Abstract

A study was conducted by the U.S Geological Survey in cooperation with the Arkansas State Highway and Transportation Department and the Federal Highway Administration to characterize the source and hydrogeologic conditions responsible for thermal water in a domestic well 5.5 miles east of Hot Springs National Park, Hot Springs, Arkansas, and to determine the degree of hydraulic connectivity between the thermal water in the well and the hot springs in Hot Springs National Park. The water temperature in the well, which was completed in the Stanley Shale, measured 33.9 degrees Celsius, March 1, 2006, and dropped to 21.7 degrees Celsius after 2 hours of pumping—still more than 4 degrees above typical local groundwater temperature. A second domestic well located 3 miles from the hot springs in Hot Springs National Park was discovered to have a thermal water component during a reconnaissance of the area. This second well was completed in the Bigfork Chert and field measurement of well water revealed a maximum temperature of 26.6 degrees Celsius. Mean temperature for shallow groundwater in the area is approximately 17 degrees Celsius. The occurrence of thermal water in these wells raised questions and concerns with regard to the timing for the appearance of the thermal water, which appeared to coincide with construction (including blasting activities) of the Highway 270 bypass-Highway 70 interchange. These concerns were heightened by the planned extension of the Highway 270 bypass to the north—a corridor that takes the highway across a section of the eroded anticlinal complex responsible for recharge to the hot springs of Hot Springs National Park.

Concerns regarding the possible effects of blasting associated with highway construction near the first thermal well necessitated a technical review on the effects of blasting on shallow groundwater systems. Results from available studies suggested that propagation of new fractures near blasting sites is of limited extent. Vibrations from blasting can result in rock collapse for uncased wells completed in highly fractured rock. However, the propagation of newly formed large fractures that potentially could damage well structures or result in pirating of water from production wells appears to be of limited possibility based on review of relevant studies.

Characteristics of hydraulic conductivity, storage, and fracture porosity were interpreted from flow rates observed in individual wells completed in the Bigfork Chert and Stanley Shale; from hydrographs produced from continuous measurements of water levels in wells completed in the Arkansas Novaculite, the Bigfork Chert, and Stanley Shale; and from a potentiometric-surface map constructed using water levels in wells throughout the study area. Data gathered from these three separate exercises showed that fracture porosity is much greater in the Bigfork Chert relative to that in the Stanley Shale, shallow groundwater flows from elevated recharge areas with exposures of Bigfork Chert along and into streams within the valleys formed on exposures of the Stanley Shale, and there was no evidence of interbasin transfer of groundwater within the shallow flow system.

Fifteen shallow wells and two cold-water springs were sampled from the various exposed formations in the study area to characterize the water quality and geochemistry for the shallow groundwater system and for comparison to the geochemistry of the hot springs in Hot Springs National Park. For the quartz formations (novaculite, chert, and sandstone formations), total dissolved solids concentrations were very low with a median concentration of 23 milligrams per liter, whereas the median concentration for groundwater from the shale formations was 184 milligrams per liter. Ten hot springs in Hot Springs National Park were sampled for the study. Several chemical constituents for the hot springs, including pH, total dissolved solids, major cations and anions, and trace metals, show similarity with the shale formations in exhibiting elevated concentrations, though mean and median concentrations for most constituents were lower in the hot springs compared to water from the shale formations. The chemistry of the hot springs in Hot Springs National Park is likely a result of rock/water interaction in the shale formations in the deeper sections of the hot springs flow path; the initially, low ionic strength waters, originating as shallow recharge through quartz formations, move through the upper section of the flow path into deeper sections of the flow path and are modified by passage through shale formations present at depth. Mixing curves for lithium, manganese, and strontium concentrations, which were greatest in the shale formations, indicate that the hot springs represent an approximate 40-percent contribution of water from the shale formations and a 60-percent contribution of water from the quartz formations.

Characterization of strontium geochemistry and analysis of strontium isotopic signatures were conducted by comparing geochemical analyses from the hot springs in Hot Springs National Park, shallow groundwater samples in the study area from the quartz and shale formations, and samples from two western hot springs 30 to 50 miles west of Hot Springs in Montgomery County. Mixing model analysis indicated that the strontium geochemistry of the hot springs results from an approximately 35-percent contribution from the shales and a 65-percent contribution from the quartz formations, similar to that found from trace-metal analysis curves. The geochemistry results of the newly discovered thermal water sites at the two domestic wells were not similar to the geochemistry results for the hot springs in Hot Springs National Park with respect to strontium chemistry, but rather resemble that of the groundwater of the local outcropping formations. The very different isotopic and geochemical signatures observed for the thermal water from the two domestic wells as compared with the Hot Springs National Park springs do not provide evidence of any direct hydraulic connection with the hot springs in Hot Springs National Park.

The occurrence of thermal water throughout the Ouachita Mountains tends to be found in similar geologic settings; along the nose of plunging anticlines and closely aligned with mapped thrust faults. The shallow flow systems have been identified as being confined within the regional surface watershed boundaries. The deep flow systems for thermal water are likely a result of the local hydrologic and geologic framework and represent an analog to the geologic framework model for propagation of groundwater to the hot springs in Hot Springs National Park, rather than being systems in direct communication with the Hot Springs National Park thermal system. Thermal-water sites across the Ouachita Mountains appear to represent discrete systems that are associated with a specific set of hydrologic and geologic conditions, which occur at numerous locations across the region, and are viewed as analogs to the hot spring flow system in Hot Springs National Park rather than connected components of the same hydraulic system. Concerns related to pirating of water from the hot springs in Hot Springs National Park because of blasting near the thermal well sites were not supported by the data gathered for this study.

Revised March 8, 2011

First posted December 17, 2009