1,050

1,100

1,150

Water level, in feet above the

North American Vertical Datum of 1988

July August September

2016

0

2

4

6

8

10

12

Daily precipitation total, in inches

Precipitation

Surficial aquifer

Confining unit

Pumped confined aquifer

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5127

Prepared in cooperation with the Legislative-Citizen Commission on Minnesota Resources and 

in collaboration with Iowa State University and the Minnesota Department of Health

Hydrogeology and Groundwater Geochemistry of 

Till Confining Units and Confined Aquifers in 

Glacial Deposits near Litchfield, Cromwell, 

Akeley, and Olivia, Minnesota, 2014–18
Cover photograph. (Clockwise from top left) Photographs showing (1) the Olivia water tower and 

the rotosonic drill rig, taken at the Olivia site, July 31, 2017, taken by Jared Trost, U.S. Geological 

Survey (USGS) hydrologist; (2) USGS hydrologic technician, Andrew Berg, and Iowa State University 

graduate student, Alyssa Witt, in the process of purging a piezometer after installation, taken at the 

Cromwell site, July 11, 2015, taken by James Stark, USGS Minnesota Water Science Center director; 

and (3) a Good Thunder formation till core sample taken at the Olivia site, August 9, 2017, taken by 

Jared Trost, USGS. (4) Plot showing precipitation and hydraulic head in piezometers during 3-month 

periods. (5) Photograph showing Minnesota Geological Survey Quaternary geologist, Maurice 

Nguyen, taking notes on till taken at the hydrogeology field camp site, May 24, 2017, taken by Jared 

Trost, USGS.
Hydrogeology and Groundwater 

Geochemistry of Till Confining Units and 

Confined Aquifers in Glacial Deposits near 

Litchfield, Cromwell, Akeley, and Olivia, 

Minnesota, 2014–18

By Jared J. Trost, Anna-Turi Maher, William W. Simpkins, Alyssa N. Witt, 

James R. Stark, Justin Blum, and Andrew M. Berg

Prepared in cooperation with the Legislative-Citizen Commission on Minnesota 

Resources and in collaboration with Iowa State University and the Minnesota 

Department of Health

Scientific Investigations Report 2020–5127

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, visit 

https://store.usgs.gov/.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. 

Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Trost, J.J., Maher, A., Simpkins, W.W., Witt, A.N., Stark, J.R., Blum, J., and Berg, A.M., 2020, Hydrogeology and 

groundwater geochemistry of till confining units and confined aquifers in glacial deposits near Litchfield, Cromwell, 

Akeley, and Olivia, Minnesota, 2014–18: U.S. Geological Survey Scientific Investigations Report 2020–5127, 80 p., 

https://doi.org/10.3133/sir20205127.

Associated data for this publication:

Maher, A.-T., Trost, J.J., Witt, A.N., Berg, A.M., Simpkins, W.W., and Stark, J.R., 2020, Geochemical data, waterlevel data, and slug test analysis results from till confining units and confined aquifers in glacial deposits near 

Akeley, Cromwell, Litchfield, and Olivia, Minnesota, 2015–2018: U.S. Geological Survey data release, https://doi.org/

10.5066/P9IXC7D3.

Trost, J.J., Feinstein, D.T., and Jones, P.M., 2020, Heuristic MODFLOW models used to evaluate the effects of 

pumping groundwater from confined aquifers overlain by till confining units: U.S. Geological Survey data release, 

https://doi.org/10.5066/P9KOI6T3.

U.S. Geological Survey, 2019, USGS water data for the Nation: U.S. Geological Survey National Water Information 

System database, https://doi.org/10.5066/F7P55KJN.

ISSN 2328-0328 (online)
iii

Acknowledgments

Funding for this study was provided by the Minnesota Environment and Natural Resources Trust 

Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources and the 

U.S. Geological Survey’s Cooperative Matching Funds program. Many people made contributions throughout this study. The authors wish to thank the numerous partners in this study that 

provided guidance in selecting field sites, including staff from the Minnesota Department of 

Natural Resources, Minnesota Geological Survey, University of Minnesota, and Minnesota 

Department of Health. Thanks to Bob Tipping, Maurice Nguyen, Amy Staley, and Caleb Wagner 

for core logging, geophysical surveys, and geologic interpretations and reports. Thanks to Scott 

Alexander for providing access to the University of Minnesota’s hydrogeology field camp and 

for completing aquifer tests at this site for this study. Thanks to the Minnesota Department of 

Health staff who collected data for the aquifer tests at Litchfield and Cromwell. Thanks to the 

cities of Cromwell, Litchfield, and Olivia, and in particular, the municipal water systems operators of these cities, for their cooperation in this study. Thanks to Jan Falteisek and Erik Smith for 

providing technical reviews of this report.

A special thanks goes to Daniel Feinstein, U.S. Geological Survey, who completed the MODFLOW models discussed in this report.

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Purpose and Scope ..............................................................................................................................2

Description of Study Sites ...................................................................................................................2

Geologic Setting ....................................................................................................................................4

Litchfield ........................................................................................................................................4

Cromwell......................................................................................................................................11

Hydrogeology Field Camp .........................................................................................................11

Olivia.............................................................................................................................................12

Methods of Study.........................................................................................................................................12

Field Study Design and Piezometer Installation ............................................................................12

Hydrology .............................................................................................................................................15

Water-Level and Precipitation Monitoring ............................................................................16

Slug Tests ....................................................................................................................................16

Aquifer Tests...............................................................................................................................17

Calculations of Groundwater Flow through Till According to Darcy’s Law .....................19

Groundwater Geochemistry ..............................................................................................................19

Groundwater Modeling ......................................................................................................................20

Characterization of Glacial Till and Aquifer Systems ............................................................................25

Hydrogeology.......................................................................................................................................25

Hydraulic-Head Responses to Pumping and Weather ........................................................25

Reverse Water-Level Fluctuations ..........................................................................................32

Hydraulic Conductivity ..............................................................................................................32

Leakage through Tills ................................................................................................................36

Groundwater Geochemistry and Water Quality ............................................................................38

Background Information ...........................................................................................................38

Groundwater Age and Evidence for Infiltration of Anthropogenic Chemicals ................40

Dissolved Nitrate, Dissolved Phosphorus, and Oxidation-Reduction 

Conditions in Groundwater .........................................................................................44

Groundwater Type .....................................................................................................................48

Sources of Uncertainty ......................................................................................................................48

Heuristic Groundwater Modeling ....................................................................................................50

Till Vertical Hydraulic Conductivity, Aquifer Size, and Middle Unit Horizontal 

Hydraulic Conductivity Effects on Groundwater Fluxes ........................................50

Pumping and Aquifer Transmissivity Effects on Groundwater Fluxes .............................54

Summary........................................................................................................................................................55

References Cited..........................................................................................................................................58

Appendix 1 Well and Piezometer Construction Details .....................................................................65

Appendix 2 Slug Test Information ..........................................................................................................70

Appendix 3 Quality Assurance for Water-Quality Samples...............................................................78
vi

Figures

1. Map showing the location of the Litchfield sites and the Cromwell, 

hydrogeology field camp, and Olivia sites in Minnesota in relation to major 

glacial lobes of late-Wisconsinan deposits, and the piezometers and 

production wells at each site .....................................................................................................3

2. Litchfield 1 site piezometer construction, stratigraphy, and mean water levels ...............5

3. Litchfield 2 site piezometer construction, stratigraphy, and mean water levels ...............6

4. Cromwell site piezometer construction, stratigraphy, and mean water levels ..................7

5. Hydrogeology field camp site piezometer construction, stratigraphy, and mean 

water levels ...................................................................................................................................8

6. Olivia site piezometer construction, stratigraphy, and mean water levels .........................9

7. Photographs showing images of till from cores extracted from sites ...............................10

8. Graph showing example graphical slug test analysis output from AQTESOLV for 

piezometer OT–20 .......................................................................................................................17

9. Heuristic MODFLOW model layout ..........................................................................................24

10. Graphs showing precipitation and hydraulic head in piezometers during 

3-month periods ..........................................................................................................................27

11. Graphs showing hydraulic information for each piezometer nest including 

hydraulic conductivity, differences in hydraulic head, and hydraulic gradients 

among piezometers ....................................................................................................................29

12. Graphs showing drawdown and water-level displacement in piezometer nests 

during constant-rate aquifer tests ...........................................................................................31

13. Trilinear plots showing the relation between sand, silt, and clay contents near 

the screened interval of piezometers and horizontal hydraulic conductivity 

estimates; the presence or absence of an aquifer test drawdown response .................35

14. Graph showing comparison of oxygen-18/oxygen-16 ratios and hydrogen-2/

hydrogen-1 ratios with the global meteoric water line for groundwater and 

pore-water samples from the Litchfield 1, Litchfield 2, Cromwell, Olivia, and 

hydrogeology field camp sites ..................................................................................................41

15. Graphs showing vertical profiles of tritium, chloride concentrations, and 

chloride/bromide mass ratios in groundwater and pore-water samples .........................42

16. Graphs showing vertical profiles of dissolved nitrate and phosphorus 

concentrations in groundwater and pore-water samples ...................................................45

17. Graphs showing vertical profiles of concentrations of oxidation-reduction 

indicators in groundwater and pore-water including dissolved oxygen, iron, 

manganese, and sulfate ............................................................................................................46

18. Piper diagram showing major ion concentrations in groundwater samples from 

piezometer nests at the Litchfield 2, Cromwell, hydrogeology field camp, and 

Olivia sites....................................................................................................................................49

19. Graphs showing heuristic MODFLOW model output including changes in 

maximum drawdown in till, pumping-induced increase in leakage to till from 

the surficial unit, and the amount of water entering the aquifer through its top face ....53

20. Graphs showing heuristic MODFLOW model output for variation model runs 

including changes in maximum drawdown in model layer 3, pumping-induced 

increase in leakage to model layer 2 from model layer 1, and the amount of 

water entering the buried sand unit through its top face for variations in the 

buried sand unit horizontal hydraulic conductivity (Kh), variations in total 

pumping, and variations in till vertical hydraulic conductivity and buried sand Kh ........55
vii

Tables

1. Well and piezometer identification, vertical placement, and mean water-level 

information ...................................................................................................................................13

2. Summary of constant-rate aquifer tests and analytical approaches used to 

determine representative till vertical hydraulic conductivities ..........................................18

3. Summary of water-quality sampling events, analytes, and analytical 

laboratories used for water sample analysis .........................................................................21

4. Model parameter values used in the heuristic groundwater model scenarios 

and the naming scheme used for each model run ................................................................22

5. Summary of physical and hydraulic properties of till and confined aquifers at 

the Litchfield 1, Litchfield 2, Cromwell, Olivia, and hydrogeology field camp sites .........26

6. Summary of horizontal hydraulic conductivity (Kh) values from slug tests, 

lithology of sediments at the piezometer screen, and the methods used to 

determine Kh values from slug test data .................................................................................33

7. Hydraulic characteristics of till, annual pumping rates, and estimates of 

vertical travel time and water flux through 1 square mile of till based on Darcy's 

law for each study site ...............................................................................................................37

8. Summary of sustained pumping rates, water fluxes, and drawdowns from 

steady-state heuristic MODFLOW model output ...................................................................51

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

gallon (gal) 0.003785 cubic meter (m3)

gallon (gal) 3.785 cubic decimeter (dm3)

million gallons (Mgal) 3,785 cubic meter (m3)

Flow rate

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

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per year (gal/yr) 0.003785 cubic meter per year (m3/yr)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)
viii

International System of Units to U.S. customary units

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

Volume

cubic meter (m3) 6.290 barrel (petroleum, 1 barrel = 42 gal)

liter (L) 33.81402 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

liter (L) 61.02 cubic inch (in3)

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

 °F = (1.8 × °C) + 32.

Datum

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

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm 

at 25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Results for measurements of stable isotopes of an element (with symbol E) in water, solids, 

and dissolved constituents commonly are expressed as the relative difference in the ratio of 

the number of the less abundant isotope (iE) to the number of the more abundant isotope of a 

sample with respect to a measurement standard.
ix

Abbreviations

‰ per mil

14C yr BP carbon-14 years before present

cal yr BP calendar years before present

δ2H a measure of the ratio of stable isotopes hydrogen-2 and hydrogen-1

δ18O a measure of the ratio of stable isotopes oxygen-18 and oxygen-16

DO dissolved oxygen

HFC hydrogeology field camp

K hydraulic conductivity

KGS Kansas Geological Survey

Kh horizontal hydraulic conductivity

Kv vertical hydraulic conductivity

MGS Minnesota Geological Survey

NWIS National Water Information System

NWQL National Water Quality Laboratory

RWF reverse water-level fluctuation

TU tritium unit

USGS U.S. Geological Survey

Hydrogeology and Groundwater Geochemistry of 

Till Confining Units and Confined Aquifers in Glacial 

Deposits near Litchfield, Cromwell, Akeley, and Olivia, 

Minnesota, 2014–18

By Jared J. Trost,1 Anna-Turi Maher,1 William W. Simpkins,2 Alyssa N. Witt,2 James R. Stark,3 Justin Blum,4

and Andrew M. Berg1

1U.S. Geological Survey.

2Iowa State University.

3Minnesota State Legislative Water Commission.

4Minnesota Department of Health.

Abstract

Confined (or buried) aquifers of glacial origin overlain 

by till confining units provide drinking water to hundreds 

of thousands of Minnesota residents. The sustainability of 

these groundwater resources is not well understood because 

hydraulic properties of till that control vertical groundwater fluxes (leakage) to underlying aquifers are largely 

unknown. The U.S. Geological Survey, Iowa State University, 

Minnesota Geological Survey, and Minnesota Department 

of Health investigated hydraulic properties and groundwater flow through till confining units using field studies and 

heuristic MODFLOW simulations. Till confining units in the 

following late-Wisconsinan stratigraphic units (with locations 

in parentheses) were characterized: Des Moines lobe till of 

the New Ulm Formation (Litchfield, Minnesota), Superior 

lobe till of the Cromwell and Aitkin Formations (Cromwell, 

Minn.), and Wadena lobe till of the Hewitt Formation (hydrogeology field camp [HFC] near Akeley, Minn.). Pre-Illinoian 

till of the Good Thunder formation (Olivia, Minn.) was also 

characterized.

Hydraulic and geochemical field data were collected 

from sediment cores and a series of five piezometer nests. 

Each nest consisted of five to eight piezometers screened at 

short vertical intervals in hydrostratigraphic units including (if 

present) surficial aquifers, till confining units, confined/buried 

aquifers, and underlying bedrock. Till hydraulic conductivity 

was estimated from slug tests (horizontal [Kh]) and constantrate aquifer tests in the confined aquifer (vertical [Kv]). Travel 

times through the till were evaluated with Darcy’s law and stable isotope concentrations. A series of heuristic MODFLOW 

simulations were used to evaluate groundwater fluxes through 

till across the range of till hydraulic properties and pumping 

rates observed at the field sites.

The field data demonstrated variability in hydraulic 

properties between and within till stratigraphic units horizontally and vertically. The variability in hydraulic properties 

within and between sites resulted in substantial differences 

in groundwater flux through till. A conceptual understanding 

that emerges from the vertical till profiles is that they are not 

homogeneous hydrostratigraphic units with uniform properties; rather, each vertical sequence is a heterogeneous mixture 

of glacial sediment with differing abilities to transmit water.

Till thicknesses varied from 60 to 166 feet, and till 

textures ranged from a sandy loam (Hewitt Formation, HFC 

site) to a silt loam/clay loam (Good Thunder formation, Olivia 

site). Till Kh varied by one to three orders of magnitude within 

each piezometer nest. Four piezometer nests had downward 

hydraulic gradients ranging from 0.04 to 0.56, and one nest 

had a slight upward hydraulic gradient of 0.02. The Cromwell, 

HFC, and Litchfield 1 sites were examples of “leaky” tills 

with high Kv (0.001 to 1.1 feet per day [ft/d]) and geometric 

mean Kh (0.03 to 0.07 ft/d) and extensive vertical hydraulic 

connectivity between the confined aquifer and the overlying 

till. Estimated groundwater travel times through these sites 

ranged from 1 to 81 years, and two of these sites had tritium 

throughout their till profiles. The tills at the other two sites, 

Olivia and Litchfield 2, were effective confining units that had 

low Kv (0.001 to 0.0005 ft/d) and geometric mean Kh (0.0002 

to 0.004 ft/d). The till piezometers at these sites had no drawdown response to short-term (up to 10 hours for Olivia and 

up to 5 days for Litchfield) high-capacity pumping from the 

confined aquifer. Estimated groundwater travel times through 

the tills at these sites ranged from 165 to nearly 1,800 years, 

and tritium was only detected in the upper one-third of these 

till profiles. Across all sites, the till vertical anisotropy (ratio of 

Kh to Kv) ranged by four orders of magnitude from 0.05 at the 

Cromwell nest to 70 at the Litchfield 1 nest. Stable isotopes of 
2 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

oxygen and hydrogen indicate that groundwater throughout all 

five till profiles is younger than the last glacial advance into 

Minnesota at about 11,000 years ago.

The heuristic modeling demonstrated that, for understanding sustainability of groundwater pumping from confined aquifers, knowledge of till hydraulic properties is just 

as important as knowledge of aquifer hydraulic properties. 

Substantial differences in groundwater fluxes into and through 

till were observed across hydrogeologic settings representative of the field sites. Over long periods of time (hundreds of 

years), pumping-induced hydraulic gradients are established in 

confined aquifer systems and, even in low hydraulic conductivity tills, these pumping-induced hydraulic gradients increase 

leakage into and through till compared to ambient conditions.

In conclusion, groundwater flowing vertically downward through till confining units (leakage) replenishes water 

pumped from confined aquifers. Till hydraulic properties, such 

as those presented in this report, provide important information that can be used to quantify leakage rates through till. Till 

hydraulic properties are variable over short distances and profoundly affect leakage rates, demonstrating the importance of 

site-specific till hydraulic data for evaluating the sustainability 

of groundwater withdrawals from confined aquifers.

Introduction

Confined aquifers of glacial origin overlain by till confining units provide drinking water to thousands of Minnesota 

residents. These till confining units are typically conceptualized as having low potential for transmitting water; thus, 

the confined aquifers below may be prone to unsustainable 

groundwater withdrawals. Quantification of the recharge 

(leakage) rate through till is essential to understanding the 

sustainability of groundwater pumping from confined aquifers. Although the well yields of these confined aquifers are 

sufficient for some Minnesota communities, sustainability 

issues can arise because of the small size of the aquifer or low 

groundwater recharge rates. Strain on the water supply can 

be the result of the water demand exceeding the recharge rate 

to the aquifer, or from reduction of the recharge rate to the 

aquifer because of changes in climate (Delin, 1986; Lindgren, 

1996, 2002).

Buried aquifers can be confined or unconfined, and the 

field investigations of this study focused solely on confined 

aquifers. Groundwater in confined aquifers is isolated from 

the atmosphere at the point of discharge by poorly conductive geologic formations (for example, overlying till confining 

units), and the confined aquifer is subject to pressures higher 

than atmospheric pressure. This means that when a well is 

drilled through an overlying confining unit into a confined 

aquifer, water rises in the well to some level above the top of 

the aquifer. The water level in the well represents the confining pressure at the top of the aquifer (Driscoll, 1986). On 

the other hand, buried, unconfined aquifers do not have a 

confining pressure. When a well is installed in a buried unconfined aquifer, the water level in the well will be below the top 

of the aquifer.

Confined aquifers may be protected from anthropogenic 

contamination by a confining unit overlying them, but properties such as the hydraulic conductivity (K) and the thickness 

of the confining unit, the presence or absence of fracture 

flow, and the confining unit geochemical environment may 

either impede the flow of contaminants or allow the flow 

of contaminants through a confining unit to an underlying 

aquifer (Bradbury and others, 2006). Investigations concerning confining unit properties are less abundant compared 

to investigations on aquifer properties (Cherry and others, 

2004). Field studies of hydrogeology and (or) geochemistry of till confining units have been completed in Alberta, 

Saskatchewan, Manitoba, Wisconsin, Iowa, and Minnesota 

(Grisak and Cherry, 1975; Fortin and others, 1991; Simpkins 

and Bradbury, 1992; Simpkins and Parkin, 1993; Witt, 2017). 

These studies reported a wide range of hydraulic properties 

and geochemical environments in till confining units. For 

instance, K in studied till confining units has been estimated to 

be as low as 6×10−6 feet per day (ft/d) (Simpkins and Parkin, 

1993) and as high as 2×10−1 ft/d (Witt, 2017). Properties may 

also vary spatially throughout a till confining unit, and the 

presence of features such as sand lenses, erosional surfaces, 

joints, and fractures is locally important to the flux of groundwater in the till (Gerber and Howard, 2000).

To help understand the sustainability of groundwater 

resources in confined aquifers overlain by till confining units, 

the U.S. Geological Survey (USGS), Iowa State University, 

Minnesota Geological Survey, and Minnesota Department of 

Health investigated hydraulic properties and groundwater flow 

through till confining units using field studies and heuristic 

MODFLOW simulations. The results of this study provide 

insight to the sustainability of the groundwater resources being 

withdrawn from confined aquifer systems in Minnesota.

Purpose and Scope

The purpose of this report is to describe the hydrogeology and groundwater geochemistry of till confining units and 

confined aquifers in glacial deposits at four representative 

sites in Minnesota. The results of field studies and modeling 

approaches designed to quantify the variability of hydrologic 

properties and groundwater fluxes through till confining units 

to confined aquifers are described. The field studies were completed during 2014–18 near Litchfield, Cromwell, Akeley, and 

Olivia, Minnesota (fig. 1). 

Description of Study Sites

Four field sites were selected for inclusion in this 

study. Field sites were representative of deposits from major 

glacial lobe extents in Minnesota. Sites were in three lateWisconsinan deposits: the Des Moines lobe, the Superior 
Introduction 3

96° 94° 92° 90°

48°

46°

44°

0 25 50 75 100 MILES

0 40 80 120 160 KILOMETERS

Glacial deposit information from Minnesota Department of Natural Resources (2006)

 derived from Hobbs and Goebel (1982) 

Hydrographic information from Minnesota Department of Natural Resources (2018), and

 county boundary information from Minnesota Department of Natural Resources (2014) 

Piezometer and well information from the National Water Information System 

 (U.S. Geological Survey, 2019)

UNITED STATES

CANADA

MANITOBA

ONTARIO

LAKE 

SUPERIOR

EXPLANATION

Des Moines

Rainy

Superior

Wadena

Study site and identifier

County boundary

Piezometer nest and identifier

Piezometer or observation well 

and identifier

Production well and identifier

MINNESOTA

WISCONSIN

Hydrogeology 

field camp (HFC)

HFC

Cromwell

Cromwell

Litchfield

Litchfield

Olivia

Olivia

Lobes Olivia

CWO2

CWO2

CW–CM3 

CWO1

CW–CM4

0 150 300 FEET

0 40 80 METERS

0 50 100 FEET

0 15 30 METERS

MW–01

HB–3

HB–1

HB–1

HT–200 HT–175

HT–115 HT–140

550 feet to

HB–2*

2,600 feet to

WL07* and WL12*

4,000 feet to

WL299*

0 1,100 2,200 FEET

0 310 620 METERS

LF–CM4

LF–CM1

LFO2

LF–CM3

LF–OB1

LF–OB1

LF–CM2 LFO1

CWO1 piezometers: CWO1–A, –B, and –C

CWO2 piezometers: CWO2–A, –B, –C, –D, and –E

LFO1 piezometers: LFO1–B, –C, –D, –E, and –F

LFO2 piezometers: LFO2–A, –B, –C, –D, –E, and –F

0 75 150 FEET

0 20 40 METERS

OT–60

OT–35

OT–20

Olivia-4

OT–13

OB–7

OT–175

OT–145

OT–105

1,000 feet 

to Olivia-5*

4,000 feet

to Olivia-6*

*Not shown on individual site maps are:

At the HFC site, WL12 is an observation well. 

HB–2, WL299, and WL07 are observation wells.

At the Olivia site, Olivia-5 and -6 are production wells

Base from Minnesota Department of Natural 

Resources digital data, 1:24,000 and 

U.S. Geological Survey digital data, 1:100,000

Universal Transverse Mercator projection, Zone 15 

Horizontal coordinate information is referenced to the 

North American Datum of 1983 (NAD 83)

Figure 1. The location of the Litchfield sites (LFO1 and LFO2) and the Cromwell (CWO1/O2), hydrogeology field 

camp (HFC), and Olivia sites in Minnesota in relation to major glacial lobes of late-Wisconsinan deposits, and the 

piezometers and production wells at each site.
4 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

lobe, and the Wadena lobe, as well as one pre-Illinoian deposit 

underlying the Des Moines lobe (fig. 1; Hobbs and Goebel, 

1982). Candidate field sites were required to have (1) a small 

number (less than five) of high-capacity production wells 

withdrawing water from a buried artesian (confined) aquifer contained in Quaternary deposits, as classified by the 

Minnesota Geological Survey (MGS); (2) a confined aquifer 

within 300 feet (ft) of land surface; (3) a completed wellhead 

protection plan (or comparable form of local site hydrogeological characterization); (4) a completed county geologic 

atlas (or comparable detailed geological data compilation); 

and (5) information on the integrity of the high-capacity well 

construction. Sites meeting these minimum criteria were identified and then municipalities or land owners were contacted 

to gauge their willingness in partnering with the USGS in 

the study.

The Litchfield and Olivia study sites are within the 

footprint of the Des Moines lobe in central Minnesota. The 

city of Litchfield, where the Litchfield site is located, has 

a population of 6,726 and is in central Minnesota (Meeker 

County) (U.S. Census Bureau, 2012). The population of 

Litchfield relies on four municipal wells that pump approximately 340 million gallons per year (Mgal/yr) (Haglund and 

Robertson, 2000). The Litchfield site had two piezometer nests 

installed for this study, referred to as the Litchfield 1 (LFO1) 

site and the Litchfield 2 (LFO2) site.

The town of Olivia, where the Olivia site is located, 

has a population of 2,484 and is 35 miles (mi) southwest of 

Litchfield (Renville County) (U.S. Census Bureau, 2012). The 

population of Olivia draws its water supply from two confined 

aquifers. The water use in the town of Olivia, from the confined aquifer in this study, is around 64 Mgal/yr (Robertson, 

2011). The Olivia site had one piezometer nest installed for 

this study. The towns of Litchfield and Olivia draw municipal 

water from glacigenic confined aquifers of limited areal extent. 

The physical setting at both sites consists of low-relief ground 

moraine typical of the Des Moines lobe. Row-crop agriculture 

is the dominant land use in the region. The land area surrounding both sites usually receives about 27–29 inches (in.) 

of precipitation annually (Minnesota Department of Natural 

Resources, 2020).

The town of Cromwell has a population of 231 and is in 

the footprint of the Superior lobe in Carlton County (fig. 1) 

(U.S. Census Bureau, 2012). The population of Cromwell 

relies on two production wells pumping approximately 

6 Mgal/yr from a glacigenic confined aquifer (Walsh, 2012). 

The Cromwell field site had two piezometer nests installed for 

this study, the Cromwell 1 (CWO1) and Cromwell 2 (CWO2) 

nests; however, the two nests are only about 160 ft apart and 

are primarily discussed as a single, merged site (CWO1/O2) 

throughout this report. The CWO1/O2 site is on a topographic 

high of hummocky topography consisting primarily of sand 

and gravel. Land cover consists of moderately forested 

woodlands and some agriculture. The annual precipitation 

around the town of Cromwell is about 29–31 in. (Minnesota 

Department of Natural Resources, 2020).

The hydrogeology field camp (HFC) site is within 

the footprint of the Wadena lobe on the far eastern edge of 

Hubbard County and is not located in a town (fig. 1). The town 

of Akeley, Minn., (not shown) is to the northwest of the field 

site. There are more than 60 observation wells at this location that are operated as part of the University of Minnesota’s 

HFC. The HFC site had one piezometer nest installed for this 

study. The area is highly wooded, and numerous lakes are near 

the site. The annual precipitation in the area surrounding the 

HFC site is about 26–28 in. (Minnesota Department of Natural 

Resources, 2020).

Geologic Setting

The following is a summary of detailed geologic reports 

produced during this study (Wagner and Tipping, 2016; Staley 

and Nguyen, 2018; Staley and others, 2018) and the glacial 

history of the sites. Naming conventions and descriptions of 

the Quaternary-aged formations and members listed in this 

report are described in the Minnesota Geological Survey’s 

definitive reference for Minnesota Quaternary geology titled 

“Quaternary lithostratigraphic units of Minnesota” (Johnson 

and others, 2016). Generalized lithologies are presented in the 

completion diagram figures (figs. 2, 3, 4, 5, and 6). The depths 

and thicknesses shown in the generalized lithologies in these 

figures are simplified compared to the detailed stratigraphy 

presented in the geologic reports (Wagner and Tipping, 2016; 

Staley and Nguyen, 2018; Staley and others, 2018).

Litchfield

At the Litchfield site, till of the Villard Member of the 

New Ulm Formation overlies the confined aquifer (Wagner 

and Tipping, 2016). The glaciofluvial deposit that composes 

the confined aquifer is most likely outwash of the Hewitt 

Formation (Wagner and Tipping, 2016). Till of the Villard 

Member was deposited by glacial ice (and its meltwater) that 

moved into Minnesota from the Winnipeg provenance to the 

north, eventually depositing the Pine City moraine (Johnson 

and others, 2016). The till age is not exactly known but is estimated as about 12,300 carbon-14 years before present (14C yr 

BP) (about 14,450 calendar years before present [cal yr BP]) 

(Clayton and Moran, 1982; Johnson and others, 2016). More 

recent publications indicate that the deposition of the Pine City 

moraine is older, about 13,000 14C yr BP (about 16,000 cal yr 

BP) (Johnson and others, 2016). The lobe eventually advanced 

as far south as Des Moines, Iowa, by 14,000 14C yr BP.

The mean particle-size distribution of the till for the 

Litchfield site, determined by the MGS from two continuous till cores from the LFO1 and LFO2 sites, sampled at 

about 4-ft intervals, is 49-percent sand, 33-percent silt, and 

18-percent clay (Wagner and Tipping, 2016). This distribution is similar to the equivalent till of the Alden Member of 

the Dows Formation near Ames, Iowa (Helmke and others, 

2005b). Particle-size distribution of the LFO1 and LFO2 till 
Introduction 5

Elevation

(feet above 

NAVD 88)

Depth

(feet below 

land surface)

Lithology and 

geologic unit

HydroLFO1–B LFO1–C LFO1–D LFO1–E LFO1–F

stratigraphy

Litchfield 1 (LFO1)—Piezometer construction, 

stratigraphy, and mean water levels

1,110

1,100

1,090

1,080

1,070

1,060

1,050

1,040

1,030

1,020

1,010

1,000

990

980

970

120

110

100

90

80

70

60

50

40

30

20

10

0

130

140

150

QNUL deltaic

deposit

QNUL glaciolacustrine

deposit

QNUL

deltaic

deposit

QNUL

till

QNUL

till

QHWU

glaciofluvial

deposits

Surficial

aquifer

Confining

unit

Confined

aquifer

Lithology

Sand and gravel

Till

Fine-grained sand,

silt, and clay

Soil

Artificial fill

Mean water level

Piezometer construction

Concrete

Bentonite grout

Bentonite chips

Sand pack

Screen

EXPLANATION

NOTE: Not to scale

NAVD 88, North American Vertical Datum of 1988

QNUL, Quaternary-age Villard Member 

 of the New Ulm Formation 

QHWU, Quaternary-age Hewitt Formation

Figure 2. Litchfield 1 (LFO1) site piezometer construction, stratigraphy, and mean water levels. Stratigraphic information 

is based on the Minnesota Geological Survey analysis (Wagner and Tipping, 2016; Staley and others, 2018).
6 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

LFO2–A LFO2–B LFO2–C LFO2–D LFO2–E LFO2–F

Litchfield 2 (LFO2)—Piezometer construction, 

stratigraphy, and mean water levels

QNUL

till

QNUL

till

QNUL glaciofluvial

deposit

QHWU

glaciofluvial

deposits

Unidentified 

till

QNUL

till

Lithology and 

geologic unit

Confining

unit

Confined

aquifer

Confining

unitHydrostratigraphy

1,130

1,120

1,110

1,100

1,090

1,080

1,070

1,060

1,050

1,040

1,030

1,020

1,010

1,000

990

980

970 170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Depth 

(feet below

land surface)

Elevation

(feet above

NAVD 88)

NOTE: Not to scale

NAVD 88, North American Vertical Datum of 1988

QNUL, Quaternary-age Villard Member 

 of the New Ulm Formation 

QHWU, Quaternary-age Hewitt Formation

Mean water level

Concrete

Bentonite grout

Collapse

Bentonite chips

Sand pack

Screen

Piezometer construction

Lithology

Sand and gravel

Till

EXPLANATION

Figure 3. Litchfield 2 (LFO2) site piezometer construction, stratigraphy, and mean water levels. Stratigraphic information is 

based on the Minnesota Geological Survey analysis (Wagner and Tipping, 2016; Staley and others, 2018).
Introduction 7

CWO2–A CWO2–B CWO2–C CWO2–D CWO2–E CWO1–A CWO1–B CWO1–C

Cromwell (CWO1/O2)—Piezometer construction,

stratigraphy, and mean water levels

1,320

1,300

1,280

1,260

1,240

1,220

1,200

1,180

1,160

1,140

1,120

1,100

1,080

1,060

1,040

1,020

1,000

980

360

340

320

300

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

Depth

(feet below

land surface)

Elevation

(feet above

NAVD 88)

QCMU

 till

QCMU

glaciofluvial

deposit

Thomson 

Formation slate

QAIA till

QCMU

glaciofluvial

deposit Lithology and geologic unit

Hydrostratigraphy

Surficial

aquifer

Confining

unit

Confined

aquifer

Bedrock

aquifer

Mean water level

Piezometer construction

Concrete

Bentonite grout

Bentonite chips

Sand pack

Screen

Lithology

Sand and gravel

Till

Slate

Soil

Collapse

NOTE: Not to scale

NAVD 88, North American Vertical Datum of 1988

QAIA, Quaternary-age Alborn Member of the 

 Aitkin Formation

QCMU, Quaternary-age Cromwell Formation

EXPLANATION

Figure 4. Cromwell (CWO1/O2) site piezometer construction, stratigraphy, and mean water levels. Stratigraphic 

information is based on Knaeble and Hobbs (2009) and the Minnesota Geological Survey analysis (Staley and others, 

2018).
8 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

MW–01HT–115 HT–140 HT–175 HT–200 HB–1

Hydrogeology field camp (HFC)—Piezometer 

construction, stratigraphy, and mean water levels

240

220

200

180

160

140

120

100

80

60

40

20

0

250

1,440

1,420

1,400

1,380

1,360

1,340

1,320

1,300

1,280

1,260

1,240

1,220

1,200

Depth 

(feet below

land surface)

Elevation

(feet above

NAVD 88)

QHWU 

glaciofluvial

deposits

QHWU 

debris flow

QHWU 

till

QBVU glaciolacustrine

deposit

QBVU glaciofluvial

deposits

QBVU 

till Lithology and geologic unit

Surficial

aquifer

Confining

unit

Confined

aquifer

Confining

unitHydrostratigraphy

EXPLANATION

Piezometer construction

Concrete

Bentonite grout

Bentonite chips

Sand pack

Screen

Lithology

Sand and gravel

Till/diamicton

Fine-grained sand,

silt, and clay

Mean water level

NOTE: Not to scale

NAVD 88, North American Vertical Datum of 1988

QHWU, Quaternary-age Hewitt Formation

QBVU, Quaternary-age Browerville Formation

Figure 5. Hydrogeology field camp (HFC) site piezometer construction, stratigraphy, and mean water levels. 

Stratigraphic information is based on the Minnesota Geological Survey analysis (Staley and others, 2018).
Introduction 9

Olivia—Piezometer construction, stratigraphy, 

and mean water levels

OT–13 OT–20 OT–35 OT–60 OT–105 OT–145 OT–175 OB–7

840

820

1,060

1,040

1,020

1,000

980

960

940

920

900

880

860

240

250

220

200

180

160

140

120

100

80

60

40

20

0Depth 

(feet below

land surface)

Elevation

(feet above 

NAVD 88)

Holocene stream deposits

QGTA

till

QGTB

till

QGTC

till

QGTC

till

QGTC

till

QGTD

till

QUPS 

glaciofluvial

deposits

Cretaceous shale 

bedrock Lithology and geologic unit

Surficial

aquifer

Confining

unit

Confined

aquifer

Bedrock Hydro- stratigraphy

EXPLANATION

Mean water level

Piezometer construction

Concrete

Bentonite grout

Bentonite chips

Sand pack

Screen

Lithology

Sand and gravel

Till

Fine-grained sand,

silt, and clay

Shale

Repeat soil

Artificial fill

NOTE: Not to scale

NAVD 88, North American Vertical Datum of 1988

QGT, Quaternary-age Good Thunder formation 

 (A, member 1; B, member 2; C, member 3; 

 D, member 4)

QUPS, undefined Pleistocene-age sediment

Figure 6. Olivia site piezometer construction, stratigraphy, and mean water levels. Stratigraphic information is based on 

the Minnesota Geological Survey analysis (Staley and others, 2018).
10 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

cores, from the MGS report by Staley and others (2018), was 

used to calculate separate mean particle-size distributions for 

the LFO1 and LFO2 sites. At LFO1, the mean particle size 

is 47-percent sand, 34-percent silt, and 19-percent clay, and 

at the LFO2 site, the mean particle size is 52-percent sand, 

31-percent silt, and 17-percent clay (fig. 7A).

Sediment of the New Ulm Formation is yellowish brown 

and oxidized in the upper 15 ft, and grayish brown and unoxidized below this depth. Carbonate clasts and a calcareous matrix 

are present throughout, except in the top 3 ft of the LFO1 core. 

Fractures were described in the LFO1 and LFO2 cores to depths 

of about 60 and 90 ft, respectively. Most fractures lacked iron 

A 1 INCH

2.54 CENTIMETERS

A. Till of the New Ulm Formation from the Litchfield site. 

The mean particle size distribution of the till at the 

LFO1 site is 47 percent sand, 34 percent silt, and 

19 percent clay; and at the LFO2 site is 52 percent

sand, 31 percent silt, and 17 percent clay.

B 1 INCH

2.54 CENTIMETERS

B. Till of the Cromwell Formation from the Cromwell site. 

The till has a mean particle size distribution of 57 percent 

sand, 31 percent silt, and 13 percent clay (greater than 

100 percent because of rounding). 

C 1 INCH

2.54 CENTIMETERS

C. Till of the Hewitt Formation from the HFC site. The 

till has a mean particle size distribution of 67 percent 

sand, 22 percent silt, and 11 percent clay. 

D 1 INCH

2.54 CENTIMETERS

D. Till of the Good Thunder formation from the Olivia site. 

The till has a mean particle size distribution of 37 percent

sand, 40 percent silt, and 23 percent clay. 

Figure 7. Images of till from cores extracted from sites. A, till of the New Ulm Formation from the Litchfield site (LFO1 and 

LFO2); B, till of the Cromwell Formation from the Cromwell site; C, till of the Hewitt Formation from the hydrogeology field 

camp (HFC) site; D, till of the Good Thunder formation from the Olivia site. Mean particle-size information is from Wagner 

and Tipping (2016) and Staley and Nguyen (2018). [in, inch; cm, centimeter]
Introduction 11

staining common to fracture surfaces in the equivalent till in 

Iowa (Helmke and others, 2005b). Some fractures may be artifacts of the coring process and subsequent unloading; however, 

McKay and Fredericia (1995) determined that till fractures can 

occur below depths where oxidation staining occurs.

Sediment sequences differ between the LFO1 (fig. 2) and 

LFO2 (fig. 3) sites. At the LFO1 site, fine-grained, sandy and 

silty deltaic and glaciolacustrine sediment with some gravel 

occurs above the till. Wagner and Tipping (2016) interpreted 

this as a deltaic deposit resulting from a series of meltwater 

plumes into Glacial Lake Litchfield (Meyer, 2015). The sand 

and gravel unit is not detected at the LFO2 site, which lies at 

about 25 ft higher in elevation than the LFO1 site (Wagner 

and Tipping, 2016). The confined sand and gravel aquifer unit 

begins at approximately 98 and 117 ft below land surface at 

the LFO1 site and the LFO2 site, respectively. Till thickness 

varies between the two piezometer nests. At the LFO1 site, 

the till is about 60 ft thick, and at the LFO2 site, the till is 

about 115 ft thick. The aquifer is about 44 ft thick at the LFO2 

site, based on borehole geophysical logs and the generalized 

borehole lithostratigraphy (Wagner and Tipping, 2016). The 

confined aquifer sediments were not included in core samples 

at either LFO1 or LFO2, which is why alternative methods were used to determine confined aquifer thickness. The 

confined aquifer at the Litchfield sites may be underlain by 

pre-Wisconsinan till of the Sauk Centre Member of the Lake 

Henry Formation (Meyer, 2015).

Cromwell

The stratigraphic sequence at the Cromwell site (fig. 4) 

is more complex than that at the Litchfield site. The Superior 

lobe advanced and retreated from the Lake Superior Basin 

multiple times during the late-Wisconsinan glacial episode. 

As the climate warmed, the extent of those advances into 

Minnesota became successively smaller. The Cromwell 

Formation, which consists of till and glaciofluvial and 

glaciolacustrine sediment of the Superior provenance, is the 

primary glacial lithostratigraphic unit at the Cromwell site 

and in northeastern Minnesota. The exact age of the unit at 

Cromwell is not well constrained (Johnson and others, 2016). 

The St. Croix phase of the Superior lobe advanced ice over 

the Cromwell site in west-central and south-central Minnesota 

between 15,000 and 20,000 14C yr BP. The Superior lobe 

advanced over the Cromwell site later during the Automba 

phase between 13,500 and 14,000 14C yr BP (Jennings and 

Johnson, 2011). It was during ice retreat at the end of the 

Automba phase that till of the Cromwell Formation was likely 

deposited on top of the sand and gravel of the Cromwell 

Formation that compose the confined aquifer at the site. After 

the retreat of the Superior lobe, the St. Louis sublobe advanced 

over the Cromwell site from the northwest at about 12,500 14C 

yr BP (about 15,000 cal yr BP) and deposited the Alborn 

Member of the Aitkin Formation (Johnson and others, 2016; 

Staley and others, 2018).

Core samples were not retrieved from the CWO1 site, 

and the MGS reconstructed the geology through analysis of 

downhole gamma ray logs. Core samples were collected at 

the CWO2 site; however, the high frequency of clasts greater 

than 2 in. in diameter interfered with the coring process and 

resulted in the collection of fewer core samples than expected. 

Two glacigenic units were identified at the Cromwell site. 

Starting at land surface, 4 ft of silt loam till of the Alborn 

Member of the Aitkin Formation overlies 40 ft of sand and 

gravel outwash of the Cromwell Formation deposited during 

the Automba phase of the Superior lobe. The Alborn Member 

is likely responsible for the hummocky topography at the site. 

Below the sand and gravel deposits lies about 126 ft of sandy 

loam to loam till with cross-stratified, fine- to very coarse-sand 

and gravel layers, also likely deposited during the Automba 

phase. The confined aquifer below these deposits consists 

of a sand and gravel unit within the Cromwell Formation, 

underlain by Paleoproterozoic slate of the Thomson Formation 

(Boerboom, 2009).

Sediment of the Cromwell Formation and the Aitkin 

Formation was typically reddish brown, and a calcareous 

matrix was present in the core below 43.5 ft. The till of the 

Cromwell Formation had a mean particle-size distribution of 

57-percent sand, 31-percent silt, and 13-percent clay (fig. 7B, 

greater than 100 percent because of rounding), which is about 

8 percent more sand than till of the New Ulm Formation. Till 

of the Aitkin Formation was not analyzed for particle-size 

distribution.

Hydrogeology Field Camp

The HFC site (fig. 5) has glacial sediment deposits of the 

Wadena lobe. Glacial ice brought northeast sourced sediments 

from the Rainy provenance and deposited till, outwash, and 

lake sediments of the Hewitt Formation (Johnson and others, 2016). Two depositional events known as the Alexandria 

and Itasca phases have been identified. The Alexandria phase 

represents the first ice advance, and the Itasca phase is associated with a later, second ice advance (Knaeble and Hougardy, 

2018). Deposits of both phases are assigned to the Hewitt 

Formation, which has an estimated age of about 30,000 14C yr 

BP (Johnson and others, 2016; Knaeble and Hougardy, 2018). 

This age indicates that the Wadena lobe was actively depositing sediments in the early part of late-Wisconsinan time 

(Johnson and others, 2016).

The till at the HFC site underlies a 105-ft-thick coarsegrained sand and gravel outwash deposit of the Hewitt 

Formation. The till of the Hewitt Formation is a sandy loam, 

with a mean particle size of about 67-percent sand, 22-percent 

silt, and 11-percent clay (fig. 7C; Staley and Nguyen, 2018). 

The till is brown in color and lacks shale clasts but has moderate carbonate clasts of around 10–25 percent (Staley and 

Nguyen, 2018). The thickness of the till unit is about 102 ft. 

The entirety of the till of the Hewitt Formation is considered 

one unit (Staley and Nguyen, 2018).
12 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Below the till of the Hewitt Formation to a depth of 

250 ft is pre-Wisconsinan lake sediments, outwash, and glacial 

till of the Browerville Formation (Staley and Nguyen, 2018). 

The confined aquifer below the Hewitt Formation is composed of fine-grained sand and gravel glaciolacustrine (glacial 

lake) sediments and outwash sediments of the Browerville 

Formation, and is about 23 ft in thickness (Staley and Nguyen, 

2018). Immediately below the confined aquifer is till of the 

Browerville Formation, which starts at a depth of 230 ft below 

land surface and has a thickness of about 20 ft (Staley and 

Nguyen, 2018).

Olivia

At the Olivia site (fig. 6), till of the New Ulm Formation 

was expected to be present because thick sequences of Des 

Moines lobe tills have been mapped around the Olivia area 

(Knaeble, 2013; Bradt, 2017; Staley and Nguyen, 2018). 

During the Pleistocene, several glacial advances and retreats 

occurred in the study site area, with the most recent till deposition being the New Ulm Formation from the Des Moines lobe 

(Knaeble, 2013). The age range for Des Moines lobe glaciation in Minnesota is 16,000 to 12,000 cal yr BP (Knaeble, 

2006); however, the till sequence at Olivia is interpreted to be 

of the Good Thunder formation, an informally named preWisconsinan till (Staley and Nguyen, 2018). Radiocarbondated wood deposits from the Good Thunder formation 

provide an age estimate as greater than 48,500 14C yr BP 

(Knaeble, 2013).

Till of the Good Thunder formation present at Olivia is 

typically gray in color, with a loam to silty and clayey loam 

texture and a mean particle size of around 37 percent for 

sand, 40 percent for silt, and 23 percent for clay (fig. 7D). 

The till is high in carbonates (usually greater than 50 percent) 

and low in gray shale percentage (between 0 and 10 percent) 

and Cretaceous grains are present (between 1 and 10 percent) (Staley and Nguyen, 2018). Four possible members 

of the Good Thunder formation are present at Olivia, based 

on Cretaceous percentage and density changes through the 

till formation (Staley and Nguyen, 2018). The overall thickness of till of the Good Thunder formation at Olivia is about 

166 ft. Sand bodies are also often present in the formation, 

stratigraphically dividing the different members (Staley and 

Nguyen, 2018).

Above the Good Thunder formation is a thin layer of 

Holocene sediment that is about 10 ft thick and is topped by 

about 4 ft of fill (Staley and Nguyen, 2018). Below the Good 

Thunder formation is a silt and fine- to coarse-grained sand 

aquifer, likely a glaciofluvial outwash deposit of uncertain 

origin (Staley and Nguyen, 2018). The confined aquifer is 

about 48 ft thick. Underneath the aquifer at a depth of 229.5 ft 

below land surface lies Cretaceous shale bedrock (Staley and 

Nguyen, 2018).

Methods of Study

The following section describes the study design, field 

sampling and analytical procedures, and the MODFLOW 

modeling approach. Drilling operations for core sample collection and installation of wells and piezometers are presented. 

Hydrologic data collection and analysis methods are discussed. Procedures for collecting and analyzing water quality 

samples are described. Finally, descriptions of the heuristic 

MODFLOW models are presented.

Field Study Design and Piezometer Installation

Piezometer “nests” were installed at each site to assess 

the vertical flux of water and transport of chemicals from land 

surface to the underlying confined aquifer system. A piezometer nest is a series of piezometers installed adjacent to one 

another and screened at separate short intervals below land 

surface. The nest design enables vertically discrete observations throughout the geologic profile from near land surface 

through the till into the confined aquifer. The nest design has 

been commonly used to investigate hydrologic properties 

of tills (for example, Simpkins and Parkin, 1993; Shaw and 

Hendry, 1998). Small diameter (1.25-in.) piezometers were 

installed in the confining units to reduce the volume of water 

required for observable water-level fluctuations in geologic 

materials with low K. Piezometers (or wells) with a 2-in. 

diameter were installed in the confined aquifers.

At the Litchfield and Cromwell sites, two nests were 

installed at each site, one of which was near a well field and 

one which was farther from a well field. After the initial installation, the two Cromwell nests (the CWO1 and CWO2 sites) 

were considered together as a single nest (CWO1/O2) and are 

mostly presented as such throughout this report. The near and 

far nest design was intended to facilitate aquifer test analyses. 

At the Olivia and HFC sites, only one piezometer nest was 

installed near a production well. 

A total of 19 piezometers were installed in 2015 at 

the Litchfield and Cromwell sites for this study (table 1). 

The LFO1 site consisted of five piezometers and was about 

1,500 ft from the nearest production well (figs. 1 and 2). 

The LFO2 site consisted of six piezometers, was within a 

well field, and was about 500 ft from the nearest production 

well (figs. 1 and 3). Five production wells are near the LFO1 

and LFO2 sites (fig. 1). The CWO1 site consisted of three 

piezometers and was about 150 ft from the nearest production well (figs. 1 and 4). The CWO2 site consisted of five 

piezometers and was about 50 ft from the nearest production well (figs. 1 and 4). Two production wells were near the 

CWO1 and CWO2 sites. The CWO1 and CWO2 sites were 

160 ft apart from each other and had piezometers that were 

sequential in depth.
Table 1. Well and piezometer identification, vertical placement, and mean water-level information.[USGS, U.S. Geological Survey; ID, identifier; ft, foot; NAVD 88, North American Vertical Datum of 1988; BLS, below land surface; MN040, agency code representing the MiU.S. Geological Survey's National Water Information System database; --, not calculated; HFC, hydrogeology field camp near Akeley, Minnesota]

Well or 

piezometer 

short name

Field site Agency codea USGS site IDa

Minnesota 

unique 

well 

number

Installed 

during 

this 

study

Hydrostratigraphy of 

screened interval

Land surface 

elevation 

(ft above 

NAVD 88)a

Screened 

interval 

(ft BLS)

M

h

LFO1–B Litchfield USGS 450814094315001 773062 Yes Surficial aquifer 1,115.22 22.4–25.06 LFO1–C Litchfield USGS 450814094315002 773060 Yes Till 1,115.45 50.23–52.89 LFO1–D Litchfield USGS 450814094315003 773059 Yes Till 1,115.34 72.4–75.06 LFO1–E Litchfield USGS 450814094315004 773058 Yes Till 1,115.15 92.41–95.07 LFO1–F Litchfield USGS 450814094315006 773057 Yes Confined aquifer 1,115.19 117.5–127.12 LFO2–A Litchfield USGS 450832094321201 773056 Yes Till 1,139.45 17.12–19.78 LFO2–B Litchfield USGS 450832094321202 773055 Yes Till 1,139.29 32.26–34.92 LFO2–C Litchfield USGS 450832094321203 773054 Yes Till 1,139.72 56.97–59.63 LFO2–D Litchfield USGS 450832094321204 773053 Yes Till 1,139.18 82.27–84.93 LFO2–E Litchfield USGS 450832094321205 773052 Yes Till 1,139.64 110.95–113.61 LFO2–F Litchfield USGS 450832094321206 773051 Yes Confined aquifer 1,139.47 149.56–159.18 LF–OB1 Litchfield MN040 450821094320601 607417 No Confined aquifer 1,123.14 122–127 LF–CM1 Litchfield MN040 450837094321601 764258 No Confined aquifer 1,145.14 136.5–161.5 LF–CM2 Litchfield MN040 450820094320801 607420 No Confined aquifer 1,123.23 107–132 LF–CM3 Litchfield MN040 450828094320601 632077 No Confined aquifer 1,121.20 108–136 LF–CM4 Litchfield MN040 450851094321201 632078 No Confined aquifer 1,142.83 123–147 CWO1–A Cromwell USGS 464110092531401 773071 Yes Till 1,326.28 144.56–147.36 CWO1–B Cromwell USGS 464110092531402 773070 Yes Confined aquifer 1,326.29 220.91–230.53 CWO1–C Cromwell USGS 464110092531403 773069 Yes Bedrock aquifer 1,326.25 329.63–339.25 CWO2–A Cromwell USGS 464112092531401 773068 Yes Surficial aquifer 1,332.28 32.3–34.96 CWO2–B Cromwell USGS 464112092531402 773067 Yes till 1,332.59 56.75–59.41 CWO2–C Cromwell USGS 464112092531403 773066 Yes Till 1,332.33 78.7–81.36 CWO2–D Cromwell USGS 464112092531404 773065 Yes Till 1,332.13 103.58–106.24 CWO2–E Cromwell USGS 464112092531405 773064 Yes Till 1,332.44 125.78–128.44 CW–CM3 Cromwell MN040 464109092530701 519761 No Confined aquifer 1,327 180–190 CW–CM4 Cromwell MN040 464111092531401 593593 No Confined aquifer 1,327.88 210–230 HT–115 HFC USGS 465652094394801 773075 Yes Till 1,452.00 112.35–114.83 HT–140 HFC USGS 465652094394802 773076 Yes Till 1,452.39 137.77–140.25 HT–175 HFC USGS 465652094394803 773077 Yes Till 1,452.04 172.47–174.95 
Table 1. Well and piezometer identification, vertical placement, and mean water-level information.—Continued [USGS, U.S. Geological Survey; ID, identifier; ft, foot; NAVD 88, North American Vertical Datum of 1988; BLS, below land surface; MN040, agency code representing the MiU.S. Geological Survey's National Water Information System database; --, not calculated; HFC, hydrogeology field camp near Akeley, Minnesota] Well or piezometer short name Field site Agency codea USGS site IDa Minnesota unique well number Installed during this study Hydrostratigraphy of screened interval Land surface elevation (ft above NAVD 88)a Screened interval (ft BLS) M

h

HT–200 HFC USGS 465652094394804 773078 Yes Till 1,452.04 195.22–197.7 HB–1 HFC MN040 465653094394701 809697 No Confined aquifer 1,451.98 210–230 HB–2 HFC MN040 465651094394001 819726 No Confined aquifer 1,454.83 214–224 HB–3 HFC MN040 465652094394701 825587 Yes Confined aquifer 1,453.03 213.33–223.53 MW–01 HFC MN040 465652094394501 569489 No Surficial aquifer 1,453.54 80–85 WL07 HFC MN040 465711094392601 243680 No Surficial aquifer 1,502.20 126.3–128.3 WL12 HFC MN040 465712094404201 243843 No Confined aquifer 1,467.42 340–344 WL299 HFC MN040 465725094403207 243849 No Confined aquifer 1,400.46 -- OT–13 Olivia USGS 444630095002202 773086 Yes Surficial aquifer 1,070.51 7.89–13.03 OT–20 Olivia USGS 444630095002203 773085 Yes Till 1,070.75 17.44–19.91 OT–35 Olivia USGS 444630095002204 773084 Yes Till 1,070.60 32.08–34.55 OT–60 Olivia USGS 444630095002205 773083 Yes Till 1,070.67 56.74–59.77 OT–105 Olivia USGS 444630095002206 773082 Yes Till 1,071.61 101.95–104.94 OT–145 Olivia USGS 444630095002207 773081 Yes Till 1,071.44 141.15–144.13 OT–175 Olivia USGS 444630095002208 773080 Yes Till 1,071.46 172.26–175.26 OB–7 Olivia USGS 444630095002209 773079 Yes Confined aquifer 1,071.39 204.92–209.71 Olivia-4 Olivia USGS 444630095002201 228797 No Confined aquifer 1,071.00 204–228 Olivia-5 Olivia USGS 444639095002201 228796 No Confined aquifer 1,075.01 196–218 Olivia-6 Olivia MN040 444637095013701 241525 No Confined aquifer 1,087.00 333–343 

aData can be accessed from the USGS National Water Information System database (U.S. Geological Survey, 2019) using the USGS site ID. bThis water level is not a mean; it is the final water-level reading. The water level in this well took the study period to recover from being drawn down for well development, a

level best represented an approximate “static” water level.
Methods of Study 15

A total of 13 piezometers and wells were installed in 

2017 at the Olivia and HFC sites for this study (table 1). At 

the Olivia site, eight piezometers were installed about 60 ft 

from the nearest production well (figs. 1 and 6; table 1). Two 

other production wells were near the site, one about 1,000 ft 

from the piezometer nest and the other about 4,000 ft from 

the piezometer nest. At the HFC site, four piezometers were 

installed in the till about 20 ft from the nearest production 

well and one well was installed in the confined aquifer about 

50 ft from the nearest production well (figs. 1 and 5). The HFC 

site also included several wells previously installed by the 

University of Minnesota, including two in the surficial aquifer 

and five in the confined aquifer (table 1).

Drilling operations for sediment core collection and 

piezometer installation varied across the four sites. The following is a general description, and detailed drilling and 

piezometer construction information is provided in appendix 

table 1.1. A hollow-stem auger rig was used for sediment core 

collection and installation at the LFO1, LFO2, and CWO2 

sites. Hollow-stem methods are commonly used for till 

investigations because sediment core samples can be collected 

during drilling, and drilling fluids, which could contaminant 

the till formation, are not required (Simpkins and Bradbury, 

1992; Shaw and Hendry, 1998). Sediment core samples were 

collected into acetate liners with a cutter head and split core 

barrel assembly. Rocks in the till impeded the installation of 

piezometers at the CWO1 site, so a direct mud rotary rig was 

used to install the three piezometers (CWO1–A, CWO1–B, 

and CWO1–C). Sample cuttings were collected from the drilling mud at the CWO1 site (Witt, 2017).

Rotary-sonic drilling methods were used for core collection and piezometer installations at the Olivia and HFC 

sites. Rotary-sonic drilling methods enabled continuous core 

collection and eliminated problems caused by cobbles and 

boulders in the till but did require water and drilling fluids. 

At the Olivia site, untreated water from the municipal supply system was used during drilling operations, and at the 

HFC site, water from the surficial aquifer was used during 

drilling operations. At each of the Olivia and HFC sites, 

one continuous sediment core profile extending from land 

surface to the confined aquifer was collected. Core samples 

were extruded from the core barrel directly into plastic 

sleeves (Staley and others, 2018). All installed piezometers 

were developed with an inertial pump to establish a good 

connection between the well screen and the surrounding 

geologic material.

At all sites, the piezometer screened intervals were 

determined with consideration of the site geology, the vertical 

distribution of sample points, and the driller’s confidence in 

successful piezometer completion. Lithologic changes and 

oxidation state were documented from the sediment core 

samples collected during drilling operations. Piezometer 

screens were generally placed directly above lithologic 

boundaries, as recommended by Hart and others (2008). 

Lithological changes selected for piezometer screen placement were spaced somewhat uniformly within the till units. 

In some cases, the screened interval was determined by where 

the drillers were confident that a piezometer completion 

would be successful.

Hydrology

Several techniques were used to assess the hydrologic 

properties and leakage through till confining units at the four 

study sites: continuous and discrete water-level monitoring, 

slug tests, aquifer tests, and calculations according to Darcy’s 

law to estimate recharge rates and travel times. Different techniques were used to evaluate the scale dependency of hydrologic measurements. Previous studies have demonstrated that 

K values increase with measurement scale; for example, laboratory measurements of K in till are significantly lower than 

field measurements of the same materials (Grisak and Cherry, 

1975; Grisak and others, 1976; Bradbury and Muldoon, 1990).

Continuous and discrete monitoring of water-level 

responses to pumping and precipitation events can be used to 

qualitatively assess hydraulic connectivity between aquifers 

and till confining units (as was done for this study), but they 

can also be used to quantitatively estimate the vertical hydraulic conductivity (Kv) of till confining units (Cherry and others, 

2004). Previous studies have used hydraulic head variations 

in confined aquifers and confining units induced by pumping 

over long periods (years to decades) as evidence for extremely 

low confining unit Kv values (for example, Husain and others, 

1998). Other studies have monitored hydraulic head in surficial aquifers and confining unit material to determine confining unit Kv values (for example, Keller and others, 1989).

Laboratory tests and slug tests are commonly used to 

assess the hydraulic properties of till confining units, although 

these tests represent smaller volumes of till than aquifer tests. 

Vertical fractures or stratigraphic windows (zones of higher K

through low-K material) can be important transport features 

through till, but the results of laboratory measurements on core 

samples rarely reflect these features (Cherry and others, 2004). 

Slug tests, in combination with sediment core samples, can 

indicate the presence and nature of important transport features, such as fractures or high-permeability zones, in till confining units if the slug tests happen to intersect those features 

(Cherry and others, 2004). Beyond potential identification of 

important transport features, slug tests have limited usefulness 

for determining the Kv of the till matrix because, in vertical 

holes, the slug response primarily depends on the horizontal 

component of the hydraulic conductivity (Kh). However, slug 

tests can indicate the presence of permeable zones, providing 

valuable insight concerning the internal nature of the confining 

unit (Cherry and others, 2004).

Aquifer tests designed with the specific purpose of determining till confining unit properties are another, larger scale 

approach to estimating the Kv of tills. Aquifer tests measure a 

much larger volume of till than slug tests and are more likely 

to capture the effects of features most important for transport through till (Cherry and others, 2004). The piezometers 
16 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

installed as part of this study were used during an aquifer test 

at each site to measure hydraulic-head responses within the 

till confining unit and the pumped aquifer (Cherry and others, 2004). Several analytical methods, such as Neuman and 

Witherspoon (1972), can be used to determine confining unit 

properties from properly executed aquifer tests.

Water-Level and Precipitation Monitoring

Water levels in the piezometers and production wells 

were measured at discrete intervals by hand and logged every 

15 minutes with pressure transducers in a subset of piezometers. These data were collected to determine how water 

levels and hydraulic gradients vary through time in surficial 

aquifers, till confining units, and confined aquifers. Manual 

water-level measurements were made in piezometers and 

wells using a Solinst or Keck electric tape or a Lufkin steel 

tape between July 2015 and April 2017 for the Litchfield and 

Cromwell sites and intermittently for the Olivia and HFC 

sites between October 2017 and October 2018. Submersible 

pressure transducers (OTT Orpheus Mini) recorded waterlevel and temperature data in 12 piezometers at the Litchfield 

and Cromwell sites between December 2015 and April 2017 

(appendix table 1.1). OTT Orpheus Mini submersible pressure 

transducers also recorded water-level and temperature data in 

14 piezometers and wells at the Olivia and HFC sites between 

October 2017 and October 2018 (appendix table 1.1).

Precipitation was also monitored continuously (every 

15 minutes) with HOBO RG3 tipping bucket rain gages at 

the LFO2 site and the Cromwell site between December 2015 

and April 2017, at the Olivia site between August 2017 and 

October 2018, and at the HFC site between June 2017 and 

October 2018.

All discrete and continuous water-level and precipitation data collected throughout this study were reviewed and 

approved according to USGS technical policies, which are 

available at https://water.usgs.gov/admin/memo/GW. The data 

can be retrieved from https://doi.org/10.5066/F7P55KJN by 

searching for the USGS site identifiers (USGS site ID column) 

listed in table 1. An R script for downloading these data is 

provided in the data release accompanying this report (Maher 

and others, 2020).

Mean water levels were calculated for each piezometer and then used to compute the mean hydraulic gradients 

for each piezometer nest. Water-level measurements were 

included in the calculation of the mean only when measurements were made at all piezometers in a nest within a short 

period. Discrete water-level measurements were used at 

the Litchfield and Cromwell sites because not all piezometers had continuous water-level data. A total of 16 synoptic 

water-level measurement runs between September 26, 2015, 

and April 24, 2017, were used to compute the mean water 

levels for the piezometers in the LFO1 and LFO2 nests. A 

total of 13 synoptic water-level measurement runs between 

August 12, 2015, and April 25, 2017, were used to compute 

the mean water levels for the piezometers in the CWO1/

O2 nest. A “synoptic water-level measurement run” is when 

water levels in all piezometers at a given nest were measured 

within a 36-hour period. Continuous water-level data from 

transducers (recorded every 15 minutes or every hour) were 

used for calculating the mean water level for each piezometer in the Olivia and HFC nests (except OT–35, which was 

excluded from hydraulic gradient calculations). The period 

during which continuous water-level data were recorded 

in all piezometers in the Olivia nest is December 12, 2017, 

to October 10, 2018. The periods during which continuous 

water-level data were recorded in all piezometers in the HFC 

nest are September 11, 2017, to July 18, 2018, and August 26 

to October 11, 2018.

Slug Tests

Rising-head and falling-head slug tests were completed 

in each piezometer to estimate K. Generally, three rising-head 

and three falling-head slug tests were completed for each 

piezometer, though some piezometers had fewer tests completed because of field conditions or slow recoveries. For each 

rising- or falling-head slug test, a solid polyvinyl chloride 

slug was rapidly added (falling-head test) or removed (risinghead test) from the piezometer and water-level measurements 

were recorded either manually or with a submersible pressure 

transducer. A Druck PDCR 1800 transducer and Campbell 

Scientific CR10X datalogger were used to record water levels 

at most piezometers, except a few piezometers at the Litchfield 

and Cromwell sites, which had only manual water-level measurements made with an electric tape. The manual measurements provided sufficient data quality in these piezometers 

because these piezometers are screened in units with K values 

less than 32 ft/d (Butler and others, 1996).

Slug test results were analyzed with the AQTESOLV 

program (version 4.5; Duffield, 2007) using the most appropriate methods, which included the Kansas Geological Survey 

(KGS) method for unconfined or confined settings (Hyder 

and others, 1994), the Butler method (Butler, 1998), and 

the Springer-Gelhar method (Springer and Gelhar, 1991). 

Analytical methods for each slug test were selected based on 

the hydrostratigraphic placement of the piezometers (unconfined versus confined), piezometer construction (all piezometers are partially penetrating), and the water-level response 

to the slug (nonoscillatory versus oscillatory). A discussion of 

method selection for slug test analyses is provided in appendix 2, and an example of an AQTESOLV analysis done for 

OT–20 at the Olivia site is shown in figure 8. The accompanying data release (Maher and others, 2020) contains all of the 

slug test water-level data and AQTESOLV analyses done for 

this study.

Data inputs of the AQTESOLV analyses are detailed 

in appendix tables 2.1 and 2.2. Field observations recorded 

during the slug tests, including static water levels, the volume 

of the slug used, the initial water-level displacement when 

the slug was added or removed from the piezometer, and 

the theoretical displacement volume of the slug, are listed in 
Methods of Study 17

0

1

2

Water-level displacement, in feet

0.01 0.1 1 10 100 1,000

Elapsed time, in minutes

EXPLANATION

Observed data

Kansas Geological Survey model

fit (Hyder and others, 1994)

Figure 8. Example graphical slug test analysis output from AQTESOLV for 

piezometer OT–20. The plot shows observed water-level displacement versus 

elapsed time and the Kansas Geological Survey model fit to those data.

table 2.1. The theoretical displacement volume of a slug is the 

possible volume of water that can be displaced in a piezometer/well, calculated based on diameter and length of the slug, 

and the diameter of the piezometer/well (Cunningham and 

Schalk, 2011). The AQTESOLV parameters, information on 

water levels and aquifer properties, as well as piezometer 

construction, that were entered into the AQTESOLV program 

are detailed in table 2.2.

Aquifer Tests

The vertical arrangement of well screens near highcapacity production wells provided an opportunity to evaluate 

the Kv of tills from aquifer test data. Constant-rate pumping 

aquifer tests were completed at all sites to estimate the hydraulic properties of the aquifers and overlying till confining units 

at the Litchfield, Cromwell, Olivia, and HFC sites (table 2). 

An aquifer test was completed at the Cromwell site on 

May 24, 2017, and at the Litchfield site on June 29, 2017. An 

aquifer test was completed at the Olivia site from July 10 to 

July 13, 2018. Two aquifer tests were completed at the HFC 

site, but only the second test was valid. This second test was 

completed between July 18 and July 22, 2018. Water levels 

during the aquifer tests were measured with pressure transducers (OTT Orpheus Mini or Solinst) recording data at 1-minute 

intervals. Staff from the Minnesota Department of Health led 

the aquifer test planning and data analysis.

Several approaches were used to analyze the aquifer test 

data, many of which are available in AQTESOLV (table 2). 

Only the methods used to determine a representative bulk Kv

for each aquifer test are listed in table 2; many more methods 

of analysis and more complete documentation are described 

in the detailed reports (Blum and Woodside, 2017; Lund and 

Blum, 2017; Blum, 2019a, b; Blum, 2020). Most of the methods used to determine the representative bulk Kv were “leaky 

confined aquifer” type tests including Neuman-Witherspoon 

(Neuman and Witherspoon, 1969), Hantush-Jacob (Hantush 

and Jacob, 1955), and Moench (1985).
Table 2. Summary of constant-rate aquifer tests and analytical approaches used to determine representative till vertical hydraulic conductivities (BLund and Blum, 2017; Blum, 2019a, b). [min, minute; gal, gallon; gal/min, gallon per minute; Kv, vertical hydraulic conductivity; --, unknown] Site Aquifer test name(s) Minnesota unique number of pumped well(s) Date/time pumping start Date/time recovery start Pumping duration (min) Total discharge (gal) Rate (gal/min) Analytical m

to determ

representativ

Cromwell CW–CM4 593593 5/24/2017 12:10 5/25/2017 12:25 1,454.90 242,350 167 Neuman-Wither

(Neuman and Witherspoon, 

Litchfield Litchfield nest 1, composite -- 7/5/2017 19:00 7/10/2017 10:55 6,715 15,444,500 2,300 Neuman-Wither

(Neuman and Witherspoon, 

Hydrogeology field camp (HFC) HB–1 809697 7/20/2018 10:44 7/22/2018 12:02 2,958.15 Not reported 75 Hantush-Jacob 

and Jacob, 19

Olivia Olivia composite (Olivia-4 and Olivia-5) 228797 and 228796 7/11/2018 7:54 7/12/2018 17:46 1,210 Not reported 232–227 Moench (1985) Olivia Olivia-4 and Olivia-5 time- delay analysis 228797 and 228796 7/11/2018 7:54 7/12/2018 17:46 1,210 Not reported 232–227 Wang (2001) 
Methods of Study 19

The Olivia site was a special case because there were no 

detectable vertical hydraulic (drawdown) responses observed 

in any till piezometers; only reverse water-level fluctuations (RWFs) were observed. This means that application of 

traditional “leaky confined aquifer” approaches that rely on 

drawdown responses for determining aquifer and confining 

unit properties is questionable. Blum (2020) applied a second 

type of test, a time-delay analysis based on Wang (2001), to 

the data from the Olivia site. The basic idea of the time-delay 

analysis is that the delay in time of the pressure wave reaching 

a point in the confining unit from the base of the confining unit 

is related to the hydraulic diffusivity of the confining unit. The 

hydraulic diffusivity can then be used to calculate the Kv of 

the confining unit if the specific storage of the confining unit 

is known or estimated (Wang, 2001; Blum, 2020); therefore, 

two representative bulk Kv values were determined for the 

Olivia site using different conceptual models, each with their 

own limitations. A more complete discussion of the time-delay 

analysis is available in Blum (2020).

Calculations of Groundwater Flow through Till 

According to Darcy’s Law

Calculations based on Darcy’s law were used to estimate 

travel times and leakage through till confining units into the 

confined aquifer (Simpkins and Bradbury, 1992; Hendry and 

Wassenaar, 1999; Witt, 2017; Maher, 2020). The following 

equations were used to compute discharge and travel time 

through till confining units. According to Darcy’s law, discharge, Q, is calculated as follows:

Q=−KiA, (1)

and the specific discharge (q) is calculated by dividing the 

discharge by the cross-sectional area:

Q_

A =q=−Ki, (2)

and a mean linear velocity (Vz) is calculated by dividing the 

specific discharge by the effective porosity of the till:

Vz = _q

ne, (3)

and finally, a travel time through till (T) is calculated by dividing the till thickness by the mean linear velocity:

 T = _

L

Vz

, (4)

where

Q is discharge (synonyms in this report 

include leakage through till and recharge 

to confined aquifer) (length cubed per 

unit time);

K is hydraulic conductivity (length per 

unit time);

i is the hydraulic gradient (length/length);

A is the cross-sectional area of flow (length 

× length);

q is specific discharge (length per unit time);

Vz is the mean linear velocity (length per 

unit time);

ne is effective porosity (unitless);

T is travel time (time); and

L is the thickness of the till confining unit 

(length).

For all sites, an effective porosity of 0.25 was used for 

travel time calculations, which is within the range of values 

used in calculations for fluxes of groundwater or solutes 

through till (McKay and others, 1993). Because of large 

uncertainties in the sizes of the confined aquifers at each site, 

the calculations were done using a cross-sectional area (A) of 

1 square mile (mi2). The hydraulic gradient used in the calculation was the mean hydraulic gradient at each piezometer 

nest, determined by taking the mean of the hydraulic gradients 

among all till piezometers at a given site. The till thickness (L) 

was determined from cores at each well nest. Two calculations 

of travel time, specific discharge, and discharge were done 

for each piezometer nest: the first calculation was done using 

the geometric mean of all Kh values from slug tests, and the 

second calculation was done using the representative Kv value 

determined from each site’s aquifer test. For the calculations 

done using the geometric mean Kh, isotropy between Kh and 

Kv was assumed.

Groundwater Geochemistry

Groundwater samples and pore-water samples were collected to evaluate the vertical distribution of anthropogenic 

chemicals, groundwater ages, and oxidation-reduction (redox) 

conditions from land surface through till confining units to the 

underlying confined aquifer. All groundwater sampling procedures and methods were completed according to the USGS 

“National Field Manual for the Collection of Water-Quality 

Data” (U.S. Geological Survey, variously dated).

Groundwater samples for laboratory analyses were collected after three well volumes were purged and water-quality 

field properties (dissolved oxygen [DO], pH, specific conductance, and temperature) were stable. Water quality properties 

were measured during the purging process with a YSI 6820 

multiparameter sonde. Samples for stable isotopes of oxygen and hydrogen (δ18O and δ2H, respectively) and tritium 

analyses were collected raw, without filtration. Samples collected for cation analysis were filtered through a 0.45-micron 

filter into a polyethylene bottle, acidified to a pH less than 

2 with nitric acid, and chilled on ice until analysis. Samples 

for anion analysis and alkalinity were filtered through a 

0.45-micron filter into a polyethylene bottle and chilled on ice 
20 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

until analysis. Samples for nutrients (ammonia, nitrite, nitrate, 

and phosphorus) were filtered through a 0.45-micron filter 

into a brown polyethylene bottle and chilled on ice until analysis. Alkalinity (in milligrams per liter as calcium carbonate) 

was determined on filtered samples within 24 hours of sample 

collection using a Hach Digital Titrator and the inflectionpoint method. Listed in table 3 are the analyses completed on 

groundwater samples.

Pore-water samples were extracted from till core samples 

to evaluate differences in water chemistry between hydraulically conductive flow paths (groundwater) and water bound 

within the till matrix (pore water). Pore water was extracted 

from 6-in.-long subsamples of core sections extracted from 

boreholes during drilling operations. These subsamples were 

collected at or near the screened interval of a piezometer 

and prepared for storage and analysis in a similar manner to 

Gerber and Howard (1996). Core subsamples were scraped 

clean on the outside to remove potential contamination 

from drilling equipment or fluid. The subsamples were then 

wrapped in at least two layers of plastic wrap, taped, wrapped 

in at least two layers of aluminum foil, taped again, labeled, 

and then bagged. The core subsamples were then sent to the 

San Diego Geochemistry Laboratory at the USGS California 

Water Science Center where a hydraulic press was used to 

extract pore fluid. Pressures between 8,000 and 9,500 pounds 

per square inch were used to extract the pore fluid. Listed in 

table 3 are the analyses completed on pore-water (interstitial 

water) samples.

The following are brief descriptions of analytical 

methods used to determine concentrations of analytes in 

groundwater and pore-water samples. Ammonia concentrations measured in samples at the USGS National Water 

Quality Laboratory (NWQL) were determined with a 

salicylate-hypochlorite colorimetry method (Fishman, 1993). 

Dissolved phosphorus concentrations measured in samples at 

the USGS NWQL were determined by colorimetry according to U.S. Environmental Protection Agency method 365.1 

(Odell, 1993). Anion concentrations measured in samples at 

either the University of Minnesota Geochemistry Laboratory 

or the Ion Chrom Analytical Laboratory were determined 

by anion chromatography using a Dionex ICS 5000 with an 

AS19 4-micron (2-x 250-millimeter) column (Maher and others, 2020). At the USGS NWQL, anion concentrations were 

determined by ion chromatography, and cation concentrations 

were determined by inductively coupled plasma atomic emission spectroscopy (Fishman and Friedman, 1989; Fishman, 

1993; American Public Health Association and others, 1998). 

Nitrate plus nitrite concentrations measured in samples at the 

USGS NWQL were determined with an enzyme reductiondiazotization colorimetry method (Patton and Kryskalla, 

2011), and nitrite concentrations were determined by colorimetry (Fishman, 1993). Stable isotope analyses were done at the 

Iowa State Stable Isotope Laboratory on a Picarro L2130–i 

Isotopic Liquid Water Analyzer with autosampler and ChemCorrect software. Reference standards for isotopic corrections 

varied between runs and are identified in the accompanying 

data release (Maher and others, 2020). Tritium concentrations in samples at the University of Waterloo Environmental 

Isotope Laboratory were determined by electrolytic enrichment and an LKB Wallace 1220 Quantulus counter (Maher 

and others, 2020).

All geochemical data from non-USGS laboratories are 

provided in a data release accompanying this report, along 

with an R script to retrieve geochemistry data from the USGS 

National Water Information System (NWIS; Maher and others, 2020). Alternatively, USGS NWIS water-quality data are 

available at https://doi.org/10.5066/F7P55KJN and can be 

retrieved using the USGS site identifiers listed in table 1.

Quality assurance samples were collected during the field 

study, including field replicates, field blanks, and split samples 

sent to separate laboratories. A summary of quality assurance 

at USGS laboratories and comparisons between USGS and 

non-USGS laboratories is provided in appendix 3. A summary 

of the quality assurance information from non-USGS laboratories is included in the metadata of the data release accompanying this report (Maher and others, 2020).

Groundwater Modeling

Assessing the sustainability of groundwater withdrawals 

from buried confined aquifers in glacial sediments is challenging because their hydrogeologic settings at locally relevant 

scales are highly uncertain. The field investigations at the 

Litchfield site, in particular, established that the hydrologic 

properties of till overlying confined aquifers can be highly 

variable over short distances. Furthermore, the extent of confined aquifers and their connections to other buried and possibly confined aquifer systems are not well understood because 

of the complex glacial geologic history of Minnesota. The 

MGS has mapped buried aquifers (sand bodies that may or 

may not be confined) using the best available data (well logs 

from well installations) through the County Geologic Atlas 

Program; however, there are still large uncertainties about the 

connectivity and extent of buried aquifer systems. The field 

studies presented in this report could not address questions 

about water movement with and without pumping because 

the sites were near production wells that consistently pumped 

groundwater. To better understand how till properties, aquifer 

properties, and pumping affect fluxes of water through till, a 

series of heuristic steady-state groundwater-flow models was 

developed (table 4). The heuristic models do not represent the 

actual physical locations that were sampled in this study and 

thus the models are not “calibrated.” Rather, the models utilize 

a generalized structure in which a subset of system properties 

was varied in order to evaluate how hydrogeologic setting 

and groundwater pumping affect fluxes of water through till 

into confined aquifers. The software package, Groundwater 

Vistas (Environmental Simulations Incorporated), was used 

to develop MODFLOW-2005 (Harbaugh, 2005) models for 

this analysis. The specific goal of the modeling effort was to 

evaluate the variability in water fluxes into and through till 
Table 3. Summary of water-quality sampling events, analytes, and analytical laboratories used for water sample analysis.[USGS, U.S. Geological Survey; WG, groundwater sample; WI, interstitial (pore) water; --, no samples collected]

Laboratory Analytes

Well nest

Litchfield 1 (LFO1) Litchfield 2 (LFO2) Cromwell (CWO1/O2) Hydrogeolo

camp (H

Sample medium and month of sampling event

USGS field staff Alkalinity, water-quality field properties: 

specific conductance, dissolved oxygen, temperature, pH

WG: July 2015

WG: May 2016

WG: July 2015

WG: May 2016

WG: July 2015

WG: May 2016

WG: Sept.–O

Rick Knurr (University 

of Minnesota and Ion Chrom Analytical Laboratory)

Major anions: bromide (Br), chloride (Cl), acetate (CH3CO2), fluoride (F), sulfate (SO4), 

thiosulfate (S2O3); nutrients: nitrite (NO2), nitrate (NO3), phosphate (PO4)

WI: June 2015

WG: July 2015WG: May 2016

WI: June 2015

WG: July 2015WG: May 2016

WG: July 2015

WI: July 2015WG: May 2016

WI: May 20

USGS National Water 

Quality Laboratory

Major anions: Br, Cl, F, SO4; major cations: 

potassium (K), calcium (Ca), magnesium (Mg), manganese (Mn), sulfur (S), iron (Fe), sodium (Na); nutrients: ammonia (NH3), total phosphorus (P), NO2, NO3

-- WG: May 2016 WG: May 2016 WG: Sept.–O

Iowa State University 

Stable Isotope Laboratory

Stable isotopes: oxygen-18 (δ18O) and 

hydrogen-2 (δ2H)

WI: June 2015

WG: July 2015WG: May 2016

WI: June 2015

WG: July 2015WG: May 2016

WG: July 2015

WI: July 2015WG: May 2016

WI: May 20

WG: Sept2017

University of Waterloo, 

Ontario, Canada, Environmental Isotope Laboratory

Enriched tritium (3H) WG: May 2016 WG: May 2016 WG: May 2016 WG: Sept.–O

San Diego Geochemistry 

Laboratory (USGS California Water Science Center)

Pore-water extraction from cores, specific 

conductance, pH

WI: June 2015 WI: June 2015 WI: July 2015 WI: May 20

aPiezometer OT–35 was never sampled at this nest.
Table 4. Model parameter values used in the heuristic groundwater model scenarios and the naming scheme used for each model run. [Kv, vertical hydraulic conductivity; Kh, horizontal hydraulic conductivity] Model parameter value Unit Low parameter value Base model parameter value High parameter 

(Naming convention) 

Permutation model run Kv of upper till and lower unit Feet per day 0.001 (Lv) 0.05 (Mv) 2 (Hv) 

Lateral connectivity of buried aquifer to adjacent till and aquifers (Kh of middle unit) Feet per day 0.05 (Lc) 5 (Mc) 30 (Hc) Buried sand body (aquifer) size Mile × mile 1.0×0.5 (Ls) 3.0×1.5 (Ms) 5.0×2.5 (Hs) 

Variation model run Buried sand body (aquifer) Kh Feet per day 30 (BSkh_L) 100 (MsMvMc) 400 (BSkh_H) 

Thickness of upper till Feet 40 (UTtk_L) 80 (MsMvMc) 160 (UTtk_H) Total pumping rate Gallons per minute 300 (TOTq_L) 900 (MsMvMc) 2,250 (TOTq_H) Screen length and penetration of production wells Screen length and location in aquifer 40-foot screen in lower aquifer layer (Ppen_L) 80-foot screen across both aquifer layers (full penetration) (MsMvMc) 40-foot screen in 

aquifer layer (P

Kh of surficial unit; Kv of surficial unit Feet per day; feet per day 5.0; 0.5 (SURF_L) 70; 7.0 (MsMvMc) 400; 40 (SURF_HRecharge rate Inches per year 2 (SURF_L) 4 (MsMvMc) 8 (SURF_H) Thickness of surficial unit Feet 80 (SURF_L) 40 (MsMvMc) 40 (SURF_H) Transmissivity of buried sand body (aquifer) and Kh used to determine transmissivity Feet squared per day; feet per day 4,400; Kh=55.5 (CRtrlk) 8,000; Kh=100 (MsMvMc) 8,990; Kh=112.4 Low and high parameter values are calculated from the leakance of upper till as observed in Litchfield and Cromwell aquifer tests (expressed as vertical hydraulic conductivity) Feet per day 0.6769 (CRtrlk) 0.05 (MsMvMc) 0.0016 (LFtrlk) 

aLund and Blum, 2017. bBlum and Woodside, 2017. cMeyer, 2015. dWagner and Tipping, 2016. eWitt, 2017. fMinnesota Department of Natural Resources, 2017.gMinnesota Department of Health, 2017.hSmith and Westenbroek, 2015.
Methods of Study 23

to confined aquifers that are being pumped across the range 

of hydrogeologic settings observed at the field sites. All of 

the models and output data are available through the model 

archive (Trost and others, 2020).

The basic structure of the heuristic model domain was 

about 20 mi by 20 mi with a cell size of 500 ft by 500 ft 

(shown in fig. 9). The model contained seven layers: a surficial 

unit that contained several rivers and lakes, three layers of 

“upper” till that represented the confining unit, two layers 

that contained the buried sand aquifer and a “middle” unit, 

and a layer of “lower” till. Under nonpumping conditions, the 

hydraulic head in the buried sand aquifer indicated a confined aquifer; however, the persistence of confined conditions 

throughout all model runs was not tracked, so the sand unit 

is referred to as a “buried sand unit” or “buried aquifer” to 

encompass the possibility of confined or unconfined conditions for all model runs. For most models runs, the surficial 

unit (layer 1) was 40 ft thick, the till unit was 80 ft thick 

(layers 2–4), the buried sand unit and surrounding middle 

unit (layers 5–6) were 80 ft thick, and the lower till unit 

was 200 ft thick (layer 7, fig. 9). Differences in layer thicknesses for specific model runs are listed in table 4. The buried 

aquifer was in the middle of the model domain to minimize 

the potential for boundary conditions to directly affect water 

fluxes in the aquifer. Three hypothetical production wells were 

screened in the buried sand aquifer. The north and south model 

boundaries were specified head boundaries, and the east and 

west model boundaries were no-flow boundaries. A regional 

north-to-south horizontal hydraulic gradient of 0.001 was 

specified. A vertical downward hydraulic gradient of 0.15 was 

assigned to model boundary cells. A constant recharge rate 

of 4 inches per year (in/yr) was applied at the surface of the 

model for all but two model runs, which is the statewide mean 

from Smith and Westenbroek (2015). Lakes and streams were 

generally simulated as groundwater discharge features with 

head-dependent flux boundaries using the MODFLOW RIV 

and DRN packages, respectively (Harbaugh and others, 2000). 

Lakes and streams were assigned bed conductances of 1 ft/d 

and 5 ft/d, respectively. All model input files, output files, and 

executables are available through a model archive (Trost and 

others, 2020).

Several model parameters, including the Kv and Kh of till 

and aquifer material, till thickness, buried aquifer size, pumping rate, and penetration of production wells, were varied in 

the model scenarios (table 4). The range of model parameter 

values chosen for evaluation were informed by the observations made at the field sites and other applicable studies and 

datasets. Also listed in table 4 is the naming convention for the 

model runs, which corresponds to figures and tables of model 

output later in the report.

Steady-state model runs beginning with Ls, Ms, or Hs 

compose a set of “permutation runs” in which ranges of 

parameters for specific parts of the model system were evaluated (table 4). The names of the permutation model runs are 

six-letter codes representing the relative values (H = high, 

M = middle, and L = low) of the three hydraulic properties 

varied among simulations. The high (H) and low (L) model 

parameter values are inclusive of Litchfield and Cromwell, 

typically slightly greater than or less than observations at 

these sites. The three hydraulic properties that varied were 

the maximum lateral dimensions of the buried sand unit in 

model layers 5 and 6 (naming convention = “s”), the upper till 

Kv in model layers 2–4 (naming convention = “v”), and the 

Kh in the middle unit in model layers 5 and 6 surrounding the 

buried sand unit (naming convention = “c”). In all permutation runs, the Kh of the upper till (layers 2–4) was fixed at 

0.05 ft/d. The Kv of the middle unit (layers 5–6) was assigned 

the same value as the Kv of the till in layers 2–4. For example, 

from table 4, a model run titled LsMcHv means the buried 

sand unit in layers 5 and 6 was assigned the low size of 1.0 mi 

by 0.5 mi, the middle unit (layers 5 and 6) was assigned the 

“middle” Kh of 5 ft/d, and the till units (layers 2–4) were 

assigned the “high” Kv of 2 ft/d. The “base model” is labeled 

MsMvMc and contained model parameter values that represented an approximate midpoint among observations from the 

field study sites.

A set of “variation” steady-state model runs was also 

completed (table 4). In this set of model runs, six additional 

properties were evaluated through comparison to the base 

model, MsMvMc. Model run names ending in “_H” indicate the “high” parameter value, and names ending in “_L” 

indicate the low parameter value. These high and low values 

were also informed by field data collected as part of this study. 

The model runs titled CRtrlk and LFtrlk stand for “Cromwell 

transmissivity like” and “Litchfield transmissivity like.” In 

these model runs, the transmissivity of the buried aquifer and 

the leakance of the upper till unit were assigned values determined from the Cromwell and Litchfield aquifer test results 

(Blum and Woodside, 2017; Lund and Blum, 2017).

Several response variables were extracted from the model 

output and compared among the model runs. To check for 

boundary effects on water fluxes, the change in flux from constant head cells on the north and south model boundaries was 

compared between ambient (pumping turned off) and stressed 

(pumping turned on) periods. The following response variables were compared: (1) the source of water to buried aquifer, 

(2) pumping-induced leakage of water from the surficial unit 

in layer 1 to the till in layer 2, and (3) the maximum drawdown in the surficial unit (layer 1) and the till unit (layer 3). 

The programs for extracting model output are provided in the 

model archive (Trost and others, 2020).

For the source of water to the buried aquifer, the relative 

contributions of water entering the buried aquifer from above, 

from the sides, and from below were compared among model 

runs. The leakage of water from the surficial unit in layer 1 to 

the till in layer 2 was quantified within a 5-mi by 5-mi “local 

area” (red outline in fig. 9) centered on the production wells 

and buried aquifer. The following equation was used to compute leakage as a percentage of total inputs into layer 1 within 

the 5-mi by 5-mi local area:
24 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

SOUTH

Cross section along column 106 NORTH

Surficial unit

Upper till

Upper till

Upper till

Buried sand aquifer // Middle unit

Buried sand aquifer // Middle unit

Lower unit

10,000 FEET 50 FEET

Pumping wells

20 MILES

No-flow boundary

No-flow boundary

Specified head boundary

Specified head boundary

5 MILES

North

40 feet

80 feet

80 feet

200 feet

Buried aquifer

Local area

A

B

Model layer 1

2

3

4

5

6

7

A

A'

A

A'

EXPLANATION

Buried sand body

Lake

Line of section

Local area boundary

River

Pumping well

A A'

Figure 9. Heuristic MODFLOW model layout. A, top view; B, cross-sectional view.
Characterization of Glacial Till and Aquifer Systems 25

LD,PCT = _VD 

(VR + VL + VI) × 100, (5)

where

LD,PCT is the percentage of downward leakage from 

layer 1 to layer 2,

VD is the volume of water flowing downward 

from layer 1 to layer 2,

VR is the volume of groundwater recharge within 

the local area (water reaching the water 

table from precipitation and percolating 

through soil),

VL is the volume of groundwater inputs 

entering the local area from the sides and 

below, and

VI is the volume of induced flow from local 

streams into layer 1 within the local area 

(typically zero or very small).

The pumping-induced increase in leakage was then calculated as the difference between the percentage of downward 

leakage during ambient and stressed (pumped) periods. The 

recharge rate was 4 in/yr for all but two model runs (SURF_L, 

SURF_H, table 4), so increases in the percentage of downward leakage from ambient to stressed conditions indicated a 

pumping-induced reduction in lateral groundwater flow out of 

the local area and (or) a reduction in the contribution of groundwater discharge to lakes and streams within the local area (fig. 9).

Characterization of Glacial Till and 

Aquifer Systems

The following section presents the hydrogeological 

and geochemical findings along with sources of uncertainty 

associated with field methods. Evaluations of the MODFLOW 

heuristic models are also presented. The “Hydrogeology” subsection describes hydraulic properties of the till and confined 

aquifer units. The “Groundwater Geochemistry and Water 

Quality” subsection presents the results and interpretations 

of chemical analyses. The “Sources of Uncertainty” subsection describes important factors related to field procedures 

that may affect the conclusions of this study. The “Interpretive 

Groundwater Modeling” subsection describes variations in 

groundwater flux through till caused by pumping from buried 

aquifers in different hydrogeologic settings.

Hydrogeology

Several physical and hydrogeological properties of the 

till and confined aquifer units at each site are summarized in 

table 5. In the following sections, a qualitative evaluation of the 

vertical profiles of hydraulic-head responses to pumping and 

weather, a discussion of K distributions in till, and calculations 

of leakage (recharge) through till confining units are presented.

Hydraulic-Head Responses to Pumping and 

Weather

The piezometer nests at the Litchfield, Cromwell, and 

Olivia sites were installed near high-capacity production 

wells, and aquifer tests were completed at all the study sites, 

providing an opportunity to observe hydraulic-head fluctuations to pumping in vertical till profiles (fig. 10). Water levels 

in piezometer screens hydraulically isolated from the aquifer being pumped were not expected to have a drawdown 

response from pumping stress over the short-term (several 

hours up to 5 days) pumping cycles that occurred during this 

study (Cherry and others, 2004). If water levels in till piezometers demonstrate a drawdown response to short-term pumping, it indicates a likely hydraulic connection between the 

confined aquifer and the till piezometer. Hydraulic connections 

are possible because of a conductive matrix with high percentages of sand (for example, tills at the HFC and Cromwell 

sites) or from fractures in more clayey till (Cherry and others, 

2004). If, for example, a piezometer intersected or was near 

a fracture that is hydraulically connected to the aquifer being 

pumped, then it is likely that a drawdown response would 

be observed in that piezometer (Cherry and others, 2004). 

Similarly, hydraulic connectivity from the surface downward 

can be examined by water-level responses in till piezometers 

to snowmelt or other substantial infiltration events (Cherry and 

others, 2004).

The LFO1 and LFO2 sites showed decreasing hydraulichead values with depth and an overall downward hydraulic 

gradient (figs. 10A,B and 11A,B). At both sites, the downward hydraulic gradients increased with depth through the 

till, with the largest hydraulic-head losses occurring near the 

base of the till (fig. 11A,B). The downward hydraulic gradient was larger at the LFO2 site compared to the LFO1 site. 

Continuous water-level data at the LFO1 and LFO2 sites 

show varying responses to the pumping of the high-capacity 

production wells (fig. 10A,B). In the two aquifer piezometers, 

LFO1–F and LFO2–F, a clear daily to subdaily oscillation in 

hydraulic head from the high-capacity wells is evident. The 

LFO2 site is nearer to the high-capacity production wells, 

and as expected, LFO2–F shows a much larger oscillation in 

hydraulic head, as much as 4 ft, from pumping than LFO1–F, 

which shows only about 1-ft variations in hydraulic head 

(fig. 10A,B). Both confined aquifer piezometers show three 

large decreases in hydraulic head in July and August 2016 

(fig. 10A,B). These large drops occurred during dry periods 

and ended during or just before precipitation events, indicating that these water-level fluctuations are likely caused by 

a high-capacity irrigation system that withdrew water from 

a confined aquifer system hydraulically connected to the 

confined aquifer used by high-capacity production wells. 

According to the Minnesota Department of Natural Resources 

(2020), there are several agricultural irrigation wells within 

about 1 mi from the Litchfield site, with the closest irrigation 

well about one-half mi away.
Table 5. Summary of physical and hydraulic properties of till and confined aquifers at the Litchfield 1 (LFO1), Litchfield 2 (LFO2), Cromwell (CWO1/O2field camp (HFC) sites. [--, no data or not applicable; Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; <, less than] Property Unit Site Litchfield 1 (LFO1)a Litchfield 2 (LFO2)a Cromwell (CWO1/O2)b Hydrogeology fi

camp (HFCc) 

Glacial lobe -- Des Moines Des Moines Superior Wadena Agef -- Late-Wisconsin Late-Wisconsin Late-Wisconsin Late-Wisconsin 

Till properties Geologic formationf -- New Ulm, Villard Member New Ulm Villard Member Cromwell Hewitt 

Mean till grain sizef Percent (sand:silt:clay) 47:34:19 52:31:17 57:31:13 67:22:11 Till texturef -- Loam to sandy loam Loam to sandy loam Sandy loam Sandy loam to loa

sand 

Mean lithologic compositionf Percent (crystalline:carbonate:shale) 56:28:16 56:28:16 98:3:0 97:3:0 Till thickness Feet 60 115 120 100 Hydraulic gradient through till Dimensionless 0.56 downward 0.48 downward 0.02 upward 0.04 downward Slug test geometric mean Kh Feet per day 0.07 0.0002 0.06 0.03 Slug test Kh range Feet per day 0.02–0.4 0.00001–0.001 0.006–0.3 0.0003–0.4 Aquifer test representative Kv Feet per day 0.001 0.001 1.1 0.031 Aquifer test Kv range Feet per day <0.0001–0.02 <0.0001–0.02 0.8–4.1 0.011–0.037 Vertical anisotropy (representa- tive Kv/geometric mean Kh) Dimensionless 70 0.20 0.05 1.0 Hydraulic gradient across till/ confined aquifer boundary Dimensionless 0.01 downward 0.02 downward 0.05 upward 0.02 downward 

Confined aquifer properties (determined from aquifer tests) Transmissivity Feet squared per day 9,000 9,000 4,400 1,850 

Thickness Feet 29 29 145 14 Kh Feet per day 310 310 30 132 Vertical anisotropy Dimensionless 1 1 0.5 1 Storativity Dimensionless 0.000075 0.000075 0.0002 0.000058 Leakage factor Feet 21,000 21,000 -- 2,630 Well efficiency Percent -- -- -- 0.1 

aAquifer test results from Blum and Woodside, 2017. bAquifer test results from Lund and Blum, 2017. cAquifer test results from Blum, 2019a.dAquifer test results from Blum, 2019b.eAquifer test results from Blum, 2020.fStaley and others, 2018.
Characterization of Glacial Till and Aquifer Systems 27

1,050

1,100

1,150

Water level, in feet above the

North American Vertical Datum of 1988

July August September

2016

0

2

4

6

8

10

12

Daily precipitation total, in inches

A. LFO1

EXPLANATION

LFO1–B

LFO1–D

LFO1–F

Piezometer 

short name

Screened interval

(feet below 

land surface)

Hydrostratigraphy

of screened 

interval

Precipitation

22.4–25.06

72.4–75.06

117.5–127.12

Surficial aquifer

Till

Confined aquifer

1,050

1,100

1,150 0

2

4

6

8

10

12

Daily precipitation total, in inches

Water level, in feet above the

North American Vertical Datum of 1988

July August September

2016

B. LFO2

LFO2–A

LFO2–C

LFO2–D

LFO2–F

17.12–19.78

56.97–59.63

82.27–84.93

Till

Till

149.56–159.18 Confined aquifer

Till

EXPLANATION

Piezometer 

short name

Screened interval

(feet below 

land surface)

Hydrostratigraphy

of screened 

interval

Precipitation

1,300

1,305

1,310

1,315

1,320 0

2

4

6

8

10

12

Daily precipitation total, in inches

Water level, in feet above the

North American Vertical Datum of 1988

June July August

2016

C. CWO1/O2

CWO2–A

CWO2–D

CWO1–A

CWO1–B

32.3–34.96

103.58–106.24

144.56–147.36

Surficial aquifer

Till

220.91–230.53 Confined aquifer

Till

EXPLANATION

Piezometer 

short name

Screened interval

(feet below 

land surface)

Hydrostratigraphy

of screened 

interval

Precipitation

Figure 10. Precipitation and hydraulic head in piezometers during 3-month periods. A, the LFO1 nest at the Litchfield site; 

B, the LFO2 nest at the Litchfield site; C, the CWO1/O2 nest at the Cromwell site; D, the hydrogeology field camp (HFC) site; 

E, the Olivia site.
28 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

2018

April May June

0

2

4

6

8

10

12

Water level, in feet above the

North American Vertical Datum of 1988

Daily precipitation total, in inches

1,380

1,385

1,390

1,395

1,400

MW–01

HT–115

HT–140

HT–175

HT–200

HB–3

80.0–85.0

112.35–114.83

137.77–140.25

Surficial aquifer

Till

Confined aquifer

172.47–174.95

Till

Till

195.22–197.77 Till

213.33–223.53

D. HFC

E. Olivia

950

960

970

980

990

1,000

1,010

1,020

1,030

1,040

1,050

1,060

1,070

1,080

2018

June July August

0

2

4

6

8

10

12

Water level, in feet above the

North American Vertical Datum of 1988

Daily precipitation total, in inches

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

OB–7

7.89–13.03

17.44–19.91

56.74–59.77

Surficial aquifer

Till

Confined aquifer

101.95–104.94

Till

Till

141.15–144.13 Till

172.26–175.26 Till

204.92–209.71

EXPLANATION

Piezometer 

short name

Screened interval

(feet below 

land surface)

Hydrostratigraphy

of screened 

interval

Precipitation

EXPLANATION

Piezometer 

short name

Screened interval

(feet below 

land surface)

Hydrostratigraphy

of screened 

interval

Precipitation

Figure 10. Precipitation and hydraulic head in piezometers during 3-month periods. A, the LFO1 nest at the Litchfield site; 

B, the LFO2 nest at the Litchfield site; C, the CWO1/O2 nest at the Cromwell site; D, the hydrogeology field camp (HFC) site; 

E, the Olivia site.—Continued

Hydraulic-head data from the LFO2 site demonstrate the 

presence of a till confining unit that limits hydraulic connectivity through the till profile (fig. 10B). Water-level fluctuations from pumping stress are not apparent at LFO2–D, 30 ft 

above the till/aquifer boundary. There is not a large sand lens 

that could dampen the hydraulic-head fluctuation response 

between the till/aquifer boundary and LFO2–D (fig. 3). This 

piezometer also did not demonstrate a drawdown response 

during the aquifer test after 24 hours of pumping at 787 gallons per minute (gal/min) (Blum and Woodside, 2017). This 

indicates there is a confining unit within 30 ft of the till/aquifer 

boundary that limits the hydraulic connectivity between the 

aquifer and the till.

Water levels in LFO2–A (screened about 17–20 ft 

below land surface) and LFO2–C (screened about 57–60 ft 

below land surface) responded similarly to large precipitation events (fig. 10B), indicating hydraulic connectedness 

through the till from 20 to 60 ft below land surface. Patterns in 

water levels at LFO2–D did not resemble those of LFO2–A, 

indicating that LFO2–D is also reasonably hydraulically 

isolated from surficial processes. Taken together, this indicates 

that the most effective confining unit at LFO2 exists above and 

below LFO2–D and that at least the upper 60 ft of till at the 

LFO2 site are hydraulically connected.

A different “leaky” response was observed at the far nest, 

the LFO1 site. LFO1–D is screened in till about 25 ft above 

the top of the confined aquifer/till boundary, and water-level 

patterns in this piezometer closely resemble those observed 

in the confined aquifer. Even the daily oscillations from the 

cycling on and off of the production wells are evident at 

LFO1–D, indicating a reasonable hydraulic connection from 

the aquifer through the bottom 25 ft of till (fig. 10A). During 

the aquifer test at the Litchfield site, drawdown was observed 

in all the till piezometers (LFO1–C, LFO1–D, and LFO1–F), 
Characterization of Glacial Till and Aquifer Systems 29

LFO1–B

LFO1–C

LFO1–D

LFO1–F

LFO1–E

LFO2–B

LFO2–A

LFO2–C

LFO2–D

LFO2–F

LFO2–E

CWO2–A

CWO2–B

CWO2–C

CWO2–D

CWO2–E

CWO1–A

CWO1–B

CWO1–C

MW–01

HT–115

HT–140

HT–175

HT–200

HB–3

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

OB–7

EXPLANATION

Surficial aquifer

Confining unit (till)

Confined aquifer

Bedrock aquifer

Bedrock 

Hydrostratigraphy

Downward

Hydraulic gradient (i) between 

adjacent piezometers

Upward

Zero line

Axis break

No information

Horizontal hydraulic conductivity 

(Kh

) from slug tests

Drawdown response observed in 

piezometer during aquifer test

Confining unit vertical hydraulic 

conductivity (Kv

) range—Dashed 

line means less than (Blum and 

Woodside, 2017; Lund and Blum, 

2017; Blum, 2019a, b)

Difference in hydraulic head 

E. Olivia

A. LFO1 B. LFO2

C. CWO1/O2 D. HFC 0

 50

100

150

200

250

300

350

 0

 20

 40

 60

 80

100

120

140

160

170

Depth, in feet below land surface Depth, in feet below land surface

Logarithmic hydraulic 

conductivity, 

in feet per day

Difference in hydraulic

head from uppermost 

piezometer, in feet

Hydraulic gradient,

in foot per foot

−5 −4 −3 −2 −1 0 1 2 3 −100 −80 −60 −40 −20 0 20 −1.0 −0.5 0.0 0.5 1.0

Logarithmic hydraulic 

conductivity, 

in feet per day

Difference in hydraulic

head from uppermost 

piezometer, in feet

Hydraulic gradient,

in foot per foot

−5 −4 −3 −2 −1 0 1 2 3 −100 −80 −60 −40 −20 0 20 −3.0 −2.5 −2.0 −1.0 −0.5 0.0 0.5 1.0

Logarithmic hydraulic 

conductivity, 

in feet per day

Difference in hydraulic

head from uppermost 

piezometer, in feet

Hydraulic gradient,

in foot per foot

−5 −4 −3 −2 −1 0 1 2 3 −100 −80 −60 −40 −20 0 20 −1.0 −0.5 0.0 0.5 1.0

 0

 20

 40

 60

 80

100

120

140

160

170

Depth, in feet below land surface Depth, in feet below land surface

Logarithmic hydraulic 

conductivity, 

in feet per day

Difference in hydraulic

head from uppermost 

piezometer, in feet

Hydraulic gradient,

in foot per foot

−5 −4 −3 −2 −1 0 1 2 3 −100 −80 −60 −40 −20 0 20 −1.0 −0.5 0.0 0.5 1.0

Logarithmic hydraulic 

conductivity, 

in feet per day

Difference in hydraulic

head from uppermost 

piezometer, in feet

Hydraulic gradient,

in foot per foot

−5 −4 −3 −2 −1 0 1 2 3 −100 −80 −60 −40 −20 0 20 −1.0 −0.5 0.0 0.5 1.0

 0

 50

100

150

200

250

Depth, in feet below land surface

 0

 50

100

150

200

250

Figure 11. Hydraulic information for each piezometer nest including hydraulic conductivity, differences in hydraulic head, and 

hydraulic gradients among piezometers. A, at the LFO1 nest at the Litchfield site; B, at the LFO2 nest at the Litchfield site; C, at the 

CWO1/O2 nest at the Cromwell site; D, at the hydrogeology field camp (HFC) site; E, at the Olivia site.
30 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

indicating a hydraulic connection through most of the till 

layer. Water-level patterns at LFO1–D bear a stronger resemblance to the confined aquifer than to the surficial aquifer, 

which is monitored by LFO1–B. Sharp water-level rises in 

LFO1–B are linked to rainfall events. Further time-series 

analysis is needed to determine if the routine pumping signal 

is apparent in the LFO1–B well. The till at the LFO1 site is 

only about 58 ft thick, and nearly one-half of this sequence 

is hydraulically connected between the top of the confined 

aquifer and LFO1–D.

The hydraulic-head data from the Cromwell site (CWO1/

O2) demonstrate a “leaky” till unit. At this site, a slight 

upward hydraulic gradient (fig. 11C) was observed. All the 

piezometers with continuous water-level data showed similar 

seasonal patterns in water levels (fig. 10C). Throughout the 

profile, from the surficial aquifer (CWO2–A) down to the 

bedrock (CWO1–C), an increase in water levels was observed 

in July 2016 (fig. 10C). Subdaily oscillations in hydraulic head 

caused by pumping from nearby high-capacity production 

wells are evident in the bedrock aquifer (CWO1–C, data not 

shown but are available at U.S. Geological Survey [2019]), 

the confined aquifer (CWO1–B), and two till piezometers 

(CWO1–A and CWO2–D) but not in the surficial aquifer 

(CWO2–A). Drawdowns were observed in all till piezometers 

installed at this site (CWO2–B through CWO1–A) during the 

aquifer test (fig. 11C; Lund and Blum, 2017). The till at the 

CWO1/O2 site is about 130 ft thick, and CWO2–B is screened 

about 14 ft below the top of the till. This means that hydraulic 

connectivity was observed through 90 percent of the till thickness at the Cromwell site.

The hydraulic-head data from the Olivia site demonstrate that parts of the till are an effective confining unit, 

limiting hydraulic connectivity between the confined aquifer 

and overlying till. The Olivia site has an overall downward 

hydraulic gradient (fig. 11E). The vertical hydraulic gradient 

between OT–13 in the surficial aquifer and OT–20 near the 

top of the till (confining) unit is small (0.03) and, depending on the time of year, can have a slightly upward vertical 

gradient. Mainly, the lack of a vertical hydraulic gradient 

between OT–13 and OT–20 indicates that there is primarily 

horizontal groundwater flow through the surficial aquifer in 

which OT–13 is screened. The vertical hydraulic gradient 

between OT–20 and OT–60 is also small (0.02) but is downward through the till throughout the year. Larger downward 

vertical hydraulic gradients exist from OT–60 through 

OT–175. An extremely large hydraulic gradient (2.26) 

exists between the piezometer screened in the bottom of the 

till, OT–175, and the piezometer screened in the confined 

aquifer, OB–7 (fig. 11E). Large vertical hydraulic gradients 

such as this have been observed in confining units before, 

and at the Olivia site, the large vertical hydraulic gradient 

may be due to the presence of thin layers of glaciolacustrine 

sediments near the bottom of the till unit (Hart and others, 

2008). During a 10-hour aquifer test, the confined aquifer 

was pumped at 232 gal/min, and no drawdown response was 

observed in any till piezometers, including OT–175, which 

is only 12 ft above the till/aquifer boundary (Blum, 2019b); 

however, the lack of a hydraulic response was confounded 

because of RWFs (fig. 12A).

The upper part of the tills at the Olivia site, to a depth 

of at least 60 ft, are hydraulically connected. Groundwater 

recharge events, especially in spring/summer, cause hydraulichead increases (of as much as about 3–5 ft) in piezometers 

OT–13, OT–20, and OT–60 (fig. 10E). Geologic descriptions 

and textural analyses of the till from about 30 to 60 ft are not 

appreciably different from 60 to 150 ft, indicating that fractures may explain the hydraulic connections in the upper part 

of the till. No visible fractures were reported in the geologic 

description of the cores (Staley and Nguyen, 2018), but fractures without visible staining are present in tills (Cherry and 

others, 2004; Helmke and others, 2005a, b).

Hydraulic-head observations at the Olivia site were 

unique in that water levels in several till piezometers demonstrated RWFs in response to pumping (see RWF discussion in the next section). Routine pumping by high-capacity 

production wells from the confined aquifer typically caused 

hydraulic-head changes of about 10 ft at well OB–7, which is 

screened in the confined aquifer. At first glance, OT–175 and 

OT–145 seem to be responding hydraulically to the pumping (fig. 10E); however, the fluctuations at these piezometers 

are RWFs, meaning water levels increased in response to 

pumping. Shown in figure 12A are hydraulic heads during 

the aquifer test completed in July 2018. Well OB–7 demonstrates a typical drawdown response when the pump turns on; 

however, the piezometers OT–175, OT–145, OT–105, and 

OT–60 all demonstrate varying degrees of RWF in that water 

levels in these wells increase as the hydraulic head in the 

aquifer decreases. Analysis of the RWF from the aquifer test 

data reveals no hydraulic response from any of these piezometers to the pumping; rather, the hydraulic-head changes are 

attributed to a poroelastic response of the system to pumping 

(Blum, 2019b).

The hydraulic-head data from the HFC site demonstrate 

a somewhat “leaky” till unit. The overall vertical hydraulic 

gradient at the HFC site is also downward but small (fig. 11D). 

The largest hydraulic gradients occur in the bottom half of the 

till profile. In contrast to the other sites, the confined aquifer 

at the HFC site is not continuously pumped, so the hydraulic 

heads measured at this site represent a static condition. The 

hydraulic-head difference between HT–115, which is screened 

about 10 ft below the top of the till confining unit, and 

HT–140, which is screened about 25 ft lower in the till unit is 

small (0.01 ft). Indeed, the hydrographs for these two piezometers overlap in figure 10D and the mean hydraulic gradient 

between these two piezometers is near zero (fig. 11D). There is 

a narrow band of higher sand content between these piezometers (Staley and Nguyen, 2018), which could be a zone of 

increased horizontal groundwater flow and might explain 

why there is such a small hydraulic gradient between these 

piezometers. There is a small vertical hydraulic gradient of 

0.005 between MW–01 (in the surficial aquifer) and HT–115 

(uppermost till piezometer).
Characterization of Glacial Till and Aquifer Systems 31

250

0

250

50

100

150

200

50

100

150

200

0

A. Olivia site B. Hydrogeology field camp (HFC) site

+0.5

−0.5

0

+0.5

−0.5

0

−20

+0.5

−0.5

+0.5

−0.5

0

−5

−10

EXPLANATION

Surficial aquifer

Confining unit (till)

Confined aquifer

Hydrostratigraphy

+0.5

−0.5

+0.5

−0.5

+0.5

−0.5

1

0

Drawdown or water-level

displacement

0

0

0

0.5

−10

0

0

0

−2.5

Midpoint of screened 

interval, in feet below 

land surface

Bedrock NOTE: Vertical drawdown or water-level 

displacement scales differ among plots

1

0

0.5 0

−5

−2.5

0

−5

Drawdown and water-level displacement plots

Drawdown and water-level displacement plots

Hydrostratigraphy

Hydrostratigraphy

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

OB–7

Date, in 2018 Date, in 2018

July 11 July 12 July 13

Olivia-4 pump on

Olivia-4 pump off

Olivia-5 pump on

Olivia-5 

pump off

HB–1 pump on

HB–1 pump off

HB–1 pump on

HB–1 pump off

HT–200

July 18 July 19 July 20 July 21 July 22

MW–01

HT–115

HT–140

HT–175

HB–3

Depth, in feet below land surface

Depth, in feet below land surface

Drawdown or water-level displacement, in feet

Drawdown or water-level displacement, in feet

Figure 12. Drawdown and water-level displacement in piezometer nests during constant-rate aquifer tests. A, at the Olivia site; 

B, at the hydrogeology field camp (HFC) site near Akeley, Minnesota.
32 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Hydraulic connections were observed throughout the 

till profile at the HFC site, although these were difficult to 

observe until the system was pumped. The unsaturated zone 

above the surficial aquifer is thick (over 60 ft) compared to the 

other sites in this study, and immediate responses to rainfall 

events were not noticeable; there is little to no variability in 

hydraulic head from April to June 2018 (fig. 10D). The only 

time water was pumped at this site was during aquifer tests. 

When the surficial aquifer was pumped for an aquifer test in 

2017, a drawdown response was observed down to HT–175, 

which is 71 ft below the top of the till unit. During the 2018 

aquifer test, when the confined aquifer below the till was 

pumped, a drawdown response was observed up through the 

till at piezometers HT–200, HT–175, and HT–140 (figs. 11D

and 12B). This indicates that there are hydraulic connections 

throughout the till profile, and if this confined aquifer were 

pumped regularly such that a stronger downward hydraulic gradient was established, there could be fast downward 

groundwater flow through the sandy till.

Reverse Water-Level Fluctuations

RWF responses were observed in till piezometers during 

aquifer tests at three of the four study sites. At the Cromwell 

and HFC sites, brief RWF responses were observed when 

pumps were turned on or off. At the Olivia site, a prolonged 

RWF response that lasted the duration of the aquifer test was 

observed in several till piezometers (fig. 12A; Lund and Blum, 

2017; Blum, 2019a, b). These RWF observations are usually 

attributed to a poroelastic response, or a “deformation-induced 

effect” (Hsieh, 1996), where pumping in the aquifer causes 

a reduction in pressure, which then leads to the expansion 

of water and a compression of the aquifer skeleton (Kim 

and Parizek, 2005; Berg and others, 2011). Deformation of 

the aquifer skeleton can then lead to strains that can cause 

confining units to have RWF responses. Often, these are 

either a brief response at the beginning of the pumping, 

where there is a temporary increase in the water level in the 

confining unit, called the Noordbergum effect, or a quick, 

temporary drop in water level at the end of pumping, called 

the Rhade effect (Kim and Parizek, 2005; Berg and others, 

2011). Noordbergum and Rhade effects are clearly visible 

in HT–115 during the aquifer test at the HFC site (fig. 12B). 

At the Cromwell site, piezometers CWO2–B and CWO2–C 

demonstrated the Noordbergum and Rhade effects as the pump 

cycled on and off during the aquifer test period (Lund and 

Blum, 2017; Blum, 2020).

Evaluation of the Cromwell RWF data indicated hysteretic RWF responses to pumping and recovery when steadystate hydraulic gradient conditions in till were not achieved, 

but the hysteresis disappears when steady-state conditions 

are achieved (Blum, 2020). Under nonsteady-state conditions 

at the Cromwell site, the recovery RWF response is much 

larger than the pumping RWF response. At the Olivia site, 

the opposite is true. At the Cromwell site, there is an upward 

hydraulic gradient, and at the Olivia site, there is a downward 

hydraulic gradient; therefore, the apparent hysteresis of the 

RWF response is caused by the direction and strength of the 

ambient vertical hydraulic gradient (when inelastic deformation of the confining unit is negligible) (Blum, 2020). The 

prolonged RWF response in the till (confining unit) at the 

Olivia site is unusual because it lasted for the entire aquifer 

test (fig. 12A). The pumping time (10 hours) for the Olivia 

site aquifer test was not long enough to achieve a steady-state 

hydraulic gradient, and thus nonhysteretic RWF responses, 

within the till confining unit. It is estimated that a minimum 

pumping period of 20–30 days would be required to achieve 

steady-state conditions in the confining unit till at the Olivia 

site (Blum, 2019b).

Hydraulic Conductivity

A total of 141 slug tests were completed on the piezometers for this study. The calculated Kh values are summarized 

in table 6. Slug test model fit residual statistics and confidence 

intervals of Kh estimates are provided in appendix table 2.3. 

All the water-level data, AQTESOLV files, and graphical outputs are available in the data release accompanying this report 

(Maher and others, 2020).

Generally, Kh estimates from slug tests were repeatable 

with narrow confidence intervals. Repeated slug test estimates 

of Kh for a single till piezometer generally varied by one order 

of magnitude or less (table 6). For till piezometers, the magnitude of the 95-percent confidence interval of Kh estimates 

was small relative to the estimated Kh value, having a mean of 

12 percent (appendix table 2.3). The model fit to the slug test 

data for one till piezometer, LFO2–A, was an outlier; the magnitude of the confidence interval value was large, 180 percent, 

relative to the Kh value. Estimates of Kh for aquifers (confined 

and unconfined) were more variable than estimates of Kh for 

till (table 6; appendix table 2.3).

The estimates of Kv from aquifer tests at each study 

site are shown in table 5 and figure 11. Two types of values 

are presented, a “representative bulk Kv” and a range of Kv

values. These numbers are taken from several reports from 

the Minnesota Department of Health (listed in table 5). The 

representative bulk Kv is considered the single best Kv estimate 

for each site and is used for travel time estimates and comparisons with Kh values. The range of reasonable Kv estimates is 

shown in figure 11. The range in Kv estimates (table 5) results 

from variations in simplifying assumptions and application of 

different analytical solutions to estimating Kv from the aquifer 

test data. The detailed methodology is available in the reports 

referenced in table 5.

At the Litchfield site, mean Kh values from slug tests 

range from 306 ft/d for the sand and gravel aquifer to about 

1×10−5 ft/d for till (fig. 11 and table 6). The geometric mean 

Kh values of till at the LFO1 and LFO2 sites are 7×10−2 ft/d 

and 2×10−4 ft/d, respectively (table 5). These values for Kh

are within previously observed values for Des Moines lobe 

till, although the Kh values at the LFO1 site were slightly 

higher than expected (Simpkins and Parkin, 1993; Helmke and 
Characterization of Glacial Till and Aquifer Systems 33

Table 6. Summary of horizontal hydraulic conductivity (Kh) values from slug tests, lithology of sediments at the piezometer screen, and 

the methods used to determine Kh values from slug test data.

[Kh, horizontal hydraulic conductivity; E, scientific notation denoting exponentiation; for example 1.78E+02 = 1.78×102; Springer-Gelhar, Springer and Gelhar, 

1991; KGS, Kansas Geological Survey model, Hyder and others, 1994; Butler, Butler, 1998]

Piezometer

Mean Kh Minimum Kh Maximum Kh

Fallinghead testsRisinghead tests Lithology at well 

screen

Slug test analysis 

method

Feet per day Count

Litchfield field sites

LFO1–B 1.78E+02 1.05E+02 2.44E+02 3 3 Silty to coarse sand Springer-Gelhar

LFO1–C 1.57E−02 1.50E−02 1.63E−02 1 1 Till KGS

LFO1–D 3.40E−01 2.53E−01 4.27E−01 1 1 Till KGS

LFO1–E 8.88E−01 5.04E−01 1.55E+00 3 3 Till/sand and gravel KGS

LFO1–F 3.34E+02 2.78E+02 3.89E+02 2 2 Sand and gravel Butler

LFO2–A 1.10E−04 2.17E−05 1.99E−04 1 1 Till KGS

LFO2–B 5.52E−04 2.09E−04 8.95E−04 1 1 Till KGS

LFO2–C 1.34E−03 1.34E−03 1.34E−03 1 1 Till KGS

LFO2–D 1.22E−05 1.22E−05 1.22E−05 1 0 Till KGS

LFO2–E 1.95E−04 1.11E−04 2.80E−04 1 1 Till KGS

LFO2–F 3.06E+02 2.28E+02 3.74E+02 3 3 Sand and gravel Butler

Cromwell field sites

CWO1–A 2.84E−01 2.63E−01 3.06E−01 1 1 Till KGS

CWO1–B 2.31E+01 1.91E+01 2.81E+01 3 3 Sand and gravel Butler (1), KGS (5)

CWO1–C 3.49E−01 2.48E−01 5.62E−01 3 3 Slate KGS

CWO2–A 6.88E+00 4.71E+00 9.57E+00 3 3 Sand and gravel KGS

CWO2–B 6.21E−02 5.89E−02 6.54E−02 1 1 Till KGS

CWO2–C 1.12E−01 1.02E−01 1.23E−01 1 1 Till KGS

CWO2–D 8.97E−03 6.15E−03 1.18E−02 1 1 Till KGS

CWO2–E 3.60E−02 3.55E−02 3.65E−02 1 1 Till KGS

Hydrogeology field camp field sites

MW–01 2.49E+01 1.94E+01 3.15E+01 3 3 Sand and gravel Springer-Gelhar

HT–115 3.64E−01 3.37E−01 3.77E−01 3 3 Till KGS

HT–140 4.16E−04 2.61E−04 5.06E−04 3 3 Till KGS

HT–175 2.78E−02 1.51E−02 6.45E−02 3 3 Till KGS

HT–200 1.36E−01 1.24E−01 1.54E−01 3 3 Till KGS

HB–3 7.27E+01 6.50E+01 8.03E+01 3 3 Sand and gravel Butler

Olivia field sites

OT–13 1.55E+00 1.12E+00 1.97E+00 3 3 Sand and gravel KGS

OT–20 1.09E−02 6.50E−03 1.46E−02 3 3 Till KGS

OT–35 6.07E−06 6.07E−06 6.07E−06 0 1 Till KGS

OT–60 1.38E−03 6.47E−04 2.27E−03 3 2 Till KGS

OT–105 2.11E−04 1.00E−04 3.41E−04 3 3 Till KGS

OT–145 1.71E−02 6.45E−04 3.32E−02 3 3 Till KGS

OT–175 2.09E−02 1.28E−02 3.02E−02 3 3 Till KGS

OB–7 3.07E+00 1.44E+00 4.17E+00 3 3 Sand and gravel KGS
34 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

others, 2005a, b). Only two piezometers were used to estimate 

the geometric mean Kh value of till at the LFO1 site. LFO1–E, 

which was intended to be screened solely in till, seems to be 

connected to the aquifer and was excluded from the geometric 

mean calculations. The large difference in mean Kh values 

between the two study sites in Litchfield was unexpected. 

The mean sand content in the till at the LFO1 site (47 percent) was lower than the mean sand content of the till at the 

LFO2 site, yet the Kh was two orders of magnitude higher at 

LFO1 compared to LFO2. The large difference between these 

sites could be due to differences in till deposition, a greater 

effect of till fractures on the hydraulic observations at the 

LFO1 site compared to the LFO2 site, or localized effects on 

the borehole wall from drilling operations (see “Sources of 

Uncertainty” section). The LFO2 site Kh values may be more 

indicative of the properties of the till matrix rather than fractures. Alternatively, installation procedures may have affected 

some piezometers more than others and may explain some of 

the large differences between these two nests (see “Sources of 

Uncertainty” section).

At the Olivia site, mean Kh values ranged from about 

3 ft/d for sand and gravel to a low of about 2×10−4 ft/d for till 

(table 6, fig. 11E). The geometric mean Kh of the slug tests in 

the till confining unit is 4×10−3 ft/d (table 5), which is higher 

than at the LFO2 site, despite the lower sand and higher 

clay content at the Olivia site compared to the LFO2 site. As 

previously discussed, the upper part of the till at the Olivia 

site (to a depth of 60 ft) is hydraulically connected, with some 

data indicating that fractures may be present. The till deposit 

at the Olivia site is older (pre-Illinoian) than the deposit at 

the Litchfield site and could therefore be more weathered 

and may explain, in part, the higher K values observed at the 

Olivia site. One extremely low Kh outlier, about 6×10−6 ft/d, 

observed at piezometer OT–35 (table 5), was not considered 

for the calculation of the geometric mean Kh or plotted on 

figure 11. This piezometer had only one “slug” test completed that lasted for 14 months. The well was purged after 

installation in August 2017, and the water levels had not 

fully recovered before the study’s data collection ended. It is 

hypothesized that the screen of this piezometer was affected 

by bentonite during the installation process; however, water 

samples were not collected to evaluate this hypothesis. An 

alternative hypothesis is that this till piezometer does not 

intersect any fractures and is a demonstration of a clayey till 

matrix response.

At the Cromwell site, mean Kh values ranged from about 

23 ft/d for sand and gravel to about 9×10−3 ft/d for till (table 6, 

fig. 11C). The geometric mean Kh value for all slug tests in 

the till confining unit is 0.06 ft/d (table 5). The till Kh values 

at this site were remarkably similar to the till Kh values at the 

LFO1 site, despite a 10-percent difference in sand content and 

6-percent difference in clay content (table 5).

At the HFC site, mean Kh values ranged from about 

73 ft/d in sand and gravel to about 4×10−4 ft/d in till for the 

slug test results (table 6, fig. 11D). The geometric mean Kh

value in the till confining unit is 3×10−2 ft/d (table 5), which 

again is similar to the LFO1 site despite a 20 percent higher 

sand content at the HFC site compared to the LFO1 site 

(table 5).

The material near the piezometer/well screen is the main 

material controlling the Kh value during a slug test (Cherry 

and others, 2004). Slug tests measure the horizontal hydrologic properties of a small (compared to aquifer tests) volume 

of till surrounding the sand pack, on the order of cubic meters 

(Bradbury and Muldoon, 1990). Studies have reported that 

a standard slug test has a sample volume of about 24 cubic 

meters for depths of 1–3.7 meters (Seo, 1996; Beckie and 

Harvey, 2002; Young and others, 2020). Thus, slug tests can be 

greatly affected by anything that affects the piezometer’s connection to the geologic material being measured. Some such 

factors are discussed in the “Sources of Uncertainty” section.

Indeed, site-averaged textural compositions and till geometric mean Kh values were not well correlated in this study. 

This indicates that, at the localized scale measured with slug 

tests, textural compositions were not the primary driving factor for estimates of till hydraulic properties; for example, the 

correlation coefficient of the logarithm of the geometric mean 

Kh and site-average percentage of sand (number of samples 

= 5 sites) is 0.27. If the mean Kh value of each individual 

piezometer is paired with the nearest textural composition data 

from cores (Wagner and Tipping, 2016; Staley and Nguyen, 

2018), the correlation coefficient between the logarithmic Kh

and percentage of sand does not substantially improve; the 

correlation coefficient is 0.29 across 21 till piezometers. For 

this comparison, textural samples were collected within 2 ft 

vertically of the well screen, on average.

To further illustrate that textural composition is not a 

good indicator of Kh estimates in till, Kh values were plotted 

on a soil texture trilinear diagram in figure 13A. The Kh of 

till piezometers varied substantially across the range of soil 

textural compositions measured in this study (fig. 13A); for 

example, sediments classified as loam had Kh estimates that 

varied by four orders of magnitude. Two samples that were 

classified as clay loam or silt loam had higher Kh values than 

several samples classified as loam despite having higher clay 

and silt contents (fig. 13A).

In contrast, during aquifer tests, hydraulic responses in a 

much larger volume of till, on the order of hundreds of cubic 

meters, are measured. The bulk Kv values determined from 

aquifer tests for each site correlate much more strongly with 

the site-averaged percentage of sand in the till. Across the five 

sites, the correlation coefficient between percentage of sand 

and the logarithm of the bulk Kv is 0.61. The bulk Kv values 

from aquifer tests are even more strongly correlated with 

the percentage of clay at each site with a correlation coefficient of −0.73. These correlations improve if the logarithm 

of time-delay bulk Kv at the Olivia site (0.0005 ft/d) is used 

rather than the leaky confined aquifer bulk Kv at the Olivia 

site (0.0012 ft/d). These two correlations indicate that bulk till 

textural composition is an important factor controlling leakage 

through till to underlying aquifers at the scale measured with 

aquifer tests.
Characterization of Glacial Till and Aquifer Systems 35

The vertical anisotropy, herein defined as the ratio of 

Kh to Kv, varies widely across the sites and the geometric 

mean Kh estimates are not well correlated with the bulk 

Kv estimates (correlation coefficient of 0.48). In modeling 

applications, anisotropy is commonly a specified parameter, 

and this study provides measured values of anisotropy in 

till. Confining units can have higher anisotropy compared to 

aquifers, with Kh possibly being higher because of stratification or Kv possibly being higher because of fractures (Cherry 

and others, 2004). Overall, the vertical anisotropy ranges by 

four orders of magnitude from 0.05 at the Cromwell site to 70 

at the LFO1 site (table 5). In a single till unit at the Litchfield 

site, the vertical anisotropy varies by three orders of magnitude, from 0.20 to 70. Values of vertical anisotropy greater 

than 1 indicate tills that are more horizontally conductive than 

vertically conductive whereas vertical anisotropy values of 

less than 1 indicate the opposite. The textural composition is 

not well correlated with anisotropy. Only one site, the HFC 

site, had nearly equal Kv and Kh values, as indicated by its 

anisotropy value near 1.

Percent clay

10

20

30

40

50

60

70

80

90

Percent silt

10

20

30

40

50

60

70

80

90

90

80

70

60

50

40

30

20

10

Percent sand

Clay

Silty

clay

Silty clay

loam

Clay loam

Sandy

clay

Sandy clay

loam

Sandy loam

Sand sand

Loam

Loamy

Silt loam

Silt

EXPLANATION

Color of symbol represents the site

Shape of symbol represents the aquifer test response

Aquifer well

Till piezometer with drawdown observed during an aquifer test

Deepest till piezometer at a site with no drawdown observed 

during an aquifer test 

Shallower (than the deepest till piezometer) till piezometer 

with no drawdown response during an aquifer test

A

B

Litchfield 1 (LF01)

Litchfield 2 (LF02)

Olivia

Cromwell (CW01/02)

Hydrogeology field camp (HFC)

Percent clay

10

20

30

40

50

60

70

80

90

Percent silt

10

20

30

40

50

60

70

80

90

90

80

70

60

50

40

30

20

10

Percent sand

Clay

Silty

clay

Silty clay

loam

Clay loam

Sandy

clay

Sandy clay

loam

Sandy loam

Loamy

Sand sand

Loam

Silt loam

Silt

Size of symbol corresponds with logarithmic (log) hydraulic conductivity, 

in feet per day 

Log horizontal hydraulic conductivity (Kh

) = −4.9 (0.00001 foot per day)

Log horizontal hydraulic conductivity (Kh

) = −1.5 (0.02 foot per day) 

Log horizontal hydraulic conductivity (Kh) = 1.9 (80 feet per day) 

o o

o

o

o o

o

o

ooo

o

o

oo

oo

o

o

o

o

o

oo

o

o

o

Figure 13. Trilinear plots showing the relation between sand, silt, and clay contents near the screened interval of piezometers and 

(A) horizontal hydraulic conductivity (Kh) estimates; (B) the presence or absence of an aquifer test drawdown response.
36 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Although K values provide information about a formation’s ability to transmit water, evidence of hydraulic connections during periods of stress provide information about actual 

flow paths that may behave differently than those predicted 

from estimated K values. Qualitatively, the importance of till 

textural composition to hydraulic connections between till and 

underlying aquifers is shown in figure 13B; connections that 

only become apparent when a large pumping stress is present 

in the system. The aquifer tests completed for this study were 

a much more prolonged, continuous pumping stress compared 

to the routine pumping by municipalities. The triangle symbols in this plot represent piezometers that did not demonstrate 

a drawdown response during the aquifer test whereas square 

symbols represent piezometers that did demonstrate a drawdown response. Almost every piezometer screened in till with 

at least 55-percent sand and less than 15-percent clay had a 

drawdown response. Conversely, almost every piezometer 

with less than 55-percent sand and more than 15-percent clay 

did not demonstrate a drawdown response. There are a couple 

of outliers to this general classification, but overall, there is a 

consistent separation among these tills as to which materials 

demonstrate hydraulic connectivity during pumping stress.

Leakage (Recharge) through Tills

Calculations of travel time through the till, specific discharge, and leakage from till according to Darcy’s Law vary 

by three to four orders of magnitude across the sites (table 7). 

At three of four sites, a downward hydraulic gradient through 

the till was observed, so the calculations represent the downward flux of water through till into the confined aquifer. At 

the Cromwell site, there was an upward hydraulic gradient, so 

the calculations represent the upward flux of water from the 

confined aquifer through the till to the surficial aquifer.

Specific discharge is given in inches and represents 

leakage from till as volume divided by a cross-sectional 

area (eq. 2). In most cases, the calculated specific discharge 

is constrained by the properties of the till (table 7). In other 

words, specific discharge from the till is less than or equal 

to precipitation-driven annual groundwater recharge to the 

water table aquifer, as estimated by Smith and Westenbroek 

(2015). Two specific discharge rates, 176 in/yr at LFO1 and 

86 in/yr at the Cromwell site, are substantially greater than 

the precipitation-driven groundwater recharge to the water 

table aquifer (table 7). In these cases, the specific discharge 

across the till/confined aquifer boundary cannot be realized in 

a simple one-dimensional flow system where the only input 

to groundwater is diffuse recharge from precipitation directly 

above the vertical profile of interest. Lateral groundwater flow 

could supply additional water such that these fluxes could 

be realized. In fact, this is likely occurring at the Cromwell 

site. At the Cromwell site, the hydraulic gradient is slightly 

upward, and one hypothesis is that the groundwater is flowing laterally from a distant recharge location to the confined 

aquifer at the Cromwell site (Witt, 2017).

At all sites except for the HFC site, mean linear velocities and travel times vary widely between calculations using 

the geometric mean Kh from slug tests and the Kv from aquifer 

tests. The two types of calculations (Kh and Kv) require slightly 

different assumptions and represent different volumes of till 

and water. Calculations of travel time that use the geometric 

mean Kh assume isotropy between Kh and Kv. Calculations of 

travel time that use the bulk Kv assume that the hydraulic gradient and till thicknesses determined at each piezometer nest 

are representative of a large till volume. 

When the geometric mean Kh from slug tests is used 

to calculate travels times through till, the resulting travel 

times vary from 1 year to more than 900 years (table 7). 

Interestingly, these extremes were observed at LFO1 and 

LFO2, respectively, which are one-half mi apart. The geometric mean Kh calculations represent localized flow conditions, 

and the wide range of travel times at the Litchfield site demonstrates two different vertical flow paths through which water 

travels to the confined aquifer. 

When the bulk Kv from aquifer tests is used to calculate 

travel times through till at all sites, estimated travel times vary 

over a much larger range, from 4 to nearly 1,800 years. The 

minimum estimated travel time occurs at the Cromwell site 

and the maximum occurs at the Olivia site. For the Litchfield 

sites, calculations with the bulk Kv produce far different travel 

times compared to calculations with the geometric mean Kh. 

Using the bulk Kv, the estimated travel time through till at 

LFO1 is 75 years and at LFO2 is 165 years. The same Kv

value was used for both nests, and the difference in travel 

time estimates results from different till thicknesses (60 ft at 

LFO1, 115 ft at LFO2) and hydraulic gradients (0.56 at LFO1 

and 0.48 at LFO2). The Olivia and Cromwell sites also have 

a large difference in travel time estimates from the bulk Kv

compared to the geometric mean Kh.

The Olivia site has two representative bulk Kv values, one 

from a traditional leaky confined aquifer approach and one 

from a time-delay analysis (tables 2, 5). Both of these methods 

have limitations, but given the complicated RWF responses 

observed at the site, both Kv estimates are considered reasonable. Varying the bulk Kv by only one order of magnitude has 

implications for estimating the travel time through the till 

at the Olivia site. The leaky confined aquifer method Kv is 

0.001 ft/d, which when used in Darcy calculations, produces a 

travel time through the till of 738 years. The time-delay analysis Kv is 0.0005 ft/d, which produces a travel time through till 

of 1,772 years (table 7).

A comparison between the areal extent of till/aquifer surface required to meet the pumping rates of the high-capacity 

wells (table 7) and the mapping of confined aquifers reveals 

uncertainty about aquifer geometry and (or) the source of 

water to wells. The areal estimates assume (1) the only source 

of water to the confined aquifer is leakage from overlying 

till and (2) the bulk Kv from aquifer tests is a representative K for the leakage that occurs in response to pumping for 

water supply.
Table 7. Hydraulic characteristics of till, annual pumping rates, and estimates of vertical travel time and water flux through 1 square mile of till basestudy site.[i, mean hydraulic gradient; ft, foot; in/yr, inch per year; ft/d, foot per day; q, specific discharge (leakage); Q, recharge (leakage) from till; gal/yr, gallon per year; mi2, square milductivity; E, scientific notation denoting exponentiation; for example 3.1E+09 = 3.1×109; --, no data or not applicable; Kv, vertical hydraulic conductivity]

Site name i (dimensionless) Till thickness (ft) Potential groundwater recharge (in/yr)a Hydraulic conductivity source Hydraulic conductivity (ft/d) Mean linear velocity (ft/d) Travel time through till (years) q (in/yr) Q (gal/yr) 

Litchfield 1 

(LFO1)

0.55 downward 60 4 to 8 Slug test geometric mean 

Kh

0.07 0.2 1 176 3.1E+09 

Aquifer test Kv (Blum and 

Woodside, 2017)

0.001 0.002 74 2.4 4.2E+07 

Litchfield 2 

(LFO2)

0.48 downward 115 4 to 8 Slug test geometric mean 

Kh

0.0002 0.0003 913 0.4 6.6E+06 

Aquifer test Kv (Blum and 

Woodside, 2017)

0.001 0.002 165 2.1 3.6E+07 

Cromwell 

(CWO1/O2)

0.02 upward 120 4 to 8 Slug test geometric mean 

Kh

0.06 0.004 80 4.5 7.8E+07 

Aquifer test Kv (Lund and 

Blum, 2017)

1.10 0.08 4 86 1.5E+09 

Hydrogeology 

field camp (HFC)

0.04 downward 100 4 to 8 Slug test geometric mean 

Kh

0.03 0.005 57 5.3 9.2E+07 

Aquifer test Kv (Blum, 

2019a)

0.03 0.005 50 6.0 1.0E+08 

Olivia 0.13 downward 166 2 to 6 Slug test geometric mean 

Kh

0.004 0.002 217 2.3 4.0E+07 

Aquifer test Kv (Blum, 

2019b)

0.001 0.0006 738 0.7 1.2E+07 

Aquifer test Kv, time-delay 

analysis Blum, 2020)

0.0005 0.000257 1,772 0.3 4.9E+06 

aSmith and Westenbroek, 2015.
38 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

According to the Darcy’s law calculations, and assuming that overlying till contributes all the water to the aquifer, 

Litchfield’s groundwater pumping requires between 8 and 

9 mi2 of till to meet the pumping demands and Olivia’s pumping requires 6 to 13 mi2 (table 7). These areal extents are larger 

than the areal extents of the aquifers as mapped in geologic 

maps based on well and borehole logs. The Renville County 

Geologic Atlas shows that the areal extent of the confined 

aquifer at Oliva is only 0.08 mi2 (Bradt, 2017), and the Meeker 

County Geologic Atlas shows an approximate areal extent of 

the aquifer at Litchfield of 3 mi2 (Meyer, 2015). The Olivia 

aquifer test indicated an aquifer boundary within 350 ft of the 

production well, and well records indicate that the aquifer is 

most likely a buried alluvial channel with a complex shape 

(Blum, 2019b). The glacial deposits at Litchfield are a complex mixture of till layers and sand bodies with many possible 

lateral connections between buried sand bodies (Meyer, 2015).

The source of water to the confined aquifers is much 

more complex than the simple conceptual system discussed 

here of one-dimensional flow from land surface through till 

to a confined aquifer. Uncertainty of the distribution of till 

hydraulic properties, the extent of confined aquifers, and the 

connections between aquifers make evaluations of the sustainability of groundwater pumping from confined aquifers challenging. This uncertainty about the structure of the subsurface 

provided the impetus for developing the heuristic models, 

rather than site-specific models for this study. In a later section of this report, heuristic MODFLOW models are used to 

evaluate the flux of water into and through till in a variety of 

hydrogeologic settings and pumping rates representative of the 

field sites.

Groundwater Geochemistry and Water Quality

Several laboratories provided analytical services for this 

study. The data from each laboratory were acceptable for the 

purposes of our study, unless otherwise noted in the text of 

this report. A summary of the quality assurance evaluation 

for these laboratories is provided in appendix tables 3.1 and 

3.2 and in the metadata of the data release accompanying this 

report (Maher and others, 2020). Included in table 3.1 is information on replicates and blanks compiled during laboratory 

analyses, and included in table 3.2 are comparisons between 

split samples of analytes analyzed at two laboratories. All 

the geochemical data are available through the USGS NWIS 

database (U.S. Geological Survey, 2019) and the data release 

accompanying this report; guidance for accessing the data in 

NWIS is also provided in the data release (Maher and others, 2020).

Background Information

Background information and context for interpreting the 

geochemical constituents evaluated in this study are presented 

in this section. Information is included for stable isotopes of 

water (18O and 2H), enriched tritium, chloride, chloride to bromide ratios, nitrate, phosphorus, oxidation-reduction (redox) 

conditions, and major ions.

During the Wisconsinan glaciation, glacial ice locked up 

a large part of the 16O and H from precipitation in the northern 

hemisphere, thus leaving most of the 18O and 2H in the oceans, 

where it became enriched in those isotopes. Till deposited by 

that ice under a cold climate may retain some of that isotopic 

signature, manifested by δ18O values approaching −30 per 

mil (‰) and δ2H values approaching −200 ‰ (Remenda and 

others, 1994). Unfractured, thick, and unweathered confining 

units in North America have been determined to hold glacialage groundwater when the residence time of the groundwater 

is long enough (Remenda and others, 1994).

Enriched tritium was released into the atmosphere during 

the hydrogen bomb testing in the 1950s and 1960s. Today, 

it is used as an indicator of relative groundwater “age.” 

Groundwater age is an indication of when water in the groundwater system was last exposed to the atmosphere. If there 

are detectable levels of tritium (greater than 0.8 tritium unit 

[TU]), then at least some of the water in the sample is considered to have been in the atmosphere “postbomb” and likely 

reached the groundwater system sometime after the 1950s. 

Samples with detectable concentrations could be a mixture 

of old, prebomb water and postbomb water, but at least some 

postbomb water is present. If there is no detectable tritium, 

then the sample water is considered to be “prebomb” and the 

water likely reached the groundwater system before the 1950s. 

For this report, samples with detectable tritium are considered 

to have “modern,” post-1950s water, and samples without 

detectable tritium are considered to be “old,” pre-1950s water. 

The special case of high tritium concentrations (greater than 

15 TU) is also considered because groundwater tritium peaks 

related to the bomb peak that occurred in the mid-1960s have 

a tritium concentration of 15 TU or greater (Berg, 2019).

Chloride concentrations can be naturally occurring in 

groundwater or affected by anthropogenic activities. Salt 

(sodium chloride) can occur naturally in groundwater from 

the presence of halite deposits, weathering of bedrock, briny 

water, seawater, surficial materials, soils, and volcanic activity 

(Mullaney and others, 2009). In the United States, salt use has 

been increasing, mainly because of deicing activities, since 

the 1950s (Mullaney and others, 2009). Minnesota is known 

to have an influx of chloride from road salt contamination 

or wastewater correlated with urban land use (Kroening and 

Ferrey, 2013). Anthropogenic salt contamination in groundwater can come from sources other than deicing, including landfills that have food waste and products containing salt, water 

softeners, septic systems, household use, and agricultural use 

(Mullaney and others, 2009). Agricultural salt groundwater 

contamination can come from animal feed, fertilizers, and 

pesticides. Some fertilizers have potassium chloride, which 

raises chloride levels but not sodium levels in water resources 

(Mullaney and others, 2009).

In a survey of sand and gravel aquifers in Minnesota, 

chloride concentrations in groundwater were generally less 
Characterization of Glacial Till and Aquifer Systems 39

than 25 milligrams per liter (mg/L) in rural settings and greater 

than 25 mg/L in urban settings, with some urban groundwater having chloride concentrations greater than 250 mg/L 

(Kroening and Ferrey, 2013). Even though groundwater in 

rural settings generally had low chloride concentrations, 

anthropogenically sourced chloride was commonly present, 

making establishment of accurate background (nonanthropogenic) chloride concentration challenging (Kroening and 

Ferrey, 2013). Background chloride levels in Quaternary 

sediments in Canada and Illinois are generally between 15 

and 20 mg/L (Howard and Beck, 1993) and between 1 and 

15 mg/L (Kelly and others, 2012), respectively. Some aquifers 

contained in Cretaceous-age formations in southwestern and 

south-central Minnesota are known to have naturally high 

concentrations of chloride, as much as 1,500 mg/L (Kroening 

and Ferrey, 2013). Upward flux of groundwater from aquifers 

in older formations, such as the Red River-Winnipeg aquifer 

contained in the Ordovician-age Red River and Winnipeg 

Formations in northwestern Minnesota, has been known to 

increase salinity in groundwater in the overlying sediments 

(Ruhl and Adolphson, 1986), though this type of flow system 

is not represented at the piezometer nests of this study.

Chloride and bromide concentrations and ratios have 

been used by many scientists to evaluate the presence of 

groundwater contamination, such as from septic tanks and 

road salt (for example, Katz and others, 2011). The anions 

chloride and bromide are conservative tracers in water because 

neither anion has substantial ion exchange reactions at low 

temperatures, both are soluble, both are not likely to be 

adsorbed to mineral surfaces, and they only form minerals during extreme evaporation (Alcalá and Custodio, 2008).

A broad range of thresholds of chloride concentrations 

and chloride/bromide ratios have been used to classify samples 

as containing anthropogenically sourced chloride or not. In 

a national study that examined thousands of wells, groundwater samples with a chloride concentration between 20 and 

100 mg/L and a chloride/bromide ratio between 400 and 1,100 

were statistically more likely to contain other contaminants 

indicative of septic discharge; thus, chloride and chloride/

bromide were determined to be a useful screening tool for 

identifying wells affected by septic systems (Katz and others, 

2011). The Katz and others (2011) study was solely focused 

on screening for septic contamination rather than the effect of 

other sources such as road salt and fertilizers. In a statewide 

evaluation of chloride in groundwater in Minnesota, Kroening 

and Ferrey (2013) concluded that groundwater with a chloride 

concentration of less than 7 mg/L and a chloride/bromide ratio 

less than 200 was unaffected by anthropogenically sourced 

chloride, groundwater with a chloride concentration between 

7 and 30 mg/L and a chloride/bromide ratio between 200 and 

1,000 was “intermediate,” and groundwater with a chloride 

concentration greater than 30 mg/L and a chloride/bromide 

ratio greater than 1,000 was definitely affected by anthropogenically sourced chloride. The term “intermediate” was used 

ambiguously and could indicate a mixed sample that contained 

some anthropogenically sourced chloride, or it could indicate 

samples that were difficult to classify as affected by anthropogenically sourced chloride. A report by Berg (2018, p. 23) for a 

northwestern Minnesota county (Clay County) states “chloride 

is elevated if concentrations are greater than or equal to 5 [parts 

per million] ppm. It is anthropogenic if chloride/bromide ratios 

are greater than 250.” In Renville County, Minn. (the county 

containing the Olivia site), samples with a chloride concentration greater than 5 mg/L, a bromide concentration greater than 

0.07 mg/L and a chloride/bromide ratio greater than 200 were 

used to classify samples as having anthropogenically sourced 

chloride (Bradt, 2017). At low bromide concentrations, defined 

as three times the minimum reporting limit, the chloride/bromide ratio becomes a much less reliable indicator of anthropogenically sourced chloride (Brian G. Katz, Environmental 

consultant and former USGS research hydrologist, written 

commun., July 2, 2020).

The chloride concentration and the chloride/bromide ratio 

are simply screening metrics to identify locations potentially 

affected by anthropogenic inputs. It is clear from the diversity 

of classification schemes discussed previously that there is 

plenty of room for interpreting chloride and chloride/bromide 

ratio data. The data are best used in conjunction with other 

chemical data to determine the presence of anthropogenic 

contamination at a given site. For this study, the classification scheme of chloride greater than 5 mg/L and a chloride/

bromide ratio of 250 proposed by Berg (2018) was used to 

classify samples. Samples with concentrations less than these 

criteria were considered to not have anthropogenically sourced 

chloride. Samples with concentrations slightly greater than 

these criteria were classified as “intermediate,” in that they 

may or may not have anthropogenically sourced chloride. 

Samples with values greater than these criteria, but with 

concentrations of bromide less than 0.03 mg/L (three times the 

reporting limit of bromide) were also classified as not having 

anthropogenically sourced chloride. Concentrations of more 

analytes indicative of anthropogenic contamination would 

be beneficial to improve the classification of the samples as 

affected by anthropogenic chemicals.

Nitrogen fertilizers are the primary cause of increasing 

nitrate concentrations in groundwater throughout the United 

States (Spalding and Exner, 1993; Sebilo and others, 2013). Other 

anthropogenic sources of increased nitrate include waste from 

animals and contaminated rainfall because of the combustion of 

fossil fuels (Kroening and Ferrey, 2013). Nitrate concentrations in 

groundwater are 1.1 mg/L or less in areas of Minnesota without 

nitrate from anthropogenic activities (Kroening and Ferrey, 2013). 

In Minnesota, the aquifers most affected by nitrate contamination 

are shallow sand and gravel aquifers underlying agricultural areas 

(Kroening and Ferrey, 2013).

Phosphorus concentrations in Minnesota groundwater 

have been determined to be more strongly related to geology, rather than land use (Kroening and Ferrey, 2013). Rocks 

and sediments that contain phosphorus-bearing minerals can 

weather and contribute phosphorus to groundwater (Kroening 

and Ferrey, 2013). Shale is one such rock, and sand and gravel 

aquifers in Minnesota deposited by glacial lobes sourced 
40 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

from the west/northwest tend to have higher phosphorus 

concentrations because of the presence of carbonates and 

shale, compared to aquifer material deposited by northeast 

glacial lobes (Kroening and Ferrey, 2013). Anthropogenically 

sourced phosphorus in groundwater is usually from fertilizer 

use or waste (Minnesota Pollution Control Agency, 1999). 

Concentrations of phosphorus can also increase with increasing residence time, which may be associated with elevated 

iron and manganese (Minnesota Pollution Control Agency, 

1999). Groundwater with low redox potentials can result in the 

dissociation of iron-phosphorus minerals, releasing adsorbed 

phosphorus (Burkart and others, 2004). The median phosphorus concentration for buried aquifers in Quaternary-age deposits in Minnesota is 0.124 mg/L (Minnesota Pollution Control 

Agency, 1999).

The oxidation-reduction (redox) state of groundwater is an important factor in determining the presence of 

harmful constituents in groundwater (McMahon and others, 2011). Some common byproducts of reducing environments are manganese, ferrous iron, hydrogen sulfide, and 

methane (McMahon and others, 2009). Zones can form in 

the subsurface where one electron-accepting process can 

dominate. Zones commonly form in subsurface recharge 

areas of aquifers. Near the point of recharge, at the beginning 

of a groundwater-flow path, DO reduction is the dominant 

process, followed by a zone of nitrate reduction below, 

followed by a zone of ferric iron reduction, and then the 

deepest zone is usually one of sulfate reduction (McMahon 

and others, 2011). These zones follow flow paths, and commonly, increasing age in groundwater systems coincides with 

increased reducing conditions (McMahon and others, 2011). 

Under reducing conditions, nitrate is typically not of concern 

because it is readily reduced, but other constituents become 

problematic for human health. The most common contaminants are geogenic; that is, they are “naturally” occurring in 

the system and are not introduced from anthropogenic activities. For example, under reducing conditions, phosphorus 

concentration can increase because of dissociation of ironphosphorus minerals (Burkart and others, 2004), arsenic can 

be released, and certain metals can become soluble (McMahon and others, 2011).

Characterizing the major ion geochemistry of groundwater provides information about how groundwater geochemical 

composition evolves with residence time, through contact 

with different geologic units, and effects from anthropogenic 

activities. Major ion chemistry is useful for defining hydrochemical facies (Blanchette and others, 2010). Previous 

research has indicated cation exchange of sodium on sediments with calcium and magnesium along the direction of 

groundwater flow, leading to more calcium- and magnesiumdominated groundwater in water near the beginning of a flow 

path and more sodium-dominated groundwater associated 

with older water that is farther along a flow path (Hendry and 

Schwartz, 1990). 

Groundwater Age and Evidence for Infiltration of 

Anthropogenic Chemicals

Stable isotopes of oxygen and hydrogen indicated no 

evidence of glacial-age groundwater greater than 11,000 years 

old in the till confining units at any site. This aligns with the 

range of travel times predicted from the Darcy calculations 

using either geometric mean Kh or Kv, which ranged from 1 

to nearly 1,800 years across the sites. Values of δ18O ranged 

from −7.4 to −12.0 ‰, and values of δ2H ranged from −84.1 

to −55.7 ‰ (fig. 14), both of which are well outside the range 

of glacial-aged water. All but one sample fell along the global 

meteoric water line (fig. 14). The two northern sites, HFC 

and Cromwell, generally had lower δ18O and δ2H values than 

the two southern sites, Olivia and Litchfield. This pattern is 

expected because fractionation increases with distance from the 

Gulf of Mexico, causing δ18O and δ2H values to decrease with 

increasing latitude. The only sample that plots distant from the 

global meteoric water line (Craig, 1961) is from a surficial aquifer well, MW–01, at the HFC site. This sample is most likely 

an evaporative signal, indicating that surface water from nearby 

lakes is recharging the surficial aquifer (Palmer and others, 

2007). Stable isotope values from pore water are consistent with 

the groundwater samples from piezometers (fig. 14). These data 

indicate that the groundwater values mostly reflect what is in the 

till rather than an artifact left from the drilling process.

Enriched tritium indicates differences in travel times 

through till among the sites that generally corroborate travel 

time estimates from Darcy’s law. Tritium data indicate faster 

fluxes of groundwater through till at the HFC and LFO1 sites 

compared to the Olivia and LFO2 sites (fig. 15). Tritium 

values greater than 1 TU were detected in all till piezometers 

at the HFC and LFO1 sites, indicating the presence of modern 

water throughout these till profiles. Both sites have a tritium 

peak within the till confining units. The tritium peak is near 

the bottom of the till at the LFO1 site, whereas at the HFC 

site, the tritium peak is near the top of the till unit. The peak 

value at the LFO1 site was 16.1 TU, and the peak at the HFC 

site was 21.1 TU, both of which fall in the range of a mid1960s bomb peak tritium value.

At the LFO1 site, assuming piston flow and the tritium 

peak at LFO1–E represents water that reached the surficial 

aquifer in 1966, groundwater took 50 years (1966–2016) to 

travel from the water table surface to the deepest till piezometer (LFO1–E). This is within the range of travel times (1 to 

74 years) estimated with hydraulic data according to Darcy’s 

law (table 7). At the LFO1 site, travel time estimates made 

using tritium data are expected to be toward the high side of 

the Darcy calculations because the tritium travel time estimate 

includes vertical travel through a 30-ft thickness of surficial aquifer in addition to the till profile, whereas the Darcy 

calculations in table 7 only include travel through the till 

profile. Assuming the distance traveled is from the mean water 

table surface (11.20 ft below land surface, measured in the 
Characterization of Glacial Till and Aquifer Systems 41

shallowest well of the nest, LFO1–B, table 1) to the midpoint 

of the LFO1–E screen (93.74 ft below land surface, table 1), 

the mean linear velocity according to the tritium data is 

0.0045 ft/d. This is within the range of mean linear velocities of 

0.002 to 0.2 ft/d computed according to Darcy’s law (table 7). 

At the HFC site, assuming that the tritium peak at 

HT–115 represents water that reached the surficial aquifer in 

1967, the travel time from the water table surface through a 

53-ft thickness of saturated material, including 42 ft of surficial aquifer and 11 ft of till, was 50 years (1967–2017). This 

translates to a mean vertical linear velocity of 0.0029 ft/d. The 

Darcy’s law calculation indicates a faster vertical travel time 

through till: 50–56 years to travel a vertical distance of 100 ft 

with a mean vertical linear velocity of 0.005 ft/d.

At the LFO2 and Olivia sites, tritium concentrations of 

about 5–6 TU are only seen in the upper one-third of the till 

units (fig. 15B,E). At the LFO2 site, tritium was only detected 

in the two uppermost till piezometers, to about 35 ft into the 

till, and at the Olivia site, tritium was only detected in the 

uppermost till piezometer, at a depth of about 8 ft into the till; 

therefore, most of the till thickness at these sites contained 

nonmodern water (older than 1953). The Darcy’s law calculations of travel time through till at these sites are in the hundreds of years, so in a general sense, the tritium data corroborate the Darcy’s law travel time calculations.

At the Cromwell site, tritium concentrations of around 

5 TU are present in the surficial aquifer and the confined aquifer, but not the till in between (fig. 15C). The hydraulic gradient data and the tritium data indicate that recharge to the confined aquifer enters the system somewhere upgradient in the 

same buried aquifer system or perhaps through a stratigraphic 

window in the overlying till confining unit where the hydraulic 

gradient in the till is downward. The till sequence observed 

near the production well may therefore have little direct effect 

on the quality and quantity of water at the Cromwell site. 

Rather, anthropogenic activities and geologic materials at a 

distal recharge area (yet to be defined) may affect the water 

observed in the confined aquifer at the Cromwell site.

−90

−85

−80

−75

−70

−65

−60

−55

−50

δ2H (per mil) Vienna Standard Mean Ocean Water

−13 −12 −11 −10 −9 −8 −7

δ18O (per mil) Vienna Standard Mean Ocean Water

δ18O

δ2

H

EXPLANATION

LFO2 groundwater LFO2 pore water

LFO1 groundwater LFO1 pore water

Olivia groundwater Olivia pore water

HFC groundwater HFC pore water

CWO1/O2 groundwater

Global meteoric water line (Craig, 1961)

Oxygen-18/oxygen-16 ratio

Hydrogen-2/hydrogen-1 ratio

Figure 14. Comparison of oxygen-18/oxygen-16 ratios (δ18O) and 

hydrogen-2/hydrogen-1 ratios (δ2H) with the global meteoric water 

line for groundwater and pore-water samples from the Litchfield 

1 (LFO1), Litchfield 2 (LFO2), Cromwell (CWO1/O2), Olivia, and 

hydrogeology field camp (HFC) sites.
42 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

 0

 50

100

150

200

250

Depth, in feet below land surface

E. Olivia

 0 5 10 15 20 25 0 20 40 60 80 100 0 500 1,000 1,500 2,000

Tritium, in

 tritium units

Chloride, in

 milligrams per liter

Chloride/bromide

mass ratio

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

Olivia-5

Olivia-4

 0

 50

100

150

200

250

D. HFC

Depth, in feet below land surface

 0 5 10 15 20 25 0 20 40 60 80 100 0 500 1,000 1,500 2,000

Tritium, in

 tritium units

Chloride, in

 milligrams per liter

Chloride/bromide

mass ratio

MW–01

HT–115

HT–140

HT–175

HT–200

HB–3

 0

 50

100

150

200

250

300

350

Depth, in feet below land surface

 0 5 10 15 20 25 0 20 40 60 80 100 0 500 1,000 1,500 2,000

Tritium, in

 tritium units

Chloride, in

 milligrams per liter

Chloride/bromide

mass ratio

C. CWO1/O2

CWO2–A

CWO2–B

CWO2–C

CWO2–D

CWO2–E

CWO1–A

CWO1–B

CWO1–C

 0

 20

 40

 60

 80

100

120

140

160

170 0 5 10 15 20 25 0 20 40 60 80 100 0 500 1,000 1,500 2,000

Tritium, in

 tritium units

Chloride, in

 milligrams per liter

Chloride/bromide

mass ratio

B. LFO2

Depth, in feet below land surface

LFO2–B

LFO2–A

LFO2–C

LFO2–D

LFO2–F

LFO2–E

 0

 50

100

150

 0 5 10 15 20 25 0 500 1,000 1,500 2,000

Tritium, in

 tritium units

Chloride, in

 milligrams per liter

Chloride/bromide

mass ratio

A. LFO1

Depth, in feet below land surface

 0 20 40 60 80 100 260 280 300

LFO1–B

LFO1–C

LFO1–D

LFO1–F

LFO1–E

EXPLANATION

Surficial aquifer

Confining unit (till)

Confined aquifer

Bedrock aquifer

Bedrock 

Hydrostratigraphy Plots

Axis break

Groundwater

Pore water

Chloride/bromide

mass ratio—

Groundwater

bromide below 

detection limit

Chloride/bromide

mass ratio—250 line 

(Berg, 2018)

Figure 15. Vertical profiles of tritium, chloride concentrations, and chloride/bromide mass ratios in groundwater and 

pore-water samples. A, from the Litchfield 1 (LFO1) site; B, from the Litchfield 2 (LFO2) site; C, from the Cromwell (CWO1/O2) 

site; D, from the hydrogeology field camp (HFC) site; E, from the Olivia site.
Characterization of Glacial Till and Aquifer Systems 43

Anthropogenically sourced chloride inputs are variable 

among the four study sites. Three of the sites, Litchfield, 

Cromwell, and Olivia, are in developed municipalities with 

high road densities and likely higher road salt applications 

compared to the HFC site, which is in the middle of a large 

forested area with few roads. Furthermore, the land surrounding the towns of Litchfield and Olivia is predominantly 

row-crop agriculture, which likely receives additional fertilizer 

inputs containing chloride.

Chloride concentrations, as well as chloride/bromide 

mass ratios in the groundwater at the Litchfield, Cromwell, 

and Olivia sites, indicate some anthropogenically sourced 

chloride is present in the till profiles, though the depth of penetration varies by site. A chloride/bromide ratio greater than 

250 is considered to be an indicator of possible anthropogenically sourced chloride for this discussion (Berg, 2018). Except 

for the LFO1 site, all pore-water chloride/bromide mass ratios 

indicate no anthropogenically sourced chloride. At the LFO1 

site, groundwater and pore-water chloride/bromide ratios are 

greater than or near 250 (fig. 15A).

Chloride concentration profiles at the LFO1 and LFO2 

sites were different (fig. 15A,B). At the LFO1 site, pore-water 

and groundwater chloride concentrations in the till unit were 

all around 20–40 mg/L, except for one pore-water outlier 

that was near 300 mg/L. Most pore-water chloride concentrations are higher than groundwater chloride concentrations. 

Groundwater from the confined aquifer has stronger indicators of anthropogenically sourced chloride compared to the 

values observed in the till. The groundwater chloride/bromide 

mass ratios are between 250 and 261 in the till unit whereas in 

the confined aquifer, the groundwater chloride/bromide mass 

ratio is 336 and the chloride concentration is 47.07 mg/L. At 

LFO2, the highest concentration of chloride in groundwater, 

about 40 mg/L, was in the shallowest piezometer, and then 

chloride decreases in groundwater through the till unit. Pore 

water has concentrations that range from 20 to slightly greater 

than 40 mg/L in the till unit. In contrast to the LFO1 site, most 

pore-water concentrations and chloride/bromide mass ratios 

at LFO2 are lower than groundwater concentrations. Similar 

to the LFO1 site, the chloride concentration in the confined 

aquifer at the LFO2 site was greater than the concentration in 

the deepest till piezometer just above the confined aquifer. The 

chloride/bromide mass ratio in the confined aquifer at LFO2 

was 278, higher than any ratio observed in the till at LFO2. At 

both nests, LFO1 and LFO2, the confined aquifer had stronger evidence for anthropogenically sourced chloride than the 

lower portion of the till, a possible indication of a chloride 

source reaching the confined aquifer in an area not represented 

by LFO1 or LFO2. In the confined aquifer, groundwater flow 

will primarily be horizontal toward the production wells. The 

water sampled from the confined aquifer piezometers may be 

representative of a different, perhaps larger, area than the till 

directly above and may explain why confined aquifer chloride 

concentrations are higher than till concentrations.

At the CWO1/O2 site, which has an overall upward 

hydraulic gradient through the vertical transect, groundwater 

chloride concentrations ranged from 1 to 45 mg/L and 

decreased with depth to near background values in the till 

and confined aquifer (fig. 15D). Chloride/bromide mass ratios 

are near or below 250 in the till and confined aquifer, but the 

surficial unit demonstrates a chloride/bromide mass ratio nearing 2,000, which is a strong indication of anthropogenically 

sourced chloride (fig. 15D). Because of the upward gradient 

at this well nest site, the chloride in the surficial aquifer at the 

location of the CWO1/O2 nest is not likely to reach the confined aquifer, though chloride could reach the confined aquifer 

where recharge to that aquifer occurs.

At the HFC site, the pore-water samples had higher 

chloride concentrations than groundwater samples. The chloride concentrations ranged from 0.46 to 50 mg/L (fig. 15D). 

All groundwater samples had chloride concentrations less 

than 5 mg/L, whereas chloride concentrations in pore-water 

samples were all greater than 20 mg/L. The pore-water 

chloride concentrations at the HFC site are comparable to 

pore-water chloride concentrations at other sites; however, 

the groundwater chloride concentrations at the HFC site are 

consistently lower than the chloride concentrations at the 

other sites. The groundwater concentrations of chloride at 

several piezometers at the CWO1/O2 site are also low among 

the sites, indicating that background chloride tends to be 

lower in sandier tills.

The Olivia site chloride concentrations in groundwater 

and pore water were fairly consistent throughout the till, with 

the exception of samples collected at about 20 ft below land 

surface. Groundwater and pore water concentrations were 

about double at this depth compared to all other till concentrations (fig. 15E). Chloride concentrations at this site ranged 

from 7 to 86 mg/L. Similar to the HFC and LFO1 sites, all 

pore-water samples at the Olivia site have higher chloride 

concentrations than the groundwater samples at similar depth 

intervals. Groundwater chloride concentrations were generally 

less than 20 mg/L whereas pore-water chloride concentrations 

were between 30 and 40 mg/L (fig. 15). Groundwater in the 

three deepest till piezometers at the Olivia site had the highest 

chloride/bromide mass ratios, ranging from 652 to 1,150, of 

all of the till piezometers at all the sites. The high ratios at 

depth were generally caused by extremely low or nondetectable bromide concentrations as opposed to high chloride 

concentrations. At low bromide concentrations, defined as 

three times the minimum reporting limit, the chloride/bromide ratio becomes a much less reliable indicator of anthropogenically sourced chloride (Brian G. Katz, Environmental 

consultant and former USGS research hydrologist, written 

commun., July 2, 2020). The minimum reporting limit at the 

USGS NWQL for bromide is 0.01 mg/L, so the chloride/

bromide ratio becomes less reliable at bromide concentrations of 0.03 mg/L or lower. In Renville County, chloride/bromide ratios were determined to be a less reliable indicator of 

anthropogenically sourced chloride at a bromide concentration 

of less than about 0.07 mg/L (Bradt, 2017); therefore the high 

chloride/bromide ratios at the Olivia site do not necessarily 

indicate anthropogenically sourced chloride.
44 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

One hypothesis for the low bromide concentrations at 

the Olivia site is bromide sorption to clay. To evaluate this 

hypothesis, bromide concentrations of pore water and groundwater were compared with clay content of colocated samples 

from detailed grain size analyses across all sites (Staley and 

others, 2018). No strong correlations between bromide and 

clay content were evident. Bromide can sorb to clays in certain 

circumstances. At some pH levels, bromide can adsorb to 

clay minerals such as kaolinite and montmorillonite as well 

as iron and aluminum oxides (Goldberg and Kabengi, 2010). 

The mineral adsorption of bromide can occur at a pH of as 

much as 8; however, there is minimal adsorption at a pH of 

greater than 7 (Goldberg and Kabengi, 2010). Almost all 

piezometers had groundwater with a pH greater than 7. Only 

two piezometers had a groundwater pH slightly lower than 7. 

These two piezometers were shallow (greater than 20 ft below 

land surface) and possibly affected by a more recent influx of 

precipitation. Given all of these observations, bromide adsorption was probably not an important factor for the low bromide 

concentrations observed in till at the Olivia site.

Differences in chloride concentrations between groundwater and pore water were observed at the sites where both types 

of water were sampled. At three sites (LFO1, HFC, and Oliva), 

pore-water chloride was generally greater than groundwater 

chloride and at one site (LFO2), the opposite was observed. 

The differences in chloride concentrations between pore water 

and groundwater may be due to several processes. One hypothesis is that the sampled water came from different sampling 

scales. The till cores collected and squeezed for pore water 

were 6 in. long and 4 in. in diameter and contained a water volume of about 0.31 liter (L; 0.08 gallon [gal]) assuming a porosity of 25 percent, whereas groundwater samples represented a 

5-ft screen and a borehole annulus diameter of 6.75 in., which 

contained about 8.8 L (2.3 gal) of water. Thus, the chloride 

concentration of pore water represented a much smaller sample 

volume compared to groundwater. A second hypothesis is that 

evaporation or other effects from sample handling procedures 

could have affected the chloride concentration of pore-water 

samples differently than groundwater samples because the two 

sample types were handled differently. Cores were wrapped 

in plastic and shipped to a laboratory where pore water was 

extracted weeks to months after sample collection. The pore 

water was then shipped to a second laboratory for analysis. 

Groundwater samples were collected in bottles, sealed immediately, and then shipped to a laboratory for analysis. The δ18O 

and δ2H values did not indicate an evaporation effect in the 

pore water from the till core samples. A third hypothesis is that 

drilling operations introduced groundwater with chloride concentrations different than ambient conditions to the formation. 

The pore-water samples would have been minimally affected 

by drilling fluids because the cores were collected ahead of the 

fluid injection point. The groundwater samples were collected 

after completion of drilling, and so could have been much more 

affected by the water used during the drilling process. This 

hypothesis cannot be fully evaluated because the water used for 

drilling operations at each site was not sampled.

Dissolved Nitrate, Dissolved Phosphorus, and 

Oxidation-Reduction (Redox) Conditions in 

Groundwater

Nitrate was detected infrequently and at low concentrations in groundwater samples across all study sites. The 

highest concentration of 1.78 mg/L as nitrogen was measured in the shallowest piezometer at the Cromwell site. 

This is the only sample with a concentration higher than 

the statewide ambient nitrate concentration of 1.1 mg/L 

(Kroening and Ferrey, 2013). The shallowest piezometer at 

the Olivia site had a concentration of 1.06 mg/L as nitrogen, 

and all other samples had nitrate concentrations less than 

0.4 mg/L as nitrogen. The HFC site had no detectable nitrate 

(less than 0.01 mg/L as nitrogen) present in any groundwater samples (fig. 16D). The low nitrate concentrations were 

somewhat surprising, especially at the Litchfield and Olivia 

sites where the broader landscape is dominated by row-crop 

agriculture. In Iowa, for example, nitrate concentrations of 

greater than 10 mg/L as nitrogen have been measured in 

confined aquifers (Eidem and others, 1999; Rodvang and 

Simpkins, 2001).

Given the low concentrations of nitrate at all sampling 

locations, it is difficult to evaluate if denitrification is occurring along the vertical till profiles at the study sites. If, for 

example, anthropogenically sourced chloride was present 

through a vertical profile and nitrate concentrations decreased 

with depth in the same profile, this would be indirect evidence 

that denitrification could be occurring, particularly if redox 

conditions in the profile are reducing. At the Cromwell site, 

the highest nitrate concentration (1.78 mg/L as nitrogen) is 

coincident with the highest chloride concentration in the shallowest piezometer (CWO2–A). Below this depth, chloride 

and nitrate concentrations are lower. Because the conservative 

analyte concentration (chloride) decreases as well as the reactive analyte concentration (nitrate), this indicates there could 

be hydrologic reasons (for example, dilution) for lower nitrate 

concentrations at depth at the Cromwell site rather than denitrification. There is slight indirect evidence for denitrification 

occurring in till at the Olivia site. The only detection of nitrate 

occurred in the shallowest piezometer, OT–13. The highest 

chloride concentration occurred in the next deepest piezometer, OT–20, and this concentration is marginally indicative of 

anthropogenically sourced chloride. DO, iron, and manganese 

indicate reducing conditions that would support denitrification at the depth of OT–20 (about 20 ft below land surface; 

fig. 17E).

Though not observed in this study, other studies of the 

Des Moines lobe tills in Iowa (Simpkins and Parkin, 1993; 

Parkin and Simpkins, 1995) have demonstrated nitrate 

removal by denitrification in till. Simpkins and Parkin (1993)

detected extremely reducing, methanogenic conditions in Des 

Moines lobe tills and reported that denitrification would occur 

in the Des Moines lobe tills unless recharge rates are unusually high.
Characterization of Glacial Till and Aquifer Systems 45

A. LFO1 B. LFO2

C. CWO1/O2 D. HFC

E. Olivia 0

 50

100

150

200

250

 0 2 4 6 8 10 0.0 0.1 0.2 0.3 0.4 0.5

Depth, in feet below land surface

 0

 50

100

150

200

250

 0 2 4 6 8 10 0.0 0.1 0.2 0.3 0.4 0.5

Depth, in feet below land surface

 0

 50

100

150

200

250

300

350

 0 2 4 6 8 10 0.0 0.1 0.2 0.3 0.4 0.5

Depth, in feet below land surface

 0

 50

100

150

 0 2 4 6 8 10 0.0 0.1 0.2 0.3 0.4 0.5

Depth, in feet below land surface

Dissolved phosphorus, 

in milligrams

per liter as phosphorus

Dissolved nitrate, 

in milligrams 

per liter as nitrogen

Dissolved phosphorus, 

in milligrams

per liter as phosphorus

Dissolved nitrate, 

in milligrams 

per liter as nitrogen

Dissolved phosphorus, 

in milligrams

per liter as phosphorus

Dissolved nitrate, 

in milligrams 

per liter as nitrogen

Dissolved phosphorus, 

in milligrams

per liter as phosphorus

Dissolved nitrate, 

in milligrams 

per liter as nitrogen

Dissolved phosphorus, 

in milligrams

per liter as phosphorus

Dissolved nitrate, 

in milligrams 

per liter as nitrogen

 0

 20

 40

 60

 80

100

120

140

160

170

0.0 0.1 0.2 0.3 0.4 0.5

Depth, in feet below land surface

 0 2 4 6 8 10 12 14

Groundwater 

Pore water

Nitrate

Soluble inorganic phosphorus

Total dissolved phosphorus

Nitrate

Soluble inorganic phosphorus

EXPLANATION

Surficial aquifer

Confining unit (till)

Confined aquifer

Bedrock aquifer

Bedrock

Hydrostratigraphy

Axis break

LFO1–B

LFO1–C

LFO1–D

LFO1–F

LFO1–E

LFO2–B

LFO2–A

LFO2–C

LFO2–D

LFO2–F

LFO2–E

CWO2–A

CWO2–B

CWO2–C

CWO2–D

CWO2–E

CWO1–A

CWO1–B

CWO1–C

MW–01

HT–115

HT–140

HT–175

HT–200

HB–3

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

Olivia-5

Olivia-4

Figure 16. Vertical profiles of dissolved nitrate and phosphorus concentrations in groundwater and 

pore-water samples. A, from the Litchfield 1 (LFO1) site; B, from the Litchfield 2 (LFO2) site; C, from the 

Cromwell (CWO1/O2) site; D, from the hydrogeology field camp (HFC) site; E, from the Olivia site.
46 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

A. LFO1 B. LFO2

C. CWO1/O2

D. HFC

E. Olivia 0

 50

100

150

200

250 0 2 4 6 8 10 0 0.5 1.0 1.5 2.02.2 0 1.0 2.0 3.0 3.4 0 50 100 150 200 240

Dissolved oxygen, 

in milligrams per liter

Dissolved sulfate, 

in milligrams per liter

Dissolved iron,

in milligrams per liter

Dissolved manganese, 

in milligrams per liter

Depth, in feet below land surface Depth, in feet below land surface Depth, in feet below land surface

 0

 20

 40

 60

 80

100

120

140

160

Depth, in feet below land surface

 0

 50

100

150

200

250

300

350

 0

 50

100

150

200

250

 0

 50

100

150 0 2 4 6 8 10

Dissolved oxygen, 

in milligrams per liter

 0 50 100 150 200 240

Dissolved sulfate, 

in milligrams per liter

Depth, in feet below land surface

EXPLANATION

Surficial aquifer

Confining unit (till)

Confined aquifer

Bedrock aquifer

Bedrock

Hydrostratigraphy

Groundwater

Pore water

OT–13

OT–20

OT–60

OT–105

OT–145

OT–175

Olivia-5

Olivia-4

LFO1–B

LFO1–C

LFO1–D

LFO1–F

LFO1–E

LFO2–B

LFO2–A

LFO2–C

LFO2–D

LFO2–F

LFO2–E

CWO2–A

CWO2–B

CWO2–C

CWO2–D

CWO2–E

CWO1–A

CWO1–B

CWO1–C

MW–01

HT–115

HT–140

HT–175

HT–200

HB–3

Figure 17. Vertical profiles of concentrations of oxidation-reduction (redox) indicators in groundwater and 

pore-water including dissolved oxygen, iron, manganese, and sulfate. A, at the Litchfield 1 (LFO1) site; B, at the 

Litchfield 2 (LFO2) site; C, at the Cromwell (CWO1/O2) site; D, at the hydrogeology field camp (HFC) site; E, at the 

Olivia site.
Characterization of Glacial Till and Aquifer Systems 47

Pore-water nitrate concentrations tended to be higher than 

the corresponding groundwater samples (fig. 16). The higher 

pore-water nitrate values are likely from inadequate sample 

handling procedures and, therefore, are not an accurate representation of in situ pore-water nitrate. This difference is probably due to the processing of the core samples. Groundwater 

samples were held to strict holding times and were chilled 

until laboratory analyses were completed. The cores were 

stored for months before being squeezed. After the pore water 

was extracted from the core, the pore water was not chilled 

continuously before being analyzed for nitrate. Given that 

there were reducing conditions in the sediments and that cores 

were not processed in an anoxic environment, it is possible 

that ammonia was present in the pore water and was oxidized 

to nitrate during the sample handling.

Profiles of phosphorus concentrations along with the 

chloride, tritium, and travel time calculations indicate a 

potential geologic source of dissolved phosphorus rather than 

vertical penetration of anthropogenically sourced phosphorus 

from the surface into the subsurface (fig. 16). For example, at 

the LFO2 and Olivia sites, phosphorus was detected at depth 

in the till (fig. 16B,E) in piezometers where there was no 

evidence of anthropogenically sourced chloride and tritium, 

and Darcy calculations indicated vertical travel times of 

hundreds of years. At a leaky till site, LFO1, phosphorus was 

not detected in any piezometers despite modern water being 

present throughout the till profile. Phosphorus was detected 

throughout the tills at the HFC and Cromwell (CWO1/O2) 

sites (fig. 16C,D). Generally, pore-water phosphorus concentrations were lower than groundwater phosphorus concentrations, though there are a few exceptions (fig. 16). Overall, 

groundwater phosphorus concentrations varied from less than 

0.003 to 0.147 mg/L as phosphorus. Groundwater concentrations of phosphorus in till piezometers were equal to or greater 

than concentrations of phosphorus in surficial aquifer piezometers. Groundwater samples from all the confined aquifers had 

detectable phosphorus (fig. 16), with concentrations generally lower than the statewide median (0.124 mg/L) for buried 

aquifers contained in Quaternary-age deposits in Minnesota 

(Minnesota Pollution Control Agency, 1999).

Concentrations of redox indicator species including DO, 

iron, manganese, and sulfate generally indicate that reducing conditions exist in the tills at each site, though there was 

substantial variability in concentrations of these species with 

depth. One consistent anomaly was the existence of higher 

than expected DO concentrations in several till piezometers. 

Concentrations of DO in six of the eight till piezometers at 

the LFO1 and LFO2 sites and in piezometer OT–60 at the 

Olivia site were higher than expected given the presence of 

other redox species; furthermore, a near-zero DO sample was 

collected in the uppermost piezometer at the LFO1, LFO2, 

and Olivia sites. The six Litchfield till piezometers had DO 

concentrations near 2 mg/L, and OT–60 had an extremely 

high DO concentration of 6.0 mg/L (fig. 17). For the instruments used in this study, the uncertainty of the probe DO 

readings is plus or minus 0.2 mg/L, which means that any 

reading greater than 0.2 mg/L indicates the presence of oxygen (Lewis, 2006).

Higher than expected DO in till piezometers may be 

explained in part by the multiday purging and low-flow 

methods used for sampling till piezometers in low K settings. 

At least three well volumes were purged before collecting 

water-quality samples. In some cases, it took multiple days 

to complete this purging process. This means that the water 

in the piezometers could have been exposed to oxygen for 

many hours before sampling. The pumping flow rate used for 

sampling the till piezometers was low (often less than 0.2 gal/

min) and often resulted in dozens of feet of drawdown, allowing more time for water to be exposed to oxygen before being 

measured by the probe on the sonde. Therefore, the field DO 

measurements from till piezometers are less reliable than field 

DO measurements from sampling procedures used for aquifer 

wells. Groundwater from aquifer wells had far less opportunity for oxygen exposure because the wells could be purged 

and sampled with higher flow rates (typically greater than 

0.7 gal/min) all on the same day.

The conditions at the Litchfield site seem to be reducing, 

despite the previously mentioned elevated DO concentrations 

in most till piezometers (fig. 17). At the LFO1 site, sulfate 

concentrations are low in the uppermost piezometer, corresponding to low DO. Sulfate concentrations then increase in 

the second piezometer and then gradually decrease down into 

the confined aquifer. A similar pattern in sulfate concentrations occurred at LFO2. This could mean that sulfate reduction is occurring along a vertical flow path. Aside from DO 

and sulfate concentrations, no other redox data are available 

for the LFO1 site. At the LFO2 site, dissolved iron is variable 

with depth; the highest values (greater than 2.0 mg/L) occur 

at LFO2–D (till) and LFO2–F (confined aquifer). Dissolved 

manganese gradually increases in concentration from the top 

of the till (LFO2–A) to the approximate midpoint of the till 

profile (LFO2–C). Below LFO2–C, the manganese concentrations then gradually decrease again.

Redox species indicate reducing conditions at the 

Cromwell (CWO1/O2) site. Concentrations of DO were 

low throughout the profile; the highest DO concentration 

(0.5 mg/L) was measured in the surficial aquifer. Dissolved 

iron and manganese were present throughout the profile. The 

highest manganese concentration (0.749 mg/L) occurred 

in the surficial aquifer, and the highest iron concentration 

(1.24 mg/L) occurred in the confined aquifer. The presence of 

manganese and iron and the absence of sulfate in the confined 

aquifer indicate the confined aquifer is the most reduced environment at this site. Manganese, iron, and sulfate concentrations were variable in the till piezometers at this site. Sulfate 

concentrations were less than 3 mg/L at the depth of CWO1–A 

(about 140 ft below land surface) and lower.

Redox species indicate reducing conditions at the HFC 

site (fig. 17D). Concentrations of DO were low at all piezometers, ranging from 0.2 to 0.3 mg/L (fig. 17D). Dissolved 

iron, manganese, and sulfate were comparatively low at this 

site relative to the other sites. These analytes all had peak 
48 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

concentrations at different depths. The highest dissolved iron 

concentration occurred at HT–115 (0.5 mg/L), which is about 

10 ft below the top of the till unit. Below this piezometer, dissolved iron concentrations were less than 0.1 mg/L. Dissolved 

manganese concentrations gradually increased through the till, 

peaking at a concentration of 0.9 mg/L at HT–175, about 70 ft 

below the top of the till, and then gradually decreased down 

into the confined aquifer. Dissolved sulfate concentrations 

gradually increased until HT–140, about 35 ft below the top of 

the till, and then gradually decreased down into the confined 

aquifer (fig. 17D).

At the Olivia site, conditions were generally reducing, 

though there were anomalously high DO and low iron concentrations at piezometer OT–60 (about 60 ft below land surface) 

(fig. 17E). The surficial aquifer may have been only slightly 

reducing as indicated by the data from OT–13. At OT–13, DO 

was low (0.4 mg/L), but so were dissolved iron and manganese concentrations. Nitrate was also detected at this well in 

the surficial aquifer (fig. 16E). At OT–20, DO went up slightly 

to 0.7 mg/L, but dissolved iron and manganese concentrations 

increased substantially to 1.32 mg/L and 3.3 mg/L, respectively. The next deepest piezometer, OT–60, had extremely 

high DO (6.0 mg/L) and low iron (0.15 mg/L) concentrations. 

In the remainder of the till piezometers at this site, dissolved 

iron concentrations increased with depth, manganese generally decreased with depth, and sulfate concentrations were low 

and variable (fig. 17E). The confined aquifer was definitely 

a reduced environment because of low DO and elevated iron 

and manganese concentrations. The steady increase in dissolved iron indicates that there may be an iron-reducing zone 

in the bottom half of the till confining unit.

Groundwater Type

A Piper plot (Piper, 1944) was used to characterize the 

groundwater type and evolution at each site (fig. 18). Major 

ion composition data from the two sites with the longest travel 

times, Olivia and LFO2, provided evidence for the evolution of groundwater geochemical composition along vertical 

flow paths (shallowest to deepest). At these sites, shallow, 

young groundwater in till had distinct major ion compositions 

compared to older groundwater deeper into the till profile. 

Groundwater in the two shallowest piezometers at the Olivia 

site was predominantly calcium bicarbonate and magnesium 

bicarbonate, whereas all till piezometers below 60 ft (OT–60 

and below) contained groundwater samples that were predominantly sodium bicarbonate, or nearly so. At the LFO2 

site, groundwater from the two shallowest piezometers had 

higher calcium and magnesium and lower sodium compared 

to groundwater from piezometers deeper in the till. However, 

at the LFO2 site, all groundwater samples were still in the 

water type of calcium bicarbonate and magnesium bicarbonate 

(or between calcium and magnesium) except for groundwater 

from LFO2–E, the deepest till piezometer, which approaches 

the sodium bicarbonate water type.

Most of the remaining piezometers at the Cromwell, 

LFO1, and HFC sites had calcium or magnesium as the 

dominant cation and bicarbonate as the dominant anion. The 

HFC site had one outlier groundwater sample from piezometer 

HT–140 that is a sodium bicarbonate water type. It is not clear 

if this is a unique signature caused by hydrogeological interactions or if it is caused by sodium-bentonite contamination 

from the piezometer installation process.

Sources of Uncertainty

Though the geology of these sites was well characterized by the MGS, there are still some uncertainties in this 

information. At each site, one or two continuous cores from 

boreholes were analyzed by the MGS and were used for 

understanding the stratigraphy and hydrostratigraphy for 

the vertical transects. Heterogeneity in the geology of the 

systems, such as sand or clay lenses, may have been missed 

because of the inability to analyze all cores from piezometer installation. Possibly, an important geologic layer with 

distinct hydraulic properties that affects the vertical flux 

of groundwater could have been missed. Fractures can be 

an important feature for the flux of water through till but 

fractures that lack staining are difficult to identify in cores. 

Even if unstained fractures are observed, it is unknown if 

they were present in situ or if they were introduced by the 

coring process.

Uncertainty in K estimates from slug tests can come from 

a variety of sources. The geometric mean Kh values for the 

till units are based on slug test results from the piezometer 

screened intervals, but Kh between the screened intervals is 

unknown. Drilling and installation of the piezometers may 

have affected the K values estimated from slug and aquifer 

tests. For instance, drilling, especially auger drilling, can 

produce a smear zone along a borehole that creates a zone 

of lower Kh along the wall of the borehole when the actual 

formation has a higher Kh (Cherry and others, 2004). A variety 

of drilling methods were used in this study (hollow-stem 

auger, mud rotary, and rotary-sonic; table 1.1), but the effects 

of these methods on the estimates of Kh were not evaluated. As 

previously discussed, there is also uncertainty in the modeling operations used to determine Kh from slug test data and Kv

from aquifer test data.

This study relied on geochemistry to corroborate hydraulic data; however, each site was sampled within months of 

drilling, and most sites were only sampled once. It is therefore possible that water-quality samples were affected by the 

drilling and installation of piezometers, especially in low K

zones. Sodium-bentonite contamination of groundwater and 

intrusion into screened intervals may have occurred during 

the piezometer installation process for some piezometers. 

Analytes that are associated with bentonite contamination 

include chloride, bromide, sodium, and sulfate (Remenda 

and van der Kamp, 1997). Other studies have indicated that 

these analytes will be elevated in concentration initially after 
Characterization of Glacial Till and Aquifer Systems 49

installation and then decrease through time (Remenda and 

van der Kamp, 1997). For this study, funding was insufficient 

to do repeated samplings, so it is unknown of the geochemical results were affected by drilling operations. Additional 

samples would be beneficial for a more complete evaluation 

of the effect of bentonite on water-quality samples presented 

in this report.

Assuming that the drilling effects on water quality will be 

smaller in aquifer wells compared to till piezometers (because 

water fluxes are higher through pumped aquifers than tills), a 

comparison between aquifer chemistry and till chemistry may 

provide some insight into the potential for bentonite to affect 

water chemistry. Confined aquifer groundwater samples have 

a sodium concentration similar to till piezometer groundwater 

samples for the Olivia, HFC, and Cromwell sites (data available on https://doi.org/10.5066/F7P55KJN; U.S. Geological 

Survey, 2019). At the LFO2 site, there is less sodium in the 

confined aquifer compared to till piezometers, indicating a 

potential effect of bentonite on water-quality samples at the 

LFO2 till piezometers. At the HFC site, HT–140 groundwater may have sodium-bentonite contamination because the 

groundwater from this sample has a high sodium concentration compared to all other piezometer and well groundwater samples.

EXPLANATION

Litchfield 2 (LF02)

Cromwell (CW01/02)

Hydrogeology field camp (HFC)

Olivia

Percent

 100

 80

 60

 40

 20

 0

Calcium

 0

 20

 40

 60

 80

 100

Magnesium

 0

 20

 40

 60

 80

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Sodium plus potassium

 0

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Chloride, fluoride, nitrite plus nitrate

 100

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 0

Carbonate plus bicarbonate

 100

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 0

Sulfate

 0

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Sulfate plus chloride

 0

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Calcium plus magnesium

 100

 80

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 40

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 0

 100

 80

 60

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 0

Percent

Percent

Figure 18. Piper diagram of major ion concentrations in groundwater samples from piezometer nests at the Litchfield 2 (LFO2), 

Cromwell (CWO1/O2), hydrogeology field camp (HFC), and Olivia sites.
50 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Heuristic Groundwater Modeling

A series of model scenarios demonstrated that pumping groundwater from buried aquifers affected water levels 

and groundwater fluxes through the till. The magnitude of 

the pumping effects varied substantially across the range of 

hydrogeological properties observed at the field sites in this 

study. Response variables extracted from steady-state model 

outputs are given in table 8. These models demonstrated that 

for understanding sustainability of groundwater pumping from 

confined aquifers, knowledge of till hydraulic properties is just 

as important as knowledge of aquifer hydraulic properties.

The north and south constant head boundary conditions 

minimally contributed to the water pumped from the buried 

sand unit for all but three model scenarios (table 8). In the 

LsLvHc, MsLvHc, HsLvHc scenarios, about 10 percent of the 

pumped water originated from the boundary cells (table 8). 

Calculations of water emanating from boundary cells were 

implemented as a quality assurance measure but still yielded 

useful information about the water source to the wells. These 

three scenarios all had a low Kv for till overlying the buried 

sand unit and a high Kh for the middle unit (LvHc). A total 

of 80 to 90 percent of the water entered the buried sand unit 

through its sides. The implication is that in this hydrogeologic 

setting, the contributing area for pumping is laterally extensive, extending farther than 10 mi from the well field.

The small buried sand unit size of 1.0 mi by 0.5 mi (Ls) 

generally could not sustain pumping at 900 gal/min, unless the 

buried sand unit was surrounded by conductive till (Hc). The 

most comparable field site to this situation is the Olivia site 

where two high-capacity production wells were completed in 

a buried sand and gravel aquifer that is mapped to be 0.08 mi2

(Bradt, 2017), though the aquifer could be larger because its 

extent has been estimated with limited data. These wells pump 

at an approximate rate of 123 gal/min, and the geometric mean 

Kh of the till confining unit was 0.004 ft/d and the bulk Kv of 

the till confining unit was between 0.0005 and 0.001 ft/d. The 

pumping rate has been maintained for many years in this small 

confined aquifer, but substantially increasing the pumping 

may not be feasible under the assumption that the aquifer truly 

is small.

Across all the model scenarios, little water entered 

the buried sand unit from the till below as water was being 

pumped from the buried sand unit. The maximum contribution 

from below among all the scenarios was 9.7 percent, which 

occurred in an extreme case with a small, isolated buried sand 

unit (0.5 mi2, Ls), under low-conductivity till (0.001 ft/d, 

Lv), and surrounded on all sides by low-conductivity material 

(0.05 ft/d, Lc). When the aquifer size was increased and surrounded by low-conductivity till, the percentage contribution 

from below decreased to about 4 to 5 percent. When the Kv

of the overlying till and Kh of the middle unit were increased, 

the percentage contribution from below generally decreased 

to less than 1 percent of the total water entering the buried 

sand unit.

Leakage from the surficial unit into the till within the 

local area under ambient (no pumping) conditions varied in 

response to several hydrogeological factors and indicated that 

till properties as well as the fluxes of water through material 

underlying the till were important. Not surprisingly, as till Kv

became smaller, the amount of leakage also decreased and 

lateral flow through the surficial unit increased; however, the 

size of this response was affected by the Kh of the material 

underlying the till. For example, in the LsHvLc (small sand 

unit, high till Kv, low middle unit Kh), the ambient leakage 

into the till was small, only about 6 percent of inputs into the 

surficial unit, but when the Kh of the middle unit was increased 

(Hc, 30 ft/d), the leakage into the top of the till increased to 

37 percent (table 4). The ambient leakage into the upper till 

also increased as the size of the buried sand unit increased.

Till Vertical Hydraulic Conductivity, Aquifer 

Size, and Middle Unit Horizontal Hydraulic 

Conductivity Effects on Groundwater Fluxes

Three response variables across the full range of permutation parameter values are presented in figure 19: (1) maximum drawdown in the till (plots A–C), (2) pumping-induced 

increase in leakage to till from surficial unit (plots D–F), and 

(3) the amount of water entering the buried sand unit across 

the top face of the unit (plots G–I). For the following discussion, the term “sensitivity” will be used to describe, in relative 

terms, how much a given response variable changed across the 

range of model parameter values used in the series of permutation model runs. Comparing the same-colored lines across 

rows of graphs in figure 19 provided a visual for the sensitivity 

of response variables to aquifer size. The slope of the samecolored lines in each graph provided a visual for the sensitivity 

of each response variable to the Kv of the till unit. The vertical 

separation between points for a given Kv value provided a 

visual for the sensitivity of each response variable to the Kh of 

the middle unit surrounding the buried sand unit.

A couple of general patterns in response variable sensitivity to parameter changes are apparent across most of the plots 

in figure 19 (not D). First, the response variables were much 

more sensitive to changes in the middle unit Kh with low overlying till Kv, as demonstrated from the vertical separation of 

points when till Kv=0.001 ft/d. Second, the response variables 

tended to be more sensitive to changes in till Kv between 0.001 

and 0.05 ft/d than between 0.05 and 2.0 ft/d.

The maximum drawdown in the till in model layer 3 

was highly variable across the permutation runs (fig. 19, 

plots A–C). In plot A, the dotted lines indicated that the specified pumping rate of 900 gal/min was not sustained and therefore cannot be compared directly to the other model runs. In 

the remaining model runs, the maximum drawdown decreased 

as the Kv of the till increased. This seemed counterintuitive, 

especially in light of field results from this study where drawdown in response to short-term (several hours to a maximum 

of 5 days) pumping were not apparent in low K tills; however, 
Table 8. Summary of sustained pumping rates, water fluxes, and drawdowns from steady-state heuristic MODFLOW model output (from Trost and ot[gal/min, gallon per minute]

Model run 

name

Sustained 

pumping 

(gal/min)

Percent of 

water pumped 

by wells from 

boundary 

conditions

Percent of inputs to surficial unit 

(layer 1) that leaks into upper till 

(layer 2) within local area

Pumpinginduced 

increase in 

percent of inputs leaking 

into upper till 

(layer 2)

Percent of water entering buried sand 

unit Dra

in 

(surf

i

Ambient Stressed From below Through the side faces Through the top face

LsLvLc 291 0.4 0.5 8.8 8.3 9.7 21.9 78.0 LsLvMc 501 0.3 1.8 10.4 8.6 0.9 80.7 18.4 LsLvHc 900 10.3 3.6 11.5 7.9 0.4 90.0 9.6 LsMvLc 304 0.0 2.7 8.9 6.2 2.5 3.5 94.1 LsMvMc 577 0.0 11.9 22.8 10.9 0.4 41.6 58.0 LsMvHc 900 0.1 22.2 35.5 13.3 0.2 61.7 38.2 LsHvLc 304 0.0 6.3 10.2 3.9 0.7 2.6 96.7 LsHvMc 625 0.0 18.7 29.2 10.5 0.4 35.6 63.9 LsHvHc 900 0.0 37.1 48.8 11.8 0.2 70.8 29.0 MsLvLc 900 0.0 1.3 24.0 22.7 4.9 27.0 68.2 MsLvMc 900 0.4 2.5 17.6 15.1 0.5 72.4 27.1 MsLvHc 900 10.6 3.9 11.3 7.4 0.2 89.0 10.8 MsMvLc 900 0.0 11.2 25.8 14.6 0.7 15.2 84.1 MsMvMc 900 0.0 17.5 31.1 13.6 0.2 34.7 65.1 MsMvHc 900 0.1 24.2 35.8 11.6 0.1 52.2 47.7 MsHvLc 900 0.0 21.8 32.5 10.6 0.2 21.2 78.6 MsHvMc 900 0.0 28.3 38.7 10.4 0.1 28.9 70.9 MsHvHc 900 0.0 39.1 49.1 10.0 0.1 42.3 57.6 HsLvLc 900 0.0 2.9 22.8 20.0 4.1 8.9 87.0 HsLvMc 900 0.5 3.5 15.8 12.3 0.6 56.5 42.9 HsLvHc 900 10.8 4.5 11.3 6.9 0.2 78.9 20.9 HsMvLc 900 0.0 19.9 31.7 11.8 0.6 1.5 97.9 HsMvMc 900 0.0 22.2 33.5 11.3 0.2 15.0 84.8 HsMvHc 900 0.1 25.4 35.7 10.3 0.7 30.4 69.5 HsHvLc 900 0.0 33.1 41.6 8.5 0.1 1.7 98.1 HsHvMc 900 0.0 34.7 43.3 8.6 0.1 7.4 92.5 HsHvHc 900 0.0 39.8 48.6 8.8 0.0 17.2 82.8 
Table 8. Summary of sustained pumping rates, water fluxes, and drawdowns from steady-state heuristic MODFLOW model output (from Trost and ot[gal/min, gallon per minute] Model run name Sustained pumping (gal/min) Percent of water pumped by wells from boundary conditions Percent of inputs to surficial unit (layer 1) that leaks into upper till (layer 2) within local area Pumping- induced increase in percent of in- puts leaking into upper till (layer 2) Percent of water entering buried sand unit Dra

in 

(surf

i

Ambient Stressed From below Through the side faces Through the top face BSkh_L 900 0.0 14.6 30.2 15.6 0.3 28.2 71.5 

BSkh_H 900 0.0 19.9 33.1 13.2 0.2 37.5 62.3 LFtrlk 900 0.1 4.1 20.4 16.3 0.5 70.3 29.3 CRtrlk 900 0.0 23.3 35.6 12.3 0.1 19.7 80.2 Ppen_L 900 0.0 17.5 31.1 13.6 0.2 34.7 65.1 Ppen_H 900 0.0 17.5 31.1 13.6 0.2 34.7 65.1 SURF_L 900 0.0 39.0 60.4 21.3 0.3 49.0 50.7 SURF_H 900 0.0 6.3 14.1 7.8 0.1 26.3 73.5 TOTq_L 300 0.0 17.5 21.3 3.8 0.2 33.2 66.6 TOTq_H 2,250 0.0 17.5 59.7 42.2 0.3 45.4 54.4 UTtk_L 900 0.0 20.1 32.7 12.7 0.2 30.4 69.4 UTtk_H 900 0.0 14.7 29.3 14.6 0.2 39.8 59.9 
Characterization of Glacial Till and Aquifer Systems 53

it is important to note that the results discussed here were 

from steady-state models and demonstrate conditions after 

1,000 years of pumping. The maximum drawdown in the till 

generally decreased as the buried sand body size increased, 

and the maximum drawdown was especially sensitive to 

aquifer size less than 4.5 mi2 (compare green lines in fig. 19, 

plots A and B). At low till Kv, there is a consistent inverse relation between the middle unit Kh and drawdown in the till.

In all cases, the introduction of pumping stress to the 

model system increased the amount of water leaking from the 

surficial unit into the top of the till unit (fig. 19, plots D–F). 

These pumping-induced increases in leakage represented a 

reduction in groundwater discharge out of the local area and 

groundwater discharge to local streams and lakes that is not 

met by increased fluxes from outside the local area (fig. 9, 

model layout). In some model scenarios, more than 20 percent 

of the inputs within the 5 mi local area were lost from the 

surficial unit because of pumping. Generally, the magnitude 

of the increase in leakage increased as the ambient leakage 

(no pumping) decreased (table 4). In other words, the largest 

pumping-induced increases in leakage occurred in low Kv tills. 

As with the drawdown results, this seemed counterintuitive 

based on our field study, but this modeling represented longterm (hundreds of years) conditions.

There is a strong interaction between the middle unit Kh

and the overlying till Kv on pumping-induced leakage (fig. 19, 

plots D–F). Pumping-induced leakage into the till was highly 

sensitive to the middle unit Kh at low till Kv (0.001 ft/d) and 

0

5

10

15

20

25

Buried sand unit size = 0.5 mi2 (Ls) Buried sand unit size = 12.5 mi2

 (Hs) 

0

10

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40

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70

Maximum drawdown in layer 3,

in feet 

0

20

40

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0.001 0.01 0.1 1 10

Water entering buried sand

through top face, in percent 

0.001 0.01 0.1 1 10

Log Kv of till, in feet per day

0.001 0.01 0.1 1 10

Pumping induced increase in 

leakage to till, in percent

Buried sand unit size = 4.5 mi2 (Ms) 

Middle unit Kh = 0.05 ft/d (Lc)

Middle unit Kh = 5.0 ft/d (Mc)

Middle unit Kh = 30 ft/d (Hc)

EXPLANATION

mi2 

Kh

Middle unit K gal/min h = 0.05 ft/d (Lc, 900 gal/min pumping not sustained)

Middle unit Kh = 5.0 ft/d (Mc, 900 gal/min pumping not sustained) ft/d 

Square mile

Horizontal hydraulic conductivity

Logarithmic vertical hydraulic conductivity

Gallon per minute

Foot per day 

Log Kv

A B C

D E F

G H I

Figure 19. Heuristic MODFLOW model output including changes in maximum drawdown in till (A–C), pumping-induced increase in 

leakage to till from the surficial unit (D–F), and the amount of water entering the aquifer through its top face (G–I).
54 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

insensitive to middle unit Kh at high till Kv. Pumping-induced 

leakage was insensitive to changes in aquifer size. The practical implication of this result is that to accurately simulate these 

fluxes, which are important for managing water resources near 

the surface, reliable information on either the till Kv or the 

connectivity among buried aquifers is needed.

The relative amounts of water reaching a confined aquifer 

from above (and by implication, from the sides) changed 

drastically with the range of hydrogeological characteristics 

observed in this study (fig. 19G–I). As previously discussed, 

water entering the buried sand unit from below was almost 

always less than 1 percent of the total. Not surprisingly, the 

percentage of water entering the aquifer from above increased 

as the lateral extent of the aquifer increased. At one extreme, 

98 percent of the water entered the aquifer directly from the 

till above. This model run had a conductive till overlying a 

large sand body surrounded by poorly conductive materials 

(HsHvLc). In the other extreme case, only about 10 percent 

of the water entered the aquifer directly from the till above. 

This model run had poorly conductive till overlying a small 

sand body that was adjacent to conductive till (LsLvHc). This 

model run had an extensive contributing area, as indicated by 

the increased water supplied by the model boundary 10 mi 

away from the well field.

Pumping and Aquifer Transmissivity Effects on 

Groundwater Fluxes 

For the next set of model runs, one model parameter or 

set of parameters was varied and the remainder of the model 

parameters were from the base model (MsMvMc) (fig. 20; 

table 8).

Increases in pumping from 300 to 2,250 gal/min caused 

higher drawdowns in the till, substantially increased the 

percentage of water leaking into the till from the surficial 

unit, and only moderately decreased the percentage of water 

entering the aquifer directly from the overlying till (fig. 20). 

At the 300 gal/min pumping rate, pumping only increased 

the leakage by about 4 percent, but at the 2,250 gal/min 

pumping rate, the leakage increased to more than 40 percent 

of water inputs to the surficial unit within the local area. 

These pumping-induced increases in leakage represented a 

reduction in groundwater discharge out of the local area and 

a reduction in groundwater discharge to local streams and 

lakes that were not met by increased fluxes from outside the 

local area (fig. 9, model layout). These results indicated that 

the effect of pumping on surface-water resources depended 

on the pumping rate. The 900 gal/min rate was representative 

of the pumping rate from the confined aquifer at Litchfield. 

The city of Litchfield pumps at a mean rate of 630 gal/min, 

or 340 Mgal/yr, and there are other high-capacity permits 

within the same buried aquifer, as was evident from the 

large summer drawdowns in the buried aquifer hydrographs 

(fig. 10) and from the aquifer test data (Blum and Woodside, 

2017). At the 900 gal/min pumping rate, pumping increased 

leakage into the upper till by about 14 percent, as compared 

to ambient conditions.

Two model runs, CRtrlk and LFtrlk, were intended to be 

“Cromwell transmissivity like” and “Litchfield transmissivity like.” These models were not intended to replicate actual 

observed responses at these sites; rather, these model runs 

were used to compare and contrast responses to pumping. In 

these two scenarios, the till Kv and aquifer transmissivity were 

set to approximate results from aquifer tests completed at each 

site. The steady-state models indicate, over long periods of 

time, a greater drawdown in till for a Litchfield-like setting 

compared to a Cromwell-like setting. This was consistent with 

the model results presented previously, where less conductive 

tills demonstrated higher drawdowns in steady-state conditions. The pumping-induced increase in leakage to till was 

about the same between the two model runs. About 80 percent 

of the water in the Cromwell-like scenario entered the buried 

aquifer directly from the overlying till, compared to only about 

30 percent in the Litchfield-like scenario. This meant that 

there was a more laterally extensive contributing area for a 

Litchfield-like setting than a Cromwell-like setting, which has 

implications for managing drinking water quality. Because till 

Kv and buried aquifer Kh were varied simultaneously in these 

model runs, it was difficult to tell which parameter was exerting a greater effect on the response variables. In a separate set 

of variation runs (BSkh), the response variables were sensitive 

to changes in the buried sand Kh from 30 to 100 ft/d, and the 

response variables were sensitive to changes in till Kv between 

0.0001 and 0.05, indicating that both attributes played a role 

in the different responses of the Cromwell-like and Litchfieldlike settings.
Summary 55

0

5

10

15

20

25

0

10

20

30

40

50

0 0

Maximum drawdown in 

100 200 300 400 500

layer 3 (till), in feet 

200 400 600

Kh of buried sand unit, in feet per day 

0

20

40

60

80

100

0 200 400 600

0

5

10

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25

0 1,000 2,000 3,000

0

10

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0

Pumping from buried sand, in gallons per minute 

0

20

40

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100

0

5

10

15

20

25

0

10

20

30

40

50

0

20

40

60

80

100

CRtrlk MsMvMc LFtrlk CRtrlk MsMvMc LFtrlk CRtrlk MsMvMc LFtrlk

Variation in till Kv and buried sand unit Kh—Cromwell like (CRtrlk), 

base model (MsMvMc), and Litchfield like (LFtrlk) Pumping-induced increase in leakage to till, in percent

Water entering buried sand

through top face, in percent

Maximum drawdown in 

layer 3 (till), in feet 

Maximum drawdown in 

layer 3 (till), in feet 

Pumping-induced increase 

in leakage to till, in percent

Pumping-induced increase 

in leakage to till, in percent

Water entering buried sand

through top face, in percent

Water entering buried sand

through top face, in percent

Variation in buried sand unit Kh (BSkh)

1,000 2,000 3,000

1,000 2,000 3,000

Variation in pumping (TOTq)

A B C

D E F

G H I

Figure 20. Heuristic MODFLOW model output for variation model runs including changes in maximum drawdown in model layer 3 (till), 

pumping-induced increase in leakage to model layer 2 (till) from model layer 1 (surficial unit), and the amount of water entering the 

buried sand unit through its top face for variations in the buried sand unit horizontal hydraulic conductivity (Kh, A–C), variations in total 

pumping (D–F), and variations in till vertical hydraulic conductivity (Kv) and buried sand Kh (G–I).

Summary

Confined (or buried) aquifers of glacial origin overlain 

by till confining units provide drinking water to hundreds 

of thousands of Minnesota residents. The sustainability of 

these groundwater resources is not well understood because 

hydraulic properties of till that control vertical groundwater 

fluxes (leakage) to underlying aquifers are largely unknown. 

The U.S. Geological Survey, Iowa State University, Minnesota 

Geological Survey, and Minnesota Department of Health 

investigated hydraulic properties and groundwater flow 

through till confining units using field studies and heuristic 

MODFLOW simulations. Till confining units in the following 

late-Wisconsinan stratigraphic units (with locations in parentheses) were characterized: Des Moines lobe till of the New 

Ulm Formation (Litchfield, Minnesota); Superior lobe tills of 

the Cromwell and Aitkin Formations (Cromwell, Minn.); and 

Wadena lobe till of the Hewitt Formation (hydrogeology field 

camp [HFC] near Akeley, Minn.). Pre-Illinoian till of the Good 

Thunder formation (Olivia, Minn.) was also characterized.

Hydraulic and geochemical field data were collected from 

sediment cores and a series of five piezometer nests. Each nest 

consisted of five to eight piezometers screened at short vertical 
56 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

intervals in hydrostratigraphic units including (if present) surficial aquifers, till confining units, confined/buried aquifers, and 

underlying bedrock. Till hydraulic conductivity (K) was estimated from slug tests (horizontal [Kh]) and constant-rate aquifer 

tests in the confined aquifer (vertical [Kv]). Pressure transducers were emplaced to measure hydraulic-head fluctuations. 

Groundwater samples from the piezometers and pore-water 

samples from till cores were analyzed for major ions, nutrients, 

enriched tritium, and stable isotopes (oxygen [δ18O] and hydrogen [δ2H]) of water. Travel times through the till were evaluated 

with Darcy’s law and stable isotope concentrations. A series 

of heuristic MODFLOW simulations were used to evaluate 

groundwater fluxes through till across the range of till hydraulic 

properties and pumping rates observed at the field sites.

The first Litchfield piezometer nest (LFO1) was in a 

leaky 60-foot (ft)-thick till profile with a mean composition 

of 47-percent sand, 34-percent silt, and 19-percent clay. The 

site had a mean downward hydraulic gradient of 0.55 in the 

till, and the largest hydraulic gradient was near the base of the 

till. Drawdown responses were observed in all till piezometers 

during the aquifer test at the site. The geometric mean Kh of 

two till piezometers was 0.07 foot per day (ft/d), whereas 

the bulk Kv determined from an aquifer test at Litchfield was 

0.001 ft/d. These two K values along with other site information were used to calculate specific discharge and travel 

time through till according to Darcy’s law. Specific discharge 

ranged from 2.4 (Kv) to 176 inches per year (in/yr) (Kh) and 

travel times varied from 1 (Kh) to 74 years (Kv). Tritium was 

detected in all till piezometers, indicating the presence of 

modern water throughout the till profile. “Modern” indicates 

groundwater that was last exposed to the atmosphere sometime after 1953. Chloride to bromide ratios were near 250 

throughout the till, indicating a possible presence of anthropogenically sourced chloride throughout the till.

The second Litchfield piezometer nest (LFO2) was only 

one-half mile away from LFO1 and in the same New Ulm 

Formation but contained an effective confining unit in the 

lower part of the till. The 115-ft-thick till profile at this site 

had a mean composition of 52-percent sand, 31-percent silt, 

and 17-percent clay. The site had a mean downward hydraulic 

gradient of 0.48 in the till, and the largest hydraulic gradient was near the base of the till. Drawdown responses were 

not observed in any till piezometers during the aquifer test at 

the site. The geometric mean Kh of five till piezometers was 

0.0002 ft/d, whereas the bulk Kv determined from an aquifer 

test at Litchfield was 0.001 ft/d. These two K values along 

with other site information were used to calculate specific 

discharge and travel time through till according to Darcy’s 

law. Specific discharge ranged from 0.4 (Kh) to 2.1 in/yr 

(Kv) and travel times varied from 165 (Kv) to 913 years (Kh). 

Tritium was only detected in the two uppermost till piezometers, to about 35 ft into the till, indicating nonmodern water 

(older than 1953) was present through most of the till profile. 

Chloride to bromide ratios were near 250 to a depth of about 

60 ft into the till, indicating a possible presence of anthropogenically sourced chloride to this depth.

The Cromwell nest was in a leaky 120-ft-thick till profile 

with a mean composition of 57-percent sand, 31-percent silt, 

and 13-percent clay (greater than 100 percent because of 

rounding). The site had a mean upward hydraulic gradient of 

0.02 in the till, but hydraulic gradient directions were variable 

throughout the till. Drawdown responses were observed in all 

till piezometers during the aquifer test at the site. The geometric mean Kh of five till piezometers was 0.06 ft/d, whereas the 

bulk Kv determined from an aquifer test was 1.1 ft/d. These 

two K values along with other site information were used to 

calculate specific discharge and travel time through till according to Darcy’s law. Because of the upward hydraulic gradient 

at this site, fluxes are from the base of the till to the overlying 

surficial aquifer. Specific discharge ranges from 4.5 (Kh) to 

86 in/yr (Kv), and travel times vary from 4 (Kv) to 80 years 

(Kh); however, these calculations ignore the broader flow 

environment at this site. Tritium was not detected in till but 

was detected in the surficial aquifer and the confined aquifer. 

The hydraulic gradient data and the tritium data indicate that 

recharge to the confined aquifer enters the system somewhere 

upgradient in the same buried aquifer system or perhaps 

through a stratigraphic window in the overlying till confining 

unit where the hydraulic gradient in the till is downward. This 

indicates that the till sequence observed near the production 

well may have little direct effect on the quality and quantity of 

water at the Cromwell site. Rather, the anthropogenic activities and geologic materials at a distal recharge area (yet to be 

defined) may affect the water observed in the confined aquifer 

at the Cromwell site. Chloride to bromide ratios were variable 

in the till and were greater than 250 at multiple locations.

The HFC nest was in a leaky 100-ft-thick till profile with 

a mean composition of 67-percent sand, 22-percent silt, and 

11-percent clay. The site had a mean downward hydraulic gradient of 0.04 in the till. Drawdown responses were observed 

in all but the uppermost till piezometer during the aquifer test 

at the site. The geometric mean Kh of till piezometers was 

0.03 ft/d, and the bulk Kv determined from an aquifer test was 

0.031 ft/d. These two K values along with other site information were used to calculate specific discharge and travel 

time through till according to Darcy’s law. Specific discharge 

ranged from 5.3 (Kh) to 6.0 in/yr (Kv), and travel times varied 

from 50 (Kv) to 57 years (Kh). Tritium was detected in all till 

piezometers, indicating the presence of modern water throughout the till profile. Chloride to bromide ratios were much less 

than 250 throughout the till, indicating that anthropogenically sourced chloride is not a major factor at this site, likely 

because this site is in a remote forested region.

The Olivia nest contained an effective confining unit 

in the lower part of the till above the confined aquifer. The 

166-ft-thick till profile at the Olivia site had a mean composition of 37-percent sand, 40-percent silt, and 23-percent 

clay. The site had a mean downward hydraulic gradient of 

0.13 in the till, with the largest hydraulic gradients at the 

base of the till and an extremely large hydraulic gradient of 

2.26 across the till/confined aquifer boundary. Drawdown 

responses during an aquifer test were not observed in any 
Summary 57

till piezometers because of a prolonged reverse water-level 

fluctuation response in most till piezometers. The geometric 

mean Kh of five till piezometers was 0.004 ft/d, and the bulk 

Kv determined from an aquifer test was 0.0012 ft/d according 

to a leaky confined aquifer analytical approach or 0.0005 ft/d 

according to a time-delay analytical approach. These K estimates along with other site information were used to calculate 

specific discharge and travel time through till according to 

Darcy’s law. Specific discharge ranged from 0.7 (Kv) to 2.3 in/

yr (Kh), and travel times varied from 217 (Kh) to 1,770 years 

(Kv). Tritium was only detected in the uppermost till piezometer, at a depth of about 8 ft into the till, indicating nonmodern water (older than 1953) was present through most of the 

till profile. In this same piezometer, chloride to bromide ratios 

were greater than 250, indicating anthropogenically sourced 

chloride may be present. Chloride to bromide ratios were far 

greater than 250 in the lower one-half of the till, but primarily because of extremely low bromide concentrations and 

not from elevated chloride concentrations compared to the 

uppermost till piezometer.

Diverse tills were evaluated at the four study sites, 

and comparisons across sites yield some useful insights 

for generalizing these results. The field data demonstrated 

variability in hydraulic properties between and within till 

stratigraphic units horizontally and vertically. The variability 

in hydraulic properties resulted in substantial differences in 

groundwater flux through till across the sites. A conceptual 

understanding that emerges from the vertical till profiles is 

that they are not homogeneous hydrostratigraphic units with 

uniform properties; rather, each vertical sequence is a heterogeneous mixture of glacial sediment with differing abilities 

to transmit water.

Till thicknesses varied from 60 to 166 ft, and till textures 

ranged from a sandy loam (Hewitt Formation, HFC site) to 

a silt loam/clay loam (Good Thunder formation, Olivia site). 

Till Kh varied by one to three orders of magnitude within each 

piezometer nest. Four piezometer nests had downward hydraulic gradients ranging from 0.04 to 0.56, and one nest had a 

slight upward hydraulic gradient of 0.02. The Cromwell, HFC, 

and LFO1 sites were examples of “leaky” tills with high Kv

(0.001 to 1.1 ft/d) and geometric mean Kh (0.03 to 0.07 ft/d), 

and extensive vertical hydraulic connectivity between the 

confined aquifer and the overlying till. Estimated groundwater 

travel times through these sites ranged from 1 to 81 years, and 

two of these sites had tritium throughout their till profiles. The 

tills at the other two sites, Olivia and LFO2, were effective 

confining units that had low Kv (0.001 to 0.0005 ft/d) and 

geometric mean Kh (0.0002 to 0.004 ft/d). The till piezometers at these sites had no drawdown response to short-term 

(up to 10 hours for Olivia and up to 5 days for Litchfield) 

high-capacity pumping from the confined aquifer. Estimated 

groundwater travel times through the tills at these sites ranged 

from 165 to nearly 1,800 years, and tritium was only detected 

in the upper one-third of these till profiles. Across all sites, 

the till vertical anisotropy (ratio of Kh to Kv) ranged by four 

orders of magnitude from 0.05 at the Cromwell nest to 70 

at the LFO1 nest. Stable isotopes of oxygen and hydrogen 

indicate that groundwater throughout all five till profiles is 

younger than the last glacial advance into Minnesota about 

11,000 years ago.

A drawdown response to pumping from the confined 

aquifer was typically observed in till piezometers screened 

in sediments with at least 55-percent sand and less than 

15-percent clay. The mean percentages of sand and clay for a 

given till profile were correlated with the logarithm of the bulk 

Kv determined from aquifer tests; however, the percentages of 

sand or clay from core sections at the same depth as piezometer screens were not correlated with the Kh determined from 

slug tests. This indicates that factors in addition to textural 

composition are affecting the hydraulic properties of till (or 

measurement thereof) at the localized scale of slug tests, but at 

the larger aquifer-test scale, textural composition is indicative 

of bulk till hydraulic characteristics.

The heuristic modeling demonstrated that, for understanding sustainability of groundwater pumping from confined aquifers, knowledge of till hydraulic properties is just 

as important as knowledge of aquifer hydraulic properties. In 

particular, it is important to have information about the Kv of 

overlying till, the areal extent of the buried aquifer, and the 

lateral connectivity of the buried aquifer to other aquifers. 

Three response variables were examined in detail from the 

steady-state models: maximum drawdown in till, pumpinginduced leakage from the surficial unit into the till confining 

unit, and the source of water to the buried sand unit (expressed 

as the percentage of water entering the aquifer directly from 

the overlying till). Over long periods of time (hundreds of 

years), pumping-induced hydraulic gradients can be established in buried aquifer systems, and even in low K tills, these 

hydraulic gradients increased leakage into and through till. 

The percentage of water entering a buried aquifer directly 

from the overlying till ranged from 10 to 98 percent among the 

model scenarios; when only 10 percent of the water entered 

the aquifer from above, water fluxes increased at the model 

boundary, 10 miles away from the well field. The percentage 

of water entering the aquifer from above was demonstrated to 

be sensitive to buried aquifer size, Kv of till, and Kh of material adjacent to the buried aquifer. In almost all cases, less 

than 1 percent of the water entered the buried sand from the 

underlying till.

In conclusion, groundwater flowing vertically downward through till confining units (leakage) replenishes water 

pumped from confined aquifers. Till hydraulic properties, such 

as those presented in this report, provide important information that can be used to quantify leakage rates through till. Till 

hydraulic properties are variable over short distances and profoundly affect leakage rates, demonstrating the importance of 

site-specific till hydraulic data for evaluating the sustainability 

of groundwater withdrawals from confined aquifers.
58 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

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Appendix 1 Well and Piezometer Construction Details 65

Appendix 1 Well and Piezometer Construction Details

This appendix contains table 1.1, which has detailed well 

and piezometer construction information.

References Cited

Staley, A., and Nguyen, M., 2018, Core descriptions and 

unit interpretations in support of hydrologic properties 

of till investigation, Olivia and University of Minnesota 

hydrogeology-field site: St. Paul, Minn., Minnesota 

Geological Survey, 20 p.

Staley, A.E., Wagner, K., Nguyen, M., and Tipping, R., 2018, 

Core descriptions, borehole geophysics, and unit interpretations in support of phase I and II USGS hydrologic 

properties of till investigation: St. Paul, Minn., Minnesota 

Geological Survey Open File Report 18–03, 29 p.

Wagner, K., and Tipping, R., 2016, Core descriptions and 

borehole geophysics in support of USGS hydrologic 

properties of till investigation, Litchfield and Cromwell, 

Minnesota: University of Minnesota and Minnesota 

Geological Survey, 15 p., accessed November 20, 2017, at 

https://www.lccmr.leg.mn/projects/2014/finals/2014_03h_

MGS_report.pdf.
Table 1.1. Construction information, types of data collected, and adjacent sediment texture properties for wells and piezometers sampled at the Litchydrogeology field camp field sites in Minnesota.[ft, foot; BLS, below land surface; N, no; Y, yes; WQ, water quality; WL, water level; --, unknown or data not collected]

Well or 

piezometer 

name

Drilling method Completion date Tipping bucket precipitation Continuous water levels Types of discrete data collected Screen slot size Mesh size of silica sand for filter pack Top of filter pack (ft BLS) Filter pack length (ft) Textural description of material at well screen

Number 

of well 

volumes 

purged to 

develop 

well

Litchfield field sites

LFO1–B Hollow-stem auger 6/12/2015 N Y WQ, WL 10 20/40 19.0 6.3 Fine-grained 

sand

51 

LFO1–C Hollow-stem auger 6/12/2015 N N WQ, WL 10 20/40 47.1 6.1 Sandy loam 

till

9 

LFO1–D Hollow-stem auger 6/11/2015 N Y WQ, WL 10 20/40 70.0 5.3 Loamy till 14 LFO1–E Hollow-stem auger 6/10/2015 N N WQ, WL 10 20/40 89.8 5.5 Sandy loam 

till

53 

LFO1–F Hollow-stem auger 6/15/2015 N Y WQ, WL 20 12/20 94.8 32.7 Sand and 

gravel

14 

LFO2–A Hollow-stem auger 6/24/2015 Y Y WQ, WL 10 20/40 15.0 5.0 Loamy till 9 LFO2–B Hollow-stem auger 6/24/2015 N N WQ, WL 10 20/40 30.1 5.0 Sandy loam 

till

5 

LFO2–C Hollow-stem auger 6/22/2015 N Y WQ, WL 10 20/40 52.0 8.7 Loamy till 4 LFO2–D Hollow-stem auger 6/23/2015 N Y WQ, WL 10 20/40 79.3 5.9 Sandy loam 

till

4 

LFO2–E Hollow-stem auger 6/20/2015 N N WQ, WL 10 20/40 108.8 5.0 Sandy loam 

till

6 

LFO2–F Hollow-stem auger 6/16/2015 N Y WQ, WL 20 12/20 137.5 24.9 Sand and 

gravel

21 

LF–OB1 Auger (nonspecified) 12/18/1997 N N WL 10 -- -- -- Sand and gravel -- LF–CM1 Nonspecified rotary 10/30/2008 N N WL 60 -- -- -- Sand and 

gravel

-- 

LF–CM2 Nonspecified rotary 2/19/1998 N N WL 115 -- -- -- Sand and 

gravel

-- 

LF–CM3 Nonspecified rotary 12/9/1999 N N WL 70 -- -- -- Sand -- LF–CM4 Nonspecified rotary 12/9/1999 N N WL 70 -- -- -- Sand and 

gravel

-- 
Table 1.1. Construction information, types of data collected, and adjacent sediment texture properties for wells and piezometers sampled at the Litchydrogeology field camp field sites in Minnesota.—Continued[ft, foot; BLS, below land surface; N, no; Y, yes; WQ, water quality; WL, water level; --, unknown or data not collected]

Well or 

piezometer 

name

Drilling method Completion date Tipping bucket precipitation Continuous water levels Types of discrete data collected Screen slot size Mesh size of silica sand for filter pack Top of filter pack (ft BLS) Filter pack length (ft) Textural description of material at well screen

Number 

of well 

volumes 

purged to 

develop 

well

Cromwell field sites

CWO1–A Mud rotary 7/21/2015 N Y WQ, WL 10 12/20 142.0 6.0 Sandy loam 

till

14 

CWO1–B Mud rotary 7/20/2015 N Y WQ, WL 20 12/20 220.9 14.8 Sand and 

gravel

15 

CWO1–C Mud rotary 7/18/2015 N Y WQ, WL 20 12/20 329.6 16.0 Slate 7 CWO2–A Hollow-stem auger 7/9/2015 Y Y WQ, WL 10 20/40 30.2 7.5 Sand and 

gravel

71 

CWO2–B Hollow-stem auger 7/13/2015 N N WQ, WL 10 20/40 54.4 5.5 Sandy loam 

till

10 

CWO2–C Hollow-stem auger 7/10/2015 N N WQ, WL 10 20/40 74.4 7.5 Sandy loam 

till

65 

CWO2–D Hollow-stem auger 6/29/2015 N Y WQ, WL 10 20/40 93.2 13.5 Sandy loam 

till

4 

CWO2–E Hollow-stem auger 7/12/2015 N N WQ, WL 10 20/40 123.6 5.3 Sandy loam 

till

14 

CW–CM3 Nonspecified rotary 10/21/1992 N N -- 25 -- -- -- Sand and 

gravel

-- 

CW–CM4 Cable tool 4/16/1999 N N WL 50 -- -- -- Sand and 

gravel

-- 

Hydrogeology field camp field sites

HT–115 Rotary sonic 5/30/2017 N Y WQ, WL 10 20/40 109.0 6.0 Sand 21 HT–140 Rotary sonic 5/31/2017 N Y WQ, WL 10 20/40 134.0 6.4 Sandy loam 

till

4 

HT–175 Rotary sonic 5/26/2017 N Y WQ, WL 10 20/40 170.0 5.1 Sandy loam 

till

6 

HT–200 Rotary sonic 5/25/2017 N Y WQ, WL 10 20/40 191.0 6.9 Sandy loam 

till

10 

HB–1 Rotary sonic 12/19/2014 N N WL 10 -- -- -- Sand and 

clay

-- 
Table 1.1. Construction information, types of data collected, and adjacent sediment texture properties for wells and piezometers sampled at the Litchydrogeology field camp field sites in Minnesota.—Continued [ft, foot; BLS, below land surface; N, no; Y, yes; WQ, water quality; WL, water level; --, unknown or data not collected] Well or piezometer name Drilling method Completion date Tipping bucket precipitation Continuous water levels Types of discrete data collected Screen slot size Mesh size of silica sand for filter pack Top of filter pack (ft BLS) Filter pack length (ft) Textural description of material at well screen Number of well volumes purged to develop well 

HB–2 Nonspecified rotary 6/22/2016 N Y WL 10 -- -- -- Sand -- HB–3 Mud rotary 7/18/2017 N Y WL 7 12/20 200.0 23.7 Sand and gravel -- MW–01 Unknown 7/20/1995 Y Y WQ, WL -- -- -- -- Sand and gravel -- WL07 Unknown 10/19/1977 N N WL -- -- -- -- Sand and gravel -- WL12 Unknown 6/13/1978 N N WL -- -- -- -- Sand -- WL299 Unknown 6/25/1978 N N WL -- -- -- -- Sand and gravel -- 

Olivia field sites OT–13 Rotary sonic 8/9/2017 Y Y WQ, WL 10 20/40 8.0 5.2 Sand and gravel 178 

OT–20 Rotary sonic 8/9/2017 N Y WQ, WL 10 20/40 15.0 5.2 Silty to clayey loam till 17 OT–35 Rotary sonic 8/9/2017 N N WQ, WL 10 20/40 30.0 4.8 Silty to clayey loam till -- OT–60 Rotary sonic 8/9/2017 N Y WQ, WL 10 20/40 55.0 4.9 Silty to clayey loam till 3 OT–105 Rotary sonic 8/8/2017 N Y WQ, WL 10 20/40 100.0 5.1 Silty to clayey loam till 6 OT–145 Rotary sonic 8/3/2017 N Y WQ, WL 10 20/40 140.0 4.3 Silty to clayey loam till 4 
Table 1.1. Construction information, types of data collected, and adjacent sediment texture properties for wells and piezometers sampled at the Litchydrogeology field camp field sites in Minnesota.—Continued[ft, foot; BLS, below land surface; N, no; Y, yes; WQ, water quality; WL, water level; --, unknown or data not collected]

Well or 

piezometer 

name

Drilling method Completion date Tipping bucket precipitation Continuous water levels Types of discrete data collected Screen slot size Mesh size of silica sand for filter pack Top of filter pack (ft BLS) Filter pack length (ft) Textural description of material at well screen

Number 

of well 

volumes 

purged to 

develop 

well

OT–175 Rotary sonic 8/2/2017 N Y WQ, WL 10 20/40 169.5 5.9 Silty to 

clayey loam till

5 

OB–7 Rotary sonic 8/1/2017 N Y WL 10 12/20 201.0 8.8 Sand and 

gravel

11 

Olivia-4 Unknown 9/3/1964 N N WQ, WL 40, 30 -- -- -- Sand and 

gravel

-- 

Olivia-5 Unknown 5/18/1972 N N WQ, WL 15, 30, 

40

-- -- -- Sand and 

gravel

-- 

Olivia-6 Unknown 1977 N N WL -- -- -- -- Sand and 

gravel

-- 

aData provided by the Minnesota Geological Survey. Data are summarized in Wagner and Tipping (2016), Staley and Nguyen (2018), and Staley and others (2018).
70 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Appendix 2 Slug Test Information

All water-level data used for slug test analyses, AQTESOLV files, and plots are available in the accompanying 

data release for this report (Maher and others, 2020). Three 

tables (tables 2.1, 2.2, and 2.3) are included in this appendix. 

Detailed in table 2.1 is the field information gathered on the 

slug tests completed at all sites, including the number of tests 

done, the type of slug test (rising or falling), the volume of 

the slug used in the tests, and water-level information (static 

water levels, observed initial displacements, and the calculated theoretical water-level displacement of the slug used). 

Detailed in table 2.2 are the water-level (static water column 

height and the saturated thickness) and piezometer construction parameters entered into AQTESOLV for the model runs. 

Detailed in table 2.3 are several statistical analyses outputted 

by AQTESOLV for each model run completed, including the 

mean residual, standard deviation of the residuals, and the 

95-percent confidence interval for Kh results.

The program AQTESOLV (Duffield, 2007) has curve 

matching solutions for slug tests done in confined and unconfined aquifers. During the slug test analyses, the Kansas 

Geological Survey (KGS) model (Hyder and others, 1994) 

was used to analyze the water-level response to slug tests. 

The KGS model has a solution for confined and unconfined 

aquifers. This method applies a curved solution to declining 

or rising water-level data collected during a single-well slug 

test in an unconfined or a confined aquifer with a completely 

or partially penetrating well. The KGS method assumes the 

following:

1. the unconfined or confined aquifer is infinite in extent, 

homogeneous, and of uniform thickness;

2. the potentiometric surface of the aquifer is initially 

horizontal;

3. the slug is introduced or removed instantaneously to/

from the well;

4. head losses during the test are negligible;

5. the water-level response from the slug test is classified as 

unsteady or overdamped (nonoscillating); and

6. water is released instantaneously from storage with 

decline of hydraulic head.

The KGS model provides corrections for low permeability materials around the well screen, such as mud residue 

from well installation, and can consider hydraulic conductivity 

anisotropy (Arnold, 2015).

A confined KGS model without wellbore skin is identical to the Dougherty-Babu (Dougherty and Babu, 1984) 

solution (Duffield, 2007). This equation takes into account 

the following physical parameters of where the piezometer 

is emplaced: confined aquifer thickness/saturated thickness, 

the distance from the water table to the top of the screen, the 

distance from the water table to the bottom of the piezometer 

screen, the screen thickness, the elevation of the piezometer from the base of the aquifer, and the elevation of the 

top of the piezometer screen from the base of the aquifer 

(Duffield, 2007). For the unconfined KGS model without a 

wellbore skin, the following physical parameters of where the 

piezometer is emplaced are considered: the depth to the top 

of the well screen, the depth below top of aquifer, the screen 

length, and the saturated thickness of the unconfined aquifer 

(Duffield, 2007).

Because the piezometers in the till are not below the 

confining unit (the piezometers are in the confining unit), they 

are not confined and do not have a potentiometric surface as 

a piezometer screened in a confined aquifer does. Therefore, 

because the till piezometers are physically not confined, 

the unconfined solution was used to estimate hydraulic 

conductivity.

Different solutions were used for the piezometers 

screened in the confined aquifers and the unconfined aquifers 

at the sites. The KGS model for confined aquifers without a 

wellbore skin was used for analyzing the results from slug 

tests from piezometers in the confined aquifers that did not 

have an oscillatory response. The KGS model for confined 

aquifers has the same assumptions as for unconfined aquifers, except that the aquifer is confined instead of unconfined 

(Duffield, 2007). The Butler (1998) inertial solution was used 

for results in confined aquifers that indicated inertial effects 

because of an oscillatory water-level response. The Butler 

(1998) inertial solution accounts for oscillatory responses in 

confined aquifers because of a high hydraulic conductivity 

(Duffield, 2007). Assumptions of the Butler (1998) solution 

were the same as the KGS solution, except that the flow is in a 

quasi-steady state and the wells must be partially penetrating 

(Duffield, 2007).

For unconfined aquifer results that indicated inertial 

effects present during the slug tests, the Springer-Gelhar 

(Springer and Gelhar, 1991) inertial solution was used to 

account for the oscillatory effects. The assumptions of the 

Springer-Gelhar (Springer and Gelhar,1991) inertial solution are the same as the Butler (1998) solution, except that 

the aquifer is unconfined and the wells can be either fully or 

partially penetrating.

References Cited

Arnold, L.R., 2015, Monitoring-well installation, slug testing, and groundwater quality for selected sites in South 

Park, Park County, Colorado, 2013: U.S. Geological Survey 

Open-File Report 2014–1231, 32 p., accessed June 2020 at 

https://doi.org/10.3133/ofr20141231.

Butler, J.J., Jr., 1998, The design, performance, and analysis of 

slug tests: Boca Raton, Fla., CRC Press, 280 p.
Appendix 2 Slug Test Information 71

Dougherty, D.E., and Babu, D.K., 1984, Flow to a partially 

penetrating well in a double-porosity reservoir: Water 

Resources Research, v. 20, no. 8, p. 1116–1122. [Also available at https://doi.org/10.1029/WR020i008p01116.]

Duffield, G.M., 2007, AQTESOLV for Windows—Version 4.5 

user’s guide: Reston, Va., HydroSOLVE, Inc., 529 p.

Hyder, Z., Butler, J.J., Jr., McElwee, C.D., and Liu, W., 1994, 

Slug tests in partially penetrating wells: Water Resources 

Research, v. 30, no. 11, p. 2945–2957. [Also available at 

https://doi.org/10.1029/94WR01670.]

Maher, A.-T., Trost, J.J., Witt, A.N., Berg, A.M., Simpkins, 

W.W., and Stark, J.R., 2020, Geochemical data, waterlevel data, and slug test analysis results from till confining 

units and confined aquifers in glacial deposits near Akeley, 

Cromwell, Litchfield, and Olivia, Minnesota, 2015–2018: 

U.S. Geological Survey data release, https://doi.org/

10.5066/P9IXC7D3.

Springer, R.K., and Gelhar, L.W., 1991, Characterization 

of large-scale aquifer heterogeneity in glacial outwash 

by analysis of slug tests with oscillatory responses, Cape 

Cod, Massachusetts, in Mallard, G.E., and Aronson, D.A., 

eds., U.S. Geological Survey Toxic Substances Hydrology 

Program—Proceedings of the technical meeting, Monterey, 

California, March 11–15, 1991: U.S. Geological Survey 

Water-Resources Investigations Report 91–4034, p. 36–40. 

[Also available at https://doi.org/10.3133/wri914034.]
Table 2.1. Water-level and displacement data from slug tests done in piezometers at the Litchfield, Cromwell, Olivia, and hydrogeology field camp fie[ft, foot; BLS, below land surface; gal, gallon; H(0), observed initial displacement; ±, plus or minus; − (negative value), water level change below the initial water level; --, unkn

Piezometer name Slug test number Approximate static water level before falling-head slug test (ft BLS) Approximate static water level before rising-head slug test (ft BLS) Volume of slug (gal) Falling head observed initial displacement, H(0) (ft) Rising head observed initial displacement, H(0) (ft) 

Litchfield field sites LFO1–B 1 11 11 0.080 0.29 −0.44 

LFO1–B 2 11 11 0.080 0.37 −0.50 LFO1–B 3 11 11 0.080 0.55 −0.62 LFO1–C 1 13 13 0.161 2.23 −1.97 LFO1–D 1 26 26 0.160 2.35 −2.47 LFO1–E 1 35 35 0.081 0.68 −0.57 LFO1–E 2 35 35 0.081 1.21 −1.33 LFO1–E 3 35 35 0.081 1.18 −1.50 LFO1–F 1 35 35 0.199 1.05 −1.27 LFO1–F 2 35 35 0.199 1.26 −1.64 LFO2–A 1 11 11 0.080 0.97 −0.93 LFO2–B 1 13 13 0.080 1.13 −1.04 LFO2–C 1 16 16 0.160 2.71 −2.84 LFO2–D 1 36 -- 0.160 2.75 -- LFO2–E 1 62 62 0.160 2.76 −2.94 LFO2–F 1 63 63 0.378 2.63 −3.15 LFO2–F 2 63 63 0.378 2.65 −2.42 LFO2–F 3 63 63 0.378 2.84 −4.02 

Cromwell field sites CWO1–A 1 20 20 0.389 2.37 −2.31 

CWO1–B 1 16 16 0.378 2.87 −3.86 CWO1–B 2 16 16 0.378 2.64 −3.15 CWO1–B 3 16 16 0.378 0.56 −3.72 CWO1–C 1 16 16 0.378 5.85 −3.13 CWO1–C 2 16 16 0.378 3.86 −2.60 CWO1–C 3 16 16 0.378 3.78 −2.65 CWO2–A 1 26 26 0.081 1.52 −1.19 CWO2–A 2 26 26 0.081 1.12 −2.46 CWO2–A 3 26 26 0.081 3.18 −4.56 
Table 2.1. Water-level and displacement data from slug tests done in piezometers at the Litchfield, Cromwell, Olivia, and hydrogeology field camp fieContinued[ft, foot; BLS, below land surface; gal, gallon; H(0), observed initial displacement; ±, plus or minus; − (negative value), water level change below the initial water level; --, unkn

Piezometer name Slug test number

Approximate static

water level before 

falling-head slug 

test (ft BLS)

Approximate static 

water level before 

rising-head slug test 

(ft BLS)

Volume of 

slug (gal)

Falling head observed 

initial displacement, 

H(0) (ft)

Rising head observed 

initial displacement, 

H(0) (ft)

CWO2–B 1 28 28 0.162 2.64 −2.81 CWO2–C 1 26 26 0.175 2.48 −2.51 CWO2–D 1 23 23 0.162 2.77 −2.83 CWO2–E 1 24 24 0.161 2.70 −2.65 

Hydrogeology field camp field sites

HT–115 1 60 60 0.161 2.64 −2.44 HT–115 2 60 60 0.161 2.47 −2.49 HT–115 3 60 60 0.161 2.51 −2.28 HT–140 1 61 61 0.080 1.24 −1.31 HT–140 2 61 61 0.080 1.22 −1.22 HT–140 3 61 61 0.080 1.28 −1.04 HT–175 1 63 63 0.080 1.18 −1.28 HT–175 2 63 63 0.080 1.23 −1.42 HT–175 3 63 63 0.080 1.34 −1.47 HT–200 1 64 64 0.161 2.5 −2.39 HT–200 2 64 64 0.161 2.57 −2.43 HT–200 3 64 64 0.161 2.58 −2.39 HB–3 1 66 66 0.411 2.21 −2.01 HB–3 2 66 66 0.411 1.95 −1.94 HB–3 3 66 66 0.411 2.81 −2.34 MW–01 1 62 62 0.411 2.99 −2.25 MW–01 2 62 62 0.411 3.29 −1.87 MW–01 3 62 62 0.411 3.99 −2.11 

Olivia field sites

OT–13 1 1 1 0.161 1.71 −2.24 OT–13 2 1 1 0.080 0.91 −0.97 OT–13 3 1 1 0.080 0.84 −1.07 OT–20 1 5 5 0.080 1.07 −0.86 OT–20 2 5 5 0.080 0.97 −0.97 
Table 2.1. Water-level and displacement data from slug tests done in piezometers at the Litchfield, Cromwell, Olivia, and hydrogeology field camp fieContinued [ft, foot; BLS, below land surface; gal, gallon; H(0), observed initial displacement; ±, plus or minus; − (negative value), water level change below the initial water level; --, unkn

Piezometer name Slug test number Approximate static water level before falling-head slug test (ft BLS) Approximate static water level before rising-head slug test (ft BLS) Volume of slug (gal) Falling head observed initial displacement, H(0) (ft) Rising head observed initial displacement, H(0) (ft) 

OT–20 3 5 5 0.080 1.07 −1.05 OT–60 1 6 6 0.080 1.18 −1.24 OT–60 2 6 6 0.080 1.17 −0.91 OT–60 3 6 6 0.080 1.19 -- OT–105 1 15 15 0.080 1.27 −1.37 OT–105 2 15 15 0.080 1.27 −1.32 OT–105 3 15 15 0.080 1.27 −1.36 OT–145 1 22 21 0.080 1.21 −1.33 OT–145 2 18 18 0.080 1.24 −1.19 OT–145 3 17 17 0.080 1.43 −1.03 OT–175 1 28 28 0.080 1.22 −1.27 OT–175 1 28 28 0.080 0.88 −1.09 OT–175 3 28 28 0.080 1.09 −1.19 OB–7 1 100 100 0.161 0.90 −0.98 OB–7 2 100 100 0.161 1.71 −1.45 OB–7 3 100 100 0.161 1.36 −1.46 
Appendix 2 Slug Test Information 75

Table 2.2. Aquifer and piezometer parameters specified in AQTESOLV for the analysis of slug test data for piezometers at the Litchfield, 

Cromwell, Olivia, and hydrogeology field camp field sites in Minnesota.

Piezometer name

Static water 

column 

height

Aquifer 

saturated 

thickness

Depth to top 

of filter pack 

(or screen for 

aquifers) 

below reference 

pointa

Length of 

filter pack 

(or screen for 

aquifers)

Radius 

casing

Radius of 

borehole (or 

well for 

aquifers)

Feet

Litchfield field sites

LFO1–B 14 27.94 11.34 2.7 0.0521 0.0521

LFO1–C 40.45 86.71 35.76 6.1 0.0521 0.3438

LFO1–D 49.54 86.71 58.84 5.3 0.0521 0.3438

LFO1–E 59.46 86.71 78.79 5.5 0.0521 0.3438

LFO1–F 91.56 44.00 19.50 9.6 0.0833 0.085

LFO2–A 8.61 105.83 3.82 5.0 0.0521 0.3438

LFO2–B 21.84 105.83 19.12 5.0 0.0521 0.3438

LFO2–C 43.29 105.83 62.91 8.7 0.0521 0.3438

LFO2–D 51.04 105.83 82.35 5.9 0.0521 0.3438

LFO2–E 51.38 105.83 96.83 5.0 0.0521 0.3438

LFO2–F 99.96 44.00 32.54 9.6 0.0833 0.085

Cromwell field sites

CWO1–A 128.38 150.69 119.82 6.0 0.0833 0.2813

CWO1–B 211.28 147.00 47.91 9.6 0.0833 0.085

CWO1–C 323.91 187.03 156.66 16.0 0.0833 0.2813

CWO2–A 6.81 15.85 4.15 2.7 0.0521 0.0521

CWO2–B 31.88 136.54 25.94 5.5 0.0521 0.3438

CWO2–C 55.39 136.54 46.95 7.5 0.0521 0.3438

CWO2–D 83.39 136.54 65.23 13.5 0.0521 0.3438

CWO2–E 104.95 136.54 95.29 5.3 0.0521 0.3438

Hydrogeology field camp field sites

HT–115 52.07 144.00 49.54 6.4 0.055 0.28125

HT–140 76.97 144.00 74.14 6.5 0.055 0.28125

HT–175 109.59 144.00 110.50 6.0 0.055 0.28125

HT–200 133.23 144.00 131.50 9.0 0.055 0.28125

HB–3 157 22.00 8.33 10.2 0.0863 0.08500

MW–01 24 44.00 19.00 5.0 0.0833 0.08500

Olivia field sites

OT–13 9.61 9.61 4.46 5.1 0.055 0.0575

OT–20 16.26 177.00 11.22 5.2 0.055 0.28125

OT–35 28.13 177.00 26.37 5.0 0.055 0.28125

OT–60 50.62 177.00 51.30 5.0 0.055 0.28125

OT–105 87.24 177.00 95.46 5.2 0.055 0.28125

OT–145 121.94 177.00 135.53 5.5 0.055 0.28125

OT–175 145.14 177.00 165.01 6.0 0.055 0.28125

OB–7 109.71 54.00 24.92 4.8 0.0863 0.08333

aReference point is the water table surface for piezometers screened in unconfined aquifers and till; reference point is the till-aquifer boundary for wells 

screened in confined aquifers. 
76 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Table 2.3. Residual statistics and confidence interval of each horizontal hydraulic conductivity estimate made from slug tests done in 

piezometers at the Litchfield, Cromwell, Olivia, and hydrogeology field camp field sites in Minnesota.

[mean residual, the mean difference between the recorded water-level recovery by modeled water-level recovery; %, percent; Kh, horizontal hydraulic conductivity; E, scientific notation denoting exponentiation; for example −2.30E−03 = −2.30×10−3; --, unknown]

Piezometer 

name

Slug test 

number

Falling head Rising head

Mean residual

Standard 

deviation of 

the residuals

95% confidence 

interval of Kh

(plus or minus)

Mean residual

Standard 

deviation of 

the residuals

95% confidence 

interval of Kh

(plus or minus)

Feet Feet per day Feet Feet per day

Litchfield field sites

LFO1–B 1 −2.30E−03 3.76E−02 1.22E+02 1.38E−03 3.60E−02 7.36E+01

LFO1–B 2 2.81E−03 1.09E−02 4.08E+01 4.18E−11 9.28E−11 9.15E−07

LFO1–B 3 −1.99E−01 5.84E−01 7.22E+02 9.05E−03 2.67E−02 6.70E+01

LFO1–C 1 6.01E−03 2.04E−02 9.75E−04 6.57E−02 2.23E−02 1.60E−03

LFO1–D 1 2.27E−02 5.86E−02 6.17E−02 6.89E−03 2.04E−02 1.37E−02

LFO1–E 1 3.27E−03 2.91E−02 2.33E−01 −8.06E−03 2.95E−02 2.45E−01

LFO1–E 2 2.05E−02 7.86E−02 1.43E−01 −5.41E−03 4.75E−02 9.80E−02

LFO1–E 3 −9.36E−02 1.51E−01 2.41E−01 1.71E−02 6.00E−02 7.96E−02

LFO1–F 1 −3.47E−02 9.63E−02 6.37E+01 1.60E−02 9.80E−02 4.94E+01

LFO1–F 2 −2.49E−02 1.36E−01 6.81E+01 3.40E−03 1.21E−01 3.76E+01

LFO2–A 1 −8.31E−03 6.95E−02 8.21E−05 −2.62E−02 6.14E−02 4.09E−05

LFO2–B 1 −3.40E−02 1.12E−01 2.57E−04 −2.74E−03 3.25E−02 5.82E−05

LFO2–C 1 1.84E−01 3.28E−01 7.64E−04 1.79E−01 3.39E−01 5.81E−04

LFO2–D 1 7.01E−04 2.49E−02 1.99E−06 -- -- --

LFO2–E 1 3.70E−03 5.26E−02 4.63E−05 2.60E−02 1.88E−01 7.45E−05

LFO2–F 1 −9.34E−02 4.37E−01 9.45E+01 −8.80E−02 2.41E−01 3.45E+01

LFO2–F 2 −1.93E−01 3.16E−01 7.16E+01 −2.26E−01 2.55E−01 5.77E+01

LFO2–F 3 −5.10E−02 4.34E−01 9.72E+01 2.79E−02 3.36E−01 3.50E+01

Cromwell field sites

CWO1–A 1 9.01E−03 7.95E−02 3.36E−02 5.06E−02 9.23E−02 3.32E−02

CWO1–B 1 2.08E−02 3.19E−01 8.15E+02 −3.39E−02 1.90E−01 8.84E+00

CWO1–B 2 −8.73E−03 7.29E−02 1.97E+00 −1.20E−02 5.21E−02 8.73E+01

CWO1–B 3 9.56E−03 2.21E−01 6.01E+02 8.54E−02 3.84E−01 5.05E+00

CWO1–C 1 −2.36E−01 6.09E−01 2.66E−01 −2.63E−02 9.88E−02 3.36E−02

CWO1–C 2 −1.23E−01 2.43E−01 8.34E−02 2.51E−02 8.94E−02 2.74E−02

CWO1–C 3 −1.80E−01 2.64E−01 8.76E−02 −1.82E−01 4.35E−01 1.31E−01

CWO2–A 1 −3.79E−02 1.62E−01 2.15E+00 −3.37E−03 1.53E−02 1.84E−01

CWO2–A 2 −1.94E−03 1.38E−02 1.52E−01 2.78E−01 4.44E−01 4.08E+00

CWO2–A 3 −5.42E−01 8.06E−01 4.08E+00 −7.36E−01 1.16E+00 6.15E+00

CWO2–B 1 1.46E−02 3.79E−02 3.18E−03 2.27E−02 5.75E−02 3.72E−03

CWO2–C 1 7.33E−03 3.32E−02 5.75E−03 1.74E−02 1.86E−01 4.74E−02

CWO2–D 1 −1.20E−02 5.59E−02 7.00E−04 3.52E−02 9.32E−02 1.40E−03

CWO2–E 1 −8.24E−03 3.44E−02 2.42E−03 7.95E−04 2.87E−02 1.87E−03

Hydrogeology field camp field sites

HT–115 1 −5.01E−02 3.50E−01 5.44E−02 −2.73E−02 2.63E−01 3.62E−02

HT–115 2 −2.13E−02 1.96E−01 2.80E−02 −2.74E−02 2.96E−01 4.04E−02

HT–115 3 −1.90E−02 2.03E−01 3.05E−02 −7.96E−03 1.90E−01 2.71E−02
Appendix 2 Slug Test Information 77

Table 2.3. Residual statistics and confidence interval of each horizontal hydraulic conductivity estimate made from slug tests done in 

piezometers at the Litchfield, Cromwell, Olivia, and hydrogeology field camp field sites in Minnesota.—Continued

[mean residual, the mean difference between the recorded water-level recovery by modeled water-level recovery; %, percent; Kh, horizontal hydraulic conductivity; E, scientific notation denoting exponentiation; for example −2.30E−03 = −2.30×10−3; --, unknown]

Piezometer 

name

Slug test 

number

Falling head Rising head

Mean residual

Standard 

deviation of 

the residuals

95% confidence 

interval of Kh

(plus or minus)

Mean residual

Standard 

deviation of 

the residuals

95% confidence 

interval of Kh

(plus or minus)

Feet Feet per day Feet Feet per day

HT–140 1 3.28E−03 4.82E−02 1.89E−05 −5.89E−03 5.33E−02 1.71E−05

HT–140 2 −2.96E−03 9.24E−02 2.71E−05 −1.37E−02 1.41E−01 5.06E−05

HT–140 3 −1.32E−02 1.43E−01 5.67E−05 −2.76E−03 3.96E−02 7.02E−06

HT–175 1 −1.21E−02 9.99E−02 7.35E−04 −2.88E−03 8.53E−02 8.68E−04

HT–175 2 −2.70E−03 5.88E−02 3.47E−04 −8.54E−03 1.01E−01 7.05E−04

HT–175 3 −9.66E−04 3.85E−02 2.18E−04 −7.29E−03 9.02E−02 6.56E−04

HT–200 1 −4.37E−02 2.95E−01 1.66E−02 −1.70E−02 1.56E−01 8.54E−03

HT–200 2 −2.16E−02 2.03E−01 1.09E−02 −1.02E−02 1.39E−01 8.32E−03

HT–200 3 −1.94E−02 1.98E−01 1.32E−02 −1.44E−02 1.44E−01 9.61E−03

HB–3 1 −1.62E−02 1.30E−01 7.34E+00 6.45E−03 1.14E−01 8.27E+00

HB–3 2 −1.01E−02 1.57E−01 1.10E+01 2.24E−02 1.18E−01 7.88E+00

HB–3 3 −1.80E−03 3.08E−01 1.58E+01 4.74E−03 1.13E−01 5.43E+00

MW–01 1 −4.45E−02 5.59E−01 8.56E+00 −6.70E−03 1.27E−01 1.55E+00

MW–01 2 −3.37E−02 4.38E−01 6.47E+00 −1.09E−02 1.50E−01 1.80E+00

MW–01 3 2.68E−03 3.86E−01 5.50E+00 −9.94E−03 1.94E−01 1.95E+00

Olivia field sites

OT–13 1 −4.45E−02 2.65E−01 2.73E−01 −1.40E−02 1.54E−01 1.18E−01

OT–13 2 −1.78E−02 1.11E−01 1.23E−01 7.96E−04 7.59E−02 1.02E−01

OT–13 3 −1.02E−02 8.95E−02 1.08E−01 3.28E−05 4.90E−02 9.74E−02

OT–20 1 −1.27E−03 7.96E−03 1.90E−05 −2.94E−03 1.59E−02 3.48E−05

OT–20 2 −1.85E−03 9.95E−03 2.11E−05 −1.10E−02 3.52E−02 2.87E−04

OT–20 3 −2.65E−03 1.43E−02 1.89E−05 −1.42E−03 4.73E−02 2.35E−04

OT–60 1 1.32E−03 6.02E−02 4.95E−05 −2.37E−03 5.66E−02 1.28E−05

OT–60 2 −2.81E−03 3.10E−02 1.20E−05 −5.68E−03 2.57E−02 6.45E−06

OT–60 3 −9.82E−03 5.18E−02 2.09E−05 -- -- --

OT–105 1 −3.33E−03 3.85E−02 5.58E−07 −2.36E−03 2.61E−02 9.91E−07

OT–105 2 −6.50E−05 1.09E−02 3.08E−07 −1.32E−04 6.57E−02 2.13E−06

OT–105 3 −2.36E−03 2.36E−02 4.99E−07 −2.07E−04 2.75E−02 8.88E−07

OT–145 1 −4.45E−03 3.38E−02 1.44E−04 −7.67E−03 3.82E−02 1.10E−04

OT–145 2 1.45E−03 1.50E−02 2.73E−05 −4.81E−02 1.00E−01 2.64E−04

OT–145 3 1.36E−03 1.94E−02 3.98E−05 −5.36E−02 9.34E−02 2.62E−04

OT–175 1 −5.40E−03 2.92E−02 8.46E−05 −2.48E−03 3.15E−02 1.06E−04

OT–175 2 3.58E−03 1.88E−02 1.09E−04 2.07E−03 1.68E−02 8.43E−05

OT–175 3 1.51E-03 9.99E−03 5.17E−05 −2.45E−04 1.32E−02 5.69E−05

OB–7 1 −1.24E−03 4.39E−02 4.03E−01 2.13E−03 2.07E−02 8.87E−02

OB–7 2 −3.12E−02 1.13E−02 1.17E+01 5.49E−03 3.07E−02 6.41E−02

OB–7 3 −2.05E−02 5.58E−02 5.79E+00 5.18E−03 2.99E−02 6.26E−02

OT–35 1 -- -- -- 3.01E−02 1.32E+00 8.77E−12
78 Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits

Appendix 3 Quality Assurance for Water-Quality Samples

Two tables (tables 3.1 and 3.2) are included in appendix 3. Listed in table 3.1 is information on replicates of 

groundwater samples for three piezometers, HT–200 from the 

hydrogeology field camp (HFC) site, OT–145 from the Olivia 

site, and LFO2–F from the Litchfield site. Also included in 

table 3.1 is information on equipment blanks analyzed for 

three piezometers, CWO2–C from the Cromwell site, HT–175 

from the HFC site, and OT–60 from the Olivia site. Detailed 

in table 3.2 are laboratory comparisons done for groundwater 

samples from the OT–13, OT–20, and OT–35 piezometers, 

which were collected at the same time for the U.S. Geological 

Survey National Water Quality Laboratory (USGS NWQL) 

and for Rick Knurr (who worked at the University of 

Minnesota Laboratory during the Litchfield and Cromwell 

site analyses and then worked at the Ion Chrom Analytical 

Laboratory during the Olivia and HFC site analyses).

Most analyte concentrations in replicate samples have a 

relative percent difference from the field sample of less than 

10 percent. The following analytes from the HT–200 replicate 

sample had a relative percent difference from the field sample 

of greater than 10 percent: filtered, inflection-point titration 

carbonate concentration with a relative percent difference of 

80 percent, filtered iron concentration with a relative percent 

difference of 11.1 percent, filtered phosphorus concentration 

with a relative percent difference of 11.1 percent, and filtered 

nitrate with a relative percent difference of 10.5 percent. Analyte concentrations in replicate groundwater samples collected 

at OT–145 had no relative percent differences greater than 

10 percent. The following analyte from the LFO2–F replicate 

sample had a relative percent difference from the field sample 

of greater than 10 percent: filtered, inflection-point titration 

carbonate with a percentage difference of 18.2 percent. 

Almost all equipment blanks analyzed were less than 

detection limits for the analytes sampled. The only exception 

was the filtered, inflection-point titration alkalinity for all three 

piezometers, ranging from 1 to 3 milligrams per liter (mg/L) 

as calcium carbonate; filtered, inflection-point titration bicarbonate where the piezometers ranged from 1.6 to 5.2 mg/L; 

and manganese where two of the piezometers (CWO2–C 

and OT–60) had detectable manganese ranging from 0.22 to 

0.24 microgram per liter. These concentrations are still low, 

especially when compared to the concentrations observed in 

the samples.

Three split replicate groundwater samples collected at 

the Olivia site were sent to the USGS NWQL and the Ion 

Chrom Analytical Laboratory and analyzed for major anions 

(table 3.2). Relative percent differences may be large, especially with lower concentrations of analytes. The holding 

time between when the whole samples were first delivered 

to the USGS NWQL and the split samples were sent to the 

Ion Chrom Analytical Laboratory was most likely exceeded 

for the nitrate and nitrite samples. Because of the sensitivity on nutrient anions, this may have led to the presence of 

nitrate or nitrite occurring in the sample analyzed by the Ion 

Chrom Analytical Laboratory, when the sample originally had 

no occurrence of the analytes, as seen in the USGS NWQL 

analysis.

Chloride and bromide concentrations between the two 

laboratories indicate that relative percent differences were low 

for chloride concentrations (less than 2 percent) and tended 

to be higher for bromide concentrations (9 to 20 percent), 

perhaps because of the lower concentrations of bromide 

compared to chloride. Fluoride concentrations tended to be 

slightly higher for the USGS NWQL compared to Ion Chrom 

Analytical Laboratory. Sulfate concentrations were similar 

between both laboratories and all had a relative percent difference of less than 5 percent. Only one piezometer groundwater 

sample can be compared for nitrate as nitrogen (OT–13), 

because the rest of the samples had one or both concentrations less than the detection limit. The resulting comparison 

of nitrate as nitrogen had a low relative percent difference, at 

3.5 percent. The relative percent differences may represent 

variation between the laboratory analyses and uncertainty with 

laboratory procedures.

Reference Cited

U.S. Geological Survey, 2019, USGS water data for the 

Nation: U.S. Geological Survey National Water Information 

System database, accessed May 20, 2020, at https://doi.org/

10.5066/F7P55KJN.
Table 3.1. Quality assurance data, including field replicates and field blanks, for groundwater samples collected from piezometers at the Litchfield, Chydrogeology field camp field sites in Minnesota.[HT–200 and HT–175, hydrogeology field camp piezometers; OT–145 and OT–60, Olivia piezometers; LFO2–F, Litchfield piezometer; CWO2–C, Cromwell piezometer; mg/Lcalcium carbonate; <, less than; --, unknown; μg/L, microgram per liter; N, nitrogen; P, phosphorus; SiO2, silica]

Constituent

HT–200 OT–145 LFO2–F CWO2

aGroundwater replicate value (brelative percent difference) E

Alkalinity, water, filtered, inflection-point titration (mg/L as CaCO3) 190 (2.1) 359 (1.7) 219 (3.7) 3 Ammonia (NH3 + NH4+), water, filtered (mg/L) 0.14 (0) 1.28 (0.8) 0.66 (1.5) <0.0Bicarbonate, water, filtered, inflection-point titration (mg/L) 230 (1.8) 434 (1.8) 266 (3.4) 5.2 Bromide, water, filtered (mg/L) <0.01 (0) 0.023 (9.1) 0.045 (6.5) <0.0Calcium, water, filtered (mg/L) 52.4 (0.9) 46.2 (3.1) 76.6 (0) <0.0Carbon dioxide, water, unfiltered (mg/L) 11 (0) 24 (4.1) 18 (0) -- Carbonate, water, filtered, inflection-point titration (mg/L) 0.7 (80) 1.7 (6.1) 0.6 (18.2) <0.1 Chloride, water, filtered (mg/L) 0.56 (3.5) 13.4 (2.2) 13.6 (0.7) <0.0Dissolved oxygen, water, unfiltered, percent of saturation (%) 2 (0) 2 (0) 2 (0) -- Dissolved solids dried at 180 degrees Celsius, water (mg/L) 213 (2.4) 465 (1.5) 316 (7.9) <20 Dissolved solids, water, filtered, sum of constituents (mg/L) 219 (0.5) 452 (1.8) 342 (1.5) <3 Dissolved oxygen, water, unfiltered (mg/L) 0.2 (0) 0.2 (0) 0.2 (0) -- Fluoride, water, filtered (mg/L) 0.1 (0) 0.69 (2.9) 0.21 (4.7) <0.0Hardness, water mg/L as CaCO3 186 (1.6) 198 (2.6) 299 (0) <0.1 Iron, water, filtered (μg/L) 71.8 (11.1) 1630 (4.2) 2120 (1.4) <4 Magnesium, water, filtered (mg/L) 13.4 (2.2) 20 (1.0) 26.1 (0.4) <0.0Manganese, water, filtered (μg/L) 485 (1.7) 100 (5.8) 92.1 (0.4) 0.2Nitrate, water, filtered (mg/L as N) <0.009 (10.5) <0.01 (0) <0.01 (0) <0.0Nitrite, water, filtered (mg/L as N) <0.001 (0) <0.001 (0) <0.001 (0) <0.0Nitrate plus nitrite, water (mg/L as N) <0.01 (0) <0.01 (0) <0.01 (0) <0.0pH, water, unfiltered, field, standard units 7.5 (0) 7.5 (0) 7.4 (0) -- Phosphorus, water, filtered (mg/L as P) 0.019 (11.1) 0.134 (--) 0.119 (5.2) <0.0Potassium, water, filtered (mg/L) 1.53 (1.3) 1.84 (4.3) 3.52 (0.6) <0.0Silica, water, filtered (mg/L as SiO2) 17.6 (1.1) 26.1 (0) 29.1 (1.0) <0.0Sodium, water, filtered (mg/L) 7.08 (0.6) 104 (2.8) 9.91 (0) <0.0Sulfate, water, filtered (mg/L) 11.5 (0.9) 21.3 (7.7) 48.1 (0) <0.0

aOnly the replicate value is shown in this table, the regular sample concentrations can be retrieved from the U.S. Geological Survey National Water Information System (U.S. b(Concentration1 – concentration2)/[(concentration1 + concentration2)/2].
Table 3.2. Comparison of anion concentrations measured in split replicate groundwater samples by Rick Knurr (University of Minnesota and Ion Chrand the U.S. Geological Survey National Water Quality Laboratory (USGS NWQL). [lab, laboratory; mg/L, milligram per liter; N, nitrogen; <, less than; --, not calculated] Constituent Samples collected October 24, 2017 Sample OT–13 Sample OT–20 S

R. Knurr lab USGS NWQL aRelative percent difference R. Knurr lab USGS NWQL Relative percent difference R. Knurr lab 

Concentration Concentration Concentrat

Bromide, water, filtered (mg/L) 0.041 0.034 18.7 0.112 0.125 11.0 0.274 

Chloride, water, filtered (mg/L) 7.242 7.12 1.7 39.714 40 0.7 13.196 

Fluoride, water, filtered (mg/L) 0.368 0.48 26.4 0.242 0.34 33.7 0.625 

Nitrate, water, filtered (mg/L as N) 1.098 1.06 3.5 0.022 <0.009 -- 0.013 <

Nitrite, water, filtered (mg/L as N) <0.003 0.012 -- <0.237 <0.001 -- 0.677 <

Sulfate, water, filtered (mg/L) 19.331 19.2 0.7 41.826 42.2 0.9 13.755 

a(Concentration1 – concentration2)/[(concentration1 + concentration2)/2]. 
For more information about this publication, contact

Director, USGS Upper Midwest Water Science Center

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Mounds View, MN 55112

(763) 783–3100

For additional information visit https://www.usgs.gov/centers/

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Trost and others—Hydrogeology and Groundwater Geochemistry of Till Confining Units and Confined Aquifers in Glacial Deposits—Scientific Investigations Report 2020–5127

ISSN2328-0328 (online)

https://doi.org/10.3133/sir20205127