HYDROGEOLOGY AND WATER QUALITY OF THE HOT SPRINGS OF HOT SPRINGS NATIONAL PARK, ARKANSAS

The U.S. Geological Survey and the National Park Service have worked jointly in a series of studies to characterize the hydrogeology of the Hot Springs National Park area and identify recharge and water-quality vulnerabilities. Urbanization in the area of the hot springs has increased the potential for degradation of groundwater recharge amount and water quality for this nationally recognized resource. A coupled groundwater and surface-water flow model--integrating data from more than 30 years of study--is being developed to estimate effects of varying land-use, such as increased urbanization, as well as changing climate. Data indicate a two-component source of water recharging the hot springs: A primary, hot-water source about 4,400 years old and a secondary, cold-water source less than 20 years old.

Water samples collected during varying hydrologic conditions and continuous spring temperature data show that the thermal springs exhibit a distinct and significant geochemical and temperature response to storm events with as much as 35% of the flow from hot springs being contributed by young, cold-water recharge. Hydrogeologic and geochemical characterization indicates that the cold-water component of recharge is derived from a local area of about 0.2 km², whereas the thermal-water recharge area encompasses approximately 23 km². The relatively young, shallow, surface-influenced origin of the cold water renders it susceptible to surface-derived contamination related to human activities—in sharp contrast with the hot water.

Protection and management strategies have been implemented to address this threat to water quality and ensure the continued pristine nature of these thermal waters. Recharge of the hot springs occurs primarily in exposures of the highly fractured Bigfork Chert and Arkansas Novaculite. Two primary water types occur in the area—a low ionic strength, mixed-type water found in the quartz-dominated lithologies, and a high ionic strength calcium-bicarbonate water type in shale formations. Water chemistry of the hot springs is likely a result of initially, low ionic strength waters, originating as shallow recharge through quartz formations, moving very slowly from shallow to deeper sections of the flow path (as much as 2400 meters deep), where fault conduits traverse shale formations.