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U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005-5120

National Water-Quality Assessment Program

Water-Quality Assessment of Part of the Upper Mississippi 

River Basin, Minnesota and Wisconsin—Ground-Water 

Quality along a Flow System in the Twin Cities 

Metropolitan Area, Minnesota, 1997-98
Water-Quality Assessment of Part of the 

Upper Mississippi River Basin, Minnesota 

and Wisconsin—Ground-Water Quality 

along a Flow System in the Twin Cities 

Metropolitan Area, Minnesota, 1997-98

By William J. Andrews, James R. Stark, Alison L. Fong, and James D. Fallon

Prepared as part of the

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005-5120
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Additional earth science information is available on the Internet via the World Wide Web.

You may connect to the National Water Quality Assessment Program (NAWQA) Home Page 

using the Universal Resource Locator (URL) at http://water.usgs.gov/nawqa, 

to the U.S. Geological Survey Home Page at http://www.usgs.gov,

to the U.S. Geological Survey Water Science Center of Minnesota Home Page at http://mn.water.usgs.gov, 

or to the NAWQA Upper Mississippi River Basin Project Home Page at http://mn.water.usgs.gov/umis/index.html
iii

Foreword

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely 

scientific information that helps enhance and protect the overall quality of life, and facilitates 

effective management of water, biological, energy, and mineral resources. (http://www.usgs.

gov/). Information on the quality of the Nation’s water resources is of critical interest to the 

USGS because it is so integrally linked to the long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable for industry, irrigation, and habitat for fish 

and wildlife. Escalating population growth and increasing demands for the multiple water uses 

make water availability, now measured in terms of quantity and quality, even more critical to the 

long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support 

national, regional, and local information needs and decisions related to water-quality management and policy. (http://water.usgs.gov/nawqa/). Shaped by and coordinated with ongoing 

efforts of other Federal, State, and local agencies, the NAWQA Program is designed to answer: 

What is the condition of our Nation’s streams and ground water? How are the conditions changing over time? How do natural features and human activities affect the quality of streams and 

ground water, and where are those effects most pronounced? By combining information on 

water chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program 

aims to provide science-based insights for current and emerging water issues and priorities. 

NAWQA results can contribute to informed decisions that result in practical and effective waterresource management and strategies that protect and restore water quality.

Since 1991, the NAWQA Program implemented interdisciplinary assessments in more than 50 of 

the Nation’s most important river basins and aquifers, referred to as Study Units. (http://water.

usgs.gov/nawqa/nawqamap.html). Collectively, these Study Units account for more than 60 percent of the overall water use and population served by public water supply, and are representative of the Nation’s major hydrologic landscapes, priority ecological resources, and agricultural, 

urban, and natural sources of contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling 

and analysis. The assessments thereby build local knowledge about water-quality issues and 

trends in a particular stream or aquifer while providing an understanding of how and why water 

quality varies regionally and nationally. The consistent, multi-scale approach helps to determine 

if certain types of water-quality issues are isolated or pervasive, and allows direct comparisons 

of how human activities and natural processes affect water quality and ecological health in the 

Nation’s diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic ecology are developed at 

the national scale through comparative analysis of the Study-Unit findings. (http://water.usgs.

gov/nawqa/natsyn.html).

The USGS places high value on the communication and dissemination of credible, timely, and 

relevant science so that the most recent and available knowledge about water resources can 

be applied in management and policy decisions. We hope this NAWQA publication will provide 

you the needed insights and information to meet your needs, and thereby foster increased 

awareness and involvement in the protection and restoration of our Nation’s waters.
iv

The NAWQA Program recognizes that a national assessment by a single program cannot 

address all water-resource issues of interest. External coordination at all levels is critical for a 

fully integrated understanding of watersheds and for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The Program, therefore, depends extensively on the advice, cooperation, and information from other Federal, State, interstate, Tribal, 

and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
v

Contents

Foreword ..................................................................................................................................................... iii

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

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

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

Acknowledgments ............................................................................................................................... 2

Description of the Study Area .................................................................................................................... 2

Population and Water Use ................................................................................................................. 2

Land Use ................................................................................................................................................ 2

Climate................................................................................................................................................... 4

Methods ......................................................................................................................................................... 4

Wells and Multipiezometers .............................................................................................................. 4

Sediment Samples and Analyses ...................................................................................................... 4

Water Samples and Analyses ............................................................................................................ 4

Ground-Water-Flow Modeling ........................................................................................................... 9

Hydrogeology ................................................................................................................................................ 9

Estimated Hydraulic Conductivities ................................................................................................ 10

Sediment pH, and Organic and Inorganic Carbon Content ........................................................ 10

Water Quality ............................................................................................................................................... 11

Physical Properties ........................................................................................................................... 11

Major Ions ........................................................................................................................................... 13

Iron and Manganese ......................................................................................................................... 13

Nutrients and Organic Carbon ......................................................................................................... 13

Nitrogen Isotopes and Dissolved Gases ........................................................................................ 22

Hydrogen and Oxygen Isotope Ratios ............................................................................................ 22

Radon ................................................................................................................................................. 22

Pesticides ........................................................................................................................................... 23

Volatile Organic Compounds ............................................................................................................ 25

Ground-Water Flow Modeling .................................................................................................................. 28

Model Description and Calibration ................................................................................................. 28

Model Limitations .............................................................................................................................. 30

Recharge Age Dating ................................................................................................................................. 30

Implications for Ground-Water Management ........................................................................................ 33

Summary and Conclusions ........................................................................................................................ 33

References ................................................................................................................................................... 35

Appendix—Quality Assurance/Quality-Control Samples .................................................................... 39
vi

Figures

1. Maps showing location of the urban flow-system study area, surficial geology, 

wells, and water-table altitude in surficial aquifers in the vicinity of the study. ............... 3

2. Hydrogeologic sections showing dissolved-oxygen concentrations in water 

samples from the flow-system well and multipiezometers ................................................ 12

3. Boxplots showing physical and chemical properties of water samples from the 

flow-system well and multipiezometers. ............................................................................... 14

4. Hydrogeologic sections showing specific conductance of water samples from 

the flow-system well and multipiezometers. ......................................................................... 15

5. Boxplots showing major ion concentrations in water samples from the 

flow-system well and multipiezometers. ............................................................................... 16

6. Trilinear diagram showing major ion concentrations in water samples from the flowsystem well and multipiezometers, July 1997 ....................................................................... 17

7. Hydrogeologic sections showing chloride concentrations in water samples 

from the flow-system well and multipiezometers. ............................................................... 18

8. Boxplots showing nutrient concentrations in water samples from the 

flow-system well and multipiezometers. ............................................................................... 19

9. Hydrogeologic sections showing dissolved nitrite plus nitrate as nitrogen concentrations in water samples from the flow-system well and multipiezometers .................. 20

10. Hydrogeologic section showing dissolved ammonia plus organic nitrogen concentrations in water samples from the flow-system well and multipiezometers .................. 21

11. Graph showing nitrogen and argon gas concentrations in water samples from the 

flow-system well and multipiezometers, July 1997. ............................................................. 23

12. Graph showing correlation between δ2H and δ180 in water samples from the flowsystem well and multipiezometers. ......................................................................................... 23

13. Hydrogeologic section showing total detected dissolved pesticide concentrations 

in water samples from the flow-system well and multipiezometers ................................. 24

14. Graph showing frequencies of detection of pesticides in water samples from the 

flow-system well and multipiezometers. ............................................................................... 25

15. Hydrogeologic sections showing total detected volatile organic compound concentrations in water samples from the flow-system well and multipiezometers .................. 26

16. Graph showing frequencies of detection of volatile organic compounds in water 

samples from the flow-system well and multipiezometers. ............................................... 27

17. Schematic of model configuration, simulated water-table altitude contours, flow 

system, and simulated flow system. ...................................................................................... 29

18. Hydrogeologic section showing recharge-age dates of water samples from the 

flow-system well and multipiezometers, July 1997 .............................................................. 31

19. Graph showing relations of selected dates of recharge, as determined by chlorofluorocarbon concentrations, to selected properties and constituents in water 

samples from the flow-system well and multipiezometers. ............................................... 32
vii

Tables

1. Laboratory analytical methods for measured water-quality constituents for

flow-system study, Twin Cities metropolitan area, Minnesota. ........................................... 5

2. Pesticide compounds analyzed in water samples for flow-system study, 

Twin Cities metropolitan area, Minnesota. .............................................................................. 6

3. Volatile organic compounds analyzed in water samples for flow-system study, 

Twin Cities metropolitan area, Minnesota. .............................................................................. 7

4. Estimated horizontal hydraulic conductivity values calculated fromsediment-size 

distributions collected from borings for a well and multipiezometers, Twin Cities 

metropolitan area, Minnesota. ................................................................................................ 10

5. Sampling depths, depths to water table, sediment pH, and organic and inorganic

carbon contents for the flow-system well and multipiezometers, Twin Cities

metropolitan area, Minnesota. ................................................................................................ 11

6. Hydraulic properties of the modeled study area, Twin Cities metropolitan area,

Minnesota. .................................................................................................................................. 30

7. Recharge-age dates and model estimates for ground water sampled from the

flow-system well and multipiezometers, Twin Cities metropolitan area, Minnesota. .... 31

8. Laboratory reporting levels and method detection limits and concentrations of

volatile organic compounds detected in three volatile organic compound

source-solution blanks for the flow-system study, Twin Cities metropolitan area,

Minnesota. .................................................................................................................................. 39

9. Laboratory reporting levels, method detection limits, and concentrations of

constituents in field blanks for flow-system study, Twin Cities metropolitan area,

Minnesota. .................................................................................................................................. 40

10. Recovery percents from Schedule 2010 pesticide spikes for multipiezometers, 

Twin Cities metropolitan area, Minnesota. ............................................................................ 41

11. Recovery percents from Schedule 2020 volatile organic compound spikes for

multipiezometers, Twin Cities metropolitan area, Minnesota. ........................................... 42

12. Laboratory reporting levels and concentrations of compounds with greater than 10

percent relative percent differences in replicate concentrations for

multipiezometers, Twin Cities metropolitan area, Minnesota. ........................................... 44
viii

Conversion Factors, Water-Quality Units, and 

Vertical Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

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

)

Flow rate

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

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

 °F=(1.8×°C)+32

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

 °C=(°F-32)/1.8

Chemical concentrations: Chemical concentrations of substances in water are given in metric 

units of milligrams per liter (mg/L) and micrograms per liter (μg/L). Milligrams per liter is a unit 

expressing the concentration of chemical constituents in solution as mass (milligrams) of solute 

per unit volume (liter) of water. Micrograms per liter is a unit expressing the concentration of 

chemical constituents in solution as mass (micrograms) of solute per unit volume (liter) of water. 

One milligram is equivalent to 1,000 micrograms.

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

(NAVD 88).

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

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

25°C).
Water-Quality Assessment of Part of the Upper 

Mississippi River Basin, Minnesota and Wisconsin—

Ground-Water Quality along a Flow System in the Twin 

Cities Metropolitan Area, Minnesota, 1997-98

By William J. Andrews1, James R. Stark1, Alison L. Fong2, and James D. Fallon1

Abstract

As part of a national analysis of the effects of land use on 

ground-water quality for the National Water-Quality Assessment Program, the U.S. Geological Survey sampled wells 

along a flow system in surficial glacial aquifers in the northwestern part of the Twin Cities metropolitan area during 1997 

and 1998. In addition, a reconnaissance steady-state groundwater model was developed to estimate flowpaths and dates of 

ground-water recharge using a particle-tracking routine.

Sediment samples collected during drilling had high 

horizontal hydraulic conductivities (ranging from about 3.1 to 

about 190 feet per day, based on sediment-size analysis of well 

cuttings), small organic carbon concentrations (ranging from 

less than 0.2 to 160 grams per kilogram), and pH values that 

were mostly alkaline (ranging from 4.9 to 8.2).

Water samples were analyzed for physical properties, 

major ions, iron, manganese, nutrients, organic carbon, radon, 

pesticides, volatile organic compounds (VOCs), chlorofluorocarbons, tritium, and isotopes of nitrogen, hydrogen, and 

oxygen. Most of the water samples had small dissolved-oxygen concentrations (less than 1 milligram per liter). Calcium, 

magnesium, sodium, bicarbonate, sulfate, and chloride were 

the primary dissolved constituents in water samples. Nitrite 

plus nitrate as nitrogen (nitrate) concentrations were less 

than the U.S. Environmental Protection Agency (USEPA) 

Maximum Contaminant Level of 10 mg/L. Nitrogen isotope 

ratios indicated that the sources of nitrate primarily were soils, 

animal waste, or denitrification that increased nitrogen isotope 

ratios in nitrate.

Small concentrations of pesticides were detected in the 

shallow parts of the aquifer. The herbicide prometon was 

the most frequently detected pesticide. Herbicides applied to 

control grasses and weeds in corn (atrazine, simazine, and 

metolachlor) also were frequently detected in water samples. 

All pesticide and VOCs detected were below USEPA Maximum Contaminant Levels or Health Advisory Limits. Chlorofluorocarbon compounds and tritium concentrations were used 

to estimate dates of recharge of ground-water samples. 

In general, shallower ground-water samples were more 

recently recharged although most water sampled from the 

aquifer was recharged after 1955.

Although land use had substantial effects on groundwater quality, the distribution of contaminants in the aquifer 

also is affected by complex combinations of factors and 

processes that include sources of natural and anthropogenic 

contaminants, three-dimensional advective flow, physical 

and hydrologic settings, age and evolution of ground water, 

and transformation of chemical compounds along the flow 

system. Compounds such as nitrate and dissolved oxygen 

were greatest in water samples from the upgradient end of the 

flow system and near the water table. Specific conductance 

and dissolved solids increased along the flow system and with 

depth due to increase in residence time in the flow system and 

dissolution of aquifer materials.

Introduction

In 1991, the U.S. Geological Survey (USGS) began 

implementation of the National Water-Quality Assessment 

(NAWQA) Program. Long-term goals of NAWQA include 

describing the status and trends in the quality of large representative parts of the Nation’s surface- and ground-water 

resources, and identifying the major natural and anthropogenic 

factors that affect the quality of the Nation’s water resources. 

To meet these goals, nationally consistent data useful to 

policy makers, scientists, and managers are being collected 

and analyzed. Because assessment of the water quality in the 

entire Nation is impractical, major activities of the NAWQA 

Program take place within a set of hydrologic systems called 

Study Units, which contain diverse hydrologic systems of river 

basins or aquifers. 1

U.S Geological Survey

2

City of Minneapolis, Environmental Management
The Upper Mississippi River Basin Study (UMIS) Unit 

(fig. 1) encompasses about 47,000 mi2 in Minnesota, Wisconsin, South Dakota, Iowa, and North Dakota, and includes the 

Twin Cities (Minneapolis and St. Paul, Minn.) metropolitan 

area (TCMA). The three major rivers in the UMIS are the 

Mississippi, Minnesota, and St. Croix. In 1990, about 3.7 million people resided in the UMIS, mostly in the TCMA (U.S. 

Bureau of Census, 1991). Most ground-water withdrawals 

in the TCMA are from surficial glacial aquifers or from the 

Prairie du Chien-Jordan bedrock aquifer.

Study-unit activities involving ground water included 

assessment of ground water and land use combinations using 

three sampling strategies: (1) regional studies of selected 

major aquifers (subunit surveys), (2) targeted-area studies of 

selected land-use areas (land-use studies), and (3) this localized study of processes occurring along a shallow groundwater-flow system (flow-system study). This study emphasizes 

shallow ground water, the quality of which is likely to be 

influenced by land use and land cover. 

The flow-system study described in this report is 

within the area of an urban land-use study conducted in 

1996 (Andrews and others, 1998). The urban land-use study 

sampled shallow wells completed in surficial (glacial) aquifers. This flow-system study was designed to obtain information about ground-water flow and chemical processes occurring throughout the thickness of the surficial aquifers. This 

study had four objectives: (1) determine the presence, spatial 

distribution, and temporal variability of selected constituents 

in ground water along a general flow system from areas of 

recharge to discharge; (2) relate the presence and distribution of selected constituents in ground water to land use and 

physical factors; (3) develop an understanding of the transport 

of the selected constituents from land surface through the flow 

system; and (4) provide an estimate of the time needed for 

dissolved constituents to move through the flow system.

Purpose and Scope

This report summarizes hydrologic and water-quality data 

collected in 1997 and 1998 from one existing monitoring well 

and six multiport samplers (multipiezometers) installed along 

a transect about parallel to ground-water flow in Brooklyn 

Park and Brooklyn Center, Minnesota. Collected data characterize spatial changes in ground-water quality along the 

flow system within the urban land-use study area described in 

Andrews and others (1998). Water-quality data are presented 

for physical properties, major ions, iron, manganese, nutrients, 

organic carbon, radon, pesticides, volatile organic compounds, 

and isotopes of nitrogen, hydrogen and oxygen. Data also 

are presented for chlorofluorocarbons and tritium, which 

were used to estimate the age of recharge water. Results of a 

reconnaissance steady-state, ground-water-flow model used to 

estimate flowpaths and dates of ground-water recharge, using 

a particle-tracking routine, also are presented.

Acknowledgments

The authors express appreciation to the following people 

and agencies for allowing the installation of monitoring wells 

on their properties and for their interest in the project: 

Mr. Steve Shelstead of the Anoka-Hennepin Independent 

School District #279 and Mr. Scott Brink of the City of 

Brooklyn Center Engineering Department. The authors also 

thank Dr. James Piegat and Ms. Leigh Harrod, formerly of the 

Hennepin County Conservation District, for their assistance 

in sampling and for evaluating land uses, ages of development, geology, and significant local features of the study area. 

Appreciation is extended to Larry Puckett of the U.S. Geological Survey who assisted with sampling. Dr. Mark Pearson 

and Emily Youcha, formerly of the University of Minnesota; 

Alan Burns, Timothy Cowdery, and Geoffrey Delin of the 

U.S. Geological Survey who assisted in the preparation of the 

ground-water-flow model. Paul Hansen, formerly of the U.S. 

Geological Survey, and Robert Borgstede of the U.S. Geological Survey for assistance with the preparation of illustrations. 

Ginger Amos, formerly of the U.S. Geological Survey, helped 

with editorial suggestions and direction.

Description of the Study Area

The study area for this report (fig. 1) is located in the 

northwestern part of the TCMA. The area consists primarily 

of suburban residential and commercial development. Shingle 

Creek and several major roadways cross the flow system.

Population and Water Use

Population density in the study area during 1995 was 

about 1,530 people per mi2 (Metropolitan Council, 2003). 

Public water supplies included 12.6 Mgal/d with greater than 

90 percent of withdrawals from ground water. The cities of 

Brooklyn Center and Brooklyn Park withdraw ground water 

from wells completed in the surficial aquifers composed of 

sand and gravel terrace deposits and in underlying bedrock 

aquifers. In addition to municipal wells, there are approximately 200 low-capacity domestic or lawn-irrigation wells 

completed in surficial aquifers in the study area that pump an 

estimated 15,000 gal/d during summer months (James Piegat, 

Hennepin Conservation District, written commun., 1997).

Land Use

Land use and land cover can affect the presence of contaminants in ground water. Fertilizers and pesticides applied 

to lawns can leach to ground water. Roadways and railroads 

are sources of sodium and chloride from de-icing salt. Pesticides and volatile organic compounds (VOCs) emitted to the 

atmosphere and applied to the land surface also are potential 

2 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Figure 1. Location of the urban flow-system study area, surficial geology, wells, and water-table altitude in surficial aquifers in the 

vicinity of the study.

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Description of the Study Area 3
contaminants. Commercial and industrial establishments may 

also discharge VOCs, metals, and other substances to the 

atmosphere or to the land surface in the course of their activities.

Climate

Average monthly temperatures range from 11 degrees 

Fahrenheit in January to 74 degrees Fahrenheit in July (Minnesota State Climatologist, electronic commun., 1995). Average 

annual precipitation is between 28 and 30 inches (Minnesota 

State Climatologist, 1995). About three-fourths of annual 

precipitation falls from May through September (Baker and 

others, 1979). Mean annual evaporation is approximately 

36 inches (Farnsworth and others, 1982). Seasonal fluctuations 

in temperature and rainfall can affect solubilities of VOCs in 

rainfall, volatilization of VOCs to the atmosphere, and seasonal loadings of pesticides in rainfall.

Methods

Hydrologic and water-quality data for this report were 

from one existing monitoring well and six multiport samplers 

(multipiezometers). Collected data characterized groundwater hydrology and water quality along the flow system. A 

reconnaissance steady-state ground-water flow-model was 

constructed to estimate flowpaths and dates of ground-water 

recharge using a particle tracking routine.

Wells and Multipiezometers

A transect of sites along the ground-water-flow system 

including an existing well (HN-K) and multipiezometers 

samplers (UFP-1 to UFP-6) (fig. 1) was selected on the basis 

of estimated ground-water flow directions and the location 

of streams and wetlands. Multipiezometer samplers (Delin 

and Landon, 1996) were constructed of 0.167 ft insidediameter (ID) by 10-ft long schedule 40 polyvinyl chloride 

(PVC) casings with 3.12 x 10-3 ft diameter stainless steel 

sampling tubes. The annular space between the casing and the 

borehole was packed with sand. In cases where natural sand 

collapse did not completely fill the borehole surrounding the 

open interval, commercially prepared washed sand was added 

to a level approximately 1 ft above the top of the ports. The 

remaining annular spaces were grouted with bentonite slurry 

to within 2 ft of the land surface. The top 2 ft of the boreholes 

were grouted with concrete. One-half foot ID, schedule 40 

steel protective casings, with locking aluminium caps, were 

set into mortar to a depth of 4 ft, and extended 2 ft above land 

surface. All wells and multipiezometers were constructed in 

accordance with Minnesota Department of Health regulations 

(Minnesota Department of Health, 1994) and NAWQA specifications (Lapham and others, 1995). Multipiezometer ports 

were developed by surging with a peristaltic pump. Sampling 

ports were designated as follows: (1) all multipiezometer 

wells were denoted as “UFP-X” with “X” ranging from 1 

downgradient from well HN-K to 6 at the downgradient end of 

the flow system, near the Mississippi River. At each multipiezometer, individual ports were designated with letters A-D. 

“A” ports were completed above the water table for possible 

sampling of gases in the unsaturated zone, “B” ports were 

completed just below the water table, “C” ports were approximately equidistant between the water table and the bottom of 

the aquifer, and “D” ports were near the bottom of the aquifer. 

Water levels were measured before water samples were collected using the wetted-tape method.

Sediment Samples and Analyses

Sediment samples from the unsaturated zone, and from 

the first few feet of the saturated zone, were collected during 

well drilling with split-spoon samplers. Sediment was analyzed for pH, sediment-size distribution, and organic-carbon 

content. Sediment-size distributions were analyzed by sieving 

(Guy, 1969) for sandy samples and by pipet analysis (Guy, 

1969) for fine-grained samples at the USGS Sediment Laboratory in Iowa City, Iowa. Organic and inorganic carbon contents 

of sediment were analyzed by induction furnace and a modification of the Van Slyke Method (Wershaw and others, 1987) 

at the USGS National Water-Quality Laboratory in Lakewood, 

Colorado. Sediment sample pH values were measured in the 

field by mixing samples with deionized water on a 1:1 weight 

basis and measuring the pH of the resulting solution.

Hydraulic conductivity values were estimated from sediment-size distributions using Hazen’s equation (Freeze and 

Cherry, 1979):

(1) � ����

� �

where K is hydraulic conductivity in centimeters per second 

(cm/s), A equals 1.0 centimeter per second per square millimeter (cm/s/mm2) and d10 is the sediment-size diameter (in 

millimeters) at which 10 percent of the particles by weight are 

finer and 90 percent are coarser.

Water samples and analyses

Physical properties of specific conductance, pH, water 

temperature, turbidity, dissolved oxygen, and alkalinity, as 

well as air temperature and barometric pressure, were measured in the field with instruments calibrated with a mercury 

manometer and standard solutions (obtained commercially 

for pH and from the USGS Quality of Water Service Unit in 

Ocala, Florida for specific conductance and dissolved oxygen). An equivalent of five standing volumes of water in each 

access tube was withdrawn through the ports, and stability of 

water chemistry was verified through periodic measurements 

of properties made while purging the ports prior to sampling. 

Water samples were collected during July 1997, October 1997, 

4 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
and August 1998, using a sealed system of viton or nylon 

tubing according to protocols described in Koterba and 

others (1995). Water samples were collected from most of 

the 16 sampling ports once during each of the three sample 

periods. Water samples were shipped overnight to the USGS 

National Water-Quality Laboratory in Lakewood, Colorado 

where analyses were done according to USGS protocols for 

analysis, quality assurance, and quality control (table 1). 

Water-quality results are available in the USGS National Water 

Information System (NWIS) at http://water.usgs.gov/nwis. 

Quality-assurance sample data are listed and described in the 

appendix.

Samples were analyzed for a variety of constituents. 

These included major ions, nutrients, selected pesticides, 

and VOCs. The groups of constituents analyzed are listed in 

table 1. The water-soluble pesticides analyzed are shown in 

table 2. The VOCs analyzed are shown in table 3.

Samples analyzed for chlorofluorocarbons (CFCs) 

CFC-11, CFC-12, CFC-l3, and CFC-113 were collected using 

sampling methods and protocols described by Busenberg 

and Plummer (1992). Analyses were by electron-capture 

gas chromatography, with a detection limit of 0.3 picogram 

per kilogram of water (pg/kg) for CFC-11 and CFC-12, and 

1.0 pg/kg for CFC-113. Sample ages were assigned based on 

comparison of CFC equilibrium partial pressures, corrected 

for recharge temperature, with a chronology of atmospheric 

partial pressures (Thompson and Hays, 1979). Dissolved-gas 

analyses, used to determine redox states of water samples, 

were made using gas chromatographic methods.

Minimum laboratory reporting limits used in this report 

are based on the Method Detection Limit (MDL) and the 

Laboratory Reporting Level (LRL). The MDL is the minimum concentration of a substance that can be measured and 

reported with 99-percent confidence that the analyte concentration is greater than zero. It is determined from the analysis 

of a sample in a given matrix containing the analyte. At the 

MDL concentration, the risk of a false positive is predicted to 

be less than or equal to 1 percent. The LRL is generally equal 

to twice the yearly determined Long-Term Method Detection 

Level (LT-MDL). The LT-MDL is derived by determining the 

standard deviation of a minimum of 24 MDL spike sample 

measurements over an extended period of time. LT-MDL 

data are collected on a continuous basis to assess year-to-year 

variations in the LT-MDL. The LT-MDL controls false positive 

error. The chance of falsely reporting a concentration at or 

greater than the LT-MDL for a sample that does not contain 

the analyte is predicted to be less than or equal to 1 percent. 

These definitions can be accessed at URL http://nwql.usgs.

gov/Public/ltmdl/glossary.html.

Comparisons of water-quality data were made using the 

nonparametric Kruskal-Wallis test, a statistical test which 

compares groups of data based on data ranks, and which is 

well suited to data that are normally distributed (Helsel and 

Hirsch, 1992; Miller, 1985) and because sample sizes were 

small and variable. For comparisons between categories, multiple stage tests were performed on Kruskal-Wallis statistics 

(Helsel and Hirsch, 1992). Results were considered statistically significant between p <0.05 during the first step of the 

test, and p <0.025 for the final step between any two categories (Helsel and Hirsch, 1992).

Table 1. Laboratory analytical methods for measured water-quality constituents for flow-system study, Twin Cities metropolitan area, Minnesota.

[USGS, U.S. Geological Survey]

Constituent or constituent group Reference

Major ions (USGS schedule 2750) Fishman and Friedman (1989)

Nutrients (USGS schedule 2752) Fishman and Friedman (1989)

Dissolved organic carbon (USGS schedule 2085) Brenton and Arnett (1993)

Nitrogen isotopes (USGS procedure 1718) Bremner (1965), Bremner and Edwards (1965), Liu (1979), and 

Minagawa and others (1984)

Hydrogen and oxygen isotopes in water (USGS labcode 489) Coplen and others (1991), and Epstein and Mayeda (1953)

Radon (USGS labcode 1369) American Society for Testing and Materials (2002)

Pesticides (USGS schedule 2010) Zaugg and others (1995)

Volatile organic compounds (USGS custom method 9090) Rose and Schroeder (1995)

Chlorofluorocarbons Busenberg and Plummer (1992)

Dissolved gases Busenberg andPlummer (1992)

Tritium (USGS labcode 1565) Ostlund and Dorsey (1975)

Methods 5
Table 2. Pesticide compounds analyzed in water samples for flow-system study, Twin Cities metropolitan area, 

Minnesota.

[All units in micrograms per liter; CAS, Chemical Abstract Service]

Compound CAS Number

Laboratory 

reporting level

Linuron 330-55-2 0.035

Malathion 121-75-5 .027

Parathion-methyl 298-00-0 .006

Metolachlor 51218-45-2 .013

Metribuzin 21087-64-9 .006

Molinate 2212-67-1 .0016

Napropamide 15299-99-7 .007

p,p’-DDE 72-55-9 .0025

Parathion 56-38-2 .010

Pebulate 1114-71-2 .0041

Pendimethalin 40487-42-1 .022

Phorate 298-02-2 .011

Prometon 1610-18-0 .015

Propachlor 1918-16-7 .010

Propanil 709-98-8 .011

Propargite 2312-35-8 .023

Simazine 122-34-9 .005

Tebuthiuron 34014-18-1 .016

Terbacil 5902-51-2 .034

Terbufos 13071-79-9 .017

Thiobencarb 28249-77-6 .0048

Triallate 2303-17-5 .0023

Trifluralin 1582-09-8 .009

Compound CAS Number

Laboratory 

reporting level

alpha-HCH 319-84-6 0.0046

Acetochlor 34256-82-1 .006

Alachlor 15972-60-8 .0045

2,6-Diethylaniline 579-66-8 .006

Atrazine 1912-24-9 .007

Deethylatrazine 6190-65-4 .006

Azinphos-methyl 86-50-0 .05

Benfluralin 1861-40-1 .010

Butylate 2008-41-5 .002

Carbaryl 63-25-2 .041

Carbofuran 1563-66-2 .020

Chlorpyrifos 2921-88-2 .005

cis-Permethrin 54774-45-7 .006

Cyanazine 21725-46-2 .018

Dacthal 1861-32-1 .0030

Diazinon 333-41-5 .005

Dieldrin 60-57-1 .0048

Disulfoton 298-04-4 .021

EPTC 759-94-4 .0020

Ethalfluralin 55283-68-6 .009

Ethoprophos 13194-48-4 .005

Fonofos 944-22-9 .0027

Lindane 58-89-9 .0040

6 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Table 3. Volatile organic compounds analyzed in water samples for flowsystem study, Twin Cities metropolitan area, Minnesota.

[μg/L, micrograms per liter; pct, percent; CAS, Chemical Abstract Service; --, none]

Compound CAS Number

Laboratory 

reporting level Unit

1,1,1,2-Tetrachloroethane 630-20-6 0.03 μg/L

1,1,1-Trichloroethane 71-55-6 .032 μg/L

1,1,2,2-Tetrachloroethane 79-34-5 .09 μg/L

1,1,2-Trichloroethane 79-00-5 .064 μg/L

1,1,2-Trichlorotrifluoroethane 76-13-1 .06 μg/L

1,1-Dichloroethane 75-34-3 .035 μg/L

1,1-Dichloroethylene 75-35-4 .044 μg/L

1,1-Dichloropropene 563-58-6 .05 μg/L

1,2,3,4-Tetramethylbenzene 488-23-3 .23 μg/L

1,2,3,5-Tetramethylbenzene 527-53-7 .2 μg/L

1,2,3-Trichlorobenzene 87-61-6 .27 μg/L

1,2,3-Trichloropropane 96-18-4 .16 μg/L

1,2,3-Trimethylbenzene 526-73-8 .12 μg/L

1,2,4-Trichlorobenzene 120-82-1 .12 μg/L

1,2,4-Trimethylbenzene 95-63-6 .056 μg/L

1,2-Dibromo-3-chloropropane 96-12-8 .5 μg/L

1,2-Dibromoethane 106-93-4 .036 μg/L

1,2-Dichlorobenzene 95-50-1 .048 μg/L

1,2-Dichloroethane 107-06-2 .13 μg/L

1,2-Dichloroethane-d4 17060-07-0 .1 pct

1,2-Dichloropropane 78-87-5 .029 μg/L

1,3,5-Trimethylbenzene 108-67-8 .044 μg/L

1,3-Dichlorobenzene 541-73-1 .030 μg/L

1,3-Dichloropropane 142-28-9 .12 μg/L

1,4-Bromofluorobenzene 460-00-4 .1 pct

1,4-Dichlorobenzene 106-46-7 .050 μg/L

2,2-Dichloropropane 594-20-7 .05 μg/L

2-Butanone 78-93-3 5.0 μg/L

2-Chlorotoluene 95-49-8 .04 μg/L

2-Hexanone 591-78-6 .7 μg/L

3-Chloropropene 107-05-1 .12 μg/L

4-Chlorotoluene 106-43-4 .05 μg/L

4-Isopropyl-1-methylbenzene 99-87-6 .12 μg/L

4-Methyl-2-pentanone 108-10-1 .37 μg/L

Acetone 67-64-1 7.0 μg/L

Acrylonitrile 107-13-1 1.2 μg/L

Benzene 71-43-2 .021 μg/L

Bromobenzene 108-86-1 .036 μg/L

Bromochloromethane 74-97-5 .12 μg/L

Methods 7
Compound CAS Number

Laboratory 

reporting level Unit

Bromodichloromethane 75-27-4 0.048 μg/L

Bromoethene 593-60-2 .10 μg/L

Bromoform 75-25-2 .10 μg/L

Bromomethane 74-83-9 .26 μg/L

n-Butylbenzene 104-51-8 .19 μg/L

Carbon disulfide 75-15-0 .07 μg/L

Chlorobenzene 108-90-7 .028 μg/L

Chloroethane 75-00-3 .12 μg/L

Chloroform 67-66-3 .024 μg/L

Chloromethane 74-87-3 .17 μg/L

cis-1,2-Dichloroethylene 156-59-2 .038 μg/L

cis-1,3-Dichloropropene 10061-01-5 .09 μg/L

Dibromochloromethane 124-48-1 .18 μg/L

Dibromomethane 74-95-3 .050 μg/L

Dichlorodifluoromethane 75-71-8 .18 μg/L

Dichloromethane 75-09-2 .16 μg/L

Diethyl ether 60-29-7 .17 μg/L

Diisopropyl ether 108-20-3 .10 μg/L

Ethyl methacrylate 97-63-2 .18 μg/L

Ethyl tert-butyl ether 637-92-3 .05 μg/L

Ethylbenzene 100-41-4 .03 μg/L

Hexachlorobutadiene 87-68-3 .14 μg/L

Hexachloroethane 67-72-1 .19 μg/L

Isopropylbenzene 98-82-8 .06 μg/L

m- and p-Xylene -- .06 μg/L

Methyl acrylate 96-33-3 2.0 μg/L

Methyl acrylonitrile 126-98-7 .57 μg/L

Methyl iodide 74-88-4 .35 μg/L

Methyl methacrylate 80-62-6 .35 μg/L

Naphthalene 91-20-3 .5 μg/L

n-Propylbenzene 103-65-1 .042 μg/L

ortho-Ethyl toluene 611-14-3 .06 μg/L

ortho-Xylene 95-47-6 .07 μg/L

sec-Butylbenzene 135-98-8 .06 μg/L

Styrene 100-42-5 .042 μg/L

tert-Butyl methyl ether 1634-04-4 .17 μg/L

tert-Butylbenzene 98-06-6 .10 μg/L

tert-Amyl methyl ether 994-05-8 .08 μg/L

Tetrachloroethylene 127-18-4 .027 μg/L

Tetrachloromethane 56-23-5 .060 μg/L

Table 3. Volatile organic compounds analyzed in water samples for flowsystem study, Twin Cities metropolitan area, Minnesota.—Continued

8 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Compound CAS Number

Laboratory 

reporting level Unit

Tetrahydrofuran 109-99-9 2.2 μg/L

Toluene 108-88-3 .05 μg/L

Toluene-d8 2037-26-5 .1 pct

trans-1,2-Dichloroethylene 156-60-5 .032 μg/L

trans-1,3-Dichloropropene 10061-02-6 .09 μg/L

trans-1,4-Dichloro-2-butene 110-57-6 .70 μg/L

Trichloroethylene 79-01-6 .038 μg/L

Trichlorofluoromethane 75-69-4 .09 μg/L

Vinyl chloride 75-01-4 .11 μg/L

Ground-Water-Flow Modeling

A reconnaissance steady-state, ground-water-flow model 

was used to simulate the flow system and to aid in understanding ground-water flow. The finite-difference program 

MODFLOW (McDonald and Harbaugh, 1988) was used with 

Ground Water Modeling System (GMS) software as the graphical user interface. MODPATH, a post-processing program 

developed for MODFLOW, was used to estimate flowpaths 

and dates of ground-water recharge using the particletracking routine (Pollock, 1994). A three-dimensional, conceptual, variable-spaced, cell-centered, finite-difference model 

grid was developed to emulate hydrologic conditions. Cell 

dimensions were varied to allow for the discretization needed 

along the flow system, in rivers, and in areas of low permeability. Construction of the model assumed equilibrium or steadystate conditions. Data collected for this study and existing data 

(Norvitch and others, 1978; Palen and others, 1989) were used 

to prepare input files of hydrologic unit thickness, hydraulic 

properties, and to identify inactive cells of low permeability. 

Hydrographic data and water levels from existing wells were 

used to estimate hydraulic head. Boundary flux general-head 

cells, river cells, and drain cells were used to simulate internal and external boundary conditions. Flow between surficial 

aquifers and bedrock aquifers was considered negligible 

(Schoenberg, 1990). Drain cells represented springs and seeps, 

river cells represented rivers, and general-head cells represented lakes and wetlands. General-head boundaries were 

used to simulate subsurface flow at the margins of the model. 

No-flow boundaries were used to simulate the limits of the 

modeled area where ground-water flow was assumed to be 

nonexistent or insignificant (McDonald and Harbaugh, 1988). 

A no-flow boundary at the base of the model represented 

glacial till overlying bedrock. Calibration of the model was 

accomplished by comparing water-level altitudes measured in 

observation wells to model-simulated water-level altitudes and 

by flow conditions.

Hydrogeology

Permeability of soils and glacial deposits in the study 

area are conducive to leaching of contaminants from the land 

surface to ground water (Lueth, 1974; Chamberlain, 1977). 

The presence and concentrations of many contaminants in 

ground water also are affected by the chemical composition of 

soils. Contaminants are less likely to seep to the water table in 

areas with peaty soils, due to adsorption by organic materials 

and small permeabilities that impede ground-water flow and 

contaminant advection.

The study area is mantled with sand, gravel, and clay 

deposited as river terraces and overbank or by glacial outwash 

from the Des Moines glacial lobe during the Pleistocene 

Epoch (Norvitch and others, 1978; Bloomgren and others, 

1989; Meyer and Hobbs, 1989; Meyer, 1993; Anderson, 

1993; and Andrews and others, 1998). The terrace overbank 

and outwash deposits typically are composed of brown to 

gray, coarse to fine, gravelly sand, with local clayey deposits 

along streams. Pebbles, cobbles, and sands consist of mafic 

and felsic igneous and metamorphic minerals, dolomite, and 

quartz. Those sands and gravels commonly are underlain by 

silty, sandy clay.

Ground water in the surficial aquifers flows toward the 

Mississippi River or to tributaries of the Mississippi River 

(Kanivetsky, 1989; Palen and others, 1989) (fig. 1). The water 

table in the surficial aquifers generally is within 20 ft of 

land surface. Helgeson and Lindholm (1977) estimated that 

recharge to these aquifers can be as great as 9-11 in./yr based 

on analyses of hydrographs of water levels in shallow wells. 

Therefore, the surficial sand and gravel aquifers are highly 

susceptible to contamination from activities at the land surface 

(Piegat, 1989; Meyer, 1993).

The study area also is underlain by as much as 1,000 ft 

of sedimentary strata. Sedimentary strata are underlain by 

so-called “basement” units of metamorphic and igneous rocks 

Table 3. Volatile organic compounds analyzed in water samples for flowsystem study, Twin Cities metropolitan area, Minnesota.—Continued

Hydrogeology 9
of Precambrian age. The primary bedrock aquifer underlying 

the study area is the Prairie du Chein-Jordan aquifer consisting of fractured sandy dolomite of the Prairie du Chien Group 

of Ordovician age, and the underlying quartzite sandstone 

of the Jordan Sandstone of Cambrian age (Setterholm and 

others, 1991). The Prairie du Chien Group is karstic, with 

ground-water flow occurring mainly through joints, fractures, 

and solution cavities, whereas flow in the Jordan Sandstone is 

primarily through the intergranular pore spaces and some joint 

partings (Delin and Woodward, 1984). The Prairie du Chien 

Group and the Jordan Sandstone are in effective hydraulic connection and have traditionally been considered a single aquifer 

(Delin and Woodward, 1984; Young 1992a; Young, 1992b).

Estimated Hydraulic Conductivities

Estimated horizontal hydraulic conductivity values 

derived from sediment-size analyses ranged from 3.1 to 

190 ft/d (table 4), which are typical for materials ranging from 

glacial till to sand (Freeze and Cherry, 1979). Due to layering 

of sand and clay, vertical hydraulic conductivities of the surficial sand and gravel aquifers may be as much as five orders 

of magnitude less than horizontal hydraulic conductivities 

(Schoenberg, 1990).

Sediment pH, and Organic and Inorganic Carbon 

Content

Acidity of glacial sediment overlying the water table can 

affect the degradation and transport of organic compounds 

from the land surface to the saturated zone, because acidic 

organic compounds are more likely to be sorbed in alkaline 

sediments and basic organic compounds are more likely to be 

sorbed in acidic sediments. Sediment pH had a greater range 

(4.9–8.2) than pH in ground water, and increased slightly with 

depth (table 5).

Organic carbon in sediments can affect organic compounds such as pesticides and VOCs, which can leach from 

the land surface to ground water, because organic compounds 

can be sorbed to organic substances. Organic carbon also 

may provide a growth substrate for bacteria, which reduce 

sulfate and nitrate in ground water. Organic-carbon contents 

ranged from less than 3.2 to 160 grams per kilogram (g/kg) 

in the unsaturated zone (shallow) sediments, and from 0.5 to 

110 g/kg in sediments near the top of the saturated zone (deep) 

(table 5). Because low organic-carbon contents were measured 

in most sediment samples, organic compounds that are transported through surface soils could readily seep through the 

unsaturated zone to the water table.

Inorganic carbon primarily is an indication of carbonate 

minerals in sediments, which can buffer pH and affect major 

ion chemistry of water samples. Inorganic carbon concentrations generally were greater in deep sediment samples than 

shallow sediment samples, indicating progressive dissolution 

of carbonate minerals as acidic water seeps from the land 

surface.

Table 4. Estimated horizontal hydraulic conductivity (K) values calculated from sediment-size 

distributions collected from borings for a well and 

multipiezometers, Twin Cities metropolitan area, 

Minnesota.

[ft/d, feet per day; NA, not applicable]

Well or multipiezometers

Sampling 

depth

interval 

(feet)

Estimated 

horizontal 

K value 

(ft/d)

HN-K (shallow) 3-5 24

HN-K (deep) 20-22 82

UFP-1 (shallow) 3-5 4

UFP-1 (deep) 13-15 150

UFP-2 (shallow) 3-5 102

UFP-2 (deep) 12-14 190

UFP-3 (shallow) 3-5 9

UFP-3 (deep) 15-17 74

UFP-4 (shallow) 3-5 60

UFP-4 (deep) 7-9 180

UFP-5 (shallow) 3-5 5.1

UFP-5 (deep) 23-25 3.1

UFP-6 (shallow) 3-5 9.0

UFP-6 (deep) 13-15 16

Median (shallow) NA 9.0

Median (deep) NA 82

10 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Table 5. Sampling depths, depths to water table, sediment pH, and organic and inorganic carbon contents for the flow-system well 

and multipiezometers, Twin Cities metropolitan area, Minnesota.

[The 15-digit station identification (ID) number can be used to access water-quality results in the U.S. Geological Survey National Water Information System. 

Local well or multipiezometer names are shown in figure 1. NA, not analyzed; shaded rows indicate sample in the unsaturated zone; <, less than]

Well HN-K or 

multipiezometers

Site identification 

number

Sampling depth, 

in feet below 

land 

surface

Depth to water 

table, 

in feet below 

land surface

Date of 

measurementSediment 

pH

Organic 

carbon 

content 

(grams/

kilogram)

Inorganic 

carbon 

content 

(grams/

kilogram)

HN-K (shallow) 450430093220801 3-5 NA 7/15/1997 7.0 <0.20 0.1

HN-K (deep) 450430093220801 20-22 17.32 7/15/1997 7.9 2.9 9.1

UFP-1 (shallow) 450424093210501 3-5 NA 7/15/1997 4.9 4.2 1

UFP-1 (deep) 450424093210505 13-15 8.32 7/15/1997 6.6 .5 4.0

UFP-2 (shallow) 450425093195303 3-5 NA 7/10/1997 5.6 160 2.6

UFP-2 (deep) 450425093195304 12-14 11.96 7/10/1997 6.6 1.6 4.0

UFP-3 (shallow) 450423093194303 3-5 NA 7/9/1997 6.9 4.8 .4

UFP-3 (deep) 450423093194305 15-17 8.80 7/9/1997 6.8 1.0 .1

UFP-4 (shallow) 450415092184801 3-5 NA 7/7/1997 7.1 13 .7

UFP-4 (deep) 450414092184803 7-9 5.37 7/7/1997 8.2 NA NA

UFP-5 (shallow) 450413093184301 3-5 NA 7/24/1997 7.4 3.2 .1

UFP-5 (deep) 450413093184305 23-25 1.63 7/24/1997 7.2 110 73

UFP-6 (shallow) 450406093170204 3-5 NA 7/9/1997 6.9 3.3 .1

UFP-6 (deep) 450406093170205 13-15 10.60 7/9/1997 7.6 .9 1.6

Median for all wells and 

piezometers (shallow)

NA NA NA NA 6.9 4.2 0.1

Median for all wells and 

piezometers (deep)

NA NA 8.80 NA 7.0 1.3 4.0

Water Quality

Water-quality constituents summarized in this report 

include: physical properties; major ions, iron, manganese, 

nutrients, organic carbon, radon, pesticides, VOCs, and isotopes of nitrogen, hydrogen, and oxygen. Concentrations of 

chlorofluorocarbons, tritium, and dissolved gases also were 

determined for recharge-age dating.

Physical Properties

Physical properties measured while purging water 

from access tubes to sampling ports, or during sample collection, included depth to water, specific conductance, pH, 

water temperature, turbidity, dissolved-oxygen concentration, and alkalinity (milligrams per liter (mg/L) as calcium 

carbonate (CaCO3)). The depth of the water table below land 

surface generally was less than 9 ft. Values of pH tended to 

be alkaline, and specific conductances and alkalinities were 

substantial, probably due to dissolution of carbonate minerals. Dissolved-oxygen concentrations generally were less than 

1 mg/L, except on the upgradient end of the flow system at 

monitoring well HN-K (fig. 2). Dissolved-oxygen concentrations along much of the flow system were low and indicate 

that reduction processes such as denitrification, sulfate reduction, and reduction of metallic oxides may affect water quality 

in this aquifer.

Two-sided p-values from Kruskal-Wallis non-parametric 

tests (Helsel and Hirsch, 1992) of data from ports sampled 

three times (July 1997, October 1997, and August 1998) indicated that pH, turbidity, and dissolved-oxygen concentrations 

were significantly different (fig. 3) between the three samplings (two-sided p-value less than 0.025 means a greater than 

95 percent probability that the sample groups are not statistically different). Decreases in dissolved-oxygen concentrations 

and turbidities with time could indicate stabilization of conditions in the aquifer after drilling and well installation. The 

reason for greater pH values in the samples collected October 

1997, relative to the other two samplings, is unknown.

Specific conductances were significantly positively correlated to dissolved-solids concentrations (determined by evaporation of water samples at 180 degrees Celsius) (p=0.0001). 

Specific conductances and dissolved solids generally increased 

in the downgradient direction along the flow system, and with 

depth, except at UFP-6, where ground-water upwelling from 

Water Quality 11
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Figure 2. Dissolved-oxygen concentrations in water samples from the flow-system well and multipiezometers (trace of 

hydrogeologic section A-A’ shown on figure 1).

12 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 3. Physical and chemical properties of water samples from the flow-system well and multipiezometers.

Water Quality 13
14 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality

underlying aquifers to the Mississippi River, or water originating from the Mississippi River, may have lower specific 

conductance than ground water in the surficial aquifer (fig. 4). 

Major Ions

Major ions analyzed in water samples included calcium, 

magnesium, sodium, potassium, sulfate, chloride, fluoride, 

bromide (fig. 5), and bicarbonate (alkalinity). The primary 

major ions in water samples were calcium, magnesium, 

sodium, bicarbonate, sulfate, and chloride (figs. 5 and 6). 

Concentrations of these ions generally increase along the flow 

system (fig. 6) and with depth, with the exception of UFP-6, 

where ground water upwelling from underlying aquifers or 

water from the Mississippi River may have lower concentrations of these ions than ground water in the surficial aquifers. 

Calcium and bicarbonate generally increases along the flow 

system as a result of dissolution of carbonate minerals in the 

surficial aquifer. Sodium and chloride may be related to leaching from road salts and fertilizers. Sulfate probably is derived 

from the oxidation of sulfide minerals, oxidation of organic 

materials in soils and peat, leachate from concrete, or leachate 

from small amounts of gypsum in the surficial aquifer. With 

the exception of bromide, two-sided p-values of the KruskalWallis test comparing major-ion concentrations between samplings were less than 0.025 for the major ions, indicating that 

concentrations were similar for the three samplings (fig. 5).

Granato and Smith (1999) found a positive correlation of 

chloride and sodium concentration with specific conductance 

in highway runoff containing road-salt constituents. A significant positive correlation between chloride and total dissolvedsolids concentration (p-value=0.0001) in the urban flowsystem study is probably a result of salt applied to de-ice 

roads. The positive correlation between sodium and total dissolved-solids concentrations also was significant 

(p-value=0.0001). Chloride concentrations were greatest at 

UFP-3, UFP-5, and UFP-6 (fig. 7). UFP-3 and UFP-6 are 

downgradient from Brooklyn Boulevard and Interstate 94, 

respectively, and UFP-5 is downgradient from Shingle Creek, 

which has substantial chloride concentrations during winter 

snowmelt (J.D. Fallon, U.S. Geological Survey, oral commun., 

January 6, 2005; Granato and Smith, 1999).

Iron and Manganese

Because of concerns about stainless steel used in 

multipiezometers, a full suite of trace metals was not analyzed. Concentrations of dissolved iron and manganese were 

analyzed. Iron and manganese form insoluble oxide minerals 

in the presence of oxygen, which tend to dissolve in oxygen-depleted reducing conditions. Therefore, concentrations 

of these metals tend to increase in oxygen-depleted ground 

water. Iron (ranging from 0.033 to 18,600 micrograms per liter 

(μg/L)) and manganese (ranging from 543.7 to 2,606 μg/L) 

concentrations exceeded the Secondary Maximum Contaminant Levels (SMCLs) set by the U.S. Environmental Protection Agency (USEPA) of 300 μg/L for iron and 50 μg/L for 

manganese (U.S. Environmental Protection Agency, 1996).

Mineral saturation indices (Parkhurst, 1995) indicate 

minerals that may be present in an aquifer and minerals 

that are likely to dissolve or precipitate. If ground water is 

undersaturated with respect to a mineral, the ground water 

would theoretically dissolve the mineral if present. Conversely, if ground water is oversaturated with respect to a 

mineral, then the mineral would theoretically precipitate 

from the ground water. Equilibrium equations indicate that 

ground-water samples were oversaturated with respect to the 

iron and manganese oxide and hydroxide minerals hematite 

(Fe2O3), goethite (FeO(OH)), pyrolusite (MnO2), hausmannite 

(Mn2O4), and magnetite (MnO(OH)). Oversaturation determined for these minerals may be due to (1) overestimates of 

the concentrations of these metals due to the presence of colloids (particles with diameters less than the filter pore size of 

0.45 microns), (2) high activation energies of mineral formation, or (3) overestimation of redox states that were calculated 

from the ratios of nutrient compound concentrations. Most 

of the water samples were oversaturated or nearly saturated 

with respect to the silicate minerals quartz (SiO2) and chalcedony (cryptocrystalline quartz) and the carbonate minerals 

rhodochrosite (MnCO3), aragonite (CaCO3), calcite (CaCO3), 

and dolomite (Ca,Mg(CO3)

2

), probably as a result of the presence or predominance of most of these minerals in clasts or 

grains of the surficial aquifer and underlying aquifers.

Nutrients and Organic Carbon

Concentrations of nitrite plus nitrate as nitrogen rarely 

contained a substantial nitrite component and these nutrients 

hereinafter are termed nitrate. Concentrations of nitrate in 

drinking water exceeding the USEPA Maximum Contaminant 

Level (MCL) of 10 mg/L as nitrogen have been associated 

with blue-baby syndrome (methemoglobinemia), and with 

increased rates of stomach cancer, birth defects, miscarriage, 

and leukemia (Dorsch and others, 1984: Forman and others, 

1985; National Research Council, 1985; Fan and others, 

1987). Nitrogen and phosphorus in ground water that discharges to surface water also can lead to eutrophication, which 

depletes oxygen in lakes and rivers, leading to increased 

mortality of oxygen- and temperature-sensitive native aquatic 

species.

Most nitrate concentrations were less than 3 mg/L (fig. 8) 

and ranged from less than 0.050 to 4.15 mg/L. Nitrate in 

ground water principally is derived from animal wastes, soils, 

fertilizers, and nitrate-mineral deposits. Nitrate-mineral deposits do not occur in the study area. Most nitrate is oxidized 

(nitrified) in soils from the ammonia form of nitrogen (NH4

+

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which is leached from animal wastes, soils, and fertilizers. 

Rainwater also is a source of nitrate to soils and ground water. 

Nitrate concentrations in rainfall in the Upper Midwest ranged 

from 1.0 to 1.5 mg/L in 1995 (National Atmospheric Deposi-
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Figure 4. Specific conductance of water samples from the flow-system well and multipiezometers (trace of hydrogeologic 

section A-A’ shown on figure 1).

Water Quality 15
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Figure 5. Major ion concentrations in water samples from the flow-system well and multipiezometers.

16 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 6. Major ion concentrations in water samples from the flow-system well and multipiezometers, July 1997 (expressed 

as percentages of milliequivalents per liter).

tion Program/National Trends Network, 1997). Nitrate can be 

removed from water by the processes of denitrification and 

assimilation. Denitrification is a process in which nitrate is 

reduced to gases such as nitrous oxide (N2O) or nitrogen 

gas (N2). Bacteria or reduced forms of metals in oxygendepleted water facilitate denitrification. Assimilation is the 

uptake of nitrate and other nutrients by plant roots or by biota 

such as algae and bacteria. Nitrite (NO2) is an unstable intermediate product of nitrification, and does not commonly occur 

in substantial concentrations in ground water.

Two-sided p-values from the Kruskal-Wallis test indicate that concentrations of nitrogen compounds were similar 

in water samples collected during the three sampling events 

(fig. 8). Phosphorus compound concentrations increased during the last sampling (August 1998).

Concentrations of ammonia plus organic nitrogen (ranging from 0.10 to 5.20 mg/L) (reduced forms) were similar to 

those of nitrate (oxidized form), but concentrations of these 

constituents differed along the flow system (figs. 9 and 10). 

Nitrate concentrations generally were greatest in water samples collected from the upgradient end of the flow system and 

near Shingle Creek (fig. 9). As ground water flows through the 

aquifer, nitrate probably is both diluted and denitrified. Where 

conditions were reducing (deeper in the aquifer), reduced 

forms of nitrogen (ammonium plus organic nitrogen) were the 

dominant nitrogen compounds, for example UFP-3 (fig. 10).

Water Quality 17
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18 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Water Quality 19
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Figure 9. Dissolved nitrite plus nitrate as nitrogen concentrations in water samples from the flow-system well and 

multipiezometers (trace of hydrogeologic section A-A’ shown on figure 1).

20 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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multipiezometers (trace of hydrogeologic section A-A’ shown on figure 1).

Water Quality 21
Nitrogen Isotopes and Dissolved Gases

The isotopic ratio of 15N/14N of nitrogen in nitrate 

molecules can indicate the source of nitrate and whether 

denitrification has occurred. The ratio between these two 

isotopes is reported as δ15N in parts per thousand (ppt) and is 

expressed as follows:

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Atmospheric nitrogen gas consists principally of 14N 

(~99.632 percent, δ15N=~0) (Nier, 1955). The nitrogen-isotope 

ratio of nitrate derived from synthetic fertilizers generally 

is near that of atmospheric nitrogen gas (δ15N= -3 to +2 ppt) 

(Krietler, 1975). Nitrate leached from soils is somewhat 

enriched with 15N (δ15N=+2 to +8 ppt) as a result of preferential uptake of lighter 14N by plants and microbes (Krietler, 1975). Nitrate derived from animal wastes has greater 

amounts of 15N (δ15N=+10 to +20 ppt) (Krietler, 1975) as a 

result of preferential uptake of 14N and preferential evaporation of 14N as ammonia gas from human and animal wastes. 

Denitrification also increases δ15N values because denitrifying 

bacteria preferentially uptake lighter nitrate molecules due to 

greater thermodynamic efficiency.

Because analyses of nitrogen isotope ratios requires a 

minimum concentration of nitrate (3 mg/L in this case), water 

samples with nitrate concentrations less than that concentration were not analyzed for isotope ratios. Four of the 15 samples collected during July 1997 had sufficient nitrate concentrations to be analyzed. Nitrogen-isotope ratios (δ15N) in those 

water samples ranged from 6.5 to 10.3 ppt, indicating that the 

sources of nitrogen primarily were soils or animal wastes, or 

that denitrification that had increased nitrogen isotope ratios 

from an original, much lighter, fertilizer end member.

Concentrations of methane (CH4), carbon dioxide (CO2), 

nitrogen (N2), oxygen (O2), and argon (Ar) were determined 

to evaluate the redox state of water and the related likelihood 

of degradation of CFC concentrations used for age-dating 

of ground-water recharge. As oxygen is depleted by biotic 

or abiotic reactions in aquifers, bacteria can derive energy 

(through oxidation of carbon substrate) by removing oxygen 

atoms from nitrate (denitrification), sulfate (sulfate reduction), 

and carbon dioxide (methanogenesis). Greater ratios of the 

concentrations of dissolved nitrogen gas to argon gas (fig. 11) 

than the ratio of those gases in the atmosphere can indicate 

production of excess amounts of nitrogen gas in water by 

denitrification. In water samples from most of the sampling 

ports, nitrogen to argon ratios were similar to the atmospheric 

ratio, indicating little denitrification. Water samples most 

likely to have undergone denitrification, as indicated by that 

dissolved gas ratio, were from ports UFP-1C, 5D, 6C, and 

6D (fig. 11), which are located in deeper parts of the surficial 

aquifers. Nitrate concentrations were less than 0.2 mg/L in 

water samples from those same ports. Methane concentrations 

exceeding 0.5 mg/L in water samples from multipiezometer 

wells UFP-3 and 5 probably were related to very low redox 

conditions at those wells.

Hydrogen and Oxygen Isotope Ratios

The ratios of the isotopes of hydrogen (2H and 1H) and 

oxygen (18O and 16O) in water molecules can be used to 

determine the extent of evaporation in water samples based on 

comparison to the average ratios of those isotopes in precipitation (the meteoric water line). As with nitrogen isotopes, the 

ratios of the isotopes (R) are expressed in delta units (δ) as ppt 

as shown in the following equation:

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The standard is an arbitrary standard, known as Vienna 

Standard Mean Ocean Water (VSMOW). Because precipitation over continental areas originally is evaporated from ocean 

or lake water, it tends to be lighter than VSMOW as indicated 

by negative δ ppt values. For the most part, isotope ratios of 

hydrogen and oxygen (δ2H and δ18O) were well correlated 

with the regional meteoric water line (Craig, 1961) (fig. 12), 

indicating little evaporation of precipitation at the land surface 

prior to recharge. These results indicate rapid recharge in the 

study area.

Radon

Radon (222Rn) is a radioactive noble gas created by the 

decay of radium isotopes 223, 224, and 226 (Hem, 1985). 

Radon is soluble in water and readily volatilizes, with a halflife of 3.8 days. The primary health threat posed by radon in 

ground water is through atmospheric exposure (Wanty and 

Nordstrom, 1993). Radon can be absorbed by lung tissues, and 

alpha particles emitted during radioactive decay can damage 

those tissues. Prolonged respiration of high concentrations 

of radon gas have been related to increased incidence of lung 

cancer (National Research Council, 1988). Because of this 

risk, the USEPA has proposed a MCL for drinking water of 

300 picocuries per liter (pCi/L) in states with an active indoor 

air program and an alternative MCL of 4,000 pCi/L in states 

that do not have an active indoor air program (U.S. Environmental Protection Agency, 2004).

Radon concentrations were analyzed in samples collected 

from multipiezometers during July 1997. Radon concentrations in those 15 samples ranged from 195 to 374 pCi/L, some 

exceeding the proposed MCL of 300 pCi/L in samples from 

ports UFP-3D, UFP-4B, UFP-4D, and UFP-6C. These radon 

concentrations generally were less than those reported in water 

samples from the deeper aquifers in the surrounding area 

(Fong and others, 1998).

22 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 11. Nitrogen and argon gas concentrations in water 

samples from the flow-system well and multipiezometers, 

July 1997.

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Figure 12. Correlation between δ2H and δ180 in water samples 

from the flow-system well and multipiezometers.

Pesticides 

Several different classes of pesticides, including triazines, 

organophosphorus, organochlorines, and carbamates, were 

analyzed in ground-water samples (table 2). Most pesticides 

are designed to decompose in the soil zone. Many pesticides 

sorb to organic carbon or clay in soil and are degraded by 

abiotic and biotic processes in soils and the unsaturated zone, 

and have relatively low solubilities in water (Rao and Alley, 

1993). Soil and unsaturated-zone properties including hydraulic conductivity, organic-carbon content, pH, and cationexchange capacity may affect leaching of a pesticide to ground 

water. Pesticides are more likely to be sorbed to less permeable soils with greater organic-carbon contents. Information 

about uses and application of pesticides in the study area is 

presented by Andrews and others (1998).

Water samples from 8 of the 15 ports sampled during 

July 1997 and in 5 of the 15 ports sampled during October 1997 had detectable concentrations of one or more pesticides (fig. 13). None of the concentrations of the 10 pesticides 

detected exceeded USEPA MCLs or Health Advisory Limits 

(HALs). Pesticide concentrations greater than the method 

detection limits, but less than the laboratory reporting levels, 

were reported as estimated concentrations.

Prometon, a nonselective, long-lasting herbicide used to 

control weeds and grasses on rights-of-way (Sine, 1993), was 

the most frequently detected pesticide (fig. 14). There is no 

MCL established for prometon, but the USEPA has established 

a lifetime HAL of 100 μg/L for drinking water (U.S. Environmental Protection Agency, 1996), which was not exceeded in 

any of the water samples (maximum concentration detected 

was 0.0598 μg/L). Prometon is relatively slow to degrade in 

soils, with an estimated half-life of 200 to 500 days (Rao and 

Alley, 1993; Weber, 1994). Prometon has a relatively low 

sorption coefficient to organic carbon in soils, making it more 

likely to leach to ground water (Weber, 1994).

Some herbicide compounds associated with agricultural 

applications were detected in some water samples. The source 

of these herbicides is not known. They may be associated 

with atmospheric deposition or agricultural applications prior 

to suburban development in the study area (Andrews and 

others, 1998). Tebuthiuron also is a herbicide labeled for use 

on rights-of-way and was the detected in water samples from 

three of the ports during July 1997 (UFP-5C, UFP-5D, and 

UFP-2C) (fig. 14) (maximum concentration of 0.0418 μg/L). 

Tebuthiuron has a greater solubility and a lower sorption 

coefficient than prometon and atrazine. Its half-life in soil is 

estimated to be 360 days (Weber, 1994). The USEPA established a lifetime HAL of 500 μg/L for tebuthiuron in drinking water (U.S. Environmental Protection Agency, 1996). 

Simazine, an herbicide commonly applied to corn, was also 

detected in water samples from two of the ports sampled during July 1997 (UFP-4C, UFP-4D). Metolachlor (a maximum 

concentration of 0.0418 μg/L), a pre-emergence herbicide used 

on corn, soybeans, grain sorghum, and potatoes (Sine, 1993), 

was detected in water samples from two of the ports (UFP-4B 

Water Quality 23
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Figure 13. Total detected dissolved pesticide concentrations in water samples from the flow-system well and 

multipiezometers (trace of hydrogeologic section A-A’ shown on figure 1).

and UFP-4D) sampled during July 1997 and in water from 

port UFP-4C during October 1997. Metolachlor is classified as 

a possible human carcinogen with a lifetime HAL of 70 μg/L 

for drinking water (U.S. Environmental Protection Agency, 

1996). Metolachlor is relatively soluble in water, is moderately 

volatile, has a relatively low sorption coefficient to organic 

carbon, and has a half-life of 40 days (Weber, 1994).

Atrazine, used to control annual grasses in corn (Winkelmann and Klaine, 1991), and its metabolite deethylatrazine 

were detected in water samples collected during July 1997 

from two ports (UFP-1B and UFP-4D) and from well HN-K. 

During October 1997, atrazine and deethylatrazine were 

detected in a water sample from well HN-K. The maximum 

concentration detected was 0.00885 μg/L.

EPTC, an herbicide applied to beans, forage legumes, 

potatoes, and corn (Sine, 1993), was detected in a sample from 

UFP-5D during July 1997, and in water from UFP-5C and 

UFP-5D during October 1997 (estimated maximum concentration of 0.00262 μg/L). EPTC is highly volatile, moderately 

soluble in water, has a relatively low partition coefficient 

to organic carbon in soils, has a moderately short half-life 

of 7 days, and is readily degraded by soil micro-organisms 

(Weber, 1994). EPTC in ground water may be related to seepage from Shingle Creek during periods of high streamflow. 

Average estimated application rates for EPTC in 1987 were 

39.9 and 56.9 lb/mi2 in Anoka and Hennepin Counties (Gianessi and Puffer, 1991), respectively, making EPTC the most 

heavily-applied registered pesticide in or adjacent to the study 

area. Due to the lack of cropland in the study area, actual 

applications of this compound are probably less than the averages for the counties.

24 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 14. Frequencies of detection (in percent) of pesticides in 

water samples from the flow-system well and multipiezometers.

The compound p,p’-DDE, an impurity and breakdown 

product of the insecticide DDT (Verschueren, 1983), can damage nervous systems of humans and other animals and is listed 

by the USEPA as a probable human carcinogen (Agency for 

Toxic Substances and Disease Registry, 2003). The compound 

p,p’-DDE was detected in water from port UFP-2C during 

July 1997, and in water from well HN-K during October 1997 

(estimated maximum concentration of 0.00206 μg/L). The 

insecticide alpha-HCH, was detected in the water sample 

collected from UFP-1B during July 1997 (estimated concentration of 0.0012 μg/L). Napropamide, an herbicide used 

to control annual grasses and weeds in vegetables and turf 

(Extension Toxicology Network, 2003), was detected 

(0.00915 μg/L) in the water sample collected from UFP-4B 

during July 1997.

Volatile Organic Compounds

Volatile organic compounds (VOCs) are carbon-containing compounds that evaporate rapidly at normal temperatures 

and pressures. VOCs are contained in many commercial 

products including gasoline, paint, adhesives, solvents, wood 

preservatives, dry-cleaning agents, pesticides, cosmetics, 

correction fluids, and refrigerants. VOCs may reach ground 

water through spills and leaks at or near the land surface, 

from atmospheric dispersion into ground water, and through 

recharge of rainwater containing VOCs sorbed during passage 

through the atmosphere. Concentrations of 88 VOCs (table 3) 

were analyzed in water samples from the flow-system well and 

multipiezometers. None of the concentrations of the 

26 detected VOCs exceeded MCLs set by the USEPA for 

drinking water (U.S. Environmental Protection Agency, 1996). 

VOC concentrations greater than the method detection limits, 

but less than laboratory reporting levels, were reported as 

estimated concentrations.

VOC concentrations generally were greatest at UFP-1 

through UFP-4 (fig. 15). During July 1997, rainfall had 

seeped into an underground tank containing fuel oil, located 

approximately 100 ft to the northwest of UFP-1, forcing oil to 

the land surface (Anoka-Hennepin School District Staff, oral 

commun., 1997). Minor amounts of VOCs related to leakage 

of oil from that tank may have led to the detection of several 

VOCs in water samples from UFP-1 during July and October 

1997. Detection of VOCs in water from UFP-3 may be associated with the proximity of a dump site (located about 300 ft 

west/upgradient) (Chris Malec, Minnesota Pollution Control 

Agency, oral commun., 2003), or nearby roadways.

Twenty-three VOCs, most of which were solvents or 

fuel constituents, were detected in water samples collected 

during July 1997 (fig. 16) and 14 VOCs were detected in 

water samples collected during October 1997 (fig. 16). Only 

six VOCs were detected in water samples collected from the 

multipiezometers during August 1998 (fig. 16) due, at least 

in part, to a practice begun in 1998 of diluting samples by a 

factor of 2 prior to analysis at the laboratory. That practice 

probably contributed to the lack of detection of several VOCs, 

limiting comparability of data from the third sampling event 

to the previous sampling events. For more information on 

the uses and sources of each of the detected VOCs, refer to 

Andrews and others (1998) or Verschueren (1983).

The most frequently detected VOCs included carbon 

disulfide, chloromethane, toluene, and trichloroethylene. 

Carbon disulfide is a colorless liquid with a slightly pungent, 

sulfurous odor used in the manufacture of rayon, cellophane, 

carbon tetrachloride, rubber, soil disinfectants, soil conditioners, grain fumigants, herbicides, paper, pharmaceuticals, and 

electronic vacuum tubes (Versheuren, 1983). Carbon disulfide 

also is used as a solvent for phosphorus, sulfur, bromide, 

iodine, selenium, fats, and resins (Verscheuren, 1983). In 

addition to a wide variety of industrial uses, carbon disulfide is 

naturally emitted by bacteria in many soils, particularly soils 

rich in organic matter (Versheuren, 1983). At airborne concentrations exceeding 100 parts per million, carbon disulfide can 

cause illness or be toxic to humans, but no MCL for drinking 

water has been established for the compound. The substantial 

frequency of detection of low concentrations of this compound in ground water (maximum estimated concentration of 

0.01 μg/L) indicates a widespread source of this compound, 

such as emissions from soils. Lack of knowledge about 

anthropogenic emissions or measurement of airborne concentrations of this compound precludes assessment as to whether 

the origins of this compound in ground water are natural or 

anthropogenic in the study area.

Chloromethane (CH3Cl), also known as methyl chloride, 

is used in manufacturing silicones, tetraethyllead, synthetic 

rubber, methyl cellulose, refrigerants, chloromethane, chloroWater Quality 25
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Figure 15. Total detected volatile organic compound concentrations in water samples from the flow-system well and 

multipiezometers (trace of hydrogeologic section A-A’ shown on figure 1).

26 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 16. Frequencies of detection (in percent) of volatile organic compounds in water samples from the flow-system well and 

multipiezometers.

form, carbon tetrachloride, and fumigants (Versheuren, 1983). 

Chloromethane also is used as a low-temperature solvent, 

in medicines, as a fluid in thermostatic equipment, as an 

extractant, a propellant, and in herbicides, and also is present 

in cigarette smoke (Versheuren, 1983). Low concentrations 

of chloromethane may have been created through reactions 

between naturally occurring organic matter in the water 

samples and the drops of 50 percent hydrochloric acid used 

to preserve the samples analyzed for VOCs. The USEPA has 

determined that chloromethane is a possible human carcinogen 

and has established an HAL for this compound of 3 μg/L for 

drinking water (U.S. Environmental Protection Agency, 1996), 

which was not exceeded in any of the water samples (maximum estimated concentration of 0.08 μg/L).

Toluene (C6H5CH3), also known as methyl benzene, can 

be emitted by petroleum refining and coal tar distillation and 

is used in the manufacture of benzene derivatives, saccharin, 

medicines, dyes, perfumes, TNT, and detergents (Verscheuren, 

1983). Toluene is a component of gasoline and is used as a 

solvent for paints, coatings, gums, resins, rubber, lacquer, and 

adhesives (Verscheuren, 1983). In 1993, in Anoka and Hennepin Counties, 99,202 and 443,311 lbs, respectively, of this 

compound were released to air according to Toxic Release 

Inventory records (U.S. Environmental Protection Agency, 

written commun., 1995). Similar concentrations of toluene in 

ground water and equilibrium concentrations in air indicate 

that dispersion from the atmosphere or entrainment in rainfall 

may be the sources of this compound in ground water in the 

study area. The maximum estimated concentration in samples 

collected for this study was 0.22 μg/L.

The chlorinated alkene trichloroethene (CCl2CHCl), also 

known as TCE, is used for dry cleaning and metal degreasing; as a solvent for fats, greases, waxes, caffeine, and dyes; 

and as a refrigerant, fumigant, and anesthetic (Verscheuren, 

1983). In 1993, in Anoka and Hennepin Counties, 55,417 

and 421,458 lbs, respectively, of TCE were released to air 

according to records of the Toxic Release Inventory (U.S. 

Environmental Protection Agency, electronic commun., 1995). 

Water Quality 27
In ground water, TCE degrades to less chlorinated alkene compounds such as dichloroethene (DCE) and vinyl chloride. The 

maximum estimated concentration in water samples collected 

for this study was 0.015 μg/L.

Ground-Water Flow Modeling

A reconnaissance steady-state, ground-water-flow model 

was used to simulate the flow system in the study area and 

along the sampling transect to aid in understanding groundwater flow. The finite-difference program MODFLOW 

(McDonald and Harbaugh, 1988) was used with GroundWater Modeling System (GMS) software (Environmental 

Modeling Systems, Incorporated, 2002) as the graphical user 

interface. MODPATH, a post-processing program developed 

for MODFLOW, was used to estimate flowpaths and dates of 

ground-water recharge using the particle tracking routine (Pollock, 1994).

Model Description and Calibration

A three-dimensional, conceptual, variable-spaced, cellcentered, finite difference model grid was developed to emulate hydrologic conditions along the flow system (fig. 17). The 

grid contained sixteen 10-foot-thick layers of cells, 35 cells in 

the east-west direction, and 33 cells in the north-south direction. Cell dimensions varied to allow for the discretization 

needed along the flow system and in rivers and areas of low 

permeability. Construction of the model assumed equilibrium 

or steady-state conditions when inflow to the system equals 

outflow from the system. Hydrologic conditions from 1996 

and 1997 were considered to be acceptable estimates of the 

flow system at equilibrium for purposes of the model because 

pumpage from the system was minimal and precipitation was 

near average.

Data collected for this study and existing data (Norvitch 

and others, 1978; Palen and others, 1989) were used to prepare 

input files of hydrologic unit thickness, hydraulic properties, 

and to identify inactive cells of low permeability. Peat and 

silty sand deposits of low permeability were incorporated into 

appropriate model cells.

Hydrographic data and water levels from existing wells 

were used to calibrate the model to hydraulic head. Boundary 

flux general-head cells, river cells, and drain cells were used 

to simulate internal and external boundary conditions. Flow 

between surficial aquifers and bedrock aquifers was considered negligible (Schoenberg, 1990).

Drain cells represented springs and seeps, river cells 

represented rivers, and general-head cells represented lakes 

and wetlands. River cells allowed leakage through the bottom of the river based on assumed differences in water levels 

and assumed streambed conductance. River cells provided 

or received as much water as the model required to reach a 

mathematical solution. Drain cells allowed leakage from the 

uppermost layers to springs or seeps. It was assumed that the 

stage in the springs or seeps reflected the altitude of the water 

table and that flow in the springs and seeps was predominantly 

from ground water. General-head boundaries were used to 

simulate subsurface flow at the margins of the model. They 

allowed ground water to enter or leave the simulated system 

based on relative head differences between the model cells and 

the regional potentiometric surface. No-flow boundaries were 

used to simulate the limits of the modeled area where groundwater flow was assumed to be nonexistent or insignificant 

(McDonald and Harbaugh, 1988). A no-flow boundary at the 

base of the model represented glacial till overlying bedrock.

River and general-head cells were assigned a value for 

the altitude of the respective river, lake, or wetland and for 

conductance. Altitude of the river, lake, or wetland bed was 

determined from USGS 7.5-minute topographic maps (U.S. 

Geological Survey, 1967). A bed thickness of 1 ft and a vertical hydraulic conductivity of 0.5 ft/d were used in the streambed conductance calculations (McDonald and Harbaugh, 

1988). Drain cells were assigned values in a similar way.

Hydraulic heads in active model cells were assumed to 

represent steady-state conditions. Grain-size information from 

samples obtained during drilling were used to estimate horizontal and vertical hydraulic conductivities assigned. Evapotranspiration was not simulated. Net recharge (average infiltration minus average evapotranspiration) to the uppermost active 

cells was assumed to be 0.002 ft/d (Lindholm, 1980). The final 

estimates of hydraulic conductivity (table 6) for the aquifer 

were substantially greater than estimates based on samples 

from drill cuttings. This difference may indicate that the 

modeled value for recharge was less than actual recharge, that 

the thickness of the aquifer was not precisely known, or that 

soil-sample permeabilities were not representative estimates of 

average aquifer hydraulic conductivity.

Calibration of the model was accomplished by comparing water-level altitudes measured in observation wells to 

model-simulated water-level altitudes and comparing average 

baseflow to model-simulated flux. The mean absolute error 

was 2.61 ft. Simulated hydraulic heads were greater than measured hydraulic heads on average. This difference is within 

the range of annual fluctuation of the water table for surficial 

aquifers in Minnesota. Mass balance in recharge compared to 

discharge calculated by the model was within less than 0.01 

percent. Recharge was from precipitation and from certain 

reaches of Shingle and Ryan Creeks. Discharge was to other 

reaches of the creeks and to the Mississippi River. Measured 

and simulated ground-water flow generally flowed to the east 

toward the Mississippi River. Simulated discharge to streams 

was used to help evaluate the model by comparing these values with stream discharge in Shingle Creek measured during 

baseflow. Average measured baseflow in Shingle Creek near 

well HN-U (fig. 1) varied from about 3 to about 6 ft3/s (Mitton 

and others, 1978; 1999), whereas model-simulated discharge 

to the river at this location was 6.3 ft3/s.

28 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 17. Schematic of model configuration, simulated water-table altitude contours, flow system, and simulated flow system. 

Ground-Water Flow Modeling 29
Table 6. Hydraulic properties of the modeled study area, 

Twin Cities metropolitan area, Minnesota.

Stratigraphic unit 

(figure 1)

Horizontal 

conductivity (ft/d)

Vertical/horizontal 

anisotrophy ratio

Aquifer (terrace 

and overbank 

deposits and 

outwash)

500 0.2

Silty sand 10 .5

Peat 0.004 .003

multipiezometers indicate older water (fig. 18). Because of 

possible anaerobic degradation of CFCs, recharge dates at 

UFP-5 were estimated by comparing tritium concentrations at 

other ports with CFC-determined dates. The peak of tritium 

concentrations in rainfall, which coincided with the peak of 

nuclear testing during the early to mid-1960s, is reflected in 

the graph comparing tritium concentrations to CFC-determined dates of recharge from water samples collected during 

July 1997 from the flow-system well and multipiezometers 

(fig. 19a). The presence of many chemical constituents is 

related to dates of recharge and residence time in the surficial aquifers. For example, water from the ports near the 

water table and those with the greatest hydraulic conductivity generally were recharged most recently (figs. 18, 19b). 

Concentrations of calcium and magnesium, which are likely 

to be dissolved from the major minerals in the aquifer, tended 

to be less in more recently recharged ground water (fig. 19c). 

Concentrations of nutrients also related to dates of recharge. 

The largest concentrations of nitrate only occurred in ground 

water recharged since 1985 (fig. 19d).

Prometon, metolachlor, and simazine were only detected 

in ground water recharged after 1985, and the greatest detected 

total pesticide concentrations were in water recharged after 

1975 (fig. 19e). Unlike pesticides, VOC concentrations were 

generally greater in older ground water (fig. 19f). Although 

the total detected concentrations of VOCs and date of recharge 

were not significantly correlated (p-value=0.49), the greatest detected concentrations of VOCs were in water samples 

recharged before 1975.

Reverse particle tracking was done at the flow-system 

well and multipiezometers to obtain an approximate particle 

flowpath traveltime (table 7, fig.17). A porosity of 0.2 was 

assumed for all model cells for particle tracking. An example 

of a simulated flow path through the flow system transect is 

shown in figure 17. The model predictions of recharge-age 

dates for HN-K, UFP-4, UFP-5 (port D) and UFP-6 (port D) 

were similar to the CFC/tritium dates (table 7). At well UFP-6 

(port C), however, the model did not predict an accurate 

date, possibly due to misrepresentation of the model simulation near the boundary representing the Mississippi River. 

Simulated recharge-age at UFP-1, UFP-2, UFP-3, and UFP-5 

(port C) were less consistent when compared with dates from 

water samples. Water from well UFP-1, located downgradient 

from a tributary to Shingle Creek, had a CFC/tritium age that 

was older than the corresponding model-computed age date, 

indicating that interaction between the creek and ground water 

was not as great as was simulated by the model. At UFP-2 

(port C), the CFC/tritium age date also was older than the 

dates estimated from the model.

At UFP-3, completed in the low permeability silty sand 

and peat, the CFC/tritium date may have substantial error 

because of interference from carbon and methane in water 

from that well. The CFC/tritium dates were much older in the 

water in UFP-3 than the model predicted dates. Well UFP-5, 

completed in silty sand and peat deposits, also contained substantial carbon and methane, which may have affected CFCModel Limitations

The steady-state ground-water simulation used herein 

represents a simplification of actual ground-water conditions, 

but this tool can provide insights about ground-water flow 

based on what is known about the aquifer system. A finitedifference method is an approximation of the ground-water 

flow equations, and in certain conditions the method may not 

be able to approximate the equations accurately. This model 

assumed steady-state hydrologic conditions and horizontal 

homogeneity of aquifer properties. Hydraulic conductivity 

probably varies throughout the aquifer, and in the model, most 

areas were assigned generalized (constant over a broad area) 

hydraulic conductivity values. Streambed hydraulic properties 

also were estimated, and it is possible that the conductances 

change throughout the modeled streambeds. Streamflow 

data for calibration were sparse and the effects from pumping wells were assumed to be negligible on ground water and 

streamflow.

Recharge Age Dating

Simulated ground-water flow and model-simulated 

hydraulic heads, coupled with measured ground-water 

recharge-age dates, were used to aid in understanding groundwater flow. Concentrations and ratios of CFCs can be used 

to estimate the year that the sampled water reached the water 

table (for water that has been recharged since the 1940s) 

(Thompson and Hayes, 1979). CFCs are dissolved in water as 

it falls through the atmosphere and flows through the unsaturated zone to the water table (Johnston and others, 1998). 

Because of the relatively inert nature of CFCs, their atmospheric concentrations and ratios can be maintained for long 

periods in water containing dissolved oxygen. In water with 

little to no dissolved oxygen, bacteria may metabolize CFCs, 

affecting estimated ground-water recharge dates.

CFC concentrations at the upgradient end of the flow 

system (well HN-K) and in the vicinity of Shingle Creek 

indicate that ground water was recharged about 1990 (fig. 

18). However, CFC concentrations in samples from other 

30 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Figure 18. Recharge-age dates of water samples from the flow-system well and multipiezometers, July 1997 (trace 

hydrogeologic section A-A’ shown on figure 1).

calculated recharge-age dates. UFP-5A is completed in a peat 

unit and simulated flow indicates very slow traveltime indicating that ground water at that site probably was recharged at 

relatively early dates.

Table 7. Recharge-age dates and model estimates for ground water sampled from the flow-system well and 

multipiezometers, Twin Cities metropolitan area, Minnesota.

[TU, tritium units; CFC, chlorofluorocarbon; --, no sample]

Site identifier (local well or 

multipiezometer name)

CFC/Tritium 

Date Tritium (TU) Model date from reverse particle tracking Model comments

450430093220801 (HN-K) 1990 45.0 1992 --

450424093210502 (UFP-1B) 1972 37.8 1994 Stream fed

450424093210504 (UFP-1C) 1959 26.9 1990 --

450424093210505 (UFP-1D) 1956 42.2 1986 --

450425093195303 (UFP-2C) 1977 31.7 1990 --

450425093195304 (UFP-2D) -- -- 1969 --

450423093194303 (UFP-3B) 1976 33.6 1986 --

450423093194304 (UFP-3C) 1973 39.4 1985 --

450423093194305 (UFP-3D) 1954 26.2 1976 --

450415092184801 (UFP-4B) 1995 23.0 1994 --

450415092184802 (UFP-4C) 1989 36.2 1989 --

450414092184803 (UFP-4D) 1985 36.5 1986 --

450413093184301 (UFP-5A) -- -- 1950 

or older

Very slow traveltimes 

through peat unit

450413093184303 (UFP-5B) -- -- 1981 --

450413093184304 (UFP-5C) 1990 34.6 1979 --

450413093184305 (UFP-5D) 1973 38.7 1975 --

450406093170204 (UFP-6C) 1961 70.1 1990 --

450406093170205 (UFP-6D) 1966 91.8 1965 --

Recharge Age Dating 31
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Figure 19. Relations of selected dates of recharge, as determined by chlorofluorocarbon concentrations, to selected 

properties and constituents in water samples from the flow-system well and multipiezometers.

32 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Implications for Ground-Water 

Management

Surficial sand and gravel aquifers, similar to those in 

the study area for this report, are sources of ground-water 

discharge that sustain base flow to streams. In addition, 

these aquifers commonly are important sources of drinking 

water to supply wells because they are near land surface, and 

because drilling wells into these aquifers is relatively easy 

and inexpensive. However, surficial aquifers generally are 

highly susceptible to contamination because precipitation and 

contaminants can quickly infiltrate to the water table from the 

land surface.

Ground-water vulnerability to contamination is a combination of an aquifer’s natural susceptibility to contamination and the potential availability of chemicals that could be 

transported to aquifers through infiltration. Identifying areas 

that are vulnerable to ground-water contamination, as well as 

understanding how contaminants move through and are transformed in aquifers, can help water-resource managers make 

land-use decisions that minimize the potential for contamination. The surficial aquifers described in this report are susceptible to contamination because the water table is near land 

surface, the aquifers have a high hydraulic conductivity, and 

contaminants from urban land uses are potentially available. 

Results from this study provide insights about factors and processes that affect ground-water quality of susceptible aquifers 

in urban areas.

Land use is an important factor in assessing the potential 

for ground-water contamination. During the last 40 years, the 

study area has been converted from an agricultural area to an 

area that is suburban. These changes have resulted in uses of 

complex mixtures of chemicals that are potential sources of 

ground-water contamination. Water samples from the aquifer 

had small concentrations of many anthropogenic compounds. 

Some nutrient and pesticide compounds may be relicts of past 

agricultural activity. Small concentrations of sodium, chloride, 

nutrients, and certain herbicides are more likely to be related 

to applications of chemicals used in urban and suburban areas. 

Small concentrations of VOCs detected during sampling probably are likely to be due to industrial and commercial activities, and to disposal of wastes from these activities.

In addition to directly affecting the presence and distribution of chemicals in ground water, land uses also are related 

to each other, to hydrologic characteristics in the study area, 

and to the presence and distribution of contaminants in ground 

water. For example, residential land uses comprise about 

one-half of the land area surrounding the flow-system well 

and multipiezometers in the eastern part of the study area. In 

relatively undeveloped areas of the western part of the study 

area, the water table is near land surface and home builders avoid those areas because they are prone to flooding or 

seepage of water into basements. Industrial development and 

road builders also avoid those areas because of poor drainage. Conversely, parks are present in greater percentages in 

those areas due to the attractiveness of water features, the low 

cost of the land, and the unsuitability of the land for other 

purposes. These factors result in differences in the kinds of 

compounds and concentrations of compounds detected locally 

in the aquifer.

Some compounds such as nitrate and dissolved oxygen 

are greatest in water samples from the upgradient end of the 

flow system and near the water table. As ground water flows 

through the aquifer, nitrate is diluted and denitrified. Specific 

conductance, dissolved solids, and ammonia increase and dissolved oxygen decreases along the flow system and with depth 

due to increase in residence time of water within the aquifer 

and dissolution of aquifer materials. Calcium and bicarbonate 

also increase along the flow system probably as a result of dissolution of carbonate minerals in the aquifer. Some constituent 

concentrations varied with depth and with location due to variability in the flow system. Distributions of sodium, chloride, 

certain pesticides, and VOCs were complex and appear to 

be related to local sources, such as roadways, petroleum 

spills, dump sites, and interactions with streams and wetlands 

receiving runoff that contains contaminants that locally affect 

ground water.

Although land use has a substantial effect on groundwater quality, the distribution of contaminants in the aquifer 

is affected by a complex combination of factors and processes 

that include sources of natural and anthropogenic contaminants, three-dimensional advective flow, physical and hydrologic settings, age, evolution, and transformation of chemical 

compounds along a flow system. Water quality is stratified 

vertically and along the flow system as indicated by the age of 

ground water, and physical and chemical transformations in 

soil and water systems. Older and more reduced ground water 

are likely to occur near the base and downgradient parts of the 

aquifer. Results from this study also indicate that monitoring 

that focuses on wells completed near the water table is useful 

in identifying the first arrival of contaminants to the aquifer 

but deeper wells are necessary to understand how groundwater contaminants are transported throughout flow systems, 

including to surface waters and deeper aquifers.

Summary and Conclusions

As part of a national analysis of the effects of urban land 

use on ground-water quality for the National Water-Quality 

Assessment Program, the U.S. Geological Survey sampled 

wells along a flow system in surficial glacial aquifers in the 

northwestern part of the Twin Cities metropolitan area during 

1997 and 1998. In addition, a reconnaissance, steady-state 

ground-water model was developed to estimate flowpaths 

and dates of ground-water recharge using a particle-tracking 

routine.

Sediment samples from drilling were analyzed for sediment size, organic-carbon content, and pH. Sediment samples 

had high horizontal hydraulic conductivities (ranging from 3.1 

Summary and Conclusions 33
to 190 ft/d, based on sediment-size analysis of well cuttings), 

small organic carbon concentrations (ranging from less than 

0.2 to 160 g/kg), and mostly alkaline pH values (ranging from 

4.9 to 8.2).

Water samples were analyzed for physical properties; 

concentration of major ions, iron, manganese, nutrients, 

organic carbon, radon, pesticides, volatile organic compounds 

(VOCs), chlorofluorocarbons, and tritium; and isotopes of 

nitrogen, hydrogen, and oxygen. Most of the water samples 

had small dissolved-oxygen concentrations (less than 1 milligram per liter (mg/L)). Calcium, magnesium, sodium, bicarbonate, sulfate, and chloride were the primary dissolved constituents in water samples. Mineral saturation indices indicated 

oversaturation with respect to oxides and hydroxides of iron 

and manganese. Iron and manganese concentrations in water 

samples exceeded Secondary Maximum Contaminant Levels 

set by the U.S. Environmental Protection Agency (USEPA). 

Concentrations of nitrite plus nitrate as nitrogen (nitrate) 

were less than the USEPA Maximum Contaminant Level 

of 10 mg/L. Nitrate concentrations generally were greatest 

in water samples collected from the upgradient end of the 

flow system and near the water table. Nitrogen-isotope ratios 

indicated sources of nitrogen primarily were soils or animal 

wastes, and that denitrification may have increased nitrogen 

isotope ratios.

Isotope ratios of hydrogen and oxygen indicated that little 

evaporation of precipitation at the land surface occurs before 

recharge, thus indicating rapid recharge in the study area. 

Radon concentrations in some samples exceeded the proposed 

USEPA Maximum Contaminant Level of 300 picocuries per 

liter.

Small concentrations of pesticides were detected in the 

shallow parts of the aquifers. The herbicide prometon was 

the most frequently detected pesticide. Herbicides applied to 

control grasses and weeds in corn (atrazine, simazine, and 

metolachlor) also were frequently detected in water samples. 

All pesticide detections were less than USEPA Maximum 

Contaminant Levels or Health Advisory Limits. Twenty-six 

VOCs were detected in water samples, and all concentrations 

were below USEPA Maximum Contaminant Levels for drinking water. 

Chlorofluorocarbon compounds and tritium concentrations were used to estimate dates of recharge of ground-water 

samples. In general, shallower ground-water samples were 

more recently recharged, although most water sampled from 

the aquifer water was recharged after 1955.

A three-dimensional, variable-spaced, cell-centered 

finite difference model was developed to estimate flowpaths 

and recharge age-dates of ground-water in the study area. 

Water-level altitudes, measured in observations wells, were 

compared to model-simulated water-level altitudes. Simulated hydraulic heads were within the range of fluctuation 

of measured ground-water heads. Mass balance in recharge 

compared to discharge calculated by the model was within 

less than 0.01 percent. Recharge was from precipitation and 

from certain reaches of Shingle and Ryan Creeks. Discharge 

was to other reaches of the creeks and to the Mississippi River. 

Measured and simulated ground water generally flows to the 

east toward the Mississippi River.

The model predictions of recharge-age dates at most 

sampling location and depths were similar to the CFC/tritium 

dates. At multipiezometer UFP-6 (port C) the model did not 

predict an accurate date possibly due to misrepresentation of 

the model simulation near the boundary representing the Mississippi River. Water from well UFP-1, located downgradient 

from a tributary to Shingle Creek, had a CFC/tritium age that 

was older than the corresponding model-computed age date, 

indicating that interaction between the creek and ground water 

was not as great as was simulated by the model. At some 

multipiezometers (UFP-3 and UFP-5) completed in low permeability silty sand and peat deposits, CFC/tritium dates may 

have substantial error because of interference from carbon and 

methane in water from those multipiezometer ports or degradation of CFC’s in the flow system. Regardless of these problems, the conclusion that can be drawn for age-dating results is 

that ground-water age increases with depth in the aquifer and 

downgradient in the flow system.

Although land use has a substantial potential affect on 

ground-water quality, the distribution of contaminants in the 

aquifer is affected by a complex combination of factors and 

processes that include sources of natural and anthropogenic 

contaminants, three-dimensional advective flow, physical and 

hydrologic settings, age and evolution of ground water, and 

transformation of chemical compounds along the flow system. 

Water quality is stratified vertically and along the flow system, 

as indicated by the age of the ground water, the age of the land 

use, and physical and chemical transformations of the soil and 

water systems. Some compounds such as nitrate and dissolved 

oxygen are greatest in water samples from the upgradient end 

of the flow system and near the water table. Other constituents, such as specific conductance, dissolved solids, and 

ammonia increase along the flow system and with depth due to 

increase in residence time of water within the aquifers, reducing conditions, and dissolution of aquifer materials.

34 Water-Quality Assessment of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
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Appendix 39

Appendix—Quality Assurance/Quality-Control Samples

A total of 18 quality-assurance/quality-control (QA/QC) 

samples were collected in accordance with NAWQA protocols (Koterba and others, 1995). These samples included 

three VOC source-solution blanks; one field blank for major 

ions, nutrients, and VOCs; two field blanks for major ions, 

nutrients, pesticides and VOCs; three VOC trip blanks; three 

pesticide spikes; four VOC spikes; and two replicates.

Blanks collected were one of three types: source solution, trip, or field. Source solution blanks consist of speciallyprepared deionized, distilled, nitrogen-purged water, which is 

decanted directly to sample bottles and shipped to the laboratory for analysis. The purpose of source solution blanks is to 

serve as a reference for the field blanks. Trip blanks consist of 

vials filled with VOC free blank water prepared at the USGS 

National Water-Quality Laboratory, which are sealed and kept 

in the sampling van for a defined period of time. Trip blanks 

verify that VOC vials are not being contaminated during storage, sampling, and shipment to the laboratory. One VOC trip 

blank (composed of three vials) was placed in the sampling 

van during each of the 3 weeks that samples were collected. 

Those trip blanks were subsequently shipped to the laboratory with other samples. Field blanks consist of three types of 

water prepared to be free of analyzed major ions and nutrients, 

pesticides, and VOCs, that have been pumped through the 

sampling system after standard post-sampling cleaning. Field 

blanks are used to check the efficacy of cleaning procedures, 

and that field methods, sample shipment, and laboratory procedures are not contaminating samples.

Two source solution blanks for VOCs were submitted 

on December 16, 1997, and one source-solution blank was 

submitted on August 13, 1998. Only 3 of the 88 analyzed 

VOCs were detected in the source solution blanks and those 

compounds were only detected at concentrations at or below 

the laboratory reporting level (table 8).

Trip blanks for VOCs were submitted on July 7, 1997; 

October 27, 1997; and August 12, 1998. The only VOC 

detected in trip blanks was 1,4-dichlorobenzene, which was 

detected at an estimated concentration of 0.01 μg/L (less than 

the reporting limit) in the trip blank submitted on July 7, 1997.

Calcium and magnesium were detected at small concentrations in most of the field blanks (table 9), but at concentrations much less than were typical for ground-water 

samples (fig. 5). Ammonia nitrogen and orthophosphorus were 

detected in low concentrations in the field blank collected on 

August 31, 1998. No pesticides were detected in any of the 

field blanks. Organic carbon was detected in all field blanks, 

with elevated concentrations in the blank collected on October 

30, 1997 (table 9). Sixteen of the 88 analyzed VOCs were 

detected in one or more of the three field blanks. Three of 

those compounds—acetone, 1,4-dichlorobenzene, and toluene, 

also were detected in one or more of the VOC source solution 

blanks (table 8). Most of the detected concentrations of VOCs 

were less than the laboratory reporting levels. The field blank 

collected on October 30, 1997, had large concentrations of 

2-butanone and toluene. The concentration of toluene also was 

elevated in the field blank collected on July 17, 1997. Contamination of these samples by those compounds may have 

occurred during collection, processing, and storage prior to 

analysis.

Table 8. Laboratory reporting levels and method detection limits and concentrations of volatile organic compounds detected in three volatile organic compound source-solution blanks for the flow-system study, Twin 

Cities metropolitan area, Minnesota.

[all units are in micrograms per liter; NA, not applicable; <, less than; e, estimated]

Compound

Reporting level/ 

method detection 

limit December 17, 1997 December 17, 1997 August 13, 1998

Acetone 7.0/4.90 <5.0 <5.0 e3.0

1,4-Dichlorobenzene 0.50/.050 e.02 e.02 <.05

Toluene .05/.05 e.04 e.02 e.02
Table 9. Laboratory reporting levels, method detection limits, and concentrations of constituents in field 

blanks for flow-system study, Twin Cities metropolitan area, Minnesota.

[mg/L, milligrams per liter; μg/L, micrograms per liter; na, not analyzed; --, none; >, less than

Compound and concentration units

Laboratory 

reporting level/

method 

detection limit July 17, 1997

October 30, 

1997 August 31, 1998

Specific conductance (microsiemens per 

centimeter at 25 degrees Celsius)

0.5/-- 3.0 3.0 3.0

pH --/-- 7.6 7.7 7.3

Calcium (mg/L) .02/-- .15 .14 .05

Magnesium (mg/L) .001/.01 <.01 .023 .004

Acid neutralizing capability (mg/L as 

CaCO3)

0.5/-- 2.0 3.0 .02

Dissolved solids (mg/L) 1.0/10 4.0 2.0 <10

Ammonia-nitrogen (mg/L) .015/.01 <.01 <.015 .042

Dissolved orthophosphorus-phosphorus 

(mg/L)

.01/-- <.01 <.01 .013

Organic carbon (mg/L) .1/-- .2 13 .3

Acetone (μg/L) 5.0 /-- 16 9.0 3.0

1,2,4-trimethylbenzene (μg/L) .056/-- .01 .01 <.06

1,4-dichlorobenzene (μg/L) .05/-- <.05 .1 <.05

Benzene (μg/L) .021/0.1 .04 .06 <.03

Carbon disulfide (μg/L) .07/0.37 .05 <.08 <.08

Chloroform (μg/L) .024/-- .04 <.05 <.05

Ethylbenzene (μg/L) .03/-- <.03 .01 <.03

Methyl iodide (μg/L) .35/0.2 na .2 <.2

Bromomethane (μg/L) .26/-- <.1 .1 <.1

Dichloromethane (μg/L) .16/-- <.4 .5 <.4

2-Butanone (μg/L) 5.0/-- <2.0 28 <2.0

m- and p-Xylene (μg/L) .06/-- .02 <.06 <.06

ortho-Xylene (μg/L) .07/-- <.06 .01 <.06

Styrene (μg/L) .042/-- <.04 .01 <.04

Toluene (μg/L) .05 /-- .52 .25 .02

Trichloroethylene (μg/L) .038/-- 00<.04 .01 <.04

40 Water-Quality Assessment of Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Appendix 41

Spiked samples are collected to assess analytical recovery 

and precision of pesticides and VOCs. Water samples from 

UFP-5C on July 24, 1997, and from UFP-2D on October 30, 

1997, were spiked with known volumes of solutions containing known concentrations of pesticides. Most pesticides had 

mean recoveries ranging from about 50 to 120 percent (table 

10). Of the compounds with poor recoveries, deethylatrazine 

was the only pesticide compound that was frequently detected 

in environmental samples. The relatively poor recoveries may 

indicate that its frequency of occurrence and reported concentrations may have been greater than described in this report. 

The pesticide-spiked sample collected on October 30, 1997, 

generally had poorer recoveries than the samples collected on 

July 24, 1997 (table 10).

Pesticide

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-2D

(October 30, 

1997)

Acetochlor 90 98 49

Alachlor 82 95 47

Atrazine 88 94 64

Deethylatrazine 19 21 12

Azinphos-methyl 51 43 2

Benfluralin 99 99 56

Butylate 94 92 54

Carbaryl 110 110 40

Carbofuran 110 110 57

Chlorpyrifos 90 90 57

Cyanazine 73 78 34

DCPA (dacthal) 89 91 60

p,p’-DDE 72 74 43

Diazinon 110 110 62

Dieldrin 280 310 230

Disulfoton 28 50 34

EPTC 100 98 81

Ethalfluralin 90 87 60

Ethoprop 100 105 67

Fonofos 94 95 62

alpha-HCH 94 95 54

Lindane 88 87 45

Linuron 110 120 58

Malathion 91 86 35

Parathion, methyl 150 150 74

Pesticide

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-2D

(October 30, 

1997)

Metolachlor 94 98 53

Metribuzin 100 110 51

Molinate 110 110 78

Napropamide 88 93 68

Parathion 0 0 98

Pebulate 92 87 67

Pendimethalin 35 16 91

cis-Permethrin 39 39 19

Phorate 58 71 40

Prometon 82 77 45

Propachlor 93 88 35

Propanil 110 110 79

Propargite 79 76 31

Simazine 82 88 58

Tebuthiuron 81 73 27

Terbacil 54 64 31

Terbufos 71 68 23

Thiobencarb 99 100 80

Triallate 110 110 75

Trifluralin 110 100 66

Surrogates

Diazinon-d10 104 110 69

Alpha D6 HCH 103 111 82

Terbuthylazine 105 106 98

Table 10. Recovery percents from Schedule 2010 pesticide spikes for multipiezometers, Twin Cities metropolitan area, Minnesota.

Three water samples collected from well UFP-5C on July 

24, 1997, and one water sample collected from well UFP-2D 

on October 30, 1997, were spiked with identical volumes of 

VOC solutions. Mean recoveries in VOC spike samples generally ranged from 70 to 110 percent (table 11). As with the 

pesticide-spiked samples, recoveries generally were less in the 

sample spiked on October 30, 1997.

In addition to spike samples, surrogates comprised of 

compounds similar in character to the standard analytes were 

added to every pesticide (diazinon-d10, alpha D6 HCH, and 

terbutylazine) and VOC (1,2-dichloroethane-d4, toluene-d8, 

1,4-bromofluorobenzene) sample before analysis to assess 

recoveries (tables 10 and 11).
Table 11. Recovery percents from Schedule 2020 volatile organic compound spikes for multipiezometers, 

Twin Cities metropolitan area, Minnesota.

Compound

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-2D

(October 30, 1997)

1,1,1-2-Tetrachloroethane 77 94 98 72

1,1,1-Trichloroethane 81 110 110 73

1,1,2,2-Tetrachloroethane 95 110 110 85

1,1,2-Trichloroethane 91 110 110 82

1,1,2-Trichlorotrifluoromethane 98 120 130 62

1,1-Dichloroethane 82 100 110 79

1,1-Dichloropropene 89 110 120 80

1,2,3,5-Tetramethylbenzene 75 94 100 69

1,2,3-Trichlorobenzene 91 110 110 78

1,2,3-Trichloropropane 97 110 110 66

1,2,3-Trimethylbenzene 78 96 100 78

1,2,4-Trichlorobenzene 74 91 91 63

1,2,4-Trimethylbenzene 74 94 100 69

1,2-Dibromo-3-chloropropane 91 100 100 66

1,2-Dibromoethane 82 99 100 76

1,2-Dichlorobenzene 88 100 100 80

1,2-Dichloroethane 88 100 100 80

1,2-Dichloropropane 83 100 110 76

1,3,5-Trimethylbenzene 74 96 100 66

1,3-Dichlorobenzene 74 94 98 64

1,3-Dichloropropane 74 94 98 64

1,4-Dichlorobenzene 73 92 97 90

2,2-Dichloropropane 61 77 88 42

2-Butanone 93 100 93 110

2-Chlorotoluene 83 100 100 66

2-Hexanone 100 110 110 100

3-Chloropropene 110 120 140 48

4-Chlorotoluene 78 92 100 67

4-Methyl-2-pentanone 88 97 97 79

Acetone 120 130 130 87

Acrylonitrile 83 83 83 83

Benzene 88 110 110 83

Bromobenzene 78 95 100 70

Bromochloromethane 77 98 98 76

Bromodichloromethane 78 97 97 80

Bromoethene 110 130 140 75

Bromoform 72 85 88 75

Bromomethane 97 97 130 32

n-Butylbenzene 63 86 91 57

Carbon disulfide 130 140 130 79

Chlorobenzene 82 100 110 76

Chloroethane 61 52 88 61

Chloroform 77 98 100 75

Chloromethane 120 120 140 54

cis-1,2-Dichloroethene 91 110 110 84

cis-1,3-Dichloropropene 77 98 100 65

42 Water-Quality Assessment of Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality
Appendix 43

Compound

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-5C

(July 24, 1997)

Well UFP-2D

(October 30, 1997)

Dibromomethane 80 99 100 77

Dibromochloromethane 85 100 100 85

Dichlorodifluoromethane 160 140 190 65

Dichloromethane 100 130 120 100

Diethyl ether 94 110 110 63

Diisopropyl ether 75 97 110 86

Ethyl methacrylate 81 93 97 70

tert-Ethyl-butyl ether 83 98 98 89

Ethylbenzene 82 100 100 74

Hexachlorobutadiene 74 97 97 58

Hexachloroethane 70 91 97 55

Isopropylbenzene 81 100 110 66

m-and p-Xylene 83 100 110 71

Methyl acrylate 97 110 110 96

Methyl acrylonitrile 92 100 100 104

Methyl iodide 70 97 120 89

Methyl methacrylate 82 91 93 84

tert-butyl methyl ether 87 110 110 87

n-Propylbenzene 73 94 98 64

Naphthalene 120 140 140 71

ortho-Ethyl toluene 80 98 100 62

ortho-Xylene 83 100 110 71

sec-Butylbenzene 65 83 91 53

Styrene 70 86 96 71

tert-Amyl methyl ether 77 96 96 77

tert-Butylbenzene 73 93 97 63

Tetrachloroethylene 94 110 110 100

Tetrachloromethane 74 96 100 68

Tetrahydrofuran 75 74 90 75

Toluene 82 100 100 77

trans-1,2-Dichloroethene 94 120 120 79

trans-1,3-Dichloropropene 81 95 100 71

trans-1,4-Dichloro-2-butene 75 86 100 51

Trichloroethylene 90 110 120 82

Trichlorofluoromethane 98 120 130 62

Vinyl chloride 120 130 140 57

Surrogates

1,2-Dichloroethane-d4 100 100 110 97

Toluene, D8 100 100 100 96

1,4-Bromofluorobenzene 100 100 100 85

Table 11. Recovery percents from Schedule 2020 volatile organic compound spikes for multipiezometers, 

Twin Cities metropolitan area, Minnesota.—Continued
Replicates for this study are ground-water samples collected sequentially at the same well and are analyzed for the 

same compounds using the same methods. The purpose of 

replicate samples is to assess variability that may result during 

sample collection and laboratory analysis. One set of replicate 

samples was collected from port UFP-5C on July 24, 1997, 

and from UFP-3C on October 29, 1997 (table 12). Precision 

is calculated from two replicate samples, expressed as relative 

percent difference (RPD):

(4)

���

�� �� �

�� �� �

� �������������������

� ��������������������� � ���

where C1 is the larger of the two concentrations and C2 is the 

smaller of the two concentrations.

If the estimated concentration of a constituent in one 

sample of a pair was less than the laboratory reporting level, 

the reporting level was substituted for that concentration to 

calculate the RPD. If both constituent concentrations in the 

replicate pair were less than the reporting level, then the RPD 

was not calculated and the concentrations were assumed to be 

the same.

Constituents with concentrations varying by more than 

10 percent between the samples and replicates are shown in 

table 12. The relatively few compounds (of the more than150 

compounds measured) with greater than 10 percent relative 

percent differences and the small relative differences of those 

constituents, in terms of concentration, indicate that results 

were highly reproducible.

Table 12. Laboratory reporting levels and concentrations of compounds with greater than 

10 percent relative percent differences in replicate concentrations for multipiezometers, 

Twin Cities metropolitan area, Minnesota.

[mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than]

Compound and concentration units

Laboratory 

reporting 

level Concentration

Replicate 

concentration

Relative 

percent 

difference

UFP-5C July 24, 1997

Dissolved phosphorus (mg/L) 0.01 1.44 1.67 15

Dissolved orthophosphorus (mg/L) .01 1.34 1.69 23

Organic carbon (mg/L) .5 8.3 9.3 11

Tebuthiuron (μg/L) .01 .042 <.01 123

Benzene (μg/L) .03 .05 .04 22

Carbon disulfide (μg/L) .01 .08 .06 29

UFP-3C October 29, 1997

Bromide (mg/L) .01 .08 .09 12

Dissolved phosphorus (mg/L) .01 .013 <.01 26

Dissolved orthophosphorus (mg/L) .01 .23 .17 30

1,4-Dichlorobenzene (μg/L) .2 .12 .08 40

Toluene (μg/L) .22 .08 .06 29

44 Water-Quality Assessment of Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality

For more information concerning the research in this report, 

contact:

Director

U.S. Geological Survey

Water Science Center of Minnesota

2280 Woodale Drive

Mounds View, MN 55112
Andrews and others—Water-Quality Assessment of Part of the Upper Mississippi River Basin, Minnesota and Wisconsin—Ground-Water Quality along a Flow System in the Twin 

Cities Metropolitan Area, Minnesota, 1997-98—Scientific Investigations Report 2005-5120

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