STRONTIUM AND CARBON ISOTOPE GEOCHEMISTRY OF WATER FROM THERMAL SPRINGS AND COLD-WATER SOURCES IN THE HOT SPRINGS RECHARGE AREA, HOT SPRINGS, ARKANSAS

Geochemical analyses were conducted on water samples from 10 hot springs, 15 cold-water wells, and 2 cold springs located in or near the suspected recharge area for the thermal springs at Hot Springs National Park (HSNP), Arkansas, focusing on strontium and carbon isotopes as important tracers.   Strontium and carbon isotopes serve as useful tracers to delineate flow paths, because of the distinctive mineralogy and isotopic signatures of specific geologic units and the varying degree and mode of rock-water interaction.   Strontium isotopes have been previously used to show that cold-water recharge from storm events contribute to the total flow from the thermal springs.  This present study uses strontium isotopic analyses 1) to better conceptualize ground-water flow paths, 2) to determine flow path lengths and water residence time, and ultimately 3) to facilitate delineation of the recharge area for the thermal springs. 

Preliminary results indicate that substantial geochemical differences exist between strontium isotopes within different geologic units; likewise, flow paths and recharge zones have the potential to be delineated using this technology.  Carbon-14 (¹⁴C) and Carbon-13 (¹³C)   data from HSNP, incorporate measured values for soil carbon, bedrock carbonate, dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC).  Previous age dating estimated the age of the thermal waters at approximately 4,500 years old.  This age estimate has inherent uncertainty because it was based only on measured ¹⁴C and DIC values, whereas ¹³C   for the country rock, soil carbon, and DOC was assumed.  Deriving a geochemical model-based water age estimate using measured ¹⁴C   and ¹³C   will refine water age estimate accuracy and will provide for a better understanding flow path length as directly related to recharge area.