Hydrology and Relation of Selected Water-Quality 

Constituents to Selected Physical Factors in Dakota 

County, Minnesota, 1990-91

By James E. Almendinger and Gregory B. Mitton____________

U.S. Geological Survey

Water-Resources Investigations Report 94-4207

Prepared in cooperation with the

Minnesota Department of Natural Resources, the 

Legislative Commission on Minnesota Resources and the 

Dakota County Soil and Water Conservation District

Mounds View, Minnesota 

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Earth Science Information Center

District Chief Open-File Reports Section

U.S. Geological Survey Box 25286, MS 517

2280 Woodale Drive Denver Federal Center

Mounds View, MN 55112 Denver, CO 80225
Contents

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

Introduction.........................................................................._ 1

Purpose and scope............................................................................................................................................ 3

Geology of study area...................................................................................................................................... 3

Previous investigations..................................................................................................................................... 3

Methods of investigation.................................................................................................................................. 5

Hydrology................................................................................................................................................ 5

Water quality........................................................................................................................................... 5

Contributing areas....................................^ 8

Physical factors........................................................................................................................................ 8

Acknowledgments.......................................................^^ 9

Hydrology..............................................................................................^^ 9

Surface water.................................................................................................................................................... 10

Ground water.................................................................................................................................................... 10

Water quality.......................................................................................................................................................... 11

Differences among sample types...................................................................................................................... 11

Variability with time........................................................................................................................................ 14

Relation of selected water-quality constituents to selected physical factors......................................................... 17

Source factore..............................................................................................................................^ 17

Point sources............................................................................................................................................ 17

Spatially distributed sources.................................................................................................................... 17

Agricultural land............................................................................................................................... 17

Quaternary deposits.......................................................................................................................... 18

Bednx*......................................................................................................^ 19

Transport factors.............................................................................................................................................. 21

Soil-leaching potential............................................................................................................................. 21

Water-table depth..................................................................................................................................... 21

Summary.....................................................................................................................................^ 23

References..............................................................^ 25

Illustrations

Figure 1. Map showing Vermillion River Basin, outwash deposits, and major

municipalities, Dakota County, Minnesota............................................................................. 2

2. Diagram showing selected physical factors and water-flow paths that can 

affect water quality: A. Selected source factors, B. Selected transport 

factors and water-flow paths................................................................................................... 4

3. Map showing locations of five Vermillion River sampling sites, five 

subbasins of the Vermillion River, 20 domestic wells, and a waste-water 

treatment plant in Dakota County, Minnesota......................................................................... 6

4. Diagram showing plan view of assumed contributing area for a well..................................... 9

5-12. Graphs showing:

5. Daily average streamflow of the Vermillion River at site 2 during 1990

and 1991 water years, Dakota County, Minnesota........................................................... 12

111
Illustrations Continued

5-12. Graphs showing (continued):

6. Frequency of relative water-level measurements for (A) water table 

relative to stream, and (B) deeper wells relative to water table for 

five sites in the Vermillion River Basin during 6 to 12 sampling 

trips in 1990-91................................................................................................................. 13

7. Relation of specific conductance values and nitrate concentrations 

in stream water to streamflow in the Vermillion River Basin, 

Minnesota, 1990-91.......................................................................................................... 16

8. Relation of median nitrate concentrations in water to percentages of 

agricultural land in contributing area of sampling points in Dakota 

County, Minnesota, 1990-91............................................................................................. 18

9. Relation of median calcium concentrations in water to percentages 

of Des Moines lobe drift in contributing area of sampling points in 

Dakota County, Minnesota, 1990-91................................................................................ 19

10. Relation of median calcium concentrations in water to percentages of 

Prairie du Chien Group bedrock in contributing area of sampling 

points in the Vermillion River Basin, Minnesota, 1990-91.............................................. 20

11. Relation of nitrate concentrations in ground water to soil-leaching 

potential category for sampling points with equal to or greater than 

75 percent agricultural land use in contributing area of sampling 

points in Dakota County, Minnesota, 1990-91................................................................. 22

12. Relation of nitrate concentrations in ground water to water-table 

depths for sites with equal to or greater than 75 percent agricultural 

land use in contributing area of sampling points in Dakota County, 

Minnesota, 1990-91.......................................................................................................... 23

Tables

Table 1. Description of sampling sites, sampling points, and sample types of

water samples collected in Dakota County, Minnesota........................................................... 7

2. Summary of subbasin and streamflow data from five sites in the

Vermillion River Basin, Dakota County, Minnesota, 1990-91............................................... 11

3. Specific conductance values in water for three sample types collected at

five sampling sites in the Vermillion River Basin, Minnesota, 1990-91................................ 14

4. Nitrate concentrations in water for three sample types collected at five

sampling sites in the Vermillion River Basin, Minnesota, 1990-91........................................ 14

IV
Conversion Factors, Vertical Datum, and Abbreviations

Multiply inch-pound unit Bv To obtain metric unit

inch (in.) 25.4 millimeter

foot (ft) .3048 meter

mile (mi) 1.609 kilometer

cubic foot per second (ftVs) .02832 cubic meter per second

Sea level: In this report, sea level refers to the National Geodetic Vertical Datum of 1929 (NG VD of 1929) geodetic 

datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly 

called Sea Level Datum of 1929.

Chemical concentrations: Chemical concentrations of substances in water are given in metric units of milligrams per 

liter (mg/L) or micrograms per liter (|J.g/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. One thousand micrograms per 

liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L, the numerical values are the same 

as for concentrations expressed in parts per million.

Water year: A water year is from October 1 of the previous year through September 30 of the stated year. For example, 

the 1990 water year is from October 1,1989 through September 30,1990.
HYDROLOGY AND RELATION OF SELECTED WATER-QUALITY

CONSTITUENTS TO SELECTED PHYSICAL FACTORS IN

DAKOTA COUNTY, MINNESOTA, 1990-91

By James E. Almendinger and Gregory B. Mitton

Abstract

Selected water-quality constituents were determined in water from 5 surface-water sites and 29 wells in Dakota 

County, Minnesota, to search for possible relations to selected physical factors, including waste-water discharge, 

agricultural land, Quaternary deposits, bedrock, soil-leaching potential, and water-table depth. All surface-water 

samples were from the Vermillion River Basin, whose hydrologic setting was studied to determine its relation to the 

ground-water flow in the surrounding surficial sand aquifer. Each site was sampled from 1 to 12 times during 1990- 

91. A total of 198 samples were collected; selected samples were analyzed for major inorganic ions, nutrients, and 

triazine content. Physical factors within the area of land assumed to be contributing water to each sampling site were 

determined from existing mapped or digitized sources. Nitrate concentrations in ground water were related to 

agricultural land and soil-leaching potential. Nitrate concentrations were large (median 13.2 milligrams per liter as 

nitrogen) where the percentage of agricultural land in the contributing area was large (equal to or greater than 75 

percent) and where the soils had a large soil-leaching potential. Nitrate concentrations were small (median 3.2 

milligrams per liter as nitrogen) where the soils had a small soil-leaching potential, despite a large percentage of 

agricultural land. The statistical relation was not particularly strong, however: the null hypothesis that sites with 

different soil-leaching potentials had the same nitrate concentrations in ground water was rejected by the KruskalWallis test at only the probability P = 0.15 level. Water-table depth was not an important factor in the relation between 

nitrate concentrations in ground water and agricultural land. Discharge from a waste-water treatment plant provided 

most of the downstream loading of nitrate into the Vermillion River mainstem. Triazines were found in small 

concentrations (less than 2 micrograms per liter) in the Vermillion River and its tributaries. No relation was apparent 

between selected water-quality constituents and either Quaternary deposits or bedrock.

Introduction

State and local officials in Minnesota are concerned 

about the potential effects of agricultural chemicals such 

as fertilizers and pesticides on water quality in Dakota 

County (fig. 1). The central part of the county is covered 

by a surficial sand aquifer. This area is farmed 

extensively and the aquifer is susceptible to 

contamination by nutrients and pesticides applied to the 

fields. These chemicals could contaminate the water in 

the surficial sand aquifer and eventually migrate into the 

underlying bedrock Prairie du Chien-Jordan aquifer, 

which is widely used for domestic drinking-water 

supplies (Balaban and Hobbs, 1990). These 

contaminants could also move to the Vermillion River in 

central Dakota County and consequently affect surfacewater quality there.

Effective water-quality management requires 

knowledge of the local hydrology and of the physical 

factors that affect water quality. The U.S. Geological 

Survey, in cooperation with the Minnesota Department 

of Natural Resources, the Legislative Commission on 

Minnesota Resources, and the Dakota County Soil and 

Water Conservation District, initiated a study in 1989 to 

investigate the hydrology of the Vermillion River Basin 

and to relate selected water-quality constituents to 

selected physical factors in Dakota County.

Physical factors that can affect water quality are 

categorized in this report as source factors and transport 

factors (fig. 2). Source factors include both natural and 

human-caused potential sources of chemical 

constituents in water. Some source factors, such as a 

waste-water discharge, are present at known locations 

and are called point sources. Other source factors are
44°45'

Non-contributing 

area

Burnsvllle

^:^N^:-:-x-^V:f^*x->:.w?^B:-!-:v.:-:.:.!.^:-N%a??5.:.:A

Geologic data from 

Hobbs and Goebel, 1982.

01234 SMILES

I I 'l .' I 'l ' ' 

01234 5 KILOMETERS

Base from U.S. Geological Survey digital data 

1:100.000 1985. Universal Transverse Mercator 

Projection, icone 15

EXPLANATION

Outwash

Vermillion River Basin 

boundary

Study area boundary

Minneapolis/St. Paul

Dakota County

Figure 1.--Vermiilion River Basin, outwash deposits, and major municipalities,

Dakota County, Minnesota.
present over larger land surface areas or underground 

volumes and are called spatially distributed sources. 

Agricultural land, Quaternary deposits, and bedrock are 

examples of spatially distributed sources. Transport 

factors affect the flow path or travel time of water as it 

flows from a given source to the water sampling 

location. Soil-leaching potential and water-table depth 

are examples of transport factors. Other transport factors 

not examined in this report include hydraulic 

conductivity and hydraulic gradient.

Purpose and Scope

The purpose of this report is to investigate the 

hydrology of the Vermillion River Basin and to relate 

selected water-quality constituents to selected physical 

factors in Dakota County, both within and beyond the 

Vermillion River Basin. The study area includes Dakota 

County plus the part of the Vermillion River Basin that 

extends into Scott County to the west. Physical factors 

examined were waste-water discharge, agricultural land, 

Quaternary deposits, bedrock, soil-leaching potential, 

and water-table depth. The scope is limited to data from 

five surface-water sampling sites and 29 wells, and to 

information from the literature and other public sources. 

A total of 49 samples of surface water and 149 samples 

of ground water were collected from spring 1990 to 

summer 1991. Selected water samples were analyzed 

for major inorganic ions, nutrients, and triazine 

herbicides.

Geology of Study Area

The surficial geologic deposits (drift) of Dakota 

County are dominated by unconsolidated glacial 

deposits, although bedrock outcrops are common in the 

southern part of the county (Hobbs and Goebel, 1982; 

Balaban and Hobbs, 1990). The glacial history of 

Dakota County is complex because mineralogically 

distinct deposits originated from different ice lobes. The 

southeastern part of the county has exposures of noncalcareous early or pre-Wisconsin drift. In later 

Wisconsin time (about 20,000 years ago), ice from the 

Superior lobe advanced from the northeast and 

deposited non-calcareous drift as end moraine in the 

northern and northwestern parts of the county and as 

outwash in the central part. Finally, calcareous drift was 

deposited by the Des Moines lobe, which advanced 

from the northwest about 18,000 to 14,000 years ago. 

This lobe lapped onto the existing moraine along the 

northwestern boundary of the county and continued 

south to form a low moraine along the western boundary 

of the county. An eastward-trending corridor of outwash

was deposited at that time along the present course of 

the Vermillion River (Wright, 1972).

Small areas of Cambrian sandstone and dolostone 

subcrop in buried bedrock valleys in northern and 

eastern Dakota County. These valleys were channels of 

the Mississippi River during previous interglacial 

periods and are mostly filled with drift (Wright, 1972). 

The Prairie du Chien Group dolostone of Ordovician 

age is the most extensive bedrock subcrop unit in 

Dakota County (Balaban and Hobbs, 1990). The Prairie 

du Chien Group and the underlying Jordan Sandstone of 

Cambrian age function as a single aquifer over much of 

their extent (Adolphson and others, 1981). Overlying 

the Prairie du Chien Group is the St. Peter Sandstone, a 

poorly cemented sandstone of Ordovician age that 

forms the second-most extensive bedrock subcrop unit 

in Dakota County. Small areas of younger Ordovician 

carbonate and shale bedrock units overly the St. Peter 

Sandstone, especially in the northern part of the county 

(Balaban and Hobbs, 1990).

Previous Investigations

The general hydrology of the Minneapolis-St. Paul 

area is discussed by Norvitch and others (1973). They 

evaluated the water-resource availability for the area 

and calculated overall water budgets for various regions 

in their study area, which included the northern and 

central parts of Dakota County.

The U.S. Geological Survey conducted a sampling 

program from 1972 to 1978 to assess lake-water quality 

in relation to urbanization in northern Dakota County 

(Have, 1980; Ayers and others, 1980; Payne, 1980; 

Have and others, 1981). The reports concluded that 

urbanization generally increases nutrient loadings to 

lakes, notably phosphorus loading. Several of the 

reports tested regression models that could be used to 

predict phosphorus loadings. The reports also 

documented that,urban runoff increased chloride 

concentrations and specific conductances of lake water. 

Tornes (1989) did follow-up sampling during 1982-83 

and showed that chloride and specific conductance had 

increased in a number of lakes since the earlier 

sampling.

Ayers and others (1985) investigated runoff from 

small watersheds in the Minneapolis-St. Paul area to 

characterize loads from spatially distributed sources and 

to investigate transport mechanisms of various 

dissolved constituents. The authors concluded that 

amount of streamflow determined loadings of most 

constituents, in spite of negative correlations between
A. Selected source factors

Spatially distributed sources

Waste-water

treatment

plant

V: ::-{':-; Quaternary deposits ' '

B. Selected transport factors and water-flow paths

Water-flow paths

Overland 

flow path

Transport factors

Soil-leaching 

potential Surface-water 

flow path Unsaturated zone 

flow path

 Water-table depth '£:'-;::$:'^^

'.'.v.'.'.'.'. i I I\i I I I I

rxj i i i i i i i

Figure 2,-Selected physical factors and water-flow paths that can affect water quality: 

A. Selected source factors, B. Selected transport factors and water-flow paths.
concentration and streamflow for many constituents. 

One of the watersheds studied was the South Branch 

Vermillion River, which is included in the present study 

area. Ayers and others (1985) found this watershed to be 

different from many of the others, possibly because of 

the large amount of irrigation in the basin and the large 

baseflow component of streamflow. Anderson and 

others (1987) studied rainfall-runoff relations for the 

Vermillion River and determined that 73 percent of the 

basin had a small runoff potential.

Ground water in drift and bedrock aquifers in Dakota 

County tends to be of the calcium-magnesiumbicarbonate type (Winter, 1974; Ruhl and others, 1983; 

Ruhl and Wolf, 1983; and Ruhl, 1987). Reeder and 

Norvitch (1974) examined a contaminated ground-water 

plume in a heavily industrialized area in northeastern 

Dakota County. They found elevated concentrations of 

many chemical constituents, including phenols, and 

smaller pH values in the contaminated plume. None of 

the wells in the present study are located in the 

contaminated area delineated by Reeder and Norvitch 

(1974).

The present study produced two earlier reports. 

Lorenz and Trotta (1991) examined computer methods 

of mapping water-table depth in Dakota County. 

Almendinger (1991) described the relation of nitrate 

concentrations in ground water to agricultural land and 

soil-leaching potential in Dakota County.

Methods of Investigation

This section describes the methods used to 

investigate the hydrology, water quality, contributing 

areas, and physical factors in the study area.

Hydrology

Five sampling sites were established in the 

Vermillion River Basin at two locations along the main 

stem (sites 2 and 5, fig. 3) and near the mouths of three 

tributaries (sites 1,3, and 4, fig. 3). These sites will be 

referred to as the five Vermillion River sites in this 

report. A water-table well is a well with a screen 

intersected by the water table. A water-table well was 

installed in the drift within about 50 feet of the river at 

each of the five Vermillion River sites. In this report, the 

term "deeper well" refers to a well located within 10 

horizontal feet of a water-table well and constructed 

with a 3-foot screen placed about 10 to 30 feet below the 

water table. A deeper well was installed in the drift at 

each of sites 1-4. Water levels in the river, water-table 

well, and deeper well were compared to estimate

whether ground water was moving toward or away from 

the river at each site.

Streamflow was measured at each of the five 

Vermillion River sites from 9 to 12 times during 1990- 

91. At other times during the 1990 water year (October 

1,1989 through September 30,1990) streamflow was 

estimated with stage-streamflow relations for sites with 

stage recorders. Site 2 was instrumented with a 

continuous stage recorder that was operated year-round; 

sites 3 and 5 were instrumented with continuous stage 

recorders for the ice-free periods of 1990-91. Subbasins 

above the five sampling sites were delineated on 

l:24,000-scale topographic maps and digitized to obtain 

areas by using geographic information system software.

Water quality

Selected samples were analyzed for major inorganic 

ions, nutrients, and tiiazine content. In this report, water 

quality refers primarily to four selected water-quality 

constituents: specific conductance, calcium, nitrate, and 

triazines. Specific conductance was chosen because it is 

an overall measure of the ionic content of the water. 

Calcium was chosen because it can originate from 

dissolution of minerals in the Quaternary deposits and 

bedrock. Nitrate and triazines were chosen because they 

commonly originate from agricultural land. Other 

water-quality constituents sometimes showed patterns 

similar to these four in their variability with time and 

relation to selected physical factors.

In this report, sampling "site" refers to a small area 

(radius about 50 feet or less) where water samples are 

collected, sampling "point" refers to a location within a 

sampling site where a certain sample type was collected, 

and sample "type" refers to the kind of water sample 

collected within a site (table 1). The 25 sampling sites 

include the five Vermillion River sites plus 20 domestic 

wells. The three sampling points within the Vermillion 

River sampling sites included the river or tributary 

stream, a water-table well, and a deeper well (except at 

site 5). Each domestic well sampling site had only one 

sampling point, namely the well itself. The four sample 

types included surface water collected from the river or 

tributary stream, shallow ground water collected from 

the water-table wells, deeper ground water collected 

from the deeper wells, and domestic well water 

collected from the domestic wells.

Samples for water-quality analyses were collected 10 

to 12 times during 1990-91 at the five Vermillion River 

sites (fig. 3). Depth-integrated surface-water samples
44°45' 

01234 SMILES

I .'..'.'. ' ' 

01234 5 KILOMETERS

Base from U.S. Geological Survey digital data 

1:100,0001985, Universal Transverse Mercator 

Projection. Zone 15.

EXPLANATION

Subbasins:

Site 1 (tributary)

Site 2 (mainstem)

Site 3 (tributary)

Site 4 (tributary)

Site 5 boundary (mainstem)

Study area boundary

Vermillion River sampling 

site, number indicates site 

identifier

Domestic well 

Waste-water treatment plant

Figure 3.--Locations of five Vermlllion River sampling sites, five subbaslns 

of the Vermlllion River, 20 domestic wells, and a waste-water treatment 

plant in Dakota County, Minnesota.
Table 1 .-Description of sampling sites, sampling points, and sample types of water samples collected

in Dakota County, Minnesota

Sampling site Sampling point Sample type

Vermillion River 1 Vermillion River 1 

Water-table well 1 

Deeper well 1

Surface water 

Shallow ground water 

Deeper ground water

Vermillion River 2 Vermillion River 2 

Water-table well 2 

Deeper well 2

Surface water 

Shallow ground water 

Deeper ground water

Vermillion River 3 Vermillion River 3 

Water-table well 3 

Deeper well 3

Surface water 

Shallow ground water 

Deeper ground water

Vermillion River 4 Vermillion River 4 

Water-table well 4 

Deeper well 4

Surface water 

Shallow ground water 

Deeper ground water

Vermillion River 5 Vermillion River 5 

Water-table well 5

Surface water 

Shallow ground water

20 domestic wells 

Total 25 sampling sites

20 domestic wells Ground water

34 sampling points

(5 surface-water sampling points, and

29 ground-water sampling points)

(49) were collected in mid-channel. Ground-water 

samples from the five water-table wells (59 samples) 

and four deeper wells (47 samples) were collected with 

a submersible pump. At least three well volumes of 

water were purged before sampling began. Groundwater samples from 20 domestic wells screened in the 

drift were collected in September 1990, and from May 

to June 1991. Of these wells, 17 were outside the 

Vermillion River Basin. Most (15) domestic wells were 

sampled twice, four were sampled three times, and one 

was sampled once, for a total of 43 samples from 

domestic wells.

Water temperature, pH, specific conductance, and 

dissolved-oxygen concentrations were measured at each

site. Samples were filtered in the field, chemically 

preserved, and placed on ice in coolers for transport to 

the laboratory. Chilled samples were shipped to the U.S. 

Geological Survey National Water Quality laboratory in 

Arvada, Colorado, for analyses of major inorganic ions 

and nutrients. In this report, nitrate concentrations are 

given in mg/L (milligrams per liter) as nitrogen and 

assumed to be equal to combined nitrite-plus-nitrate 

concentrations because nitrite concentrations were 

commonly negligible. Nitrite was determined on a 

subset of the samples; the median nitrite concentration 

was less than 1 percent of the combined nitrite-plusnitrate concentration for samples that had both nitrite 

and nitrate concentrations above the reporting limits. 

Triazine herbicide content was determined for 92
samples by an immunoassay technique. Major inorganic 

ions were determined only on samples collected in 

1991. The results of all water-quality analyses are 

available in the National Water Information System 

(NWIS) computer data base of the U.S. Geological 

Survey and the STORET computer data base of the U.S. 

Environmental Protection Agency.

Quality control was effected by calculation of ion 

balances and by analysis of field duplicates and filter 

blanks, which accounted for more than 10 percent of the 

samples analyzed. The average ion imbalance was only 

1.4 percent, and only two samples had imbalances 

greater than 5 percent. Analyses of field duplicates 

agreed to within an average of 2 percent for major 

dissolved constituents (calcium, magnesium, sodium, 

potassium, alkalinity, sulfate, chloride, and silica). The 

minor constituents (nitrite, ammonia, Kjeldahl nitrogen, 

phosphorus, and fluoride), which were often in 

concentrations near the reporting limit, had larger 

imprecisions, ranging from about 10 to 30 percent. 

Analyses of filter blanks indicated no significant 

contamination by any constituent. Small amounts of 

calcium and alkalinity in the blanks indicated that the 

filter apparatus was not perfectly rinsed between 

samples, but the amount of potential contamination was 

trivial (about 0.5 percent) compared to the calcium and 

alkalinity values of the samples being collected.

Contributing areas

Water quality at a sampling point depends on the 

source factors contributing water-quality constituents 

along the flow path leading to the sampling point (fig. 

2). For point sources, such as discharge from a wastetreatment plant into a river, the river is clearly the 

primary flow path between the source and a downstream 

sampling point. For spatially distributed sources, the 

flow path between source factors and sampling points 

may not be readily apparent. The relation of spatially 

distributed sources to water quality at a sampling point 

requires the concept of a contributing area, which is 

defined in this report as that area of land or aquifer (as 

projected onto a horizontal plane) that contributes water 

to a sampling point. Water quality at a sampling point 

may then be related to the spatially distributed source 

factors that are present within the contributing area for 

that sampling point.

Contributing areas for surface-water sampling points 

were assumed to be the upstream subbasins. A subbasin 

comprises the area that contributes overland flow to a 

stream. No data were available to delineate the area of 

aquifer that contributes ground water to the stream in

each subbasin. This aquifer area was assumed to be the 

same as the subbasin area, although this assumption 

may not be valid, especially for the smaller tributaries.

Contributing areas for ground-water samples were 

arbitrarily assumed to be a sector spanning a 30-degree 

arc and extending upgradient from the well to a distance 

corresponding to a 10-year horizontal ground-watertravel time (fig. 4). This distance was calculated from 

the estimated pumping rate of each well, the magnitude 

and direction of the horizontal hydraulic gradient at the 

well, and the aquifer transmissivity and porosity (Bear, 

1979). A 10-year travel time was chosen partly because 

this time period encompasses the age of the land-use 

data, which were from 1984. The choice of a 30-degree 

arc to span the sector was arbitrary, but should account 

for some variability or uncertainty in the direction of 

ground-water flow near each well. The orientation of the 

sector upgradient from the well allowed knowledge of 

ground-water-flow directions to be incorporated in 

delineating contributing areas. Other studies have 

chosen circular contributing areas (buffer zones) of a 

fixed radius centered around each well (Vowinkel and 

Battaglin, 1989; Almendinger, 1991), which ignore 

information concerning ground-water flow directions 

and travel times. The previous investigation in Dakota 

County that used circular contributing areas for each 

well (Almendinger, 1991), however, produced results 

similar to those in this report, and thus the choice of the 

shape of the contributing area was not critical.

Physical factors

Geographic information system software was used to 

estimate the percent coverage of selected source factors, 

including agricultural land, Quaternary deposits, and 

bedrock, in each contributing area. Land-use data as of 

1984 were obtained in digital form from the Minnesota 

Land Management Information Center. Information on 

the Quaternary deposits and bedrock was digitized by 

Dakota County personnel from 1:100,000-scale maps in 

the Dakota County geologic atlas (Balaban and Hobbs, 

1990).

The transport factors examined were soil-leaching 

potential and water-table depth. Soil types in Dakota 

County were rated according to the soil-leaching 

potential, which is a measure of the likelihood that 

chemicals could be leached through the soil layer. For 

this study, the soil layer is defined as being the topmost 

60 inches of unconsolidated material. The U.S. 

Department of Agriculture Soil Conservation Service 

rates the soil-leaching potential by a method that 

depends on the combined effects of soil permeability,
Approximate direction

of horizontal 

ground-water flow

Well

Figure 4.--Plan view of assumed contributing area for a well.

land-surface slope, depth to the seasonal high water 

table (which may include perched water), and organic 

matter content of the upper soil (Soil Conservation 

Service, 1992). Each soil series may be scored 

according to soil-leaching potential on a 3-point scale. A 

score of 1 corresponds to a large soil-leaching potential, 

2 to a moderate soil-leaching potential, and 3 to a small 

soil-leaching potential. Soil series are grouped into soil 

associations, and each soil association in Dakota County 

was assigned an average soil-leaching potential score 

weighted by the percent areal coverage of the soil series 

within that soil association. Each contributing area was 

then assigned an average soil-leaching potential score 

weighted by the percent areal coverage of soil 

associations within that contributing area as determined 

by planimetry. Each contributing area was categorized 

as having a large, medium, or small soil-leaching 

potential, according to the weighted average soilleaching potential score. A large soil-leaching potential 

corresponds to a weighted average score between 1 and 

1.67, medium to a score between 1.68 and 2.33, and 

small to a score between 2.34 and 3.

A map of water-table depth in Dakota County was 

constructed by subtracting the water-table altitude 

mapped originally at 1:125,000 (Balaban and Hobbs,

1990) from land-surface altitude mapped originally at 

1:62,500 on U.S. Geological Survey topographic 

quadrangles (L.C. Trotta, U.S. Geological Survey, St. 

Paul, Minn., written commun., 1990). This map of 

water-table depth was used to estimate the water-table 

depths at the 20 domestic wells. Water-table depths at 

the five Vermillion River sites were determined from the 

water-table wells there.

Acknowledgments

The help of landowners who permitted sampling of 

well water is gratefully acknowledged. The Soil 

Conservation Service provide instruction and literature 

on the method of calculating soil-leaching potentials. 

The Metropolitan Waste Control Commission provided 

data on wastewater discharges. Both Dakota County and 

the Minnesota Geological Survey provided data in 

digitized or mapped form.

Hydrology

A brief overview of the surface-water and groundwater hydrology in Dakota County provides the basis
for understanding the flow paths of water and associated 

chemical constituents.

Surface Water

Dakota County receives about 28 inches of 

precipitation each year, of which about 23 inches is 

returned to the atmosphere by evapotranspiration 

(Norvitch and others, 1973). The remaining 5 inches of 

water, the climatic moisture surplus (Sellers, 1965), 

moves to streams either by overland flow or by groundwater flow and becomes runoff.

Streamflow comprises both overland flow and 

baseflow. Baseflow presumably is derived primarily 

from ground water that discharges into the channel. The 

flow of the Vermillion River tends to be stable because 

of the large baseflow component, which has gradual 

seasonal trends. Rain or snowmelt can produce enough 

overland flow, however, to cause occasional peaks in 

flow that are in excess of the baseflow component (fig. 

5).

The average streamflow of the Vermillion River at 

site 2 (fig. 3) was 53.4 ftVs (cubic feet per second) for 

18 years of record (1974-91). Long-term data are not 

available for the other four sampling sites. The 18-year 

average streamflow at site 2 corresponds to an average 

annual runoff of about 5.2 inches, which is similar to the 

estimated annual climatic moisture surplus of 5 inches 

(Norvitch and others, 1973). During water year 1990, 

however, the annual meui streamflow at site 2 was only 

41.5 ftVs, about 78 percent of the 18-year average 

streamflow and corresponding to a runoff of about 4.1 

inches (table 2). Runoff estimated at the other 

Vermillion River sites during water year 1990 ranged 

from 0.5 inches at site 4 to 5.6 inches at site 3 (table 2). 

Differences in runoff among nearby basins may result 

from differences in precipitation, evapotranspiration, or 

interbasin transfers of ground water. In particular, a 

basin with less runoff than surrounding basins may be 

losing ground water to those basins.

Ground Water

Ground water moving from upland recharge areas to 

discharge points such as streams and other surface-water 

bodies typically has a large horizontal component of 

flow. In a surficial aquifer near a stream, if the water 

table is higher than a stream, then shallow ground water 

is moving to the stream (fig. 2). If the water table is 

lower than the stream, then stream water may be moving 

into the aquifer, and ground water is moving to some 

other point of discharge. The direction of ground-water

flow can change depending on fluctuations in the watertable and stream levels. Comparison of stream levels 

with the water levels in the water-table wells at the five 

Vermillion River sites (fig. 6) showed that the water 

table was usually higher than or equal to the stream 

level at sites 1,2, and 3, and consequently shallow 

ground water was usually moving to the stream at these 

sites. The water table was usually lower than the stream 

level at sites 4 and 5, and consequently shallow ground 

water was usually not discharging into the stream at 

these sites but was moving to some other point of 

discharge. The stream at site 4 was usually dry, and the 

water table was below the dry stream bed.

Ground water also can have a vertical component of 

flow. Beneath recharge areas ground water moves 

downward, and beneath discharge points ground water 

moves upward (fig. 2). Comparison of water levels in 

water-table wells and deeper wells can indicate whether 

ground-water flow is upward or downward (Winter, 

1984). If a water-table well has a higher water level than 

that in an adjacent deeper well, then some ground water 

is moving downward. If a water-table well has a lower 

water level than that in a deeper well, then some ground 

water is moving upward. The direction of vertical 

ground-water flow can change depending on 

fluctuations of the water table relative to water level in 

the deeper well. Comparison of water levels in watertable wells and deeper wells at sites 1-4 allowed 

estimation of the direction of vertical ground-water flow 

during 6 to 12 sampling trips in 1990-91 (fig. 6). Ground 

water was usually moving upward at site 3 between the 

deeper well (about 30 feet below the water table) and 

the water table. This deeper ground water may discharge 

into the stream (South Fork Vermillion River) at site 3, 

as does the shallow ground water. Ground water was 

usually moving downward at site 4 between the water 

table and the deeper well. Ground water usually does 

not discharge into the stream at site 4, but moves toward 

some other point of discharge. At sites 1 and 2 the water 

levels in the water table wells and deeper wells showed 

no consistent pattern, indicating that vertical groundwater flow between the water table and the deeper wells 

was neither consistently upward nor downward. The 

deeper wells were screened about 10 feet below the 

water table at site 1 and about 30 feet below the water 

table at site 2. Downward-moving deeper ground water 

can bypass the river at sites 1 and 2 and flow to some 

other point of discharge (for illustration, see deeper 

ground-water flow path, fig. 2).

Site 5 differed from the rest because the water table 

about 50 horizontal feet from the Vermillion River is 

more than 40 feet below the bottom of the channel,

10
Table 2.-Summary of subbasin and streamflow data from five sites in the Vermillion River Basin, Dakota County,

Minnesota, 1990-91 

[streamflow in cubic feet per second]

Streamflows measured during sample trips, 1990-91

Location1

Sitel 

tributary 

Site 2

main stem 

Site 3

tributary 

Site 4

tributary 

Site 5

main stem

Subbasin 

area, square 

miles

31 

138

32

6

193

Minimum 

(month/year)

1.2 

(Februaiy 1991) 

19

(February 1990) 

5.9

(January 1990) 

0

(usually) 

25

(January 1990

Median

15

47

12

0

70

Maximum 

(month/year)

63 

(June 1990) 

149

(June 1990) 

51

(March 1990) 

1

(March 1990) 

183

(March 1990)

Number of 

observations

10 

11

11

12

9

For 1990 water year

Estimated 

annual 

mean 

streamflow

Z7.6 

341.5

4 13.3

2 .2

458.5

Estimated 

annual 

runoff, 

inches

3.4 

4.1

5.6

.5

4.1

1 Site locations shown in figure 3.

2 Calculated from correlation of instantaneous streamflow measurements to streamflow at site 2.

rj

Calculated from continuous stage data and stage-streamflow relation.

4 Calculated from continuous stage data and stage-streamflow data during ice-free months, and by correlation of instantaneous 

streamflow measurements to streamflow at site 2 during winter months.

indicating that an unsaturated zone may exist at some 

depth below the channel. The level of the water table 

relative to the Vermillion River in the vicinity of site 5 

may be related to the proximity of the deep Mississippi 

River trench less than two miles to the northeast. Nearby 

buried bedrock valleys filled with drift may lower the 

water table by being permeable conduits leading to the 

Mississippi River trench. The vertical movement of 

ground water at site 5 could not be determined with 

certainty because a deeper well was not installed below 

the water table. Ground water is probably not moving 

upward, however, because the site is not a point of 

ground-water discharge. Site 5 is more likely an area of 

ground-water recharge from water seeping out of the 

Vermillion River into the surficial aquifer, and the 

ground-water flow probably has a downward 

component, in addition to a horizontal component 

leading toward the Mississippi River.

Water Quality

The water quality measured at a site varies among 

sample types and with time. These variations are

important considerations in obtaining representative 

water-quality values for each site. Differences among 

sample types and variability with time were examined 

with respect to specific conductance, nitrate, and 

triazines.

Differences Among Sample Types

Water quality differed among surface-water, shallow 

ground-water, and deeper ground-water sample types 

collected at Vermillion River sites 1-4. The null 

hypothesis that the specific conductance values (table 3) 

and nitrate concentrations (table 4) were the same in 

these three sample types was rejected at the probability 

P = 0.01 level with the Kruskal-Wallis test applied 

separately to each of these four sites. Median specific 

conductance values and nitrate concentrations were 

larger in shallow ground water than in deeper ground 

water, except for nitrate concentrations at site 4. 

Differences between the quality of shallow ground 

water and surface water were not so consistent among 

sites. For example, median specific conductance values 

in shallow ground water were larger than those in

11
A. 1990 water year

1,000

N 

1989

B. 1991 water year

05

1,000

10

O IN D 

1990

Figure 5.--Daily average streamflow of the Vermillion River at site 2 during (A) 1990, 

and (B) 1991 water years, Dakota County, Minnesota (U.S. Geological Survey 

station 05345000, Vermillion River near Empire, Minnesota).

12
A. WATER TABLE RELATIVE 

TO STREAM

B. WATER LEVEL IN DEEPER WELLS 

RELATIVE TO WATER TABLE

234 

SITE NUMBER

234 

SITE NUMBER

EXPLANATION

Water table higher 

than stream level

Water table same as 

stream level

Water table lower 

than stream level

Stream dry

EXPLANATION

Water table higher 

than stream level

Water table same as 

stream level

Water table lower 

than stream level

Site locations shown in Figure 3

Deeper wells are defined as

wells 10 to 30 feet below the

water table at sites 1-4.

Figure 6.-Frequency of relative water-level measurements for (A) water table relative 

to stream, and (B) deeper wells relative to water table for five sites In the 

Vermllllon River Basin, Minnesota during 6 to 12 sampling trips In 1990-91.

13
Table 3.-Specific conductance values in water for three sample types collected at five sampling sites in the

Vermillion River Basin, Minnesota, 1990-91

[values in microsiemens per centimeter at 25 degrees Celsius; n, number of measurements; min, minimum; med, median;

max, maximum; IQR, interquartile range, the difference between the upper and lower quartiles; upper quartile, the value greater than or

equal to 75 percent of analyses; lower quartile, the value greater than or equal to 25 percent of analyses; n/a, not applicable;

--, no data;**, significant at the 0.01 level; ns, not significant at the probability P = 0.01 level;

statistical significance based on Kruskal-Wallis test]

Sample type

Surface water

Site n

1 11 

2 12

3 12

4 3

5 11

min med

402 594 

482 759

446 500

430 654

547 709

max

632 

1060

544

694

883

Average IQR values:

IQR

185 

193

44

n/a

115

134

Shallow ground water

n

12 

12

12

12

11

min

787 

521

534

372

572

med max

1090 1570 

699 749

584 595

595 855

722 952

IQR

578 

74

29

166

253

220

Deeper ground water

n

12 

12

11

12

~

min

567 

311

473

366

~

med max

605 631 

505 662

501 518

377 406

__

IQR

19 

30

23

23

24

Statistical 

significance 

of differences 

among 

sample types

** 

**

**

**

ns

Table 4.-Nitrate concentrations in water for three sample types collected at five sampling sites 

in the Vermillion River Basin, Minnesota, 1990-91

[values in milligrams per liter as nitrogen; n, number of measurements; min, minimum; med, median;

max, maximum; IQR, interquartile range, the difference between the upper and lower quartiles; upper quartile, the value greater than or

equal to 75 percent of analyses; lower quartile, the value greater than or equal to 25 percent of analyses; n/a, not applicable;

<, less than; --, no data; **, significant at the 0.01 level; ns, not significant at the probability P = 0.01 level;

statistical significance based on Kruskal-Wallis test]

Sample type

Surface water

Site n

1 11

2 12

3 12

4 3

5 11

min med

0.5 1.1

3.1 5.0

2.7 4.8

<.l <.l

3.9 5.5

max

4.0

10.0

5.9

2.0

7.4

Average IQR values:

IQR

0.9

.4

1.3

n/a

1.6

1.0

Shallow ground water

n

12

12

12

12

11

min

<0.1

5.5

2.1

.5

2.3

med

0.3

8.6

7.6

3.2

5.2

max

1.6

25.0

11.0

11.0

9.6

IQR

0.5

3.9

4.7

1.9

3.9

3.0

Deeper ground water

n

12

12

11

12

min med max IQR

<0.1 <0.1 <0.1 0.0

<.l <.l <.l 0

<.l <.l <.l 0

1.0 4.7 5.0 .4

._

.1

Statistical 

significance 

of differences 

among 

sample types

**

**

**

**

ns

14
surface water at sites 1 and 3, and smaller at sites 2 and 

4 (table 3). Median nitrate concentrations in shallow 

ground water were larger than those in surface water at 

sites 2-4, and smaller at site 1 (table 4).

In contrast, water quality was not significantly 

different between surface-water and shallow groundwater sample types at site 5 (tables 3 and 4). The water 

quality of shallow ground water and surface water may 

be similar at site 5 because movement of surface water 

into the aquifer may be a primary source of shallow 

ground water at this site. The shallow ground water 

from site 5 was excluded from analyses relating water 

quality to source factors because of the dominating 

influence of surface-water quality at this site.

Triazine herbicide content in 49 filtered water 

samples from the Vermillion River sites was measured 

with an immunoassay technique that categorizes the 

triazine concentration into the following ranges: not 

detected, less than 0.1 |Ag/L (micrograms per liter); 

small, from 0.1 to 0.25 |Ag/L; moderate, from greater 

than 0.25 to 2 (ag/L; and large, greater than 2 |Ag/L. The 

U.S. Environmental Protection Agency proposed 

maximum contaminant level for atrazine, one of the 

triazine herbicides, is 3 |Ag/L (U.S. Environmental 

Protection Agency, 1989). None of the samples had a 

triazine concentration greater than 2 |Ag/L. Of 18 surface 

water samples, nine had moderate, six had low, and 

three had non-detectable amounts of triazines. Of 18 

shallow ground water samples, two had moderate, four 

had low, and 12 had non-detectable amounts of 

triazines. Triazines were not detected in any of the 13 

samples of deeper ground water.

Water-quality differences among sample types may 

arise because of differences in the contributing areas. 

The surface-water sample integrates contributions from 

overland flow and ground-water discharge in the 

upstream subbasin, the shallow ground-water sample is 

from the surficial aquifer immediately upgradient from 

the site, and the deeper ground-water sample could have 

originated far from the sampling site. For example, 

nitrate was not detected in water from the deeper wells 

at sites 1-3 (table 4) despite the proximity of agricultural 

land, which presumably is the source of the nitrates 

found in the shallow ground water and surface water. 

The ground-water flow paths intercepted by the deeper 

well screen may have originated beneath a point on the 

land surface where nitrate fertilizers were not applied, or 

nitrates may not have had time to reach the deeper well 

because of the length of the flow paths, or bacteria may 

have denitrified the water along the flow paths.

Variability with Time

Variability of water quality with time is related to 

sample type. A measure of variability with time is the 

interquartile range. The interquartile range is the 

difference between the upper quartile (the value equal to 

or greater than 75 percent of analyses) and the lower 

quartile (the value equal to or greater than 25 percent of 

analyses). The averages of interquartile ranges of both 

specific conductance values (table 3) and nitrate 

concentrations (table 4) are smaller for deeper ground 

water than for either surface water or shallow ground 

water at the five Vermillion River sites. Therefore the 

variability of these selected water-quality constituents is 

smaller for deeper ground water than for either surface 

water or shallow ground water. The quality of deeper 

ground water may not vary much because it is moving 

too slowly to change appreciably at that point during the 

period of sampling. Alternatively, the small variability 

may result from chemical homogenization by dispersive 

mixing along long ground-water flow paths deeper in 

the aquifer.

Stream-water quality varies with time as streamflow 

changes. Hem (1985) indicates that specific 

conductance of stream water typically is related 

inversely to streamflow, which varies with the season. 

Specific conductance values tended to have an inverse 

relation with streamflow at the five Vermillion River 

sites (fig. 7). Streamflow in the Vermillion River Basin 

was greater during spring and summer months (MarchAugust) in 1990-91 (fig. 5), than during autumn and 

winter months (September-February) and thus in 

general the stream water most sites had slightly smaller 

values of specific conductance in spring and summer 

than during autumn and winter. Nitrate concentrations 

were not so simply related to streamflow, and more 

samples than were collected for this study may be 

needed to distinguish trends at any one site.

Triazine content of surface-water samples varied 

seasonally with the streamflow. Of the nine surfacewater samples with moderate triazine content, eight 

were collected during spring, and six were collected 

during flows greater than the annual mean streamflow. 

Triazines apparently moved to the river with overland 

runoff during high flow conditions. Squillace and 

Thurman (1992) found similar results in the Cedar River 

Basin in northeastern Iowa, where 94 percent of the 

river load of atrazine originated from overland flow and 

only 6 percent came from ground-water discharge into 

the river.

15
DC 

UJ

O

82

U- UJ 

OCO

wo

1,200

1-000

800

600

0 400

200

x

X

O

n xD

+ SITE 1 O SITE 4

x SITE 2 O SITE 5

D SITE 3

200 400 600 800

STREAMFLOW IN 1990-91, AS PERCENTAGE OF ANNUAL MEAN

STREAMFLOW FOR 1990 WATER YEAR

1,000

z »

UJ

o

O

-f

Q 2 6

cc

H- CC 

Z UJ

UJ 1-

** 4

o u-1

m oj

K £ rt

f DC 2

1

z

 n

i i i ' i

x + SITE 1 O SITE 4

D D x SITE 2 O SITE 5

- q^ x o o n SITE 3

o n +

x

X

+ + ^

" + + +

. /S i , i , i . i ,

200 400 600 800

STREAMFLOW IN 1990-91, AS PERCENTAGE OF ANNUAL MEAN

STREAMFLOW FOR 1990 WATER YEAR

1,000

Site locations shown in Figure 3

Figure 7.-Relatlon of specific conductance values and nitrate concentrations 

in stream water to streamflow In the Vermlllion River Basin, Minnesota, 1990-91.

16
Relation of Selected WaterQuality Constituents 

to Selected Physical Factors

The purpose of this section of the report is to relate 

water quality to selected physical factors that affect 

chemical constituents in water at a sampling point.

Source Factors

Source factors are categorized in this report as either 

point sources or spatially distributed sources.

Point sources

The only point source examined was a waste-water 

treatment plant on the main stem of the Vermillion River 

about 2.5 river miles upstream from site 2 and about 13 

river miles upstream from site 5 (fig. 3). Monthly 

average discharge and nitrate-concentration data (Melba 

Hensel, Metropolitan Waste Control Commission, 

written commun., 1991) were multiplied to obtain a 

monthly average nitrate loading from January 1990 

through June 1991. Nitrate load from the waste-water 

discharge accounted for an average of 92 percent (n = 

11) of the nitrate load measured in stream water at site 2, 

and 60 percent (n = 9) of the nitrate load measured in 

stream water at site 5. The waste-water discharge 

apparently was the primary source of nitrates in the 

main stem of the Vermillion River downstream from the 

point of discharge from January 1990 through June 

1991. Because the nitrate concentrations of surfacewater samples from sites 2 and 5 were so influenced by 

the waste-water discharge, these samples were excluded 

from analyses examining the relations between nitrate 

concentrations and other physical factors.

Spatially distributed sources

In the following analyses, the median values of 

water-quality constituents at each sampling point were 

used as the measure of water quality in examining 

bivariate relations between selected water-quality 

constituents and selected physical factors. The use of 

median values, rather than the raw data, avoids overrepresentation of sampling points where more samples 

were collected than at other sampling points. 

Overlapping contributing afeas could cause overrepresentation of physical factors within the area of 

overlap, because the same set of physical factors there 

would be included more than once in the statistical 

analyses. In particular, the method of estimating 

contributing areas for ground-water sampling points

does not take vertical gradients into account, and 

consequently the estimated contributing areas for the 

water-table wells and deeper wells are exactly 

overlapping at each of the Vermillion River sites 1-4. 

Data from either the water-table wells or the deeper 

wells were excluded from certain analyses to remove 

the overpresentation caused by overlapping contributing 

areas. Data from the water-table wells were used where 

the ground-water flow paths between the source factor 

(such as agricultural land or Quaternary deposits) and 

the sampling point had predominantly horizontal and 

downward components. Data from the deeper wells 

were used where the ground-water flow paths between 

the source factor (such as bedrock) and the sampling 

point had some upward component.

Agricultural land

Agricultural practices can introduce pesticides and 

fertilizers into surface and ground waters (Freeze and 

Cherry, 1979; Goolsby and others, 1991). Most of the 

contributing areas for sampling points in the present 

study were classified as having predominantly 

agricultural land. Surface-water sampling points at 

Vermillion River sites 2 and 5 and the water-table well 

at site 5 were excluded from analysis because of 

influence from the waste-water treatment plant; deeper 

wells at Vermillion River sites 1-4 were excluded from 

analysis because their contributing areas overlapped 

with those of the water-table wells at the same sites. 

Fifteen of the remaining 27 sampling points had equal to 

or greater than 90 percent agricultural land in the 

contributing areas, and seven had between 50 and 90 

percent.

Nitrate concentrations in water were related to the 

percentage of agricultural land in the contributing area 

(fig. 8). Sampling points with large percentages (equal 

to or greater than 75 percent) of agricultural land can 

have larger nitrate concentrations (median 3.6 mg/L as 

nitrogen) than do sampling points with a smaller 

percentage of agricultural land (median below the 

detection limit of 0.1 mg/L as nitrogen). Many sampling 

points, however, had small nitrate concentrations 

despite having a large percentage of agricultural land in 

the contributing area. Concentrations of dissolved 

Kjeldahl nitrogen and dissolved phosphorus appeared to 

be related to agricultural land in the same way that 

nitrate concentrations were. Specific conductance,

17
30

HI

O

20

uj t 

O -J

2 «- 

Oft

< 

cc

10

5 5

O

A SURFACE WATER 

O GROUND WATER

O

A iQ

20 40 60

O O'OOOQ

OQ C)

o:

80 100

PERCENTAGE OF AGRICULTURAL LAND IN 

CONTRIBUTING AREA

Figure 8.-Reiation of median nitrate concentrations in water to 

percentages of agricultural land in contributing area of 

sampling points in Dakota County, Minnesota, 1990-91.

chloride concentration, and other water-quality 

constituents showed no apparent relation to percent 

coverage of agricultural land in the contributing areas.

Triazine herbicides were detected at all five of the 

surface-water sampling points. Areal coverage of 

agricultural land in the contributing subbasins ranged 

from 16 to 98 percent; four of the five subbasins had 85 

percent or greater. Triazine was detected in reportable 

concentrations at five of the ground-water sampling 

points, and agricultural land accounted for more than 90 

percent coverage of the contributing area for four of 

these sampling points.

Quaternary deposits

Quaternary deposits are a source of dissolved 

minerals in the water. Unweathered glacial deposits of 

the Des Moines ice lobe are calcareous, whereas other 

glacial deposits in Dakota County typically are not 

calcareous. Three of the subbasins (contributing areas 

for surface-water sampling points) in this study have 

more than 50 percent coverage by Des Moines lobe 

deposits, and two subbasins have less than 50 percent 

coverage. Contributing areas of 13 of the 25 groundwater sampling points have more than 80 percent 

coverage by Des Moines lobe deposits, and 10 have less 

than 50 percent coverage. Deeper wells at Vermillion 

River sites 1-4 were excluded from the analysis because

18
Z DC

oy

o co 80

S 60

40

C)

A SURFACE WATER

O GROUND WATER

2 Indicates overlapping points

°

Q

O

iOQ

20 40 60 80 100

PERCENTAGE OF DEB MOINES LOBE DRIFT 

IN CONTRIBUTING AREA

Figure 9.-Relatlon of median calcium concentrations In water 

to percentages of Des Molnes lobe drift in contributing area 

of sampling points In Dakota County, Minnesota, 1990-91.

their contributing areas overlapped with those of the 

water-table wells at the same sites. One domestic well 

was excluded from the analysis because necessary data 

were not collected there.

Dissolution of carbonate minerals from calcareous 

deposits could result in elevated levels of dissolved 

calcium, magnesium, bicarbonate (alkalinity), and pH. 

No relation was apparent, however, between any of 

these water-quality constituents and the percent 

coverage of the contributing area by Des Moines lobe 

deposits. For example, the median concentrations of 

dissolved calcium at each sampling point ranged from 

47 to 100 mg/L, and no trend in calcium concentrations 

with percentage of Des Moines lobe drift was evident

(fig. 9). For some of the ground-water sampling points, 

the geology at the depth of the well screen may differ 

from the mapped Quaternary deposits at the land 

surface.

Bedrock

Bedrock can be a source of dissolved minerals in 

water and can affect the quality of surface water and 

ground water in the drift where ground water is moving 

upward from bedrock aquifers. Only sampling points 

with indication of upward-moving ground water in the 

contributing areas were included in the analysis. These 

were the surface-water sampling points from the 

Vermillion River sampling sites 1-5, and the deeper

19
120

£ 100

80

3 s

? 60

40

A SURFACE WATER

O GROUND WATER FROM DEEPER WELLS

Deeper wells are defined as wells screened about 

10 to 30 feet below the water table at sites 1-4

O 

6

20 40 60 80 100

PERCENTAGE OF PRAIRIE DU CHIEN GROUP BEDROCK 

IN CONTRIBUTING AREA

Figure I0.--Relation of median calcium concentrations in water to 

percentages of Prairie du Chien Group bedrock in contributing area 

of sampling points in the Vermillion River Basin, Minnesota, 1990-91.

wells at sites 1-4. Even for these sampling points, the 

upward movement of the ground water was determined 

just within the drift and demonstrates only the 

possibility of ground water moving upward from the 

bedrock.

Subbasins for the Vermillion River surface-water 

sampling points 2,3, and 5 have about 50 percent 

coverage by subcrops of the Prairie du Chien Group. 

Subbasins for the Vermillion River surface-water 

sampling points 1 and 4 have more than 50 percent 

coverage by subcrops of the St. Peter Sandstone. Each 

of the contributing areas for the deeper ground-water 

sampling points at the Vermillion River sites 1-3 has 

over 1Q percent coverage by subcrops of the Prairie du 

Chien Group.

Dissolution of calcite and dolomite in the Prairie du 

Chien Group could result in elevated levels of dissolved 

calcium, magnesium, bicarbonate, and pH. No relation 

was evident, however, between these water-quality 

constituents and percent coverage of Prairie du Chien 

Group in the contributing area. For example, the median 

concentrations of dissolved calcium at the sampling 

points were not proportional to coverage of the 

contributing area by subcrops of Prairie du Chien Group 

(fig. 10). Surface-water quality and ground-water 

quality at the Vermillion River sites did not appear to be 

controlled by the type of bedrock subcrop in the 

contributing area.

20
Transport Factors

Transport factors affect the direction and rate of the 

movement of chemical constituents from a source factor 

to the sampling point. Because transport factors do not 

add chemical constituents to the water, transport factors 

alone cannot be causally related to chemical 

constituents found in the water at the sampling point. 

Instead, examining transport factors may help explain 

residual data variability in a known relation between a 

source factor and water quality. In this study, the most 

likely relation between a source factor and water quality 

in Dakota County was between agricultural land and 

nitrate concentrations in ground water. Consequently, 

the effect of transport factors on this relation was 

analyzed. This analysis includes 24 ground-water 

sampling points: the 20 domestic wells and the watertable wells at Vermillion River sites 1-4. The water-table 

well at site 5 was excluded because of possible influence 

from surface water and the waste-water treatment plant, 

and the deeper wells at sites 1-4 were excluded because 

their contributing areas overlapped those of the 

corresponding water-table wells.

Soil-leaching potential

Soil composition and surface slope affect the 

leaching of dissolved constituents from the soil layer. 

Soil-leaching potentials of soil types in Dakota County 

were categorized and scored according to Soil 

Conservation Service (1992) procedures, and are based 

on characteristics of only the uppermost 60 inches of 

unconsolidated materials. The contributing areas for five 

of the ground-water sampling points had large soilleaching potentials, 13 had medium soil-leaching 

potentials, and seven had small soil-leaching potentials.

Nitrate concentrations in ground water were related 

to soil-leaching potential at sampling points with large 

areal coverage (equal to or greater than 75 percent) of 

agricultural land in the contributing areas (fig. 11). The 

relation was not strong, however: the null hypothesis 

that sites with different soil-leaching potential 

categories had the same nitrate concentrations in ground 

water was rejected by the Kruskal-Wallis test at only the

probability P = 0.15 level. Sampling points with small 

soil-leaching potential had relatively small nitrate 

concentrations (median 3.2 mg/L as nitrogen) in the 

ground water. A small soil-leaching potential indicates 

reduced or retarded infiltration and percolation, and thus 

the transport of nitrates from the land surface to the 

water table is reduced or at least delayed. Sampling 

points with large soil-leaching potential had large nitrate 

concentrations (median 13.2 mg/L as nitrogen) in the 

ground water because transport of nitrates from the land 

surface to the water table is relatively unhindered. 

Sampling points with medium soil-leaching potential 

had intermediate nitrate concentrations (median 7.6 

mg/L as nitrogen) in the ground water.

Water-table depth

Water-table depth affects the travel time of water and 

dissolved constituents through the unsaturated zone. For 

the 25 ground-water sampling points in this study (table 

1), water-table depth is greater than 100 feet for seven, 

between 50 and 100 feet for twelve, and less than 50 feet 

for six. A large water-table depth should result in a long 

travel time for water percolating from the land surface 

to the water table, all other transport factors being equal. 

A long travel time allows time for transformation 

processes, such as denitrification, to reduce the 

concentration of certain dissolved constituents, such as 

nitrate. Thus nitrate concentrations in ground water 

conceivably could be inversely related to water-table 

depth.

Nitrate concentrations in ground water, however, 

apparently were not related to water-table depth at sites 

with large areal coverage (equal to or greater than 75 

percent) of agricultural land in the contributing area (fig. 

12). Possibly other factors are more important than 

water-table depth in controlling the travel time of 

percolating water. The texture and moisture content of 

the material that composes the unsaturated zone are 

factors that are particularly important in controlling the 

travel time of percolating water. The relation shown in 

the previous section between nitrate concentrations in 

ground water and soil-leaching potential, which 

typically is proportional to the coarseness of the soil 

texture, indicates that texture is an important control on 

the travel time of percolating water and dissolved 

nitrates through the unsaturated zone.

21
30 

u3 25

8oc I* 20

I- DC

cclO

SMALL MEDIUM LARGE 

(N = 6) (N = 10) (N = 4)

SOIL-LEACHING POTENTIAL CATEGORY 

EXPLANATION

+ Median for each sampling point with agricultural land 

equal to or greater than 75 percent

O Median of the medians within each soil-leaching potential 

category

For area-weighted soil-leaching potential:

Small 3.00 > SLP > 2.34 

Medium 2.33 > SLP > 1.68 

Large 1.67 > SLP > 1.00

Figure 11.-Relation of nitrate concentrations in ground water to soil-leaching 

potential category for sampling points with equal to or greater than 75 percent 

agricultural land In contributing area of sampling points in Dakota County, 

Minnesota, 1990-91.

22
DC 

O

ic

Z HI

m b 

o _i

O* 8s

in

30

25

20

15

10

O

O

O

"O

O

O

-©-

O

20 40

O

O ' O

O O O

60 80

-©- O

100 120 140 160

WATER-TABLE DEPTH, IN FEET 

BELOW LAND SURFACE

Figure 12.--Relatlon of nitrate concentrations in ground water to 

water-table depths for sites with equal to or greater than 75 

percent agricultural land in contributing area of sampling points 

in Dakota County, Minnesota, 1990-91.

Summary

State and local officials in Minnesota are concerned 

about the potential effects of agricultural chemicals on 

water quality in Dakota County. Drift aquifers in Dakota 

County are susceptible to infiltrating contaminants such 

as fertilizers and pesticides. These contaminants can 

also move to streams, such as those in the Vermillion 

River Basin in central Dakota County. Effective 

management of water resources requires knowledge 

about hydrology and physical factors that can affect 

water quality. The U.S. Geological Survey, in 

cooperation with the Minnesota Department of Natural 

Resources, the Legislative Commission on Minnesota 

Resources, and the Dakota County Soil and Water

Conservation District, began a study in 1989 to 

investigate water quality in Dakota County. As part of 

this study, the purpose of this report is to investigate the 

hydrology of the Vermillion River Basin and to relate 

selected water-quality constituents to selected physical 

factors in Dakota County, both within and beyond the 

Vermillion River Basin. Physical factors examined were 

waste-water discharge, agricultural land, Quaternary 

deposits, bedrock, soil-leaching potential, and watertable depth.

Streamflow was measured periodically at two sites on 

the mainstem Vermillion River and at three sites on 

tributaries near their mouths; these five sites are referred 

to as the Vermillion River sites. A water-table well was

23
constructed within about 50 horizontal feet of the stream 

at each of the five sites, and a deeper well was screened 

about 10 to 30 feet below the water table at four of the 

sites. Water levels in these nine wells were used to 

determine whether ground water was moving toward or 

away from the stream at these sites. Surface-water 

samples were collected at the five Vermillion River 

sites. Ground-water samples were collected from 29 

drift wells, 9 of which were those at the five Vermillion 

River sites and 20 of which were domestic wells. 

Seventeen of the domestic wells were located outside 

the Vermillion River Basin. A total of 198 water 

samples were collected in 1990-91 and analyzed for 

nutrients; selected samples were also analyzed for major 

inorganic ions and triazine herbicide content. 

Contributing areas for surface-water samples were 

assumed to be the upstream subbasin. Contributing 

areas for ground-water samples were assumed to be a 

sector spanning 30 degrees and extending upgradient 

from the well to a distance corresponding to a 10-year 

horizontal ground-water travel time. Physical factors 

inside these contributing areas were determined from 

existing mapped or digitized data.

Annual average streamflow of the Vermillion River 

near the middle of its basin was 41.5 cubic feet per 

second during the 1990 water year (October 1,1989, 

through September 30,1990), about 78 percent of the 

18-year average. Shallow ground water at three sites in 

the Vermillion River Basin was moving toward the 

stream during at least half of the sampling trips to those 

sites in 1990-91. At one site in the lower portion of the 

basin and at another site along an intermittent tributary, 

however, water from the river and tributary (when 

flowing) can seep into the aquifer. Deeper ground water 

10 to 30 feet below the water table was moving 

horizontally or downward at four of the sites during 

more than half of the sampling trips. This ground water 

may pass under the Vermillion River and discharge into 

the Mississippi River to the east.

Selected water-quality constituents typically differed 

among sample types at the Vermillion River sites. The 

quality of surface water from the streams, water from 

the water-table wells, and water from the deeper wells 

differed at sites 1-4. The most consistent difference was 

in nitrate concentrations at sites 1-3, where ground 

water moved to the stream during at least half of the 

sampling trips. At these sites, nitrate was not detected at 

the 0.1 milligrams per liter (mg/L) as nitrogen level in 

water from the deeper wells. In contrast, nitrates were 

present at these three sites in water from water-table 

wells (median values of 0.3,8.6, and 7.6 mg/L as 

nitrogen for sites 1-3, respectively) and surface water

(median values of 1.1,5.0, and 4.8 mg/L as nitrogen for 

sites 1-3, respectively). Most surface-water samples had 

detectable levels of triazines, whereas fewer than half of 

the water samples from water-table wells and none of 

the water samples from deeper wells had detectable 

levels of triazine.

Selected water-quality constituents varied with time 

at the Vermillion River sites. The quality of water from 

deeper wells was much less variable with time than the 

quality of either surface water or water from water-table 

wells. For example, the average of the interquartile 

ranges for specific conductance of water from each of 

the deeper wells is only about 24 \iS/cm (microsiemens 

per centimeter), in contrast to 220 \iS/cm and 134 

(4,S/cm for water from the water-table wells and the 

streams, respectively. The quality of stream water varied 

with time as flow varied. Specific conductance values of 

stream water were inversely related to flow. Nitrate 

concentrations in stream water, however, were not 

simply related to flow. Most surface-water samples with 

moderate levels of triazine (greater than 0.25 to 2 

micrograms per liter) were collected during spring and 

during flows greater than the mean annual streamflow.

Selected water-quality constituents were related to 

selected physical factors. A waste-water treatment plant 

was a point source that accounted for most of the 

downstream nitrate loading in the Vermillion River 

mainstem. Water from sampling points with a large 

percentage (equal to or greater than 75 percent) of 

agricultural land in the contributing area had larger 

nitrate concentration (median 3.6 mg/L as nitrogen) than 

did water from sampling points with a smaller 

percentage of agricultural land (median below the 

detection limit of 0.1 mg/L as nitrogen). Water from 

areas with calcareous Quaternary or bedrock deposits 

could not be distinguished on the basis of calcium 

concentration from water from areas with noncalcareous 

deposits. Soil-leaching potential affected the relation 

between nitrate concentration in ground water and 

agricultural land. Nitrate concentrations were large 

(median 13.2 mg/L as nitrogen) where the percentage of 

agricultural land in the contributing area was large 

(equal to or greater than 75 percent) and where the soils 

had a large soil-leaching potential. Nitrate 

concentrations were small (median 3.2 mg/L as 

nitrogen) where the soils had a small soil-leaching 

potential, despite a large percentage of agricultural land. 

The statistical relation was not particularly strong, 

however: the null hypothesis that sites with different 

soil-leaching potentials had the same nitrate 

concentrations in ground water was rejected by the 

Kruskal-Wallis test at only the probability P = 0.15

24
level. Water-table depth was not an important factor in 

the relation between nitrate concentrations in ground 

water and agricultural land.

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