Geological Survey Professional Paper 1044—C The Waters of Hot Springs National Park, Arkansas—Their Nature and Origin

The hot springs of Hot Springs National Park, Arkansas, issue from the Hot Springs Sandstone Member of the Stanley Shale of Mississippian age at the crest of a plunging overturned anticline, along the southern margin of the Ouachita anticlinorium. The combined flow of the 47 hot springs ranges from 750,000. to 950,000 gallons per day (3.28x10^-2 to 4.16x10^-2 cubic meters per second). The temperature of the combined hot-springs water is about 62° Celsius. Using silica concentration as an indicator, the maximum temperature of the water at depth is estimated to be not more than a few degrees Celsius higher than the temperature at the surface. Silica concentrations of the water since 1901 and maximum temperature measurements since 1804 indicate a small decline in temperature with time.

Tritium and Carbon-14 analyses of the water indicate that the water is a mixture of a very small amount of water less than 20 years old and a preponderance of water about 4,400 years old. The radioactivity and chemical concentration of the hot springs are similar to that of cold-water springs in the area. The dissolved solids concentrations range from 175 to 200 milligrams per liter. The main differences in the quality of the hot water, compared with the nearby cold ground water, are the high temperature and higher silica concentrations of the hot springs.

The geochemical data, flow measurements, and geologic structure of the region support the concept that virtually all the hot-springs water is of meteoric origin recharged locally. Recharge to the hot-springs artesian system is by infiltration of rainfall in the outcrop areas of the Big fork Chert and Arkansas Novaculite. The water moves slowly to depth where it is heated by contact with rocks of high temperature. Highly permeable zones, related to jointing or faulting, collect the heated water in the aquifer and provide avenues for the water to travel to the surface.