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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