U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3393

Sheet 1 of 2

Delineation of the Hydrogeologic Framework of the Big Sioux Aquifer near Sioux Falls, South Dakota, Using Airborne Electromagnetic Data

By Kristen J. Valseth, Gregory C. Delzer, and Curtis V. Price

2018

Abstract

The U.S. Geological Survey, in cooperation with the City of Sioux Falls, South Dakota, began developing a groundwater-fl ow model of the Big Sioux aquifer in 2014 that will enable the City to make more 

informed water management decisions, such as delineation of areas of the greatest specifi c yield, which 

is crucial for locating municipal wells. Innovative tools are being evaluated as part of this study that can 

improve the delineation of the hydrogeologic framework of the aquifer for use in development of a groundwater-fl ow model, and the approach could have transfer value for similar hydrogeologic settings. The fi rst 

step in developing a groundwater-fl ow model is determining the hydrogeologic framework (vertical and 

horizontal extents of the aquifer), which typically is determined by interpreting geologic information from 

drillers’ logs and surfi cial geology maps. However, well and borehole data only provide hydrogeologic 

information for a single location; conversely, nearly continuous geophysical data are collected along fl ight 

lines using airborne electromagnetic (AEM) surveys. These electromagnetic data are collected every 3 

meters along a fl ight line (on average) and subsequently can be related to hydrogeologic properties. AEM 

data, coupled with and constrained by well and borehole data, can substantially improve the accuracy of 

aquifer hydrogeologic framework delineations and result in better groundwater-fl ow models. 

AEM data were acquired using the Resolve frequency-domain AEM system to map the Big Sioux 

aquifer in the region of the city of Sioux Falls. The survey acquired more than 870 line-kilometers of AEM 

data over a total area of about 145 square kilometers, primarily over the fl ood plain of the Big Sioux River 

between the cities of Dell Rapids and Sioux Falls. The U.S. Geological Survey inverted the survey data 

to generate resistivity-depth sections that were used in two-dimensional maps and in three-dimensional 

volumetric visualizations of the Earth resistivity distribution. Contact lines were drawn using a geographic 

information system to delineate interpreted geologic stratigraphy. The contact lines were converted to 

points and then interpolated into a raster surface. The methods used to develop elevation and depth maps of 

the hydrogeologic framework of the Big Sioux aquifer are described herein.

The fi nal AEM interpreted aquifer thickness ranged from 0 to 31 meters with an average thickness 

of 12.8 meters. The estimated total volume of the aquifer was 1,060,000,000 cubic meters based on the 

assumption that the top of the aquifer is the land-surface elevation. A simple calculation of the volume 

(length times width times height) of a previous delineation of the aquifer estimated the aquifer volume at 

378,000,000 cubic meters; thus, the estimation based on AEM data is more than twice the previous estimate. The depth to top of Sioux Quartzite, which ranged in depth from 0 to 90 meters, also was delineated 

from the AEM data.

Introduction

The city of Sioux Falls is in the southeastern corner of South Dakota at the junction of Interstate 90 

and Interstate 29 (fi g. 1). Sioux Falls is a rapidly growing metropolitan area with a population of about 

172,000 as of 2015. Between 2000 and 2010, the population of Sioux Falls increased 24 percent, and an 

11 percent population increase was estimated between 2010 and 2015 (U.S. Census Bureau, 2015). The 

population growth, combined with a desire to attract new industries to the city, has motivated the City of 

Sioux Falls to reappraise the local groundwater resources, including the Big Sioux aquifer.

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-96°48' -96°46' -96°44' -96°42'

-96°48' -96°46' -96°44' -96°42'

43°48'

43°46'

43°44'

43°42'

43°40'

43°38'

43°36'

43°34'

43°48'

43°46'

43°44'

43°42'

43°40'

43°38'

43°36'

43°34'

SOUTH DAKOTA

Study 

area

Sioux Falls

Dell Rapids

Big Sioux R

Big

Baltic

§ ̈¦I-90

§ ̈¦I-29

Sioux Falls Regional Airport boundary

Direct current resistivity transect

Line of section (fig. 2)

Airborne electromagnetic delineated section

( South Dakota Department of Environment and Natural 

Resources well

Map image is the intellectual property of Esri and is used 

herein under license. Copyright © 2017 Esri and its 

licensors. All rights reserved.

Web Mercator Auxiliary Sphere, 1984

0 1.5 3 KILOMETERS

0 0.75 1.5 MILES

A A'

A A'

SIOUX FALLS

REGIONAL

AIRPORT

DITCH ROAD

Sioux R

EXPLANATION

Figure 1. The study area, Sioux Falls Regional Airport, cross section A–A’, airborne electromagnetic delineated cross sections, 

direct current resistivity transects, and South Dakota Department of Environmental and Natural Resources wells, South Dakota.

The U.S. Geological Survey (USGS), in cooperation with the City of Sioux Falls, South Dakota, began 

developing a groundwater-fl ow model of the Big Sioux aquifer in 2014 that will enable the City to make 

more informed water management decisions, such as delineation of areas of the greatest specifi c yield, 

which is crucial for locating municipal wells. Innovative ArcGIS (Esri, 2017) tools are being evaluated as 

part of this study that can improve the delineation of the hydrogeologic framework of the aquifer for use in 

development of a more detailed groundwater-fl ow model, and the approach could have transfer value for 

similar hydrogeologic settings. Groundwater-fl ow models also support the development of future monitoring networks, help to understand hydrologic responses to potential groundwater management scenarios and 

climate extremes, determine potential contaminant source and transport, and determine aquifer storage and 

sustainable development. Groundwater-fl ow models that start with improved aquifer geometries and properties, such as those derived from airborne electromagnetic (AEM) surveys, can provide the best information possible to various water-resource managers well into the future.

Traditionally, aquifer boundaries for groundwater-fl ow models are determined by interpolation of 

surface observations of geology, recorded well and borehole geology, and geophysical logs (Arihood, 

2008; Casey, 1996; Rothrock and Otton, 1947; Thamke and others, 2014). The well geology and geophysical data used in this method only provide hydrogeologic information at a single location, which could 

be the only point of information for several square miles. The Big Sioux aquifer is composed of material 

deposited during glacial retreat and advancement, which poses a challenge for mapping buried aquifer 

channels because these channels cannot be identifi ed from surface geology (Valder and others, 2016). In 

the last two decades, geophysical techniques, such as AEM surveys, are increasingly being used to support 

groundwater-fl ow modeling efforts around the world (Abraham and others, 2012; Ball and others, 2011, 

2015; Christiansen and Christensen, 2003; Danielsen and others, 2003). AEM surveys offer nearly continuous measurements that can be used to map alluvial aquifer boundaries and improve estimates of aquifer 

properties, such as hydraulic conductivity. This information, coupled with well and borehole records, can 

improve the accuracy of the delineated geometry of aquifer boundaries for input into a groundwater-fl ow 

model. By improving the groundwater-fl ow model, more accurate and representative information of the 

aquifer can be obtained to make informed decisions on placement of future wells and sustainable water 

withdrawal rates.

To obtain the most accurate data possible to inform a groundwater-fl ow model of the Big Sioux aquifer, the City of Sioux Falls contracted with CGG Canada Services to collect AEM data in 2015. This fl ight 

was the fi rst application of AEM in South Dakota to characterize water resources. Overall, 870 line-kilometers were fl own, and 145 cross-section profi les were generated. A map of the study area depicting the 

location of the collected AEM data as well as nearby cities, wells, interstates, and rivers are shown in fi gure 

1. CGG provided the fi nal AEM dataset to the City and the USGS Crustal Geophysics and Geophysical 

Science Center, who inverted the data using the program EM1DFM (Farquharson, 2000) to produce resistivity depth sections for three-dimensional visualization of subsurface electrical variations. A data release 

document (Smith and others, 2018) is available that contains raw and processed AEM and magnetic data, 

processed AEM inversions, and a contractor’s report describing survey parameters. 

The purpose of this map is to describe and show the delineation of the hydrogeologic framework Big 

Sioux aquifer near Sioux Falls, South Dakota, using AEM methods. The use of innovative ArcGIS (Esri, 

2017) tools has provided a qualitative assessment of the hydrogeologic framework and a lower cost solution for characterizing alluvial aquifers for future projects with other municipalities. The methods described 

herein, combined with the developed ArcGIS tools, offer vastly improved aquifer delineation compared to 

traditional methods.

Geology and Hydrogeology of the Big Sioux Aquifer

The Big Sioux aquifer, for the most part, underlies the Big Sioux River fl ood plain, which meanders 

north to south in eastern South Dakota. The section of the Big Sioux aquifer of interest for this map is situated between Dell Rapids and Sioux Falls (fi g. 1). The study area is about 32.2 kilometers (km) by 4.5 km 

(Valder and others, 2016). The region is characterized by hilly topography, generated by at least two periods 

of glacial advance and retreat, that varies from smooth rolling plains to very rough knobby hills (Rothrock 

and Otton, 1947). The geologic units of interest within the study area include Quaternary-age alluvial material (alluvium), Quaternary-age glacial till, and Precambrian-age Sioux Quartzite. The alluvium within the 

study area is composed of clay, silt, sand, gravel, boulder-size clasts, and locally abundant organic material 

(U.S. Geological Survey, 2015). Glacial till includes tills from several glacial periods, such as Pre-Illinoian 

1, Pre-Illinoian 2, Pre-Illinoian 3, and Late Wisconsin (Tomhave, 1994). Till layers have been characterized 

as heterogeneous “clay with silt to boulder-size clasts of glacial origin” (U.S. Geological Survey, 2015). 

Because of the similarity in composition, differentiation between the various till layers was not attempted 

for this study and these till layers are referred to as the glacial till layer. The glacial till layer is not entirely 

impervious but will retain water on its surface for a considerable time (Rothrock and Otton, 1947); consequently, the glacial till layer is considered the bottom of the Big Sioux aquifer, except for a few locations 

where the till was eroded down to the underlying Sioux Quartzite. The Sioux Quartzite has been described 

as pink or reddish- to tan-colored, siliceous, iron-stained orthoquartzite (sandstone cemented by silica) 

with mudstone and minor conglomerate layers (U.S. Geological Survey, 2015). The Sioux Quartzite is so 

thoroughly lithifi ed that the pore spaces of the sandstone have almost entirely fi lled with cement, which 

causes the geologic layer to be impermeable (Rothrock and Otton, 1947). If glacial till is not present, then 

the Sioux Quartzite acts as an impermeable bottom of aquifer layer. Because the two different geologies act 

as the bottom of aquifer, it was necessary generate two AEM-delineated raster surfaces for input into the 

groundwater-fl ow model.

Groundwater resources include both glacial and Precambrian aquifers (Lindgren and Niehus, 1992). 

The glacial aquifers are composed predominantly of unconsolidated sand and gravel glacial outwash. The 

Big Sioux and Spilt Rock Creek aquifers are the only glacial aquifers within the study area; however, the 

aquifers have no known hydraulic connection. The Brandon, Skunk Creek, Pipestone Creek, Beaver Creek, 

and Slip Up Creek glacial aquifers are nearby but do not affect the study area. The Precambrian aquifer in 

the study area is contained in the Sioux Quartzite, which is a locally well-fractured and jointed crystalline 

rock that is used as a source of water in western and east-central Minnehaha County (Lindgren and Niehus, 

1992). The amount of water contained within the Precambrian aquifer in the Sioux Quartzite is unknown 

because the depth and development of the fracture system in the aquifer is not known (Lindgren and Niehus, 1992).

Previous Work on the Big Sioux Aquifer

Rothrock and Otton (1947) described an investigation carried out by the USGS and the South Dakota 

State Geological Survey (SDGS) in cooperation with the City of Sioux Falls. The volume and shape of 

the Big Sioux aquifer was determined primarily from test wells drilled for the investigation. Eight lines of 

wells were in the Big Sioux River Basin. The bottom of the aquifer was drawn by linearly interpolating 

between the points where information was given by the various wells (Rothrock and Otton, 1947). The 

hydrogeologic framework results from the Rothrock and Otton (1947) study present the Big Sioux aquifer 

as a body of sand and gravel averaging 12.2 meters (m) deep, 3.2 km wide, and 29 km long. It is resting 

in a trough with a clay bottom and sides with bedrock (Sioux Quartzite) boundaries at the north and south 

end. A simple calculation of the volume (length times width times height) of the delineated aquifer from 

Rothrock and Otton (1947) estimated the aquifer volume at 378,000,000 cubic meters (m3), assuming the 

land surface as the top of aquifer.

A digital model of the Big Sioux aquifer was constructed in 1982 (Koch, 1982). The model used a 0.4-

km grid network of 77 rows and 18 columns, which resulted in a total of 585 cells within the aquifer. The 

hydrogeologic framework was based on the known thicknesses of sand and gravel deposits from available 

well logs. Based on the well data collected by Koch (1982), the Big Sioux aquifer ranged in thickness from 

1.2 to 14.6 m.

O’Connor (2017) delineated the Big Sioux aquifer using a subset of the same AEM data collected by 

CGG used in the study described in this report. The thesis only delineated the bottom of the aquifer, not the 

Sioux Quartzite, and did not delineate the study area south of Interstate 90, which is included in this study. 

70 to 130

20 to 40

10 to 20

20 to 40

70 to 130

Resistivity ,

in ohm-meters

Depth on profile,

in meters

2.0

4.4

24.4

26.5

1,000 to 10,000

500 to <1,000

200 to <500

150 to <200

130 to <150

100 to <130

70 to <100

50 to <70

40 to <50

30 to <40

20 to <30

15 to <20

10 to <15

7 to <10

5 to <7

2 to <5

1 to <2

A A'

Not to scale 

475

450

425

400

375

350

325

300

A

METER

0 0.5 1.0 KILOMETERS

0 0.25 0.5 MILES

Well R3–38–89

Big

Sioux River

Resistivity, in ohm-meters

DATUM IS GEODETIC REFERENCE SYSTEM 1980

VERTICAL EXAGGERATION ×13

EXPLANATION

Airborne electromagnetic delineation of the

bottom of the Big Sioux aquifer

Well distance from cross section, in meters

0 to 25.0

25.1 to 50.0

50.1 to 100.0

100.1 to 150.0

150.1 to 300.0

300.1 to 1,00.0

Figure 2. Example of relating resistivity to lithology of well (well number R3–38–89) within 25 meters of a resistivity profile, Big Sioux aquifer, South Dakota.

The fi nal AEM hydrogeologic framework ranged from 0 to 36.5 m, and the average thickness was 12.3 m. 

The total volume of the delineated aquifer was 771,000,000 m3 based on the assumption that the top of the 

aquifer is the land surface elevation (O’Connor, 2017).

Airborne Electromagnetic Methods

AEM surveys have been used for several different purposes. Examples include the measurement of the 

depth of permafrost in Alaska (Ball and others, 2011), the distribution of brine in groundwater in Colorado 

and Montana (Ball and others, 2015; Thamke and Smith, 2014), and the mapping of the lower boundaries 

of alluvial aquifers in Nebraska (Abraham and others, 2012). AEM surveys are carried out by either helicopter or fi xed-wing aircraft to characterize aquifers and their geologic setting. AEM systems operate by 

transmitting radio-frequency signals that interact with the underlying Earth, inducing secondary currents. 

The secondary currents are a function of the electrical resistivity of the subsurface materials. Materials, 

such as soils and rocks, have an intrinsic resistivity that controls the relation between the current density 

and the gradient of the electrical potential (U.S. Environmental Protection Agency, 2016). The electrical 

resistivity of subsurface geology is affected by the water content and quantity of metallic mineralization, 

clay, and gravel in the subsurface materials (Abraham and others, 2012). Variations in these properties, in 

any direction, create variations in the resistivity and, thus, the resulting induced current and the electric 

potential distribution.

Two types of AEM methods are used to measure the secondary (eddy) currents induced by the transmitted radio-frequency signals. One is the frequency-domain method, in which the signal is a continuous 

sinusoidal wave and measurements are taken at the same frequency of the original signal (Abraham and 

others, 2012; Ball and others, 2015). The second method is the time-domain method, in which the radio-frequency signal is a string of pulses and measurements are taken immediately after the pulse is turned off 

as a function of time. The investigation depth becomes a function of the time interval between pulses and 

decay rate based on ground conductivity (Lange and Seidel, 2007). Desired investigation depth and level of 

human development in an area generally determines which of the two methods to use at a particular study 

area.

To determine the suitability of the Big Sioux aquifer area for use of electromagnetic methods and to 

help determine which of the two AEM methods to use, the USGS investigated direct current (DC) resistivity along fi ve transects (fi g. 1) in April 2015. Electromagnetic methods rely on distinctive contrasts in 

conductivities (reciprocal of resistivity) in overlying geologic layers to differentiate between them. Results 

of the DC resistivity investigation revealed distinctive resistivity ranges for each geologic layer that did not 

overlap, which supported the use of electromagnetic methods. Results from the DC resistivity investigation 

estimated the alluvial deposits to have resistivities of between 30 and 70 ohm-meters (ohm-m), and the 

underlying glacial till was generally less than 10 ohm-m.

Based on the results of the preliminary resistivity investigation of the study area and the advantages 

and disadvantages of the two AEM methods, a frequency domain airborne electromagnetic survey was 

selected to survey the aquifer (Valder and others, 2016; O’Connor, 2017). A helicopter was equipped with 

the Resolve frequency domain system (Smith, 2010), which has the capability to image resistivity, dependent on resistivity of materials, to depths of about 80 m (Abraham and others, 2012). The electromagnetic 

measurements were made at six frequencies ranging from about 400 hertz to about 140,000 hertz, operating 

with paired transmitter and receiver coils contained in a “bird” (torpedo shaped instrument) suspended from 

the helicopter about 38 m above the ground.

Once the AEM frequency data were collected and preprocessed, they were then numerically inverted 

into depth-dependent subsurface resistivity cross sections, also referred to as resistivity-depth sections. Preprocessing included removing noise because of sferics (naturally occurring atmospheric signals), fi ltering 

out system noise (system noise is recorded during fl ight for this purpose), correcting drift, and leveling to 

correct signal levels for elevation variations.

Resistivity depth-sections and geo-referencing data were exported from Geosoft Oasis montaj (Geosoft, 2017). Python script tools were used to (1) import images into ArcGIS Desktop software, (2) convert 

profi le line data to geographic line features with route measures (along profi le), (3) reference profi le images 

to “X-Z space” (X-axis, profi le distance; Z-axis, elevation), and (4) use the ArcGIS Desktop software 

“Near” analysis tool to calculate the perpendicular distance between wells and fl ight lines. Well data were 

retrieved online from the Lithologic Logs Database (South Dakota Department of Environment and Natural 

Resources, 2016). Wells were categorized into six bins based on the perpendicular distance from fl ight 

lines. The six bins were 0–25 m, 25–50 m, 50–100 m, 100–150 m, 150–300 m, and 300–1,000 m. The 

bottoms of all wells (South Dakota Department of Environment and Natural Resources, 2016) within a 

1,000-m radius were plotted on the profi le images. The predominant resistivity range and color determined 

from well lithology was used to qualitatively locate the till and quartzite. Emphasis was given to wells with 

a perpendicular distance of 0–25 m and 25–50 m from fl ight lines.

An example of relating the resistivity of a profi le to geology with a well that is within 25 m (hexagon with dot in the middle) is shown in fi gure 2. The profi le on the far left of fi gure 2 is resistivity-depth 

cross section. On the far right of fi gure 2 is a scale depicting the resistivity ranges of the colors on the 

resistivity-depth cross section. Located between the resistivity-depth cross section and the resistivity scale 

is a diagram of the example well that shows the depth (not to scale) of where the resistivity range (color) 

changes. The lithology of the well shown on the profi le is listed in table 1. According to the lithologic log, 

example well R3–38–89 completely extends through the glacial till to the Sioux Quartzite, which made 

it an ideal well to relate the resistivity of a profi le to known geology. By noting where the resistivity and 

lithology changed, target geologic units were associated to a resistivity range. In fi gure 2 the “top soil” and 

“sand, fi ne” geologic layers were indistinguishable from each other, which was expected. According to the 

lithologic log in table 1, the lithology changed from “sand, fi ne” to “clay, brown, sandy, pebbly (till)” at 3.0 

m, which is in the 20–40 ohm-m (light yellow/dark yellow) interval, immediately before it starts changing 

to 10–20 ohm-m (light green/green). The glacial till layer extends to 26.2 m, which is where the resistivity 

changes from 10–20 ohm-m (light green/green) to 20–40 ohm-m (light yellow/dark yellow). This extension indicates that the glacial till layer is associated with the 10–20 ohm-m (light green/green) resistivity 

interval. This well ends a few centimeters into the Sioux Quartzite at 26.3 m, which is where the resistivity 

changes from 20–40 ohm-m (light yellow/dark yellow) to 70–130 ohm-m (red). This associates the Sioux 

Quartzite with the 70–130 ohm-m (red) resistivity but the Sioux Quartzite unit rapidly increases in resistivity, to as much as 1,000 ohm-m.

The ArcGIS Desktop software “Editor” tool was used to digitize hydrogeologic and geologic contacts (in X-Z space) at the selected resistivity ranges to delineate the top of till and top of Sioux Quartzite. 

Because the till and Sioux Quartzite act as the bottom of the aquifer, both geologic units needed to be 

delineated for the groundwater-fl ow model. The delineation (black line on resistivity cross section in fi g. 2) 

of the top till was drawn as a bottom of the aquifer layer. The Sioux Quartzite delineation was drawn as the 

top of Sioux Quartzite. Python script tools converted X-Z profi le lines to XYZ points. All the profi le XYZ 

points were then appended to a single feature layer that was then converted to a raster fi le using the 3D 

Analyst ArcGIS tool “IDW” (Inverse Distance Weighted). The resulting raster fi le is referred to as the “bottom of aquifer raster surface.” This process was duplicated for the delineation of the top of Sioux Quartzite. 

The resulting raster fi le is referred to as the “top of Sioux Quartzite raster surface.”

One of the challenges in the use of AEM data was cultural interference. Manmade structures, such as 

railroads, buried pipelines, well casings, and other metallic objects, often produce signals that jumble or 

complicate electromagnetic signals because of the irregular three-dimensional responses of these objects 

(Watson and others, 2001). Because of the presence of roads, utilities, railroads, wells, and other highly 

conductive structures in the study area, cultural interference was identifi ed on 55 of the profi les that required 

correction. Profi le correction consisted of linearly interpolating from the point at the start of the cultural 

interference to the end of the cultural interference. The cultural interference was particularly evident at Sioux 

Falls Regional Airport (about 4 km north of Sioux Falls) and the railroad track following Ditch Road South 

from Baltic, South Dakota, toward Sioux Falls (fi g. 1). The Sioux Falls Regional Airport had substantial cultural interference from wells and other manmade structures, and from operational radio-frequency transmitters. More than 30 percent of the AEM data collected within the airport area was unusable for interpretation. 

To increase the number of elevation data points in the airport area, well lithology was used to locate points of 

top of till and top of Sioux Quartzite. These points were combined with the AEM data to generate the raster 

fi les, “bottom of aquifer raster surface” and “top of Sioux Quartzite raster surface.”

The aquifer extents (east and west boundaries) were user-defi ned by examining the resistivity 

ranges associated with till and aquifer materials to determine where the aquifer ended, and marking 

the location with a point feature class on the profi le view that was plotted on an aerial view map of the 

study area.

Once the initial interpretations were completed, a group of groundwater hydrologists reviewed the 

interpretations and selected aquifer boundaries. Each profi le was re-examined with information such as 

fl ood plain boundaries, river location, cultural interference, and other AEM profi les nearby to see if the 

interpretation or aquifer boundaries should be modifi ed. If any changes were warranted, notes were made 

on the profi le with what needed to change and the reasons they were changed, and then they were modifi ed 

manually in ArcGIS. Final raster surfaces were generated once all modifi cations were completed. An aquifer boundary polygon was drawn by connecting each revised extent pick point sequentially using the “edit 

tool” in ArcGIS. This aquifer boundary was used to constrain the new bottom of the aquifer raster surface 

to the AEM observed east and west boundary extents.

In addition to the generation of elevation maps of the bottom of the aquifer and the top of Sioux 

Quartzite, thickness of the aquifer and depth to top of Sioux Quartzite maps were generated. Because the 

Big Sioux aquifer is an alluvial aquifer, the top of the aquifer was assumed to be the land surface elevation. 

A land surface elevation raster layer was created from 1/9 arc-second (3 m) elevation data from the USGS 

National Map (U.S. Geological Survey, 2017). The thickness of the aquifer and depth to top of Sioux 

Quartzite maps were calculated by subtracting the bottom of the aquifer elevations and top of Sioux Quartzite elevations from the land surface elevation.

A few elevation discrepancies in top of Sioux Quartzite maps required correction or discussion. 

Because of the presence of Sioux Quartzite outcrops in the area and observed small errors in delineation, 

plotting, or both, the top of Sioux Quartzite raster surface was corrected using the ArcGIS “Raster Calculator” and “Mosaic” tools. This correction helped ensure that the top of Sioux Quartzite is never at a higher 

elevation than the land surface elevation raster, and that the outcropping of Sioux Quartzite matches the 

land surface elevation raster. It is important to note that there are areas in the map that have a lower level of 

confi dence because of cultural interference and steep drop-offs of Sioux Quartzite that are beyond the AEM 

investigation depth. These areas of lower level confi dence were not delineated, but the raster surface in 

these areas was generated using interpolation.

Delineation Results

The maps shown in fi gure 3 illustrate the delineated elevation of the bottom of the Big Sioux aquifer 

(fi g. 3A) and the delineated thickness of the Big Sioux aquifer within the study area (fi g. 3B). The total 

volume of the Big Sioux aquifer was estimated to be 1,060,000,000 m3 based on the assumption that the 

land surface elevation is the top of the aquifer. The surface area of the aquifer was estimated as 85,000,000 

square meters. The predicted aquifer thickness ranged from 0 to 31 m, and the average thickness was 12.8 

m. At the north end of the study area (townships 103N and 104N), the bottom of the aquifer maps reveal 

thick deposits confi ned to narrow channels that quickly thin out at the edges. At the south end of the study 

area (townships 101N and 102N), the aquifer extents widen and are characterized by thinner deposits with 

pockets of thicker deposits (fi g. 3B).

The maps shown in fi gure 4 illustrate the delineated elevation of the top of Sioux Quartzite (fi g. 4A) and 

the delineated depth to the top of the Sioux Quartzite (fi g. 4B). Depth to top of Sioux Quartzite was predicted 

to range from 0 to 90 m. The top of Sioux Quartzite maps reveal Sioux Quartzite near the land surface at 

the north end of the study area, but glacial till layers become thicker toward the middle and south end of the 

study area, burying the Sioux Quartzite as much as 90 m in depth. Lower confi dence areas (cultural interferences and steep drop-offs) are outlined with hatching in the fi nal top of Sioux Quartzite maps (fi g. 4).

A three-dimensional fence diagram of the delineated hydrogeologic framework of the Big Sioux aquifer, represented as cross-sectional slices running north to south and east to west, is shown in fi gure 5. The 

changes of depth in the alluvium, glacial till, and Sioux Quartzite layers throughout the study area can be 

visualized in a single fi gure.

Table 1. Lithologic log of example well in figure 1 (well number 

R3–38–89), Big Sioux aquifer, South Dakota.

Depth, in meters Lithology description

0.3–0.6 Topsoil, black.

0.6–3.0 Sand, fi ne.

3.0–10.7 Clay, brown, sandy, pebbly (till).

10.7–26.2 Clay, gray, sandy, pebbly (till).

26.2–26.3 Quartzite; only few centimeters penetrated.

Prepared in cooperation with the 

City of Sioux Falls, South Dakota

Bottom of Big Sioux aquifer elevation, in meters 

above Geodetic Reference System 1980

408 to 414

415 to 420

421 to 426

427 to 432

433 to 438

439 to 444

445 to 450

451 to 457

EXPLANATION

-96°46'

-96°44'

-96°42'

43°48'

43°46'

43°44'

43°42'

43°40'

43°38'

43°36'

0 0.75 1.5 MILES

0 1.5 3 KILOMETERS

R. 50 W. R. 49 W.

T. 101 N.

T. 102 N.

T. 103 N.

T. 103 N.

T. 104 N.

T. 102 N.

A

Base modified from the Bureau of Land Management 

Geographic Coordinate Data Base coordinate data

Albers Equal-Area Conic projection

EXPLANATION

Big Sioux aquifer thickness, in meters

0 to 4

5 to 8

9 to 12

13 to 16

17 to 20

21 to 23

24 to 27

28 to 31

-96°46'

-96°44'

-96°42'

0 1.5 3 KILOMETERS

0 0.75 1.5 MILES

43°44'

43°42'

43°40'

43°38'

43°36'

43°46'

43°48'

R. 50 W. R. 49 W.

T. 101 N.

T. 102 N.

T. 102 N.

T. 103 N.

T. 103 N.

T. 104 N.

B

Base modified from the Bureau of Land Management 

Geographic Coordinate Data Base coordinate data

Albers Equal-Area Conic projection

Figure 3. Delineated elevation and thickness of the Big Sioux aquifer, South Dakota. A, elevation of the bottom of the Big Sioux aquifer. B, thickness of the Big Sioux aquifer.

-96°46'

-96°44'

-96°42'

43°48'

43°46'

43°44'

43°42'

43°40'

43°38'

43°36'

0 1.5 3 KILOMETERS

0 0.75 1.5 MILES

T. 101 N.

R. 50 W. R. 49 W.

T. 102 N.

T. 102 N.

T. 103 N.

T. 103 N.

T. 104 N.

Lower confidence area

Big Sioux aquifer boundary

Top of Sioux Quartzite elevation, in meters 

above Geodetic Reference System 1980

EXPLANATION

368 to 380

381 to 393

394 to 405

406 to 417

418 to 430

431 to 442

443 to 455

456 to 467

A

Base modified from the Bureau of Land Management 

Geographic Coordinate Data Base coordinate data

Albers Equal-Area Conic projection

Lower confidence area

Big Sioux aquifer boundary

Depth to top of Sioux Quartzite, in meters

0 to 10

11 to 20

21 to 30

31 to 40

41 to 50

51 to 60

61 to 70

71 to 80

81 to 90

EXPLANATION

43°48'

43°46'

43°44'

43°42'

43°40'

43°38'

43°36'

0 1.5 3 KILOMETERS

0 0.75 1.5 MILES

T. 101 N.

R. 50 W. R. 49 W.

T. 102 N.

T. 102 N.

T. 103 N.

T. 103 N.

T. 104 N.

96°46'

96°44'

96°42'

B

Base modified from the Bureau of Land Management 

Geographic Coordinate Data Base coordinate data

Albers Equal-Area Conic projection

Figure 4. Delineated elevation and depth to the top of Sioux Quartzite, South Dakota. A, elevation of the top of Sioux Quartzite. B, depth to top of Sioux Quartzite.

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

endorsement by the U.S. Government.

This map or plate is offered as an online-only, digital publication. Users should be aware that, because of 

differences in rendering processes and pixel resolution, some slight distortion of scale or color may occur when 

viewing it on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at 

its intended publication scale.

Digital files available at https://doi.org/10.3133/sim3393.

Suggested citation: Valseth, K.J., Delzer, G.C., and Price, C.V., 2018, Delineation of the hydrogeologic framework of 

the Big Sioux aquifer near Sioux Falls, South Dakota, using airborne electromagnetic data: U.S. Geological Survey 

Scientific Investigations Map 3393, 2 sheets, https://doi.org/10.3133/sim3393. 

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3393