U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5001
By Daniel S. Yeatts
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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
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.
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