, with well identifier and compound detected and concentration in micrograms per liter in bold text

metolachlor (1976) pesticide detected in one sample from a well, with well identifier and compound detected and concentration in micrograms per liter in bold, italicized text

USGS well 

identifier

Well 

Depth 

(feet)

Depth to 

top of well 

screen for 

wells with 

pesticide 

detections 

(feet)

Minimum vertical groundwater flow velocity computed 

from pesticide detected with 

youngest registration date 

(feet/year) 

Pesticide detected in one or 

more samples from a well and 

year of registration in the United States1, pesticides detected 

in one or more samples used 

for time-of-travel estimate

Maximum time of travel from land 

surface to the Mahomet aquifer for 

the part of one or more water samples with detections of a pesticide 

(Difference of the year of sampling 

for the earliest pesticide detection 

in a sample, minus the registration 

date of that pesticide1)

Tritium samples

Year of tritium 

sample collection 

Tritium concentration, in 

tritium units 

(TU)

Tritium-based 

groundwater 

age classification2
54 Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater 401106088300801 275 NPD NPD NPD NPD 2007 0.03 TU Pre-1953 400842089065701 234 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate 400827088332601 208 NPD NPD NPD NPD 1996 <0.3 TU Pre-1953 400805088531501 250 242 5 prometon (1959) detected in successive samples 46 years or less, based on two detections of prometon 1997 <1.8 TU Indeterminate 400603088300901 238 234 11 atrazine (1958) and metola- chlor (1976) detected in one sample 38 years or less based on one detection of atrazine, 20 years or less based on one detection of metolachlor 1998 <1.8 TU Indeterminate 400817088593401 273 269 13 metolachlor (1976) detected in one sample 20 years or less based on one detection of metolachlor 1996–1998 <0.3 TU <1.8 TU Pre-1953 400433088495801 273 NPD NPD NPD NPD 2007 0 TU Pre-1953 400422088575101 280 276 13 diazinon (1956) detected in successive samples, atrazine (1958) and metolachlor (1976) detected in one sample 40 years or less based on two detections of diazinon, 40 years or less based on one detection of atrazine, 22 years or less based on one detection of metolachlor 1997 <1.8 TU Indeterminate 400357088444401 258 NPD NPD NPD NPD 2007 0.2 TU Pre-1953 400317088512701 257 253 8 trifluralin (1963) detected in one sample 33 years or less based on one detection of trifluralin 2007 -0.03 TU Pre-1953 400134088433101 228 NPD NPD NPD NPD 2007 0.05 TU Pre-1953 400057088550101 270 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate 400009088484601 210 NPD NPD NPD NPD 1997 <1.8 TU Indeterminate 395853088441301 220 NPD NPD NPD NPD 2007 0.05 TU Pre-1953

1Pesticide registration date references: U.S. Environmental Protection Agency (1995, 1996, 2006a, 2006b, 2008, 2014).2Tritium-based groundwater age classification reference: Clark and Fritz (1997), adapted to decay the classifications to 2012.3Molinate did not have an estimated use in Illinois, as reported in Stone (2013) and as displayed in maps at http://water.usgs.gov/nawqa/pnsp/usage/maps/.

Table 5. Minimum vertical groundwater flow velocities and maximum time of travel from land surface to the Mahomet aquifer for water samples collected by U.S. Geological Survey from domestic wells open to the Mahomet aquifer in east-central Illinois, 1996 to 2011, with detections of a pesticide and tritium-based groundwater age classification.—Continued[<, less than, indicates constituent not detected and detection limit; TU, tritium unit; NPD, no pesticides detected; NS, not sampled; A negative tritium concentration is referred to in the report as a very low,near zero tritium concentration. A negative tritium concentration indicates that tritium derived decay counts yielded from the sample analysis were less than the analytical background concentation of zero tritium units]

Color coding of table cells and type of concentration reported in sample: 

5.7 TU indicates tritium detection, with well identifier and concentration in bold text if greater than 0.03 TU

atrazine (1958) indicates pesticide detected in at least two samples from a well, with well identifier and compound detected and concentration in micrograms per liter in bold text

metolachlor (1976) pesticide detected in one sample from a well, with well identifier and compound detected and concentration in micrograms per liter in bold, italicized text

USGS well 

identifier

Well 

Depth 

(feet)

Depth to 

top of well 

screen for 

wells with 

pesticide 

detections 

(feet)

Minimum vertical groundwater flow velocity computed 

from pesticide detected with 

youngest registration date 

(feet/year) 

Pesticide detected in one or 

more samples from a well and 

year of registration in the United States1, pesticides detected 

in one or more samples used 

for time-of-travel estimate

Maximum time of travel from land 

surface to the Mahomet aquifer for 

the part of one or more water samples with detections of a pesticide 

(Difference of the year of sampling 

for the earliest pesticide detection 

in a sample, minus the registration 

date of that pesticide1

)

Tritium samples

Year of tritium 

sample collection 

Tritium concentration, in 

tritium units 

(TU)

Tritium-based 

groundwater 

age classification2
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Kay and Buszka—Application of Hydrogeology and Tritium-Based Groundwater-Age Estimates to Assess the Travel Time of Groundwater—Scientific Investigations Report 2015–5159

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155159