Characteristics of Thermal Springs and the Shallow 

Ground-Water System at Hot Springs National Park, 

Arkansas

Prepared in cooperation with the

National Park Service

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006-5001
Front Cover: Provided courtesy of the National Park Service, Hot Springs National Park collections. Lithograph published in 1844 by William Clowes and Sons of London. It was used as the 

frontispiece of English geologist G.W. Featherstonhaugh’s “Excursion through the Slave States, 

Volume II.” 
Characteristics of Thermal Springs and the 

Shallow Ground-Water System at Hot 

Springs National Park, Arkansas

By Daniel S. Yeatts

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

National Park Service

Scientific Investigations Report 2006-5001
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.
Contents

 iii

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

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

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

Acknowledgments ........................................................................................................................................................................ 4

Thermal Springs Development ................................................................................................................................................... 4

Previous Literature ....................................................................................................................................................................... 6

Description of Study Area....................................................................................................................................................................... 7

Methods of Investigation ........................................................................................................................................................................ 7

Thermal and Hydrologic Monitoring.......................................................................................................................................... 7

Hydrogeologic Assessment ...................................................................................................................................................... 10

Estimating Cold-Water Recharge Area ................................................................................................................................... 11

Dye Tracing .................................................................................................................................................................................. 11

Geologic Setting ..................................................................................................................................................................................... 14

Lithology........................................................................................................................................................................................ 14

Structure....................................................................................................................................................................................... 16

Characteristics of Thermal Springs .................................................................................................................................................... 16

Collection System Discharge and Water Temperature......................................................................................................... 16

Discharge.......................................................................................................................................................................... 16

Water Temperature ......................................................................................................................................................... 19

Thermal Springs Temperature Monitoring.............................................................................................................................. 21

Group 1 Thermal Springs................................................................................................................................................ 22

Group 2 Thermal Springs................................................................................................................................................ 23

Characteristics of the Shallow Ground-Water System ................................................................................................................... 24

Hydrologic Properties of Local Lithologies............................................................................................................................. 26

Principal Conduits and Barriers ............................................................................................................................................... 26

Shallow Aquifers and Cold-Water Recharge ......................................................................................................................... 26

Ground-Water Levels ................................................................................................................................................................. 27

Thermal Springs Hydraulic Connection .................................................................................................................................. 28

Alternative Sources of Cold-Water Recharge ....................................................................................................................... 28

Cold-Water Recharge Area....................................................................................................................................................... 28

Recharge Area Size......................................................................................................................................................... 29

Recharge Area Boundaries ........................................................................................................................................... 29

Dye Tracing ....................................................................................................................................................................... 29

Summary .................................................................................................................................................................................................. 33

References .............................................................................................................................................................................................. 34

Figures

1. Map showing location of Hot Springs National Park, Arkansas.......................................................................................... 3

2. Map showing study area and monitoring sites, Hot Springs National Park, Arkansas ................................................... 8

3. Map showing location of thermal springs, collection system, and monitoring sites, Hot Springs National Park,

Arkansas........................................................................................................................................................................................ 9

4. Map showing dye release sites and dye recovery points, Hot Springs National Park, Arkansas............................... 12

5. Photograph showing collection box 2 with fabricated lid to provide access for dye trace monitoring, Hot Springs 

National Park, Arkansas ........................................................................................................................................................... 13

6. Map showing geology in the vicinity of Hot Springs Mountain, Hot Springs National Park, Arkansas...................... 15
iv 

7. Graph showing daily and annual average discharge and temperature of the thermal springs at the inflow of the 

collection system reservoir ...................................................................................................................................................... 19

8. Graph showing thermal springs collection system hourly temperature and discharge response to 3.5 inches of 

rainfall on February 15-16, 2001 ............................................................................................................................................... 20

9. Graph showing hourly air temperature at Hot Springs Memorial Field Airport, Hot Springs, Arkansas, 2005 .......... 21

10. Graph showing daily average water temperature of group 1 springs and rainfall, 2000-2005 ..................................... 22

11. Graph showing hourly air temperature fluctuations compared to hourly water temperature at group 1 springs,

July 10-20, 2004........................................................................................................................................................................... 23

12. Graph showing daily average water temperature of group 2 springs and rainfall, 2000-2005 ..................................... 24

13. Diagram showing conceptual model of the thermal water flow system .......................................................................... 25

14. Graph showing hourly water levels in wells W2 and W4 and daily rainfall, 2004-2005 ................................................. 27

15. Map showing estimated recharge area for shallow ground-water contribution to the thermal springs................... 30

16. Map showing dye release and detection points and implied flow paths of the dyes .................................................... 31

17. Photograph showing rhodamine dye release at site 2 on Hot Springs Mountain, Hot Springs National Park,

Arkansas...................................................................................................................................................................................... 32

Tables

1. Thermal springs identified in Hot Springs National Park ...................................................................................................... 5

2. Generalized stratigraphy of sedimentary rocks in the vicinity of the thermal springs................................................... 14

3. Historical record of thermal springs temperature and discharge..................................................................................... 17

4. Results of rhodamine dye release at site 2 on December 4, 2004 ..................................................................................... 32

Conversion Factors, Vertical Datum, and Abbreviations,

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare

Flow rate

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

gallon per minute (gal/min) 0.06308 liter per second (L/s)

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

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

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

°F = (1.8 x °C) + 32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD of 1929).
Characteristics of Thermal Springs and the Shallow 

Ground-Water System at Hot Springs National Park, 

Arkansas

By Daniel S. Yeatts

Abstract

The thermal springs of Hot Springs National Park have 

been valued for the recreational and therapeutic benefits of the 

thermal baths, as a source of drinking water, and a destination 

of attraction since the history of the area was first recorded. The 

future of the park and the city of Hot Springs depends greatly on 

maintaining and protecting this unique natural resource from 

degradation and contamination. To maintain and protect the 

thermal springs, it is imperative to understand the character of 

the springs, monitor changes in spring characteristics, and evaluate the source of the thermal springs.

The thermal springs are situated in the Ouachita Mountains of central Arkansas. The springs emerge in a gap between 

Hot Springs Mountain and West Mountain in an area about 

1,500 feet long by 400 feet wide. The springs predominantly are 

composed of a deep thermal ground water component with a 

lesser but qualitatively substantial component of shallow cold 

ground water. Currently, there are 43 thermal springs in the park 

that are presumed to be flowing. Thermal water from 33 of the 

thermal springs is collected and monitored at a central reservoir, 

which distributes the combined discharge for public use and 

consumption.

The average collection system discharge over the period of 

record 1990 through 1995 and 1998 through 2005 was 658,000 

gallons per day and ranged from 518,000 to 763,000 gallons per 

day, not including about 131,000 gallons per day from springs 

43 and 43a that emerge from the bottom of the collection system 

reservoir. The overall pattern of the collection system discharge 

from 1990 through 2005 shows an increasing rate of discharge. 

Changes in the collection system temperature showed a positive 

relation to changes in discharge from 1990 through 1995, and 

an inverse relation to changes in discharge from 1998 through 

2005. The collection system usually increases in discharge during rainfall events. 

Continuous water temperature monitoring at the collection 

system reservoir inflow pipe shows that there has not been a 

substantial long-term temperature change during the past 15 

years. The daily water temperature ranged from 59.1 to 62.1 

degrees Celsius and the average daily temperature was 61.4 

degrees Celsius. The collection system water temperature 

shows a strong seasonal pattern, with highs and lows about 1 

month delayed from air temperature highs and lows. The collection system temperature also shows strong response to rainfall. 

The water temperatures at four thermal springs were monitored from August 2000 through June 2005, and four additional 

thermal springs and one thermal spring collection box were 

monitored from September 2003 through June 2005. Springs of 

relatively higher elevation (defined as group 1) generally 

showed a greater temperature response to changes in air temperature and rainfall. Springs of relatively lower elevation (defined 

as group 2) generally showed a smaller temperature response to 

changes in air temperature and rainfall. Springs 17 and 46 were 

exceptions that displayed unique water temperature responses 

that differed somewhat from group 1 and 2 springs.

Rock types exposed in the vicinity of the thermal springs 

are shale, chert, novaculite, sandstone, and conglomerate. Shale 

units generally impede ground-water movement, while fractured chert, novaculite, and sandstone units generally support 

ground-water movement. The thermal-water component hypothetically enters the ground-water system as regionally derived 

recharge from rainfall and flows to estimated depths of 4,500 to 

7,500 feet, where the water is heated and rises along fault and 

fracture conduits. The cold-water component enters the groundwater system primarily as locally derived recharge from rainfall 

and flows along shallow northeast trending faults, joints, and 

fractures to the thermal springs. The thermal springs are 

bounded on the southwest, southeast, and northwest by shale 

barriers. The lower member of the Arkansas Novaculite is probably the primary aquifer of shallow ground-water flow. 

Ground-water levels generally indicate that ground-water flow 

is towards Hot Springs Creek.

The size of the shallow cold-water recharge area was estimated from the general concept of the hydrologic budget, where 

the average annual ground-water recharge (input) is equal to the 

average annual cold-water discharge (output) of the thermal 

springs. Based on the thermal springs estimated cold groundwater baseflow discharge of 17.8 million gallons per year, and 

an estimated ground-water recharge rate of 5 to 10 inches per 

year, the estimated size of the shallow cold-water recharge area 

computes to 0.10 to 0.20 square mile. The shallow cold-water 

recharge area appears to be bounded on three sides by low-permeability barriers, and extends approximately to the topographic divide. The estimated shallow ground-water recharge 

area based on the boundaries is about 0.14 square mile. 

Rhodamine dye released on Hot Springs Mountain, about 

1,000 feet east of Central Avenue, was detected above background levels at several thermal water recovery sites over a 

period of several weeks. The flow path of the rhodamine dye to 
2 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

the thermal springs is probably along the western boundary contact with the Stanley Shale or along northeast-trending fractured 

lineaments. Presence of the dye verifies that this area is part of 

the recharge area and that surface water enters the ground-water 

system at some point along the pathway of the rhodamine dye. 

Time of travel from the release point to the thermal springs was 

1 to 3 weeks, depending on where the dye was detected.

Introduction

The thermal springs of Hot Springs National Park (HSNP) 

have been valued for the recreational and therapeutic benefits of 

the thermal water baths, as a source of drinking water, and as a 

destination of attraction since the history of the area was first 

recorded. After becoming a territory in 1803, the Arkansas Territorial Legislature recognized the importance of the thermal 

springs resource and requested in 1820 that the springs and 

adjoining mountains be set aside as a Federal reservation. The 

value of the thermal springs received national recognition when 

President Andrew Jackson signed legislation in 1832 to set 

aside “...four sections of land including said (hot) springs, 

reserved for the future disposal of the United States (which) 

shall not be entered, located, or appropriated, for any other purpose whatsoever.” The Federal reservation was designated as a 

national park on March 4, 1921. In 2004, about 1.4 million people made recreational visits to HSNP, more than 100,000 people 

bathed in the thermal waters, and additional tens of thousands 

of people collected water for drinking. The thermal springs of 

today provide the stimulus and attraction that form the base for 

local tourism and businesses.

HSNP is situated in the Ouachita Mountains of central 

Arkansas; about 55 miles southwest of Little Rock and about 80 

miles east of the Oklahoma State line (fig. 1). The mountains of 

HSNP divide the city of Hot Springs into northern and southern 

areas: the northern area lying in a valley on the southern edge of 

the Zigzag Mountains; and the southern area lying on the northern border of the Mazarn Basin, which drains to the Ouachita 

River. The north and south areas are connected by a pass 

between West Mountain and North and Hot Springs Mountains. 

This pass is the only throughway for drainage of the northern 

valley area to the southern Mazarn Basin area via Hot Springs 

Creek, and for vehicular and pedestrian traffic between the two 

areas. It is also the area of occurrence for 43 thermal springs of 

HSNP.

Flow from the thermal springs originates in two distinct 

recharge areas that yield water from two flow paths in HSNP. 

The predominant component of flow from the springs is supplied from thermal water that takes 4,000 years or more to move 

from the recharge area to the discharge point at the springs 

(Bedinger and others, 1979). Another component of flow is 

derived from a short, shallow flow path representing a locally 

recharged, cold-water component of flow that is more readily 

susceptible to contamination from development and land use in 

the area. The exact location and extent of this recharge area are 

unknown. An investigation conducted in 2000 and 2001 by the 

USGS has provided temperature and geochemical data that confirms the presence of a substantial locally derived cold-water 

component from recharge (Richard W. Bell and Phillip D. 

Hays, U.S. Geological Survey, written commun., 2005). The 

recharge area for the cold-water component is within or in proximity of an urban environment. Potential sources of contamination occurring in an urban setting include leaking sewer systems; leaking underground storage tanks; chemical-intensive 

commercial activities where spills or improper disposal may 

occur; and chemically-charged non-point source runoff from 

lawns, parking lots, roads, and other surfaces. 

Direct infiltration of contaminants from above and adjacent to the springs is also a concern because of the past construction history of the springs and the park. The springs today 

have been disturbed by intensive excavation throughout the 

spring discharge area to reroute the path of Hot Springs Creek, 

renovate spring catchments, and construct the bathhouses and 

pedestrian walkways. Excavations and thermal spring catchments that have altered tufa and other surficial formations allow 

pathways for surface water to infiltrate into the springs (Charles 

G. Stone, Arkansas Geological Commission, written commun., 

2005). Hot Springs Creek, which drains a large urban area, 

flows about 50 feet (ft) west of some thermal springs at the base 

of Hot Springs Mountain, and the water level in the creek can 

rise above the level of the thermal springs during storm events, 

posing a concern for lateral infiltration into the springs.

The future of HSNP and the city of Hot Springs depends 

greatly on maintaining and protecting the thermal springs from 

degradation. Types of degradation that could affect the park and 

the city include: (1) increased contributions of colder surface 

water into the thermal water system affecting temperature, (2) 

introduction of contaminants from point and non-point sources 

affecting water quality, and (3) changes in ground-water 

hydraulics affecting the discharge of the springs. To maintain 

and protect the thermal springs, it is imperative to understand 

the character of the springs, monitor changes in spring characteristics over time, and evaluate the source of the thermal 

springs and the thermal springs system. In October 2003, a 2-

year study was initiated in cooperation with the National Park 

Service to monitor and trace local, shallow ground-water flow 

to the thermal springs and estimate its recharge area. 

Purpose and Scope

The purpose of this report is to describe the characteristics 

of the thermal springs and shallow ground-water system, and 

approximate the size and boundaries of the recharge area for the 

cold-water component of the thermal springs. Several sources 

of information and techniques were used to achieve this purpose. Geologic and structural data from previous studies (Purdue, 1910; Bryan, 1922; Arndt and Stroud, 1953; Bedinger and 

others, 1979), topographic data from existing maps, and data 

collected in the field were inventoried, compiled, and reviewed 

to determine the surficial controls on drainage and infiltration,
Introduction 3

Figure 1. Location of Hot Springs National Park, Arkansas.

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4 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

the ground-water flow paths, and the boundaries to groundwater flow. Four wells were logged using borehole geophysics 

to provide more detailed information on lithology, distribution 

of permeability, fracture orientation, and thermal flow within 

the shallow ground-water system. The water temperature at 

eight thermal springs and a collection box, the water temperature and discharge at the collection system reservoir, the rainfall 

in downtown Hot Springs, and the ground-water levels at two 

wells were monitored to identify and evaluate the relation 

between water temperature, discharge, precipitation, and 

ground-water level. Geochemical data from previous or contemporaneous studies were evaluated to quantify and characterize the nature of the cold-water component of the thermal 

springs (Richard W. Bell and Phillip D. Hays, U.S. Geological 

Survey, written commun., 2005; Bedinger and others, 1979). 

Dye tracing was performed to identify flow paths from points of 

release to points of dye recovery and to approximate the time of 

travel between release and recovery points. 

Acknowledgments

Appreciation is extended to the staff of the National Park 

Service for accompanying and assisting USGS personnel on 

many of our trips to HSNP, and for their understanding in dealing with the delicate nature of the Hot Springs thermal resource. 

Appreciation is especially expressed to Steve Rudd and 

Leonard Lawson for their coordination of the project and their 

patience. Special thanks to Sharon Shugart, the museum curator 

of HSNP, for the plethora of information she maintains and for 

her kind assistance.

The author would like to acknowledge the assistance of 

other USGS personnel that made this study possible: to Rene 

Freret and Marsha Gipson for installing, operating, and maintaining the gage station network; to Greg Stanton and Brian 

Clark for conducting the geophysical logging, and processing 

and interpreting the logs; and to personnel of the reports unit for 

preparing the illustrations and editing the report. The author 

also is grateful to Charlie Stone and Doug Hansen of the Arkansas Geological Commission for their kind assistance in supplying information for the report, for their field assistance, and for 

their wealth of knowledge about Hot Springs geology. 

Thermal Springs Development

The importance of the thermal springs resource was first 

recognized when Congress created the Hot Springs Reservation 

in 1832; the first Federal reservation established to protect natural resources for public enjoyment. The presence of the Hot 

Springs Reservation attracted people and businesses that developed into the city of Hot Springs, incorporated in 1876. 

Accounts of development of the thermal springs began some 

time before 1877, describing that some springs were walled in 

and covered by masonry arches to protect them from contamination (Scully, 1966). 

In 1883, excavation and reconstruction began on Hot 

Springs Creek to contain and cover the creek. This tunnel, 

known as “the creek arch,” still exists and extends about 4,600 

ft through a gap between Hot Springs Mountain and West 

Mountain. The creek arch conveys the normal flow of Hot 

Springs Creek and has sufficient volume to contain stormflows 

of 3-year recurrence interval (U.S. Army Corps of Engineers, 

1993). Five thermal springs that were reported to rise from the 

bed of Hot Springs Creek were covered by the creek arch (Purdue and Miser, 1923). Only one thermal spring, Alum Spring, 

was reported to issue from the west bank of the creek (Bryan, 

1922; Haywood, 1902), across from the Hale Bathhouse. Alum 

Spring was captured with stone walls in 1885, refurbished in 

1914, and capped in 1921 (Shugart, 2000). All other thermal 

springs issue from the east side of the creek arch, at the base of 

the southwest sloping Hot Springs Mountain.

Extensive development of the thermal springs and area 

continued through the 1900’s. By 1901, most of the thermal 

springs were covered and a complex system of piping evolved 

to supply hot water to the bathhouses. Major development took 

place from 1912 to 1922 to construct eight bathhouses. In 1931, 

some springs were deepened and the collection system was 

reconstructed. Bedinger and others (1979) reported that the collection system diverted the flow of 44 springs to the central reservoir system. Some springs and the spring distribution system 

were reconstructed again in three phases: Arlington Lawn area, 

1976; upper promenade, 1979; and lower promenade, 1981. No 

major construction has taken place since then, with the exception of the present ongoing reconstruction of the bathhouses. 

An attempt was made to inventory the correct number of 

thermal springs and assess their condition and use, but the 

results should be considered with some uncertainty because 

many of the springs and collection system components were 

buried, secured, or unmarked, and available information was 

difficult to decipher. According to the author’s assessment, a 

total of 55 thermal springs have been identified since 1901 

(table 1). Interestingly, 12 of the 55 springs have been abandoned and are either covered or destroyed (12, 14, 16, 18, 20, 

21, 30, 35, 36, 37, 40, 41), while 11 new springs were identified 

after 1901 (4a, 6a, 6b, 6c, 9a, 21c, 43a, 47, 48, 49, 50). Therefore, there are presently 43 thermal springs that are presumed to 

still be flowing. Of these 43 springs, the present collection system consists of 33 springs, 2 of which (43, 43a) discharge 

directly from the bottom of the collection system reservoir 

(Jacobs, 1988). 

The present collection system carries thermal water 

through 2- to 6-inch pipes into collection boxes that lead to the 

collection main in the creek arch (Jacobs, 1988). The collection 

main carries the thermal water to the collection system reservoir 

under the Park Administrative Building on Reserve Avenue. 

Ten springs that are not collected are drained to the creek arch, 

including several display springs (27, 28, 29, 32, 33, and 34), 

Haywood Spring (31, used for water fountain), Maurice Bathhouse Spring (50), Army and Navy Spring (39), and Cave 

Spring (10).
Introduction 5

Table 1. Thermal springs identified in Hot Springs National Park .

Spring

number Spring name Condition Water use

1 Egg Reconstructed 1980 Collection system

2 Arsenic South Reconstructed 1977 Collection system

3 Arlington Reconstructed 1980 Collection system

4 Cliff Reconstructed 1977 Collection system

4a Cliff New Constructed 1977 Collection system

5 Avenue Reconstructed 1980 Collection system

6 Boiler House North Reconstructed 1977 Collection system

6a Cooler North Reconstructed 1977 Collection system

6b Cooler South Reconstructed 1977 Collection system

6c Boiler House South Constructed 1977 Collection system

7 Imperial North Reconstructed 1980 Collection system

8 Crystal Reconstructed 1980 Collection system

9 Rector Reconstructed 1977 Collection system

9a Rector North Constructed 1977 Collection system

10 Cave Covered with fill material Drains to creek arch

11 Little Iron North Reconstructed 1977 Collection system

12 Little Geyser Abandoned Not collected

13 Little Iron South Reconstructed 1977 Collection system

14 Ral Abandoned Not collected

15 Big Iron Reconstructed 1977 Collection system

16 Imperial South Abandoned Not collected

17 Arsenic North Reconstructed 1977 Collection system

18 Hitchcock Abandoned Not collected

19 Superior Bath Reconstructed 1977 Collection system

20 Superior North Abandoned Not collected

21 Alum Abandoned Not collected

21c Alum East Constructed 1980 Collection system

22 Superior South Reconstructed 1980 Collection system

23 Twin North Reconstructed 1980 Collection system

24 Twin South Reconstructed 1980 Collection system

25 Hale Bath Reconstructed 1980 Collection system

26 Palace Reconstructed 1980 Collection system

27 Tunnel Display tunnel Drains to creek arch

28 Maurice Covered Drains to creek arch

29 Dripping Display fountain To fountain and creek arch

30 Arch Abandoned Not collected

31 Haywood Reconstructed 1980 To fountain and creek arch

32 Noble Display spring Drains to creek arch
6 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

33 Lamar Display spring Drains to creek arch

34 Wiley Display spring Drains to creek arch

35 Hardin Abandoned Not collected

36 Eisele Abandoned Not collected

37 Stevens Abandoned Not collected

38 Horseshoe Reconstructed 1980s Collection system

39 Army and Navy Drilled and cased Drains to creek arch

40 W.J. Little Abandoned Not collected

41 Mud Abandoned Not collected

42 Quapaw Bath Reconstructed 1980s Collection system

43 Reservoir North In collection reservoir Collection system

43a Reservoir South In collection reservoir Collection system

46 Fordyce Bath Reconstructed 1980s Collection system

47 New North Reconstructed 1980 Collection system

48 New South Reconstructed 1980 Collection system

49 USGS Reconstructed 1980 Collection system

50 Maurice Bath Reconstructed 1980s Drains to creek arch

Table 1. Thermal springs identified in Hot Springs National Park—Continued.

Spring

number Spring name Condition Water use

Previous Literature

Numerous scientific investigations directly or indirectly 

involved the thermal springs at HSNP. Although each study 

generally had a different specific objective, most addressed the 

issue of the origin of the thermal spring waters. Several theories 

on the source of the thermal water and the source of the heat for 

the thermal waters have been proposed, but only a few survived 

subsequent inquiries. The earliest recorded theory on the origin 

of the heat source dates back to the Lewis and Clark expedition 

in 1804. As part of this expedition to explore the Ouachita 

Mountains, William Dunbar suggested the heat resulted from 

chemical reactions in the water because he saw no evidence of 

volcanic activity in the vicinity (Bergfelder, 1976). Owen 

(1860) rejected the chemical theory proposed by Dunbar and 

attributed the cause of the high water temperature to the internal 

heat of the earth. Branner (1892), Arkansas State Geologist, 

also discounted the chemical reaction theory among other less 

proven theories, and added that the probable cause of the water 

being hot is from contact with masses of hot rocks; the cool 

edges of which may or may not be exposed at the surface. 

Haywood (1902) and Weed (1902) performed the most 

comprehensive early investigations of the thermal springs, measuring the temperature and discharge of all accessible thermal 

springs and performing a thorough chemical analysis on each 

spring. Weed (1902) supported the theory that the heat source 

was from a “great body of still heated igneous rocks.” Weed 

(1902) strengthened this hypothesis with evidence of surficial 

volcanic occurrences at Potash Sulfur Springs and at Magnet 

Cove, and the occurrence of intrusive dikes at various locations 

around the thermal springs. The dikes trend below Hot Springs 

to a possible source of heat. From the chemical analyses, Haywood (1902) and Weed (1902) also deduced that because the 

thermal springs contain so little mineral matter, particularly silica, and the gases given off closely correspond to the ratio of 

gases in atmospheric air, that the thermal waters are of meteoric 

origin, or waters derived from rainfall.

Purdue (1910) supported that the thermal waters were of 

meteoric origin, based on Weed’s studies and the general consensus of geologists. Purdue (1910) went further to suggest that 

the Bigfork Chert in the anticlinal valley between North Mountain and Sugarloaf Mountain is the most probable collection 

area for the meteoric water that is “conducted through the Bigfork Chert beneath the North Mountain syncline, and forced up 

into the Hot Springs anticline, at the western end of which it 

emerges in the hot springs.” 

Bryan (1922 and 1924) distinguished between an east and 

west body of the Hot Springs Sandstone Member of Stanley 

Shale based on the extreme difference in strike and dip orientation of the two bodies, and postulated that a thrust fault separated them. The west body contained the thermal springs and 

the east body contained a new well with mostly thermal water. 

Bryan (1922) also postulated that there are four lines of open 

joints where the thermal springs emerge that are parallel to the 

postulated thrust fault. 
Description of Study Area 7

Arndt and Stroud (1953) contributed to the understanding 

of stratigraphy and structure in the vicinity of the HSNP through 

field studies conducted in 1952. The authors described two 

major thrust faults, called Alpha and Beta, between which the 

thermal springs emerged, and defined structural features in 

more depth than in previous studies. The authors theorized that 

the thermal waters rose from juvenile water and mixed with 

artesian meteoric cold water in the lower member of the Arkansas Novaculite before emerging along the faults, fissures, and 

joints at the thermal springs. 

Fellowes (1968) expanded on the geology of the Hot 

Springs area with more detailed field investigations, geologic 

mapping, and petrographic analyses. His findings generally 

agreed with Arndt and Stroud (1953) where their studies overlapped. Fellowes further hypothesized that the position of the 

thermal springs near the southwest-plunging nose of the Hot 

Springs Mountain Anticline suggests a possible relation 

between the structural trend and ground-water flowpaths. Fellowes also introduced an abnormally high geothermal gradient 

as an alternative hypothesis for the source of the heat to the thermal springs.

Perhaps the most comprehensive study of the thermal 

springs was conducted by Bedinger and others (1974 and 1979). 

The authors’ findings supported the meteoric origin of the thermal water and the flow pathway through the Bigfork Chert as 

presented by Purdue (1910). The high temperature of rocks is 

attributed to the normal geothermal gradient, which is estimated 

to be between 0.006 and 0.01 degree Celsius (°C) per foot. The 

minimum depth of fluid circulation would range from 4,500 to 

7,500 ft for the geothermal gradient theory to work. Silica analyses made on samples collected in 1901, 1952, and 1972 suggested that the spring source temperature had been decreasing 

at a rate of about 0.08 °C per year since 1901. 

Bedinger and others (1974 and 1979) presented evidence 

of a secondary cold-water component to the thermal springs 

through their studies. Tritium and carbon-14 dating of the thermal springs indicated that the water is a mixture of a small 

amount of water less than 20 years old and a preponderance of 

water about 4,400 years old. In addition, the study indicated that 

a component of flow to some cold-water springs and wells in 

the Hot Springs area showed evidence of contamination, such as 

elevated concentrations of nitrate and chloride. 

More recent studies presented further evidence of a coldwater component. C. Shane Barks (U.S. Geological Survey, 

written commun., 1995) indicated that storm events strongly 

affected thermal springs discharge and temperature. Using data 

collected during storms, calculations showed that water from 

locally derived recharge areas might contribute 25 percent of 

the total flow to the thermal springs during and after storm 

events. Richard W. Bell and Phillip D. Hays (U.S. Geological 

Survey, written commun., 2005) performed chemical analyses 

at nine thermal springs and two cold water springs in 2000 and 

2001 to determine the influence of locally derived cold water 

recharge on the thermal springs. They estimated that the proportion of cold-water recharge ranged from 0 to 16 percent during 

baseflow conditions and 21 to 31 percent during stormflow conditions, based on silica concentrations.

Description of Study Area

The local study area for this report includes Hot Springs 

Mountain and adjacent area (fig. 2). The thermal springs are situated in the gap between Hot Springs Mountain and West 

Mountain. The thermal springs emerge at the base of Hot 

Springs Mountain in a belt about 1,500 ft long, between Reserve 

Avenue and Fountain Street, and about 400 ft wide from Central 

Avenue to the lower part of Hot Springs Mountain Drive (fig. 

3). The altitude is 600 ft above National Geodetic Vertical 

Datum of 1929 (NGVD 1929) at the corner of Reserve and Central Avenue, which is about the lowest surface point in the area 

of occurrence of the thermal springs, and is also the southern 

end of the block of bathhouses known as “Bathhouse Row.” 

Some springs along Bathhouse Row have been excavated and 

emerge below ground surface. Altitudes of the thermal springs 

range from 576 ft above NGVD 1929 in the collection system 

reservoir (springs 43 and 43a) at the extreme southern end of 

occurrence, to 683 ft above NGVD 1929 at springs 47 and 5 at 

the extreme northeastern end of occurrence (Hamilton and 

Blood, 1931).

Methods of Investigation

Methods of investigation for this report included monitoring the thermal springs characteristics, performing a hydrogeologic assessment to define the physical characteristics of the 

thermal springs system, estimating the cold-water recharge area 

of the thermal springs, and conducting dye tracing to verify 

inferred cold-water recharge area boundaries. 

Thermal and Hydrologic Monitoring

Several types of monitoring equipment were used to collect information on the physical characteristics of the thermal 

springs and study area to assist in defining the hydrogeology 

and estimating the recharge area. The monitoring equipment 

consisted of nine water-temperature probes with recorders, two 

ground-water observation well recorders, a rain gage, and a 

monitoring station at the collection system reservoir that monitored inflow, overflow, water level, and water temperature at 

the inflow of the reservoir (fig. 3). 

Nine water-temperature probes were placed at eight thermal springs and at collection box 1 (CB1). Four of the probes 

were installed in August 2000 at thermal springs 8, 17, 25, and 

46. Five additional probes were installed in September 2003 at 

thermal springs 15, 19, 42, 49, and at CB1. CB1 monitored the 

composite temperatures of thermal springs 1, 3, 47, and 48. All 

temperature probes remained in place from installation through
Figure 2. Study area and monitoring sites, Hot Springs National Park, Arkansas.

 







 



  







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10 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

June 2005, although there are some gaps in data because of 

equipment malfunction and onsite disturbances such as construction activities. All temperatures were measured to one-hundredth of a degree Celsius using a thermistor, and digital 

recorders collected data at 15-minute intervals. The temperature 

probes were checked periodically using a calibrated digital thermometer. Recorder data were corrected to the temperature taken 

with the calibrated digital thermometer. The methods used followed the guidelines by Wagner and others (2000), and Radtke 

and others (1998).

Of the two ground-water level recorders, one was placed in 

a well above and east of the thermal springs (W4 in fig. 3), and 

one was placed in a well at the base of Hot Springs Mountain, 

just north of the thermal springs (W2 in fig. 3). The groundwater observation well with recorder at the upper well (W4) has 

been monitoring continuously from October 1996 through June 

2005 and uses a float attached to an encoder that translates 

water-level measurements to digital format. The ground-water 

observation well with recorder at the lower well (W2) was operated from June 2004 through May 2005, and consisted of a transducer that monitored water pressure converted to water level. 

Both water-level recorders measured the water levels to onehundredth of a foot and recorded values at 1-hour intervals. Data 

periodically were downloaded and the water level checked using 

a calibrated steel or electrical tape. The methods used followed 

Garber and Koopman (1968) and the Quality-Assurance Plan for 

Ground-Water Level Activities of the U.S. Geological Survey 

(Arkansas Water Science Center, U.S. Geological Survey, written commun., 2002).

The rain gage was operated from September 25, 2003, 

through June 2005 on the southeastern corner of the roof of the 

Lamar Bathhouse (fig. 3). The gage consisted of a single automatic tipping bucket connected to the collection reservoir 

recorder in the basement of the adjacent park administration 

building. Data were transmitted by satellite from this recorder to 

the USGS Arkansas Water Science Center in Little Rock. Rainfall was measured to one-hundredth of an inch and values 

recorded at 15-minute intervals. Rainfall data prior to September 

25, 2003, was obtained from Hot Springs Memorial Field Airport (Elizabeth Sanders, Southern Regional Climate Center, 

written commun., 2005).

The monitoring station located in the basement of the park 

administration building has been monitoring collection system 

discharge from October 1988 through June 2005 and temperature from October 1990 through June 2005. The monitoring station consists of a weir to measure the inflow from the collection 

system to the collection reservoir; a float device to measure the 

water level in the reservoir; a weir to measure the overflow from 

the reservoir to Hot Springs Creek; and a temperature probe 

located at the inflow weir to measure water temperature from the 

thermal springs. The inflow and overflow weirs were identical in 

design and construction and were installed side by side, but were 

oriented in opposite directions and at different elevations. The 

weirs were calibrated in a flume for discharges from 0.01 to 2.12 

cubic feet per second (ft3/s), or 4.5 to 952 gallons per minute 

(gal/min), and rating equations were established to convert the 

water level in the weir boxes to discharge of water passing 

through the weirs. The water levels in the weirs and the reservoir 

were measured to one-hundredth of a foot, and the temperature 

measured to one-hundredth of a degree Celsius. All data were 

recorded at 15-minute intervals and transmitted every 4 hours. 

The methods used for measuring discharge followed Rantz and 

others, (1982), and the methods used for measuring temperature 

followed Wagner and others (2000).

The total discharge of the thermal springs for the period 

from October 1990 through June 2005 was determined by adding the total inflow to the collection reservoir and the estimated 

discharge from springs 43 and 43a that emerge from the open 

bottom of the collection reservoir. The discharge from springs 

43 and 43a was estimated from three reservoir recovery tests 

conducted in 1989 (C.S. Barks, U.S. Geological Survey, written 

commun., 1989). The tests were conducted by diverting collection-system inflow to Hot Springs Creek and pumping down the 

water in the reservoir, allowing only springs 43 and 43a to discharge into the reservoir. The total discharge of springs 43 and 

43a were calculated from the recovery of the reservoir water 

level.

The historical total discharge and average temperature of 

the thermal springs were determined from investigations conducted in 1901 (Haywood, 1902) and 1976 (Jacobs, 1988). The 

total discharge of the thermal springs was determined by adding 

the discharge of each spring measured. The average temperature 

of the thermal springs was determined by calculating the average 

of each spring’s discharge-weighted temperature. The discharge-weighted temperature of each spring is the product of the 

spring discharge and temperature, divided by the total discharge 

of all the thermal springs measured.

Hydrogeologic Assessment

The first step in delineating the cold-water recharge area for 

the thermal springs was to develop a conceptual model of the 

hydrogeology. Features of the conceptual model that define the 

hydrogeology include the rock type and physical properties, the 

orientation and thickness of rock units, structural features such 

as folds and faults, and the capacity of the rock and soil to hold, 

transmit, and deliver water. This information was obtained from 

geologic maps that show rock types, lithologic boundaries, faults 

and fractures; topographic maps that show surface relief, drainage pattern, and direction of surface-water flow; drillers’ logs 

and geophysical logs that show subsurface hydrogeologic features; aquifer tests that help define subsurface porosity and permeability; geochemical analyses that characterize the makeup 

and source of the ground water; and ground-water levels that 

infer the direction of ground-water flow.

Additional field mapping and geophysical logging were 

conducted for the investigation of this report. Field mapping was 

conducted to identify surface rock types and determine the orientation of rock units, determine release and collection points for 

dye tracing, and also inventory the location of all thermal 

springs, cold springs, and wells in the study area (fig. 3). Geo-
Methods of Investigation 11

physical logging was conducted on four wells in the study area: 

Wells W1, W2, W3, and W4. These logs provided information 

about the lithology, distribution of permeability, fracture orientation, and thermal flow within the ground-water system. The 

geophysical measurements included well caliper, fluid specific 

conductance and temperature, natural gamma, long (64 inch) 

and short (16 inch) normal resistivity, single-point resistivity, 

electromagnetic induction resistivity, spontaneous potential, 

and secondary porosity/structural features using an acoustic 

televiewer.

Estimating Cold-Water Recharge Area

The size of the cold-water recharge area was estimated 

from the general concept of the hydrologic budget (input=output), where the average annual ground-water recharge (input) is 

equal to the average annual cold-water discharge (output) of the 

thermal springs. If ground-water recharge is the amount of rainfall over an area that recharges the water table, then the recharge 

area for the cold-water discharge of the thermal springs can be 

expressed as:

A = (3.5 x 10-8) Q / R (1)

Where A is the cold-water recharge area, in square miles;

Q is the annual cold-water discharge from the thermal 

springs, in cubic feet per year;

R is the annual ground-water recharge from rainfall, in 

feet per year; and

3.5 x 10-8 converts square feet to square mile.

The cold-water discharge of the thermal springs (Q) was 

determined during baseflow or low-flow conditions following 

the methods of Brahana (1997) and Quinlan and Ray (1995). 

Baseflow discharge is generally a better estimator of the total 

recharge area of a spring than stormflow because stormflow 

generally is derived near the discharge point of the springs and 

may consist of some unsaturated flow and surface runoff, 

whereas, baseflow generally is derived from ground-water storage.

Total annual baseflow of the thermal springs first was estimated from the total discharge of the collection system and 

springs 43 and 43a during low-flow conditions in September 

2000. Then, the cold-water component of the total thermal 

springs discharge was estimated from the average silica concentration of water samples collected from nine thermal springs 

(springs 8, 9, 17, 25, 33, 42, 46, 47, and 49) during low flow 

from September 18 to 22, 2000 (Richard W. Bell and Phillip D. 

Hays, U.S. Geological Survey, written commun., 2005). Silica 

concentration of water can be used as a measure of the maximum temperature reached by the water (Fournier and Rowe, 

1966). The average silica concentration of the nine thermal 

springs then was compared to the silica concentration typical of 

cold-water springs and wells (8 mg/L of silica), representing the 

cold-water component, and the maximum thermal spring silica 

concentration measured (47 mg/L of silica), representing the 

thermal-water component. A binary mixing equation for silica 

was used to estimate the proportion of cold water in the thermal 

springs samples:

Xmix = (Xa)fa + (Xb)fb (2)

Where Xmix is the silica concentration or isotopic composition of the mixture, in milligrams per liter;

 Xa is the silica concentration or isotopic composition 

of the cold-water component (a) contributing to the 

mixture, in milligrams per liter;

 fa is the proportion of the cold-water component (a) 

present in the mixture;

 Xb is the silica concentration or isotopic composition 

of the thermal-water component (b) contributing to 

the mixture, in milligrams per liter; and

 fb is the proportion of the thermal-water component 

(b) present in the mixture.

The proportion of cold water in the thermal springs samples 

then was applied to the total annual baseflow discharge of the 

thermal springs to obtain the annual cold-water discharge from 

the thermal springs. 

The average annual ground-water recharge (R) was estimated by Dugan and Peckenpaugh (1985) using a soil-moisture 

computer model with input of soil properties, vegetation types, 

monthly rainfall, and monthly potential evapotranspiration. The 

average annual recharge in the general area north of Hot Springs 

for woodland and range conditions was estimated to be 10 to 15 

inches. The factors used for the model were generalized and the 

actual ground-water recharge for the study area is presumed to 

be less than 10 inches because of the steep gradient on Hot 

Springs Mountain (20 percent) and extensive road drainage for 

HSNP not considered in the model. Recharge values (R) of 5 to 

10 inches per year (in/yr) were considered in equation 1 to compute a range of sizes for the recharge area.

The cold-water recharge area boundaries were delineated 

based on the lithology, structure, and hydrology of the study 

area. The boundaries define the shape of the recharge area. The 

size of the recharge area determined from the recharge boundary assessment also was computed using Geographical Information System (GIS) tools. The size of the recharge area determined from the recharge area boundary was compared to the 

size of the recharge area as estimated using the hydrologic budget. 

Dye Tracing

Dye tracing was conducted from December 2004 through 

March 2005 in an attempt to verify inferred cold-water recharge 

area boundaries and identify shallow ground-water flow paths. 

Fluorescent dyes certified for use in drinking water were 

released at three locations (fig. 4) to trace the flow path of the
Figure 4. Dye release sites and dye recovery points, Hot Springs National Park, Arkansas.













 



















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Methods of Investigation 13

ground water. The general methods used are described in Mull 

and others (1988).

The lithology and surface structure in the area of the thermal springs did not present favorable conditions to transmit the 

dyes in the quantities needed and in the amount of time feasible 

for the investigation. The lithology consisted of well-cemented, 

fine-grained chert, sandstone, and shale that supported diffuse 

ground-water flow rather than conduit flow. Structural faults 

and fractures that could present conduit flow, if present in the 

subsurface, were weathered and covered by vegetation and soil 

on the surface. The few visible areas of surface structure along 

drainage features dictated where the dye release points were 

placed. 

Three types of fluorescent dyes were released consecutively into surface-drainage features at three different points 

upgradient from the thermal springs. The three dyes (and color 

index) used were Eosine OJ (acid red 87), Rhodamine WT (acid 

red 388), and Fluorescein (acid yellow 73) released at sites 1, 2, 

and 3 respectively (fig. 4). One liter of 10 percent concentration 

eosine dye, 2 liters of 5 percent concentration rhodamine dye, 

and 4 liters of 10 percent fluorescein dye were released. The 

quantities of dyes desired for detection were reduced to minimize the likelihood of discoloring the public water system supplied by the thermal springs. Reducing dye quantities also 

reduced the quantities of dye detected at recovery points.

Activated charcoal was used as a dye receptor at 24 recovery points (selected springs, wells, creeks, and drainageways) 

around Hot Springs Mountain (fig. 4) based on accessibility and 

location. Activated charcoal is a passive detector that indicates 

if dye was present, but is not used to quantify the concentration 

of the dyes. Charcoal packets were fabricated by placing the 

charcoal in nylon mesh screen that was stapled together. Prior 

to releasing the dyes, the charcoal packets were placed at the 

recovery points for about 1 week to obtain background florescence readings. New charcoal packets were placed at the recovery sites 1 day prior to releasing the dyes, and the packets were 

replaced about weekly after the dye release.

The charcoal packets were placed in the center of flow, 

suspended on string and wire at spring boxes, drainageways, 

creeks, and wells. Four spring box lids were fabricated and 

installed on collection boxes and thermal springs (fig. 5) to provide access for the charcoal packets and protect the spring 

source from contamination. Upon collection of the charcoal 

packets, the packets were rinsed and dried, then immersed in a 

solution of 90 percent isopropyl alcohol and 10 percent ammonium hydroxide for 2 to 6 hours to extract the dyes from the 

charcoal (Imes and Fredrick, 2002). Presence or absence of fluorescence in the solution was analyzed using a spectrofluorophotometer (fluorometer) as described by Wilson and others 

(1986). The three dyes fluoresce at different wavelengths, and 

therefore, could be differentiated when present.

Several laboratory tests were performed on the capability 

of the charcoal to absorb and retain dyes in hot water. The tests 

showed that the dyes were absorbed and retained similar to how 

they would have been retained in cold water. Heated solutions 

(about 90 °C) of dye concentration greater than 1 microgram per 

liter (μg/L) were detected by the fluorometer. Solutions of dye 

concentration less than 1 μg/L showed low detection peaks, or 

no peaks at all. The results varied depending on the type of dye 

used, length of time the charcoal packet was exposed to the dye 

solution, and time exposed to hot distilled water after absorbing 

the dye.

Figure 5. Collection box 2 with fabricated lid to provide access for dye trace monitoring, Hot Springs National Park, Arkansas. 
14 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Geologic Setting

The exposed rock types in the vicinity of the thermal 

springs are sedimentary rocks of Mississippian to Ordovician 

age (table 2), with the exception of younger igneous rocks (Cretaceous age) exposed in two small areas about 6 and 11 miles 

southeast of the thermal springs (Potash Sulphur Spring and 

Magnet Cove, respectively), and in many very small dikes and 

sills (Bedinger, 1979). Most dikes are less than 5 ft wide (Purdue and Miser, 1923). Purdue (1910) noted 80 dikes about 4 

miles southeast of Hot Springs, on and near the Ouachita River. 

There is no indication that igneous rock occurs where the thermal springs discharge.

Lithology

The sedimentary rocks in the vicinity of the thermal 

springs consist of shale, chert, novaculite, sandstone, and conglomerate (fig. 6). The Womble Shale is the oldest geologic unit 

that underlies all other exposed units. It is exposed a few miles 

northeast of the study area. It is black, hard, and argillaceous 

shale, with interbedded lenses of limestone 20 ft or more in 

thickness (Purdue and Miser, 1923). The Bigfork Chert overlies 

the Womble Shale and consists almost entirely of chert and silty 

chert in layers 2 to 12 inches thick, separated by minor thin beds 

of black shale. The chert is very brittle and intensely fractured 

from folding (Purdue, 1910).

The Polk Creek Shale and Missouri Mountain Shale overlie the Bigfork Chert and generally consist of shale with minor 

thin layers of quartzitic sandstone. The contact between the 

Missouri Mountain Shale and Polk Creek Shale at Hot Springs 

is inconspicuous. The Polk Creek Shale is a black, fissile, graphitic shale. The Missouri Mountain Shale varies in color, and 

is soft and argillaceous (Purdue and Miser, 1923).

The Arkansas Novaculite consists of lower, middle, and 

upper members. The lower member is a massive fractured novaculite, and is the dominant member on Hot Springs Mountain, 

with a thickness of about 275 ft. The middle member is a black 

clay shale interbedded with novaculite, about 10 ft thick on Hot 

Springs Mountain (Purdue and Miser, 1923). The upper member is chiefly a massive, highly calcareous light gray to black 

novaculite. It reaches a maximum thickness of 180 ft in the 

study area, and the rock weathers to a soft, porous, fine-grained 

material (Purdue and Miser, 1923). 

The Hot Springs Sandstone Member of the Stanley Shale, 

hereafter referred to as the Hot Springs Sandstone, consists of 

fine- to medium-grained sandstone with some shale and conglomerate. The sandstone is gray, hard and quartzitic, reaching 

thicknesses up to 6 ft. The shale predominantly occurs at the top 

of the unit, and the principal bed of the conglomerate occurs at 

the bottom (Purdue and Miser, 1923). 

The Stanley Shale is predominantly a clayey, thinly fissile, 

black to green shale, with large amounts of sandstone interbedded throughout the formation. The sandstone, when freshly 

exposed, is a hard, fine-grained, feldspathic, silty sandstone, but 

weathers easily to a soft, clayey porous material ranging from 

green to brown in color (Purdue and Miser, 1923). Almost all of 

the low-lying areas in the city of Hot Springs are composed of 

the Stanley Shale, and it surrounds Hot Springs Mountain on the 

south, east, and west sides. 

Table 2. Generalized stratigraphy of sedimentary rocks in the vicinity of the thermal springs (modified from Bedinger and others, 1979).

System Unit Character of rocks

Maximum

thickness in

Hot Springs 

area1

(feet)

1Charles G. Stone, Arkansas Geological Commission, written commun., 2005.

 Mississippian Stanley Shale Bluish-black shale, and gray sandstone 2,500

Devonian

Hot Springs Sandstone

Member of the Stanley Shale

Hard, gray, quartzitic sandstone, and conglomerate 200

Arkansas Novaculite Massive and thin bedded novaculite interbedded with

black argillaceous and siliceous shale.

465

Silurian Missouri Mountain Shale,

Polk Creek Shale 

Green to black shale, and few thin sandstones 225

Ordovician Bigfork Chert Thin-bedded chert, highly fractured, and interbedded thin shale 700

Womble Shale Black shale, and thin bedded lenses of limestone 1,200
Figure 6. Geology in the vicinity of Hot Springs Mountain, Hot Springs National Park, Arkansas.









































































  





















 







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16 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Structure

Rocks in the Hot Springs area have undergone at least 

three episodes of compressional deformation, resulting in a 

series of thrust faults and overturned complexly folded strata 

trending in a northeast-southwest direction (Bedinger and others, 1979). The thermal springs emerge from the plunging crest 

line of a large overturned anticline in the Zigzag Mountains of 

the Ouachita anticlinorium. The overturned anticline plunges 

toward the southwest into the Mazarn Basin. 

Arndt and Stroud (1953) and Fellowes (1968) recognized 

two major thrust faults trending nearly parallel to fold axes that 

define the northern and southern limits of the thermal springs 

discharge area (fig. 6). The southern fault, identified as Beta 

Fault, extends northeastward about 9,000 ft roughly along the 

axis of the Hot Springs anticline, and dips about 44 degrees 

north. The northern fault, identified as Alpha fault, extends 

nearly parallel to Fountain Street northeastward about 9,200 ft 

onto the southeast flank of North Mountain, and dips about 26 

degrees north. At the northern extent of the thermal springs, 

Alpha fault is suggested to form along the bedding contact of 

the Hot Springs Sandstone and Stanley Shale, with the Stanley 

Shale forming the hanging wall of the fault. Bedinger (1979) 

proposed a fault that splits away from Beta fault, trends west 

and connects with Alpha fault (fig. 6). A natural ravine trends 

along the location of this fault.

Extensive cracks, joints, and fissures in the Bigfork Chert, 

Arkansas Novaculite, and the Hot Springs Sandstone produce 

the primary permeability in the thermal springs area. Purdue 

(1910) attributed the intensely fractured and brittle nature of the 

Bigfork Chert as a primary factor in defining it as the most probable recharge area for meteoric water infiltration. Bedinger and 

others (1979) observed that jointing is common in the few exposures of Hot Springs Sandstone in the thermal springs discharge 

area, and that these are associated with thrust faults, normal 

faults, and joints on the plunging anticline of Hot Springs 

Mountain. Bryan (1922) observed strong, nearly vertical jointing in the northeasterly direction in outcrops of the Hot Springs 

Sandstone on the hillside behind the bathhouses. He suggested 

that there are at least four lines of jointed systems that are 

arranged along lines of thermal springs, and parallel to a postulated thrust fault, later recognized as Beta fault by Arndt and 

Stroud (1953). Arndt and Stroud (1953) believed that jointing 

and shattering of Arkansas Novaculite in the North Mountain 

Syncline and adjacent to faults created preferred channels for 

migration of water directly into the thermal springs area, and 

that water in the lower member, confined by middle member 

shales of the Arkansas Novaculite, was then forced upward to 

the thermal springs through fissures.

Characteristics of Thermal Springs 

The water temperature and discharge of the thermal 

springs were monitored infrequently prior to continuous monitoring in 1988. However, a rough comparison between the 

results of the 1901 (Haywood, 1902) and 1976 (Jacobs, 1988) 

investigations indicates that temperature and discharge did not 

change substantially from 1901 to 1976 (table 3). The total discharge estimated in 1901 (826,000 gal/d) was about equal to the 

total discharge estimated in 1976 (829,000 gal/d). The average 

discharge-weighted water temperature in 1901 was 58.8 °C, and 

the average discharge-weighted water temperature in 1976 was 

59.9 °C. The lower temperature in 1901 may be attributed to 

several factors unrelated to the source of the thermal water. The 

1901 investigation included several springs of lower temperature (such as springs 27-30 and 32-37) that did not exist or were 

not of sufficient discharge to be measured in 1976. Also, the 

1901 temperatures were measured in January, during the winter 

season, whereas, the 1976 temperatures were measured at 

unspecified times, but probably during warmer periods. 

Collection System Discharge and Water Temperature

To better understand the condition of the thermal springs, 

a system of weirs, water level, and temperature devices were 

installed at the collection system reservoir in 1988 to monitor 

the combined discharge and temperature of all thermal springs 

collected. The discharge and temperature have been monitored 

continuously from October 1, 1990 through September 30, 

1995 and October 1, 1998 through June 10, 2005. The period 

from October 1, 1995 through September 30, 1998 was not 

monitored. By accurately and continuously monitoring the 

combined discharge and temperature of the collected springs 

over a long period of time, response to natural and man-induced 

effects may be documented and temporal patterns evaluated. 

Discharge

The average daily discharge at the inflow of the collection 

system reservoir was 658,000 gal/d and ranged from 518,000 to 

763,000 gal/d from 1990 through 1995, and 1998 through 2005, 

not including springs 43 and 43a that emerge from the open bottom of the collection system reservoir. The discharge from 

springs 43 and 43a was calculated in 1989 to be 131,000 gal/d 

(C. Shane Barks, U.S. Geological Survey, written commun., 

1989). Adding springs 43 and 43a discharge to the collection 

system discharge gives an average daily discharge of 789,000 

gal/d and range of discharge from 649,000 to 894,000 gal/d. 

The overall pattern of the collection system discharge from 

1990 through 2005 shows an increasing rate of discharge, 

although the majority of increase in discharge took place during 

the period from 1990 through 1995, and the discharge pattern 

during the period not monitored from 1995 through 1998 is 

unknown (fig. 7). The discharge appears to increase and 

decrease cyclically at a 4-5 year interval during the period from 

1990 through 2005, although data are insufficient to substantiate this observation.
Table 3. Historical record of thermal springs temperature and discharge.[e, estimated; °C, Celsius; gal/d, gallons per day; w/spring number, included with spring indicated; --, no data]

Spring

number Spring Name

1890

(Branner, 1892)

1900

(Haywood, 1902)

19011

(Haywood, 1902)

1931

(Hamilton, 1932)

1952

(Kuroda, 1953)

1972

(Bedinger, 1979)

1

(Jaco

Temperature

(°C)

Temperature

(°C)

Temperature

(°C)

Discharge

(gal/d)

Temperature

(°C)

Discharge

(gal/d)

Temperature

(°C)

Temperature

(°C)

Temperature

(°C)

1 Egg 62.6 61.9 61.7 28,800 -- 9,600 62.0 54.6 58.9 2 Arsenic South -- 51.9 53.9 w/17 54.4 -- -- -- 55.0 3 Arlington -- 61.7 61.3 19,938 60.0 w/5 62.2 52.2 60.0 4 Cliff -- 55.9 52.4 3,600 57.2 -- -- -- 60.6 

4a new spring 1 -- -- -- -- -- -- -- -- w/4 

5 Avenue -- 61.4 61.9 17,280 61.1 21,800 61.7 -- 60.6 6 Boiler House -- 57.5 58.3 32,400 57.2 -- 58.6 -- 61.1 

6a Cooler North -- -- -- -- -- -- -- -- 60.0 

6b Cooler South -- -- -- -- -- -- -- -- 54.4 

6c new spring 2 -- -- -- -- -- -- -- -- w/6 

7 Imperial North -- 60.1 60.8 18,514 -- 1,760 62.2 59.3 59.4 8 Crystal -- 35.2 36.2 e2,000 61.1 w/5 -- -- 53.9 9 Rector 59.6 61.1 62.4 51,840 62.2 18,000 61.2 -- 54.4 

9a new spring 3 -- -- -- -- -- -- -- -- w/0 

10 Cave -- 57.4 57.2 18,514 60.0 14,400 -- -- -- 11 Little Iron North -- -- 56.8 w/9 64.4 32,400 61.2 -- w/9 12 Little Geyser -- 36.2 36.2 524 -- 0 -- -- -- 13 Little Iron South -- -- 56.3 w/9 w/11 w/11 61.2 -- w/9? 14 Ral 59.6 60.9 62.8 8,640 -- 0 -- -- -- 15 Big Iron 63.6 63.9 63.9 201,600 64.4 -- 61.2 -- 66.7 16 Imperial South -- 60.8 60.9 w/7 -- 0 -- -- -- 17 Arsenic North -- 55.4 56.4 10,800 -- -- 56.9 56.0 57.2 18 Hitchcock -- 57.3 57.3 e35,000 52.8 90,000 59.6 -- -- 19 Superior Bath -- 56.4 56.1 13,292 -- -- -- -- 62.8 20 Superior North -- 46.3 44.5 3,677 44.4 1,690 -- -- -- 21 Alum 46.6 43.3 46.0 1,152 -- -- -- -- -- 

21c new spring 4 -- -- -- -- -- -- -- -- -- 

22 Superior South -- 57.1 56.5 1,723 56.1 2,460 -- -- -- 23 Twin North -- 62.0 62.4 w/24 50.0 2,500 59.6 56.2 62.8 24 Twin South -- 62.3 60.3 10,800 57.8 2,500 54.3 -- 63.9 
25 Hale Bath 61.6 62.7 62.9 e35,000 -- -- 63.3 -- 60.6 26 Palace -- 63.4 61.4 25,847 63.3 10,950 -- -- 63.3 27 Tunnel -- -- 51.9 800 -- -- 59.2 -- -- 28 Maurice -- -- 59.8 e21,000 -- -- 60.0 -- -- 29 Dripping -- 57.1 57.8 2,618 -- -- 61.1 -- -- 30 Arch -- 53.9 51.9 -- -- -- -- -- -- 31 Haywood -- 51.4 51.4 7,200 54.4 130 54.0 -- 57.8 32 Noble -- 46.0 46.5 28,800 45.0 -- 52.5 52.5 -- 33 Lamar -- 48.3 49.2 w/31 -- -- -- 57.6 -- 34 Wiley -- 47.9 47.3 28,800 -- -- 57.5 -- -- 35 Hardin -- 39.0 43.0 2,469 -- -- -- -- -- 36 Eisele -- 48.9 48.8 9,600 -- -- -- -- -- 37 Stevens -- 52.9 52.6 5,760 -- -- -- -- -- 38 Horseshoe -- 58.8 59.8 e40,000 w/42 w/42 60.3 -- w/42 39 Army and Navy -- 61.4 61.4 35,000 -- -- -- -- -- 40 W.J. Little -- 48.9 48.9 4,320 -- -- -- -- -- 41 Mud -- 46.8 48.3 e4,000 -- -- -- -- -- 42 Quapaw Bath 51.6 -- 58.3 e50,000 60.6 12,000 60.8 61.3 62.8 43 Reservoir North -- 46.3 46.1 e20,000 50.0 140,000 -- -- 50.0 

43a Reservoir South -- -- -- -- -- -- -- -- w/43 

46 Fordyce Bath -- 51.5 51.5 e25,000 57.2 32,200 -- 58.3 60.0 47 New North -- no flow -- -- 58.6 13,500 61.7 -- 64.4 48 New South -- no flow -- -- w/47 w/47 62.2 60.0 62.2 49 USGS -- no flow -- -- -- -- 61.0 61.8 64.4 50 Maurice Bath -- no flow -- -- 62.8 -- -- 53.3 57.2 

Total 58.84 826,3085 59.94 

1Spring 30 discharge could not be estimated; springs 47-50 were not discharging at the time of analyses. 2Springs 43 and 43a temperatures are from Hamilton (1932); spring 13 included with spring 9 is not certain. 3Temperatures in 2005 were measured on March 8, 2005, except for springs 27, 32, and 33 which were measured on October 6, 2004. 4Discharge-weighted average temperature of all measured springs. 5Total estimated discharge of all measured springs.

Table 3. Historical record of thermal springs temperature and discharge.—Continued[e, estimated; °C, Celsius; gal/d, gallons per day; w/spring number, included with spring indicated; --, no data]

Spring

number Spring Name

1890

(Branner, 1892)

1900

(Haywood, 1902)

19011

(Haywood, 1902)

1931

(Hamilton, 1932)

1952

(Kuroda, 1953)

1972

(Bedinger, 1979)

1

(Jaco

Temperature

(°C)

Temperature

(°C)

Temperature

(°C)

Discharge

(gal/d)

Temperature

(°C)

Discharge

(gal/d)

Temperature

(°C)

Temperature

(°C)

Temperature

(°C)
Characteristics of Thermal Springs 19

Figure 7. Daily and annual average discharge and temperature of the thermal springs at the inflow of the collection system reservoir.

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The annual average discharge and temperature show how 

the annual cycle has changed over the period of record (fig. 7). 

Of particular interest is how the annual discharge changed in 

comparison to the annual temperature. During the period from 

1990 through 1995, the temperature generally showed a positive relation to discharge. This would suggest that increased discharge in the thermal water component relative to the coldwater component of the thermal springs caused increased water 

temperature during this period, because increased discharge in 

the cold-water component would have an inverse effect on the 

water temperature. One possible explanation is that more thermal water was added to the system through man-induced 

changes such as collection system upgrades or adding thermal 

spring flow to the system. Another possible explanation may be 

that thermal spring discharge shifted from the upper springs to 

the lower springs. Hamilton (1932) reported that discharge of 

the upper springs decreased from 167,000 gal/d in 1901 to 

124,000 gal/d in 1931. If discharge did shift to the lower 

springs, there should be an overall increase in spring discharges 

because the springs would be subjected to less elevation pressure head. Monitoring discharge and temperature at selected 

individual springs would better show when changes are natural 

or man-induced. Currently (2005), only temperature is monitored at selected springs.

During the period from 1998 through 2005, the annual 

average temperature showed a strong inverse relation to the 

annual average discharge. This would suggest that increased 

discharge in the cold-water component relative to the thermal 

component of the thermal springs caused decreased water temperature during this period. This is the result that would be 

expected if cold-water recharge from rainfall was the main 

cause of change to thermal spring discharges.

The collection system daily discharge showed a noticeable 

immediate short-term response to rain events, increasing in discharge when rainfall contributed recharge to the ground-water 

system. The discharge increased by as much as 100,000 gal/d 

(15 percent) in response to major rain events. Water temperature also decreased considerably, indicating that the increase in 

discharge during rain events was primarily, if not wholly, from 

increase in the cold-water component of flow, although infiltration through collection system piping (leaks) also may contribute.

Water Temperature

Continuous water temperature monitoring conducted by 

the USGS from 1990 through 1995 and 1998 through 2005 at 

the collection system reservoir inflow pipe shows that there has 

not been a long-term temperature change during the past 15 

years (fig. 7). The daily water temperature ranged from 59.1 to 

62.1 °C and the average daily temperature was 61.4 °C for the 

period of record. The 1976 average discharge-weighted temper-
20 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

ature was 59.9 °C at the thermal springs. The 15-year collection 

system temperature does not include the water from springs 43 

and 43a that emerge from the bottom of the reservoir, while the 

1976 temperature calculation does include temperature from 

these springs. If springs 43 and 43a temperatures of 50.0 °C 

(Hamilton, 1932) discharging at 131,000 gal/d (C. Shane Barks, 

U.S. Geological Survey, written commun., 1989) were included 

in the collection system temperature, the average dischargeweighted collection system temperature would be about 59.5 

°C. Heat loss between the spring source and the collection system reservoir during transmission is one of many variables that 

could account for the recent collection system temperature 

being lower than the water temperature measured at the springs 

in 1976.

The collection system temperature shows a strong seasonal 

pattern, with maximum temperatures occurring from August 

through October and minimum temperatures occurring from 

January through March, about one month delayed from air temperature highs and lows. This suggests that air temperature 

influences the thermal springs and the shallow ground water 

that mixes with the thermal water. Seasonal change in temperature at the reservoir inflow also may be from heat loss in the collection system. Disregarding short-term fluctuations in water 

temperature, the seasonal temperature varied about 1 °C, from 

about 60.9 to 61.9 °C.

The collection system temperature also shows substantial 

response to rainfall. One of the largest responses recorded was 

on February 16, 2001 (fig. 8), when the hourly temperature 

decreased from 60.7 to 58.6 °C, while hourly inflow increased 

from 705,000 to 812,000 gal/d, in response to heavy rainfall; 3.5 

inches of rain was measured on February 15-16, 2000, at Hot 

Springs Memorial Field Airport, about 3 miles southwest of the 

thermal springs (Elizabeth Sanders, Southern Regional Climate 

Center, written commun., 2005).

Figure 8. Thermal springs collection system hourly temperature and discharge response to 3.5 inches of rainfall on February 15-16, 2001.

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Characteristics of Thermal Springs 21

Thermal Springs Temperature Monitoring

The water temperature of eight thermal springs and one 

thermal springs collection box was monitored during the period 

of investigation from 2000 through 2005. Springs 8, 17, 25, and 

46 were monitored from August 2000 through June 2005, and 

springs 15, 19, 42, 49, and collection box 1 were monitored 

from September 2003 through June 2005 (fig. 3). Collection 

box 1 (CB1) collected flow from springs 1, 3, 47, and 48. Each 

spring displayed different temperature magnitudes ranging 

from about 50 to 65 °C and showed a unique response to environmental influences, such as air temperature and rainfall. General similarities among springs above the promenade that differed from springs on Bathhouse Row suggest that the springs 

may be grouped for evaluation purposes. The springs in group 

1 (spring 17 and promenade springs 8, 49, and CB1) generally 

showed strong seasonal patterns and sharp responses to rainfall 

and air temperature. The springs in group 2 (Bathhouse Row 

springs 15, 19, 25, 42 and 46) generally showed more stable 

temperatures with less response to rainfall and air temperature. 

Springs 17 and 46 displayed unique temperature responses, but 

were grouped with the other springs based on the seasonal pattern of the springs. 

Diurnal and annual air temperature fluctuations (fig. 9) 

affect the water temperature of the springs. Thermal springs 

respond to air temperature because air temperature affects the 

ground temperature at the point of emergence, and air temperature affects the water temperature of shallow ground water that 

mixes with the thermal spring water. Some springs that emerge 

at depths below the land surface, such as the bathhouse springs 

and excavated springs, are less likely to be affected by surface 

temperatures. 

The effects of rain events are evident at each spring to 

varying degrees. Recharge from rainfall generally causes downward spikes in water temperature. The response of the water 

temperature to rain events is the most direct evidence that cooler 

ground water is mixing with the thermal water of the springs. 

Decreases in thermal spring temperature show a poor linear 

relation to rainfall events, though, because there are other factors involved in how the thermal springs respond, such as antecedent soil saturation conditions and the temperature, intensity, 

duration, and areal distribution of rainfall during a rain event.

Figure 9. Hourly air temperature at Hot Springs Memorial Field Airport, Hot Springs, Arkansas, 2005.

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22 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Group 1 Thermal Springs

Group 1 springs 8, 17, 49, and CB1 (composite flow from 

springs 1, 3, 47, 48) share several common attributes. All group 

1 springs lie near the north-northeastern boundary of the thermal springs area. Springs 8, 49, and CB1 are located in the same 

general area on the lawn above the promenade, and are the highest and most northeastern thermal springs in the study area. 

Spring 17 is the most northwestern spring in the study area, 

lying about 50 ft directly downgradient from CB1. Group 1 

springs generally show strong seasonal patterns and sharp 

responses to rainfall events and diurnal air temperature (fig. 10).

The temperature at spring 17 on Bathhouse Row is considerably lower than the temperature at springs 8, 49, and CB1 

above the promenade. Springs 8 and 49 are about the same temperature, averaging 61.5 °C. CB1 temperatures generally are 2 

°C lower than temperatures of springs 8 and 49, although the 

actual temperatures at the springs that CB1 collects probably 

are higher because of heat loss during transmission to CB1. 

Spring 17 on the northern edge of the thermal springs area 

exhibits the lowest temperatures of all the thermal springs monitored, averaging about 52.7 °C. Spring 17 lies about 150 ft from 

well W2; well W2 predominantly is influenced by cold water, 

with temperatures near the surface ranging from 20 to 25 °C 

(June 2004 through May 2005). Recent (2000-2005) temperatures measured at spring 17 also are lower than have been 

reported in the recorded past (table 3), suggesting that colder 

shallow ground-water influence increased sometime after 1976 

when the recorded temperature was 57.2 °C.

All group 1 springs show strong seasonal water temperature patterns that coincide with air temperature. Group 1 shows 

temperature highs from August through September and lows 

from February through March, about one month later than air 

temperature highs and lows. The differences between the seasonal high and low spring temperatures ranged from about 1.0 

°C difference at springs 8 and 49, to about 4.0 °C at spring 17. 

Springs 8, 49, and CB1 responded to most rain events that 

contributed recharge to ground water, while spring 17 response 

was more gradual and of less magnitude. After large rain events, 

the water temperature monitored hourly at spring 17 decreased 

about 0.2 °C, spring 49 decreased about 0.5 °C, CB1 decreased 

about 1.0 °C, and spring 8 dropped several degrees Celsius. 

Spring 8 hourly temperature decreased more than 5.0 °C during 

at least eight rain events for the period of record from August 

2000 through June 2005. The largest hourly temperature 

decrease at spring 8 for the same period of record was about 20 

°C on February 16, 2001. During the same rain event, spring 17 

showed a gradual decrease of only a few tenths of a degree Celsius over a 2-3 day period. This suggests that recharge from 

rainfall may take a longer ground-water path to spring 17 and is 

more thoroughly mixed than at the other springs. This spring 

also may be influenced by subsurface flow from Hot Springs 

Creek.

Hourly variations in spring temperatures because of daily 

air temperature changes are evident in group 1. Springs 8 and 

CB1 show daily temperature fluctuations of about 0.10 and 0.20 

°C, respectively, with highs and lows coinciding similarly to 

daily air temperature highs and lows (fig. 11). Daily temperature fluctuations at springs 17 and 49 are less evident, although 

apparent fluctuations between daily highs and lows of about 

0.05 °C were observed.

Figure 10. Daily average water temperature of group 1 springs and rainfall, 2000-2005.
Characteristics of Thermal Springs 23

Figure 11. Hourly air temperature fluctuations compared to hourly water temperature at group 1 springs, July 10-20, 2004.

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Group 2 Thermal Springs

Group 2 springs 15, 19, 25, 42, and 46 share several common attributes. All Group 2 springs are located along bathhouse 

row. Four of the springs (19, 25, 42, and 46) are in bathhouses. 

Springs 15, 19, 25, and 42 discharge large quantities of water 

with the highest and most stable temperatures monitored. Group 

2 springs generally show less response than group 1 springs to 

seasonal effects and environmental influences (fig. 12). Spring 

46 differs from other springs in group 2 because of a lower 

water temperature and larger responses to diurnal temperature 

changes and rainfall events. Spring 46 also shows responses to 

man-induced changes in the distribution system and spring 

chamber that did not occur at the other springs.

The average water temperature for the period of record in 

group 2 decreases towards the south except for spring 46. 

Spring 15 is the most northerly spring in group 2 and exhibits 

the highest average temperature of 64.9 °C. Spring 19, located 

about 35 ft south of spring 15, shows a slightly lower average 

temperature of 64.5 °C. The average water temperature continues to decrease towards the south with spring 25 at 63.1 °C, and 

spring 42 at 61.3 °C. Spring 46, located between springs 25 and 

42, does not follow the same pattern, showing a lower average 

temperature of 55.4 °C. The lower temperature at spring 46 suggests it may be influenced by another cold-water source, possibly Hot Springs Creek. The decreasing pattern from spring 15 

to springs 19, 25, and 42 in a southerly direction may be the 

cumulative effect of cold-water recharge as it collects at the 

base of Hot Springs Mountain and continues to accumulate as 

the cold-water recharge flows south along Central Avenue and 

Hot Springs Creek.

All group 2 springs show subdued seasonal water temperature patterns with peaks 2 to 5 months delayed from the air 

temperature cycle. The differences in spring seasonal high and 

low water temperatures range from about 0.2 °C at spring 19 to 

about 0.5 °C at spring 42. Springs 15, 19, 42, and 46 show temperature highs around September and lows around March, 

which is about 2 months later than air temperature highs and 

lows. Spring 25 exhibited a more delayed seasonal cycle of 

about 5 months, with highs in December and lows in June. The 

delayed seasonal responses at the springs suggest that the seasonal cycle also may be affected by seasonal rainfall through 

shallow ground-water paths. The delayed seasonal pattern at 

spring 25 may suggest that shallow ground water takes a longer 

pathway or has a larger recharge area than at springs 15, 19, 42, 

and 46. Spring 25 also may be more affected by subsurface flow 

along Hot Springs Creek, which is about 50 ft west of spring 25.

The seasonal temperature response at spring 46 is hard to 

discern because of responses to artificial changes in the distribution system and spring chamber, but the response to seasonal 

temperature probably is lessened at this spring because it is 

located in the basement of the Fordyce bathhouse, well insu-
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24 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Figure 12. Daily average water temperature of group 2 springs and rainfall, 2000-2005.

lated from the outside changing environment. Springs 19, 25, 

and 42 also are located in the basement of bathhouses, probably 

lessening the seasonal influence of these springs. Spring 15 is 

not located in a bathhouse, but the spring emerges from about 

15 ft below the land surface, probably providing an insulating 

effect at this spring as well.

Short-term variations in spring temperatures because of 

diurnal air temperature changes and rain events are small or 

negligible in group 2 springs. Springs 42 and 46 show slight 

daily temperature fluctuations of about 0.02 °C and 0.05 °C, 

respectively, which roughly coincide with daily air temperature 

changes. Springs 15, 19, and 25 do not appear to show a daily 

pattern. None of the group 2 springs show discernible responses 

to moderate rain events. During heavy rainfall of several inches, 

springs 15, 19, and 42 decreased less than 0.1 °C, usually over 

a several-day period. Springs 25 and 46 may drop in temperature several tenths of a degree Celsius during heavy rainfall. 

Two responses at springs 25 and 46 are especially evident during rain events on February 16 and December 16, 2001. The 

largest temperature decrease occurred on February 16, 2001, 

when spring 25 decreased 0.6 °C and spring 46 decreased 2.3 

°C. Temperature responses at springs 25 and 46 occurred during 

the wet season and after several inches of rain had fallen in previous days. This indicates that these two springs appear to only 

respond during saturated soil conditions. Both springs 25 and 

46 are about 50 ft from Hot Springs Creek, and the elevation of 

the springs is below the top of the underground creek arch, 

which fills beyond capacity about every 3 years (U.S. Army 

Corps of Engineers, 1993) and near capacity annually. Infiltration through fill material between the creek and springs or overland during flooding are possible pathways of flow to the 

springs.

Characteristics of the Shallow GroundWater System

The thermal springs consist of two components of flow: 

deep thermal-water flow and shallow cold-water flow. The purpose of this report is to describe the shallow ground-water system, although, because the deep and shallow ground-water systems are connected and interact near the surface, a brief 

discussion of the deep ground-water system also will be presented.

According to the hypotheses of past investigators, the thermal-water component at HSNP enters the ground-water system 

as meteoric water from regional recharge areas in the fractured, 

permeable Bigfork Chert and Arkansas Novaculite (Bedinger 

and others, 1979; Arndt and Stroud, 1953; Purdue, 1910). These 

formations outcrop in exposed anticlinal structures to the west, 

north, and east of the thermal springs. The waters migrate to 

estimated minimum depths of 4,500 to 7,500 ft and are heated 

in the deep section of the flow path before rising through over-
Characteristics of the Shallow Ground-Water System 25

lying geological units along fault and fracture conduits 

(Bedinger and others, 1979). Under artesian pressure, the thermal waters rise and emerge through the Hot Springs Sandstone 

between the traces of two thrust faults, along several northeasttrending lineaments (Bedinger and others, 1979; Bryan, 1922) 

(fig. 13). 

There are several theories of how the thermal waters are 

heated. The one presented by Bedinger and others (1979) 

assumes that the waters are heated by geothermal gradient. The 

recharge area in the anticlinal structure to the northwest of the 

thermal springs is about 1 mile from the thermal springs, which 

is a short distance for the Alpha and Beta faults to reach depths 

of 4,500 to 7,500 ft. Recharge areas to the east-northeast are 

more probable for the geothermal model to work (illustrated by 

the arrows of flow from the right in figure 13). Although, alternative theories of the heat source, such as uncooled masses of 

igneous rock (Purdue, 1910; Bryan, 1922), juvenile waters rising and heating shallower meteoric waters (Arndt and Stroud, 

1953), and an abnormally high geothermal gradient (Fellowes, 

1968) could be used to explain how recharge from the northwest 

anticlinal structure contributes to the thermal springs.

The ground-water composition of the shallow, cold-water 

component of the thermal springs is probably similar in origin 

to cold-water wells and cold-water springs in the immediate 

area of HSNP. Cold water enters the ground-water system as 

locally derived recharge from rainfall and primarily flows along 

faults, joints, and fractures to the thermal springs. Cold-water 

recharge from losing streams in the study area (Hot Springs 

Creek and Happy Hollow Creek) appears less likely based on 

topography, stratigraphy, and faults, except during large rain 

events when water levels are abnormally high. Surface and 

unsaturated flow probably also contribute recharge during 

storm events.

Figure 13. Conceptual model of the thermal water flow system.

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26 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Hydrologic Properties of Local Lithologies

Formations composed predominantly of shale generally 

may be considered to impede ground-water movement, contribute small quantities to recharge, and yield meager quantities to 

wells (Bedinger and others, 1979). Shale formations in the 

vicinity of HSNP include the Womble Shale, Missouri Mountain Shale, Polk Creek Shale, and Stanley Shale. The emergence 

of the thermal springs from the Hot Springs Sandstone above 

the contact with the Stanley Shale supports the concept that the 

Stanley Shale serves as a low-permeability barrier. Groundwater studies in Arkansas show the Stanley Shale generally 

yields only a few gallons per minute (Halberg and others, 1968).

Ground-water flow in the vicinity of the thermal springs 

occurs primarily in formations that are dense and hard; tending 

to crack or break under stress. These formations include the 

Bigfork Chert, Arkansas Novaculite, and Hot Springs Sandstone. The Bigfork Chert is not exposed on or adjacent to Hot 

Springs Mountain. Therefore, the contribution of the Bigfork 

Chert to cold-water recharge to the thermal springs is less 

likely, but possible from distant artesian sources.

The Arkansas Novaculite is highly resistant to erosion and 

supports the ridge of Hot Springs Mountain. It is locally 

intensely jointed (Bedinger and others, 1979) especially in the 

lower massive and dense member of the formation, which is 

presumed to be the major water-bearing unit of the formation 

(Arndt and Stroud, 1953). The lower member makes up the 

majority of surficial geology on Hot Springs Mountain and, 

therefore, probably represents a major recharge area on Hot 

Springs Mountain. The middle member of the Arkansas Novaculite consists mostly of shales with thin chert beds and may act 

as a barrier between the lower and upper members of the formation, creating conditions for a confined aquifer in the lower 

member (Arndt and Stroud, 1953). The upper member of the 

formation is less densely fractured chert with increased intergranular permeability, interbedded with minor shale and conglomerate. There is a thin shale zone between the top of the 

upper member and the basal conglomerate of the overlying Hot 

Springs Sandstone in Gulpha Gorge east of Hot Springs Mountain (Arndt and Stroud, 1953). However, fracturing and faulting 

probably supply some conduits of flow between the upper 

member and Hot Springs Sandstone.

Like the Arkansas Novaculite, the Hot Springs Sandstone 

is highly resistant to erosion with locally intense joints and fractures that increase permeability. The basal contact with the 

Arkansas Novaculite consists of conglomerate pebbles and cobbles in a dense siliceous matrix. The upper part of the Hot 

Springs Sandstone grades into the Stanley Shale by an increase 

in the amount of shale (Arndt and Stroud, 1953). Surface exposures of the Hot Springs Sandstone are enclosed by the Stanley 

Shale on all sides of Hot Springs Mountain except toward the 

northeast topographic divide. 

Principal Conduits and Barriers

The principal conduits of shallow ground-water flow 

appear to occur along lines of southwest-northeast trending 

faults, joints, and fractures. Evidence of minor and major faults 

trending northeast have been documented by Bryan (1922), 

Arndt and Stroud (1953), Bedinger and others (1979), and 

Bedinger (1994). The two major faults, Alpha and Beta (Arndt 

and Stroud, 1953) that trend along the northern and southern 

boundaries of the thermal springs area are theorized to represent 

the permeable conduits through which the thermal water rises to 

shallow depths. Thermal water is distributed to the thermal 

springs through joints and fractures in the Hot Springs Sandstone and Arkansas Novaculite. The faults, joints, and fractures 

also represent pathways where adjacent cold ground water 

could mix with the thermal water. How much mixing occurs 

would depend on the head of the thermal water in relation to that 

of the cold water and dynamics of the system.

The shallow ground-water system associated with the thermal springs is bounded on three sides—southwest, southeast, 

and northwest—by shale barriers aligned to the structural trend 

of folds and faults. To the southwest, the Stanley Shale forms a 

barrier over the southwest plunging Hot Springs Sandstone. To 

the southeast, the Stanley Shale also forms a barrier below the 

southeast limb of the overturned Hot Springs anticline, and Beta 

fault trends between the southeast limb and the thermal springs 

area. The final barrier to shallow ground-water flow lies to the 

northwest along the northeast-trending Alpha fault. North of the 

thermal springs, Alpha fault lies along the contact between the 

Stanley Shale and Hot Springs Sandstone (Bedinger, 1994; 

Arndt and Stroud, 1953). Alpha fault probably developed in the 

less competent shale beds of the Stanley Shale, thrusting the 

overturned Stanley Shale over the Hot Springs Sandstone (fig 

6). The emergence of the thermal springs from the Hot Springs 

Sandstone near to the contact with the Stanley Shale implies 

that the low permeability of the Stanley Shale creates a barrier 

that directs the springs to emerge in the Hot Springs Sandstone 

(Bedinger, 1994). 

Shallow Aquifers and Cold-Water Recharge

The lower member of the Arkansas Novaculite is probably 

the primary aquifer that contributes a continuous supply of shallow, cold ground water to the thermal springs area. It is the 

thickest of the exposed rock units with relatively large outcrops 

on Hot Springs Mountain and North Mountain and with extensive jointing and fracturing. The underlying Missouri Mountain 

Shale and the overlying shales of the middle member of the 

Arkansas Novaculite also may seal off the lower novaculite 

aquifer, creating confined conditions. If the lower novaculite is 

saturated sufficiently, the resulting pressure head may force 

confined water upward from depth, through fractures to the 

thermal springs at the surface (Arndt and Stroud, 1953).

It is unclear how much ground-water flow from the Arkansas Novaculite and Hot Springs Sandstone on the southeast limb 
Characteristics of the Shallow Ground-Water System 27

of the Hot Springs anticline reach the thermal springs area. Cold 

ground water on the southeast flank of Hot Springs Mountain 

was reported to emerge at the contact of the Hot Springs Sandstone and Stanley Shale in seepages and some small springs 

(Kirk Bryan, U.S. Geological Survey, written commun., 1922). 

Well W4 was drilled in 1922 in an effort to capture this water. 

However, the well encountered predominantly thermal water 

reaching temperatures of up to 54 °C throughout the well except 

for a small quantity of cold water reported in the upper part of 

the well (Kirk Bryan, U.S. Geological Survey, written commun., 1922). Therefore, it is evident that thermal water leaks 

into the ground-water system and mixes with cold ground water 

at least in the vicinity of the well. Thermal water probably 

migrated from the thermal springs area and possibly from Beta 

fault on the southern limits of the thermal springs area.

Recharge directly around the spring catchments and 

recharge to the Hot Springs Sandstone and Arkansas Novaculite 

directly upgradient from the thermal springs contribute an 

unknown amount of cold water to the thermal springs during 

rain events. Temperature monitoring at selected thermal springs 

indicates the water temperature decreases within minutes to 

hours of recharge from rainfall. This is especially true of the 

higher thermal springs on Hot Springs Mountain, which are less 

sheltered from weather than the lower springs. The lower thermal springs are commonly covered by the bathhouses or 

increased ground cover over the spring source.

Ground-Water Levels

The altitudes of the water table in wells W2, W3, and W5 

(well depths 200, 190, and 202 ft, respectively) were measured 

simultaneously on July 7, 1993, prior to aquifer testing 

(Bedinger, 1994). The wells are located just north of the thermal 

springs discharge area. The water-table altitudes were 604.17 ft 

(W2), 608.96 ft (W3), and 612.35 ft (W5) above NGVD 1929. 

Based on these three data points, the direction of ground-water 

flow was south-southwest, generally towards Hot Springs 

Creek. Temperatures, silica and sulfate concentrations, and the 

specific conductance of samples from wells W2 and W3 were 

between those of the thermal springs and typical cold-water 

springs, whereas, the chemistry of well W5, a few hundred feet 

farther north from the thermal springs area, was more similar to 

cold-water springs (Pearson, 1994).

The altitude of the water level at well W4 (well depth 336 

ft) on Hot Springs Mountain was considerably higher than the 

other wells, at about 629 ft above NGVD 1929 on July 7, 1993. 

This would be expected if the direction of ground-water flow 

followed the slope of the mountain southwest, towards Hot 

Springs Creek. Well W4 appears to respond to similar hydrologic conditions as well W2, according to the water-level measurements recorded at the two wells in 2004-2005 (fig. 14). The 

water level at well W2 is considerably more responsive to atmospheric conditions than well W4, probably because the depth of 

water at W2 is near land surface, whereas, the depth of water at 

Figure 14. Hourly water levels in wells W2 andW4 and daily rainfall, 2004-2005.

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28 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

W4 is about 100 ft deeper. The hydraulic head in well W4 

increased more than well W2 over the recorded period. This 

suggests that the sources of recharge to W4 are more continuous 

or retained longer, as opposed to the more flashy response to 

recharge at W2.

Thermal Springs Hydraulic Connection

There are several examples that show how many of the 

thermal springs are in hydraulic connection. Kirk Bryan (U.S. 

Geological Survey, written commun., 1922) reported that thermal springs 37 and 40 ceased to flow after spring 39, converted 

to a well, began to be used. Bryan also reported that spring 49, 

created during spring collection systems excavations, caused 

the drying up of adjacent springs. Other springs that were 

reported to develop as a result of excavation include springs 5, 

7, 16, 37, 47, and 48 (Weed, 1902). Springs 12 and 14 reportedly lost most of their outflow when spring 16 was deepened 

(Weed, 1902). Weed (1902) also reported that spring 36 did not 

flow when water was pumped from the well on the military reservation (Hot Springs Rehabilitation Center on Reserve Avenue). Bedinger (1994) assessed during aquifer tests that wells 

W2, W3, and W5 were all hydraulically connected. Furthermore, Bedinger (1994) attributed a recharge boundary indicated 

by the drawdown plot at well W2 “to a partial hydraulic connection between the aquifer at the west well (W2) and Hot Springs 

Creek.”

A hydraulic connection appears to be present between the 

lower springs on Bathhouse Row and the upper springs above 

the promenade, as evidenced by a decrease in the number of 

upper springs and increase in the number of lower springs over 

the last 100 years, presumably because of excavating and other 

earth-moving activities. Hamilton (1932) reported a decrease in 

discharge of the upper group of springs from about 167,000 gal/

d in 1901 to 124,000 gal/d in 1931. Three old springs in the 

group had gone dry and others showed a decrease in flow. 

Hamilton (1932) commented that “It seems clear that the drilling of wells or the opening of new springs under certain bathhouses has lessened the flow from this group of springs.” 

Alternative Sources of Cold-Water Recharge

Although the source of shallow ground-water flow to the 

thermal springs has been proposed to originate on or adjacent to 

Hot Springs Mountain, alternative paths of ground-water flow 

are plausible. First, vertical infiltration through fill material 

around the spring catchments is possible if the fill material is 

not properly sealed. Most of the entire hillside that lies east of 

Central Avenue has been excavated at some time for construction of the Hot Springs Creek arch, new bathhouses, walkways, 

and new spring catchments and distribution lines. 

Second, lateral infiltration of water from Hot Springs 

Creek to adjacent springs on Bathhouse Row is possible. This is 

a concern because Hot Springs Creek drains a large urban area 

that may contain potentially harmful chemicals, substances, and 

biological hazards. The creek arch was constructed over 100 

years ago with an open floor and fill material between the creek 

and some of the springs. Four thermal springs collected for public use (springs 25, 42, 43, 46) and well W2 are within 45 to 60 

ft of the creek. Bedinger (1994) presents results from an aquifer 

test on well W2 that indicates the well is in hydraulic connection with the Hot Springs Creek and that the creek is a source of 

recharge, supplying water to the well during the aquifer test. 

Heavy rainfall events are more of a concern because the water 

level in the creek rises above the level in many springs and 

flooding may inundate some of the springs. 

Third, ground-water flow in the Stanley Shale across Central Avenue is possible along major faults, such as the Alpha 

and Beta faults discussed. Arndt and Stroud (1953) and 

Bedinger and others (1979) proposed that the Alpha and Beta 

faults do extend across Central Avenue and further southwest, 

although there is no evidence to suggest that sufficient permeability and pressure head exists in the Stanley Shale to conduct 

flow into the thermal springs area. 

Fourth, it is entirely possible that deeper, cold-water flow 

paths exist, that are harder to identify and evaluate. One such 

flow path may be the same path as the proposed thermal water 

flow path through the Bigfork Chert, but instead of meteoric 

water traveling to deep, heated depths, some meteoric water 

may take shorter, shallower flow paths across the Alpha fault 

that extends into the massive lower Arkansas Novaculite, providing a pathway to the surface (Arndt and Stroud, 1953). 

Another flow path may be from the flanks of North and Hot 

Springs Mountains through preferred channels in the trough of 

the plunging North Mountain syncline and adjacent faults, 

directing flow into the thermal springs area.

Cold-Water Recharge Areas

Delineating the cold-water recharge area to the thermal 

springs involves numerous factors such as the hydrologic 

boundaries, soil properties, vegetation types, rainfall frequency 

and distribution, runoff, evapotranspiration, spring discharge, 

and surface-water influence. Many of these factors and their 

associated processes are not well defined for the study area, and 

it was beyond the scope of this investigation to evaluate all of 

the factors involved. Therefore, the information for estimating 

the recharge area was simplified from previous studies to make 

a preliminary estimate of the recharge area possible. Further 

study that adds to the knowledge and understanding of any of 

these factors will help to better define the recharge area.

As discussed previously, the thermal-water component of 

the thermal springs is theoretically derived from distant sources, 

and travels to depths greater than 4,500 ft over periods greater 

than 4,000 years; whereas, the cold-water component of the 

thermal springs is derived locally and travels to shallow depths 

over relatively short periods of time. Therefore, the size and 

boundaries of the cold-water recharge area should be analogous 

to the shallow ground-water recharge area. This cold-water 

recharge area is delineated by, first, estimating the recharge area 
Characteristics of the Shallow Ground-Water System 29

size; second, approximating the recharge area boundaries; and 

third, testing the delineated recharge area using dye-tracing 

techniques.

Recharge Area Size

The size of the cold ground-water recharge area was estimated from a general form of the hydrologic budget in equation 

1 that utilizes the cold ground-water baseflow discharge of the 

thermal springs and the amount of recharge from rainfall. The 

cold ground-water baseflow discharge was estimated from 

equation 2 using silica concentrations and the average daily discharge from the collection system and springs 43 and 43a. 

Using the average silica concentration of nine thermal springs 

(44.4 mg/L sampled in September 2000) for Xmix, the estimated 

proportion of cold-water discharge (fa) during baseflow conditions computes to 6.6 percent. The average daily discharge from 

the collection system and springs 43 and 43a during September 

2000 was 738,000 gal/d. The total cold-water discharge component is 6.6 percent of 738,000 gal/d, or about 48,700 gal/d. This 

computes to about 17.8 million gallons per year (Mgal/yr).

Dugan and Peckenpaugh (1985) estimated the average 

annual ground-water recharge in the general area north of Hot 

springs for woodland and range conditions to be 10 to 15 inches. 

The actual annual ground-water recharge for the study area is 

presumed to be less than 10 inches because of the steep gradient 

on Hot Springs Mountain (20 percent) and the extensive road 

drainage system not applied specifically for HSNP in the model 

used. Recharge values ranging from 5 to 10 in/yr were considered more representative of the study area. Applying 5 to 10 in/

yr recharge from rainfall and 17.8 Mgal/yr cold-water discharge 

from the thermal springs to the hydrologic budget equation 1, 

the cold-water recharge area computes to a range from 0.10 to 

0.20 mi2

. Recharge rates greater than 10 in/yr would decrease 

the estimated size of the recharge area and rates less than 5 in/

yr would increase the estimated size of the recharge area that 

contributes to the shallow ground-water component of the thermal springs. 

Recharge Area Boundaries

Subsurface bedding attitude departs from surface topography on the southeast limb of Hot Springs Mountain and along at 

least two faults that displace subsurface bedding, creating a cold 

(shallow) ground-water recharge area that deviates from the 

surface drainage area of the thermal springs (the surface drainage area was inferred from topographic contours). Four groundwater boundaries are recognized to form the estimated recharge 

area for shallow ground-water flow to the thermal springs: 

southeast, west, northwest, and northeast boundaries (fig. 15). 

On the southeast boundary, the southeast limb of Hot Springs 

anticline is overturned and dips to the northwest towards the 

thermal springs, potentially extending the ground-water 

recharge boundary past the topographic divide, to include the 

overturned Arkansas Novaculite and Hot Springs Sandstone, up 

to the boundary of the Stanley Shale that acts as a barrier. The 

western ground-water boundary parallels the surface drainage 

boundary along Bathhouse Row, and acts as a barrier at the contact with the Stanley Shale. Most springs emerge at or above 

this contact in the Hot Springs Sandstone, although a few 

springs probably follow fractured conduits a short distance into 

the Stanley Shale. The northwest ground-water boundary is a 

fault (Alpha fault) that forms a barrier along the contact of the 

Stanley Shale that has been overthrust adjacent to the Hot 

Springs Sandstone and Arkansas Novaculite on the northwest 

limb of the Hot Springs anticline. The northeast ground-water 

boundary does not appear to have barriers to ground-water flow, 

and is presumed to closely conform to the surface drainage 

boundary, extending the recharge area to the topographic divide 

at the peak of Hot Springs Mountain near lookout point. The 

approximate size of the shallow ground-water recharge area 

based on these boundaries computes to 0.14 mi2

.

The absence of thermal water influence and the presence 

of cold-water sources just outside the projected boundaries of 

the thermal springs recharge area present further evidence of the 

location of the recharge boundaries. On the northwest faulted 

boundary, cold water at Happy Hallow Spring and at well W5 

are evidence that thermal water does not substantially cross the 

fault. On the west boundary, limited discharge of thermal water 

through the Stanley Shale in Hot Springs Creek is evidence that 

thermal water does not substantially flow southwest of the 

springs. On the southeast boundary, cold-water springs have 

been reported to seep or flow along the boundary contact with 

the Stanley Shale (Kirk Bryan, U.S. Geological Survey, written 

commun., 1922).

Dye Tracing

Dyes were released on December 4, 2004, at three sites in 

the estimated recharge area above the thermal springs (fig. 16). 

Dyes at sites 1 and 3 were not sufficiently detected above background levels at thermal-water sites to indicate these dyes were 

in the thermal springs. The rhodamine dye released at site 2 was 

detected above background levels at several thermal water sites 

over a period of several weeks (table 4). The levels of dye 

detected were low because low concentrations of dyes were 

released to prevent the water in the public water system from 

discoloring. 
Figure 15. Estimated recharge area for shallow ground-water contribution to the thermal springs.









































































  





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32 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

Eosine dye (1 liter, 10 percent concentration) was released 

at site 1 about 100 ft above spring 31 in a partially paved natural 

drainageway for road drainage. About 30 ft above spring 31, the 

drainageway flows into a culvert, then underground past display 

springs 32 and 33, between the Fordyce and Maurice bathhouses, and to Hot Springs Creek. Positive identification of the 

eosine dye was detected in the surface drainage at the culvert 

entrance, but detection levels were not sufficiently above background levels at thermal water sites to conclusively determine 

presence of the dye. 

Fluorescein dye (4 liters, 10 percent concentration) was 

released at site 3 about 1,500 ft east of Central Avenue in a natural drainageway below Hot Springs Mountain Drive. Surface 

drainage flows northwest until Hot Spring Mountain Drive, 

where drainage flows underground to Fountain Street drainage, 

then to Hot Springs Creek. Positive identification of the fluorescein dye was detected in the surface drainage near the bottom of 

Hot Springs Mountain, but dye was not detected at any of the 

thermal water recovery sites. Dye release site 3 was the site farthest from the thermal springs and it is possible that more dye 

or more time was needed to reach detection levels at the recovery sites. 

Rhodamine dye (2 liters, 5 percent concentration) was 

released at site 2 about 1,000 ft east of Central Avenue in a partially paved natural drainageway used for road drainage (fig. 

17). The drainageway lies near and perhaps along a fault proposed by Bedinger and others (1979). The flow path of the dye 

was probably along drainage west until Hot Springs Mountain 

Drive just above the upper thermal springs. At that point, the 

drainage flows into a culvert and underground, apparently to 

Fountain Street drainage and Hot Springs Creek. It was 

observed by the author that the culvert had open gaps between 

sections where water could filter into the ground. The culvert 

terminates in the area of wells W2 and W3. The dye could have 

either flowed to wells W2 and W3 through the culvert across the 

fault, or the dye could have seeped into the fault, which intercepted wells W2 and W3 downdip.

The flow path of the rhodamine dye to the thermal springs 

is probably along the western boundary contact with the Stanley 

Shale. Bedinger (1994) suggested that this contact is a thrust 

fault. It also is possible that the dye flowed into northeast trending fractured lineaments higher up on the mountain that lead to 

the thermal springs. Bryan (1922) proposed four such lineaments that appear to run along lines of thermal spring occurrence (fig. 16). The dye did show up at springs near each of the 

four proposed lineaments. 

Figure 17. Rhodamine dye release at site 2 on Hot Springs Mountain, Hot 

Springs National Park, Arkansas.

Table 4. Results of rhodamine dye release at site 2 on December 4, 2004.

[Res., collection reservoir inflow; W2, Arlington lawn well west; W3, Arlington lawn well east; CB5, collection box 5; Rainfall is total rainfall for period 

between date charcoal packet installed and date charcoal packet removed]

Date

charcoal

packet

installed

Date

charcoal

packet

removed

Sites where dye

was detected 

Rainfall

(inches)

12/03/04 12/09/04 Res., W3 0.56

12/09/04 12/15/04 W3 0

12/15/04 12/22/04 Res., W2, W3,

Springs 25, 34, 42, CB5

0.66

12/22/04 12/28/04 Res., W3, Springs 25, 42 0.07

12/28/04 01/06/05 Res., Springs 34, 42 2.28

01/06/05 01/14/05 not detected 0.88

01/14/05 01/20/05 not detected 0

01/20/05 02/04/05 not detected 0.76

02/04/05 03/03/05 not detected 1.37
Summary 33

Presence of the rhodamine dye at the thermal springs, 

wells, and reservoir verifies that this area is part of the recharge 

area and that runoff from this area enters the ground-water system at some point along the pathway of the rhodamine dye. Several implications can be drawn from this conclusion. The drainage for the dye also drains runoff from the road. Therefore, road 

drainage enters the ground-water system that is connected to the 

thermal springs. Time of travel from release point to the thermal 

springs was 1 to 3 weeks, depending on where the dye was 

detected (table 4). Time of travel also would be dependent on 

other environmental conditions such as the soil saturation condition, and the amount, intensity, and duration of rainfall. The 

dye was detected only at well W3 and the collection reservoir 

after the first week of observation. Well W3 is not part of the 

thermal water collection system, therefore, the dye that was 

detected at the collection system reservoir must have come 

through a spring that was not monitored with a charcoal packet. 

Summary

The thermal springs of Hot Springs National Park have 

been valued for the recreational and therapeutic benefits of the 

thermal baths, as a source of drinking water, and a destination 

of attraction since the history of the area was first recorded. The 

future of the park and the city of Hot Springs depends greatly on 

maintaining and protecting this unique natural resource from 

degradation and contamination. To maintain and protect the 

thermal springs, it is imperative to understand the character of 

the springs, monitor changes in spring characteristics, and evaluate the source of the thermal springs.

The thermal springs are situated in the Ouachita Mountains of central Arkansas. The springs emerge in a gap between 

Hot Springs Mountain and West Mountain in an area about 

1,500 feet long by 400 feet wide at altitudes from 576 to 683 

feet. The springs predominantly are composed of a deep thermal 

ground water component with a lesser but qualitatively substantial component of shallow cold ground water. Currently, there 

are 43 thermal springs in the park that are presumed to be flowing. Thermal water from 33 of the thermal springs is collected 

and monitored at a central reservoir, which distributes the combined discharge for public use and consumption.

Rock types exposed in the vicinity of the thermal springs 

are shale, chert, novaculite, sandstone, and conglomerate. Shale 

units generally impede ground-water movement, while fractured chert, novaculite, and sandstone units generally support 

ground-water movement.

The average daily collection system discharge over the 

period of record 1990 through 1995 and 1998 through 2005 was 

658,000 gal/d and ranged from 518,000 to 763,000 gal/d, not 

including 131,000 gal/d from springs 43 and 43a that emerge 

from the bottom of the collection system reservoir. The overall 

pattern of the collection system discharge from 1990 to 2005 

shows an increasing rate of discharge; the majority of the 

increase took place from 1990 through 1995. Changes in the 

collection system temperature showed a positive relation to 

changes in discharge from 1990 through 1995, and an inverse 

relation to changes in discharge from 1998 through 2005. The 

period 1995 through 1998 was not monitored. The collection 

system discharge shows a good response to rain events, increasing in discharge when rainfall contributes recharge to the 

ground-water system. 

Continuous water temperature monitoring conducted by 

the USGS from 1990 through 1995 and from 1998 through 2005 

at the collection system reservoir inflow pipe shows that there 

has not been a significant long-term temperature change during 

the past 15 years. The daily water temperature ranged from 59.1 

to 62.1 °C and the average daily temperature was 61.4 °C. The 

collection system water temperature shows a strong seasonal 

pattern, with highs and lows about 1 month delayed from air 

temperature highs and lows. The collection system temperature 

also shows strong response to rainfall. 

The water temperatures were monitored at four thermal 

springs from August 2000 through June 2005, and at four additional thermal springs and one thermal spring collection box 

from September 2003 to June 2005. Springs 8, 17, 49, and CB1 

(group 1) generally showed strong seasonal patterns and sharp 

responses to changes in air temperature and rainfall. Group 1 

showed water temperature highs from August through September and lows from February through March, about 1 month later 

than air temperature highs and lows. Springs 8, 49, and CB1 

water temperature responded to practically all rain events that 

contributed recharge to ground water, while spring 17 response 

was more gradual and of less magnitude. Spring 8 water temperature dropped several degrees Celsius after large storm 

events. Springs 8, CB1, 17, and 49 showed daily water temperature fluctuations of about 0.10, 0.20, 0.05, and 0.05 °C, respectively, with highs and lows coinciding similarly to air temperature highs and lows. 

Springs 15, 19, 25, 42, and 46 (group 2) generally showed 

more stable water temperatures and less response to changes in 

air temperature and rainfall than group 1. The water temperatures in group 2 decreased towards the south except for spring 

46. Springs 15, 19, 42, and 46 show water temperature highs 

around September and lows around March, which is about 2 

months later than air temperature highs and lows. Spring 25 

water temperature exhibited a more delayed seasonal cycle of 

about 5 months, with highs in December and lows in June. 

Short-term variations in spring water temperatures because of 

diurnal air temperature changes and rain events are small or 

negligible in group 2 springs. Springs 42 and 46 showed daily 

water temperature fluctuations in response to diurnal air temperature changes of about 0.02 °C and 0.05 °C, respectively. 

Springs 25 and 46 may drop in water temperature several tenths 

of a degree Celsius during heavy rainfall. 

The source of the thermal water component of the thermal 

springs hypothetically enters the ground-water system as rainfall from regional recharge areas in the fractured, relatively permeable Bigfork Chert, Arkansas Novaculite, and Hot Springs 

Sandstone. The meteoric water migrates to estimated minimum 

depths of 4,500 to 7,500 ft and achieves high temperatures in 
34 Characteristics of Thermal Springs and the Shallow Ground-Water System at Hot Springs National Park, Arkansas

the deep section of the flow path before rising along fault and 

fracture conduits. Under artesian pressure, the thermal waters 

rise and emerge through the Hot Springs Sandstone between the 

traces of two thrust faults, along several northeast-trending lineaments.

The cold-water component enters the ground-water system 

as locally derived recharge from rainfall, and flows primarily 

along shallow northeast trending faults, joints, and fractures to 

the thermal springs. The thermal springs are bounded on the 

southwest, southeast, and northwest by shale barriers. The 

lower member of the Arkansas Novaculite is probably the primary aquifer of shallow ground-water flow. Water-level observations made at four wells in the thermal springs area indicate 

that shallow ground-water flow generally follows the slope of 

Hot Springs Mountain, flowing southwest and towards Hot 

Springs Creek. A hydraulic connection appears to be present 

between the lower and upper thermal springs, as evidenced by 

a decrease in the number of upper springs over the last 100 

years, presumably because of excavation and other earth-moving activities at or near the thermal springs. 

Alternative sources of cold-water recharge that are plausible include: (1) vertical infiltration through fill material around 

the spring catchments, (2) lateral infiltration of water from Hot 

Springs Creek to adjacent springs on bathhouse row, (3) 

ground-water flow in the Stanley Shale across Central Avenue 

along major faults, and (4) longer, cold-water flow paths such 

as the proposed thermal water flow path through the Bigfork 

Chert, but taking shallower flow paths to the thermal springs.

The size of the cold-water recharge area was estimated 

from the general concept of the hydrologic budget, where the 

average annual ground-water recharge (input) is equal to the 

average annual cold-water discharge (output) of the thermal 

springs. Based on the thermal springs estimated cold groundwater baseflow discharge of 17.8 Mgal/yr, and an estimated 

ground-water recharge rate of 5 to 10 in/yr, the estimated size 

of the shallow ground-water recharge area computes to 0.10 to 

0.20 mi2. The shallow ground-water recharge area appears to be 

bounded on three sides by low-permeability barriers, and 

extends approximately to the topographic divide. The shallow 

ground-water recharge area based on the boundaries is about 

0.14 mi2. 

Dyes were released at three sites in the proposed recharge 

area above the thermal springs. Two of the dyes were not sufficiently detected above background levels to indicate the dyes 

were in the thermal springs. Rhodamine dye released at site 2, 

about 1,000 ft east of Central Avenue, was detected above background levels at several recovery sites over a period of several 

weeks. The flow path of the rhodamine dye to the thermal 

springs is probably along the western boundary contact with the 

Stanley Shale or along northeast-trending fractured lineaments. 

Presence of the dye verifies that this area is part of the recharge 

area and that surface water enters the ground-water system at 

some point along the pathway of the rhodamine dye. Time of 

travel from the release point to the thermal springs was 1 to 3 

weeks, depending on where the dye was detected.

References

Arndt, R.H., and Stroud, R.B., 1953, Thrust faulting near the 

hot springs, Hot Springs National Park, Arkansas, App. 3, in 

Arndt, R.H., and Damon, P.E., Radioactivity of thermal 

waters and its relationship to the geology and geochemistry 

of uranium: Arkansas University of Institute of Science and 

Technology Annual Progress Report to U.S. Atomic Energy 

Commission, 28 p.

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Yeatts, D.S.—CHARACTERISTICS OF THERMAL SPRINGS AND THE SHALLOW GROUND-WATER SYSTEM AT HOT SPRINGS NATIONAL PARK, 

ARKANSAS—U.S. Geological Survey Scientific Investigations Report 2006-5001