Water-Quality Characteristics and Trends for

Selected Sites in or near the Earth Resources

Observation Systems (EROS) Data Center,

South Dakota, 1973-2000

Water-Resources Investigations Report 03-4148

U.S. Department of the Interior

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

U.S. Geological Survey

Water-Quality Characteristics and Trends for

Selected Sites in or near the Earth Resources

Observation Systems (EROS) Data Center,

South Dakota, 1973-2000

Water-Resources Investigations Report 03-4280

By Kathleen M. Neitzert
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and 

does not constitute endorsement by the U.S. Government.

Rapid City, South Dakota: 2003

For additional information write to:

District Chief

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
Contents III

CONTENTS

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

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

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

Description of Study Area ........................................................................................................................................... 4

 Physiography and Climate................................................................................................................................ 4

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

Hydrologic Setting............................................................................................................................................. 4

Glacial Aquifers....................................................................................................................................... 4

Bedrock Aquifers..................................................................................................................................... 5

Previous Investigations ................................................................................................................................................ 6

Acknowledgments ....................................................................................................................................................... 6

 Methods of Study.................................................................................................................................................................. 6

Sample Collection, Processing, and Analysis ............................................................................................................. 6

Methods of Trend Analysis ......................................................................................................................................... 9

ESTREND Trend-Analysis Program................................................................................................................. 9

Regression Using Smoothing-Line on Scatter Plots.......................................................................................... 10

Water-Quality Characteristics................................................................................................................................................ 10

Selected Surface-Water Sites....................................................................................................................................... 11

Field-Measured Properties and Constituents..................................................................................................... 11

Major Ions and Indicators of Major Ions........................................................................................................... 15

Nutrients ............................................................................................................................................................ 19

Trace Elements .................................................................................................................................................. 26

Bed Sediment..................................................................................................................................................... 30

Trends ................................................................................................................................................................ 30

Selected Ground-Water Sites....................................................................................................................................... 33

Field-Measured Properties and Constituents..................................................................................................... 40

Major Ions and Indicators of Major Ions........................................................................................................... 43

Nutrients ............................................................................................................................................................ 51

Trace Elements .................................................................................................................................................. 55

Trends ................................................................................................................................................................ 55

Summary................................................................................................................................................................................ 62

Selected References............................................................................................................................................................... 62

FIGURES

1. Map showing location of water-quality sites sampled by the U.S. Geological Survey within

the Earth Resources Observation Systems study area, 1973-2000........................................................................... 3

2. Boxplots of field-measured properties and constituent concentrations for selected surface-water

sites in the study area................................................................................................................................................ 14

3. Boxplots of major-ion constituent concentrations for selected surface-water sites in the study area ...................... 20

4. Trilinear diagrams showing proportions of major ions for selected surface-water sites in the study area............... 22

5. Boxplots of nutrient constituent concentrations for selected surface-water sites in the study area ......................... 25

6. Boxplots of trace-element constituent concentrations for selected surface-water sites

in the study area........................................................................................................................................................ 29

7. Trend-analysis plots showing significant trends using ESTREND in samples from the

Big Sioux River near Dell Rapids ............................................................................................................................ 32
IV Contents

FIGURES—Continued

8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites

in the study area ....................................................................................................................................................... 34

9. Boxplots of field-measured properties and constituent concentrations for selected ground-water

sites in the study area............................................................................................................................................... 42

10. Boxplots of major-ion constituent concentrations for selected ground-water sites in the study area...................... 48

11. Trilinear diagrams showing proportions of major ions for selected ground-water sites in the study area .............. 50

12. Boxplots of nutrient constituent concentrations for selected ground-water sites in the study area......................... 54

13. Boxplots of trace-element constituent concentrations for selected ground-water sites in the study area ............... 59

14. Local regression (Lowess) plots for selected ground-water sites in the study area................................................. 60

TABLES

1. Site information for water-quality sites sampled in the study area......................................................................... 7

2. Properties and constituents analyzed for in water-quality samples for the water-quality monitoring

program and used for statistics and trend analyses................................................................................................. 8

3. Available options for flow-adjusted models using the Seasonal Kendall test ........................................................ 9

4. Statistical summary of water-quality results of physical properties for selected surface-water

sites in the study area.............................................................................................................................................. 12

5. Surface-water quality standards for selected properties and constituents .............................................................. 13

6. Statistical summary of major-ion and indicators of major-ion results for selected surface-water

sites in the study area.............................................................................................................................................. 16

7. Statistical summary of nutrient results for data for selected surface-water sites in the study area ........................ 23

8. Statistical summary of water-quality results of selected trace-element constituents for selected

surface-water sites in the study area ....................................................................................................................... 26

9. Results of trend-analysis tests for selected surface-water constituents in samples from the

Big Sioux River near Dell Rapids site .................................................................................................................... 31

10. Statistical summary of selected physical properties of ground-water sites, grouped by aquifer,

in the study area...................................................................................................................................................... 33

11. Statistical summary of selected physical properties for selected ground-water sites in the study area.................. 40

12. Statistical summary of major-ion and indicators of major-ion results for data for selected

ground-water sites in the study area ....................................................................................................................... 43

13. Statistical summary of nitrogen and phosphorus nutrient results for data for selected

ground-water sites in the study area ....................................................................................................................... 51

14. Statistical summary of water-quality results of selected trace-element constituents for selected

ground-water sites in the study area ....................................................................................................................... 55

15. Analytical results for surface-water sites....................................................................................................... CD-ROM

16. Analytical results for ground-water sites ....................................................................................................... CD-ROM

17. Analytical results for bed-sediment data ....................................................................................................... CD-ROM
Contents V

CONVERSION FACTORS AND VERTICAL DATUM

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

° F = (1.8 × °C) + 32

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

°C = (° F - 32) / 1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29); horizontal coordinate information is referenced to the North American Datum of 1927 

(NAD 27).

Water year (WY): Water year is the 12-month period, October 1 through September 30, and is designated by the calendar year in which it ends. Thus, the water year ending September 30, 2000, is 

called the “2000 water year.”

Multiply By To obtain

acre 4,047 square meter

acre 0.4047 hectare

foot (ft) 0.3048 meter

gallon (gal) 3.785 liter

gallon (gal) 0.003785 cubic meter 

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

inch 2.54 centimeter

inch 25.4 millimeter

mile 1.609 kilometer

square mile (mi2) 259.0 hectare

square mile (mi2) 2.590 square kilometer 

troy ounce 0.031104 kilogram
Introduction 1

Water-Quality Characteristics and Trends for

Selected Sites in or near the Earth Resources

Observation Systems (EROS) Data Center,

South Dakota, 1973-2000

By Kathleen M. Neitzert

ABSTRACT

 This report presents data on water-quality 

samples that were collected in and near the Earth 

Resources Observation Systems (EROS) Data 

Center from 1973 through 2000. The investigation 

is a collaborated effort between the U.S. Geological Survey, Water Resources Discipline (WRD), 

and Geography (formerly National Mapping) 

Discipline, EROS Data Center. 

 A water-quality monitoring program was 

initiated in 1973, when the EROS Data Center was 

constructed, and continues at the present time 

(2003). Under this program, water-quality samples 

were collected at various sites on the EROS Data 

Center’s property and in the surrounding area. 

These sites include 4 wastewater-treatment 

lagoons, 1 site on EROS Lake located behind the 

EROS Data Center, 2 stream sites near the EROS 

Data Center, and 9 ground-water wells surrounding the EROS Data Center. Additionally, 3 sites on 

EROS Lake, 7 stream sites, and 9 ground-water 

sites are located within the study area and have 

been sampled during the period covered in the 

report. Some of these additional sites were part of 

the initial water-quality monitoring conducted 

during and immediately after the construction of 

the EROS Data Center. For other sites, some special sampling (depth-profile and bottom material) 

has occurred at times during the sampling history; 

however, these sites have little water-quality data 

and were not used for statistical or trend analysis.

A trend-analysis program, Estimate 

TREND (ESTREND), was used to analyze for 

trends for one surface-water site, the Big Sioux 

River, which was the only site that had a substantial number of samples collected during an extensive period. The ESTREND trend-analysis 

program was used to analyze 16 constituents. Specific conductance and dissolved orthophosphate 

were the only constituents determined to have 

statistically significant trends. Results showed an 

increasing trend for specific conductance and a 

decreasing trend for dissolved orthophosphate. 

 Scatter plots with regression smoothing 

lines for selected constituents are presented for 

selected surface-water and ground-water sites. 

Regression analyses using a Lowess (Locally 

Weighted Scatterplot Smoothing) smoothing line 

for Split Rock Creek, EROS Lake, the lagoon 

sites, and the ground-water sites indicated variable 

results, with some constituents indicating an 

increasing or decreasing trend, some having varied 

results, and others indicating no change during the 

sampling period. 

 INTRODUCTION

The Earth Resources Observation Systems 

(EROS) Data Center is a data management, systems 

development, and research field center for the U.S. 

Geological Survey's (USGS) Geography (formerly 

National Mapping) Discipline. The EROS Data Center 
2 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

is a large facility in a rural area, located approximately 

15 mi north of Sioux Falls, South Dakota (fig. 1). As 

part of routine operations, the EROS Data Center 

stores, processes, and distributes a variety of data, 

including cartographic, satellite, and aircraft data. 

Photographic development and processing activities 

are integral to the operations of the EROS Data Center, 

and several toxic chemicals are used in these activities. 

As a result, the EROS Data Center has taken precautions to try to ensure that operations prevent environmental degradation and has monitored the surface 

water and ground water in and near the EROS Data 

Center to evaluate that the precautions are sufficient.

 The EROS Data Center’s Photographic Laboratory uses approximately 20,000 gal of water each day 

(U.S. Geological Survey, 2002). Chemicals used in 

photographic development and processing are recycled 

within the chemical management system by a process 

of first removing silver, then treating the photographic 

chemicals for reuse. Approximately 150 troy ounces of 

silver are currently collected each month from the laboratory’s waste photographic processing chemistry 

(Daniel Wray, EROS Data Center, written commun., 

2003), and 99 percent of this silver is recovered and 

sold. Recycling these photographic chemicals extends 

their life, reduces the amount of chemicals discharged 

through the EROS Data Center's waste-management 

system, and decreases chemical costs. The chemical 

management system has saved more than $1 million 

through a reduction in chemical costs and the sale of 

recovered silver. 

A hearing was conducted on July 10, 1973, by 

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

and a discharge permit was granted to the EROS Data 

Center. The permit application requested one discrete 

discharge—a controlled seasonal discharge—to an 

unnamed intermittent stream tributary to West Pipestone Creek, a tributary of Split Rock Creek, which is a 

tributary of the Big Sioux River (U.S. Environmental 

Protection Agency, written commun., 1973). In accordance with USEPA requirements, after an in-house 

recycling process is completed, safe photographic processing chemicals that cannot be recycled are broken 

down and piped to the first of four recess ponds 

(lagoons). These lagoons, which are settling ponds, act 

as a filter to help purify the water. Effluent treatment is 

completed in each lagoon (assigned sequential numbers as names) in sequential order. The resulting water 

is discharged into EROS Lake, a 101⁄2-acre lake located 

near the lagoons, where it is held until the seasonal 

discharge occurs. This controlled seasonal discharge is 

adjusted to the flow rate in Split Rock Creek to minimize the effect on the environment. EROS Data Center 

personnel use a 90° V-notch weir to measure the discharge (Daniel Wray, EROS Data Center, written 

commun., 2003). 

 A water-quality monitoring program was initiated in 1973, when the EROS Data Center was constructed, and continues at the present time (2003). 

Under this program, water-quality samples are collected at various sites on the EROS Data Center property and in the surrounding area. These sites include 

4 wastewater-treatment lagoons, 1 site on EROS Lake 

located behind the EROS Data Center, 2 stream sites 

near the EROS Data Center, and 9 ground-water wells 

surrounding the EROS Data Center. Additionally, 

3 sites on EROS Lake, 7 stream sites, and 9 groundwater sites are located within the study area and have 

been sampled during the period covered in the report. 

Some of these additional sites were part of the initial 

water-quality monitoring conducted during and immediately after the construction of the EROS Data Center. 

Other sites have had some special sampling (depth-profile and bottom material) at times during the sampling 

history; however, all of these additional sites have little 

water-quality data and were not used for statistical or 

trend analysis. The analyzed constituents were determined by considering State and Federal water-quality 

standards (W.A. Radlinski, U.S. Environmental 

Protection Agency, written commun., 1973).

Nearly 30 years of data collected at the EROS 

Data Center are archived in the USGS’s permanent 

National Water Information System’s (NWIS) waterquality database (http://waterdata.usgs.gov/nwis), and 

the data collected for the study described in this report 

also have been published in the South Dakota District’s 

Annual Water-Data Reports (U.S. Geological Survey, 

1974-2001).

Purpose and Scope

 The purpose of this report is to describe the 

general water-quality characteristics and trends for 

selected sites located in and near the EROS Data 

Center. The selected sites were the sites with a sufficient number of water-quality samples for statistical 

analysis that have been collected from 1973 through 

2000. Comparisons of water quality by aquifer and 

comparisons of surface water and ground water by 
Introduction 3

Figure 1. Location of water-quality sites sampled by the U.S. Geological Survey within the Earth Resources 

Observation System study area, 1973-2000.

Big

Sioux

River

Silver

Split

West

Rock

Creek

Creek

Creek

Creek

Pipestone

Creek

Creek

Slip

Up

Pipestone

Unnamed

96o40' 96o30'

43o

50'

43o

40'

MINNESOTA

MINNEHAHA COUNTY

MOODY COUNTY

R. 50 W. R. 49 W. R. 48 W. R. 47 W.

T. 

102

N.

T.

103

N.

T.

104

N.

0 1 2 3 4 5 6 MILES

0123456 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:100,000, Sioux Falls,1985

Universal Transverse Mercator projection, zone 14

North American Datum of 1927

Crooks

Baltic

Sherman

Garretson

Dell Rapids

+ +

+

+

+

+ ++

+

+ +

+

Big Sioux River

near Dell Rapids

Split Rock Creek

at Corson

x

x

x

x

x

x

xx xx

x

Sioux

Falls

Big

Sioux

River

Lower Big

Sioux River

drainage basin

Study

area

SOUTH DAKOTA

SELECTED SURFACE-WATER SITES (7) AND SITE NAME (table 1)

EARTH RESOURCES OBSERVATION SYSTEM (EROS) DATA CENTER

SURFACE-WATER SITES INCLUDED IN REPORT TABLES BUT NOT

 DISCUSSED IN THE REPORT

SELECTED GROUND-WATER SITES (9) AND SITE NAME (table 1)

GROUND-WATER SITES INCLUDED IN REPORT TABLES BUT NOT

 DISCUSSED IN THE REPORT

EARTH RESOURCES OBSERVATION SYSTEM STUDY AREA

$

$

EXPLANATION

Unnamed

EROS Lake

GW8 GW9

GW7

GW5

GW3 GW6

GW2

GW4 Lagoon

1,2,3,4

GW1

Creek

+

+

+ +

+

+

+

x

x

xxx

Lagoon 1

GW1
4 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

water source are presented for sites in the EROS Data 

Center water-quality monitoring program, and also for 

sites in the EROS Data Center vicinity that have sufficient water-quality data but were not part of the monitoring program.

 Comparisons are made for physical properties, 

major-ion chemistry, nutrients, and trace elements for 

the surface-water and ground-water samples. This 

report presents spatial and temporal variations where 

sufficient data are available. Analytical results also are 

presented for 58 constituents analyzed in bottom 

material samples collected from three locations in 

EROS Lake.

Description of Study Area

 The study area is located in a rural setting in 

Minnehaha County and includes the EROS Data Center 

property and surrounding areas in southeastern South 

Dakota (fig. 1). The study area covers a small region 

within the lower Big Sioux River drainage basin. 

 Physiography and Climate

The study area is located entirely in the Coteau 

des Prairies physiographic region, a plateau-like highland occupying the area between the Minnesota RiverRed River Lowland to the east and the James River 

Lowland to the west (Koch, 1982). Land-surface altitudes within the study area range from 1,340 ft (NGVD 

29) in the southeastern part to 1,630 ft (NGVD 29) in 

the north-central part.

 The climate in the study area is subhumid with a 

mean annual precipitation of about 25 inches in the Big 

Sioux River drainage basin (Koch, 1982). Maximum 

precipitation occurs during the growing season with 

approximately 75 percent of the annual precipitation 

occurring between April and September. Annual cumulative snowfall averages 40 inches, generally occurring 

between November and March (Spuhler and others, 

1971). The mean annual temperature is about 46oF 

(8oC), and temperature extremes commonly range from 

-20oF (-29oC) in the winter to near 100oF (38oC) in the 

summer (Ohland, 1987).

Geologic Setting

The Precambrian-age Sioux Quartzite is the 

oldest bedrock formation underlying the study area 

(Lindgren and Niehus, 1992). The altitude of the top of 

the Sioux Quartzite ranges from approximately 1,300 

to 1,550 ft (NGVD 29). The Sioux Quartzite is locally 

well-fractured and jointed crystalline rock; however, 

the depth and development of the fracture system is not 

well known (Lindgren and Niehus, 1992). The Late 

Cretaceous-age Split Rock Creek Formation lies in valleys in the Sioux Quartzite surface and generally consists of thick layers of siltstone, shale, and claystone 

(Lindgren and Niehus, 1992).

Quaternary-age glacial drift and alluvium overlies the Sioux Quartzite. The glacial drift is primarily 

unconsolidated sand and gravel outwash deposited by 

meltwater from receding glaciers (U.S. Geological 

Survey, 1985). Outwash deposits can be at land surface 

or buried by till or alluvium (Lindgren and Niehus, 

1992). Till is an unsorted and unstratified mixture of 

glacial deposits containing clay, silt, sand, gravel, and 

boulders. The alluvium is composed of clay, silt, sand, 

and gravel that have been deposited by water.

Hydrologic Setting

The Big Sioux River flows through the western 

part of the study area (fig. 1) and has a drainage area of 

145.9 mi2

 in Minnehaha County. Split Rock Creek 

flows through the southeast corner of the study area, 

and has a drainage area of 54.3 mi2 in Minnehaha 

County. West Pipestone Creek is a tributary of Split 

Rock Creek that flows along the eastern edge of the 

study area and has a drainage area of 66.7 mi2

(Lindgren and Niehus, 1992). Two smaller streams, 

Slip Up Creek and Silver Creek, begin in the southwestern part of the study area before joining the Big 

Sioux River. Several unnamed, intermittent streams 

develop within the study area during wet periods. 

Ponds within the study area include stock ponds and 

small intermittent ponds that develop during wet 

months.

Glacial Aquifers

 Glacial aquifers are predominantly shallow, 

water-table aquifers and are primarily composed of 

outwash. Glacial aquifers found in the study area 

include the Big Sioux, Pipestone Creek, Beaver Creek, 

Brandon, and Slip Up Creek aquifers.

 The predominant shallow aquifer in the study 

area is a portion of the Big Sioux aquifer, which underlies the flood plain of the Big Sioux River valley (Koch, 

1982) and along the western part of the study area. The 

Big Sioux aquifer is composed of an alluvium-mantled 
Introduction 5

outwash, which consists of silt, fine to coarse sand, and 

gravel. The aquifer overlies relatively impermeable 

glacial till. Yields from the Big Sioux aquifer can be as 

much as 1,000 gal/min, depending on the thickness of 

the aquifer (Lindgren and Niehus, 1992). Withdrawals 

from the Big Sioux aquifer are widely used for 

domestic, municipal, stock, and irrigation purposes; the 

city of Sioux Falls is the principal user of water withdrawn from the Big Sioux aquifer in Minnehaha 

County.

 The Pipestone Creek aquifer generally is a 

shallow, water-table aquifer. One small portion of the 

Pipestone Creek aquifer occurs in the northeast corner 

of the study area, and a separate portion occurs in the 

eastern one-half of the study area. Yields generally are 

less than 100 gal/min in the Pipestone Creek aquifer; 

however, it can yield as much as 500 gal/min in areas 

where the thickness is greatest (Lindgren and Niehus, 

1992). Withdrawals from the Pipestone Creek aquifer 

are primarily for stock watering and irrigation 

purposes.

The Beaver Creek aquifer occurs in a small area 

in the southeast portion of the study area. Yields from 

the Beaver Creek aquifer generally are less than 

300 gal/min within the study area; however, the aquifer 

can yield as much as 500 gal/min in some areas 

(Lindgren and Niehus, 1992). Withdrawals from the 

Beaver Creek aquifer are primarily for stock watering 

and irrigation purposes.

 The Brandon aquifer occurs in the southeast part 

of the study area between the Beaver Creek aquifer and 

a portion of the Big Sioux aquifer. Withdrawals from 

the Brandon aquifer are used primarily for municipal 

and domestic purposes by the city of Brandon; to a 

lesser extent, withdrawals also are used for stock 

watering and irrigation purposes.

The Valley Springs aquifer is located near the 

city of Valley Springs, approximately 9 mi southeast of 

the study area. The aquifer is under confined conditions, with the depth to the top of the aquifer ranging 

from 93 to 207 ft. Water from the Valley Springs 

aquifer is used primarily for domestic and municipal 

purposes by the city of Valley Springs.

 Slip Up Creek aquifer, which is a minor aquifer, 

underlies the flood plains of Slip Up Creek located in 

the center of the study area. The areal extent of the sand 

and gravel deposits is less than 5 mi2

 (Lindgren and 

Niehus, 1992). Cumulative thickness of the Slip Up 

Creek aquifer generally is less than 20 ft.

Bedrock Aquifers

The Split Rock Creek aquifer extends into 

southern portions of the study area. It is predominantly composed of layers of fine to coarse, wellsorted quartzose sand interbedded with layers of siltstone, shale, and claystone of the Split Rock Creek 

Formation (Lindgren and Niehus, 1992). Depths to the 

top of the aquifer range from 21 ft within the study 

area to 337 ft west of the study area. Although the 

Split Rock Creek aquifer generally is under confined 

conditions, water-table conditions occur south of the 

study area, near an observation well located in the 

southwest quarter of 102N49W141

 (Lindgren and 

Niehus, 1992). The thickness of the sand and gravel 

within the study area generally ranges from 10 to 

40 ft, with average cumulative thickness of about 

48 ft. Recharge is likely from infiltration of precipitation that falls on Sioux Quartzite outcrops and then 

moves along fractures in the quartzite and into the 

Split Rock Creek aquifer (Lindgren and Niehus, 

1992). Sioux Quartzite outcrops occur to the north, 

south, and west of the Split Rock Creek aquifer in 

southwestern Minnehaha County. The Split Rock 

Creek aquifer also may receive recharge from the 

Valley Springs aquifer. Calcium and sulfate are the 

predominant chemical constituents in water from the 

Split Rock Creek aquifer. Withdrawals are used for 

stock watering and domestic purposes. 

The Sioux Quartzite aquifer underlies all of the 

study area. Locally, the aquifer is contained within 

well-fractured and jointed crystalline rock. It is under 

water-table conditions near the Sioux Quartzite outcrop areas, and under confined conditions where overlain by till or glacial aquifers in other parts of the study 

area (Lindgren and Niehus, 1992). Recharge to the 

Sioux Quartzite aquifer is by infiltration of snowmelt 

and precipitation, and seepage from streams (Koch, 

1982). Natural discharge is by evapotranspiration and 

seepage to surrounding streams. Withdrawals are used 

for stock watering and domestic and municipal wells.

1

The well number consists of the township number, followed 

by “N,” the range number followed by “W,” and the section number followed by a maximum of four uppercase letters that indicate, respectively, the 160-, 40-, 10-, and 2 1/2-acre tract in which 

the well is located. These letters are assigned in a counterclockwise direction beginning with “A” in the northeast quarter. A 

serial number following the last letter is used to distinguish 

between wells in the same 2 1/2-acre tract.
6 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Previous Investigations

Information about the quantity and availability of 

surface and ground water, the hydrologic system as it 

affects water availability, and the quality of surfaceand ground-water supplies in Minnehaha County is 

presented in Lindgren and Niehus (1992). Koch (1982) 

describes the Big Sioux aquifer in Minnehaha County 

and presents a digital model to simulate varying hydrologic conditions.

Additional investigations have been completed 

in Minnehaha County. Bradford (1981a, 1981b) and 

Winter (1983) presented water-level records that were 

measured in the Big Sioux aquifer in Minnehaha 

County; Koch (1983) evaluated the response of the Big 

Sioux River to extreme drought conditions; Koch 

(1984) presented a simulated artificial recharge model 

for the Big Sioux aquifer; Niehus and Lindgren (1994) 

described the major aquifers; Ohland (1990) appraised 

the water resources of the Skunk Creek aquifer; and 

Putnam (1998) described the Split Rock Creek aquifer 

in Minnehaha County and presented a numerical flow 

model.

Acknowledgments

 The author acknowledges the efforts of the 

EROS Data Center for helping develop and support the 

water-quality monitoring program, and the cooperation 

of residents of the study area for providing information 

and access concerning their private wells. The author 

also recognizes the hard work and dedication of the 

many USGS Water Resources Discipline (WRD) 

hydrologic technicians that collected the water-quality 

and streamflow data on which this report is based. 

 METHODS OF STUDY

A water-quality monitoring program was initiated in 1973 when the EROS Data Center was constructed. USGS WRD personnel in the South Dakota 

District have performed routine water-quality sampling 

on an annual basis for selected sites for the monitoring 

program. Water-quality data continue to be collected at 

the present time (2003), but data analyzed in the study 

described in this report are restricted to water years 

1973 through 2000. The USGS maintained two waterquality monitoring sites during this period on streams, 

Split Rock Creek at Corson (station 06482610) and Big 

Sioux River near Dell Rapids (station 06481000) 

(table 1). 

The 16 sites sampled for the water-quality 

monitoring program (table 1) include 4 wastewatertreatment lagoons located approximately 0.1 to 0.3 mi 

northeast of the EROS Data Center, 1 site on EROS 

Lake located approximately 0.4 mi east of the EROS 

Data Center, 2 stream sites near the EROS Data Center, 

and 9 ground-water wells surrounding the EROS Data 

Center. 

Within the study area, 19 additional sites 

(table 1), which include 7 stream sites, 3 sites on EROS 

Lake, and 9 ground-water sites, were sampled during 

the period covered in the report. Some of these sites 

were part of the initial water-quality monitoring conducted during and immediately after the construction 

of the EROS Data Center, whereas the remainder have 

been sampled for other purposes. The initial waterquality monitoring sites were analyzed to determine 

their suitability for a long-term monitoring project and 

establish which sites would be representative of various 

depths and completions. However, the additional sites 

have little water-quality data and were not used for statistical or trend analysis. Water-quality data for these 

sites are included in tables 15 (7 stream sites) and 16 

(9 ground-water sites), available on CD-ROM at the 

back of this report, and are not described further. 

Analytical data for the bed-sediment samples for the 

three sites on EROS Lake are available in table 17 on 

the CD-ROM.

Sample Collection, Processing, and Analysis

Water-quality sampling and processing methods 

varied over the duration of the monitoring program. 

Samples were collected and processed using methods 

considered acceptable at the time of collection. 

Throughout the duration of the monitoring program, 

stream water-quality and suspended-sediment samples 

were collected using isokinetic samplers and depthand width-integrating procedures described in Edwards 

and Glysson (1988). Prior to 1993, sampling procedures generally followed guidelines described in 

Brown and others (1970), Wells and others (1990), and 

Ward and Harr (1990).

In general, all water-quality sampling equipment 

was presoaked in a Liquinox solution, thoroughly 

scrubbed, rinsed with tap water, rinsed with deionized 

water, and finally rinsed with native water prior to collecting water-quality samples. Some of the sampling 
Methods of Study 7

Table 1. Site information for water-quality sites sampled in the study area

[Site type: SW, surface water. GW, ground water; EROS, Earth Resources Observation Systems Data Center; --, none]

Station name or location

Monitoring project

site name

(see fig. 1 for

location)

Latitude Longitude Site

type Period of record for study

Sites used for statistical or trend analysis

Big Sioux River near Dell Rapids, SD Big Sioux River 434725 0964442 SW 1973-2000

Split Rock Creek at Corson, SD Split Rock Creek 433659 0963354 SW 1973-96, 1997

EROS Lake EROS Lake 434405 0963655 SW 1973, 1979-80, 1982-2000

Lagoon #1 at EROS Data Center Lagoon 1 434415 0963715 SW 1979-80, 1982-2000

Lagoon #2 at EROS Data Center Lagoon 2 434415 0963715 SW 1979-80, 1982-86

Lagoon #3 at EROS Data Center Lagoon 3 434415 0963715 SW 1979-80, 1982-84, 1986

Lagoon #4 at EROS Data Center Lagoon 4 434415 0963715 SW 1979, 1982-86

103N48W 5CACA2 GW1 434508 0963727 GW 1973-84, 1986-2000

103N48W 7DAC2 GW2 434414 0963803 GW 1973-79, 1981, 1983

103N48W 8BCCB2 GW3 434435 0963748 GW 1973-78

103N48W 8ADA GW4 434432 0963642 GW 1973-82

103N48W 8DDDD GW5 434357 0963642 GW 1973-79

103N48W 9BDCB GW6 434429 0963618 GW 1973-80

103N48W 9CCDA GW7 434400 0963622 GW 1973-2000

103N48W18ACA2 GW8 434339 0963811 GW 1973-79, 1981-83

103N48W17ACCC2 GW9 434332 0963715 GW 1973-82, 1984, 1986-2000

Additional sites not used for statistical or trend analysis

EROS Lake at EROS Data Center, SD (2) -- 434405 0963656 SW 1991

EROS Lake at EROS Data Center, SD (3) -- 434408 0963658 SW 1991

EROS Lake at EROS Data Center, SD (4) -- 434409 0963655 SW 1991

Unnamed Creek near EROS Data Center -- 434433 0963640 SW 1979

Unnamed Creek East of EROS Data 

Center

-- 434402 0963640 SW 1979

Tributary to West Pipestone Creek, SD, SW 2 SW2 434137 0963415 SW 1973

West Pipestone Creek, SD, SW 3 SW3 434210 0963316 SW 1973

West Pipestone Creek, SD, SW 4 SW4 434022 0963443 SW 1973

Split Rock Creek, SW 5 SW5 433853 0963417 SW 1973

Split Rock Creek, SW 6 SW6 433939 0963304 SW 1973

103N48W 4ABCC -- 434531 0963601 GW 1986-87

103N48W 5CACA -- 434508 0963727 GW 1981

103N48W 7DAC -- 434414 0963803 GW 1981

103N48W 8DBDB -- 434457 0963734 GW 1982

103N48W 9BDCA -- 434429 0963615 GW 1975

103N48W17ACCC -- 434332 0963715 GW 1981

103N48W18ACA -- 434339 0963811 GW 1981

103N49W 5BADA -- 434532 0964429 GW 1975-77

103N49W 5BBBB -- 434538 0964500 GW 1975
8 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

equipment, including sampling bottles, also was periodically soaked in a dilute nitric-acid solution. 

Although these methods were considered acceptable, 

they sometimes were inadequate in preventing contamination of samples for certain constituents, including 

dissolved aluminum, copper, lead, mercury, and zinc 

(Alexander and others, 1996). Beginning in about 

1993, more rigorous equipment cleaning and waterquality sampling methods were implemented to prevent contamination. These methods generally followed 

the guidelines described in Horowitz and others (1994). 

All sampling equipment that would contact the native 

water first was soaked in a Liquinox solution, scrubbed 

with non-metallic brushes, rinsed with tap water, 

soaked in dilute hydrochloric acid, rinsed with deionized water, and finally rinsed with native water prior to 

collecting water-quality samples. Generally, samples 

were collected by one person, and vinyl or latex gloves 

were worn during sample collection.

Composite wading samples generally were collected from the EROS Lake and lagoons at five locations around the perimeter of each site. The three 

ground-water sites currently active are sampled at yard 

hydrants that have been flushed for 30 minutes.

Water-quality samples for the monitoring program were analyzed for selected field-measured properties, and concentrations of major ions, nutrients, and 

trace elements. Analytical constituents varied over the 

duration of the monitoring program. Water-quality 

analytical constituents and properties that were used to 

meet the objectives of this report are presented in 

table 2. The USGS National Water Quality Laboratory 

in Denver, Colorado, performed laboratory analyses of 

water samples.

Table 2. Properties and constituents analyzed for in water-quality samples for the water-quality monitoring program and 

used for statistics and trend analyses

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; oC, degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter]

Field-measured property

or constituent

Water-quality laboratory analytical constituents

(dissolved concentration unless noted otherwise1)

Constituent

Major ion

(mg/L)

Nutrient

(mg/L)

Trace element

(μg/L)

Specific conductance (μS/cm) Solids, residue at 180°C Nitrite, as nitrogen Aluminum

pH (standard units) Solids, sum of constituents Nitrite plus nitrate, as nitrogen Boron

Water temperature (°C) Calcium Ammonia, as nitrogen Chromium

Carbon dioxide (mg/L) Magnesium Ammonia plus organic nitrogen

(total), as nitrogen

Iron

Hardness, as CaCO3 (mg/L) Sodium Manganese

Noncarbonate hardness (mg/L) Sodium, percent Phosphorus (total) Silver

Alkalinity (mg/L) Sodium-adsorption ratio Orthophosphate, as phosphorus Zinc

Potassium

Bicarbonate

Carbonate

Sulfate

Chloride

Fluoride

Silica

1Dissolved” is operationally defined as that part of a water sample that passes through a 0.45-micrometer pore-size filter; “total” is operationally 

defined as an unfiltered water sample.
Methods of Study 9

Methods of Trend Analysis

Various methods are described in this section of 

the report that were used to determine trends for 

selected water-quality constituents. For data sets with 

more than 50 observations over a minimum 5-year 

period, the program Estimate TREND (ESTREND) 

was used. Scatter plots with a smoothing line were used 

for selected constituents that did not have sufficient 

data to use ESTREND.

ESTREND Trend-Analysis Program

Long-term trends in selected water-quality constituents were analyzed using both parametric and nonparametric test methods. Parametric trend tests assume 

that when a water-quality constituent is regressed with 

time, or any other independent variable, the resulting 

residuals are symmetrically distributed and form a bellshaped (normal) curve. Nonparametric trend tests are 

utilized when the assumptions of normality and equal 

variances of data are not valid. Nonparametric tests 

compare ranks of data rather than the actual data values 

(Schertz and others, 1991).

The inherent variability of a data set is the primary factor responsible for the resulting loss of power 

when a parametric test is applied to water-quality data. 

This power is defined as the ability of the test to reject 

the null hypothesis when it is false. Other factors such 

as the use of censored data (values defined as “less 

than” the laboratory reporting limit), outliers, and data 

sets with multiple laboratory reporting limits also 

diminish the power of a parametric test to determine 

trends. Nonparametric tests are not subject to distributional assumptions of parametric tests (symmetry or 

normality) and typically are more powerful than parametric tests for data that violate normality assumptions 

(Helsel, 1992).

The ESTREND program incorporates statistical 

methods that deal with the factors that may limit power. 

This computerized statistical and graphical program 

developed by the USGS to analyze for trends in surface 

water, as used in this report, is described by Schertz and 

others (1991). Depending on the existence and quantity 

of censored data, three types of trend-analysis methods 

are utilized by ESTREND—Seasonal Kendall test for 

uncensored data (uncensored Seaken), Seasonal Kendall test for censored data (censored Seaken), and Tobit 

regression (Tobit). This report utilized each of the three 

types.

The Seasonal Kendall test is used to analyze 

water-quality data for both long-term and seasonal 

trends (Hirsch and others, 1982). This nonparametric 

method is a generalization of the Mann-Kendall test 

(Mann, 1945; Kendall, 1975). It is designed to remove 

the variability in water-quality data that may be caused 

by seasonality. The Seasonal Kendall test method 

allows the model to determine the best number of 

seasons to use in analyzing the trend, from 1 to 12 

seasons, and data may be either flow-adjusted concentration (FAC) or raw concentration data (RCD). The 

FAC method reduces flow-related variability, which 

may decrease the power of the test. The FAC method 

is used for data sets that contain less than 5 percent 

censored data. This method has 16 flow-adjustment 

options available (table 3). ESTREND is used to 

determine the best option as defined by prediction sum 

of squares (PRESS) statistics (Myers, 1986) from the 

first 11 options. The RCD method does not use flowadjusted data and therefore is not the preferred 

method; however, it is used to analyze for trends in 

constituents that have more than 5 percent censored 

data and a single laboratory reporting limit.

The Tobit parametric trend test (Cohen, 1976; 

Cohn, 1988) is used to analyze data sets that contain 

values censored at multiple laboratory reporting 

limits. This method does not distinguish between 

seasons. The Tobit method uses a maximum likelihood estimation procedure (Cohn, 1988) to estimate 

the parameters of a regression model relating concentration and time (Schertz and others, 1991). The Tobit 

method uses RCD, and therefore, the data are not flow 

adjusted.

Table 3. Available options for flow-adjusted models using 

the Seasonal Kendall test

[Lowess, Locally Weighted Scatterplot Smoothing]

None Hyperbolic 7

Linear Hyperbolic 8

Log Inverse

Hyperbolic 1 Best of previous

Hyperbolic 2 Log-log

Hyperbolic 3 Lowess

Hyperbolic 4 Log-log Lowess

Hyperbolic 5 Log-flow Lowess

Hyperbolic 6
10 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

The null hypothesis for a trend test is that there is 

no trend in the data over time. Results from the 

ESTREND program may detect trends that are 

increasing or decreasing over time, allowing the null 

hypothesis to be rejected. Failure to reject the null 

hypothesis does not necessarily indicate no trend in the 

data, but simply that the test result fails to conclude that 

there is trend (Schertz and others, 1991).

The test results include a calculation of a p-value 

to determine the statistical significance of the analysis 

of trend, based on a selected significance level. Trend 

analyses that have a p-value of 0.05 or less are considered statistically significant in this report.

Regression Using Smoothing-Line on Scatter Plots

Scatter plots are presented for selected constituents that did not have sufficient data to use ESTREND. 

A smoothing line, created using the Lowess (Locally 

Weighted Scatterplot Smoothing) method (Cleveland, 

1979), is presented with the scatter plots. Lowess is a 

nonparametric smoothing technique that generalizes 

the running means, which determines a predicted value 

at each point by fitting a weighted linear regression, 

and shows where the weights decrease with distance 

from the point of interest. This method can depict slight 

changes in trend that may have occurred within a relatively small part of the period of record examined. This 

method does not use flow-adjusted data. The features of 

the Lowess regression plot are determined by: (1) the 

span (used to control the amount of smoothing), which 

was set at 0.9 for all plots because most data sets have 

less than 50 values and require a high level of 

smoothing; (2) the degree of the locally fitted polynomial, which was set as 1 degree, meaning a locally 

linear fit is used; and (3) the family, for which symmetric was selected because it combines local fitting 

with a robustness feature that guards against distortion 

by outliers.

WATER-QUALITY CHARACTERISTICS

Boxplots, trilinear diagrams, and statistical summaries are used to describe variability in water-quality 

properties and constituents at each site. Boxplots are 

used to graphically display the distribution of data. 

They provide visual summaries of the median, interquartile range, skewness, and presence or absence of 

disproportionate values. Trilinear diagrams show 

chemical analyses of water represented as percentages 

of total equivalents per liter, and are useful for visually 

describing differences in major-ion chemistry in water 

(Freeze and Cherry, 1979). Statistical summaries are 

numeric descriptive measures that can be used to 

describe the center of distribution of measurements and 

how the measurements vary about the center of distribution.

Censored data (that is, concentrations reported as 

“less than” the laboratory reporting limit) exist for several constituents in the data set. When constituents with 

censored data had concentrations reported as “ND” or 

“0,” an attempt was made to determine what reporting 

level was in use during that time, and the concentration 

was assigned as “less than” that reporting level.

Some constituents had multiple laboratory 

reporting limits over the duration of the monitoring 

program. Multiple laboratory reporting limits occur for 

some constituents due to: (1) changes in analytical 

methods used during the study period; (2) changes in 

detection limits for a given method; and/or (3) sample 

dilutions for analysis. To calculate summary statistics 

and construct boxplots, it was necessary to select a 

single reporting level when multiple reporting levels 

exist for a constituent. For these constituents, the 

largest laboratory reporting limit that did not exceed a 

substantial number (that is, about one-half) of the 

reported concentrations for that constituent was 

selected as the study reporting level. Some constituents 

had a small number of censored samples that were 

reported as less than a given laboratory reporting limit 

that was unusually large in comparison to other 

reporting limits for the same constituent. In these cases, 

these censored values were excluded from the calculations of statistics and the construction of boxplots.

All concentrations that were less than the study 

reporting level, regardless of whether they were originally reported as “less thans,” were actual concentrations, or were assigned a “less than” value, were 

considered to be less than the study reporting level for 

the calculation of summary statistics and the construction of boxplots.

The study reporting level is shown as a line 

across the boxplot for constituents that had censored 

data, and the total number of samples with values 

below the study reporting level is reported below the 

line for each site. Censored data used for the trend plots 

were assigned a concentration value of one-half the 

value of the laboratory reporting limit. Data presented 

in the supplemental tables are exactly as stored in 

NWIS.
Water-Quality Characteristics 11

Selected Surface-Water Sites

Water-quality samples were collected at 17 

surface-water sites within the study area. Of these sites, 

sufficient data were collected at 7 sites, minimal data 

were collected at 7 stream sites, and only bed-sediment 

data were collected at 3 sites on EROS Lake (table 1). 

The analytical results for the additional surface-water 

sites, excluding the three sites where only bed-sediment 

data were collected, are presented in table 15.

Field-Measured Properties and Constituents

A statistical summary of selected field-measured 

properties and constituents of surface water in the study 

area is given in table 4. Boxplots of the field-measured 

properties and constituents are presented in figure 2.

Specific conductance is a measure of the ability 

of water to conduct an electrical current. Values for the 

Big Sioux River and Split Rock Creek sites ranged from 

175 to 1,900 μS/cm (microsiemens per centimeter at 25 

degrees Celsius), which generally is typical for streams 

in this area, and below criteria for irrigation and fish and 

wildlife propagation, recreation, and livestock watering 

(table 5). Specific-conductance values for EROS Lake 

ranged from 249 to 2,520 μS/cm, and also are suitable 

for irrigation and fish and wildlife propagation, recreation, and livestock watering. 

Specific-conductance values for lagoon 1 ranged 

from 898 to 4,600 μS/cm. Specific-conductance values 

for lagoons 2, 3, and 4 ranged from 1,400 to 

4,300 μS/cm.

The pH value is a measure of the hydrogen ion 

concentration. Values for the Big Sioux River ranged 

from 7.1 to 9.1, which meet the fish and wildlife propagation, recreation, and livestock-watering criterion, 

but the maximum value slightly exceeds the maximum 

beneficial-use criteria for warmwater permanent and 

marginal fish-life propagation (table 5). This maximum 

value occurred during spring runoff and does not typify 

the normal values. The pH values for Split Rock Creek 

ranged from 7.4 to 8.6 and meet the South Dakota 

surface-water quality criterion for fish and wildlife 

propagation, recreation, and livestock watering. EROS 

Lake had pH values that ranged from 6.6 to 9.3, which, 

except for one value, meet the fish and wildlife propagation, recreation, and livestock-watering criterion; one 

pH value exceeded the maximum beneficial-use criteria 

for warmwater permanent and marginal fish-life propagation. The pH values for the lagoons ranged from 6.2 

to 10.1.

Water temperatures for the Big Sioux River and 

Split Rock Creek ranged from 0°C during winter 

months to 33oC. One value exceeded the maximum 

beneficial-use criteria for warmwater permanent and 

marginal fish-life propagation (table 5), and does not 

typify the normal maximum values. Temperatures 

ranged from 5.5 to 28.5oC for EROS Lake, and from 

5.0 to 32.0°C in the lagoons. 

Carbon dioxide is contained in the atmosphere 

in variable amounts and is relatively abundant in 

natural waters, due largely to its high coefficient of 

solubility (Cole, 1994). It is dissolved in the gaseous 

state by absorption at the water surface and utilized in 

photosynthesis. Rainwater, water seepage through 

organic soil that comes into contact with products of 

decomposition, and anaerobic decomposition of carbohydrates in bottom sediments may introduce gaseous carbon dioxide into surface water. Respiration 

and decay of aquatic organisms from within the lake 

and lagoons also contribute to the production of 

carbon dioxide. Carbon dioxide co-occurs with bicarbonate and carbonate as pH and temperatures change 

to sustain the dynamic balancing of ionic charges 

(Cole, 1994). Carbon dioxide values in the Big Sioux 

River ranged from 0.5 to 47.0 mg/L, which reflects the 

high variability of pH values; values in the Split Rock 

Creek ranged from 1.0 to 7.4 mg/L and reflect the low 

variability of pH values. Carbon dioxide values in 

EROS Lake ranged from 0.1 to 2.3 mg/L, whereas the 

lagoon values ranged from 0 to 7.8 mg/L.

Hardness is a measure of the soap-consuming 

capacity of water. The carbonate hardness of water 

includes the portion equivalent to the bicarbonate and 

carbonate in the water; noncarbonate hardness is the 

difference between total and carbonate hardness and is 

caused by the sulfates and chlorides of bivalent cations 

(Cole, 1994). Water with hardness less than 61 mg/L 

is considered soft; 61 to 120 mg/L, moderately hard; 

121 to 180 mg/L, hard; and greater than 180 mg/L, 

very hard (Heath, 1983). Hardness values for the Big 

Sioux River and Split Rock Creek ranged from 70 to 

940 mg/L, and noncarbonate hardness values ranged 

from 14 to 540 mg/L. EROS Lake had hardness values 

ranging from 110 to 400 mg/L, and noncarbonate 

hardness values ranged from 0 to 180 mg/L. Hardness 

values for the lagoon sites ranged from 280 to 

480 mg/L. Noncarbonate hardness was reported for a 

single visit at lagoons 1, 2, and 3, and those values 

ranged from 190 to 420 mg/L.
12 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 4. Statistical summary of water-quality results of physical properties for selected surface-water sites in the study area

[Statistics summarize alkalinity field and laboratory values, and are a combination of fixed-end point and inflection titrations. μS/cm, microsiemens per 

centimeter at 25 degrees Celsius; °C, Celsius; mg/L, milligrams per liter; --, not analyzed or not determined]

Property or constituent

Big Sioux River Split Rock Creek

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 391 841 850 175 1,900 179 605 620 180 1,020

pH (standard units) 197 8.0 7.9 7.1 9.1 7 8.2 8.2 7.4 8.6

Water temperature (°C) 384 10.5 9.0 0.0 29.0 221 10.5 9.5 0.0 33.0

Carbon dioxide (mg/L) 136 8.0 4.9 0.5 47 7 3.0 2.4 1.0 7.4

Hardness, as CaCO3 (mg/L) 141 420 410 70 940 7 270 280 120 380

Noncarbonate hardness (mg/L) 101 200 190 14 540 7 71 85 23 100

Alkalinity, as CaCO3 (mg/L) 142 222 219 51 396 7 201 204 95 276

Property or constituent

EROS Lake Lagoon 1

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 28 1,360 1,450 249 2,520 22 1,780 1,570 898 4,600

pH (standard units) 30 8.3 8.5 6.6 9.3 22 7.3 7.4 6.2 8.3

Water temperature (°C) 29 17.0 17.0 5.5 28.5 22 16.5 16.0 10.5 23.5

Carbon dioxide (mg/L) 11 1.1 0.9 0.1 2.3 8 2.6 2.2 0.2 7.1

Hardness, as CaCO3 (mg/L) 11 290 310 110 400 8 350 340 280 410

Noncarbonate hardness (mg/L) 5 90 130 0 180 1 -- -- 190 190

Alkalinity, as CaCO3 (mg/L) 14 185 180 111 284 11 53 29 8 144

Property or constituent

Lagoon 2 Lagoon 3

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 7 2,750 2,810 1,500 4,300 6 2,520 2,520 1,400 3,680

pH (standard units) 7 8.3 8.2 6.8 10.1 6 8.2 8.3 7.3 8.9

Water temperature (°C) 7 17.0 18.0 9.5 21.0 6 17.0 16.0 9.5 29.5

Carbon dioxide (mg/L) 7 1.8 0.3 0.0 5.9 6 2.5 1.3 0.1 7.3

Hardness, as CaCO3 (mg/L) 7 360 350 330 410 6 370 360 300 480

Noncarbonate hardness (mg/L) 1 -- -- 400 400 1 -- -- 420 420

Alkalinity, as CaCO3 (mg/L) 7 46 28 8 125 6 105 96 61 163
Water-Quality Characteristics 13

Table 4. Statistical summary of water-quality results of physical properties for selected surface-water

sites in the study area—Continued

[Statistics summarize alkalinity field and laboratory values, and are a combination of fixed-end point and inflection titrations. 

μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, Celsius; mg/L, milligrams per liter; --, not analyzed or not determined]

Property or constituent

Lagoon 4

Number

of

samples

Mean Median Minimum Maximum

Specific conductance (μS/cm) 6 2,410 2,250 1,400 3,900

pH (standard units) 6 8.8 9.0 7.4 10.0

Water temperature (°C) 6 20.0 20.5 5.0 32.0

Carbon dioxide (mg/L) 6 1.5 0.2 0.0 7.8

Hardness, as CaCO3 (mg/L) 6 350 350 280 460

Noncarbonate hardness (mg/L) 0 -- -- -- --

Alkalinity, as CaCO3 (mg/L) 6 107 102 73 160

Table 5. Surface-water quality standards for selected properties and constituents

[μg/L, micrograms per liter; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degrees Celsius; >, less than; --, not 

applicable]

Property or constituent

Selected index values

National

Irrigation

Water-Quality 

Program

Guidelines 

level of

concern1

South Dakota beneficial-use criteria2

Warmwater

permanent

fish-life

propagation

Warmwater

marginal

fish-life

propagation

Aquatic-life

criterion,

acute/chronic

(μg/L)

Irrigation

Fish and wildlife

propagation,

recreation,

livestock

watering

Specific conductance (μS/cm) -- -- 32,500/44,375 34,000/47,000

pH (standard units) -- 56.5-9.0 56.0-9.0 -- -- 56.0-9.5

Water temperature (°C) -- 27 32 -- -- --

Alkalinity, as CaCO3 (mg/L) -- -- -- -- -- 3750/41,313

Dissolved solids, residue on 

evaporation at 180°C (mg/L)

-- -- -- -- -- 32,500/44,375

Dissolved solids, sum of

constituents (mg/L)

-- -- -- -- -- 32,500/44,375

Sodium-adsorption ratio -- -- -- -- 10 --

Nitrite plus nitrate, as nitrogen 

(mg/L)

-- -- -- -- -- 350/488

Dissolved silver (μg/L) -- -- -- 63.4/-- -- --

Dissolved zinc (μg/L) -- -- -- 6110/6100 -- --

1U.S. Department of the Interior (1998); the upper value of the level of concern represents the National Irrigation Water Quality Program toxicity 

threshold. 2

South Dakota Legislative Research Council (2001).

330-day mean.

4

Daily maximum.

5

Application of this standard is dependent on specific conditions described in South Dakota Legislative Research Council (2001). 6Hardness-dependent criteria; value given is an example based on hardness of 100 milligrams per liter as CaCO3.
14 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 2. Boxplots of field-measured properties and constituent concentrations for selected surface-water sites in

the study area.

391 28 6 22 7 6 141 7 8 11 6 7 6

197 7 22 30 6 7 6 101 7 1 5 0 1 1

384 221 22 29 6 7 6

136 7 8 11 6 7 6

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

142 7 11 14 6 7 6

2,000

1,000

0

3,000

4,000

5,000

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

400

600

0

200

800

1,000

1,200

HARDNESS, IN MILLIGRAMS PER LITER

200

300

0

100

400

500

600

NONCARBONATE HARDNESS,

IN MILLIGRAMS PER LITER

200

300

0

100

400

500

ALKALINITY,

IN MILLIGRAMS PER LITER

5

7

6

8

10

9

11

pH, IN STANDARD UNITS

5

0

60

50

40

30

20

10

0

15

10

20

30

25

35

40

WATER TEMPERATURE,

IN DEGREES CELSIUS

CARBON DIOXIDE,

IN MILLIGRAMS PER LITER

179

NO DATA AVAILABLE

22

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile range

 outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than 1.5 times the

 interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile range outside

 the quartile

Individual values plotted when data are inadequate to construct boxplot
Water-Quality Characteristics 15

Alkalinity is a measure of the capacity of unfiltered water to neutralize acid. It is ordinarily considered 

an index to the nature of the rocks within a drainage 

basin and to the degree to which they are weathered; 

increased alkalinity commonly results from carbon 

dioxide and water attacking sedimentary carbonate 

rocks and dissolving some of the carbonate to form 

bicarbonate solutions (Cole, 1994). The alkalinity 

values used for statistics and graphical representations 

are a combination of field and laboratory values collected during different periods of study, based on USGS 

standard methods at the time of collection. Alkalinity 

for the Big Sioux River and Split Rock Creek ranged 

from 51 to 396 mg/L. Alkalinity measured at EROS 

Lake ranged from 111 to 284 mg/L, whereas the lagoon 

sites had much lower alkalinity, ranging from 8 to 

163 mg/L. Alkalinity for all surface-water sites were 

below the maximum limit for fish and wildlife propagation, recreation, and livestock-watering criterion 

(table 5).

Major Ions and Indicators of Major Ions

A statistical summary for selected major ions, 

including dissolved solids, calcium, magnesium, 

sodium, percent sodium, sodium-adsorption ratio, 

potassium, bicarbonate, carbonate, sulfate, chloride, 

fluoride, and silica, is given in table 6. Boxplots are 

presented in figure 3 for each of the major ions except 

carbonate.

The dissolved-solids concentration is the total of 

all dissolved mineral constituents and is a convenient 

means of comparing waters. It can be determined from 

the weight of the dry residue remaining after evaporation of the volatile portion of an aliquot of the water 

sample at 180°C, or by summing the concentrations 

reported for the various dissolved constituents (Hem, 

1992). The dissolved-solids concentration commonly 

represents salinity and is classified as follows: fresh, 

0 to 1,000 mg/L; slightly saline, 1,000 to 3,000 mg/L; 

moderately saline, 3,000 to 10,000 mg/L; very saline, 

10,000 to 35,000 mg/L; and briny, more than 

35,000 mg/L (Heath, 1983). Concentrations of dissolved solids, residue at 180oC, for the Big Sioux River 

and Split Rock Creek ranged from 126 to 1,540 mg/L 

(table 6, fig. 3), and the dissolved solids, sum of constituents, ranged from 98 to 1,380 mg/L. Dissolved solids, 

residue at 180oC, for EROS Lake ranged from 485 to 

1,170 mg/L, and the concentrations of dissolved 

solids, sum of constituents, ranged from 137 to 

1,340 mg/L. These values meet the fish and wildlife 

propagation, recreation, and livestock-watering daily 

maximum criterion (table 5).

Dissolved solids, residue at 180oC, was analyzed at lagoon 1 for 14 visits, and ranged from 606 to 

1,210 mg/L; however, it was not analyzed for at the 

other lagoons. The dissolved solids, sum of constituents, for all four lagoon sites ranged from 720 to 

2,290 mg/L. 

The median concentration for major ions in the 

Big Sioux River generally is greater than the median 

concentration for major ions in Split Rock Creek. The 

Big Sioux River also has more variability in concentrations than Split Rock Creek. Explanations for these 

contrasting characteristics may be due to: (1) the large 

number of samples for the Big Sioux River site (76-

142) compared to the Split Rock Creek site (7); 

(2) when the samples were collected; and/or (3) that 

Split Rock Creek is one of many tributaries to the Big 

Sioux River, and the Big Sioux River receives water 

from a much larger drainage area than Split Rock 

Creek. 

EROS Lake generally has low variability in 

major-ion concentrations, and excluding dissolved 

bicarbonate, median concentrations for major ions are 

lower than in the lagoon sites. Concentrations of dissolved calcium, magnesium, sodium, potassium, chloride, fluoride, and sulfate generally are similar among 

the lagoons, with substantial overlap in distributions.

Trilinear diagrams (Piper, 1944; Maddy and 

others, 1990) showing proportions of major ions in 

water samples from selected surface-water sites are 

presented in figure 4. The dominant cations are calcium and magnesium in the Big Sioux River and 

sodium (plus potassium) in the Split Rock Creek. The 

dominant anions are sulfate (plus chloride) and bicarbonate in the Big Sioux River and bicarbonate and 

mixed in the Split Rock Creek.

Cation data for EROS Lake indicate dominance 

of calcium and magnesium, and anion data indicate 

dominance of sulfate and bicarbonate. Sodium (plus 

potassium) generally is the dominant cation for the 

four lagoons; however, bicarbonate data were not 

available to determine the dominant anion.
16 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 6. Statistical summary of major-ion and indicators of major-ion results for selected surface-water sites in the study area

[All values are for dissolved constituents, in milligrams per liter, except as indicated. °C, degrees Celsius; <, less than; --, not analyzed or not determined]

Constituent

Big Sioux River

Number of

samples

Number of values 

less than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 124 0 626 594 126 1,540

Solids, sum of constituents 142 0 579 563 98 1,380

Calcium 141 0 93 90 18 210

Magnesium 141 0 45 46 6 100

Sodium 140 0 34 34 2.2 80

Sodium, percent 139 0 15 15 6 24

Sodium-adsorption ratio 140 0 0.7 0.7 0.1 1.3

Potassium 140 0 7.3 7.1 4.5 26

Bicarbonate 83 0 270 260 62 483

Carbonate 76 0 0 0 0 7

Sulfate 141 0 221 220 22 690

Chloride 141 0 30 27 4.1 260

Fluoride 141 0 0.3 0.3 0.1 1.5

Silica 142 11 10 8.7 <1.2 26

Constituent

Split Rock Creek

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 2 0 415 415 409 421

Solids, sum of constituents 7 0 354 378 166 471

Calcium 7 0 57 58 29 86

Magnesium 7 0 31 34 11 40

Sodium 7 0 23 27 7.7 28

Sodium, percent 7 0 15 16 12 17

Sodium-adsorption ratio 7 0 0.6 0.6 0.3 0.7

Potassium 7 0 5.7 5.3 3.9 8.7

Bicarbonate 7 0 238 234 116 337

Carbonate 7 0 3 0 0 13

Sulfate 7 0 87 100 27 120

Chloride 7 0 23 25 12 30

Fluoride 7 0 0.4 0.4 0.3 0.5

Silica 7 3 3.8 4.6 <1.2 7.8
Water-Quality Characteristics 17

Table 6. Statistical summary of major-ion and indicators of major-ion results for selected surface-water sites in the study 

area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. °C, degrees Celsius; <, less than; --, not analyzed or not determined]

Constituent

EROS Lake

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 17 0 813 798 485 1,170

Solids, sum of constituents 11 0 828 850 137 1,340

Calcium 11 0 55 59 24 70

Magnesium 11 0 36 37 12 54

Sodium 11 0 178 180 8.6 320

Sodium, percent 11 0 49 56 11 63

Sodium-adsorption ratio 11 0 4.4 4.6 0.3 7.8

Potassium 11 0 17 19 3.6 28

Bicarbonate 6 0 238 248 135 319

Carbonate 6 0 4 0 0 14

Sulfate 11 0 188 200 16 330

Chloride 13 0 244 240 2.2 440

Fluoride 11 0 0.7 0.7 0.4 0.9

Silica 11 3 2.6 1.9 <1.2 5.8

Constituent

Lagoon 1

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 14 0 843 805 606 1,210

Solids, sum of constituents 8 0 1,460 1,370 720 2,290

Calcium 8 0 67 65 60 81

Magnesium 8 0 43 43 28 51

Sodium 8 0 351 355 100 630

Sodium, percent 8 0 63 67 41 77

Sodium-adsorption ratio 8 0 8.1 8.5 2.6 14.0

Potassium 8 0 35 30 22 58

Bicarbonate 3 0 83 103 35 111

Carbonate 3 0 0 0 0 0

Sulfate 8 0 411 410 240 560

Chloride 9 0 464 480 44 900

Fluoride 8 0 1.0 1.0 0.9 1.3

Silica 8 2 9.9 12 <1.2 14
18 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 6. Statistical summary of major-ion and indicators of major-ion results for selected surface-water sites in the study 

area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. °C, degrees Celsius; <, less than; --, not analyzed or not determined]

Constituent

Lagoon 2

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 7 0 1,580 1,680 928 2,220

Calcium 7 0 72 71 59 95

Magnesium 7 0 43 43 40 49

Sodium 7 0 394 420 170 600

Sodium, percent 7 0 66 69.3 50.3 76

Sodium-adsorption ratio 7 0 9.0 9.8 4.1 14.0

Potassium 7 0 33 30 26 44

Bicarbonate 0 0 -- -- -- --

Carbonate 0 0 -- -- -- --

Sulfate 7 0 426 440 230 530

Chloride 8 0 535 585 170 860

Fluoride 7 0 1.1 1.1 0.9 1.3

Silica 7 0 12 14 3.5 15

Constituent

Lagoon 3

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 6 0 1,480 1,550 868 1,970

Calcium 6 0 79 75 59 110

Magnesium 6 0 42 41 37 49

Sodium 6 0 362 405 170 500

Sodium, percent 6 0 64 68 52 70

Sodium-adsorption ratio 6 0 8.1 9.3 4.2 11.0

Potassium 6 0 31 29 15 47

Bicarbonate 0 0 -- -- -- --

Carbonate 0 0 -- -- -- --

Sulfate 6 0 382 355 240 530

Chloride 7 0 496 500 170 720

Fluoride 6 0 1.1 1.0 0.8 1.4

Silica 6 1 9.1 10 <1.2 16
Water-Quality Characteristics 19

Table 6. Statistical summary of major-ion and indicators of major-ion results for selected surface-water sites in the study 

area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. °C, degrees Celsius; <, less than; --, not analyzed or not determined]

Constituent

Lagoon 4

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 6 0 1,390 1,330 884 1,990

Calcium 6 0 73 70 59 100

Magnesium 6 0 41 41 33 50

Sodium 6 0 335 340 170 490

Sodium, percent 6 0 64 67 54 69

Sodium-adsorption ratio 6 0 7.7 8.1 4.4 10.0

Potassium 6 0 31 26 21 53

Bicarbonate 0 0 -- -- -- --

Carbonate 0 0 -- -- -- --

Sulfate 6 0 370 340 310 510

Chloride 7 0 459 470 220 700

Fluoride 6 0 1.0 1.0 0.9 1.2

Silica 6 2 3.7 3.1 <1.2 11

Nutrients

Nitrogen and phosphorus are essential nutrients 

for plant growth. The enrichment of a surface-water 

body with nutrients is accompanied by a high rate of 

production of plant material in the water. Dense, 

rapidly multiplying algal growths or blooms sometimes 

occur in surface-water bodies that periodically receive 

increased concentrations of nitrogen or phosphorus. 

These dense growths are generally undesirable to water 

users and may interfere with other forms of aquatic life. 

A statistical summary of nutrients found in surfacewater sites located within the study area is given in 

table 7, and boxplots are presented for selected nutrients in figure 5. 

Concentrations of nitrite, as nitrogen, for the Big 

Sioux River, Split Rock Creek, and EROS Lake generally were low, with maximum detections of 0.08, less 

than the study reporting level of 0.01, and 0.01 mg/L, 

respectively. Lagoon 1 was the only lagoon site sampled for nitrite, as nitrogen, and concentrations were 

much higher than the other surface-water sites, and 

ranged from 0.34 to 7.1 mg/L.

Concentrations of nitrite plus nitrate, as nitrogen, 

for the stream sites ranged from less than the study 

reporting level of 0.10 mg/L to 4.0 mg/L, and concentrations for EROS Lake ranged from less than the study 

reporting level of 0.10 mg/L to 1.0 mg/L. The lagoon 

sites 1 and 2 had similar concentrations, ranging from 

0.60 to 16 mg/L, whereas the lagoon sites 3 and 4 

ranged from less than the study reporting level of 

0.10 mg/L to 2.8 mg/L. 

Ammonia, as nitrogen, for the Big Sioux River 

was only sampled on a single visit and was 1.0 mg/L. 

Ammonia was not analyzed for at Split Rock Creek. 

Concentrations of ammonia for EROS Lake ranged 

from less than the study reporting level of 0.02 mg/L to 

0.32 mg/L. Lagoon 1 had ammonia concentrations that 

ranged from less than the study reporting level of 

0.02 mg/L to 36 mg/L. Ammonia was not sampled for 

at the other lagoon sites.
20 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 3. Boxplots of major-ion constituent concentrations for selected surface-water sites in the study area.

120

100

80

60

40

20

0

DISSOLVED MAGNESIUM,

IN MILLIGRAMS PER LITER

0

1,000

500

1,500

2,500

2,000

3,000

800

200

400

600

0

1,000

1,200

1,400

1,600

1,800

DISSOLVED SOLIDS, RESIDUE ON

EVAPORATION AT 180 DEGREES CELSIUS,

IN MILLIGRAMS PER LITER

DISSOLVED SOLIDS,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

40

60

0

20

80

100

PERCENT SODIUM

8

10

2

0

4

6

12

14

16

SODIUM ADSORPTION RATIO

124 2 14 17 0 0 0

142 7 8 11 6 7 6 139 7 8 11 6 7 6

141 7 8 11 6 7 6

141 7 8 11 6 7 6

140 7 8 11 6 7 6

DISSOLVED CALCIUM,

IN MILLIGRAMS PER LITER

0

50

150

100

200

250

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

0

200

400

600

800

DISSOLVED SODIUM,

IN MILLIGRAMS PER LITER

140 7 8 11 6 7 6

0

15

30

60

45

75

DISSOLVED POTASSIUM,

IN MILLIGRAMS PER LITER

140 7 8 11 6 7 6

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek
Water-Quality Characteristics 21

Figure 3. Boxplots of major-ion constituent concentrations for selected surface-water sites in the study area.—Continued

0

200

400

600

800

DISSOLVED SULFATE,

IN MILLIGRAMS PER LITER

141 7 8 11 7 6 6

0

400

200

600

800

1,000

DISSOLVED CHLORIDE,

IN MILLIGRAMS PER LITER

141 7 9 13 7 8 7

0

10

5

15

25

20

30

DISSOLVED SILICA,

IN MILLIGRAMS PER LITER

142 11 7 8 7 6 6

0

2.0

1.6

1.2

0.8

0.4

DISSOLVED FLUORIDE,

IN MILLIGRAMS PER LITER

141 7 8 11 6 7 6

600

500

400

300

200

100

0

BICARBONATE,

IN MILLIGRAMS PER LITER

83 7 6 3 0 0 0 NO DATA AVAILABLE NO DATA AVAILABLE NO DATA AVAILABLE

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

11

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile

 range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than

 1.5 times the interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile

 range outside the quartile

Individual values plotted when data are inadequate to

 construct boxplot

2 Number of samples with concentrations less than study

 reporting level

Study reporting level—Triangle indicates data less than

 study reporting level

11 3 3 2 1 2
22 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 4. Trilinear diagrams (Piper, 1944) showing proportions of major ions for selected surface-water sites in the study 

area.

PERCENTAGE REACTING VALUES, IN MILLIEQUIVALENTS PER LITER

Calcium + Magnesium

Sulfate

Sodium + Potassium

Magnesium

Calcium

80

60

40

20

80

80

60

40

20

20

20

40

60

80

40

60

80

60

40

20

20

80

60

40

20

40

60

80

80

60

40

20

Bicarbonate 

Sulfate + Chloride

Chloride

CATIONS ANIONS

River Sites

PERCENTAGE REACTING VALUES, IN MILLIEQUIVALENTS PER LITER

Calcium + Magnesium

Sulfate

Sodium + Potassium

Magnesium

Calcium

80

60

40

20

80

80

60

40

20

20

20

40

60

80

40

60

80

60

40

20

20

80

60

40

20

40

60

80

80

60

40

20

Bicarbonate 

Sulfate + Chloride

Chloride

CATIONS ANIONS

EROS Lake and Lagoon Sites

BIG SIOUX RIVER NEAR DELL RAPIDS

SPLIT ROCK CREEK AT CORSON

EROS LAKE

LAGOON 1 AT EROS DATA CENTER

LAGOON 2 AT EROS DATA CENTER

LAGOON 3 AT EROS DATA CENTER

LAGOON 4 AT EROS DATA CENTER

EXPLANATION

EXPLANATION
Water-Quality Characteristics 23

Table 7. Statistical summary of nutrient results for data for selected surface-water sites in the study area

[All values are for dissolved constituents, in milligrams per liter, except as indicated. <, less than; --, not analyzed or not determined]

Constituent

Big Sioux River

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 12 4 0.03 0.02 <0.01 0.08

Nitrite plus nitrate, as N 138 44 0.80 0.50 <0.10 3.6

Ammonia, as N 1 0 -- -- 1.0 1.0

Ammonia plus organic, total, as N 124 0 1.9 1.7 0.07 6.8

Phosphorus, total, as P 192 0 0.27 0.24 0.03 1.0

Orthophosphate, as P 132 9 0.10 0.06 <0.01 0.46

Constituent

Split Rock Creek

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 2 2 -- -- <0.01 <0.01

Nitrite plus nitrate, as N 7 4 0.82 <0.10 <0.10 4.0

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, total, as N 2 0 1.8 1.8 1.7 1.9

Phosphorus, total, as P 4 0 0.32 0.27 0.16 0.58

Orthophosphate, as P 5 1 0.08 0.03 <0.01 0.29

Constituent

EROS Lake

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 13 9 -- <0.01 <0.01 0.01

Nitrite plus nitrate, as N 28 22 0.14 <0.10 <0.10 1.0

Ammonia, as N 17 3 0.09 0.04 <0.02 0.32

Ammonia plus organic, total, as N 5 0 2.8 2.7 1.6 4.8

Phosphorus, total, as P 5 0 0.58 0.26 0.18 1.9

Orthophosphate, as P 13 1 0.21 0.25 <0.01 0.47

Constituent

Lagoon 1

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 10 0 2.2 1.2 0.34 7.1

Nitrite plus nitrate, as N 22 0 6.9 5.1 0.60 16

Ammonia, as N 14 1 17 18 <0.02 36

Ammonia plus organic, total, as N 2 0 18 18 4.7 32

Phosphorus, total, as P 2 0 4.4 4.4 2.7 6.0

Orthophosphate, as P 10 0 3.5 3.6 1.5 4.9
24 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 7. Statistical summary of nutrient results for data for selected surface-water sites in the study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. <, less than; --, not analyzed or not determined]

Constituent

Lagoon 2

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 7 0 5.4 5.3 1.0 11

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, total, as N 2 0 19 19 5.1 32

Phosphorus, total, as P 2 0 3.2 3.2 2.2 4.1

Orthophosphate, as P 0 0 -- -- -- --

Constituent

Lagoon 3

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 6 1 0.82 0.73 <0.10 1.7

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, total, as N 2 0 2.9 2.9 2.3 3.4

Phosphorus, total, as P 2 0 2.1 2.1 1.4 2.8

Orthophosphate, as P 0 0 -- -- -- --

Constituent

Lagoon 4

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 6 2 0.88 0.58 <0.10 2.8

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, total, as N 1 0 -- -- 2.1 2.1

Phosphorus, total, as P 1 0 -- -- 1.2 1.2

Orthophosphate, as P 0 0 -- -- -- --
Water-Quality Characteristics 25

Figure 5. Boxplots of nutrient constituent concentrations for selected surface-water sites in the study area.

0.1

0.01

100

10

1

TOTAL AMMONIA PLUS ORGANIC,

IN MILLIGRAMS PER LITER

1

0.1

0.001

0.01

10

100

DISSOLVED NITRITE PLUS NITRATE,

IN MILLIGRAMS PER LITER

DISSOLVED ORTHOPHOSPHATE,

IN MILLIGRAMS PER LITER

0.01

10

1

0.1

TOTAL PHOSPHORUS,

IN MILLIGRAMS PER LITER

124 2 2 5 1 2 2

138 7 22 28 6 7 6 132 5 10 13 0 0 0

192 4 2 5 1 2 2

0.001

1

0.1

0.01

10

100

DISSOLVED AMMONIA,

IN MILLIGRAMS PER LITER

1 0 00 17 14 0

0.001

1

0.1

0.01

10

12 2 13 10 0 0 0

0.001

1

0.1

0.01

10

DISSOLVED NITROGEN, NITRITE,

IN MILLIGRAMS PER LITER

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

2

9 1 1

44 22 4 1 2

12

75th percentile

Number of samples

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

3 1

4 2 9

Outlier data value less than or equal to 3 and more than

 1.5 times the interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile

 range outside the quartile

Individual values plotted when data are inadequate to

 construct boxplot

Number of samples with concentrations less than study

 reporting level

Study reporting level—Triangle indicates data less than

 study reporting level
26 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Concentrations of ammonia plus organic total, as 

nitrogen, varied widely for stream sites (table 7, fig. 5); 

concentrations for the Big Sioux River ranged from 

0.07 to 6.8 mg/L, whereas concentrations for Split 

Rock Creek ranged from 1.7 to 1.9 mg/L in two samples. The concentration of ammonia plus organic total 

for the EROS Lake site ranged from 1.6 to 4.8 mg/L. 

The lagoon sites 1 and 2 had similar concentrations, 

ranging from 4.7 to 32 mg/L, whereas lagoon sites 3 

and 4 had much lower concentrations, ranging from 2.1 

to 3.4 mg/L. 

Phosphorus concentrations for the stream sites 

ranged from 0.03 to 1.0 mg/L, whereas EROS Lake’s 

phosphorus concentrations ranged from 0.18 to 

1.9 mg/L. Lagoons 1 and 2 had similar concentrations 

that ranged from 2.2 to 6.0 mg/L, whereas concentrations for lagoons 3 and 4 ranged from 1.2 to 2.8 mg/L. 

Maximum concentrations generally decreased from 

lagoon 1 to lagoon 4. Orthophosphate concentrations 

for the stream sites ranged from less than the study 

reporting level of 0.01 mg/L to 0.46 mg/L, and EROS 

Lake concentrations ranged from less than the study 

reporting level of 0.01 mg/L to 0.47 mg/L. Lagoon 1 

was the only lagoon site for which orthophosphate was 

analyzed for, and concentrations ranged from 1.5 to 

4.9 mg/L.

Trace Elements

Trace elements generally are defined as elements 

with concentrations in natural water less than 1 mg/L 

(Hem, 1992). Trace elements analyzed in samples from 

the various surface-water sites in the study area include 

dissolved concentrations for aluminum, boron, chromium, iron, manganese, silver, and zinc and are 

reported in micrograms per liter (μg/L). Samples were 

analyzed for additional trace-element constituents, but 

concentrations generally were below their respective 

study reporting levels (table 15).

A statistical summary of selected trace-element 

constituents in samples from selected surface-water 

sites located within the study area is given in table 8, 

and boxplots are presented in figure 6.

Only one sample from the Big Sioux River was 

analyzed for aluminum, which had a concentration of 

800 μg/L (table 8, fig. 6). As mentioned previously, 

aluminum analyses are very susceptible to contamination, and this value is questionable; however, no data 

exist to disregard it. Samples from Split Rock Creek 

were not analyzed for aluminum. Concentrations of 

aluminum for the EROS Lake site ranged from not 

detected to 60 μg/L. Concentrations of aluminum in 

samples from the lagoon sites ranged from less than the 

study reporting level of 10 μg/L to 70 μg/L, with maximum concentrations generally decreasing from 

lagoon 1 to lagoon 4. 

Concentrations of dissolved boron for the Big 

Sioux River and Split Rock Creek sites ranged from 30 

to 360 μg/L. EROS Lake had dissolved boron concentrations that ranged from 70 to 1,900 μg/L, whereas 

concentrations for the four lagoon sites ranged from 

840 to 2,500 μg/L. 

Table 8. Statistical summary of water-quality results of selected trace-element constituents for selected surface-water

sites in the study area

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Constituent

Big Sioux River

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 1 0 -- -- 800 800

Boron 129 0 130 130 30 360

Chromium 0 0 -- -- -- --

Iron 0 0 -- -- -- --

Manganese 0 0 -- -- -- --

Silver 0 0 -- -- -- --

Zinc 0 0 -- -- -- --
Water-Quality Characteristics 27

Table 8. Statistical summary of water-quality results of selected trace-element constituents for selected surface-water 

sites in the study area—Continued

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Constituent

Split Rock Creek

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 0 0 -- -- -- --

Boron 4 0 78 65 60 120

Chromium 0 0 -- -- -- --

Iron 0 0 -- -- -- --

Manganese 0 0 -- -- -- --

Silver 0 0 -- -- -- --

Zinc 0 0 -- -- -- --

Constituent

EROS Lake

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 8 2 24 20 <10 60

Boron 10 0 1,300 1,500 70 1,900

Chromium 25 22 <10 <10 <10 20

Iron 10 2 72 30 <10 450

Manganese 1 0 -- -- 2,200 2,200

Silver 28 28 <2 <2 <2 <2

Zinc 11 2 15 14 <3 30

Constituent

Lagoon 1

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 8 1 38 30 <10 70

Boron 8 0 1,610 1,600 980 2,300

Chromium 22 8 23 17 <10 100

Iron 9 0 1,080 750 50 2,500

Manganese 1 0 -- -- 160 160

Silver 23 7 4 3 <2 18

Zinc 9 0 84 90 4 160
28 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 8. Statistical summary of water-quality results of selected trace-element constituents for selected surface-water sites 

in the study area—Continued

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Constituent

Lagoon 2

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 7 1 29 30 <10 50

Boron 7 0 1,600 1,500 940 2,300

Chromium 7 2 24 20 <10 70

Iron 8 0 550 400 29 1,900

Manganese 1 0 -- -- 100 100

Silver 8 3 4 2 <2 9

Zinc 8 0 64 65 30 100

Constituent

Lagoon 3

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 6 2 18 20 <10 30

Boron 6 0 1,500 1,550 850 2,300

Chromium 6 2 18 15 <10 50

Iron 7 0 95 60 21 240

Manganese 1 0 -- -- 260 260

Silver 7 7 <2 <2 <2 <2

Zinc 7 0 64 40 10 190

Constituent

Lagoon 4

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Aluminum 6 1 18 20 <10 30

Boron 6 0 1,560 1,500 840 2,500

Chromium 6 4 <10 <10 <10 20

Iron 7 0 56 40 19 190

Manganese 1 0 -- -- 50 50

Silver 7 6 <2 <2 <2 2

Zinc 7 0 24 25 4 50
Water-Quality Characteristics 29

Figure 6. Boxplots of trace-element constituent concentrations for selected surface-water sites in the study area.

1

1,000

100

10

10,000

NO DATA AVAILABLENO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLE

ALL DATA BELOW STUDYREPORTING LEVELALL DATA BELOW STUDYREPORTING LEVEL

DISSOLVED MANGANESE,

IN MICROGRAMS PER LITER

0 0 11 1 1 1

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

Lagoon

1

Lagoon

3

EROS

Lake

Big

Sioux

River

Lagoon

2

Lagoon

Split 4

Rock

Creek

DISSOLVED IRON,

IN MICROGRAMS PER LITER

0 0 9 10 7 8 7

1

1,000

100

10

10,000

0.1

100

10

1

1,000

DISSOLVED ALUMINUM, 

IN MICROGRAMS PER LITER

1 7 0 8 8 6 6

10

10,000

1,000

100

DISSOLVED BORON,

IN MICROGRAMS PER LITER

129 4 8 10 6 7 6

DISSOLVED CHROMIUM,

IN MICROGRAMS PER LITER

0 0 22 25 6 7 6

22

2

8 2 2 4

DISSOLVED SILVER,

IN MICROGRAMS PER LITER

0 28 0 23 8 7 7

0.1

1

100

10

10

100

10,000

1,000

28 7 3 7 6

NO DATA AVAILABLENO DATA AVAILABLE

1

100

10

1,000

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

0 9 0 11 7 8 7

2

NO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLE

2

12

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

21121

Outlier data value less than or equal to 3 and more than 1.5 times the interquartile

 range outside the quartile

Outlier data value greater than 3 times the interquartile range outside the quartile

Individual values plotted when data are inadequate to construct boxplot

Number of samples with concentrations less than study reporting level

Study reporting level—Triangle indicates data less than study reporting level
30 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Samples from the stream sites were not analyzed 

for chromium. The EROS Lake site had dissolved chromium concentrations that ranged from less than the 

study reporting level of 10 μg/L to 20 μg/L. The lagoon 

sites had concentrations of chromium that ranged from 

less than the study reporting level of 10 μg/L to 

100 μg/L, with maximum values generally decreasing 

from lagoon 1 to lagoon 4. 

Samples from the stream sites were not analyzed 

for dissolved iron. The EROS Lake site had dissolved 

iron concentrations that ranged from less than the study 

reporting level of 10 μg/L to 450 μg/L. The lagoon sites 

had concentrations of dissolved iron that ranged from 

19 to 2,500 μg/L, with minimum and maximum values 

generally decreasing from lagoon 1 to lagoon 4.

Samples from the stream sites were not analyzed 

for dissolved manganese. One sample from the EROS 

Lake site had a dissolved manganese concentration of 

2,200 μg/L. One sample from each lagoon site was 

analyzed for dissolved manganese, and concentrations 

ranged from 50 to 260 μg/L.

Samples from the stream sites were not analyzed 

for dissolved silver. The EROS Lake site had dissolved 

silver concentrations that were less than the study 

reporting level of 2 μg/L. Concentrations of dissolved 

silver in samples from the lagoon sites ranged from less 

than the study reporting level of 2 μg/L to 18 μg/L, with 

maximum values generally decreasing from lagoon 1 

to lagoon 4.

Dissolved zinc was not analyzed for in samples 

from the Big Sioux River or the Split Rock Creek sites. 

The EROS Lake site had dissolved zinc concentrations 

that ranged from less than the study reporting level of 

3 μg/L to 30 μg/L, whereas concentrations of dissolved 

zinc in the lagoon sites ranged from 4 to 190 μg/L. 

Bed Sediment

Analytical results of bottom material samples 

were collected within the study area at three locations 

in EROS Lake on July 16, 1991, and the analytical 

results are presented in table 17 on the CD-ROM as 

station name EROS Lake at EROS Data Center, SD (2), 

(3), and (4). Total concentrations of aluminum ranged 

from 4,300 to 5,700 μg/g (micrograms per gram); chromium ranged from 9 to 10 μg/g; iron ranged from 9,900 

to 13,000 μg/g; and zinc ranged from 40 to 51 μg/g. 

Of the remaining 54 constituents analyzed for, only 

benzo-a-pyrene, bis(2-ethylhexyl) phthalate, di-n-butyl 

phthalate, isophorone, and n-butylbenzyl phthalate 

had concentrations above their laboratory reporting 

limits.

Trends

The trend-analysis program ESTREND was 

applied to the data set; however, only one site, Big 

Sioux River near Dell Rapids, had sufficient data to use 

the program. Water-quality concentration data for 17 

constituents collected at this site were analyzed to 

determine if trends existed using the Seasonal Kendall 

test for either RCD or FAC data, or the Tobit test for 

RCD. 

The results of the trend-analysis methods for 

each constituent are presented in table 9. Only two constituents, specific conductance and dissolved orthophosphate, had trends that were statistically significant 

(table 9). Figure 7 presents two plots for each of the 

constituents that had statistically significant trends, a 

best model regression plot derived using ESTREND, 

and a smoothing line regression plot presenting a timeseries diagram. Trend analysis, based on Seasonal 

Kendall FAC, showed a slightly increasing trend of 

0.75 percent of the median per year in specific conductance. Trend analysis of dissolved orthophosphate 

showed a decreasing trend of 10.26 percent of the 

median per year, based on the Tobit test.

For specific conductance, ESTREND used the 

uncensored Seasonal Kendall test, with FAC adjustments using 12 seasons. The ESTREND output indicates the inverted model was the best model to present 

the statistical trend, thus, although the trend was 

increasing, the diagram shows the line decreasing 

(fig. 7). The second specific-conductance plot output 

by ESTREND presents the residual values over time, 

using a Robust MM smoothing line, which uses specialized estimates that have good robustness as properties (simultaneous high breakdown point and high 

efficiency when the errors are normally distributed) 

(Yohai, 1987).

For dissolved orthophosphate, the Tobit test, 

which is used to analyze data sets that contain values 

censored at multiple reporting limits, was used to 

present a regression model of deviance of values over 

time (fig. 7). The second dissolved orthophosphate 

plot presents the values with a least-squares smoothing 

line.
Water-Quality Characteristics 31

Table 9. Results of trend-analysis tests for selected surface-water constituents in samples from the Big Sioux River near Dell Rapid[All values in milligrams per liter, except as indicated. μS/cm, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; NA, not applicable]

Constituent

Trend

analysis

method

Beginning

year

Number of

years for

trend

analysis

Number of

seasons for

trend

analysis

Total number

of values

used in

trend analysis

Trend

slope

Perce

chang

Specific conductance (μS/cm) Uncensored 1973 27 12 271 6.5695 0.746Alkalinity, as CaCO3 Uncensored 1973 7 12 70 8.0518 2.982Dissolved solids, sum Uncensored 1973 12 12 125 4.7070 0.838Nitrogen ammonia, total, as N Uncensored 1973 7 12 75 -0.0069 -3.544Nitrogen, organic ammonia, as N Censored 1973 6 6 35 -0.0500 -2.941Nitrite plus nitrate, total, as N Censored 1973 6 12 66 -0.0100 -8.000Nitrite plus nitrate, dissolved, as N Tobit 1972 12 NA 141 NA 1.384Phosphorus, total Uncensored 1973 12 12 130 -0.0095 3.938Phosphorus, dissolved Uncensored 1973 12 12 120 -0.0070 -7.019Orthophosphate, dissolved Tobit 1972 13 NA 138 NA -10.255Arsenic, total (μg/L) Uncensored 1974 6 12 48 -0.2281 -7.601Boron, dissolved (μg/L) Uncensored 1973 12 12 122 -2.6825 -2.063Copper, total (μg/L) Tobit 1974 6 NA 49 NA -13.639Lead, total (μg/L) Tobit 1974 6 NA 52 NA 29.339Manganese, total (μg/L) Uncensored 1974 6 12 51 17.9709 -5.615Zinc, total (μg/L) Censored 1974 6 12 50 0.0000 0.000
32 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 7. Trend-analysis plots showing significant trends using ESTREND in samples from the Big Sioux River near Dell 

Rapids.

100

500

1,000

5,000

Seasonal Kendall (uncensored) trend

Inverse log option, flow-adjusted Tobit method trend

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

0.1 1 10 100 1,000 10,000 100,000

-3

-1

-2

2

1

0

3

DEVIANCE OF DISSOLVED

ORTHOPHOSPHATE VALUES OVER TIME

1972 1974 1976 1978 1980 1982 1984

0

1,500

1,000

500

2,000

2,500 Robust MM smoothing Least squares smoothing

1973 1976 1979 1982 1985 1988 1991 1994 2000 1997

0

0.01

0.04

0.03

0.02

0.05

DISSOLVED ORTHOPHOSPHATE

CONCENTRATION,

IN MILLIGRAMS PER LITER

1972 1974 1976 1978 1980 1982 1984

STREAMFLOW, IN CUBIC FEET PER SECOND DATE

DATE DATE

The other surface-water sites did not have sufficient data to use ESTREND, so scatter plots with a 

local regression (Lowess) smoothing line are presented 

for selected constituents (fig. 8). This method does not 

use flow-adjusted data. Split Rock Creek and EROS 

Lake data are presented with individual plots, whereas 

the lagoon data are presented together on a single plot. 

The smoothing line displayed was created by the 

Lowess method (Cleveland, 1979) as discussed in the 

“Methods of Study” section.

Specific conductance was the only constituent 

that had sufficient values to present a Lowess plot for 

Split Rock Creek (fig. 8). The results of regression 

analyses for Split Rock Creek indicate decreasing 

specific-conductance values over time, although from 

1975 to 1985, no change was evident, and the number 

of samples and specific-conductance values decreased 

after 1990. The Lowess plot for specific conductance at 

EROS Lake indicates an increasing trend from 1973 to 

1985 but a decreasing trend after 1990. The Lowess 

plot for the four lagoon sites strongly indicates a 

decreasing trend for all sites since 1980, when data collection began for specific conductance.

An increasing trend for alkalinity was evident at 

EROS Lake. The lagoon sites had varying trend results 

for alkalinity. The plot indicates an increasing trend for 

1980 to 1984; however, there is a slight decrease after 

1987, likely due to only one site (lagoon 1) being analyzed after 1987. 

The trend results for dissolved solids, sum of 

constituents, for EROS Lake were variable because the 

first two samples, taken in 1973, were much lower than 

the subsequent samples taken during 1979 to 1986. 

Decreasing trends for dissolved solids, sum of constituents, is strongly indicated for the lagoon sites, both 

individually and as a group. 
Water-Quality Characteristics 33

The EROS Lake regression plots indicate similar 

results for selected major ions, except for silica, with 

generally low values in 1973, and higher values from 

1979 to 1986. The regression plots indicate calcium 

and potassium concentrations had a decreasing trend 

from 1979 to 1986, whereas plots for magnesium, chloride, sodium, sulfate, and fluoride generally indicate a 

slight increase in concentrations from 1979 to 1983, 

and a slight decrease from 1983 to 1986. The regression plot for dissolved silica indicates very little change 

from 1973 to 1986. The regression analyses of selected 

major-ion concentrations for the lagoon sites as a group 

indicate generally decreasing trends over time, especially from 1980 to 1986 for calcium, magnesium, 

sodium, potassium, and sulfate. Trends for chloride, 

fluoride, and silica had varying results, indicating both 

increases and decreases during the sampling period, but 

a decreasing trend overall.

Regression analyses for selected trace elements 

of aluminum, boron, and chromium strongly indicate 

decreasing trends for EROS Lake. Analysis of dissolved silver indicates no change during the sampling 

period, whereas dissolved zinc indicates variable 

results with no change overall for EROS Lake.

The regression analyses for the lagoon sites 

strongly indicate decreasing trends for dissolved boron 

and chromium, and a slightly increasing trend for dissolved silver. Variable increases and decreases appear 

evident for dissolved aluminum and zinc for the lagoon 

sites as a group.

Selected Ground-Water Sites

Water-quality samples were collected at 18 

ground-water sites within the study area; however, sufficient data were collected at only 9 ground-water sites 

(table 1). The analytical results for the 18 ground-water 

sites are presented in table 16, but the following discussions only include data from the 9 sites with sufficient 

data.

Ground-water properties are aquifer dependent. 

Two of the selected ground-water sites (GW2 and 

GW4) are completed in glacial outwash (glacial 

aquifer), and seven of the selected ground-water sites 

are completed in bedrock (Sioux Quartzite aquifer). A 

statistical summary of the physical properties of the 

ground-water data, grouped by aquifer, is presented in 

table 10.

Table 10. Statistical summary of selected physical properties of ground-water sites, grouped by aquifer, in the study area

[All values in milligrams per liter, except as indicated. μS/cm, microsiemens per centimeter at 25 degrees Celsius; °C, degrees Celsius; --, no data]

Property or constituent Number of

samples Mean Median Minimum Maximum

Glacial aquifer (glacial outwash)

Specific conductance (μS/cm) 33 1,210 1,220 790 2,100

pH (standard units) 33 7.2 7.2 6.8 8.4

Temperature (°C) 21 9.0 9 3.0 14.0

Carbon dioxide 32 84 66 2.6 208

Hardness, as CaCO3 32 650 620 390 1,500

Noncarbonate hardness 27 100 73 13 390

Well depth (feet below land surface) 2 -- 39.5 39 40

Sioux Quartzite aquifer (bedrock)

Specific conductance (μS/cm) 162 1,270 959 681 3,520

pH (standard units) 160 7.3 7.3 5.8 8.7

Temperature (°C) 94 9.5 10.0 7.5 12.0

Carbon dioxide 116 37 30 0.5 158

Hardness, as CaCO3 118 700 520 370 1,800

Noncarbonate hardness 97 360 170 27 1,400

Well depth (feet below land surface) 7 227 165 80 465
34 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study area.

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS

Split Rock Creek

EROS Lake Lagoons

EROS Lake Lagoons

ALKALINITY, IN MILLIGRAMS PER LITER

100

1,000

10,000

100

1,000

10,000

100

1,000

10,000

0

150

100

50

200

250

300

0

150

100

50

200

250

300

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

DATE DATE
Water-Quality Characteristics 35

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study 

area.—Continued

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

EROS Lake Lagoons

EROS Lake Lagoons

DISSOLVED MAGNESIUM, IN MILLIGRAMS PER LITER DISSOLVED CALCIUM, IN MILLIGRAMS PER LITER

EROS Lake Lagoons

DISSOLVED SOLIDS, SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

0

30

60

90

120

0

30

60

90

120

0

30

20

10

40

50

60

0

30

20

10

40

50

60

0

1,500

1,000

500

2,000

2,500

0

1,500

1,000

500

2,000

2,500

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

DATE DATE
36 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study 

area.—Continued

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

EROS Lake Lagoons

EROS Lake Lagoons

DISSOLVED SULFATE, IN MILLIGRAMS PER LITER DISSOLVED POTASSIUM, IN MILLIGRAMS PER LITER

EROS Lake Lagoons

DISSOLVED SODIUM, IN MILLIGRAMS PER LITER

0

10

20

30

40

50

60

0

300

200

100

400

500

600

0

300

200

100

400

500

600

0

400

500

300

100

200

600

700

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

0

400

500

300

100

200

600

700

0

10

20

30

40

50

60

DATE DATE
Water-Quality Characteristics 37

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study 

area.—Continued

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

EROS Lake Lagoons

EROS Lake Lagoons

DISSOLVED SILICA, IN MILLIGRAMS PER LITER DISSOLVED FLUORIDE, IN MILLIGRAMS PER LITER

EROS Lake Lagoons

DISSOLVED CHLORIDE,

IN MILLIGRAMS PER LITER

0

0.5

1.0

1.5

0

10

5

15

20

0

600

400

200

800

1,000

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

0

600

400

200

800

1,000

0

0.5

1.0

1.5

0

10

5

15

20

DATE DATE
38 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study 

area.—Continued

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

EROS Lake Lagoons

EROS Lake Lagoons

DISSOLVED CHROMIUM,

IN MICROGRAMS PER LITER

DISSOLVED BORON,

IN MICROGRAMS PER LITER

EROS Lake Lagoons

DISSOLVED ALUMINUM,

IN MICROGRAMS PER LITER

0

1,000

500

2,000

1,500

3,000

2,500

0

40

60

80

20

100

120

0

40

60

80

20

100

120

0

60

40

20

80

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

0

60

40

20

80

0

1,000

500

2,000

1,500

3,000

2,500

DATE DATE
Water-Quality Characteristics 39

Figure 8. Local regression (Lowess) plots for constituent concentrations for selected surface-water sites in the study 

area.—Continued

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

EROS Lake Lagoons

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

EROS Lake Lagoons

DISSOLVED SILVER,

IN MICROGRAMS PER LITER

0

100

50

200

150

0

15

10

5

20

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

Lagoon 1

Lagoon 2

Lagoon 3

Lagoon 4

0

15

10

5

20

0

100

50

200

150

DATE DATE
40 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Field-Measured Properties and Constituents

A statistical summary of the field measurements 

of selected physical properties for the nine groundwater sites is given in table 11. Boxplots of the selected 

field-measured properties and constituents for the nine 

ground-water sites are presented in figure 9. 

Specific conductance for the selected groundwater sites ranged from 681 to 3,520 μS/cm, with the 

highest values and most variability measured at GW7. 

The pH values for the ground-water sites ranged from 

5.8 to 8.7 (table 11, fig. 9).

Water seepage through organic soil that comes 

into contact with products of decomposition may 

introduce gaseous carbon dioxide into ground water. 

Subterranean water rich in carbon dioxide may 

dissolve carbonates and bring them into solution as 

bicarbonates (Cole, 1994). The carbon dioxide 

values for ground-water sites ranged from 0.5 to 

208 mg/L. 

Hardness for the ground-water sites ranged from 

370 to 1,800 mg/L, noncarbonate hardness ranged 

from 13 to 1,400 mg/L, and alkalinity ranged from 85 

to 862 mg/L. Samples from GW7 had the highest hardness and noncarbonate hardness values and the most 

variability of these properties for the ground-water 

sites.

Table 11. Statistical summary of selected physical properties for selected ground-water sites in the study area

[All values in milligrams per liter, except as indicated. Statistics summarize alkalinity field and laboratory values, and are a combination of fixed-end point 

and inflection titrations. μS/cm, microsiemens per centimeter at 25 degrees Celsius; oC, Celsius]

Property or constituent

GW1 GW2

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 35 961 920 810 1,500 14 941 915 790 1,220

pH (standard units) 35 7.3 7.3 6.3 8.3 14 7.4 7.4 7.0 8.4

Water temperature (°C) 25 9.5 9.5 8.5 11.0 10 9.0 9.0 3.0 14.0

Carbon dioxide 20 28 30 3.0 59 14 38 33 2.6 78

Hardness, as CaCO3 20 480 460 410 690 14 500 470 390 610

Noncarbonate hardness 14 170 170 93 250 12 60 50 13 170

Alkalinity, as CaCO3 23 313 308 290 385 14 429 415 313 530

Property or constituent

GW3 GW4

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 12 1,400 1,410 1,200 1,530 19 1,410 1,330 1,060 2,100

pH (standard units) 12 7.3 7.2 7.1 7.5 19 7.0 7.0 6.8 7.3

Water temperature (°C) 2 -- -- 10.0 10.0 11 9.0 9.0 4.5 12.5

Carbon dioxide 12 33 36 18 47 18 120 113 51 208

Hardness, as CaCO3 12 830 820 760 900 18 770 720 590 1,500

Noncarbonate hardness 12 530 530 460 600 15 130 92 36 390

Alkalinity, as CaCO3 12 298 300 285 302 18 613 600 491 862
Water-Quality Characteristics 41

Table 11. Statistical summary of selected physical properties for selected ground-water sites in the study area—Continued

[All values in milligrams per liter, except as indicated. Statistics summarize alkalinity field and laboratory values, and are a combination of fixed-end point 

and inflection titrations. μS/cm, microsiemens per centimeter at 25 degrees Celsius; oC, Celsius]

Property or constituent

GW5 GW6

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 11 796 804 700 840 15 762 760 710 825

pH (standard units) 11 7.3 7.3 7.1 7.7 15 7.4 7.4 7.1 7.5

Water temperature (°C) 3 10.0 10.0 10.0 10.0 5 10 10 8.5 10

Carbon dioxide 11 40 36 14 58 14 28 26 21 49

Hardness, as CaCO3 11 410 410 390 430 14 384 385 370 400

Noncarbonate hardness 11 49 45 27 77 14 54 54 27 100

Alkalinity, as CaCO3 11 364 365 350 373 14 332 340 294 343

Property or constituent

GW7 GW8

Number

of

samples

Mean Median Minimum

Maximum

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 37 1,970 1,910 941 3,520 18 1,820 1,880 1,250 2,040

pH (standard units) 37 7.1 7.1 6.2 8.0 18 8 7.5 7.2 8.7

Water temperature (°C) 27 9.5 9.5 7.5 12.0 7 9.5 9.5 8.0 10.0

Carbon dioxide 22 73 68 7.9 158 17 20 19 0.5 40

Hardness, as CaCO3 23 1,080 1,000 710 1,800 18 1,100 1,100 610 1,200

Noncarbonate hardness 16 711 610 300 1,400 14 817 820 750 880

Alkalinity, as CaCO3 26 423 424 300 542 17 289 320 85 327

Property or constituent

GW9

Number

of

samples

Mean Median Minimum

Maximum

Specific conductance (μS/cm) 34 874 880 681 1,070

pH (standard units) 34 7.3 7.3 5.8 8.2

Water temperature (°C) 25 9.5 10.0 8.0 11.5

Carbon dioxide 20 28 26 4.1 40

Hardness, as CaCO3 20 450 450 410 530

Noncarbonate hardness 16 128 125 79 180

Alkalinity, as CaCO3 23 323 322 280 346
42 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 9. Boxplots of field-measured properties and constituent concentrations for selected ground-water sites in the

study area.

200

2,200

1,200

3,200

4,200

0

1,000

500

1,500

2,000

0

800

600

400

200

1,400

1,200

1,000

1,600

0

400

200

800

600

1,000

5

8

6

7

9

10

SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

35 14 12 19 11 15 37 18 34 20 14 12 18 11 14 23 18 20

14 12 12 15 11 14 16 14 16

23 14 12 18 11 14 26 17 23

2

4

6

8

10

12

14

16

0

50

100

150

200

250

25 10 2 11 3 5 27 7 25

20 14 12 18 11 14 22 17 20

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

pH, IN STANDARD UNITS WATER TEMPERATURE,

IN DEGREES CELSIUS

CARBON DIOXIDE,

IN MILLIGRAMS PER LITER

35 14 12 19 11 15 37 18 34

HARDNESS, IN MILLIGRAMS PER LITER NONCARBONATE HARDNESS,

IN MILLIGRAMS PER LITER

ALKALINITY,

IN MILLIGRAMS PER LITER

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

22

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile range

 outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than 1.5 times the

 interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile range outside

 the quartile

Individual values plotted when data are inadequate to construct boxplot
Water-Quality Characteristics 43

Major Ions and Indicators of Major Ions

A statistical summary of selected major ions, 

including dissolved solids, calcium, magnesium, 

sodium, percent sodium, sodium-adsorption ratio, 

potassium, bicarbonate, carbonate, sulfate, chloride, 

fluoride, and silica, is given in table 12. Boxplots are 

presented in figure 10 for each of the major ions, except 

carbonate.

Concentrations of dissolved solids, residue at 

180oC, was analyzed for at GW1, GW7, and GW9, and 

concentrations ranged from 527 to 1,390 mg/L. Dissolved solids, sum of constituents, was analyzed for in 

samples from all nine ground-water sites, and concentrations ranged from 193 to 2,530 mg/L.

Site GW7, located approximately one-third mile 

east of EROS Lake (fig. 1), had the highest maximum 

value for several of the dissolved constituents for major 

ions, including calcium (matching the maximum value 

with GW4), magnesium, sodium, and chloride. GW7 is 

completed in the Sioux Quartzite aquifer, although it is 

relatively shallow at only 80 ft deep. GW4 also had 

maximum concentrations for several additional major 

ion constituents, including potassium, bicarbonate, and 

silica (matching the maximum value with GW6). GW4 

is located approximately one-third mile northeast of 

EROS Lake and, at 39 ft deep, is completed in the 

glacial aquifer.

Proportions of major ions in water samples from 

selected ground-water sites are shown in a trilinear diagram (Piper, 1944) in figure 11. The water type in the 

two wells (GW2 and GW4) completed in the glacial 

aquifer is calcium magnesium bicarbonate. Cation data 

indicate predominantly calcium- and magnesium-rich 

water for wells completed in the Sioux Quartzite 

aquifer. Anion data generally indicate predominantly 

sulfate- and bicarbonate-rich water, with the exception 

of GW7, where chloride is more dominant than sulfate.

Table 12. Statistical summary of major-ion and indicators of major-ion results for data for selected ground-water sites in the 

study area

[All values are for dissolved constituents, in milligrams per liter, except as indicated. Bicarbonate and carbonate are a combination of fixed-end point and 

inflection titrations]

Constituent

GW1

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 14 0 650 639 537 801

Solids, sum of constituents 20 0 600 609 193 936

Calcium 20 0 139 140 110 160

Magnesium 20 0 33 31 11 69

Sodium 20 0 16 16 15 22

Sodium, percent 20 0 7 7 6 8

Sodium-adsorption ratio 20 0 0.3 0.3 0.3 0.4

Potassium 20 0 3.8 3.8 3.2 4.2

Bicarbonate 16 0 377 376 350 439

Carbonate 15 0 0 0 0 0

Sulfate 19 0 199 200 130 290

Chloride 19 0 4.6 1.4 0.9 61

Fluoride 19 0 0.8 0.8 0.5 0.8

Silica 20 0 24 25 18 27
44 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 12. Statistical summary of major-ion and indicators of major-ion results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. Bicarbonate and carbonate are a combination of fixed-end point and 

inflection titrations]

Constituent

GW2

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 14 0 590 553 435 1,010

Calcium 14 0 99 93 84 130

Magnesium 14 0 60 60 28 81

Sodium 14 0 24 24 20 30

Sodium, percent 14 0 10 10 7 12

Sodium-adsorption ratio 14 0 0.5 0.5 0.4 0.6

Potassium 14 0 2.8 2.2 1.2 11

Bicarbonate 11 0 516 503 382 640

Carbonate 11 0 0 0 0 0

Sulfate 14 0 89 64 37 360

Chloride 14 0 25 20 3.5 100

Fluoride 13 0 0.9 0.9 0.6 1.0

Silica 14 0 22 23 14 28

Constituent

GW3

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 12 0 1,070 1,050 993 1,190

Calcium 12 0 220 220 200 240

Magnesium 12 0 67 67 56 73

Sodium 12 0 23 23 21 23

Sodium, percent 12 0 6 6 5 6

Sodium-adsorption ratio 12 0 0.3 0.3 0.3 0.4

Potassium 12 0 4.0 4.0 3.6 4.4

Bicarbonate 12 0 363 363 348 370

Carbonate 11 0 0 0 0 0

Sulfate 12 0 553 545 490 650

Chloride 12 0 3.4 3.4 2.8 4.1

Fluoride 12 0 0.7 0.6 0.6 0.8

Silica 12 0 17 17 16 17
Water-Quality Characteristics 45

Table 12. Statistical summary of major-ion and indicators of major-ion results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. Bicarbonate and carbonate are a combination of fixed-end point and 

inflection titrations]

Constituent

GW4

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 18 0 876 827 390 1,790

Calcium 18 0 174 165 140 340

Magnesium 18 0 81 76 59 150

Sodium 18 0 23 19 16 63

Sodium, percent 18 0 6 6 4 8

Sodium-adsorption ratio 18 0 0.4 0.3 0.3 0.7

Potassium 18 0 9.2 7.2 2.3 23

Bicarbonate 13 0 702 692 599 820

Carbonate 12 0 0 0 0 0

Sulfate 17 0 60 56 25 140

Chloride 18 0 61 53 6.7 290

Fluoride 17 0 0.5 0.5 0.4 0.6

Silica 18 0 26 26 22 30

Constituent

GW5

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 11 0 507 505 488 526

Calcium 11 0 104 100 99 110

Magnesium 11 0 37 37 35 38

Sodium 11 0 20 21 19 21

Sodium, percent 11 0 10 10 9 10

Sodium-adsorption ratio 11 0 0.4 0.4 0.4 0.5

Potassium 11 0 4.4 4.4 4.1 5.0

Bicarbonate 10 0 444 445 427 455

Carbonate 9 0 0 0 0 0

Sulfate 11 0 86 86 75 110

Chloride 11 0 5.8 5.7 4.9 7.5

Fluoride 11 0 1.3 1.3 1.1 1.3

Silica 11 0 26 26 24 28
46 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 12. Statistical summary of major-ion and indicators of major-ion results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. Bicarbonate and carbonate are a combination of fixed-end point and 

inflection titrations]

Constituent

GW6

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 14 0 479 479 456 505

Calcium 14 0 96 97 89 100

Magnesium 14 0 35 35 34 37

Sodium 14 0 21 21 19 22

Sodium, percent 14 0 10 10 10 11

Sodium-adsorption ratio 14 0 0.5 0.5 0.4 0.5

Potassium 14 0 3.1 3.0 2.8 3.7

Bicarbonate 12 0 404 412 358 418

Carbonate 11 0 0 0 0 0

Sulfate 14 0 79 78 53 110

Chloride 15 0 3.6 3.7 2.6 4.8

Fluoride 14 0 0.7 0.7 0.5 0.8

Silica 14 0 27 28 20 30

Constituent

GW7

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 14 0 1,010 1,030 625 1,390

Solids, sum of constituents 23 0 1,410 1,340 262 2,530

Calcium 23 0 217 210 140 340

Magnesium 23 0 132 120 84 230

Sodium 23 0 47 45 28 96

Sodium, percent 23 0 9 8 6 17

Sodium-adsorption ratio 23 0 0.6 0.6 0.4 1.0

Potassium 23 0 7.0 6.9 3.9 10

Bicarbonate 16 0 544 539 460 661

Carbonate 15 0 0 0 0 0

Sulfate 22 0 143 125 81 360

Chloride 23 0 161 130 73 360

Fluoride 22 0 0.5 0.5 0.4 0.7

Silica 23 0 25 25 23 27
Water-Quality Characteristics 47

Table 12. Statistical summary of major-ion and indicators of major-ion results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. Bicarbonate and carbonate are a combination of fixed-end point and 

inflection titrations]

Constituent

GW8

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 0 0 -- -- -- --

Solids, sum of constituents 17 0 1,420 1,510 618 1,580

Calcium 18 0 288 300 130 320

Magnesium 18 0 89 91 63 98

Sodium 18 0 34 33 32 41

Sodium, percent 18 0 7 6 6 13

Sodium-adsorption ratio 18 0 0.4 0.4 0.4 0.8

Potassium 18 0 5.7 5.6 5.1 6.6

Bicarbonate 13 0 380 390 300 399

Carbonate 12 0 0 0 0 0

Sulfate 17 0 842 860 620 890

Chloride 17 0 8.2 7.6 6.8 15

Fluoride 17 0 0.4 0.4 0.2 1.0

Silica 16 0 20 22 3.7 25

Constituent

GW9

Number of

samples

Number of

values less

than study

reporting level

Mean Median Minimum Maximum

Solids, residue at 180°C 14 0 585 589 527 601

Solids, sum of constituents 20 0 544 555 199 629

Calcium 20 0 134 130 120 160

Magnesium 20 0 28 28 25 32

Sodium 20 0 18 17 15 23

Sodium, percent 20 0 8 8 7 10

Sodium-adsorption ratio 20 0 0.4 0.4 0.3 0.5

Potassium 20 0 3.9 3.9 3.2 4.2

Bicarbonate 16 0 395 392 380 422

Carbonate 15 0 0 0 0 0

Sulfate 19 0 160 160 110 220

Chloride 19 0 1.5 1.1 0.6 3.6

Fluoride 19 0 0.5 0.4 0.3 1.4

Silica 20 0 23 23 19 26
48 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 10. Boxplots of major-ion constituent concentrations for selected ground-water sites in the study area. DISSOLVED MAGNESIUM, IN MILLIGRAMS PER LITER

0

1,000

500

1,500

2,500

2,000

3,000

1,000

600

800

400

1,200

1,400

1,600

DISSOLVED SOLIDS, RESIDUE ON

EVAPORATION AT 180 DEGREES CELSIUS,

IN MILLIGRAMS PER LITER

DISSOLVED SOLIDS,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

10

8

14

12

4

6

18

16

20

PERCENT SODIUM

0.6

0.2

0.4

0.8

1.0

1.2

SODIUM ADSORPTION RATIO

20 14 12 18 11 14 23 17 20

14 0 0 0 0 0 14 0 14

DISSOLVED CALCIUM,

IN MILLIGRAMS PER LITER

DISSOLVED SODIUM,

IN MILLIGRAMS PER LITER

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

GW1 GW3 GW5 GW7 GW8 GW9 GW2 GW4 GW6 GW1 GW3 GW5 GW7 GW8 GW9 GW2 GW4 GW6

0

100

200

300

400

20 14 12 18 11 14 23 18 20

0

100

50

200

150

250

20 14 12 18 11 14 23 18 20

0

60

40

20

80

100

20 14 12 18 11 14 23 18 20

20 14 12 18 11 14 23 18 20

20 14 12 18 11 14 23 18 20

DISSOLVED POTASSIUM,

IN MILLIGRAMS PER LITER

0

5

10

15

20

25

20 14 12 18 11 14 23 18 20
Water-Quality Characteristics 49

Figure 10. Boxplots of major-ion constituent concentrations for selected ground-water sites in the study area.—Continued

0

200

400

800

600

1,000

DISSOLVED SULFATE,

IN MILLIGRAMS PER LITER

0

200

100

300

400

DISSOLVED CHLORIDE,

IN MILLIGRAMS PER LITER

0

10

20

30

40

DISSOLVED SILICA,

IN MILLIGRAMS PER LITER

0

2.0

1.5

1.0

DISSOLVED FLUORIDE,

0.5

IN MILLIGRAMS PER LITER

GW1 GW3 GW5 GW7 GW8 GW9 GW2 GW4 GW6

GW1 GW3 GW5 GW7 GW8 GW9 GW2 GW4 GW6

19 14 12 17 11 14 22 17 19

19 14 12 18 11 15 23 17 19

19 13 12 17 11 14 22 17 19

20 14 12 18 11 14 23 16 20 1,000

800

900

700

600

500

400

300

BICARBONATE,

IN MILLIGRAMS PER LITER

16 11 12 13 10 12 16 13 16

11

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile

 range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than

 1.5 times the interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile

 range outside the quartile
50 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 11. Trilinear diagrams (Piper, 1944) showing proportions of major ions for selected ground-water sites in the

study area.

PERCENTAGE REACTING VALUES, IN MILLIEQUIVALENTS PER LITER

Calcium + Magnesium

Sulfate

Sodium + Potassium

Magnesium

Calcium

80

60

40

20

80

80

60

40

20

20

20

40

60

80

40

60

80

60

40

20

20

80

60

40

20

40

60

80

80

60

40

20

Bicarbonate 

Sulfate + Chloride

Chloride

CATIONS ANIONS

Glacial Aquifer Sites

PERCENTAGE REACTING VALUES, IN MILLIEQUIVALENTS PER LITER

Calcium + Magnesium

Sulfate

Sodium + Potassium

Magnesium

Calcium

80

60

40

20

80

80

60

40

20

20

20

40

60

80

40

60

80

60

40

20

20

80

60

40

20

40

60

80

80

60

40

20

Bicarbonate 

Sulfate + Chloride

Chloride

CATIONS ANIONS

Sioux Quartzite Aquifer Sites

GW2, 103N48W 7DAC2

GW4, 103N48W 8BCCB2

EXPLANATION

GW1, 103N48W 5CACA2

GW3, 103N48W 8ADA

GW5, 103N48W 8DDDD

GW6, 103N48W 9BDCB

GW7, 103N48W 9CCDA

GW8, 103N48W18ACA2

GW9, 103N48W17ACCC2

EXPLANATION
Water-Quality Characteristics 51

Nutrients

A statistical summary of nutrient concentrations 

in samples from ground-water sites located within the 

study area is given in table 13. Boxplots for selected 

nutrient constituents are presented in figure 12.

Nitrite plus nitrate, as nitrogen, was routinely 

sampled at each site; other constituents typically have 

only minimal sample results due to the requirements of 

the monitoring program. Nitrite plus nitrate concentrations ranged from less than the study reporting level of 

0.10 mg/L to 250 mg/L (table 13, fig. 12). Concentrations in samples from GW4 and GW7 generally were 

greater than 10 mg/L, and exceeded the water-quality 

standard for drinking water (U.S. Environmental 

Protection Agency, 2002). The high nitrite plus nitrate 

concentrations at GW4 and GW7 probably reflect local 

contamination and not contamination from EROS 

wastewater-treatment processes because concentrations measured in samples from EROS Lake were less 

than 1.0 mg/L. When analyzed, nitrite, as nitrogen, 

concentrations ranged from less than 0.01 to 

0.02 mg/L.

Samples from ground-water sites that were analyzed for ammonia had concentrations that ranged from 

less than the study reporting level of 0.02 mg/L to 

0.21 mg/L. Samples that were analyzed for ammonia 

plus organic, total, had concentrations that ranged from 

0.02 to 9.6 mg/L.

Total phosphorus concentrations ranged from 

less than the study reporting level of 0.01 mg/L to 

0.23 mg/L. Orthophosphate was only analyzed in three 

samples from the ground-water sites, and the concentrations ranged from less than the study reporting level 

of 0.01 mg/L to 0.07 mg/L. 

Table 13. Statistical summary of nitrogen and phosphorus nutrient results for data for selected ground-water sites in the 

study area

[All values are for dissolved constituents, in milligrams per liter, except as indicated. <, less than; --, not analyzed or not determined]

Property or constituent

GW1

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 10 8 <0.01 <0.01 <0.01 0.01

Nitrite plus nitrate, as N 34 14 5.0 0.11 <0.10 53

Ammonia, as N 14 5 0.03 0.02 <0.02 0.10

Ammonia plus organic, as N 5 0 0.34 0.37 0.02 0.59

Phosphorus, total, as P 5 1 0.03 0.01 <0.01 0.07

Orthophosphate, as P 10 2 0.02 0.02 <0.01 0.03

Property or constituent

GW2

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 14 2 1.9 1.0 <0.10 4.9

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 5 0 0.53 0.48 0.33 0.83

Phosphorus, total, as P 5 2 0.03 0.01 <0.01 0.08

Orthophosphate, as P 0 0 -- -- -- --
52 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 13. Statistical summary of nitrogen and phosphorus nutrient results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. <, less than; --, not analyzed or not determined]

Property or constituent

GW3

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 12 9 0.11 <0.10 <0.10 0.57

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 3 0 0.22 0.24 0.16 0.26

Phosphorus, total, as P 3 2 -- <0.01 <0.01 0.01

Orthophosphate, as P 0 0 -- -- -- --

Property or constituent

GW4

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 17 0 22 15 0.14 67

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 7 0 1.3 1.3 0.05 2.2

Phosphorus, total, as P 7 0 0.05 0.02 0.01 0.23

Orthophosphate, as P 0 0 -- -- -- --

Property or constituent

GW5

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 11 7 <0.10 <0.10 <0.10 0.43

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 3 0 0.33 0.33 0.25 0.40

Phosphorus, total, as P 3 1 0.01 0.01 <0.01 0.02

Orthophosphate, as P 0 0 -- -- -- --

Property or constituent

GW6

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 14 0 3.2 3.2 2.0 5.3

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 7 0 0.56 0.49 0.14 1.4

Phosphorus, total, as P 7 1 0.02 0.01 <0.01 0.04

Orthophosphate, as P 0 0 -- -- -- --
Water-Quality Characteristics 53

Table 13. Statistical summary of nitrogen and phosphorus nutrient results for data for selected ground-water sites in the 

study area—Continued

[All values are for dissolved constituents, in milligrams per liter, except as indicated. <, less than; --, not analyzed or not determined]

Property or constituent

GW7

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 10 8 -- <0.01 <0.01 0.02

Nitrite plus nitrate, as N 37 1 77 73 <0.10 250

Ammonia, as N 14 5 0.04 0.03 <0.02 0.18

Ammonia plus organic, as N 8 0 2.8 2.0 0.15 9.6

Phosphorus, total, as P 8 0 0.03 0.03 0.01 0.04

Orthophosphate, as P 10 0 0.02 0.03 0.01 0.04

Property or constituent

GW8

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 0 0 -- -- -- --

Nitrite plus nitrate, as N 17 11 0.13 <0.10 <0.10 0.83

Ammonia, as N 0 0 -- -- -- --

Ammonia plus organic, as N 5 0 0.70 0.62 0.52 1.1

Phosphorus, total, as P 5 0 0.04 0.05 0.02 0.06

Orthophosphate, as P 0 0 -- -- -- --

Property or constituent

GW9

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Nitrite, as N 10 9 <0.01 <0.01 <0.01 0.02

Nitrite plus nitrate, as N 32 19 0.09 <0.10 <0.10 0.33

Ammonia, as N 14 0 0.18 0.18 0.13 0.21

Ammonia plus organic, as N 7 0 0.40 0.37 0.29 0.62

Phosphorus, total, as P 7 4 0.02 <0.01 <0.01 0.08

Orthophosphate, as P 10 2 0.02 0.01 <0.01 0.07
54 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 12. Boxplots of nutrient constituent concentrations for selected ground-water sites in the study area.

0.1

0.01

100

10

1

TOTAL AMMONIA PLUS ORGANIC,

IN MILLIGRAMS PER LITER

1

0.1

0.01

10

100

1,000

DISSOLVED NITRITE PLUS NITRATE,

IN MILLIGRAMS PER LITER

DISSOLVED ORTHOPHOSPHATE,

IN MILLIGRAMS PER LITER

5 5 3 7 3 7 8 5 7

0.001

1

0.1

0.01

34 14 12 17 11 14 37 17 32 10 0 0 0 0 0 10 0 10

0.001

1

0.1

0.01

10

DISSOLVED AMMONIA,

IN MILLIGRAMS PER LITER

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

14 0 0 0 0 0 14 0 14

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

0.001

1

0.1

0.01

DISSOLVED NITRITE,

IN MILLIGRAMS PER LITER

NO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLENO DATA AVAILABLE

10 0 0 0 0 0 10 0 10

TOTAL PHOSPHORUS,

IN MILLIGRAMS PER LITER

5 5 3 7 3 7 8 5 7

0.01

0.001

1

0.1

2 2

14 2 9 7 1 11 19

8 89 1 22 4 1 1

5 5

3

10

75th percentile

Number of samples

Data value less than or equal to 1.5 times the

 interquartile range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than

 1.5 times the interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile

 range outside the quartile

Individual values plotted when data are inadequate to

 construct boxplot

Number of samples with concentrations less than study

 reporting level

Study reporting level—Triangle indicates data less than

 study reporting level
Water-Quality Characteristics 55

Trace Elements

A statistical summary of selected trace elements 

in samples from ground-water sites located within the 

study area is given in table 14. Concentrations of trace 

elements were highly variable among wells (table 14, 

fig. 13). Boxplots are presented for selected trace elements in figure 13. Many of the constituents in table 14 

had a high percentage of concentrations with censored 

data. All dissolved silver concentrations were reported 

below the study reporting level of 2 μg/L, thus a boxplot is not presented for dissolved silver. 

Concentrations of dissolved aluminum for 

ground-water sites ranged from less than the study 

reporting level of 10 μg/L to 230 μg/L. Concentrations 

of dissolved boron ranged from 10 to 1,700 μg/L. Dissolved chromium concentrations ranged from less than 

the study reporting level of 10 μg/L to 20 μg/L, and 

dissolved iron concentrations ranged from less than the 

study reporting level of 10 μg/L to 9,200 μg/L. Dissolved manganese concentrations ranged from less 

than the study reporting level of 20 μg/L to 1,700 μg/L. 

Concentrations of dissolved zinc ranged from less than 

the study reporting level of 3 μg/L to 3,000 μg/L.

Trends

The ESTREND trend-analysis program could 

not be used with the ground-water data set because of 

insufficient data. Scatter plots with a local regression 

(Lowess) smoothing line are presented for representative sites for selected constituents in figure 14. The 

graphical output shows the trend line for the selected 

constituents for two representative wells, based on 

sampling during the period of record. GW2 was 

selected to represent the wells developed in the glacial 

aquifer because data from the other well completed in 

outwash of the glacial aquifer (GW4) indicated contamination. GW1 was selected to represent the wells 

developed in the Sioux Quartzite aquifer because it had 

the longest period of record other than GW7, and, 

furthermore, data from GW7 also indicated contamination. The smoothing line was based on the Lowess 

method (Cleveland, 1979), as discussed previously.

Results for the representative glacial aquifer site 

(GW2) indicate increasing trends for dissolved solids, 

calcium, magnesium, sodium, potassium, sulfate, and 

boron. Results indicate generally decreasing trends for 

chloride, fluoride, silica, and zinc. Trend results are 

variable but generally indicate little change during the 

time period analyzed for specific conductance, alkalinity, aluminum, chromium, and silver. 

Trend results for the representative Sioux 

Quartzite aquifer site (GW1) indicate slightly 

increasing trends for fluoride, silica, boron, and zinc 

during the time period analyzed, whereas potassium, 

aluminum, and chromium results indicate slightly 

decreasing trends. Trend results are variable for sulfate. 

Sulfate concentrations increased from 1973 to 1978, 

but by 1986 had decreased to a slightly lower level than 

in 1973. Little or no change is evident in trend results 

for specific conductance, alkalinity, dissolved solids, 

calcium, magnesium, sodium, chloride, and silver.

Table 14. Statistical summary of water-quality results of selected trace-element constituents for selected ground-water

sites in the study area

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Property or constituent

GW1

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 14 2 28 20 <10 140

Boron 20 0 195 190 100 320

Chromium 26 24 <10 <10 <10 10

Iron 17 2 33 20 <10 80

Manganese 6 0 203 245 20 310

Silver 30 30 <2 <2 <2 <2

Zinc 17 0 355 90 20 2,600
56 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 14. Statistical summary of water-quality results of selected trace-element constituents for selected ground-water sites 

in the study area—Continued

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Property or constituent

GW2

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 8 1 29 20 <10 130

Boron 13 0 106 80 30 300

Chromium 6 5 6 <10 <10 10

Iron 11 1 106 50 <10 230

Manganese 4 4 <20 <20 <20 <20

Silver 11 11 <2 <2 <2 <2

Zinc 11 0 443 250 8 1,500

Property or constituent

GW3

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 7 0 36 30 20 70

Boron 11 0 302 300 250 360

Chromium 5 5 <10 <10 <10 <10

Iron 9 1 107 90 <10 410

Manganese 6 0 407 365 240 760

Silver 8 8 <2 <2 <2 <2

Zinc 8 0 330 150 30 1,600

Property or constituent

GW4

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 13 2 14 15 <10 20

Boron 17 2 219 80 <20 1,700

Chromium 10 9 6 <10 <10 10

Iron 16 0 111 70 13 280

Manganese 7 5 23 <20 <20 70

Silver 15 15 <2 <2 <2 <2

Zinc 13 1 130 20 <3 1,200
Water-Quality Characteristics 57

Table 14. Statistical summary of water-quality results of selected trace-element constituents for selected groundwater sites in the study area—Continued

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Property or constituent

GW5

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 8 2 25 18 <10 70

Boron 10 0 209 200 150 400

Chromium 6 5 6 <10 <10 10

Iron 9 0 950 440 58 2,200

Manganese 6 1 49 48 <20 80

Silver 9 9 <2 <2 <2 <2

Zinc 9 0 1,400 1,300 160 3,000

Property or constituent

GW6

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 9 2 25 10 <10 130

Boron 13 0 98 100 50 120

Chromium 8 6 8 <10 <10 20

Iron 13 0 120 100 20 320

Manganese 7 6 <20 <20 <20 20

Silver 13 13 <2 <2 <2 <2

Zinc 13 0 140 100 20 570

Property or constituent

GW7

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 17 3 31 20 <10 230

Boron 22 0 118 70 20 980

Chromium 28 26 <10 <10 <10 10

Iron 21 1 47 40 <10 150

Manganese 7 5 <20 <20 <20 20

Silver 32 32 <2 <2 <2 <2

Zinc 21 0 164 80 20 1,300
58 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Table 14. Statistical summary of water-quality results of selected trace-element constituents for selected ground-water sites 

in the study area—Continued

[All values are for dissolved constituents, in micrograms per liter. <, less than; --, not analyzed or not determined]

Property or constituent

GW8

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 12 0 31 25 10 90

Boron 16 0 464 460 270 750

Chromium 8 6 6 <10 <10 10

Iron 15 0 3,800 3,500 420 9,200

Manganese 7 0 1,400 1,500 900 1,700

Silver 12 12 <2 <2 <2 <2

Zinc 14 0 452 225 50 1,300

Property or constituent

GW9

Number of

samples

Number of values

less than study

reporting level

Mean Median Minimum Maximum

Aluminum 15 4 20 20 <10 40

Boron 19 0 293 290 230 360

Chromium 26 23 6 <10 <10 20

Iron 17 0 340 330 110 770

Manganese 6 0 315 325 230 380

Silver 31 31 <2 <2 <2 <2

Zinc 17 0 325 230 7 1,200
Water-Quality Characteristics 59

Figure 13. Boxplots of trace-element constituent concentrations for selected ground-water sites in the study area. DISSOLVED ALUMINUM, IN MICROGRAMS PER LITER DISSOLVED ZINC, IN MICROGRAMS PER LITER

17 11 8 13 9 13 21 14 17

10

1

10,000

1,000

100

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

GW1 GW3 GW5 GW7

GW8

GW9

GW2 GW4 GW6

1

1,000

100

10

14 8 7 13 8 9 17 12 15

100

10

1

DISSOLVED CHROMIUM,

IN MICROGRAMS PER LITER

26 6 5 10 6 8 28 8 26

1

100,000

10,000

1,000

100

10

DISSOLVED IRON,

IN MICROGRAMS PER LITER

17 11 9 16 9 13 21 15 17

1

10,000

1,000

100

DISSOLVED BORON,

10

IN MICROGRAMS PER LITER

20 13 11 17 10 13 22 16 19

1

10,000

1,000

100

10

DISSOLVED MANGANESE,

IN MICROGRAMS PER LITER

6 4 6 7 6 7 7 7 6

2 1 2 2 2 3 4 4 5 1 6 5

24 5 5 9 5 6 26 6 23

2 1 1 1

2

1

11

75th percentile

Number of samples

Data value less than or equal to 1.5 times the interquartile

 range outside the quartile

Median

25th percentile

EXPLANATION

Interquartile

range

Outlier data value less than or equal to 3 and more than

 1.5 times the interquartile range outside the quartile

Outlier data value greater than 3 times the interquartile

 range outside the quartile

Individual values plotted when data are inadequate to

 construct boxplot

2 Number of samples with concentrations less than study

 reporting level

Study reporting level—Triangle indicates data less than

 study reporting level

ALL DATA BELOW STUDYREPORTING LEVEL
60 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

Figure 14. Local regression (Lowess) plots for selected ground-water sites in the study area. DISSOLVED MAGNESIUM, IN MILLIGRAMS PER LITER

0

800

400

1,600

1,200

2,000

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

DISSOLVED SODIUM,

IN MILLIGRAMS PER LITER

0

600

400

200

800

1,000

ALKALINITY, IN MILLIGRAMS PER LITER

DISSOLVED POTASSIUM,

IN MILLIGRAMS PER LITER

0

800

600

400

200

1,000

1,200

DISSOLVED SOLIDS,

SUM OF CONSTITUENTS,

IN MILLIGRAMS PER LITER

DISSOLVED SULFATE,

IN MILLIGRAMS PER LITER

0

200

150

100

50

0

50

25

75

100

0

30

20

10

40

50

0

8

10

6

4

2

12

0

240

160

80

320

400

DISSOLVED CALCIUM,

IN MILLIGRAMS PER LITER

GW1

GW2

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

DATE DATE
Water-Quality Characteristics 61

Figure 14. Local regression (Lowess) plots for selected ground-water sites in the study area.—Continued DISSOLVED BORON, IN MICROGRAMS PER LITER

0

90

60

30

120

DISSOLVED CHLORIDE,

IN MILLIGRAMS PER LITER

DISSOLVED CHROMIUM,

IN MICROGRAMS PER LITER

0

0.8

0.4

1.2

1.6

DISSOLVED FLUORIDE,

IN MILLIGRAMS PER LITER

DISSOLVED SILVER,

IN MICROGRAMS PER LITER

20

15

10

5

0

30

35

25

DISSOLVED SILICA,

IN MILLIGRAMS PER LITER

DISSOLVED ZINC,

IN MICROGRAMS PER LITER

0

100

125

75

50

25

150

0

200

150

100

50

350

300

250

0

6

3

9

12

0

3

2

1

4

0

2,000

1,500

1,000

500

3,000

2,500

DISSOLVED ALUMINUM,

IN MICROGRAMS PER LITER

1970 1975 1980 1985 1990 1995 2000 1970 1975 1980 1985 1990 1995 2000

GW1

GW2

DATE DATE
62 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

SUMMARY

This report summarizes water-quality data that 

were collected from 1973 through 2000 for the Earth 

Resources Observation Systems (EROS) Data Center, 

and presents the long-term water-quality characteristics 

and trends. A water-quality monitoring program was 

initiated in 1973 as a collaborated effort between the 

U.S. Geological Survey, Water Resources Discipline, 

and Geography (formerly National Mapping) Discipline, EROS Data Center, and continues at the present 

time (2003). Under this program, water-quality samples were collected at various sites on EROS property 

and in the surrounding area. 

The general water-quality characteristics and 

trends for selected sites within the study area were evaluated. Comparisons of surface water by water source 

and comparisons of water quality by aquifer are presented for selected sites in the EROS water-quality 

monitoring program. Comparisons were made for 

physical properties, major-ion chemistry, nutrient concentrations, and trace elements where sufficient data 

were available. Data for sites in the EROS vicinity that 

have water-quality data, but were not part of the EROS 

water-quality monitoring program, are presented on the 

CD-ROM in the back of this report.

Trend-analysis results for water-quality concentration data are presented, as are graphical output using 

Lowess regression. Results from the ESTREND trendanalysis program on Big Sioux River data showed an 

increasing trend of 0.75 percent of the median per year 

in specific conductance and a decreasing trend of 

10.26 percent of the median per year for dissolved 

orthophosphate. 

Regression analyses using a Lowess smoothing 

line indicated variable results for the surface-water 

sites, with some constituents indicating an increasing 

or decreasing trend, some having varied results, and 

others indicating no change during the time period. 

Results for Split Rock Creek indicated a decreasing 

trend for specific conductance. Results for EROS Lake 

indicated an increasing trend in alkalinity and a 

decreasing trend in aluminum, boron, and chromium. 

Little or no change was evident for silica and silver at 

EROS Lake, and although the results were variable for 

specific conductance and zinc, results indicated that 

little change has occurred over time. Results for dissolved solids, calcium, magnesium, sodium, potassium, sulfate, chloride, and fluoride indicated an 

increase from the initial low values in 1973, but with 

generally decreasing trends following 1980. Trend 

analyses for the lagoon sites also indicated varied 

results, although no indications of increasing trends 

were evident. Lowess smoothing lines indicated generally decreasing trends in specific conductance, 

dissolved solids, calcium, magnesium, sodium, potassium, sulfate, chloride, fluoride, silica, boron, and chromium. Trend results indicated little or no change for 

silver in the lagoon sites. Alkalinity, aluminum, and 

zinc results were variable, but indicated little change 

during the time period analyzed.

Trend analyses for representative ground-water 

sites indicated mixed results. Results for the glacial 

aquifer site indicated increasing trends for dissolved 

solids, calcium, magnesium, sodium, potassium, sulfate, and boron. Results indicated generally decreasing 

trends for chloride, fluoride, silica, and zinc. Trend 

results were variable but generally indicated little 

change during the time period analyzed for specific 

conductance, alkalinity, aluminum, chromium, and 

silver.

Trend results for the Sioux Quartzite aquifer site 

indicated slightly increasing trends for fluoride, silica, 

boron, and zinc, whereas results for potassium, aluminum, and chromium indicated slightly decreasing 

trends. Little or no change was evident in trend results 

for specific conductance, alkalinity, dissolved solids, 

calcium, magnesium, sodium, chloride, and silver. 

Results were variable for sulfate for the Sioux Quartzite 

aquifer site. 

SELECTED REFERENCES

Alexander, R.B., Ludtke, A.S., Fitzgerald, K.K., and 

Schertz, T.L., 1996, Data from selected U.S. Geological 

Survey National Stream Water-Quality Monitoring 

Networks (WQN) on CD-ROM: U.S. Geological 

Survey Open-File Report 96-337, accessed on June 19, 

2001, at URL http://water.usgs.gov/pubs/dds/wqn96cd/ 

html/report/contents.htm

Bradford, W.L., 1981a, Water-level records for the Big Sioux 

aquifer, Minnehaha County, South Dakota: U.S. 

Geological Survey Open-File Report 81-222, 50 p. 

———1981b, Records of water levels in unconsolidated 

deposits in eastern South Dakota: U.S. Geological 

Survey Open-File Report 81-924, 253 p.

Brown, Eugene, Skougstad, M.W., and Fishman, M.J., 1970, 

Methods of collection and analysis of water samples for 

dissolved minerals and gases: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 5, 

chap. A1, 160 p.
Selected References 63

Cleveland, W.S., 1979, Robust locally weighted regression 

and smoothing scatterplots: Journal of American Statistical Association, v. 74, no. 368, p. 829-836.

Cohen, A.C., Jr., 1976, Progressively censored sampling in 

the three parameter log-normal distribution: Technometrics, v. 18, no. 1, p. 99-103.

Cohn, T.A., 1988, Adjusted maximum likelihood estimation 

of the moments of the parameters of lognormal populations from type I censored samples: U.S. Geological 

Survey Open-File Report 88-350, 34 p.

Cole, G. A., 1994, Textbook of Limnology (4th ed.): 

Prospect Heights, Ill., Waveland Press, Inc., 412 p.

Edwards, T.K., and Glysson, G.D., 1988, Field methods for 

measurement of fluvial sediment: U.S. Geological 

Survey Open-File Report 86-531, 118 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Heakin, A.J., 1998, Water-quality trends for the Cheyenne 

and Moreau Rivers, Cheyenne River Indian Reservation, South Dakota, 1972-94: U.S. Geological Survey 

Water-Resources Investigations Report 98-4092, 56 p.

Heath, R.C., 1983, Basic ground-water hydrology: U.S. 

Geological Survey Water-Supply Paper 2220, 84 p.

Helsel, D.R., 1987, Advantages of nonparametric procedures 

for analysis of water quality data: Journal of Hydrological Sciences, v. 32, no. 2, p. 179-189.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Amsterdam, Netherlands, Elsevier 

Science, 522 p.

Hem, J.D., 1992, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

Hirsch, R.M., Slack, J.R., and Smith, R.A., 1982, Techniques 

of trend and analysis for monthly water-quality data: 

Water Resources Research, v. 18, no. 6, p. 197-121.

Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., 

and Rickert, D.A., 1994, U.S. Geological Survey protocol for collection and processing of surface-water samples for the subsequent determination of inorganic 

constituents in filtered water: U.S. Geological Survey 

Open- File Report 94-539, 57 p. 

Kendall, M.G., 1975, Rank correlation methods (4th ed.): 

London, Charles Griffin, 202 p.

Koch, N.C., 1982, A digital-computer model of the Big 

Sioux aquifer in Minnehaha County, South Dakota: 

U.S. Geological Survey Water-Resources Investigations 

Report 82-4064, p. 49.

———1983, Evaluation of the response of the Big Sioux 

aquifer to extreme drought conditions in Minnehaha 

County, South Dakota: U.S. Geological Survey WaterResources Investigations Report 83-4234, 6 p.

———1984, Simulated artificial recharge in the Big Sioux 

aquifer in Minnehaha County, South Dakota: U.S. 

Geological Survey Water-Resources Investigations 

Report 84-4312, 8 p.

Lindgren, R.J., and Niehus, C.A., 1992, Water resources of 

Minnehaha County, South Dakota: U.S. Geological 

Survey Water-Resources Investigations Report 91-

4101, 80 p.

Maddy, D.V., Lopp, L.E., Jackson, D.L., Coupe, R.H., and 

Schertz, T.L., 1990, National Water Information 

System user’s manual, v. 2, chap. 2. Water-quality 

system: U.S. Geological Survey Open-File Report 

89-619 [variously paged].

Mann, H.B., 1945, Non-parametric test against trend: 

Econometrica, v. 13, p. 245-259.

Myers, R.H., 1986, Classical and modern regression with 

applications: Boston, Duxbury Press, 359 p.

Niehus, C.A., and Lindgren, R.J., 1994, Major aquifers in 

Minnehaha County, South Dakota: South Dakota 

Geological Survey Information Pamphlet no. 46, 22 p. 

Ohland, G.L., 1987, Appraisal of the water resources of the 

Skunk Creek aquifer in Minnehaha County, South 

Dakota: U.S. Geological Survey Water-Resources 

Investigations Report 87-4156, p. 54.

———1990, Appraisal of the water resources of the Skunk 

Creek aquifer in Minnehaha County, South Dakota: 

U.S. Geological Survey Water-Resources Investigations Report 87-4156, 54 p.

Piper, A.M., 1944, A graphical procedure in the geochemical interpretation of water analyses: American 

Geophysical Union Transactions, v. 25, p. 914-923.

Putnam, L.D., 1998, Hydrogeology of the Split Rock Creek 

aquifer with emphasis on calibration of a numerical 

flow model, southeast Minnehaha County, South 

Dakota: U.S. Geological Survey Water-Resources 

Investigations Report 97-4102, 39 p.

Schertz, T.L., Alexander, R.B., and Ohe, D.J., 1991, The 

computer program estimate trend (ESTREND), a 

system for the detection of trends in water-quality 

data: U.S. Geological Survey Water-Resources Investigations Report 91-4040, 72 p.

South Dakota Legislative Research Council, 2001, Surface 

water quality standards: chap. 74:51:01 accessed 

May 4, 2001, at URL http://legis.state.sd.us/ 

rules/rules/7451.htm#74:51:01:01

Spuhler, W., Lytle, W.F., and Moe, D., 1971, Climate of 

South Dakota: Brookings, South Dakota, Agricultural 

Experiment Site, South Dakota State University, 

Bulletin 582.

U.S. Department of the Interior, 1998, Guidelines for the 

interpretation of the biological effects of selected constituents in biota, water, and sediment: Denver, Colo., 

National Irrigation Water Quality Program Information Report No. 3., 198 p.

U.S. Environmental Protection Agency, 2002, National 

Primary Drinking Water Standards: accessed 

December 11, 2002, at URL 

http://www.epa.gov/safewater/mcl.html
64 Water-Quality Characteristics and Trends for Selected Sites in or near the EROS Data Center, South Dakota

U.S. Geological Survey, 1974-2001, Water resources data for 

South Dakota, water years 1973-2000: U.S. Geological 

Survey Water Data Reports SD-73-1 to SD-00-1 (published annually).

———1985, National water summary 1984—Hydrologic 

events, selected water-quality trends, and ground-water 

resources: U.S. Geological Survey Water-Supply Paper 

2275, p. 385-390.

———1986, National water summary 1985—Hydrologic 

events and surface-water resources: U.S. Geological 

Survey Water-Supply Paper 2300, p. 419-424.

———2002, Virtual tour of the USGS EROS Data Center, 

Sioux Falls, SD: accessed September 27, 2002, at URL 

http://edc.usgs.gov/about/tour/tour.html

Ward, J.R., and Harr, C.A., eds., 1990, Methods for collection and processing of surface-water and bed-material 

samples for physical and chemical analyses: U.S. 

Geological Survey Open-File Report 90-140, 71 p.

Wells, F.C., Gibbons, W.J., and Dorsey, M.E., 1990, Guidelines for collection and field analysis of water-quality 

samples for streams in Texas: U.S. Geological Survey 

Open-File Report 90-127, 79 p.

Winter, D.R., 1983, Records of water levels in the Big Sioux 

aquifer, Minnehaha County, South Dakota: U.S. 

Geological Survey Open-File Report 82-1020, 25 p.

Yohai, V.J., 1987, High breakdown point and high efficiency 

of robust estimates for regression: The Annals of Statistics, v. 15, p. 642-656.