St. Croix River 

In cooperation with the National Park Service 

Hydrogeologic Characteristics of the St. Croix 

River Basin, Minnesota and Wisconsin: 

Implications for the Susceptibility of Ground Water 

to Potential Contamination 

MINNESOTA Study area 91°30' 

WISCONSIN 46°30' 

93°30' 

EXPLANATION 

Schmidt susceptibility index 

1–8 

9–15 

16–23 

24–30 

31–38 

39–45 

46–53 

54–60 

61–68 

69–75 

76–83 

84–90 

91–98 

99–105 

106–113 

114–120 

121–128 

129–135 

45° 136–143 

144–150 

Water 

Extent of carbonate bedrock 

Scientific Investigations Report 2007–5112
 

U.S. Department of the Interior 

U.S. Geological Survey 

 

 

Hydrogeologic Characteristics of 

the St. Croix River Basin, Minnesota 

and Wisconsin: Implications for the 

Susceptibility of Ground Water to 

Potential Contamination 

By Paul F. Juckem 

Prepared in cooperation with the National Park Service 

Scientific Investigations Report 2007–5112 

U.S. Department of the Interior 

U.S. Geological Survey 
 

U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia: 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

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

resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Any use of trade, product, or firm names 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.
 

Suggested citation: 


Juckem, P.F., 2007, Hydrogeologic characteristics of the St. Croix River Basin, Minnesota and Wisconsin: Implications for the susceptibility of ground water to potential contamination: U.S. Geological Survey Scientific Investigations Report 2007–5112, 25 p.
 
iii 

Contents 

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

Methods for Adapting an Existing Ground-Water-Contamination-Susceptibility Map 


Methods for Determination of Ground-Water-Residence Time and Ground-Water/ 


Evaluation of Ground-Water-Residence Time and Ground-Water/ 


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

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

Physical Setting............................................................................................................................ 2
 

for Wisconsin to the St. Croix River Basin .......................................................................... 4
 

Evaluation of the Schmidt Susceptibility Index Values Map for the St. Croix River Basin..... 13
 

Surface-Water Interaction .................................................................................................. 13
 

Calculation of Mean Basin Residence Time ......................................................................... 14
 

Simulation of Areas With Relatively Short Residence Times............................................. 16
 

Simulation of Stream Leakage Into the Ground-Water System......................................... 16
 

Surface-Water Interaction .................................................................................................. 16
 

Mean Basin Residence Time Calculations............................................................................ 17
 

Simulated Areas With Relatively Short Residence Times .................................................. 21
 

Simulated Stream Leakage Into the Ground-Water System ............................................. 21
 

Limitations of the Study...................................................................................................................... 22
 

Summary............................................................................................................................................... 23
 

Acknowledgments.............................................................................................................................. 24
 

References........................................................................................................................................... 24
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

iv 

Figures 

1–11. Maps showing: 

1.	 Location of study area, St. Croix River Basin, Minnesota and Wisconsin........ 3
 

2. Soil characteristics of the St. Croix River Basin, Minnesota and Wisconsin... 6
 

3 Surficial deposits of the St. Croix River Basin, Minnesota and Wisconsin ...... 7
 

4 Type of uppermost bedrock in the St. Croix River Basin, Minnesota and 


Wisconsin .................................................................................................................... 8
 

5.	 Generalized depth from the land surface to the water table in the 


St. Croix River Basin, Minnesota and Wisconsin.................................................. 9
 

6. 	 Generalized depth from the land surface to the top of the uppermost 


bedrock in the St. Croix River Basin, Minnesota and Wisconsin ..................... 10
 

7.	 Schmidt susceptibility index values for the St. Croix River Basin, 


Minnesota and Wisconsin ...................................................................................... 12
 

8.	 Calibrated hydraulic conductivity, base elevation, and recharge values 


for tributary-basin models of the Kettle, Snake, and Sunrise River Basins, 


Minnesota, and of the Apple and Kinnickinnic River Basins, Wisconsin ....... 15
 

9.	 Distribution of water-level residuals for tributary-basin submodels of the 


Kettle, Snake, and Sunrise River Basins, Minnesota, and of the Apple and 


Kinnickinnic River Basins, Wisconsin................................................................... 18
 

10.	 Model-simulated area of relatively short ground-water residence times in 


the Kettle, Snake, and Sunrise River Basins, Minnesota, and in the Apple 


and Kinnickinnic River Basins, Wisconsin ........................................................... 19
 

11.	 Model-simulated river segments discharging surface water to the 


ground-water-flow system in the Kettle, Snake, and Sunrise River Basins, 


Minnesota, and in the Apple and Kinnickinnic River Basins, Wisconsin........ 20
 

Tables 

1.	 Susceptibility values assigned to the four resource maps used to calculate 


Schmidt susceptibility index values for the St. Croix River Basin, Minnesota 


and Wisconsin. .................................................................................................................... 5
 

2.	 Multiplier values assigned to the depth-to-bedrock map........................................... 11
 

3.	 Estimated and simulated base flow at streamflow-gaging stations in the five 


tributary basins to the St. Croix River, Minnesota and Wisconsin. ........................... 17
 

4.	 Estimated range of mean ground-water-residence times in the five tributary 

basins to the St. Croix River, Minnesota and Wisconsin. ........................................... 21
 

.
 
v 

Conversion Factors 

Multiply By To obtain 

Length 

inch (in.) 2.54 centimeter (cm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

kilometer 0.6214 mile (mi) 

Area 

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate 

foot per second (ft/s) 0.3048 meter per second (m/s) 

cubic foot per second (ft3

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

Hydraulic conductivity* 

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

Transmissivity** 

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

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

(NAVD 88). 

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

*Hydraulic conductivity: the standard unit for hydraulic conductivity is cubic foot per day 

per square foot of aquifer cross-sectional area (ft3/d/ft2). In this report, the mathematically 

reduced form, foot per day (ft/d), is used for convenience. 

**Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot 

times foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, 

foot squared per day (ft2/d), is used for convenience. 

 

 

 

 

 

Hydrogeologic Characteristics of the 

St. Croix River Basin, Minnesota and Wisconsin: 

Implications for the Susceptibility of Ground Water 

to Potential Contamination 

By Paul F. Juckem 

Abstract 

Population growth in the St. Croix River Basin in 

Minnesota and Wisconsin has intensified concerns of 

county resource managers and the National Park Service, 

which is charged with protecting the St. Croix National 

Scenic Riverway, about the potential for ground-water contamination in the basin. This report describes a previously 

developed method that was adapted to illustrate potential 

ground-water-contamination susceptibility in the St. Croix 

River Basin. The report also gives an estimate of groundwater-residence time and surface-water/ground-water 

interaction as related to natural attenuation and movement 

of contaminants in five tributary basins. 

A ground-water-contamination-susceptibility map 

was adapted from a state-wide map of Wisconsin to the 

St. Croix River Basin by use of well-driller construction 

records and regional maps of aquifer properties in Minnesota and Wisconsin. Measures of various subsurface 

properties were combined to generate a spatial index of 

susceptibility. The subjective index method developed 

for the State of Wisconsin by Schmidt (1987) was not 

derived from analyses of water-quality data or physical 

processes. Nonetheless, it was adapted for this report to 

furnish a seamless map across state boundaries that would 

be familiar to many resource managers. Following this 

method, areas most susceptible to contamination appear 

to have coarse-grained sediments (sands or gravels) and 

shallow water tables or are underlain by carbonate-bedrock aquifers. The least susceptible areas appear to have 

fine-grained sediments and deep water tables. If an aquifer 

Schmidt, R.R., 1987, Groundwater contamination susceptibility map and evaluation: Wisconsin Department of 

Natural Resources, Wisconsin’s Groundwater Management Plan Report 5, PUBL–WR–177–87, 27 p. 

becomes contaminated, the ground-water-residence time 

can affect potential natural attenuation along the groundwater-flow path. Mean basin ground-water-residence times 

were computed for the Apple, Kettle, Kinnickinnic, Snake 

and Sunrise River Basins, which are tributary basins to 

the St. Croix Basin, by use of average aquifer properties 

of saturated thickness, porosity, and recharge rates. The 

Apple River Basin had the shortest mean ground-waterresidence times (20–120 years), owing largely to the 

moderate saturated thickness and high recharge rate in the 

basin. The Kinnickinnic and Sunrise River Basins had the 

longest mean residence times (60–350 and 70–390 years, 

respectively) chiefly because of the relatively large saturated thickness of the basins. Owing to limitations of the 

residence-time calculations, actual ground-water-residence 

times will vary around the mean values within each basin 

and may range from days or weeks in karst carbonate aquifers to millennia in deep confined sandstone aquifers. 

Areas of relatively short residence time (less than the 

median residence time in each basin) were identified by 

use of ground-water-flow models for each of the five tributary basins. Results of simulations show that these areas, in 

which contaminants may have relatively less time for natural attenuation along the short flow paths, generally occur 

near streams and rivers where ground water discharges to 

the surface. Finally, the ground-water-flow models were 

used to simulate ground-water/surface-water interaction 

in the five tributary basins. Results of simulations show 

that some lakes and reservoirs leak surface water into the 

ground-water-flow system on their downgradient side, 

where the surface-water outflow has been restricted by a 

dam or a naturally constricted outlet. These locations are 

noteworthy because contaminated surface waters could 

potentially enter the ground-water-flow system at these 

locations. 
 

 

 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

Introduction 

Many of the values of the St. Croix National Scenic 

Riverway (fig. 1), including its diverse biota, are related 

to its good water quality, which is of critical concern to 

the National Park Service because of its responsibility to 

manage and protect the riverway. Water quality is also 

a critical concern for state, county, and local resource 

managers along the St. Croix River, particularly in 

watersheds bordering the river that are experiencing rapid 

population growth and development. Population in the St. 

Croix River Basin is expected to increase by as much as 

39 percent from 2000 to 2020 (Davis, 2004). This growth 

may increase the potential for ground-water contamination 

from point sources, such as spills of hazardous materials. 

The threat from nonpoint sources of contamination, such 

as those associated with fertilizers, feedlots, and animalwaste disposal, also will likely increase. 

Results of hydrologic studies over many years have 

demonstrated that surface-water and ground-water systems 

constitute a single resource (Winter and others, 1999); 

thus, assessments of vulnerability within the St. Croix 

River Basin should include understanding of how contaminants are transported in both surface water and ground 

water. Previous work by the U.S. Geological Survey 

(USGS) in cooperation with the National Park Service 

provided an integrated understanding of the regional water 

resource by assembling a series of databases related to the 

hydrogeology of the basin and by developing a groundwater-flow model that simulates surface-water/groundwater interactions within the basin (Feinstein and others, 

2006). This report builds on the work by Feinstein and 

others (2006) and describes the adaptation of a groundwater-contamination-susceptibility map of Wisconsin to 

the entire St. Croix River Basin in Wisconsin and Minnesota. The report also describes estimates of potential 

residence times for contaminants dissolved in ground 

water, and interactions between potentially contaminated 

surface water and ground water. The work described in this 

report was done by the U.S. Geological Survey in cooperation with the National Park Service. 

Purpose and Scope 

The purpose of this report is to describe the adaptation or development, and evaluation of regional-scale maps 

and tables designed to illustrate (1) possible susceptibility 

of ground water to contamination according to a previously developed index method, (2) ground-water-residence 

times, and (3) areas of surface-water leakage into groundwater-flow systems in the St. Croix River Basin. The scope 

of the work is regional in scale, yet results could be used 

as preliminary guidance for design of local and site-scale 

investigations of ground-water contamination. 

Schmidt’s (1987) ground-water-contamination susceptibility index map for Wisconsin was adapted to the 

St. Croix River Basin. This adapted map for the basin is 

referred to hereafter as the “Schmidt susceptibility index 

values map.” It has been based on readily available digital 

maps of soil and geologic material, as well as hydrogeologic databases constructed by Feinstein and others 

(2006). Simulation of ground-water-residence times and 

ground-water/surface-water interaction in five tributary 

river basins to the St. Croix River incorporated refinements to the regional ground-water-flow model (referred 

to herein as the “regional model”) developed by Feinstein 

and others (2006). Maps in this report are presented that 

show the soil, geologic, and hydrologic characteristics 

that may affect ground-water susceptibility to contamination according to the index method developed by Schmidt 

(1987). Maps also show areas of relatively short groundwater-residence times (less than the median time for the 

tributary basin), and river reaches where surface water 

enters the ground-water system. Estimates of the mean 

basin ground-water-residence time in five tributary basins 

are summarized in a table. 

Physical Setting 

The St. Croix River (fig. 1) originates near Solon 

Springs, Wis., and flows approximately 154 mi south to its 

confluence with the Mississippi River at Prescott, Wis. The 

St. Croix National Scenic Riverway includes the St. Croix 

River and the Namekagon River that together amount to 

252 river miles for recreational use. The St. Croix River 

Basin encompasses 7,730 mi2

 in Minnesota and Wisconsin (Lenz and others, 2001) and includes all or part of 19 

counties. Land cover is varied, grading from predominantly forest cover upstream in the basin to a mixture of 

forest, agriculture, and urban land use further downstream. 

Glacial deposits overlie igneous and sedimentary bedrock 

in the basin. Feinstein and others (2006) give a detailed 

description of hydrogeologic properties of the basin. 
Introduction  

Study area 

S t . C r oix River 

MINNESOTA 

WISCONSIN 

S T . C R O I X 

B A R R O N 

B U R N E T T 

P O L K 

W A S H B U R N 

D O U G L A S 

S A W Y E R 

B A Y F I E L D 

C A R L T O N 

P I N E 

A I T K I N 

K A N A B E C 

I S A N T I 

W A S H I N G T O N 

A N O K A 

P I E R C E 

C H I S A G O 

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

EXPLANATION 

St. Croix National Scenic Riverway 

Basin boundary 

Gaged river 

Other stream 

Population center 

County boundary 

Croix State boundary

Solon Springs 

Taylors Falls St. Croix Falls

Osceola 

Hudson 

Prescott 

Marine on St. Croix 

Stillwater

Snake River

Apple River 

Sunrise River 

Namekagon

River 

Eau Claire River 

T otagatic River Lower Tamarack

River

Upper Tamarack

River 

Kinnickinnic River 

Willow R i v er 

T r ade Riv e r 

W o o d River 

Knife River 

K e t tle R i ver 

N . F o r k C l a m River 

Y e l l ow River 

St. Croix River 

Snake R i v e r 

Map compiled and modified from digital data sources: U.S. 

Geological Survey, Wisconsin and Minnesota Departments of 

W IS C O NSI N 

M IN N E S O TA 

M I L L E L A C S 

Natural Resources, and National Park Service, 1:250,000. 0 25 KILOMETERS 

Figure 1. Location of study area, St. Croix River Basin, Minnesota and Wisconsin. 
 

 

 

 

 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

Methods for Adapting an Existing 

Ground-Water-ContaminationSusceptibility Map for Wisconsin to 

the St. Croix River Basin 

A map of Ground-Water-Contamination Susceptibility produced by Schmidt (1987) for the State of Wisconsin 

was adapted for the entire St. Croix River Basin (Minnesota and Wisconsin) by including well-driller construction 

data in the basin and regional maps of aquifer properties. 

This Schmidt susceptibility index values map used the 

same method and numerical ranking scheme (except where 

indicated below) as Schmidt (1987) to estimate the potential for dissolved chemicals at the land surface to infiltrate 

through geologic material to the water table. The ranking 

scheme consists of assigning susceptibility values from 1 

to 10 (1 being most susceptible, 10 being least susceptible) 

to properties identified on four physical resource maps that 

describe (1) soil material, (2) surficial deposits, (3) bedrock type, and (4) depth to the water table. In addition, a 

susceptibility multiplier, ranging from 0 to 8, was assigned 

to a map describing the depth to the uppermost bedrock 

surface. The susceptibility values assigned to properties 

of the four resource maps were weighted (multiplied) 

by values assigned to the depth to bedrock map, because 

Schmidt (1987) suggested that the depth to bedrock gave 

an indication of the relative potential for soils and surficial 

deposits to attenuate contaminants before reaching the 

water table. Finally, the four weighted resource maps were 

overlain, and their weighted susceptibility values were 

added together to produce a composite map reflecting the 

relative potential for ground water at a particular location 

to be contaminated by dissolved substances infiltrating 

with water from the land surface. The following paragraphs give details on each physical resource-map and the 

weighting scheme. 

Soil characteristics of the St. Croix River Basin 

were derived from the “Soil Atlas of Minnesota” (Minnesota Land Management Information Center, 2004) and a 

soil-characteristics map of Wisconsin (Schmidt, 1987). For 

consideration of ground-water-contamination susceptibility, Schmidt (1987) defined soils as the material extending from 0 to 5 ft below the land surface. Schmidt (1987) 

generated the soil-characteristics map of Wisconsin by 

grouping soil associations described by Hole and others 

(1968) into four categories; this map was used without 

modification for the Wisconsin part of the St. Croix River 

Basin. The four soil categories were (1) coarse texture/ 

high permeability/sands and gravels, (2) medium-coarse 

texture/high-medium permeability/sandy, (3) medium 

texture/medium permeability/loamy, and (4) fine texture/ 

low permeability/clayey. Schmidt (1987) assigned these 

soil characteristics susceptibility values of 1, 3, 6, and 

10, respectively (table 1). To produce a similar soil-characteristics map for the Minnesota part of the St. Croix 

River Basin, the Soil Landscape Units described in the 

“Soil Atlas of Minnesota” (Minnesota Land Management 

Information Center, 2004) were grouped into the same four 

susceptibility categories on the basis of predominant soil 

texture described within the first 5 ft of the land surface. 

Finally, the soil-characteristics maps covering the Wisconsin and Minnesota parts of the basin were joined together 

(fig. 2). 

Surficial deposits, or the unconsolidated deposits that 

extend from 5 ft below the land surface to the uppermost 

bedrock, were derived from Goebel and others (1983). Surficial deposits in the St. Croix River Basin were assigned 

to categories used by Schmidt (1987) in Wisconsin and are 

described as (1) sand and gravel, (2) sandy, (3) peat, (4) 

loamy, and (5) clayey (fig. 3). Surficial-deposit susceptibility values matched those used by Schmidt (1987) and are 

listed in table 1. 

Bedrock type was derived from a map of geology 

and mineral deposits of Minnesota, Wisconsin, and Michigan (Cannon and others, 1997). The geologic units were 

categorized as (1) carbonates, (2) sandstones, (3) igneous or metamorphic, and (4) shale (fig. 4), and assigned 

susceptibility values (table 1) according to Schmidt (1987). 

Carbonate rocks were considered the most susceptible to 

ground-water contamination because these rocks are prone 

to fracturing and dissolution, which produces conduits 

through which water can rapidly infiltrate to the water 

table. Because of the potential for rapid transport of contaminants through carbonate aquifers (following infiltration to the water table), the extent of carbonate bedrock 

(regardless of whether it is the uppermost bedrock or is 

buried beneath sandstone) is identified in figure 4. 

Depth to the water table was computed from more 

than 5,000 measurements reported by drillers at the time 

of well construction (appendix D in Feinstein and others, 

2006). A universal kriging technique was applied to these 

data so that a grid covering the basin with cells 1 km on 

each side could be associated with the following depth to 

water categories: (1) less than 20 ft, (2) 21 to 50 ft, and (3) 

greater than 50 ft. The categorized grid was assigned susceptibility values according to Schmidt (1987) and filtered 

to reduce the number of “cell islands,” or locations where 

a cell of one category was enclosed by cells of another 
Methods for Adapting an Existing Ground-Water-Contamination-Susceptibility Map  

Table 1. Susceptibility values assigned to the four resource maps used to calculate Schmidt 

susceptibility index values for the St. Croix River Basin, Minnesota and Wisconsin. 

[category descriptions and susceptibility values are derived from Schmidt (1987)] 

Resource map Category Susceptibility value 

Soil characteristics Coarse texture/high permeability 1 

Medium-coarse texture/high-medium permeability 3 

Medium texture/medium permeability 6 

Fine texture/low permeability 10 

Surficial deposits Sand and gravel 1 

Sandy 2 

Peat 5 

Loamy 6 

Clayey 10 

Type of bedrock Carbonate 1 

Sandstone 5 

Igneous or metamorphic 6 

Shale 10 

Depth to water table 0–20 feet 1 

21–50 feet 5 

Greater than 50 feet 10 

category. Finally, all adjoining grid cells with the same 

depth-to-water-table susceptibility value were grouped into 

a single polygon (fig. 5). 

Surface-water features were not incorporated into the 

interpolation process because of the difficulty involved 

with differentiating, at the outset, between water bodies 

that are closely connected with the ground-water-flow 

system from those that are disconnected, or “perched.” As 

a result, the depth to the water table for thin areas parallel 

to large river segments that are well connected with the 

ground-water-flow system may be less than that depicted 

in the regional-scale map (fig. 5). Locally, this would cause 

a slight decrease in the corresponding susceptibility value 

and the final Schmidt susceptibility index value. 

Depth from land surface to bedrock was also calculated from measurements reported by drillers (appendix 

A in Feinstein and others, 2006), and interpolated to a 

grid with 1-km by 1-km cells by use of a universal kriging 

technique. The depth ranges deviated slightly from the 

categories used by Schmidt (1987) and were grouped into 

categories that reflect the range in bedrock depth in the St. 

Croix River Basin. Schmidt (1987) distinguished between 

areas where bedrock is less than 5 ft below the land surface 

from areas where bedrock is between 5 and 50 ft below 

the land surface. Few drillers’ reports indicated bedrock 

within 5 ft of the land surface in the St. Croix River Basin; 

therefore, the shallowest depth to bedrock category used to 

construct the adapted Schmidt susceptibility index values 

map ranged from 0 to 25 ft. Depth to bedrock was further 

categorized as 26 to 50 ft, 51 to 100 ft, and greater than 

100 ft in accordance with Schmidt (1987). The categorized 

grid was also filtered to reduce cell islands and then converted to polygons (fig. 6). 
St. Croix River 

 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

MINNESOTA 

Study area 

WISCONSIN 

91°30' 

46°30' 

Sturgeon Lake 

93°30' 

EXPLANATION 

Soil characteristics 

Coarse texture/high permeability 

Medium-coarse texture/ Anoka Sand high-medium permeability Plain 

Medium texture/medium permeability 

Fine texture/low permeability 

45° Water 

0 25 MILES 

0 25 KILOMETERS 

Figure . Soil characteristics of the St. Croix River Basin, Minnesota and Wisconsin (modified from Hole and others, 1968; and 

Minnesota Land Management Information Center, 2004). 
 

St. Croix River

Methods for Adapting an Existing Ground-Water-Contamination-Susceptibility Map  

MINNESOTA 

Study area 

WISCONSIN 

91°30' 

46°30' 

93°30' 

EXPLANATION 

Surficial deposits 

Sand and gravel 

Sandy 

Peat 

Loamy 

Clayey 

45° 

0 25 MILES 

0 25 KILOMETERS 

Figure . Surficial deposits of the St. Croix River Basin, Minnesota and Wisconsin (modified from Geobel and others, 1983). 
 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

EXPLANATION 

Bedrock 

Carbonate 

Sandstone 

Shale 

Igneous/metamorphic 

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

0 25 KILOMETERS 

Extent of carbonate bedrock 

(buried by sandstone in some places) 

W I S C O NS IN 

M INN ESOTA 

Study area 

St. Croix River

Figure . Type of uppermost bedrock in the St. Croix River Basin, Minnesota and Wisconsin (modified from Cannon and others, 

1997). Areas where carbonate rocks are present below other rocks may be more susceptible than suggested by the Schmidt 

susceptibility index method because of the potential for rapid horizontal ground-water flow and contaminant transport from 

distant points of entry (for example, sinkholes) at the land surface. 
Methods for Adapting an Existing Ground-Water-Contamination-Susceptibility Map  

Study area 

EXPLANATION 

Depth from land surface 

to the water table 

0–20 feet 

Greater than 50 feet 

21–50 feet 

St. Croix River

W I S C O NS IN 

M INN ESOTA 

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

0 25 KILOMETERS 

Figure . Generalized depth from the land surface to the water table in the St. Croix River Basin, Minnesota and Wisconsin. 

Locally, the depth to the water table may differ from the categories shown here. 
10 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

EXPLANATION 

0–25 feet 

Greater than 100 feet 

51–100 feet 

26–50 feet 

Depth from land surface 

to the top of the uppermost bedrock 

WI S C O N S I N 

M I N NE S O TA 

Study area 

St. Croix River

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

0 25 KILOMETERS 

Figure . Generalized depth from the land surface to the top of the uppermost bedrock in the St. Croix River Basin, Minnesota 

and Wisconsin. Locally, the depth to the water table may differ from the categories shown here. 
Methods for Adapting an Existing Ground-Water-Contamination-Susceptibility Map 11 

Multiplier values assigned to the depth-to-bedrock 

map were used to weight susceptibility values of the 

other four resource maps (table 2). Following the logic of 

Schmidt (1987), the depth to bedrock indicates the amount 

of soil and surficial deposits that are present. Where 

depth to bedrock is great, surficial deposits are thick 

and, according to Schmidt (1987), are expected to have a 

greater influence on the infiltration of contaminants to the 

water table than the type of bedrock beneath the surficial 

deposits. Conversely, where bedrock is shallow, surficial 

deposits are thin and Schmidt (1987) expected the bedrock 

type to influence contamination movement. For example, 

the type-of-bedrock resource map (fig. 4) was assigned 

a higher multiplier than any other resource map in areas 

where bedrock is very shallow (table 2). The depth-to-bedrock multiplier values differed somewhat from those used 

by Schmidt (1987) because of the difference in how the 

depth to bedrock map was generated and categorized. For 

Table . Multiplier values assigned to the depth-to-bedrock map. 

example, the type-of-bedrock resource map was assigned 

a somewhat smaller multiplier for areas with shallow 

bedrock depths (0–25 ft) in the St. Croix River Basin 

compared to areas considered by Schmidt (1987) to have 

shallow bedrock (0–5 ft). 

The individual resource maps were overlain with the 

depth-to-bedrock map, and the susceptibility values (table 

1) were weighted (multiplied) by the depth-to-bedrock 

multipliers (table 2) for intersecting polygons. The resulting weighted resource maps were then overlain on one 

another, and the weighted susceptibility values were added 

together for overlapping polygons to produce a ranked 

score referred to as the Schmidt susceptibility index (fig. 

7). According to the method developed by Schmidt (1987), 

low values imply relatively higher contamination susceptibility; high values imply relatively lower contamination 

susceptibility. 

Depth to bedrock range Soil characteristics Surficial deposits Type of bedrock Depth to water table 

0–25 feet 2 3 8 2 

26–50 feet 3 4 5 3 

51–100 feet 4 5 2 4 

Greater than 100 feet 4 7 0 4 
St. Croix River

1 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

MINNESOTA 

Study area 

WISCONSIN 

91°30' 

46°30' 

93°30' 

45° 

0 25 MILES 

0 25 KILOMETERS 

EXPLANATION 

Schmidt susceptibility index 

1–8 

9–15 

16–23 

24–30 

31–38 

39–45 

46–53 

54–60 

61–68 

69–75 

76–83 

84–90 

91–98 

99–105 

106–113 

114–120 

121–128 

129–135 

136–143 

144–150 

Water 

Extent of carbonate bedrock 

Figure . Schmidt susceptibility index (Schmidt, 1987) values for the St. Croix River Basin, Minnesota and Wisconsin. Schmidt 

susceptibility index is based on a combination of soil, geologic, and hydrogeologic characteristics; lower values imply higher 

susceptibility. Areas where carbonate rocks are present below other rocks may be more susceptible than suggested by the 

Schmidt susceptibility index method because of the potential for rapid horizontal ground-water flow and contaminant transport 

from distant points of entry (for example, sinkholes) at the land surface. 
 

 

 

 

Methods for Determination of Ground-Water-Residence Time and Ground-Water/Surface-Water Interaction 1 

Evaluation of the Schmidt 

Susceptibility Index Values Map for 

the St. Croix River Basin 

The Schmidt susceptibility index values map for the 

St. Croix River Basin compares favorably to the map of 

Wisconsin constructed by Schmidt (1987). For example, 

a relatively less susceptible area (high index values) in 

western Burnett and Polk Counties, Wisconsin, occurs 

adjacent to relatively more susceptible areas to the east in 

both maps. Likewise, areas underlain by coarse-grained 

sediments (soil, surficial deposits, and bedrock) with shallow water tables generally appear to be more susceptible 

to ground-water contamination (low index values) than 

areas underlain by fine-grained sediments with deep water 

tables. Ground-water samples containing pesticides and 

nitrates in a part of the Upper Mississippi River Basin 

support this generalization (Hanson, 1998). The Anoka 

Sand Plain, which is in Anoka, Chisago, and Isanti Counties, Minn. (fig. 2), is an example of an area with coarsegrained unconsolidated material and a shallow water table 

that appears to have a relatively high susceptibility to 

ground-water contamination (low index values, fig. 7). In 

contrast, the area northeast of Sturgeon Lake in Minnesota 

(fig. 2) is underlain by fine-grained unconsolidated material and has a moderately deep water table, which suggests 

relatively less susceptibility to ground-water contamination 

according to the method presented by Schmidt (1987). 

Ground water in areas underlain by carbonate 

bedrock may be more susceptible to contamination than 

indicated by the Schmidt susceptibility index values map 

(fig. 7). Low-permeability soils (fig. 2) in this area may 

limit infiltration and enhance overland runoff toward 

sinkholes and stream sinks (Scott Alexander, University of 

Minnesota, written commun., 2007). Contaminants in this 

overland runoff could rapidly infiltrate through sinkholes 

or desiccation cracks in clay-rich soils and bypass the 

natural soil-filtration process (Tipping and others, 2001; 

Tipping, 2002). Further, because of the potential for rapid 

vertical and horizontal contaminant transport through solution cavities in karst aquifers, contamination could quickly 

spread to other parts of the aquifer that may be buried 

below tens or hundreds of feet of surficial deposits or 

sandstone bedrock. Given the unique susceptibility of karst 

aquifers to contamination, the extent of carbonate bedrock 

(regardless of whether it is the uppermost bedrock and near 

the land surface or is deeply buried beneath sandstone) has 

been identified in figure 7. 

Methods for Determination of 

Ground-Water-Residence Time 

and Ground-Water/Surface-Water 

Interaction 

The removal of contaminants from an aquifer is 

often difficult, and frequently incorporates some level of 

natural attenuation in which dilution and microbial activity 

reduce the concentration of the contaminant in the aquifer. 

The residence time of a contaminant within an aquifer is 

one factor that can affect the degree of natural attenuation along the ground-water-flow path, and is influenced 

by physical and chemical properties of both the aquifer 

and the contaminant itself. Quantifying attenuation and 

residence time of a specific chemical requires site-specific 

data. However, a first approximation of ground-waterresidence times can be estimated at a regional scale for 

nonreactive chemicals dissolved in water based on physical properties of the ground-water-flow system. Estimates 

of ground-water-residence times were evaluated for five 

tributary basins to the St. Croix River using ground-waterflow models. The models were also used to evaluate 

locations where surface water likely discharges into the 

ground-water-flow system. These locations represent a 

potential avenue for contaminated surface water to enter 

an aquifer. The methods of construction and application 

of the ground-water-flow models that were used for these 

analyses are described below. 

Feinstein and others (2006) used the analytic element (AE) code, GFLOW (Haitjema, 1995a) to develop 

a regional ground-water-flow model of the St. Croix 

River Basin that described the regional characteristics of 

the ground-water-flow system. This AE model was the 

foundation for evaluating ground-water-residence times 

and surface-water/ground-water interaction at a somewhat 

smaller scale for five tributary basins, as described in this 

report. A brief description of AE modeling is included 

here; more detailed information can be found in Haitjema 

(1995a). To construct an AE model, features important to 

ground-water flow (for example, wells) and surface-water 

features (for example, rivers and lakes) are represented 

spatially as mathematical elements or strings of elements 

called line sinks. Each mathematical element is represented by an analytic solution to the equation for groundwater flow. The effects of these individual solutions are 

added together (“superimposed”) to arrive at a solution 

for water levels and ground-water flows. In the GFLOW 

models used here, the analytic elements are two-dimen­
 

 

1 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

sional and are used to simulate steady-state conditions; 

that is, ground-water levels represent average conditions of 

water-table altitude and streamflow, and no consideration 

is given to variations in time. 

For the work described in this report, the regional 

model was divided into five tributary-basin submodels that 

were refined (that is, more hydrologic detail was added) 

to improve (1) estimation of the mean ground-water-residence time in each basin, (2) estimation of areas having 

relatively short or long ground-water-residence times, 

and (3) simulation of ground-water/surface-water interaction. The five tributary basins were the Kettle, Snake, and 

Sunrise River Basins in Minnesota and the Apple and Kinnickinnic River Basins in Wisconsin (fig. 8). Refinements 

to the tributary-basin submodels included (1) adjusting the 

base elevation of each model according to crystalline-bedrock elevations, (2) calibrating one or more recharge and 

hydraulic-conductivity values in each tributary basin, and 

(3) improving stream-elevation detail along line sinks to 

improve simulation of ground-water/surface-water interaction. 

To simulate ground-water flow using GFLOW, a 

horizontal base of the ground-water-flow system must be 

defined at a specified elevation. The crystalline-bedrock 

elevation at the mouth of each river was used to determine 

the base of the model for simulating ground-water flow in 

the Kettle, Snake, and Sunrise River Basins. For simulation 

of the Kinnickinnic River Basin, the base was assigned to 

the top elevation of the deepest regional confining unit, the 

Eau Claire Shale, at the mouth of the basin. The Eau Claire 

confining unit was used as the model base in order to limit 

the depth of the simulated ground-water flow system, 

and thereby improve simulation of tributary-basin-scale 

ground-water flow and ground-water/surface water interaction in the Kinnickinnic River Basin, which has the thickest sediments and smallest area of the five tributary basins 

(table 4). The base of the Apple River Basin submodel was 

varied among three separate zones included in the regional 

model (fig. 8), each of which was assigned the average 

crystalline-bedrock elevation within the zone. 

Line-sink segments, which represent streams and 

rivers, were divided into shorter segments in the refined 

submodels to more precisely match the stream slope and 

stream geometry. In particular, dam and reservoir outlets 

were located and line sinks were adjusted such that abrupt 

changes in stream elevation at these sites were represented 

in the tributary-basin submodels. 

New sets of recharge and hydraulic-conductivity 

values were estimated by means of a parameter-estimation program (PEST; Doherty, 2004) by matching base 

flows and water levels within each tributary basin. The 

primary benefit of using a parameter-estimation program 

is the capacity to automatically calculate parameter values 

that result in a quantified best fit between simulated and 

observed data (for example, ground-water levels and base 

flow). A subset of the water-level and base-flow targets 

used by Feinstein and others (2006) that corresponded to 

the areas within the tributary-basin submodels was used 

for calibration. The subset of base-flow targets included an 

estimate of flow near the outlet of each basin, plus a baseflow estimate along the Knife River in the Snake River 

Basin submodel. 

The submodel area for each tributary basin is shown 

in figure 8, along with zones of differing base elevations 

and calibrated recharge and hydraulic-conductivity values. 

In all basins except the Apple River Basin, a single value 

was used to simulate recharge, hydraulic conductivity, 

and base elevation. The zones delineated by Feinstein and 

others (2006) in the Apple River Basin were retained in the 

refined submodel to simulate changes in the crystallinebedrock base elevation and potentially related changes in 

recharge and hydraulic conductivity within these zones. 

Calculation of Mean Basin Residence Time 

A mean ground-water-residence time in the five 

tributary basins was calculated by use of the method of 

Haitjema (1995b). For an idealized ground-water-flow 

system, the mean basin residence time is the product of 

the aquifer’s saturated thickness and porosity, divided by 

the basin recharge rate (Haitjema, 1995b). The primary 

assumptions that characterize an idealized flow system for 

this calculation are (1) no resistance to vertical groundwater flow (only horizontal flow is considered), (2) no 

aquifer layering or regionally extensive confining units, 

and (3) constant saturated thickness and porosity of the 

aquifer and constant recharge to the aquifer over the entire 

basin. Assumption 3 is adequate for the regional-scale 

analyses described in this report. Assumptions 1 and 2 

represent limitations of the analysis for parts of all five 

tributary basins where layered sedimentary rock transmits a substantial proportion of the total ground-water 

flow through the system. Nonetheless, because this report 

provides regional estimates and insight to guide regional 

planning and subsequent local and site-scale investigations, potential violations of assumptions 1 and 2 are 

ignored for the purpose of calculating basin-wide estimates 

of the bulk mean basin residence time. The increased range 
 

 

 

 

 

 

Methods for Determination of Ground-Water-Residence Time and Ground-Water/Surface-Water Interaction 1 

EXPLANATION 

Kettle River Basin submodel: 

Base elevation: 790 ft 

Hydraulic conductivity: 5 ft/d 

Recharge: 3.9 in/yr 

Snake River Basin submodel: 

Base elevation: 760 ft (feet) 

Hydraulic conductivity: 7 ft/d (feet per day) 

Recharge: 3.0 in/yr (inches per year) 

Sunrise River Basin submodel: 

Base elevation: 545 ft 

Hydraulic conductivity: 10 ft/d 

Recharge: 5.0 in/yr 

Kinnickinnic River Basin submodel: 

Base elevation: 350 ft 

Hydraulic conductivity: 5 ft/d 

Recharge: 8.1 in/yr 

Apple River Basin submodel: 

Base elevation: 600 ft 

Hydraulic conductivity: 21 ft/d 

Recharge: 9.5 in/yr 

Apple River Basin inhomogeneities:

 Western Eastern 

Base elevation: 980 ft 940 ft 

Hydraulic conductivity: 21 ft/d 21 ft/d 

Recharge: 6.6 in/yr 9.5 in/yr 

St. Croix 

River Basin 

W I S C O N S IN 

M I NN ESOTA 

Study area 

Basin boundary 0 25 MILES 

Line sink 

0 25 KILOMETERS 

Boundary of near-field area of tributary basins 

Inhomogeneity, assigned values listed on the map 

Figure . Calibrated hydraulic conductivity, base elevation, and recharge values for tributary-basin models of the Kettle, Snake, 

and Sunrise River Basins, Minnesota, and of the Apple and Kinnickinnic River Basins, Wisconsin. [ft, feet; ft/d, feet per day; in/yr, 

inch per year] 
 

 

 

 

 

1 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

in the probable-residence-time estimates associated with 

these limitations is discussed in the evaluation section of 

this report. 

Values of saturated thickness, porosity, and recharge 

used to calculate the mean residence times for the five 

tributary basins were derived from the revised groundwater-flow models and from crystalline-bedrock-elevation information compiled from well-construction reports 

(appendix A in Feinstein and others, 2006). In areas where 

the crystalline bedrock is very deep, well-construction data 

were supplemented with estimates from a contour map of 

the top elevation of the crystalline bedrock in Minnesota 

(Mossler, 1983). The crystalline-bedrock elevation was 

subtracted from water-table elevations simulated by the 

refined ground-water-flow models to compute an average 

saturated thickness for each basin. Recharge was derived 

from calibration of the refined ground-water-flow models. 

Porosities were based on reported values in the literature 

(Domenico and Schwartz, 1998), which vary over probable values of 0.05 to 0.3 for mixed glacial sediments and 

sedimentary bedrock. Thus, a range of mean residence 

times was calculated for each basin by use of the range of 

reasonable porosities. 

Simulation of Areas With Relatively Short 

Residence Times 

Residence times can be used to estimate the time of 

arrival of contaminants at a river or lake and to estimate 

the likelihood that a contaminant will naturally degrade 

along its travel path. However, residence times can vary, 

depending upon the location of interest. The five refined 

ground-water-flow models were used to differentiate areas 

with relatively short ground-water-residence times from 

areas having relatively long ground-water-residence times. 

Simulated particles of water were traced from the water 

table through the aquifers to a point of discharge such as a 

river or pumping well. Areas of relatively short residence 

times were defined as those areas in which the length of 

time for simulated particles to reach a stream was less 

than the median traveltime of all simulated particles in the 

tributary basin. This distinction is used for convenience 

and does not mean that contaminated water inside the area 

of relatively short residence time will not undergo any natural attenuation, nor does it mean that contaminated water 

outside of the marked area will be completely attenuated 

by natural processes. Rather, contaminated water inside 

the area of relatively short residence time will, on average, 

have a shorter residence time and therefore experience less 

natural attenuation than contaminated water outside of this 

area. Aquifer porosity was set to 10 percent in each tributary submodel; this was the value used to simulate particle 

traces in the regional model. The method is subject to the 

same assumptions of horizontal flow and no aquifer layering as the calculation of mean basin residence time. 

Simulation of Stream Leakage Into the 

Ground-Water System 

Chemicals dissolved in surface waters represent a 

potential source of contamination to ground-water-flow 

systems. Where the elevation of surface water is higher 

than the elevation of the water table, surface water can leak 

through riverbed deposits and recharge the ground-waterflow system. Elevations of surface-water features simulated with line sinks in the regional model were refined in 

the five tributary-basin submodels to ensure that changes 

in stream stage were appropriately represented. The largest 

changes in stream stage typically occurred at outlets of 

natural lakes and manmade reservoirs. After calibration, 

line-sink segments were highlighted to identify those river 

reaches and lake-shoreline segments where the surface 

waters were losing water to the ground-water-flow system. 

Evaluation of Ground-WaterResidence Time and Ground-Water/ 

Surface-Water Interaction 

The calibrated recharge values in the refined tributary-basin submodels reflect trends in precipitation and 

evaporation across the region, as well as differences in 

infiltration of precipitation to individual aquifers. Recharge 

generally increases from west to east, following general 

trends in precipitation and evaporation in southeastern 

Minnesota and northwestern Wisconsin (Young and 

Hindall, 1973; Delin and Woodward, 1982; Novitzki, 

1982). The exception to this trend is the moderate recharge 

rate for part of the Apple River Basin underlain by a 

crystalline-bedrock ridge, where the thin aquifer and 

near-surface impermeable rock likely limits the amount 

of precipitation able to infiltrate to and flow through the 

ground-water system. 

Hydraulic conductivity and the model base elevation 

were treated separately for simulating the five tributary 

basins. As described previously, base elevations were 

varied among the tributary-basin submodels to represent 
 

Evaluation of Ground-Water-Residence Time and Ground-Water/Surface-Water Interaction 1 

crystalline-bedrock elevations. Model refinement included 

calibration of hydraulic conductivity for each basin so that 

differences between simulated and observed water levels 

and base flows were minimized. The pattern of calibrated 

hydraulic conductivities (fig. 8) matched the general trend 

in transmissivity estimated by Feinstein and others (2006); 

that is, in the northern Snake and Kettle River Basins, the 

base elevation is high with modest hydraulic conductivities 

of 7 and 5 ft/d, respectively, resulting in relatively lower 

transmissivities than for the southern basins. Aquifers 

in the southern Apple, Kinnickinnic, and Sunrise River 

Basins are generally thicker and have relatively higher 

hydraulic conductivities, ranging from 5 to 21 ft/d. The 

low-transmissivity area along the crystalline ridge in the 

Apple River Basin that was simulated in the regional 

model is captured in the refined tributary-basin submodel 

by the high base elevation. 

The added flexibility of representing spatial variability in recharge, hydraulic conductivity, and base 

elevation among the tributary-basin submodels resulted in 

an improved match to base flow and water-level measurements compared to simulated results from the regional 

model. Simulated base flows were within 5 percent of all 

base-flow targets (table 3). Differences between simulated 

and measured water levels (fig. 9) also were generally 

improved, with a mean error of 8 ft, a mean absolute error 

of 20 ft, and a root-mean-square-error of 28 ft for the 

total of all water-level targets in the five tributary-basin 

submodels. Simulated results were used to evaluate mean 

ground-water-residence times (table 4), the spatial distribution of areas with relatively short ground-water-residence 

times (fig. 10), and the location of river reaches where 

surface water recharges the ground-water-flow system (fig. 

11). 

Mean Basin Residence Time Calculations 

Conceptually, the mean basin residence time (table 

4) is the average amount of time required for a chemical 

application at any location on the land surface (assuming immediate transportation to the water table) to move 

through and then exit the aquifer. The calculation averages 

areas characterized by very short ground-water traveltimes 

with areas characterized by very long ground-water traveltimes. In addition, aquifer heterogeneity (variability of the 

aquifer material) and aquifer layering have been ignored. 

These simplifications tend to reduce the calculated range 

of residence times, which could vary substantially for individual aquifers; that is, ground-water-residence times are 

generally shorter in shallow aquifers than in deep confined 

aquifers because of the additional vertical flow through 

low-permeability confining units and longer regional flow 

paths typical of flow in many deep aquifers. Moreover, 

fractures in shallow carbonate aquifers, such as the karstic 

Prairie du Chien aquifer, may become enlarged and interconnected by dissolution, thereby increasing preferential 

flow through these fractures compared to the bulk aquifer 

thickness. In such karst aquifers, ground-water flow can be 

very rapid through the conduits (miles per week), yet very 

slow through the rock matrix. Thus, the actual residence 

time of a contaminant may be shorter (days or weeks) or 

longer (millennia) than the range of mean values reported 

in table 4; a site-specific investigation would be necessary 

to determine the actual residence time. 

Table . Estimated and simulated base flow at streamflow-gaging stations in the five tributary basins to the 

St. Croix River, Minnesota and Wisconsin. 

Estimated base flow*

, Simulated base flow, Streamflow-gaging station in cubic feet per second in cubic feet per second 

Apple River 369 369 

Kettle River 297 297 

Kinnickinnic River 90 90 

Snake River 221 222 

Knife River (tributary to Snake River) 21 20 

Sunrise River 71 71 

* Estimated base flow is equivalent to the Q50 flow (streamflow exceeded 50 percent of the time) at the streamflow-gaging station. 
1 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

MINNESOTA Kettle River Basin submodel Study area 

WISCONSIN 

Snake River Basin 


submodel
 

St. Croix 

River Basin 

Apple River Basin submodel 

Sunrise River Basin 

submodel 

Kinnickinnic River Basin submodel 

EXPLANATION 

Area of tributary-basin submodels 0 25 MILES 

Line sinks 0 25 KILOMETERS 

Residuals (measured water level minus simulated water level, in feet) 

Simulated water levels are too high 

Between 20 and 60 feet too high [ 8%] 

Greater than 60 feet too high [ 1%] 

Simulated water levels are closely matched [63%] 

Between 20 feet too high 

and 20 feet too low 

Simulated water levels are too low 

Between 20 and 60 feet too low [25%] 

Greater than 60 feet too low [ 2%] 

Figure . Distribution of water-level residuals for tributary-basin submodels of the Kettle, Snake, and Sunrise River Basins, 

Minnesota, and of the Apple and Kinnickinnic River Basins, Wisconsin. Percentages may not add to 100 percent because of 

rounding. 
Evaluation of Ground-Water-Residence Time and Ground-Water/Surface-Water Interaction 1 

Study area 

W ISCON SI N 

MI N N E S O TA 

Kettle River Basin 

Snake 

River 

Basin 

EXPLANATION 

Traveltime less than median 

traveltime in each basin 

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

0 25 KILOMETERS 

Sunrise River Basin 

Apple River Basin 

Kinnickinnic River Basin 

St. Croix 

River Basin 

Figure 10. Model-simulated area of relatively short ground-water residence times in the Kettle, Snake, and Sunrise River Basins, 

Minnesota, and in the Apple and Kinnickinnic River Basins, Wisconsin. 
0 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

Study area 

Snake
 

River
 

Basin
 

EXPLANATION 

Stream losing water to 

ground-water system 

93°30' 

91°30' 

45° 

46°30' 

0 25 MILES 

0 25 KILOMETERS 

Sunrise River Basin 

Kettle River Basin 

Apple River Basin 

Kinnickinnic River Basin 

St. Croix 

River Basin 

W ISCONSI N 

MI N N E S O TA 

Figure 11. Model-simulated river segments discharging surface water to the ground-water-flow system in the Kettle, Snake, and 

Sunrise River Basins, Minnesota, and in the Apple and Kinnickinnic River Basins, Wisconsin. 
 

 

Evaluation of Ground-Water-Residence Time and Ground-Water/Surface-Water Interaction 1 

Mean basin residence times are useful for evaluating the bulk response of a ground-water-flow system at a 

basinwide scale. Several general patterns can be observed 

from the estimated mean basin residence times (table 4). 

First, the two basins with the longest residence times, the 

Kinnickinnic and Sunrise River Basins, are the smallest 

(areally) of the five basins. Basin size is not a component 

in the calculation of mean basin residence time. The long 

residence times in these two relatively small basins can 

be attributed instead to their relatively large saturated 

thickness (table 4). Second, the ground-water recharge 

rate is greater in the Kinnickinnic River Basin than in the 

Sunrise River Basin and this difference results in a shorter 

residence time in the Kinnickinnic River Basin. Of the 

five tributary basins, the shortest mean residence time 

(20–120 years) was computed for the Apple River Basin, 

and the longest mean residence time (70–390 years) was 

computed for the Sunrise River Basin. The approach and 

results represent a rough guide for resource management 

that, although not appropriate for site-specific planning, 

provides an understanding of the processes that influence 

ground-water-residence time in idealized basins. 

Simulated Areas With Relatively Short 

Residence Times 

On the basis of simulated particle traces, areas of relatively short residence time generally occur where ground 

water discharges to surface-water features (fig. 10). Intuitively, contaminated water infiltrating to the water table 

near a river or stream has a relatively short distance to flow 

before discharging to the stream. A similar pattern was 

identified at the much larger scale of the entire St. Croix 

River Basin by Feinstein and others (2006). Nonetheless, the scale of investigation can influence the potential 

to identify areas of relatively short residence times; that 

is, ground water may move relatively quickly over short 

distances to seeps, ponds, agricultural tile, or headwater 

streams in localized areas that were not included in this 

investigation and therefore not identified. 

The residence time of a contaminant in a groundwater-flow system can, among other factors, affect the 

potential for natural attenuation because of geochemical 

and biological processes along the flow path. For this 

reason, delineation of areas with relatively short residence 

times is useful for resource management and planning. 

Nonetheless, the degree of actual attenuation will depend 

upon the type of contaminant, properties of the soil and 

aquifer, and implementation of any engineered approaches 

for removing or degrading the contaminant. Response to a 

specific contamination problem would require site-specific 

investigation and planning. 

Simulated Stream Leakage Into the GroundWater System 

In humid climates such as that of Minnesota and Wisconsin, ground water typically discharges to surface-water 

features that are in direct connection with the ground-water 

system. However, surface-water discharge to ground water 

can occur, and it most commonly occurs where surface 

waters are naturally or artificially restricted or elevated 

(fig. 11), such as along the downgradient shoreline of a 

lake or reservoir where the surface-water level has been 

elevated by a channel restriction or a dam, or where lakes 

and ponds are perched above the water table (perched 

water bodies were not considered in this investigation, 

however). Surface water can also enter an aquifer where 

Table . Estimated range of mean ground-water-residence times in the five tributary basins to the St. Croix 

River, Minnesota and Wisconsin. 

Saturated thickness, Porosity, Recharge, Mean residence time, Basin in feet in percent in inches per year in years 

Apple River 310 5–30 9.1* 20–120 

Kettle River 190 5–30 3.9 30–170 

Kinnickinnic River 780 5–30 8.1 60–350 

Snake River 150 5–30 3.0 30–170 

Sunrise River 550 5–30 5.0 70–390 

* Value is the area-weighted average of calibrated recharge zones in the Apple River tributary-basin submodel. 
 

 

 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

ground-water-withdrawal wells are near rivers and lakes or 

along river meanders where local hydraulic gradients can 

be complex. 

Results of simulations illustrate regional groundwater/surface-water interaction along the shoreline of 

large lakes and reservoirs in the tributary basins (fig. 11). 

The upgradient (upstream) shorelines of the simulated 

lakes and reservoirs, like most river segments, are not 

highlighted and receive ground-water discharge. However, some downgradient shorelines are highlighted (fig. 

11), indicating that surface water is discharging into the 

ground-water-flow system along those segments. Although 

only large lakes and reservoirs that are well connected to 

the ground-water system were simulated in the tributarybasin submodels, similar ground-water/surface-water interaction patterns may occur near small lakes and reservoirs. 

Small perched water bodies may lose water to the groundwater-flow system, with the amount largely dependent 

upon the permeability of the lakebed sediments. Likewise, 

the results of simulations illustrate where contaminated 

surface waters could potentially enter the ground-water 

system, but local-scale investigations would be needed 

to determine whether, and to what extent, contaminated 

surface water was entering the aquifer. 

Limitations of the Study 

The index method used by Schmidt (1987), and 

adapted in this report for the St. Croix River Basin, is 

subjective and includes uncertainties that are difficult to 

quantify (Focazio and others, 2002). Moreover, subjective methods for estimating ground-water-contamination susceptibility are not scientifically defensible in that 

categorization of physical resource maps and assignment 

of susceptibility values to those categories is not derived 

from analyses of water-quality data or process-based 

approaches, such as a ground-water-flow simulation. 

Rather, categorization is determined on the basis of “professional judgment” (Focazio and others, 2002). Nonetheless, the method presented by Schmidt (1987) is familiar 

to many resource managers in the basin and was adapted 

for the St. Croix River Basin to furnish a seamless map for 

resource managers working in both Minnesota and Wisconsin. In addition, the susceptibility patterns estimated 

by the ground-water-contamination-susceptibility maps 

for Wisconsin (Schmidt, 1987) and Minnesota (Porcher, 

1989) appear to be generally supported by pesticide and 

nitrate samples from wells in a part of the Upper Mississippi River Basin that includes the St. Croix River Basin 

(Stark and others, 1996; Hanson, 1998). Regardless, it is 

important for resource managers to realize the limitations 

of Schmidt’s (1987) method as it relates to the scientific 

method (Focazio and others, 2002). 

The Schmidt susceptibility index values map is not 

designed for site-specific use because it was derived from 

regional maps. Instead, the information contained in the 

map may be useful for regional planning and to help identify areas requiring additional study at a site-specific scale. 

The method (Schmidt, 1987) is also limited to nonreactive 

chemicals dissolved in water moving from the land surface 

to the water table; chemical-specific characteristics (for 

example, solubility and volatility) were not considered. 

Similarly, the Schmidt susceptibility index values map 

does not indicate areas that are or will be contaminated, 

nor does it identify areas that are not or cannot be contaminated. Whether the water table becomes contaminated at a 

particular location will depend upon hydrogeologic properties at the site as well as the type and amount of chemical 

released into the environment. The nature of land use and 

the intensity of that use will also influence the likelihood 

of ground-water contamination at a particular location. 

The method used to estimate mean basin residence 

times and areas of relatively short or long residence times 

incorporated assumptions that characterize idealized flow 

systems. These assumptions include: (1) no resistance 

to vertical ground-water flow, (2) no aquifer layering or 

regionally extensive confining units, and (3) constant 

saturated thickness and porosity of the aquifer and constant recharge to the aquifer over the entire basin. Idealized flow systems do not occur in nature, and as a result, 

these assumptions represent limitations of the analysis, 

especially where layered sedimentary rocks transmit a 

substantial proportion of the total ground-water flow. The 

consequence of these limitations is that the mean basin 

residence times could span a larger range of time than the 

estimated values reported in table 4 for the five tributary 

basins. Moreover, the actual residence time of an individual contaminant may be shorter (days or weeks) or longer 

(millennia) than the range of mean values reported in table 

4 depending upon the site specific characteristics of the 

aquifer, and the physical and chemical characteristics of 

the contaminant. 

Ground-water/surface-water interaction was simulated at a regional scale in the five tributary-basin submodels. Local variations in surface-water elevations, stream 

meanders, and aquifer heterogeneity, combined with 

seasonal water-level fluctuations that were not simulated in 

the steady-state submodels could induce local or temporal 

changes in the direction of water flow between ground­
Summary  

water and surface-water systems. Similarly, small water 

bodies were not evaluated in this study, but could locally 

influence ground-water/surface-water interactions. 

Summary 

The St. Croix National Scenic Riverway is recognized 

for its diverse biota and good water quality. In places, the 

St. Croix River Basin is undergoing rapid development, 

which may increase the potential for ground-water contamination from point sources such as hazardous-materials 

spills, in addition to the potential for contamination from 

nonpoint sources such as those associated with agricultural 

fertilizers or animal-waste disposal. This study by the U.S. 

Geological Survey, in cooperation with the National Park 

Service, was designed to illustrate susceptibility of the 

ground-water-flow system to potential contamination, estimate ground-water-residence times in five tributary basins 

to the St. Croix River, and identify river and lake segments 

in the five tributary basins where surface water could 

potentially enter the ground-water-flow system. These 

tributary basins included the Apple, Kettle, Kinnickinnic, 

Snake, and Sunrise River Basins. 

Following the concepts proposed by Schmidt (1987), 

the susceptibility of ground water to contamination 

depends in part on the physical properties of the material 

above the water table. Regionally available maps describing characteristics of soil, surfical deposits, and bedrock 

were compiled along with information from wells on depth 

to the water table and depth to bedrock. These data were 

used to estimate Schmidt susceptibility index values for the 

St. Croix River Basin. According to Schmidt (1987), these 

index values indicate relative susceptibility of the groundwater-flow system to contamination at the land surface. 

Based on this method, areas underlain by coarse-grained 

sediments and shallow water tables appear to be relatively 

more susceptible to contamination than areas underlain by 

fine-grained sediments and a deeper water table. Carbonate bedrock is uniquely susceptible to contamination in 

areas where karst dissolution cavities may occur (Tipping, 

2002). Therefore, the extent of carbonate bedrock is identified on the Schmidt susceptibility index values map. 

If a contaminant reaches the water table, its residence 

time within the aquifer can affect the potential for natural 

attenuation along the flow path. A method by Haitjema 

(1995b), using estimates of average saturated thickness, 

porosity, and recharge, was employed to estimate a range 

of mean residence times for the five tributary basins. 

Results were computed as a range of mean residence 

times due to uncertainty pertaining to the range of possible porosity values. The shortest mean residence time 

(20–120 years) was computed for the Apple River Basin; 

the longest mean residence time (70–390 years) was 

computed for the Sunrise River Basin. Areas of relatively 

short and relatively long residence times were evaluated by 

means of refined ground-water-flow submodels that were 

based on the regional model constructed by Feinstein and 

others (2006). Relatively short residence time was defined 

as less than the median residence time of all simulated 

particles of water in the tributary basin. Results of simulation show that contaminants that enter an aquifer in areas 

near streams and rivers, where ground water typically 

discharges from aquifers, will have shorter residence times 

than contaminants that enter the flow system in areas far 

from a point of discharge. Contaminants near streams 

and rivers could have residence times much shorter than 

the calculated minimum mean basin residence times and 

could represent a greater threat to aquatic biota because of 

limited natural attenuation or dilution. 

Just as ground-water discharge to streams could 

contaminate the surface-water system, leakage of contaminated surface water into an aquifer could contaminate the 

ground-water system. The refined tributary-basin submodels were used to identify areas where average surfacewater elevations were above average regional ground-water 

elevations. These areas, which represent locations where 

surface water recharges the underlying aquifer, typically 

occur on the downgradient side of lakes and reservoirs 

where the surface-water level has been elevated by a channel restriction or a dam. 

The results described in this report have limitations. 

In particular, the Schmidt susceptibility index values map 

is not designed for site-specific use and includes uncertainties that are difficult to quantify (Focazio and others, 

2002) due to the subjective method adapted from Schmidt 

(1987). Nonetheless, regional patterns of susceptibility 

index values appear to be generally supported by pesticide and nitrate samples from wells in a part of the Upper 

Mississippi River Basin that includes the St. Croix River 

Basin (Stark and others, 1996; Hanson, 1998). Likewise, 

estimates of contaminant residence times represent a range 

of basin-wide averages, while the actual residence time 

of a specific contaminant will vary depending upon its 

physical and chemical characteristics, the transport process 

through locally heterogeneous aquifers, and the location 

at which the contaminant enters the ground-water-flow 

system. Despite these limitations, the results described in 
 Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin 

this report provide a regional assessment of ground-watercontamination susceptibility, ground-water-residence 

times, and ground-water/surface-water interaction, which 

constitute a foundation for more detailed evaluation at 

local scales. 

Acknowledgments 

John Mossler and Robert Tipping, both from the 

Minnesota Geological Survey, furnished digital maps 

and insight on hydrogeologic properties of bedrock 

aquifers in southeastern Minnesota. Gregory Mitton and 

Geoff Delin, both of the USGS Minnesota Water Science 

Center, supplied information about characteristics of the 

Kettle, Snake, and Sunrise River Basins, and groundwater recharge estimates in Minnesota, respectively. Eric 

Porcher (Minnesota Pollution Control Agency) furnished 

documentation and a description of the methods used to 

estimate ground-water-contamination susceptibility for the 

State of Minnesota. Jana Stewart (USGS Wisconsin Water 

Science Center) helped to compile the digital maps used 

to compute ground-water-contamination susceptibility in 

the St. Croix Basin. Charles Dunning and Randall Hunt 

(USGS Wisconsin Water Science Center) gave feedback 

on the model refinements and general methods described 

in this report. James Krohelski, Daniel Feinstein (both of 

the USGS Wisconsin Water Science Center), Scott Alexander (University of Minnesota), and Thomas Reilly (USGS 

Office of Ground Water) reviewed the report and provided 

helpful comments and insights. 

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Juckem, P.F.—Hydrogeologic Characteristics of the St. Croix River Basin, Minnesota and Wisconsin: Implications for the Susceptibility of Ground Water 

to Potential Contamination—Scientific Investigations Report 2007−5112 

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